Analysis of Prerequisites for Connection of a Large-Scale

-authorIN DEGREE PROJECT ENERGY AND ENVIRONMENT, SECOND CYCLE, 30 CREDITS
, STOCKHOLM SWEDEN 2021
Analysis of Prerequisites for Connection of a Large-Scale Photovoltaic System to the Electric Power Grid
FANNY LILJA
KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE
Analysis of Prerequisites for Connection of a Large-Scale Photovoltaic System to the
Electric Power Grid
FANNY LILJA
Master’s Degree Project in Electric Power Systems School of Electrical Engineering & Computer Science KTH Royal Institute of Technology Stockholm, Sweden 2021
Supervisors: Mikaela Liss, Solkompaniet Anton Gronkvist, E.ON Energidistribution Elis Nycander, KTH Royal Institute of Technology
Examiner: Lennart Soder, KTH Royal Institute of Technology
Abstract
The deployment of large-scale photovoltaic (PV) systems is rising in the Swedish power system, both in quantity and in system size. However, the intermittent characteristics of the PV production raises questions concerning the stability in the electric power grid, and power output fluctuations from the PV systems can lead to voltage quality issues. Hence, the distribution system operator E.ON Energidistribution and the solar energy developer company Solkompaniet are interested in investigating potential challenges and possibilities related to the integration of large-scale PV systems in the electric power grid. This thesis studies fast voltage variations in the electric power grid due to output fluctuations from large-scale PV systems, and examines the possibility to mitigate the voltage variations by reactive power support strategies in the PV inverters.
Four studies are carried out to investigate the prerequisites for establishing large-scale PV systems. Firstly, a worst-case study considering eight existing substations in the electric power grid as well as a new substation is carried out, to examine the impact of different parameters on the voltage variations. Parameters such as transformer operation mode, location of the point of connection, switching mode and load capacity are compared in the study. Further, time series calculations are done to investigate the voltage variations over one year, and a study with an oversized PV system is done to investigate the possibility for increasing the PV capacity without grid reinforcements. Lastly, a study is performed with reactive power compensation from the PV inverters to examine the possibility to maintain a stabilized voltage level at the point of connection. The studies are performed in E.ONs network model in the power system simulator software PSS/E, with data for the transmission grid, the regional grid, and parts of the distribution grid included. PV systems with a rated capacity from 32 MWp and upwards are connected to substations in the regional grid, where fast voltage variations on nominal voltage levels of 20/10 kV are studied and evaluated from the perspective of the power producer.
From this thesis, it can be concluded that neither of the implemented studies results in voltage variations that violate E.ONs technical requirements on fast voltage variations in the point of connection. Further, the results from the worst-case study show the importance of analysing the specific system of interest when connecting PV systems, since the properties of the existing system have an impact on the voltage variations. The time series calculations show that the voltage variations over a time period of one year are highly influenced by the PV production and the load capacity in the substation, and the study with an oversized PV system shows the possibility for increasing the PV capacity without curtailing large amounts of active power. Finally, the study with reactive power compensation concludes that grid support strategies in the PV inverters may be a key solution for making optimal use of the existing electric power grid and enabling the continued expansion of large-scale PV systems in the Swedish power system.
Keywords: solar energy, large-scale photovoltaic system, PV, Swedish power system, power output fluctuations, voltage quality, voltage variations, reactive power compensation, PSS/E
Sammanfattning
Fyra studier genomfors for att undersoka forutsattningarna for att etablera storskaliga solcellsanlaggningar. For det forsta genomfors en varsta-fallstudie med beaktande av atta befintliga stationer i elnatet samt en ny station, for att undersoka olika parametrars paverkan pa spanningsvariationerna. Parametrar som transformatorns driftlage, plats for anslutningspunkten, omkopplingslage och lastkapacitet jamfors i studien. Vidare gors tidsserieberakningar for att undersoka spanningsvariationerna over ett ar, och en studie med en overdimensionerad solcellsanlaggning gors for att undersoka mojligheten att oka solcellskapaciteten utan elnats- forstarkningar. Slutligen genomfors en studie med reaktiv effektkompensation fran vaxelriktare for att undersoka mojligheten att uppratthalla en stabiliserad spanningsniva i anslutningspunkten. Studierna utfors i E.ONs natverksmodell i programvaran PSS/E for kraftsystemsimule- ringar, med data for transmissionsnatet, regionnatet och delar av distributionsnatet inkluderat. Solcellsanlaggningar med en nominell kapacitet fran 32 MWp och uppat ansluts till stationer i regionnatet, dar snabba spanningsvariationer pa nominella spanningsnivaer om 20/10 kV studeras och utvarderas ur kraftproducentens perspektiv.
Fran resultaten kan man dra slutsatsen att ingen av de genomforda studierna resulterar i spanningsvariationer som overskrider E.ONs tekniska krav pa snabba spanningsvariationer i anslutningspunkten. Vidare visar resultaten fran varsta-fallstudien vikten av att analysera det specifika systemet vid anslutning av solcellsanlaggningar, eftersom egenskaperna hos det befintliga systemet har en inverkan pa spanningsvarationerna. Tidsserieberakningarna visar att spanningsvariationerna over en tidsperiod av ett ar paverkas starkt av bade energiproduktionen och lastkapaciteten i stationen, och studien med en overdimensionerad solcellsanlaggning visar pa mojligheten att oka den nominella kapaciteten utan att spilla stora mangder aktiv effekt. Slutligen ger studien med reaktiv effektkompensation slutsatser om att strategier i vaxelriktare kan vara en mojlig losning for att utnyttja det befintliga elnat optimalt och mojliggora en fortsatt expansion av storskaliga solcellsanlaggningar i det svenska kraftsystemet.
Nyckelord: solenergi, storskalig solcellsanlaggning, PV, Sveriges kraftsystem, effekforandringar, spanningskvalitet, spanningsvariationer, reaktiv effektkompensering, PSS/E
Acknowledgements
I would like to thank all who have contributed and made this thesis possible to conduct. Firstly, a warm thank you to Lars Hedstrom at Solkompaniet and Andreas Svensson at E.ON Energidistribution for believing in me and giving me the opportunity to implement this thesis in cooperation with them. Next, I would like to address a special thank you to my supervisors, Mikaela Liss at Solkompaniet, Anton Gronkvist at E.ON Energidis- tribution and Elis Nycander at KTH Royal Institute of Technology, for their guidance throughout the entire thesis work. I am deeply grateful for all the support, knowledge, and great ideas I received from them. Last but not least, I would like to thank Lennart Soder, my examiner at KTH Royal Institute of Technology, for his revision and feedback throughout the thesis work.
