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    2705 West Lake Drive

    Taylor, Texas 76574

    (512) 248-6800 HTTP://WWW.ERCOT.COM

    Reactive Power and Voltage Control

    Version 0.09

    Haso Peljto

    05/15/2007

    Table of Contents

    Chapter 1 - Executive Summary ........................................................................... 3 Chapter 2 Introduction ....................................................................................... 4 Chapter 3 - Problem Statement ............................................................................ 5 Chapter 4 - Previous Options ................................................................................ 6 Chapter 5 - Solution Description ........................................................................... 6 5.1 Time Hierarchical Structure ................................................................................... 6 5.2 Day Ahead Voltage Optimization ........................................................................... 8 5.3 Real Time Voltage Optimization .......................................................................... 10 5.4 Expected Benefits................................................................................................ 12 Chapter 6 - Implementation ................................................................................ 13 6.1 Day Ahead Optimization Process ........................................................................ 13 6.2 Real Time Optimization Process .......................................................................... 14 6.3 MMS/EMS Applications ....................................................................................... 14

    6.3.1 MMS Network Constrained Unit Commitment ........................................................... 14

    6.3.2 MMS AC Power Flow Program ................................................................................. 15

    6.3.3 MMS/EMS Contingency Analysis Programs.............................................................. 18

    6.3.4 EMS Voltage Support Service Program .................................................................... 19

    6.3.5 EMS Transient Stability Analysis .............................................................................. 19

    6.3.6 EMS Voltage Stability Analysis ................................................................................. 19

    6.3.7 MMS Security Constrained Economic Dispatch ........................................................ 20 Chapter 7 Summary ......................................................................................... 20 Chapter 8 - Additional Information ...................................................................... 21

    IDA 006 Reactive Power and Voltage Control.doc Page 2

    Chapter 1 - Executive Summary

    Reactive power supply and voltage control are essential for reliability of transmission system. Inadequate reactive power causes voltage collapse and it has been one of the major causes for the August 2003 blackout in the United States and Canada.

    The main goal of reactive energy and voltage optimization is to plan, and maintain system operation security and system stability under contingency conditions at all time hierarchical levels:

     Seasonal reactive energy planning/procurement

     Weekly operation planning

     Day Ahead Market operation

     Real Time Market operation

     Transient stability analysis.

    The foundations for this proposal are derived from current ERCOT practice and the following regulatory documents:

     FERC Directives and Order No. 2003

     NERC Reliability Standards

     ERCOT Nodal Protocols and

     ERCOT Operating Guides.

    The system reliability objectives can be translated into the following requirements within the Day Ahead and Real Time Market time domain:

     Ensure sufficient transfer capabilities for active power within transmission

    line MVA limits

     Maintain voltage profiles at network buses within specified tolerances

     Ensure stable system operation and

     Prevent network voltage collapse

     Maintain and allocate reactive reserve across transmission network. To fulfill these requirements the coordinated deployment of appropriate software tools of EMS and MMS is proposed. First of all the AC network model and physical resource characteristics must be considered whenever reactive power and voltages are analyzed and optimized. Additionally, the interaction between active and reactive power flows in steady and transient states should be considered in full complexity of these physical processes. Finally, the specifics of relationship between reactive power flows and network bus voltages are the focal point of voltage control. The reactive energy and voltage optimization can be performed through manually coordinated execution of EMS/MMS software tools. This optimization process requires appropriate functionality of software tools, compatibility of data formats and data interfaces. The data transfers are usually not time critical and can be conducted manually as a part of overall reactive power and voltage optimization process.

    As a summary, the orchestrated deployment of functionally suitable software tools on integrated MMS and EMS platforms can significantly improve system reliability. Otherwise, system operational instability and unreliable operating states can be expected more frequently, and operational response is likely to be ineffective.

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    Chapter 2 Introduction

    Reactive power is an inherent part of the generation, transmission and distribution of electricity. The adequate management of reactive power is essential in order to ensure secure and reliable operation of the power system. Reactive power is tightly related to bus voltages and insufficient reactive supply can result in voltage collapse. Failure to adequately supply reactive power was identified as one of the reasons for the Northeast blackout in the United States in August 2003.

     Reactive power does not travel over long distances at high line loadings due to losses. Thus, reactive power usually must be procured near where it is needed. This limits the geographic scope and the number of suppliers that can provide reactive power.

    Reactive power may be supplied by transmission equipment such as:

     Synchronous generators within their reactive capabilities

     Static var compensators

     Synchronous condensers

     Shunt capacitors/reactors

     Tap transformers

     Phase shifters

     Transmission lines.

    Generally, reactive resources can be divided into two categories: static (capacitors and inductors) and dynamic (synchronous generators, static var compensators, synchronous condensers). The reactive power produced by static reactive resources drops when the voltage level drops. Dynamic reactive resources can change MVAR level independent of the voltage level. Thus, the production level of reactive power from Dynamic reactive resources can be increased and voltage collapse prevented even when voltage drops.

    Both the variable and fixed costs of devices producing static reactive power are much lower than those of producing dynamic reactive power. If costs were the only issue then the static reactive power equipment will be used first in procuring reactive power. Sometimes, more expensive reactive power resource can are used even if cheaper resources are idle because the dynamic reactive resource is more reliable and/or may be near the location needing the reactive power.

    The cost of producing reactive power can include lost opportunity costs associated with forgone real power production. The lost opportunity costs can arise for generators because there can be a trade-off between the amount of reactive power and real power that a generator can produce. When a generator is operating near its maximum limits, a generator can increase its production or consumption of reactive power only by reducing its production of active power. (Note that across most of a generator operating range there is not a lost opportunity cost trading off reactive power for real power production)

    The requirements for resolving voltage-constrained transmission transfer capability are addressed in this proposal within NERC and ERCOT operating standards through coordinated deployment of existing software tools. The proposed solutions consist of coordinated execution of EMS and MMS applications performing optimization of reactive power flows and voltage profiles across ERCOT power system. The final decisions are subject of approval of ERCOT, TSP and QSE operators.

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    In the following sections the reactive power and voltage optimization related issues are considered and a solution is proposed.

    Chapter 3 - Problem Statement

    The reactive energy should be managed to facilitate the transfer of active power, maintain system reliability and support local system voltages. The reactive energy and voltage optimization should be considered within Day Ahead and Real Time Market time domains to fulfill the following requirements of system reliability:

     Ensure sufficient transfer capabilities for active power within transmission

    line MVA limits

     Maintain voltage profiles at network buses within specified tolerances

     Maintain reactive reserve

     Ensure stable system operation and