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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
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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
； Prevent network voltage collapse.
There is a number of identified design issues related to reactive energy management and voltage support service. In the following chapters some of these issues are considered and appropriate solutions are proposed.
The Day Ahead and Real Time Market applications focus on active power scheduling and dispatch to accommodate energy trades and ancillary services needed to support and secure active power flows. The active energy interacts with produced and consumed reactive energy in a complex way during steady states and in a potentially hazardous way under contingency conditions. Good engineering principles supported by appropriate cost recovery for reactive energy ensure secure and stable power system operation.
， Voltage Constraints
The market clearing engines for CRR and the DAM optimization use a DC
network model. Therefore, it is impossible to set voltage/dynamic
constraints directly within the CRR auction system and the DAM trading
system.
On the other side the RUC optimization process and EMS use the AC
network model. Within these systems the voltage/dynamic constraints
should be accurately converted into constraints of thermal type to be
optimized by market clearing engines. Voltage control during contingency
and post contingency should be considered as well.
There has been discussion in ERCOT about developing surrogate
constraints outside of the RUP optimization process and entering these
constraints as “generic” constraints into the RUC process. This proposal
to have surrogate constraints is highly difficult because appropriate tools
for full reactive power and voltage optimization are not available.
， Voltage Optimization
The Nodal Protocol requires generation of voltage set-points, payment for
reactive instructions, and implementation of non-generation reactive
compensation prior to use of generation units.
Therefore, tools and processes that are consistent between Voltage
Support Service and Day-Ahead and Real Time Market applications need
to be developed to meet these requirements.
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， Reactive Power and Voltage Support Settlement
The Nodal Protocol requires that measurement of reactive power and
settlement for voltage support service should be done on resource
specific bases. The reactive dispatch instructions should be created and
sent out to each unit.
These issues are considered altogether within the combined framework of Energy Management System (EMS) and Market Management System (MMS) focusing on security and stability of power system operation as primary objective of reactive power and voltage optimization.
Chapter 4 - Previous Options
The reactive energy and voltage optimization has not been considered systematically during the design phase of Day Ahead and Real Time Market systems. The main focus has been on management of active energy and ancillary services.
The foundations for this proposal are derived from current ERCOT practices and the following regulatory documents:
； FERC Directives and Order No. 2003
； NERC Reliability Standards (Standards VAR-001-1 and VAR-002-1)
； ERCOT Nodal Protocol (Sections 3.15 and 6.6.7)
； ERCOT Operating Guide (Section 2.10).
On the other side, software tools of the Energy and Market Management Systems are already specified in business requirements, functional specifications and design documents. The intention here is to organize efficient process and deploy these available software tools for analysis and optimization of reactive energy supply and network voltage profiles.
Chapter 5 - Solution Description
5.1 Time Hierarchical Structure
The overall strategy for reactive energy management can be organized into the
following time hierarchical levels:
， Seasonal Reactive Energy Procurement
The reactive power procurement should be arranged a few months ahead
on seasonal basis. In ERCOT, most reactive resources are considered
available if not planned for an outage; thus procurement is anticipated to
consist of performing studies of reactive adequacy in a voltage set-point
analysis for each season. This long term reactive power study can be
optimized under expected operating conditions, especially on-peak and
off-peak conditions. The reactive resource setups and network voltage
profiles can be developed through engineering studies of expected
operating conditions over the following season.
Potentially, reactive energy auctions could be organized to determine
competitively procurement and prices for reactive energy and provide
economic incentives for efficient investment into reactive resources.
Note: Seasonal reactive energy procurement is out of scope of this paper.
， Weekly Operation Planning
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The near future scheduling of reactive energy begins with the operation planning for reactive energy and network voltages over the next week on hourly basis. The operation plans should respect reactive energy and reserve requirements, voltage profiles and tolerance bands, reactive resource and switch statuses, and security and stability constraints. The impact of outages for maintenance plays a key role in planning during this time frame.
The weekly operating plan can be developed according to specified rules through engineering studies. It can be provided as input data to the MMS and EMS for further adjustments and actual deployment.
ERCOT anticipates using the Weekly RUC process to provide assurance that real and reactive power adequacy is possible in the coming week. Note: The weekly operation planning is out of scope of this paper.
， Day Ahead Voltage Optimization
The weekly operating plan is a starting point for the Day Ahead Market operation. The CRR auctions and DAM clearing process use the DC network model focusing on financial aspects. Active energy and ancillary service trades are primary features of the DAM clearing engine. The only mechanism allowed for in the ERCOT Nodal Protocols for modeling stability and voltage constraints are the inclusion of generic constraints. It is the intention of ERCOT to develop generic constraints derived from stability and voltage studies to be considered by the MMS DAM clearing process.
The reactive energy supply and network bus voltages can be considered only by the MMS RUC process where full AC power flow network model is used. To confirm adequacy in reactive power and voltages, local reactive device control is performed. Additionally, generic constraints derived from stability and voltage study are considered by both the Day Ahead and Hourly RUC optimization process.
， Real Time Voltage Optimization
In real time system operation the active and reactive power dispatches are decoupled into entirely different operating time frames. The active power dispatch is performed periodically every five minutes, while reactive power is considered on hourly bases. The decoupling of active and reactive power implies that optimization of real time operation is split into two parts: the Security Economic Dispatch (SCED) for active power and Voltage Service Support (VSS) optimization for reactive power. The SCED program performs real time dispatch of active power outputs of online generators under actual operating conditions from a security as well as efficiency point of view. The system security is expressed as transmission constraints for active power flows. The active power dispatch instructions finalize the overall strategy for active power control. The VSS program performs reactive power dispatch on an hourly basis to maintain network voltage profiles within limits. The VSS program operating in “real-time” mode will recommend static reactive equipment
status changes to enforce both pre and post-contingency voltage limits to either maximize dynamic reactive reserves on hand or minimize shift. The reactive power and voltage set points are determined targeting system
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security as main objective. The reactive reserve margin should be
maintained at all times to be able to control actual fluctuations of
operating conditions.