Table of Contents
1 Introduction 8 1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.3 Research Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.4 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.5 Report Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2 Theoretical Background 13 2.1 The Power System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.1 Overview of the Electric Power Grid in Sweden . . . . . . . . . . 13 2.1.2 Characteristics for Power Transmission . . . . . . . . . . . . . . . 14
2.2 Technical Requirements for Voltage Quality . . . . . . . . . . . . . . . . 16 2.2.1 Voltage Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.2.2 The Network Operator . . . . . . . . . . . . . . . . . . . . . . . . 17 2.2.3 The Power Producer . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3 Solar Photovoltaics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.3.1 Photovoltaic Systems . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.3.2 Power Output Fluctuations . . . . . . . . . . . . . . . . . . . . . 22 2.3.3 Potential Challenges with Photovoltaics . . . . . . . . . . . . . . 23
2.4 Photovoltaic Inverters for Voltage Regulation . . . . . . . . . . . . . . . 25 2.4.1 Control Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.4.2 PQ Capability Chart . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.4.3 Implementation Strategies . . . . . . . . . . . . . . . . . . . . . . 28
2.5 Alternative Solutions for Voltage Regulation . . . . . . . . . . . . . . . . 29 2.5.1 Tap Changers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.5.2 Shunt Compensators . . . . . . . . . . . . . . . . . . . . . . . . . 30 2.5.3 Battery Energy Storage Systems . . . . . . . . . . . . . . . . . . . 30 2.5.4 Solar-Wind Complementation . . . . . . . . . . . . . . . . . . . . 31
3 Methodology 32 3.1 Power System Modelling Software . . . . . . . . . . . . . . . . . . . . . . 32 3.2 Network Model Description . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.2.1 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
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3.2.2 Switching Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.2.3 Load- and Generator Units . . . . . . . . . . . . . . . . . . . . . . 33 3.2.4 Operation Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.2.5 Point of Connection . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.3 Power Flow Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3.3.1 Power Output Fluctuations . . . . . . . . . . . . . . . . . . . . . 37 3.3.2 Load Flow Calculations . . . . . . . . . . . . . . . . . . . . . . . 38 3.3.3 Voltage Quality Assessment . . . . . . . . . . . . . . . . . . . . . 39
3.4 Worst-Case Study: Existing Substations . . . . . . . . . . . . . . . . . . 39 3.4.1 Study Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.4.2 Substations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.4.3 Operation Mode & Point of Connection . . . . . . . . . . . . . . . 42 3.4.4 Load Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 3.4.5 Short Circuit Capacity & Switching Mode . . . . . . . . . . . . . 45 3.4.6 Power Flow Simulations . . . . . . . . . . . . . . . . . . . . . . . 46
3.5 Worst-Case Study: New Substation . . . . . . . . . . . . . . . . . . . . . 47 3.6 Time Series Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.6.1 Hourly Production Data . . . . . . . . . . . . . . . . . . . . . . . 49 3.6.2 Hourly Active- and Reactive Load Data . . . . . . . . . . . . . . 50 3.6.3 Power Flow Simulations . . . . . . . . . . . . . . . . . . . . . . . 51 3.6.4 Constant Load Data . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.7 Oversized Photovoltaic System . . . . . . . . . . . . . . . . . . . . . . . . 53 3.8 Reactive Power Compensation . . . . . . . . . . . . . . . . . . . . . . . . 56
4 Results & Discussion 60 4.1 Worst-Case Study: Existing Substations . . . . . . . . . . . . . . . . . . 60
4.1.1 Parallel Operation Mode . . . . . . . . . . . . . . . . . . . . . . . 60 4.1.2 Sectioned Operation Mode . . . . . . . . . . . . . . . . . . . . . . 64 4.1.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.2 Worst-Case Study: New Substation . . . . . . . . . . . . . . . . . . . . . 75 4.3 Time Series Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.3.1 Time Series Calculations with Hourly Load Profiles . . . . . . . . 78 4.3.2 Time Series Calculations with Constant Loads . . . . . . . . . . . 86 4.3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.4 Oversized Photovoltaic System . . . . . . . . . . . . . . . . . . . . . . . . 91 4.5 Reactive Power Compensation . . . . . . . . . . . . . . . . . . . . . . . . 96
5 Conclusion 101
List of Figures
2.1 PQ capability chart for the Sungrow inverter model SG250HX [1] . . . . 28 2.2 Monthly electricity production from PV- and wind power systems during
the period 2014-2017 [2] . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.1 Parallel transformer operation mode with a direct PV-connection . . . . 35 3.2 Sectioned transformer operation mode with a direct PV-connection . . . 35 3.3 Parallel transformer operation mode with a separate PV-connection . . . 36 3.4 Sectioned transformer operation mode with a separate PV-connection . . 36 3.5 Investigated parameters in the worst-case study . . . . . . . . . . . . . . 40 3.6 Simulation chart for the power flow simulations in the worst-case study . 46 3.7 The standardized and up-scaled hourly PV production profile . . . . . . 50 3.8 Simulation chart for the time series calculations . . . . . . . . . . . . . . 51 3.9 Simulation chart for the study with an oversized PV system . . . . . . . 55 3.10 Reactive power compensation by the Q(U) curve in the Volt-Var function 57 3.11 Simulation chart for the study with reactive power compensation . . . . . 59
4.1 Voltage variation at POC in a parallel operation mode with a direct PV- connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.2 Voltage variation in a parallel operation mode with a direct PV-connection 63 4.3 Voltage variation in a sectioned operation mode with a direct PV-connection,
both sections affected by the power output fluctuation . . . . . . . . . . 65 4.4 Voltage variation in a sectioned operation mode with a direct PV-connection,
one section affected by the power output fluctuation . . . . . . . . . . . . 66 4.5 Voltage variation in a sectioned operation mode with a separate PV-
connection, both sections affected by the power output fluctuation . . . . 68 4.6 Voltage variation in a sectioned operation mode with a separate PV-
connection, both sections affected by the power output fluctuation . . . . 69 4.7 Voltage variation on each bus for each studied case for the new substation 77 4.8 Voltage variation each hour of the year for substation 1 . . . . . . . . . . 79 4.9 Voltage variation each hour of the year for substation 2 . . . . . . . . . . 80 4.10 Voltage variation each hour of the year for substation 3 . . . . . . . . . . 81 4.11 Voltage variation each hour of the year for substation 4 . . . . . . . . . . 82 4.12 Voltage variation each hour of the year for substation 5 . . . . . . . . . . 83 4.13 Voltage variation each hour of the year for substation 6 . . . . . . . . . . 84
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4.14 Voltage variation each hour of the year for substation 7 . . . . . . . . . . 85 4.15 Voltage variation each hour of the year for substation 8 . . . . . . . . . . 86 4.16 Voltage variation each hour of the year at POC 1 of substation 2 . . . . . 87 4.17 Voltage variation each hour of the year at POC 1 of substation 2 . . . . . 88 4.18 Correlation between the oversizing rate and the curtailed active power for
different load sizing rates . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 4.19 Produced respective curtailed active power each hour of the year for sub-
station 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 4.20 Produced respective curtailed active power each hour of the year for the
new substation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 4.21 Voltage variation each hour of the year at POC 1 with an oversizing rate
of 40% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 4.22 Voltage level and reactive power compensation in the new substation every
hour of the year . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 4.23 Voltage level and reactive power compensation in substation 6 every hour
of the year . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 4.24 Voltage level and reactive power compensation in substation 6 every hour
of the year . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
List of Tables
2.1 Limitations on short-term voltage drops for Un ≤ 45 kV . . . . . . . . . . 18 2.2 Limitations on short-term voltage drops for Un > 45 kV . . . . . . . . . . 18 2.3 Limitations on short-term voltage rises for Un ≤ 1 kV . . . . . . . . . . . 19 2.4 Limitations per day on the sum of rapid voltage changes and short-term
voltage drops present in area A in Table 2.1 and Table 2.2 presented previously . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.5 Compilation of the ramp rate for different amounts of installed PV capacity 23
3.1 Substation characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.2 Investigated parameters for each substation . . . . . . . . . . . . . . . . 42 3.3 Cable characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.4 Characteristics for the two cases with a new substation . . . . . . . . . . 48 3.5 Constant load capacity for each substation in each scenario . . . . . . . . 53 3.6 Load sizing rates and corresponding dimensioning load factors for substa-
tion 1 in the correlation study . . . . . . . . . . . . . . . . . . . . . . . . 54 3.7 Oversizing rates and corresponding rated PV capacity for substation 1 in
the correlation study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 3.8 Input data for the case study with an oversized PV system . . . . . . . . 54 3.9 Specified voltage limits in the Volt-Var function for each substation . . . 58
4.1 The short circuit capacity on the primary- and secondary side of each substation in a parallel operation mode with a direct PV-connection, for normal- and reserve switching mode respectively . . . . . . . . . . . . . . 61
4.2 The short circuit capacity on the PV-bus at each substation in a parallel operation mode with a separate PV-connection, for normal- and reserve switching mode respectively . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.3 The short circuit capacity on the buses at each substation in a sectioned operation mode with a direct PV-connection, for normal- and reserve switching mode respectively . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.4 The short circuit capacity on the two PV-buses at each substation in a sectioned operation mode with a separate PV-connection, for normal- and reserve switching mode respectively . . . . . . . . . . . . . . . . . . . . . 67
4.5 Compilation of the highest achieved voltage variation at POC 1 for each substation, in a sectioned operation mode with a separate PV-connection 70
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4.6 Compilation of the highest achieved voltage variation at POC 2 for each substation, in a sectioned operation mode with a separate PV-connection 70
4.7 The short circuit capacity for each studied case for the new substation . . 76 4.8 Compilation of the results from the case study with oversized PV systems 92
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List of Abbreviations
AC Alternating Current DC Direct Current DSO Distribution System Operator GW Gigawatt HV High-Voltage I Current kV kilo-Volt LV Low-Voltage MPPT Maximum Power Point Tracker MV Medium-Voltage MW Megawatt MWh Megawatt-hour MWp Megawatt-peak P Active Power POC Point of Connection PV Photovoltaic Q Reactive Power R Resistance S Apparent Power Ssc Short Circuit Capacity STATCOM Static Synchronous Compensator SVC Static VAR Compensator TSO Transmission System Operator U Voltage VA Volt-Ampere VAR Volt-Ampere reactive VSI Voltage Source Inverter W Watt X Reactance Z Impedance
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Introduction
This chapter gives the background and the motivation for the thesis. Further, the research objectives and the scope are stated, followed by an outline of the remainder of this report.
