Verification of Synchronous Generator Reactive Capability

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Open Distribution Modeling Reference Document Reporting and Verification of Generating Unit Reactive Power Capability for Synchronous Machines Revision 2015-1…
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Open Distribution Modeling Reference Document Reporting and Verification of Generating Unit Reactive Power Capability for Synchronous Machines Revision 2015-1 Disclaimer This document was created by the North American Transmission Forum (NATF) to facilitate industry work to improve the reporting and verification of generating unit reactive power capability for synchronous machines. NATF reserves the right to make changes to the information contained herein without notice. No liability is assumed for any damages arising directly or indirectly by their use or application. The information provided in this document is provided on an “as is” basis. “North American Transmission Forum” and its associated logo are trademarks of NATF. Other product and brand names may be trademarks of their respective owners. Copyright 2015. All rights reserved. This legend should not be removed from the document. Open Distribution Copyright © 2015 North American Transmission Forum. Not for sale or commercial use. All rights reserved. Open Distribution Table of Contents Section 1. Generating Unit Reactive Power Capability Modeling Reference Document Scope….…3 Section 2. Generating Unit Reactive Power Capability Modeling Reference Document Objectives….……..……………………………………………………………………………………………………….4 Section 3. Determination of Generating Unit Reactive Power Limits for Use in Transmission System Models…….…………………………………………………………………………………………….………5 Section 3.1 Generator Entity Engineering Study.…………………………………………………………………….......7 Section 3.2 Transmission Planner Engineering Study...……………………………………………....................23 Section 4. Validation of Generating Unit Reactive Power Capability..………………………………..……..29 Appendix A: Understanding Generator and Generating Unit Reactive Power Capability..…………..…38 Appendix B: Generator D-Curve Examples..………………………………………………………………………………….44 Appendix C: GSU Transformer Impacts on Generating Unit Capability..………………………………………..47 Appendix D: Potential Reactive Power Operating Limitations.….………………………………………………….51 Appendix E: Transmission Study Horizons.…………………………………………………………………………………..54 Appendix F: Representation of Generators in Power Flow Models.……………………………………………..56 Appendix G: Reporting of Test Results..………………………………………………………………………………………..58 Appendix H: Glossary of Terms.......................................................................................................60 Revisions Date Versions Notes 04/30/2015 2015-1 Original Version Modeling Reference Document - Reporting and Verification of Generating Unit Reactive Power Capability for Synchronous Machines Page 2 Open Distribution Section 1. Generating Unit Reactive Power Capability Modeling Reference Document Scope Note 1: This Reference Document applies to synchronous generators only. Note 2: This Reference Document uses the term ‘Generator Entity’ to refer to both the Generator Owner and Generator Operator. Depending on how a particular company is organized, responsibilities may be assigned to either the Generator Owner or Generator Operator. Note 3: This Reference Document does not create binding norms, establish mandatory reliability standards or create parameters by which compliance with Reliability Standards are monitored or enforced. In addition, this Reference Document is not intended to take precedence over any regional procedure. It is recognized that individual Generator Owners and Transmission Owners may use alternative and/or more specific approaches that they may deem more appropriate for their generators and transmission systems. A generating unit’s capability to provide reactive power support is essential in maintaining adequate system voltage profiles under a variety of steady-state (normal and contingency) conditions for ensuring Bulk Electric System (BES) Reliability. It is important to recognize that the “generator” reactive power capability curve only represents the capability of the electrical generator itself. The manufacturer generator reactive power capability curve or D-curve does not take into account: a) the design of the auxiliary power system and its coordination with the generator terminal voltage; b) the GSU transformer electrical characteristics; c) the strength of the transmission system to which the generator is connected; d) the transmission operating voltage and coordination with the GSU transformer tap setting; and e) generator protection system settings. These five factors will affect the “generating unit” reactive power capability. The distinction between the “Generator” reactive power capability and the “Generating Unit” reactive power capability must be understood, evaluated and reported so the generating unit can be modelled correctly in Transmission Operations and Planning studies. For