， Transient Stability
The transient network and voltage processes can not be controlled
directly due their very short time frame (seconds). But, they can be
analyzed and actions taken to avoid system instability and voltage
collapse under potential disturbances. These actions can be expressed
as additional transmission and generation constraints that are respected
during day ahead scheduling and real time dispatch.
At each of above time domains appropriate reactive energy and voltage control is performed with increasing granularity. The remaining sections of this paper are focused on reactive energy and voltage control from reliability prospective of system operation in Day Ahead and Real Time Market time domains. 5.2 Day Ahead Voltage Optimization
The reactive power flows and network bus voltages can be analyzed only using AC network model. Within the day ahead time domain the RUC optimization process deploys the AC network model only for base case within the MMS Power Flow program. The MMS Contingency Analysis program within the RUC process uses nonlinear DC network model for contingency cases. Therefore, only base case reactive power and voltages can be considered by the RUC process directly, while contingency cases can be considered by EMS reactive power and voltage study tools only. This approach is applied to both Day Ahead and Hourly RUC process.
The ERCOT market design does not include cost optimization for voltage/reactive security. This means the reactive resources can be locally controlled within the AC Power Flow program to maintain system security without optimization.
The voltage optimization process within the Day Ahead Market is illustrated on the following figure:
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EMS MMS Network and Reliability Unit Voltage Commitment Security and Generic
Stability Transmission NCUC
And Network Constrained
VSS Generation Unit Commitment Voltage Support Constraints Service
NSM Network Security NSA Monitor Network Stability Hourly Analysis AC PF Schedules Power Flow TSA Transient Stability Voltage DC CA Constraint Contingency VSA Violations Analysis Voltage Stability
Manual Action Automatic Action Figure 1: Day-Ahead Voltage Optimization
The MMS RUC optimization process consists of iterative execution of two main applications:
； Network Constrained Unit Commitment (NCUC) and
； Network Security Monitor (NSM).
The objective of the NCUC program is to minimize the startup and minimum energy costs for additionally committed generators needed to meet the system Load Forecast and reliability constraints.
The NSM package incorporates AC Power Flow (AC PF) program and the DC Contingency Analysis (DC CA) program. The reactive power and voltages are considered by the AC PF program only through local reactive resource control. At the end of NCUC-NSM iteration process the AC PF program will check the final network bus voltages for each hour and indicate network busses with violated voltage limits. Indicated hours and network buses are good candidates for detailed study of network security and voltage stability. We propose that the market operator should be capable to study security and stability of RUC solution using the EMS Network Stability Analysis (NSA) tools. The study mode of the EMS NSA package could be initialized from MMS solved power flow case for any hour within the RUC scheduling time period. These tools are based on AC network model for both normal and contingency conditions. The NSA package includes the following programs:
； Transient Stability Analysis (TSA)
； Voltage Stability analysis (VSA).
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Additionally, the study mode of the Voltage Support Services (VSS) program should be designed to allow it to be initialized from the solved power flow case for any hour within RUC scheduling time period. The VSS program performs optimal dispatch of reactive resources to maintain network voltages within tolerance bands. The VSS optimization of reactive resources is performed across the system as opposite to local control performed by the AC PF program. With these enhancements, the results of the RUC solution study can be used to formulate appropriate transmission and generation constraints. These generic constraints can be passed back to the RUC optimization model and re-optimized. With this process the additional commitment of generating resources can be performed to maintain voltage reliability as well as thermal. This is especially relevant for Hourly RUC process.
This coordination of MMS RUC optimization process and EMS NSA/VSS study analysis is fully manual process opposite to fully automated NCUC-NSM iteration process.
5.3 Real Time Voltage Optimization
Nodal Real time security and stability is designed to be maintained through the EMS Network Security and Stability Analysis of current system operation and study of potential operation disturbances. Constraints used to maintain system reliability are thermal constraints developed by real-time contingency analysis and passed into the Transmission Constraint Management application, transmission and generation generic constraints and resource limits. These formulated reliability constraints are developed within EMS and passed to Security Constrained Economic Dispatch (SCED). The SCED program respects these limits when it optimizes generation base points for active power. The reactive energy and voltage control have reliability importance which is not addressed by the SCED process described above. The set points for Automatic Voltage Regulators can be adjusted and reactive dispatch instructions issued. A generating resource can be instructed to reduce active power output to increase its reactive capability if it is needed for Voltage Support Service. Reactive energy instructions above a generator’s URL are settled on an
individual resource basis using a price set administratively in ERCOT protocols. It can be beneficial to allow a generator in an expensive load pocket to back off its real power and produce reactive power to increase transmission capacity for imports/exports for that area. Eventual lost opportunity costs are settled using real time LMPs. These costing aspects are considered after the fact without optimization within the SCED.
The real time voltage optimization consists of manual execution of security and stability analysis, constraint formulation, voltage set point adjustment, and reactive dispatch instructions. In normal operating conditions it is sufficient to optimize voltage on hourly basis in coordination with Hourly RUC process and Real Time Sequence.
The real time voltage optimization process coupled with EMS NSA/VSS study analysis is illustrated on the following diagram:
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