Project: Analysis of Indian distribution systems for the integration of high shares of rooftop PV

INTEGRATION OF RENEWABLE ENERGIES IN THE INDIAN ELECTRICITY SYSTEM (I-RE)

Final Workshop, 29-30 August 2017

Dr. Thomas Ackermann

Dr.-Ing. Eckehard Tröster

Importance of Data Collection with Growing Distributed Generation

Day 2, 3:00pm, 30 mins

MONITORING OF GENERATION CAPACITIES

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Number of generation sites increases from a few large power stations to hundreds of distributed sites. It is necessary to keep track of installed capacities to know the behavior that can be expected from the power system, both at transmission and distribution level!

Example: German regulatory agency keeps track

of all generators and publishes lists

• Generators

• Self consumption

• DSOs

• Large consumers

• Curtailment

• ….

BEFORE: LOAD ONLY DISTRIBUTION GRID

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Knowing the load here allows to adequately

assess conditions in the LV grid.

Unidirectional power flow: Voltage drop only, control at MV/HV transformer sufficient Simply measuring the load at different transformers allows to get a good idea of voltage and flows in the grid.

Knowing the load and voltage here may allow

to adequately assess conditions in the MV and

LV grid.

AFTER: PROSUMER DISTRIBUTION GRID

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Power flow may be reversed – especially reversal on one of multiple parallel feeders is hard to predict without measurements. And how many PV units are installed in this grid anyway, and what can they do? → Need for data collection increases!

500 kW load here may be 500 kW load, or 1000

kW load and 500 kW generation, or… what is

going on below?

Same here, only in MW.

How should we set the voltage controls?

This feeder may generate more than it consumes…

… this one may consume more. Who knows?

How is the voltage? Do we need to take action?

INTEGRATION ISSUES

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(This transformer station did not catch on fire because of distributed generation)

MV grid

MV grid

MV grid

MV grid

Thermal overload Critical voltage

Parallel line at 1/2 length Parallel line at 2/3 length

Source: Uhlig, CIRED Workshop 2014

Voltage range violations Thermal overloading

Result: Network reinforcements may become necessary

… and how and when do I find out?

The best way of dealing with new problems is avoiding them in the first place.

→ Conduct measurements of operational data

→ Conduct studies to assess the impact of new generation

→ Know about the probable issues

→ Take action before actual problems occur

WHAT IS GOING ON IN THE GRID…

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STUDIES (1)

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I

V

V

I

• Load Flow Study Focus of grid integration studies at distribution grid level is the same for most cases: • Are any assets overloaded, and if yes, which?

• Is the voltage too high or too low anywhere in

the grid?

• How much distributed generation can be introduced before issues appear?

• What can be done to increase the hosting capacity for distributed generation?

Given an adequate grid model, load flow studies deliver that information.

Grid modelling requires input data from real life.

DATA: LINES AND CABLES

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Parameter Remarks

Length

Coordinates Start and end substation

Rated voltage Un [kV]

Essential parameters. Ampacity Ir [kA]

Rated short-time current [kA]

Conductor type OHL only. Can be used to calculate ampacity

and short time current, otherwise not strictly

necessary. Maximum allowed conductor sag

Max. Operational Temperature [°C]

Temperature Coefficient [1/°C]

Cable type Cable only. Can be used to calculate ampacity

and short time current, otherwise not strictly

necessary. Insulation

Topology (air or ground)

Resistance R’ [Ohm/km] This data is essential for modelling, but may not

be available to the grid operator. In this case, it

can be calculated if the conductor type and

OHL/cable topology is known.

Reactance X’ [Ohm/km]

Capacity C’ [µF/km]

Conductance G’ [µS/km]

Number of Circuits on Tower

OHL only. Strictly necessary if line impedance

data is unavailable, as it can be calculated from

these parameters

Number of Earth Wires on Tower

Symmetry of conductors on tower

Distance between towers

In a meshed grid, line impedances determine the flows. In a radial distribution grid, they impact losses and voltage on the feeders. Accurate data is crucial for grid modelling. → Generally, HV and MV data is available, but the data for LV cables/lines is often lacking!

