Voltage Regulation in Distribution Systems with Distributed Generation (DG)

Voltage Regulation in Distribution Systems with

Distributed Generation (DG)

Presented by: Hao Liang

2012.11.7Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

Outline

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• Introduction

• The Voltage Rise Problem

• Optimization Based Voltage Regulation

• Estimation Based Voltage Regulation

• Combined Measurement and Estimation Based Voltage Regulation

• Remote Terminal Unit (RTU) Coordination Based Voltage Regulation

• Discussions

Broadband Communications Research (BBCR) LabSmart Grid Research Group

Introduction

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• In the recent years, distributed generation (DG) has experienced significant growth all over the world

• Potentially powered by renewable energy sources such as wind and solar, DG units are able to supply the electricity demands in an more economical and environmentally-friendly way as compared with the conventional centralized generators

• However, the adoption of DG units (which are intermittent in nature) poses new challenges on distribution system engineering

• One of the most challenging issues is the voltage regulation, which is aimed at keeping the voltage level of the distribution system within a certain range

• Distributed Generation (DG) Integration

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• WAN: Fiber optics, microwave, cellular (e.g., GPRS, 3G, HSPA+, and LTE)• HAN: ZigBee, WiFi, power line communications (PLC)• NAN: ZigBee, WiFi

Picture: http://www.fluidmesh.com

• Smart Grid Communications

Focus of This Talk

The Voltage Rise Problem

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• Causes of Voltage Rise

• The voltage (V2) at a remote point on a feeder can be approximately calculated as

V1 – Source voltage R + jX – Impedance of the line between the source and the remote point P and Q – Active and reactive power flows, respectively

• Model of a 3-bus feeder (no DG)

or in per unit

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• Causes of Voltage Rise (Cont’d)

• If the DG unit injects active power into the system (i.e., the DG unit is working at unity power factor), we have

• Model of a 3-bus feeder (with DG)

• If the output of the DG unit is large, (P − PG) becomes negative. As a result, V2 can be larger than V1, which causes the voltage rise problem

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• Possible Solutions for Voltage Rise Problem

• Network Reinforcement – The network reinforcement method aims at reducing R to mitigate the voltage rise effect. However, the cost is relatively high for upgrading the feeders

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• Possible Solutions for Voltage Rise Problem (Cont’d)

• Generation Curtailment – When the load demand is low, some of the generation capacity (PG) can be curtailed to reduce the voltage rise, at the cost of a reduction in the profit of DG. The significance of the cost is heavily dependent on the electricity price and may be less when the electricity price is low as the load demand is low

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• Reactive Power Compensation – The reactive power compensation method requires the generator to absorb a certain amount of reactive power (Q). However, this method is not very efficient (especially for cable feeders) since the X/R ratio in distribution systems is typically much smaller than that in transmission systems. At the same time, this method results in higher losses in distribution systems

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• Voltage Regulator – The voltage rise effect can also be addressed by decreasing the source voltage V1. Remote terminal units (RTUs) are deployed at strategic locations across the network for voltage and power flow measurements. The measured information is transmitted to the voltage regulator at the primary transformer (via certain communication networks in the context of Smart Grid) for voltage control

Cost of Communications? Broadband Communications Research (BBCR) LabSmart Grid Research Group

Optimization Based Voltage Regulation

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• Basic Idea

• A linear programming (LP) based formulation of the optimal power flow (OPF) is considered. The objective is to minimize the annual active generation curtailment cost, while satisfying voltage and thermal constraints. The decision variables are the generation curtailment, reactive compensation, and coordinated voltage regulation (area voltage control) using OLTC

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• Problem Formulation

Minimize the annual active generation curtailment cost

Active power injection

Reactive power injection Load flows of the branch ij

Tap setting of the tap-changer kReactive power curtailment may be correlated with the active power curtailment

Limits

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• Discussions

• The main drawback of the optimization based voltage regulation scheme is that it can hardly be implemented in real time as it is not possible to read the information of all nodes in the distribution system. How to reduce the number of measurements is the main issue to be addressed


Estimation Based Voltage Regulation

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• Basic Idea

• A reference feeder without DG unit is needed

• The key technique is the estimation of the output of generator which is connected at a remote point on the feeder

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• The Voltage Regulation Strategy • An additional current measurement IFG is used. The ratio EST represents the

load share between feeders with embedded generation to those without generators, given by

where ITL is the summation of transformer currents. The factor EST is calculated before the connection of the DG unit or when the output of the DG unit is zero

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• The Voltage Regulation Strategy (Cont’d)

• Then, during the operation of the DG unit, the generation output can be estimated, given by

• Based on the value of IG , the voltage rise at the connection point of the DG can be calculated as