1.1 Background
To mitigate climate change and reduce the carbon dioxide emissions in the atmosphere, the transition from fossil fuels to renewable forms of energy is becoming increasingly significant for the global energy system. The integration of renewable energy technologies in the power generation sector is crucial for the overall transformation, and the use of variable renewable energy resources such as wind and solar are rising. A trend for both wind- and solar power technologies is constantly declining costs, where the costs are forecasted to further decline in the coming years [3]. Out of solar power technologies, solar photovoltaic (PV) systems make an essential part of the transition, where technology improvements and policy frameworks constitute the main driving forces for the continuing development and growth of the PV market [4]. Data from the International Renewable Energy Agency (IRENA) show that the global weighted average levelized cost of electricity (LCOE) for PV systems decreased by 77% during the period 2012-2018 [3]. That declining costs is one of the drivers for the expansion of higher penetrations of PV systems across the globe, is showing in the recent increase in installed PV capacity. At the end of 2019, the global installed capacity of PV exceeded 623 GWp whereof nearly 72% have been installed over the last five years. Globally, centralized large-scale PV systems dominate over distributed PV systems, standing for approximately 63% and 37% of the cumulative installed capacity respectively [4].
In the northern hemisphere, the deployment of PV systems in Sweden is rising, and by the end of 2020 there were almost 66 000 PV facilities connected to the electric power grid. Statistics from the Swedish Energy Agency shows that, with a total installed capacity of 1090 MWp, the development of PV systems exceeded the 1 GWp boundary. Compared to a total installed capacity of 698 MWp the year before, that represented an
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increase in capacity of about 56%. However, in contrast to the global trend, the statistics show that the share of distributed PV systems outreaches large-scale PV systems in the country. By the end of 2020, the average system size corresponded to 17 kWp, and large-scale PV systems greater than 1 MWp constituted about 5% of the total installed PV capacity, an increase from 2.7% the previous year [5]. In spite of the relatively small share of large-scale PV systems in Sweden, the segment is constantly growing and expected to increase both in quantity and in system size in the near future [6].
However, even if variable renewable energy resources are considered clean with plenty of benefits, the unique topologies of the technologies raise concerns in the operation of the overall power system. For solar power technologies, the intermittent and weather dependent characteristics of the solar resource directly affect the electric power generation from grid-connected systems, where variations in the solar irradiance proportionally result in power output fluctuations. The effects of power output fluctuations affects the stability of the electric power grid, leading to challenges regarding the flexibility of the power system [7]. For now, the electric power generation in Sweden substantially re- lies on hydro- and nuclear power which have important functions in the power system, contributing to voltage stabilization solutions. Voltage stabilization is a crucial system service for a reliable power system and is achieved by reactive power support strategies, where supply and absorption of reactive power enables to regulate the voltage level [4].
Conversion-based generation such as wind- and solar do not directly contribute to voltage stabilization in the same way as power plants with synchronous rotating generators, and with more variable renewable energy sources in the power system, it becomes increasingly important how this is technically solved. However, there are possibilities. Inverters with advanced functionalities are being developed, where the inverters have the ability to contribute with reactive power support strategies and other system services for grid support [4]. To adapt for the ongoing development and expansion of centralized large-scale PV systems, the flexibility in the overall power system needs to be secured. Therefore, the impacts of power output fluctuations on the operation and management of the electric power grids need to be fully understood and addressed for each unique situation. At the same time, possible mitigating solutions for voltage stabilization purposes need to be investigated, where inverters with advanced functionalities can be a key solution [7].
1.2 Motivation
The deployment of large-scale PV systems is growing in the Swedish power system, both in quantity and in system size. In recent years, the increase in requests to connect large-scale PV systems to the electric power grid has been noticed by the distribution system operator E.ON Energidistribution, who operates at the regional- and local grid in Sweden. The size of the requested systems varies from single megawatts to hundreds of megawatts, and a common trend for developers is to search for locations close to the existing electric power grid to get an as efficient grid-connection as possible, as the largest
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economic profitability is achieved when as much as possible of the existing infrastructure is used. Connection of PV systems to the distribution grid on the medium-voltage level and transformation to the regional grid is efficient, but leads to questions about whether the existing customers will be negatively affected. For E.ON, the increase in requests of large-scale PV systems have raised questions concerning the negative effects that the connections can have on the existing customers in the electric power grid, where large challenges are voltage quality impacts such as voltage variations as a result of power output fluctuations from the systems.
A pioneer within the solar energy sector is the solar energy developer company Solkom- paniet, who works towards speeding up the overall development process by establishing large-scale PV systems to the Swedish power system. Today, the possibility to connect larger PV systems to the local grid is limited, as new infrastructures and voltage regulation devices need to be implemented to ensure optimal operation with good voltage quality. The need for mitigation solutions for voltage stabilization purposes is rising, and one possible solution is the use of advanced inverter functionalities. With functions such as active- and reactive power support strategies in the inverters, a PV system has the ability to contribute with beneficial system services to the power system, enabling grid support and voltage stability.
On behalf of E.ON Energidistribution and Solkompaniet, there is a significant interest in understanding and addressing the above stated challenges and possibilities regarding connection of large-scale PV systems to the distribution grid on medium-voltage level. Therefore, this master thesis investigates the prerequisites for ensuring an optimal operation of the existing electric power grid when establishing large-scale PV systems. The high-level goal of the thesis is to gain valuable knowledge of the impact that large- scale PV systems have on the electric power grid in terms of voltage variations, and come up with connection requirements that can work as prerequisites for the continuing deployment of large-scale PV systems.
1.3 Research Objectives
The aim of the thesis is to investigate the prerequisites for connection of a large-scale photovoltaic system to the electric power grid. The thesis aims to study the impact of voltage variations in the electric power grid in situations of power output fluctuations from large-scale photovoltaic systems, and examine the possibility to mitigate any negative effects by reactive power support strategies in the photovoltaic inverters.
To fulfill the aim, a number of objectives have been identified. Firstly, explore and identify important characteristics regarding the power system, technical requirements related to voltage quality, power output fluctuations from photovoltaic systems, and adequate mitigating solutions for voltage regulation through a literature study. Secondly, study the event of voltage variations in situations of power output fluctuations from large-scale photovoltaic systems through four studies performed in the power system
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simulator software PSS/E. The implemented studies include the following:
• A worst-case study considering eight existing substations in the electric power grid as well as a new substation, to examine the impact of different parameters on the voltage variations. Parameters such as transformer operation mode, location of the point of connection, load capacity and switching mode are compared in the study.
• Time series calculations to investigate the voltage variations over a time period of one year.
• A study with an oversized photovoltaic system to investigate the possibility for increasing the capacity of photovoltaics without grid reinforcements.
• A study with reactive power compensation from the photovoltaic inverters to examine the possibility to maintain a stabilized voltage level at the point of connection.
1.4 Scope
Geographical boundaries: The study is geographically limited to study the electric power grid in electricity area SE4 located in the south of Sweden. The network model and the obtained data resources originate from the area.
Voltage quality assessment: The thesis studies fast voltage variations in the electric power grid in situations of power output fluctuations from large-scale PV systems, and no other voltage quality parameters influenced by output fluctuations are investigated. The fast voltage variations are evaluated from the perspective of the power producer, based on the technical requirements stated in the document “Technical requirements for grid-connection in the regional grid and at the 20/10 kV substations” [8] by E.ON regarding demands on fast voltage variations on the power producer.
System boundaries: The study is carried out in E.ONs network model in the power system simulator software PSS/E, with the entire transmission grid, regional grid, and parts of the distribution grid present in the model. The voltage variations at the secondary side of the substations are examined, i.e. on nominal voltage levels of 10 or 20 kV. The study is performed in a low load network model which represents a typical summer day with low loads in the power system.
Data resolution: For the time series calculations, the resolution of the collected data for production- and consumption are based on hourly values.
Photovoltaic system size: The rated capacity of the modelled PV systems varies depending on the studied substation, ranging from 32 MWp to 160 MWp in the worst- case study. In the study with an oversized PV system, rated capacities up to 224 MWp are examined.
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1.5 Report Outline
The remainder of this report is organized as follows: Chapter 2 presents a theoretical background for the thesis, Chapter 3 addresses the methodology, Chapters 4 presents and discusses the results, Chapter 5 highlights the conclusions, and Chapter 6 gives the suggestions for future work.