DATA: TRANSFORMERS

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Parameter Remarks

Rated apparent power Sr [MVA] For three winding transformers, ratings may

vary for the three voltage sides

Rated primary voltage Ur_HV

Rated secondary voltage Ur_LV Three winding transformer data should also

include rated tertiary voltage

General winding configuration

Short circuit voltage uk [%] Necessary for the calculation of voltage drop

over loaded transformer

Commissioning date If data is unavailable, assumptions may be

based on the state of the art at the date of

commissioning/manufacture of the unit

Copper losses PCu [kW]

No-load current i0 [%]

No-load losses P0 [kW]

Transformer type Typical application

On-load tap changing, automatic

voltage control

HV/MV and MV/MV applications – voltage can

be continually controlled during operation

On-load tap changing, remote

controlled

HV/MV and MV/MV applications – voltage set

points can be changed remotely

On-load tap changing, manually

controlled

MV/MV and MV/LV applications – voltage set

points are typically changed only seasonally

Off-load tap changing MV/LV applications, ratio is re-set only at

revisions, if at all

Fixed ratio MV/LV applications, rare

Transformers, as branch elements, have an impact on flows and voltage similar to lines. OLTC transformers also control the voltage – the capabilities and control settings for those are important for a good model! The latter is also true for reactive compensators – data on those must be accurate as well.

DATA: SWITCHING STATES

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In case there are multiple different ways of supplying a feeder, these must be clearly specified: • What is the switching state

during normal operation?

• Under what conditions is it changed? Only emergency, or peak / off-peak, or seasonal?

• Can the switching states be altered to better accomodate distributed generation?

DATA: DIAGRAMS AND MAPS

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Knowing where lines and substations are is helpful: • Cross check line lengths

• Cross check load data

• Generate load data if missing

• Assess potential for distributed generation

• Determine reinforcement costs (dependent on

terrain)

Maps also allow for easier understanding of grid topology.

DATA: MEASURED DATA

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Data Typically available Additional requirements

Voltage, HV Voltage at all busbars

through SCADA system

Sufficient

Voltage, MV Voltage at secondary side

of HV/MV transformer

Sufficient for modelling. For model

validation, measurements from

other locations in the grid (feeder

end) may be useful. Voltage, LV None

Load, HV Power flow through

HV/MV transformer

Active and reactive power

measurements may be available,

can be used for model validation

Load, MV Peak load at MV/LV

transformers

Feeder load time series (active

power) are often available for MV.

For LV grids, only peak load may be

available, if at all.

Load, LV

Necessary for modelling (green): • Load of all branches • Voltage at last control instance (OLTC)

Good for validation (red): • Voltage measurements at different points

Setting up measuring infrastructure will also help to assess the grid state during real operation!

STANDARD LOAD PROFILES

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𝑓𝑐𝑜𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 = Load𝑖𝑛𝑖=1

Max (Load)𝑖𝑛𝑖=1

If only peak loads are available, standard load profiles can be used to estimate the load time series. → Standard procedure in many European countries, but not always available.

If customer peak loads are available, feeder peak load can be calculated using typical coincidence factors. → Standard procedure in many European countries. Factors can be determined from real time measurements at a higher level.

• Simulation results

LOAD FLOW MODEL

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• Load Flow Model

Voltage control regime

Measured or generic load profile

Transformer specs

Line and cable data

Switching configuration

Voltage of each point on the feeder

Loading of all elements

DATA COLLECTION AND GRID OPERATION

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Load PV

Load PV

Load PV

Load PV

OLTC

11 kV

33 kV

33 kV to 11 kV Transformer is controlled with discrete on load tap changers which keep voltage in range for multiple measurement points

Setting up measurements allow for better controllability of the grid during real life operation: • Simple: Analysis of historical measured data may identify

need for remedial action on a certain feeder

• More advanced: Data can be used for grid control in real time, see example of Wide Area Voltage Control – either manually or automatically controlled (SCADA)

• Data can also be used to assess quality of supply and improve it as necessary

• Storing data for several years can be advisable

Distributed PV integration requires a detailed

review of data management and data

collection approach

SUMMARY

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THANK YOU FOR YOUR ATTENTION!