additional information on this concept see Appendix A: Understanding Generator and Generating Unit Reactive Power Capability. In addition to the above considerations, Generating Unit reactive power factor requirements are generally governed by the Interconnection Agreement between the Generator Entity and the Transmission Owner/Operator. These Interconnection Agreement requirements often define a narrower band or range within a generating unit’s rated capability curves. Transmission planners and operators use models of generating units as part of their responsibility to ensure reliable and economic operation of the grid. Most present network analysis tools use fairly simple representations to depict each unit’s reactive power capabilities at fixed minimum and maximum reactive power limits. The lagging and leading generator reactive power capability available to the transmission system is specific to each unit and can vary significantly based on numerous factors. These simple representations do not typically take into account key variables such as transmission system voltage, generator and auxiliary bus voltages, transformer tap settings, MW output level, the impacts of excitation system controls and limiters, effects of cooling provided by variable hydrogen pressure, ambient air, etc. Modeling Reference Document - Reporting and Verification of Generating Unit Reactive Power Capability for Synchronous Machines Page 3 Open Distribution Staged reactive power tests have been used to verify and document a generating unit’s reactive power capability. It is important to recognize that the test results are representative of each unit’s reactive power capability for the prevailing conditions at the time of the test. The same unit may have significantly more or less reactive power capability under different operating conditions. Therefore, tests performed under a single set of conditions may not be adequate to provide the full picture of a unit’s reactive power capability. Consequently, the tests may result in reduced reactive power capability in comparison to the actual reactive power capability achievable under different sets of conditions. Presently, there is no consistent, industry-wide approach for ensuring that generator voltage and reactive power limits are determined and reported appropriately for use in the various types of steady-state studies performed. Developing a reference document for verification and reporting of generating unit reactive power capability for steady state studies performed by transmission planning and operations has been identified by the North American Transmission Forum’s Modeling Practices Group (NATF MPG) as a high priority initiative. The NATF Modeling Practices Group’s (MPG) Generator Reactive Modeling Working Group (GRWG) reviewed the major factors affecting the ability of a generating unit to produce lagging and leading reactive power. From this review, the group is providing information for help in determining, verifying, reporting, and modeling generating unit reactive power capabilities and associated limitations for use in steady- state evaluations of Bulk Electric System (BES). Section 2. Generating Unit Reactive Power Capability Modeling Reference Document Objectives This NATF Modeling reference document ‘Reporting and Verification of Generating Unit Reactive Power Capability for Synchronous Machines' is intended to provide guidance towards the use of consistent, industry-wide approaches for verifying, reporting and validation of generating unit reactive power capability. This document does not replace, change, or interpret any requirements in NERC Reliability Standards or other applicable criteria. The reactive power limits used in the transmission planning and transmission operations power flow simulations should be based upon the “Generating Unit” reactive power capability rather than the “Generator” reactive power capability. An understanding of and distinction between “Generator” and “Generating Unit” reactive power capability is provided in Appendix A: Understanding Generator and Generating Unit Reactive Power Capability. With proper coordination and data sharing, appropriate “Generating Unit” reactive limits can be determined via simulations performed by the Generator Entity and/or the Transmission Planner. The steps recommended for determining “Generating Unit” reactive limits are discussed in Section 3: Determination of Generating Unit Reactive Limits for Use in Transmission System Models. Implementation of these limits, i.e. using them as a guideline for operation of generators or making them mandatory, is a separate matter, however. Therefore, it is recommended that there be a joint review of these limits between the Transmission Entity and the Generator Entity prior to their use. On-Line unit performance should be reviewed and used to help validate the “Generating Unit” reactive power limits and the models developed in Section 3. Validation can be based upon Modeling Reference Document - Reporting and Verification of Generating Unit Reactive Power