Implementation of RAS Actions for Real-Time Voltage Stability Analysis (VSA) at Peak Reliability Saad Malik (Peak Reliability, USA), Michael Vaiman (V&R Energy, USA), Marianna Vaiman (V&R Energy, USA)
[email protected] United States of America I. Abstract Peak Reliability (Peak) performs the Reliability Coordinator and Interchange Authority functions in the Western Interconnection. Region Of Stability Existence (ROSE), a real-time voltage stability analysis (VSA) software by V&R Energy, was implemented at Peak’s two Reliability Coordination Offices (RCO) to improve situational awareness within the Western Interconnection. The Western Interconnection TOPs rely heavily on Remedial Action Schemes (RAS) or System Integrity Protection Schemes (SIPS) to enhance their transmission system’s transfer capability. Thus it is important for the Peak Reliability to know in real-time the impact of the operation of these RAS schemes on system voltage stability limits. This paper presents the techniques used for the implementation of Remedial Actions Schemes (RAS) as part of real-time voltage stability analysis (VSA) at Peak Reliability utilizing Peak-ROSE application. II. Introduction NERC Glossary defines remedial actions scheme (RAS) as “An automatic protection system designed to detect abnormal or predetermined system conditions, and take corrective actions other than and/or in addition to the isolation of faulted components to maintain system reliability”. The reliable and secure operation of complex interconnected power systems is dependent on proper operation of RAS when taking pre-Contingency actions is not desirable. A number of NERC standards address the issues related to RAS [1], which ensure that RAS are properly designed and coordinated with other protection systems, meet performance requirements, maintenance and test programs are developed, and misoperation is analyzed and corrected. A significant effort has been made by the industry in performing the research [2-4] as well as addressing the practical aspects of RAS implementation, [5-10]. Currently, three acronyms are equally used, which have the same meaning for remedial action schemes. Term RAS is predominantly used by the utilities in the Western part of North America. IEEE community uses the term System Integrity Protection System (SIPS), and the term System Protection System (SPS) is used by CIGRE [11]. There is a wide variety of special protection schemes. For example, in WECC transmission system there are over 190 RAS [4], and their number continues to grow, see Figure 1. The most common RAS used in WECC include generation trip, brake insertion, fast valve/generation ramp, HVDC ramp; configuration changes/islanding, load shed or rejection, excitation forcing, shunt capacitor/reactor switching, and series capacitor/reactor switching. They are designed, maintained, and evaluated in accordance with the WECC RAS Guide and Procedure, [12]. RAS in WECC are utilized to maintain or increase the transmission system capability, mitigate adverse system impact of certain low probability/high consequence system events, and prevent events from spreading out across large regions or onto wide area system. WECC issued a standard PRC-004-WECC-1, which ensures that all RAS installed at generation or transmission side of the system are analyzed [13].
Figure 1. Initial Year of RAS Operation Implemented in WECC WECC identifies three types of RAS, depending on their potential impact: 1. Local Area Protection Scheme (LAPS) LAPS constitute 62% of installed RAS at WECC. LAPS type is used to meet an owner's performance requirements within their system. 2. Wide Area Protection Scheme (WAPS) WAPS constitute 31% of installed RAS at WECC. WAPS type is needed to meet WECC performance requirements and operating standards. 3. Safety Net (SN) SN constitutes 7% of installed RAS at WECC. SN type is intended to minimize the impact of extreme events when such impacts cannot be entirely avoided.
Reliable system operation depends on proper operation of RAS, as well as coordination of RAS designs and operation between TOs and across interconnections.
The remainder of this paper is divided into five sections. Section III describes implementation of real-time voltage stability assessment at Peak Reliability. Section IV summarizes modeling of RAS during real-time VSA at Peak Reliability. Section V provides examples of RAS actions implemented during real-time VSA at Peak Reliability. Section VI presents the Impact of the RAS on VSA results. Conclusions are given in Section VII.