This value corresponds to the necessary voltage reduction at the substation in order to bring the voltage level at the point of DG connection within statutory limits


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• Discussions

• This scheme does not take advantage of the instantaneous measurements obtained from a communication network, which is expected to be deployed in the next generation power grid (also referred to as the smart grid)

• This scheme requires a reference feeder without DG

• In addition, the scheme is used to solve the voltage rise problem, while the low voltage point (below the statutory limit) can hardly be detected


Combined Measurement and Estimation Based Voltage Regulation

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• Basic Idea

• Investigate both real-time measurements and statistical estimation

• Specifically, based on the information of real-time measurements and load profile, the voltage magnitude at each network node (supplied by the primary substation) can be estimated with certain accuracy


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• Voltage Estimation

• A weighted least squares method is used, where the state variables are defined with respect to the node voltage magnitudes and relative phase angles, given by

where xj represents the state variables, Ns is the number of state variables, zi

denotes the ith measurement, Nm is the number of measurements, fi is a function relating ith measurement to state variables, and σi is the variance of the ith measurement

• Three types of function fi is used, which relates the state variables Vi, Vk, θi, and θk to the measurements Pinj, Qinj, and Vi, given by


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• Voltage Estimation (Cont’d)

• For state estimation on a distribution network, we have Nm ≪ Ns. However, the necessary condition for a weighted least squares state estimation algorithm to have a unique solution is Nm ≥ Ns

Power flow equations, which can be solved by using theNewton-Raphson method


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• Voltage Estimation (Cont’d)

• Therefore, the pseudo measurements which are derived from offline data, can be used to provide unmeasured quantities, so that Nm ≥ Ns

• In this work, the unmeasured quantities are Pinj and Qinj at load nodes and so pseudo measurements are used for these. The measurements are assumed to be independent normal random variables, with variance being greater for pseudo measurements than for real-time measurements

Power flow equations, which can be solved by using theNewton-Raphson method


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• Discussions

• The combined measurement and estimation based voltage regulation scheme is very complicated which requires the solution of non-linear equations. How to reduce the complexity and apply the scheme in real time is still an open issue

• Moreover, the performance of the scheme heavily depends on the load models. How to improve the modeling accuracy based on load profiles is interesting and requires future research

• In addition, the voltage controller used in this work is not optimized, which potentially degrades the efficiency of voltage regulation


RTU Coordination Based Voltage Regulation

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• Voltage Profile Estimation

• RTUs are deployed at the DG and capacitor connecting buses

• Theorem 1. For the voltage profile of a feeder, maximum voltage can happen only at the DG connecting buses, capacitors connecting buses, and the substation bus, provided that the R/X ratio of the feeder is constant along the whole feeder

• Theorem 2. There exists a minimum voltage point in between two DG connecting buses if and only if, for both DGs, the voltage of the DG neighboring bus, in the direction of the other DG, is less than the voltage of the DG bus


Directly Measurable

Need to be Estimated

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• Voltage Profile Estimation (Cont’d)

• Assume the load between two DG units is concentrated halfway between them. The minimum voltages calculated by DG1 and DG2 are, respectively, given by

• In order to get a better estimation of the minimum voltage, we take the average:


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• In order to achieve voltage regulation, each RTU measures the following information 1) The voltage of its element bus 2) Active and reactive power flow in lines connected to its element bus 3) The voltages of the immediate neighbor buses of its element bus

• Based on the measurements, each RTU can perform the following calculation 1) Measure a maximum voltage point 2) Check one part of the condition for the possibility of the existence of a minimum voltage point between its element and any neighbor element 3) Estimate the value of the minimum voltage point on each side of its element, if it exists


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• A communication network is established for the RTUs to pass the calculated information to the voltage regulator

• Data passing starts from the farthest RTU towards the voltage regulator• Along the data passing path, each upstream RTU updates the maximum and

minimum voltages based on the information received from its downstream RTU


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• Voltage Regulator Operation

• When the information with respect to the maximum and minimum voltages are received by the voltage regulator, the following operations are performed:

1) If the absolute maximum voltage is greater than maximum permissible voltage, then the voltage regulator will decrease the current tap position till the maximum voltage of the feeder is within the permissible range

2) If the minimum voltage of the feeder is below the minimum permissible voltage, then the voltage regulator will increase the tap position to bring the minimum voltage into the permissible range


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• Discussions

• The RTU based voltage regulation can efficiently utilizes the instant measurements at the generation buses. However, how to integrates the estimates of the loads to improve the voltage regulation performance is still an open issue

• The scheme is not optimized for certain objective functions

• The reliability of the communication network?


Thank you!

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Voltage Regulation in Distribution Systems with Distributed Generation (DG)