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Chapter 2
Theoretical Background
This chapter presents a theoretical background for the thesis, including areas of research that are of relevance for the continuing thesis work.
2.1 The Power System
This section presents an introduction to the power system, including an overview of the electric power grid in Sweden and characteristics for power transmission.
2.1.1 Overview of the Electric Power Grid in Sweden
The electric power grid is fundamental for providing electric energy services to the society, as the infrastructure enables transmission of electric power across the country. The main functionality is to transfer electricity from producers to consumers, while maintaining balance in the overall system [9]. In Sweden, the majority of the electric power generation is located in the northern parts while the demand for electricity is higher in the south. This results in large transfers of electric power from the north to the south. For a functioning electricity market, the power system is divided into four electricity areas, from north to south: SE1 (Lulea), SE2 (Sundsvall), SE3 (Stockholm), SE4 (Malmo). The borders between the electricity areas are located where there are physical restrictions on the amount of power that can be transferred. The electricity market needs to some extent take into account the limitations on transmission capacity that exist in the power system in order for the outcome from the electricity market to result in a distribution of production and consumption of electricity that is technically possible [10].
In Sweden, the electric power grid is divided into the transmission grid (220-400 kV), the sub-transmission grid (40-130 kV) and the distribution grid (≤20 kV), operating at different voltage levels [11]. The voltage levels of the electric power grid is characterized by the nominal voltage (Un), but the real voltage level that the power system is designated to operate at is called the normal operating voltage (for example 140 kV, 42.5 kV, 21.8
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kV etc). The power lines at the different voltage levels are connected at substations, where transformers adapt the voltage to the desired level on the next power line in order to transfer the electric power in the correct direction [12]. The transmission grid that operates at high-voltage (HV) level is also called the national grid, and is owned by the state and managed by the Transmission System Operator (TSO) Svenska Kraftnat who is system responsible for the entire power system in Sweden. The high voltage of the transmission grid enables the transfer of electric power over long distances with relatively small losses, and the main function for the TSO is to maintain the short-term energy balance in the power system by monitoring so that the stability and necessary safety margins are upheld. The transmission grid connects to large power producers, and transfers the generated electric power via the sub-transmission grid and the distribution grid to the final consumers. From the transmission grid, there are also connections over the national border to other European countries [9].
The sub-transmission grid which operates at a slightly lower voltage level is commonly called the regional grid, and is the connection link from the transmission grid to power producers, larger consumers such as industries and urban areas, and the distribution grids. Further, the distribution grid connects to small-scale power producers, and dis- tributes the electric power to the smaller consumers such as households, public buildings, and offices [9]. The distribution grid is also called the local grid, and is normally divided into medium-voltage (MV) grid at 10-20 kV and low-voltage (LV) grid at 230/400 V [11]. Both the sub-transmission- and the distribution grids are owned by Distribution Grid Operators (DSOs). There are approximately 170 different DSOs in Sweden, where the three largest (who together provide electricity to more than half of the electricity users in Sweden) are E.ON Energidistribution, Ellevio and Vattenfall [9].
2.1.2 Characteristics for Power Transmission
To understand the process of power transmission in the power system, there are a number of concepts which are of particular importance. In the power system, the locations where input respective output of electric power occur are referred to as nodes or buses. In each node, there is a constant voltage level, while the current changes according to Kirchhoff’s law which states that the current cannot disappear in a node. Due to the relationship between the voltage and the current, a power balance must occur in each node of the power system, meaning that the net production of power is equal to net transmission of power to the other nodes. At the same time as the power balance must be maintained, there are requirements on the current and voltage levels in different parts of the power system. In Sweden, most of the power transmission occurs in three-phase AC power, where the ideal state is symmetric three-phase which characterises that both the voltage and the current in respective phase are sinusoidal with the same amplitude and phase from each other with 120 degrees. The cycle repeats with the frequency of the power system, which is 50 Hz in Sweden [13].
Electric power is normally divided into active power (P) measured in Watts (W) and
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reactive power (Q) measured in Volt-Ampere reactive (VAR), related to each other by the phase angle (φ) which is the phase shift between the current and the voltage. The combination of active- and reactive power is called the apparent power (S) measured in Volt-Ampere (VA), and the relationship can be expressed with the Pythagorean theorem according to:
S = √ P 2 +Q2 (2.1)
The phase angle defines the power factor (PF) which is a dimensionless quantity that describes the relationship between active- and reactive power, and is expressed as:
PF = cosφ = P
( arctan
Q
P
) (2.2)
As observed from equations 2.1 and 2.2, the power factor is related to the amount of active- and reactive power. Commonly, a power factor close to 1 is desirable as it indicates a phase angle of zero, meaning that the current and voltage is totally in phase with each other (resistive loads). In this situation, the momentan power over a time period is constantly positive as the current and voltage are either both positive or both negative at the same time. A power factor of 1 indicates that the active power is at maximum and that all of the supplied power contributes to useful work i.e. transfer of electric energy to the load. However, at the same time active power is preferred in the power system, reactive power is hard to avoid due to magnetic fields (inductive loads) and electric fields (capacitive loads). The reactive power is referred to as non-useful work as it creates a flow of current in the power lines without contributing to useful work, i.e. reducing the capacity for active power transmission. Due to the increased current flow, reactive power has a large impact on the power losses in the power lines, where an increased flow of reactive power leads to increased losses. The transmission of reactive power indicates a phase shift between the current and voltage, meaning that the reactive power oscillates back-and-forth in the power line. When the phase angle increases, the power factor decreases according to equation 2.2 presented above. Depending on the character of the phase shift, the power factor is referred to as lagging/inductive when the current lags the voltage, and leading/capacitive when the current leads the voltage [14].
Due to the characteristics of power transmission, a power line can have a net production or net consumption of reactive power depending on the situation. To mitigate, it is possible to compensate by connecting devices with inductive or capacitive character that locally absorb or supply reactive power respectively. As the reactive power is strongly correlated with the voltage, the devices have the ability to both control the voltage level in the node and reduce the power losses in the line. Absorption of reactive power decreases the voltage level in the node, while supply of reactive power results in an increased voltage level [13]. As a change in reactive power has the ability to regulate the voltage level in the node, these types of strategies are commonly used
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to mitigate deviations in the voltage. The strategies are used by several devices for different purposes in the power system, which will be further presented in section 2.4 (Photovoltaic Inverters for Voltage Regulation) and section 2.5 (Alternative Solutions for Voltage Regulation). Below, equation 2.3 presents the relationship between a voltage variation and a change in active- and reactive power in a node [15]. In the equation, (U) is referred to as voltage variation, (P ) is active power change, (Q) is reactive power change, (R) is accumulated resistance, (X) is accumulated reactance, and (U) is the voltage magnitude.
U = P ·R +Q ·X
U (2.3)
Further on, rewriting equation 2.3 and setting (U) as 0, i.e. no voltage variation, it can be observed that the amount of reactive power that needs to be injected to the node in order to cancel the voltage variation is dependent on the active power change and the accumulated X/R ratio, according to the expression below.
Q = −P ·R X
R
)−1
(2.4)
As observed from equation 2.4, the accumulated X/R ratio has an impact on the need for reactive power compensation. Depending on the type of power line, i.e. overhead line or cable, the characteristics of R and X vary strongly. For example, overhead lines which are more common in the transmission grid, have a low resistance, which results in a high X/R ratio. The high X/R ratio results in the reactive power having a large influence on the voltage variation. Contrary, the distribution grid, which often consists of a mix of overhead lines and cables, usually has a lower X/R ratio, which means that the reactive power has a lower influence on the voltage variations.
Another important parameter for power transmission is the short circuit capacity which characterises the strength of the grid and shows the ability to resist disturbances. The short circuit capacity directly depends on the configuration of the grid and the impedance of components such as power lines, transformers, and the grid-connected loads and production facilities. A high short circuit capacity and a low impedance characterises a strong grid, while a weak grid is characterised by a low short circuit capacity and a high impedance. Commonly, the size of the short circuit capacity defines the limit of the allowed grid-connected loads and production facilities in a specific part of the electric power grid [13].
2.2 Technical Requirements for Voltage Quality
This section presents the concept of voltage quality together with technical requirements regarding voltage variations and reactive power exchange that applies for the network operator and the grid-connected power producer.