Capability for Synchronous Machines Page 4 Open Distribution operational history data or staged-testing. These approaches are discussed in Section 4: Validation of Generating Unit Reactive Power Capability. The test results or operational data can be compared to the results obtained in Section 3 to refine the “Generating Unit” model and reactive power limits to be used in the transmission planning and transmission operations power flow simulations. Modeling of generator reactive power capability is important for various types of power flow (also known as load flow) studies. Power flow analysis is performed for different objectives and with various time horizons. An introduction to reactive power capability modeling and differences between real-time operations, operational planning, and long-term planning studies is provided in Appendix E: Transmission Study Horizons. While there should be consistency in modeling of generating unit reactive power limits within certain types of studies, e.g. operations versus mid- and long-range planning, the most suitable generating unit representation could vary e.g., consistent with expected voltage deviations from generator voltage schedules. Operations studies may be performed in an intentionally conservative manner whereas planning studies with light or heavy loads or with multiple overlapping contingencies could be performed with generating unit reactive power capabilities expected with higher or lower transmission grid voltages than those experienced during normal system conditions. Transmission Entities perform a variety of operating and planning studies (See Appendix E for additional discussion) and the representation of the generating unit varies in the different models (See Appendix F: Representation of Generators in Power Flow models for additional discussion). Transmission Entities need the generating unit reactive power capability limits at the generator terminals and the point of interconnection to be provided in a consistent format so that they can select appropriate limits to be modeled in their planning and operating studies over a range of transmission system operating conditions including voltages. It is suggested that the limits be developed following the approach outlined in this document and communicated, as necessary, on both a generator capability curve (D-curve) as well as in a tabular format as described in Section 3.1, step 5, part 2. Note: The reference document does not provide a “one size fits all” approach to methods for determining “generating unit reactive power capability.” It does, however, provide various methods for obtaining generator and generating unit limits that can be factored into the Transmission modeling process. Examples of current processes from Forum members are provided. The goal is to ensure that generating unit reactive power limits are determined and reported appropriately for use in various types of transmission system studies. Section 3: Determination of Generating Unit Reactive Power Limits for Use in Transmission System Models Determination of the Generating Unit reactive power limits for use in transmission system models requires coordination between and cooperation of both the Generator Entity and the Transmission Planner. Two approaches are presented.  Section 3.1 describes the modeling that could be done by the Generator Entity and the data necessary from the Transmission Planner. Modeling Reference Document - Reporting and Verification of Generating Unit Reactive Power Capability for Synchronous Machines Page 5 Open Distribution  Section 3.2 describes the modeling that could be performed by the Transmission Planner and the data necessary from the Generator Entity. Modeling Reference Document - Reporting and Verification of Generating Unit Reactive Power Capability for Synchronous Machines Page 6 Open Distribution 3.1 Generator Entity Engineering Study The following outlines an approach that can be used by a Generator Entity to determine generating unit reactive power limits for use in transmission system models. This method employs an “all-the- tools-in-the-toolbox” approach which can be summarized as follows: 1. Establish Generating Unit Power Flow Model 2. Obtain Operational or Test Data 3. Validate Power Flow Model with Operational/Test Data 4. Perform Reactive Power Capability Simulations (Specific unit load points and POI transmission voltages) 5. Document Results for Reporting a. Obtain Plant Operations Staff Review & Feedback b. Transmit Results to Transmission Planner/Reliability Coordinator Each of these steps are discussed in more detail below as applied to an example generating unit. Step 1 - Establish Generating Unit Power Flow Model The following inputs are needed for development of the model: ã Unit Configuration/Drawings (Single Line (S/L), 3-Line, etc.) ã Generator Ratings & Manufacturer’s D-Curve (See Appendices A and B) ã Excitation Limiter Setpoint Study / As-Left Settings ã GSU, UAT, SST Ratings, %Z, Tap Settings (GSU-Generator Step Up transformer, UAT-Unit auxiliary transformer, SST- station