III. Implementation of Real-Time Voltage Stability Analysis at Peak Reliability Voltage stability limits and operating margins are computed at Peak Reliability in real-time, and the operator is alarmed if the current system state approaches the limit. Peak Reliability (formerly WECC RC) is the only Reliability Coordinator (RC) in the Western Interconnection. Its geographical region includes British Columbia, 14 Western US States, and Northern Baja-California/Mexico, see Figure 2. It started operation as Peak Reliability on January 1st, 2014.
Figure 2. Geographical Footprint of Peak Reliability
A number of factors contribute to Peak Reliability’s need for real-time voltage stability assessment, including:
Numerous areas in the region are prone to voltage instability;
There are multiple long transmission lines with remote generation (e.g., paths);
Operators need early detection & alarming;
Increased need for RAS Awareness.
Real-time voltage stability assessment is performed using Peak-ROSE application, [14]. It employs “hybrid” voltage stability analysis approach, which utilizes both State Estimator data (e.g., model) and PMU measurements, see Figure 3.
Figure 3. Peak-ROSE Architecture
Model-based computations include:
Node-breaker model based State Estimator (SE) case from EMS is read every 5 minutes: o
Full 12,000-bus SE model is used during AC power flow solution.
Multiple user-defined scenarios are executed;
The following analyses are performed for each scenario:
o
Determining interface limits;
o
Performing PV-curve analysis;
o
Performing QV -curve analysis.
A Model-based alarm is issued if the current system state in terms of the interface flows is close to the limit.
PMU-based computations include:
PV analysis is performed on the pre-contingency SE case at selected buses for each scenario;
The current operating point is determined using the PMU data for selected buses for each scenario;
Measurements for selected buses for each scenario are compared vs. computed PV-curve values;
A PMU measurement based alarm is issued if the current operating point is close to the limit values computed by Peak-ROSE.
ROSE Web Client displays visualization in the Client UI, as shown in Figure 4.
Figure 4. Web Client UI for Real-Time Voltage Stability Assessment RAS trigger conditions are analyzed and protective actions are taken for each Contingency (that can potentially trigger RAS action) at each transfer level within one execution run.
IV. Modeling of RAS during Real-Time Voltage Stability Analysis at Peak Reliability RAS actions are modeled in Peak ROSE as scripts, which are invoked through the scenario files. Contingencies may be executed with or without RAS based on user preference. RAS actions are automatically triggered provided that certain triggering conditions are met and RAS arming status is enabled. Since system conditions may change from one transfer step to another, RAS actions for the same contingency may be different at different transfer levels. For each scenario, a RAS log file is created which contains information on trigger conditions and RAS actions for each contingency that has RAS script associated with it.
The process for modeling RAS actions during real-time voltage stability assessment at Peak Reliability is given in Figure 5.
Figure 5. General Logic Flow Diagram to Apply RAS during Peak-ROSE Analysis
During a 5-minute SE cycle, trigger conditions and RAS actions are computed for multiple contingencies at each transfer step for all scenarios. The process described in Figure 5 is repeated every 5-minute cycle when a new State Estimator case arrives.
V. Types of RAS Implemented during Real-Time Voltage Stability Analysis at Peak Reliability The types of RAS actions that were implemented in Peak-ROSE during real-time VSA are:
Event or Contingency based;
Condition based;
Combination of event (Contingency) based and condition based;
Based on real-time RAS arming status points.
The trigger conditions for arming RAS include the following events or combination of events:
RAS status points;
A contingency;
Flows on the interface at each transfer level (pre-contingency);
Status of elements (in-service or out-of-service) in the base case, etc.
RAS actions were modeled as VB.net scripting programs which can create and save multiple operating points within one simulation run. During script execution base case flows, line statuses, and other base case conditions may be checked even after a contingency has been applied, and pre- and postcontingency/stressing values may be compared. Protective actions include:
Tripping of lines and units;
Opening breakers;
Dropping load, etc.
The value of unit/load tripping is computed based on certain conditions, including:
Status of elements in the base case,
Nomograms (functions);
Lookup tables.
Example 1. An example of a generation arming RAS based on a lookup table is given in Figure 6.