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2.2.1 Voltage Quality
In situations when the voltage deviates from the ideal state may result in disturbances which affect the reliability and stability of the power transmission. The presence of disturbances have a negative impact on the operation of the whole power system, as it can cause power outages, and harm or break electronic equipment [16]. Due to this, a number of measurable parameters have been defined, for which requirements have been set that indicate if the power transmission is of good quality. In regard to voltage quality, voltage variations are commonly measured. Voltage variations occur when the voltage level deviates from the desirable voltage level, either by an increase or decrease in voltage, and is divided into slow and fast variations handling different time scales [17].
At the different voltage levels of the electric power grid, there are specific standards and regulation frameworks that state specific requirements for good voltage quality. Among others, the configuration of the grid and the characteristics of the grid-connected production facilities have a large impact. Both the network operator and the grid- connected power producer have responsibilities for maintaining good voltage quality, where each part has different restrictions to adapt to. In the event of disturbances, both parties exchange information about the disturbances in order to ensure normal operation as soon as possible and eliminate the risk of recurrence [8].
2.2.2 The Network Operator
At the Point of Connection (POC), which is defined as the physical point in the distribution grid at which the power producer connects the production facility, the network operator holds main responsibility for maintaining good voltage quality. The Swedish Energy Markets Inspectorate has decided on the Swedish regulation EIFS 2013:1 “Requirements that must be met for the electricity transfer to be of good quality” [18] which needs to be fulfilled together with the applicable European Standard SS-EN 50160 “Voltage characteristics of electricity supplied by public distribution systems” [19] which specifies voltage parameters and permissible deviations.
Voltage variations are measured from the supply voltage at the POC, defined as the Root- Mean-Square (RMS) value of the voltage at a given moment (measured over a given time interval). The requirements on voltage variations are based on the continuous operating voltage (Uc) which is the voltage that the network operator strives to maintain in the node. Commonly, the continuous operating voltage is the same as the normal operating voltage by which the power system is designated to operate at, but depending on internal agreements these voltages may differ. In regard to slow voltage variations, the network operator is responsible for ensuring that all the 10-minute values of the supply voltage at the POC during one week maintains between 90% and 110% of the continuous operating voltage. The 10-minute value is calculated as the RMS value of the voltage over a time period of 10 minutes.
Fast voltage variations are restricted by the number of short-term voltage drops, short-
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term voltage rises, and rapid voltage changes. A short-term voltage drop is defined as a temporary decrease in the RMS value of the voltage below 90% of the continuous operating voltage. The restrictions on short-term voltage drops at time scales from 10 ms to 1 minute are characterised in Table 2.1 and Table 2.2 for Un ≤ 45 kV and Un > 45 kV respectively. The different areas (A, B and C) refer to the permission of disturbance. Voltage intervals considered acceptable (area A), voltage intervals where the network operator is obliged to take measures to improve the quality to the extent that the cost is reasonable in relation to the inconveniences from the disturbance (area B), and voltage intervals not permitted (area C).
Table 2.1: Limitations on short-term voltage drops for Un ≤ 45 kV
U [%] Duration, t [ms]
10 ≤ t ≤ 200 200 < t ≤ 500 500 < t ≤ 1000 1000 < t ≤ 5000 5000 < t ≤ 60000
90 > u ≥ 80 A
40 > u ≥ 5
5 > u C
Table 2.2: Limitations on short-term voltage drops for Un > 45 kV
10 ≤ t ≤ 100 100 < t ≤ 150 150 < t ≤ 600 600 < t ≤ 5000 5000 < t ≤ 60000
90 > u ≥ 80 A
40 > u ≥ 5
5 > u C
A short-term voltage rise is defined as a temporary increase in the RMS value of the voltage equal to or above 110% of the continuous operating voltage. The limitations on short-term voltage rises at time scales from 10 ms to 1 minute are characterised in Table 2.3 below and refer to nominal voltages equal to or below 1 kV. The same requirements apply for area A, area B and area C as stated previously.
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Table 2.3: Limitations on short-term voltage rises for Un ≤ 1 kV
u ≥ 135 C
111 > u ≥ 110 A
A rapid voltage change is a deviation in the RMS value of the voltage that is faster than 0.5% per second, and where the RMS value before, during, and after the change is between 90% and 110% of the continuous operating voltage. Rapid voltage changes are defined by the stationary voltage change (Ustat), which is the difference between the RMS voltage value before and after the voltage change, and the maximum voltage change (Umax), which is is the maximum voltage change under a voltage change process.
There are restrictions on the sum of the number of rapid voltage changes added with the number of short-term voltage drops present in area A in Table 2.1 and Table 2.2 presented previously. The obtained sum per day must not exceed the limits specified in Table 2.4 below. If there is no rapid voltage change, the limitations still apply for the number of short-term voltage drops, and vice versa if there is no short-term voltage drops but only rapid voltage changes.
Table 2.4: Limitations per day on the sum of rapid voltage changes and short-term voltage drops present in area A in Table 2.1 and Table 2.2 presented previously
Un ≤ 45 kV Un > 45 kV
Ustat ≥ 3% 24 12
Umax ≥ 5% 24 12
2.2.3 The Power Producer
As mentioned previously, the network operator holds the main responsibility for maintaining good voltage quality at the POC. However, as the operating current (Id) fed to/from the electric power grid by several production facilities affects the voltage level in the node, as well as the voltage characteristics in the rest of the grid, the network operator must enforce connection requirements on grid-connected power producers in order to maintain the requirements stated in EIFS 2013:1. The requirements on the power producer are determined by the network operator and because all the disturbances from several power producers are stored together, the requirements on the power producer are commonly stricter than the requirements set for the network operator. The network operator E.ON has specified demands on the grid-connected power producers regarding voltage quality in the document “Technical requirements for grid-connection in the regional grid and at the 20/10 kV substations” [8].
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Slow voltage variations are affected by slow variations in the operating current from the production facility, which in turn occur due to variations in the active- and reactive power exchange at the POC. Commonly, there is an agreement between the network operator and the power producer with restrictions on the exchange of reactive power at the POC. If there is no such agreement, the operating current at the POC is restricted as follows:
Id,max =
1.1 · Iref , for the 10-minutes mean value of Id (2.5)
Where the reference current (Iref ) is dependent on the subscribed active power production (Pab) and the continuous operating voltage (Uc) according to the following equation:
Iref = Pab√ 3 · Uc
(2.6)
In regard to fast voltage variations, the power producer must operate the production facility so that the voltage change at the POC does not exceed ±3% of the continuous operating voltage. In situations when the operating current causes two or more rapid voltage changes in the same direction within a time period of 10 seconds, the voltage changes are not allowed to exceed the limit on the maximum operating current (mentioned above for slow voltage variations) alone or stored together.
In addition to the requirements stated regarding voltage variations, the power producer that connects their facility to the grid must meet all the requirements of Com- mission Regulation (EU) 2016/631 “Network code on requirements for grid-connection of generators” [20] and the related Swedish regulation EIFS 2018:2 “Generally applicable requirements for grid-connection of generators” [21] by the Swedish Energy Markets Inspectorate. In the regulations, there are requirements regarding the reactive power capability. The requirements vary depending on the nominal voltage level at the point of connection and/or the maximum continuous power delivery of a production facility, divided into four categories: type A (Un <110 kV and Pmax > 800 W), type B (Un < 110 kV and Pmax ≥ 1.5 MWp), type C (Un < 110 kV and Pmax ≥ 10 MWp), type D (Un < 110 kV and Pmax ≥ 30 MWp, or Un ≥ 110 kV).
If the network operator does not announce other requirements for the power producer, power-generating facilities (power park modules such as solar- and wind power) of type C and type D need a capability for generation and absorption of reactive power corresponding to 1/3 of the momentan supplied active power. Due to the relationship between the active- and reactive power and the power factor, extraction of 1/3 corresponds to a power factor of approximately 0.95. Commonly, the reactive power is automatically supported by control methods for reactive power, power factor or voltage.
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For power producers connected to the electric power grid owned by E.ON, demands on active- and reactive power exchange is stated in the document “Technical requirements for grid-connection in the regional grid and at the 20/10 kV substations” [8]. The demands on active- and reactive power exchange may occur in situations when the grid does not have enough capacity for maximum active power production, in case of ab- normal connections in the grid, in case of disturbed operation, or in situations when the network operator lacks resources for reactive power control. Moreover, active power curtailment can occur in situations of both normal- and reserve switching mode in order to avoid thermal overload on the power lines and substations.
2.3 Solar Photovoltaics
This section introduces solar photovoltaics, including a description of photovoltaic systems, the concept of power output fluctuations, and potential challenges with the technology.