service transformer ) ã Gross MW values at which reactive power capabilities/limits are to be validated ã Auxiliary Load (MW and MVAR or PF) at which reactive power capabilities/limits are to be determined ã Transmission Bus High and Low Voltage Extremes at which reactive power capabilities/limits are to be validated. These are typically based on the applicable Transmission Owner/Operator requirements which specify the voltages at the point of interconnection ã Generating Unit Equipment Ratings & Operating Limits – Generator Bus, Breaker, and Switch Ampere Limits – Generator & Station Auxiliary Bus Voltage Limits. Generator design voltage limits are typically 105%/95% of the rated (nameplate) value (reference: IEEE C50.13). For some entities, auxiliary bus limits are typically based on 110% of motor nameplate voltage (per NEMA MG-1) and between 92.5% and 95% of nominal bus voltage. [The Generator Owner (GO) is ultimately responsible for establishing these bus voltage limits to ensure safe and reliable operation of the station auxiliary system over a wide array of operating conditions for an unlimited period.] Appendix A provides additional information on the generator and auxiliary system equipment voltage limits and their impact on unit reactive capability. The following are the basic model components needed for a comprehensive study. Determination of the specific station auxiliary system components to be modeled explicitly depend upon the specific unit design and configuration being modeled. Typically, the model should include each medium voltage bus along with low voltage buses whose operating voltage limits can potentially limit the generator operating terminal voltage range. ã Generator (See the D-curve plot below (Figure 1) for this example) Modeling Reference Document - Reporting and Verification of Generating Unit Reactive Power Capability for Synchronous Machines Page 7 Open Distribution – MVAR production limits from D-curve (consider margins, voltage impact, tolerances) at full (maximum) and minimum generator gross load – MVAR absorption limits from Minimum Excitation Limiter settings (consider margins, voltage impact, tolerances) at full (maximum) and minimum generator gross load – Review Over Excitation Limiter (OEL), Field Current Limiter, Volts per Hertz (V/Hz), armature (stator) current limiter, other limiter settings or alarms that could potentially limit operation within the Generator reactive power capability curve or operating voltage range – Factor in other known reactive power limitations (ex: generator cooling, H2 pressure, vibration, de-rated rotor, derated GSU transformer, generator bus derating, etc.) Figure 1 ã Generator Bus - include voltage & ampere ratings ã GSU Transformer – consider MVA & voltage ratings, %Z, field verified tap setting Modeling Reference Document - Reporting and Verification of Generating Unit Reactive Power Capability for Synchronous Machines Page 8 Open Distribution ã Transmission System Equivalent (typically an infinite bus is used) ã Transmission System Bus voltages (High, Schedule or Low) to use for each reactive power limit power flow simulation ã Unit Auxiliary Transformers (UATs) & Other Station Service Transformers (SST) - include MVA & voltage ratings, %Z, field verified tap settings ã Station Service Buses supplied by UATs and downstream low voltage SSTs. Include applicable voltage limits Note that equipment problems and/or reductions in equipment ratings that cause VAR limitations should be documented and reported for consideration of impacts to Operating and Planning models (ex: turbine-generator vibration, de-rated generator rotor, generator or transformer cooler degradation, bus connection hot spots or cooler issues). Below (Figure 2) is an example of a power flow model developed for a typical fossil unit: Figure 2 Note the foregoing model includes explicit representations of the generator, GSU transformer, unit auxiliary transformers, medium voltage buses, medium to low voltage station service transformers, and low voltage buses. The model also includes equivalents of the unit medium and low voltage bus loads and the transmission system at the point of interconnection. Step 2 - Obtain Operational or Test Data Modeling Reference Document - Reporting and Verification of Generating Unit Reactive Power Capability for Synchronous Machines Page 9 Open Distribution Scheduling of any tests should first be coordinated with the Transmission Operator. Data needed for validation of the power flow model can include the following: ã Transmission System Bus Voltage (high side GSU voltage) at the time of the test or operating data capture/snapshot ã Generator Voltage, MW, MVAR (or PF) ã Station Auxiliary Bus MW, MVAR (or Amp and PF) Load. This information may not be readily available in some cases, especially for LV systems* ã Station Auxiliary Bus Voltages (This would include the medium voltage buses and low voltage buses (575 V, 480 V). This information may not be readily available in some ca
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