Figure 6. A Sample Generation Arming RAS
Trigger conditions for this RAS are: 1. Opening of following breakers: a. Breaker a AND b. Breaker b AND c. Breaker c AND d. Breaker d AND e. Breaker e AND f. Breaker f AND g. Breaker g AND h. Breaker h AND 2. Path XX MWs Flow ≥ 1600 MW
Protective actions for this RAS are:
Trip units at station Station_1, Station_2, Station_3, Station_4 DROPPING ARMING TABLE” in Figure 6;
Plus an additional requirement on dropping generation at Station_5.
based on “GENERATOR
Example 2. An example of a load dropping RAS based on a nomogram is given in Figure 7.
Figure 7. A Sample Load Dropping RAS
Trigger conditions for this RAS are: 1. Line_1 is out of service in base case AND 2. Line_2 is out of service in base case AND 3. Contingency_1 occurs AND 4. Pre-contingency Path YY flow ≥ 300 MW at each transfer step
Protective actions for this RAS are:
Allocate load to each station based on its respective share of the total load at above stations;
Amount of load to drop is calculated based on the nomogram in Figure 7.
Example 3. This is an example of modeling post-contingency generation tripping RAS based on pre-contingency conditions. Trigger conditions for this RAS are: 1. Contingency_1 and Contingency_2 occur
AND 2. Check for status of Line_1 and Line_2 in the base case AND 3. Check Line_1 and Line_2 total line flow MW (dependent on status of these lines)
Protective actions for this RAS are:
If both Line_1 and Line_2 are in service in base case, execute generation tripping logic #1;
If one of the lines is initially out of service in base case, execute generation tripping logic #2.
Generation tripping logic includes:
Trip units one by one until generation dropped ≥ Arming Level;
Select units to drop such that least amount of generation is tripped to meet or exceed the arming level;
Flows are computed pre-contingency at each transfer step;
Generation is computed post-contingency at each transfer step.
Example 4. This is an example of modeling a series reactor RAS. Trigger conditions for this RAS are: 1. WHEN ANY ONE OF THE FOLLOWING LINES TRIPS: a. Line 1 OR b. Line 2 OR c. Line 3 AND 2. LINE 4 FLOW IS ≥ LIM3
Protective actions for this RAS are:
OPEN PCB abc
Upon completion of each computation cycle, trigger conditions and RAS actions for each contingency at each transfer step are recorded and archived.
VI. The Impact of the RAS on Voltage Stability Analysis Results It is necessary for calculation of precise VSA results that the RAS actions are accurately considered during the analysis. A number of RAS schemes involve post-Contingency generation dropping and/or transmission reconfiguration which may shift the operating point at which voltage collapse occurs depending on: i.
Whether post-Contingency generation is being dropped or not
ii.
The amount of generation that is being dropping
iii.
Post-Contingency system topology
An example is shown below where it can be seen how post-Contingency generation dropping due to RAS actions impacts the VSA results. A sample scenario was modeled in the ROSE application. A Contingency was defined that triggers RAS actions post-Contingency. This RAS trips generation at a number of stations when a certain line trips. A PV curve is plotted on certain selected buses as interface flow is increased in steps and the Contingency/RAS actions are applied at each step. It can be seen below how the VSA results change as impacts of post-Contingency RAS actions are considered. When VSA is performed without RAS the voltage collapse occurs at 4126 MW of interface flow, see Figure 8. Whereas when RAS actions are applied the collapse point shifts by approximately 200 MWs to 4335 MW of interface flow, as shown in Figure 9. Thus, considering RAS actions as part of the VSA, increases transfer capability by approx. 200 MWs.
Figure 8. VSA Results without RAS
Figure 9. VSA Results with RAS
VII. Conclusion ROSE application is designed to become part of suite of applications used by Peak Reliability in its operations to ensure system reliability. ROSE performs VSA which is an important aspect of situational awareness for Peak in performing its function as Reliability Coordinator. VSA results are impacted by RAS actions and ROSE application provides framework where impact of RAS actions can be seen as part of VSA using visual basic scripts, thus providing operators and engineers better situational awareness based on actual system conditions and post-Contingency RAS actions.
VIII. Acknowledgement The authors would like to acknowledge the support from the U.S. Department of Energy’s Assistance Agreement DE-OE0000364.