2.3.1 Photovoltaic Systems
Solar photovoltaic (PV) cells use the global horizontal irradiance (GHI) from the sun and convert it directly to electricity via the photovoltaic effect. PV cells take advantage of both the direct normal irradiance (DNI) which is received directly from the sun, and the diffuse horizontal irradiance (DHI) which is received from the sun after which the direction has been changed due to scattering from clouds and particles in the atmosphere. The photovoltaic effect occurs when the irradiance reaches a semiconductor material formed as a p-n junction with a positive and a negative charged side, connected to an external circuit. When the light reaches the semiconductor, a flow of electrons excites across the junction, which creates a DC current. To increase the electric output, the PV cells are connected into modules, where the cells within the module are connected in series and parallel to increase the power output from the module. The individual cells are commonly made from mono-or poly-crystalline silicon (first-generation solar cells) or thin film (second-generation solar cells). A junction box is placed at the rear of each module and terminates the connections from the individual cells to further connect the modules into arrays and form a PV system [14]. As the PV modules create DC current, a PV system uses an inverter to convert from DC to AC. For a grid-connected PV system, the inverter uses the frequency and voltage of the electric power grid as reference, which is constantly monitored by the inverter for a normal operation. Further, there are requirements and regulations set for the electric power grid which defines the conditions at which the inverter operates. There are different types of inverters, where the selection mainly depends on the layout and topology of the PV system. The most commonly used are central inverter which connects to parallel connected strings of modules, string inverter which connects to strings of modules, and micro-inverter which connects to individual modules [14].
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The ambient conditions that mainly affect the electric power output from a PV module are the irradiance and the cell temperature. To achieve maximum active power from the module, a Maximum Power Point Tracker (MPPT) is normally integrated to the inverter [14]. The relationship between the current and the voltage for a module is explained by the IV-curve, which also defines the point at which the maximum power is achieved. Via the MPPT, the electric power production from the PV module is maximized by searching for a point in the IV-curve that contributes to the highest power output [22]. The MPPT is a DC-DC converter which adjusts the voltage by varying the resistance presented to the modules, where there are MPPT methods of different complexity. The resistance is continuously adjusted so the product of the current and the voltage is at maximum, enabling maximum power. Further, the DC power from the MPPT is inverted to AC power in the inverter, which is achieved by switching transistors in the inverter. The action synthesizes a sinusoidal waveform that is synchronized with the frequency and voltage of the electric power grid [14].
2.3.2 Power Output Fluctuations
As mentioned, the electric power output from a PV system is proportionally dependent on the global horizontal irradiance from the sun. The irradiance intensity varies on both daily- and yearly timescales, mainly depending on the sun’s path across the sky, and weather patterns such as cloud coverage. On a yearly timescale, the irradiance varies seasonally, with longer days and higher irradiance in the summer, in contrast to shorter days and lower irradiance in the winter. The days of the year when the sun stands at its highest respective lowest position are called summer solstice and winter solstice, and in 2020 these days occurred on June 20 and December 21 respectively. Furthermore, at a daily timescale the irradiance varies from sunrise to sunset, usually peaking at noon. As the irradiance transfers with the speed of light, rapid variations in irradiance can occur on short timescales. For example, at the event of cloud movements, the direct normal irradiance can change in intensity between 900 W/m2 and 0 W/m2 in only a few seconds. However, despite the change in direct normal irradiance, the diffuse horizontal irradiance remains, which is why the power output from a PV system does not reduce directly from 100% to 0% due to cloud movements [23].
As the electric power output from a PV system varies according to the irradiance, the variability in the solar resource has a large impact on the power output from the PV systems. Variations on different timescales result in power output fluctuations. For grid-connected PV systems, the effects of power fluctuations is considered one of the main challenges for an increased penetration of PV in the electric power system. The variable character of the power fluctuations considerably affects the quality and safety of the power supply, leading to disturbances in the electric power grid [17]. Smoothing approaches by geographical dispersion is an interesting trend which can contribute to reduced effects of power fluctuations from the PV generation. Due to the fact that the majority of the grid-connected PV systems are widely distributed, geographical dispersion of the generation can be beneficial for balancing the power output fluctuations
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in the whole power system. When irradiance variations cause a ramp in the PV power output, the overall impact for a large PV system or for several PV systems occupying a large geographical area can be aggregated due to the smoothing effect [24].
The effects of power output fluctuations due to variability in irradiance have been investigated by several researchers. In the study [25] by Marcos et al, power output fluctuations from PV systems for time intervals shorter than 10 minutes were characterized with data of one second resolution. The study investigated the performance of seven PV systems located in Spain, with installed capacities ranging from 1 MWp to 9.5 MWp and with a total combined installed capacity of 20 MWp. It was concluded that the smoothing effect and the magnitude of the power output fluctuations were influenced by the size of the PV system, where larger PV systems indicated a larger smoothing effect and thereby lower fluctuations. Further, the sampling time had an impact on the results, where shorter sampling times gave larger smoothing effects. A sample period of 1 second resulted in ramp rates between 5-55% of the installed capacity for the investigated PV systems. For a time interval of 10 minutes, no smoothing effect was observed and power output fluctuations of 90% of the installed capacity were reached for all the investigated PV systems. Further on, the study showed evidence of larger smoothing effects due to geographical dispersion of several individual PV systems.
A similar study [26] was performed by van Haaren et al, where the fluctuations in power output from six PV systems in USA and Canada, with a total installed capacity of 195 MWp were studied. The data used for the characterization was based on minute- averaged data from each PV system and the power output from 390 inverters. It was concluded that the ramp rate of power fluctuations decreased with an increased size of the PV system, where the maximum ramp rates for PV systems of 5, 21, 48 and 80 MWp were observed as 70, 58, 53 and 43% per minute of the installed capacity respectively. Below, Table 2.5 compiles maximum ramp rates and the corresponding installed capacity of the PV system from a few studies.
Table 2.5: Compilation of the ramp rate for different amounts of installed PV capacity
Reference Installed capacity [MWp] Ramp rate [% of installed capacity/min]
[25] 2.6 85
2.3.3 Potential Challenges with Photovoltaics
Even if the solar resource is considered infinite, the intermittent character of the solar irradiance can cause problems when integrating larger capacities of solar power to
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the power system. There are several studies indicating potential challenges associated with power output fluctuations from grid-connected PV systems. In the study [27] by Shivashankar et al, technical aspects involving the voltage quality, generation dispatch control, protection, and reliability are investigated as potential problems for the power system due to the power output fluctuations. For example, there is evidence that the fluctuations influence disturbances that affect the quality of the power supply and lead to problems in the operation of the electric power grid. Depending on the situation, the disturbances can be of different characteristics, where voltage variations are explained as the most common problem. Fluctuating power output from grid-connected PV systems may result in voltage variations. The voltage is directly linked to the PV power output, where an increased output will increase the voltage and thereby risk influencing the restrictions set on the voltage level. Also, as a result of short irradiance variations, flicker induced by fast voltage variations risks to occur during shorter time periods.
Potential impacts from grid-connected PV systems are further presented by Kraiczy et al in IEA TCP Task 14 [28], where voltage variations, flicker and harmonics are highlighted together with the risk of overloaded power lines and transformers. In the report [24] by the Swedish Energy Agency, voltage variations and power flows in the power lines are presented as common problems in the power system due to a higher penetration of PV. Other voltage quality issues explained are harmonics, which mainly occurs when the PV system is connected to asymmetric three-phase with one-phase inverters. This is however not considered to be a common problem in the future as three-phase inverters are more common nowadays. Similar potential impacts were presented in the report [24] by Persson et al, where the main findings from earlier studies investigating how the quality of the power transmission is affected by grid-connected PV systems were compiled. The studies compiled in the report were performed in Sweden, and followed two main research paths. Either modelling and simulations by scenario analysis that examined how an up- scaled capacity of PV in the low-voltage grid affected the impacts of voltage quality and power outages, or measurements of how the voltage quality in the existing electric power grid was affected by PV. Voltage variations in the weak parts of the distribution grid, and asymmetrics between phases were considered as the main challenges with the grid-connected PV systems.
In the study [29] by Bagge et al, the impact on the voltage quality from the first PV system of MW-size in Sweden were analysed. The PV system was connected to the medium-voltage grid on 10 kV, and the analysis was done on a timescale of 6 seconds for the voltage and current, and on a timescale of minutes for the PV power output. Conclusions made from the study were that the PV system met the set requirements in the regulation framework EIFS 2013:1 “Regulations and general advice on requirements that must be met for the electric power transfer to be of good quality”, but that the impact on the grid was not negligible as impacts of slow voltage variations exceeded the recommendations at some occasions. Also, it was identified that the slow voltage variations were lower at days with low production, and higher at days with high production. The study concluded that the main reason for variations was that, when compared to
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the size of the installed capacity of the PV system, it was connected to a relatively weak part of the grid with a long distance to the transformer. Therefore, important lessons from the study was to check voltage variations in the grid when connecting new PV facilities.
Furthermore, in the study [15], by Trindade et al, it is stated that the increased penetration of PV systems, both in size and in quantity, connected to the low- and medium- voltage grids arise concerns regarding problems associated with cloud transients and voltage regulation. Voltage variations at different timescales, slow (steady-state) and fast (transient). According to the study, slow voltage variations were observed on cloud- less days at the maximum power output as a result of voltage rises, whereas fast voltage variations were observed on partly cloudy days as a result of cloud transients. PV systems of MW-size range connected to the distribution grid on the medium-voltage level were considered to cause main challenges in the operation of the electric power grid. As PV generators have no mechanical inertia, the variability in irradiance during cloud transients can cause rapid fluctuations in the power output. In turn, the power fluctuations result in imbalances from load-generators in the power system, which instantaneously is absorbed by the substations, resulting in the voltage variations in different time scales (duration times from a few seconds to minutes). Further, the study shows that the power output from a PV system directly affects the voltage magnitude. It is however stated that on partly cloudy days, the voltage magnitude depends on several factors such as the short circuit capacity, the rated power of the PV system and the geographical dispersion of the system. Simulations show correlation between the magnitude of voltage variations and active power variations in terms of the percentage of the rated installed PV power. At different buses of the grid, higher values of voltage variations are observed when a PV system is connected at buses with low short circuit capacity, i.e. when there is long distance between the PV system and the substation.
2.4 Photovoltaic Inverters for Voltage Regulation
Power output fluctuations that influence the voltage quality is a limiting factor for the expansion of large-scale PV systems. Due to disturbances such as voltage variations, there is a limit for how large capacity can be connected to the electric power grid before the requirements set for the voltage quality are exceeded. This can be addressed by reinforcement and strengthening of the electric power grid by new overhead lines, cables and transformers. To construct new infrastructures is however an expensive solution, and there are other possible methods that can be implemented to make optimal use of the existing grid and mitigate the problems from power output fluctuations [17]. The following section reviews advanced PV inverter functionalities that may be used to enable the integration of a higher penetration of PV to the power system. Different control schemes for grid support will be described together with possible implementation strategies.
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2.4.1 Control Schemes
Advanced functionalities in the PV inverters enable them to regulate the voltage and reduce the impact of voltage variations by strategies for active- and reactive power control. By controlling the active- and reactive power output from the PV system, the inverter has the ability to assist with grid support services in addition to the normal inverter functions [30]. The advanced functionalities include both direct control strategies that are changed manually by the operator, and autonomous control strategies that allow the inverter to make decisions automatically depending on different parameters. The control strategies can be achieved by incorporating functions that respond to variations in several parameters set in the inverter, for example the operating power factor in the inverter or the voltage level at the POC [31].
Active power control methods have the main purpose to prevent overvoltages in the electric power grid by curtailing the active power production from the PV system. The method is commonly used at high voltage levels in order to decrease the voltage level at the POC. For example, voltage regulation can be achieved by the autonomous Volt-Watt function where the active power output generated from the PV system is adjusted in response to the voltage level at the POC by a P(U) curve [31].
Reactive power control methods regulate the voltage level at the POC by the capability of reactive power support. At overvoltages, the inverter absorbs reactive power from the electric power grid in order to decrease the voltage, while it in situations of undervoltage delivers reactive power to increase the voltage level. Due to the relationship between the active- and reactive power, reactive power support reduces the ability of active power generation from the PV system. As the maximum apparent power of the inverter cannot be exceeded, the method of reactive power compensation is possible when there is excess capacity in the inverter that is not being used for generation of active power. Commonly, the inverter is sized according to the maximum capacity of the PV system. Since the peak power output of the PV system is rarely achieved, the inverter works below the maximum inverter rating over approximately 95% of the time. In these situations, the excess inverter capacity can be used to provide reactive power support. However, at times of peak power, the inverter has no excess capacity and the reactive power compensation must be achieved by curtailment of some active power production. A possible solution to ensure that there is always excess inverter capacity available for reactive power support is to oversize the inverter in relation to the capacity of the PV system [30].
There are several control schemes for reactive power control, where the two main strategies are power factor- or reactive power output control. Power factor- control is achieved by adjustment of the operating power factor, which is done by tracking of the active power in order to maintain an active-reactive power ratio according to the prevailing power factor command and within the maximum apparent power limit. For example, voltage regulation by power factor- control can be achieved by a direct control strategy using a fixed power factor function that controls the reactive power absorption/supply by specifying the power factor of the inverter to a fixed value. The power factor is
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specified as lagging/inductive or leading/capacitive depending on absorption or supply of reactive power respectively. As standard, the power factor in the inverter is designed to be as close to 1 as possible, as it generates more active power, but for voltage regulation purposes a lower power factor is desirable as it enables reactive power support [32]. Another strategy for power factor control is the autonomous Watt-cosφ function, where the power factor is varied in response to the active power output from the PV system by a cosφ(P) curve. Furthermore, reactive power output control is achieved in several ways. There are two main methods. The Watt-VAR function which uses a P(Q) curve to define the amount of reactive power that can be supported as a function of the active power output from the inverter, and the Volt-Var function which directly varies the reactive power output depending the voltage level at the POC by a Q(U) curve. Either of these functions apply for reactive power control strategies, and the response curves operate similarly but in response to different parameters [31].
In addition to active- and reactive power control methods, the PV inverters have advanced abilities that enable the PV system to stay connected to the grid as a dynamic support at minor disturbances. The need for disconnection at times when the frequency or voltage exceeds the predetermined requirements set is flexible, and the inverters stay connected unless severe disturbances occur. The dynamic support makes it possible for the PV system to deliver power to the electric power grid and contribute to stability at minor under- or overvoltages. The main functionalities which enable the control method are low/high voltage ride-through (LVRT/HVRT) functions where the inverter continuously checks the voltage level of the grid [30].
2.4.2 PQ Capability Chart
As previously mentioned in section 2.2 (Technical Requirements for Voltage Quality), there are requirements set on the capability of reactive power support in the grid codes. Normally, the power factor in a PV inverter can be adjusted between 0.8 lagging and 0.8 leading. A PQ capability chart illustrates how the active- and reactive power is limited by the apparent power. Below, Figure 2.1 presents the PQ capability chart for the string inverter model SG150HX, developed by the PV power company Sungrow [1]. The inverter model has a nominal apparent AC power (Sn) of 250 KVA, with a power factor adjustment range from 0.8 lagging to 0.8 leading.
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Figure 2.1: PQ capability chart for the Sungrow inverter model SG250HX [1]
As observed from Figure 2.1, a power factor close to 1 enables an active power output from the inverter equal to the nominal apparent power of the inverter, i.e. 250 kW active power and 0 kVAR reactive power. Moreover, the maximum reactive power output from the inverter is achieved when the power factor is 0.8, corresponding to a reactive power output of 150 kVAR (reaching 60% of the nominal apparent power of the inverter).
2.4.3 Implementation Strategies
As presented previously in this section, different control schemes in inverters can function as important system services for the power system. However, the service of reactive power support for voltage stabilization is the main interest for the grid operators, whereas the owner of the PV system aims to maximize the generation of active power. This dilemma leads to trade-offs in how the integration of grid support functions should be made, where one implementation challenge is the matter of profitability. As mentioned, reactive power support reduces the ability to generate active power, as it is only possible to provide reactive power when there is excess inverter capacity. Curtailment of active power enables reactive power support but leads to a loss of income for the active power that would otherwise be sold. To oversize the inverter according to the capacity of the PV system is a possible solution to ensure the presence of excess inverter capacity to meet the needs of reactive power support, but to invest in a larger inverter than necessary leads to increased investment costs [30]. There are discussions whether the owner of the PV system should be compensated for the service of reactive power compensation by incentives from the utilities or the network operators. In the literature, there are two main compensation models discussed. Firstly, as the owner of the PV system is only compensated for the amount of active power fed to the electric power grid, curtailment of active power for voltage stabilization purposes is an economic loss for the owner of the PV system. In these situations, one suggestion is that the owner of the PV system should be compensated for the income lost through curtailment of active power. Secondly, the solution of oversized inverters that enables it to always support reactive power without curtailment of active power generation result in additional investment costs for the over-
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all PV system. Suggestions are that the owner of the PV system should be compensated by promotions for the investment in oversized inverters [33]. Further on, National Re- newable Energy Laboratory (NREL) discusses whether alternative ownership options of the inverters would be a viable solution. The inverter is part of the PV system and is typically owned by the owner of the PV system. To consider alternative ownership to coordinate and provide the grid support functions, such as utility ownership or customer ownership of the inverters could address cost barriers [30].