IX. References [1] North Electric Reliability Corporation (NERC) Protection and Control (PRC) Standards, online: http://www.nerc.net/standardsreports/standardssummary.aspx.
available
[2] C. W. Taylor, K. E. Martin and V. Venkatasubramanian, WACS – Wide-Area Stability and Voltage Control System: R & D and Online Demonstration, Proceedings of the IEEE, Vol. 93, No. 5, May 2005. [3] Power Systems Engineering Center (PSERC) Publication 10-19, System Protection Schemes: Limitations, Risks, and Management, Final Project Report, Dec., 2010. [4] M. Vaiman, P. Hines, J. Jiang, S. Norris, M. Papic, A. Pitto, Y. Wang, G. Zweigle, “Mitigation and Prevention of Cascading Outages: Methodologies and Practical Applications”, 2013 IEEE PES GM, GM0669. [5] P.M. Anderson and B. LeReverend, “Industry experience with special protection schemes”, IEEE Trans. Power Syst., vol. 11, pp. 1167-1179, Aug. 1996. [6] D. Karlsson and X.Waymel, Eds., System protection schemes in power networks, Report, CIGRE Task Force 38.02.19, June 2001. [7] W. R. Lachs, “A New Horizon for System Protection Schemes”, IEEE Trans. Power Syst., vol. 18, No. 1, pp. 334-338, Feb. 2003. [8] M. Varghese, J. Licheng; S. Ghosh, G. Lin, B. Pek, “The CAISO experience of implementing automated Remedial Action Schemes in Energy Management Systems”, IEEE PES GM, 2009. [9] J. Adams, ERCOT Cascading Outage Practice & Possible Improvement, Presentation, May 2005. [10] CIGRE C2.02.24, “Defense Plan against extreme contingencies”, TB 316, April 2007. [11] D. Karlsson and X.Waymel, Eds., System protection schemes in power networks, Report, CIGRE Task Force 38.02.19, June 2001. [12] Western Systems Coordinating Council, Remedial Action Scheme Design Guide, February 2007. [13] Western Systems Coordinating Council Standard PRC-004-WECC-1 – Protection System and Remedial Action Scheme Misoperation, October 2011. [14] S. Malik, M. Y. Vaiman, M. M. Vaiman, “Implementation of ROSE for Real-time Voltage Stability Analysis at WECC RC”, 2014 PES T&D Conference and Exposition, 14TD0175.
X. Biographies Saad Malik is a Lead Shift Operations Engineer with Peak Reliability. He received his MASc degree in Electronics Engineering from University of Regina, Canada in 2002 and his B.Sc in Electrical Engineering from University of Engineering and Technology Peshawar, Pakistan in 1998. His responsibilities at Peak Reliability include operations planning, real-time operations support, utilization of real-time tools such as Peak-ROSE at the Peak Reliability operations centers, development of operational plans based on facility outages, analysis of real-time system events and performing system studies. Michael Vaiman received his MSEE degree from Kaunas Polytechnic University, Lithuania, Ph.D. degree from Moscow University of Transportation Engineering, Russia, and D.Sc. degree from St. Petersburg Polytechnic University, Russia. He has over 40 years of power industry experience. Dr. Vaiman is a President and Principal Engineer at V&R Energy Systems Research, Inc. (V&R Energy). His main areas of interest are power system stability and control, power flow and optimal power flow analysis, computer modeling of power system networks, selection of remedial actions for stability preservation, dynamic stability analysis, and predicting cascades. He leads the development of the POM Suite, consulting, and research and development activities at V&R Energy. He has authored over 100 publications devoted to the issues of power system stability and control. Marianna Vaiman received her BSEE and MSEE degrees from Moscow University of Transportation Engineering, Russia. She has over 20 years of experience in power system studies. In 1992 she joined V&R Energy Systems Research, Inc. (V&R Energy), where she is currently Principal Engineer and Executive Vice President. She leads the work in the following areas at V&R Energy: Software Development, Consulting Activities, Research & Development Activities. She has 20 publications devoted to the issues of power system stability and control.