In the report [34] by Vlahinic et al, it is mentioned that one viable integration strategy for enabling grid support functions from inverters could be to compensate the owner of the PV system for the service of reactive support. It is discussed that different approaches are used across the globe for charging consumers for consumption of reactive power, and that a similar solution could be used to provide incentives for PV systems with grid support services. Spain is mentioned as an example, where the owner of the PV system receives an incentive (given as a percentage of the price for active energy in kWh) for providing reactive power.
2.5 Alternative Solutions for Voltage Regulation
As a continuation of the previous section, this section presents alternative solutions for voltage stabilization, including tap changers on the transformers, installation of shunt compensating device, battery energy storage systems, and the concept of solar-wind complementation.
2.5.1 Tap Changers
A common technique for voltage regulation purposes is the use of tap changers at the transformers at the substations. A tap changer is a switching mechanism installed on the transformer which alters the connections on one of the transformer windings to vary the turns ratio on either the primary- or secondary side in order to achieve the desired voltage level [14]. The tap changer is normally placed on the primary side of the transformer (the high-voltage side), where it controls the voltage level on the secondary side (the low-voltage side) by increasing or decreasing the turns ratio on the transformer windings [17]. There are two main types of tap changers, the on-load tap changer (OLTC) and the no-load tap changer (NLTC). The tap changing mechanism for the on-load tap changer proceeds automatically while the transformer is in operation, and is more common for higher voltage levels of the grid. For this mechanism, the tap changer steps the turns ratio up/down in order to maintain an even voltage level on the secondary side of the transformer. The stepping occurs when the voltage level exceeds specified limits set on the tap changer, called the dead-band limits. The other type of tap changer, the no-load tap changer, is more used in the low-voltage grid. The changing mechanism is done manually, and the transformer must be taken out of service for manual adjustment of the turns ratio of the windings [14].
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2.5.2 Shunt Compensators
In addition to tap changers on the transformers, voltage regulation can be achieved by reactive power control strategies. As previously introduced in section 2.1.2 (Charac- teristics for Power Transmission), capacitive and inductive elements have the ability to compensate for the transmission of reactive power on a line and regulate the voltage in the node. In practice, this is achieved by connection of shunt compensators to the power line, which include shunt capacitors and shunt reactors that have different functions. Shunt capacitors are capacitive elements that produce reactive power and result in an increased voltage level in the node, while shunt reactors are inductive elements that consume reactive power and decrease the voltage level in the node [13].
Two shunt compensating power electronic devices commonly used for this voltage regulation technique are Static VAR Compensator (SVC) and Static synchronous compensator (STATCOM), which are both members in the Flexible Alternating Current Transmis- sion System (FACTS) devices family. FACTS devices have the ability to control the power flow on a power line by supporting reactive power compensation. In SVC devices, which commonly includes a thyristor-controlled reactor (TCR) in parallel with a thyristor-switched capacitor (TSC), the voltage level at the Point of Common Coupling (PCC) is adjusted depending on the situation. If the voltage level at the PCC is higher than the reference value, the SVC is predominantly reactive and absorbs reactive power, resulting in a decreased voltage level. Conversely, if the voltage level at the PCC is lower than the reference, there is a capacitive character of the SVC, and supply of reactive power occurs which increases the voltage level. The control strategy is optimized by the rating of the SVC, where the rating can be both symmetric or asymmetric with respect to capacitive and inductive reactive power. Further on, a STATCOM device is a more advanced technology based on the principle of Voltage-Source Converter (VSC). If the voltage of the VSC is higher than the voltage level at the PCC, the STATCOM device supplies reactive power, and the reverse occurs in the event of a lower voltage at the VSC in comparison to the PCC [35]. However, in contrast to inverters that have the ability to both generate active power and support with reactive power, shunt compensating devices such as VSC and STATCOM only support with reactive power [32].
2.5.3 Battery Energy Storage Systems
As mentioned previously, active power control by inverters is a possible method for voltage regulation purposes, where the active power output from the PV system is limited in order to regulate the voltage level at the POC. A complementary solution in situations of active power curtailment is the integration of battery energy storage systems. The implementation of storage is widely used for capturing the excess electricity generation and solving imbalances regarding the power supply in the power system. At peak times of high production conditions, the excess electricity is stored to later be discharged and utilized at times of high peak demands and low production conditions. The technique can solve problems of overvoltages and high currents in the power lines,
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and further lead to increased flexibility and reduced peak demands in the power system. Additionally, similarly as a battery energy storage system can be incorporated for voltage regulation purposes, electric vehicle charging at peak times is a possible strategy for taking advantage of the excess active power output from the PV system [17].
2.5.4 Solar-Wind Complementation
A strategy that may contribute to mitigation of the negative impacts from power fluctuations on a longer time scale are the concept of solar-wind complementation. Previous research presented by Fraunhofer Institute for Solar Energy Systems in the report [2] shows the importance in scaling up the share of both of the variable renewable energy technologies simultaneously. Synergies between the electric power production from PV- and wind power systems show that a balanced expansion of both technologies can be beneficial for the power system. Weather patterns for the solar irradiance and for the wind speed indicate a non-correlation, meaning that at high solar irradiance, the wind speed is low and vice versa. The negative correlation results in that the power production from the two sources can complement each other and contribute to great advantages. The research by Fraunhofer Institute for Solar Energy Systems shows that the stabilization of the power fluctuations apply on time scales from hours to months. Below, Figure 2.2 presents the monthly values of the electricity production together with the respective linear trend lines during the period 2014-2017, for combined and separate PV- and wind power systems.
Figure 2.2: Monthly electricity production from PV- and wind power systems during the period 2014-2017 [2]
As observed from Figure 2.2, the relative deviations from the linear trend lines are significantly lower for the combination of both technologies compared to the separate systems. With respect to voltage stabilization purposes, it can be concluded that a balanced integration of both PV- and wind power systems to the electric power grid can reduce the resulting effects of power output fluctuations on longer time scales.
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Methodology
This chapter addresses the methodology, starting with the power system modelling software used for the study as well as a description of the network model and important assumptions for the power flow simulations. Moreover, the four implemented studies are presented, which includes a worst-case study, time series calculations, a study with an oversized photovoltaic system, and a study with reactive power compensation.
3.1 Power System Modelling Software
The research methodology followed for the study was a quantitative approach, using the modelling software tool Power System Simulator for Engineering (PSS/E) combined with Python programming. PSS/E is a power system simulator that performs steady-state or transient calculations for power systems and is commonly used by network operators when integrating new components to the electric power grid. The software includes functions such as power flow calculations, dynamic simulations, building network equiv- alents, and fault- and contingency analysis. The application program interface (API) of PSS/E is compatible with the programming language Python, where Python is used to control the PSS/E environment.
3.2 Network Model Description
This section describes the network model used for the study, together with the principle of important components included in the model.
3.2.1 Model
The study was performed in E.ONs network model in PSS/E, representing the entire Swedish power system with data for the transmission grid, the regional grid, and parts of the distribution grid included. The network model is built as a bus-branch model which means that buses and branches respectively represent nodes and power lines,
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each characterized by a nominal voltage level. The network model includes substations with transformers that constitutes the connection between two or three voltage levels of the electric power grid, connecting several buses at different voltages to each other. Two-winding transformers operate at two voltage levels, and three-winding transformers operate at three voltage levels. Each transformer has a tap changer that regulates the voltage at a controlled bus to a specified voltage. Commonly, the tap changer controls the voltage at the secondary side of the transformer (i.e. the low-voltage side), which is the bus where components such as load- and generator units are connected to. Load- and generator units represent consumption and production of active power respectively, and both units can supply and/ or absorb reactive power. Since a power balance must be maintained in the power system at each moment, the network model includes swing-bus components that regulate their power to maintain the balance of active- and reactive power in the system during the power flow simulations.
3.2.2 Switching Mode
The electric power grid consists of meshed- and radial networks. The transmission- and regional grid are meshed networks, meaning that different parts of the electric power grid are interconnected and there are several paths between two points. Radial networks are the opposi

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