500 MW Wind Farm Technical System Impact Study

Transcription

1 FINAL REPORT Report to: El Paso Electric for 500 MW Wind Farm Technical System Impact Study Prepared by: Goran Drobnjak Sang Lee Emad Ahmed Reigh Walling Einar Larsen February 19, 2008 EPE_TechSIS_Final.doc 1

2 Foreword This document was prepared by General Electric Company in Schenectady, NY. It is submitted to El Paso Electric. Technical and commercial questions and any correspondence concerning this document should be referred to: Goran Drobnjak GE Energy Building 2, Room 640 Schenectady, New York Phone: (518) Fax: (518) EPE_TechSIS_Final.doc 2

3 Legal Notice This report was prepared by General Electric Company as an account of work sponsored by El Paso Electric. Neither El Paso Electric nor GE, nor any person acting on behalf of either: Makes any warranty or representation, expressed or implied, with respect to the use of any information contained in this report, or that the use of any information, apparatus, method, or process disclosed in the report may not infringe privately owned rights. Assumes any liabilities with respect to the use of or for damage resulting from the use of any information, apparatus, method, or process disclosed in this report. EPE_TechSIS_Final.doc 3

4 Table of Contents EXECUTIVE SUMMARY... 6 SSTI... 6 CONTROL INTERACTIONS... 7 TEMPORARY OVERVOLTAGES... 7 BREAKER TRANSIENT RECOVERY VOLTAGE INTRODUCTION SYSTEM MODEL AND METHOD OF ANALYSIS ELECTRICAL GRID Existing Network Condition Generation and Network Contingencies Short-Circuit Capacity at Windfarm Connection THREE-PHASE SHORT CIRCUIT AT THE WINDFARM 345KV PCC POWER ELECTRONICS EQUIPMENT AND ADJACENT GENERATORS PROPOSED WIND FARM Grid Changes due to Proposed Wind Farm Wind Farm Network Wind Turbines SUB-SYNCHRONOUS TORSIONAL INTERACTION (SSTI) SSTI SCREENING SSTI ELECTRICAL DAMPING CALCULATION SSTI CONCLUSIONS EQUIPMENT CONTROL INTERACTION SMALL SIGNAL DISTURBANCES LARGE-SIGNAL DISTURBANCES CONTROL INTERACTION CONCLUSIONS SYSTEM TEMPORARY OVERVOLTAGE (TOV) SCENARIO SELECTION FOR TOV EVALUATION SIMULATION RESULTS FAULTS NEAR WIND FARM POINT OF INTERCONNECTION Single Phase Fault near Wind Farm Point of Interconnection Three Phase Fault near Wind Farm Point of Interconnection Wind Farm Power Output 500 MW Wind Farm Operation with Fewer Turbines CONCLUSIONS FROM TOV EVALUATION CIRCUIT BREAKER TRANSIENT RECOVERY VOLTAGE (TRV) IEEE STANDARD FOR TRV EVALUATION CIRCUIT BREAKERS LIST FOR TRV ANALYSIS FAULT TYPE, DURATION, AND LOCATION SIMULATION CASES TRV RESULTS REFERENCES: APPENDIX A - EL PASO SYSTEM CONTROL INTERACTION EPE_TechSIS_Final.doc 4

5 APPENDIX B EL PASO SYSTEM TOV APPENDIX C EL PASO SYSTEM TRV EPE_TechSIS_Final.doc 5

6 Executive Summary An independent developer is proposing to construct a 500 MW wind plant in New Mexico that will be interconnected to the 345kV transmission system of El Paso Electric (EPE) in the vicinity of the Eddy County HVDC converter station and the Amrad static reactive power compensator (SVC) installation. The proposed wind plant is large relative to the EPE transmission system at the point of interconnection. Interactions with the existing HVDC, SVC, and conventional generation are possible. Also, the existing system experiences high temporary overvoltages in this region. The windfarm will change the character of these overvoltages, as well as being sensitive to their existence. Due to the large size of the windfarm, and unique aspects of the grid, specific issues were identified as requiring evaluation as part of the system impact study process. This study addresses the following points: Sub-synchronous torsional interaction (SSTI) with existing generation Control interaction and stability Temporary overvoltages Circuit breaker transient recovery voltage The network was represented in GE s electromagnetic transients program, including detailed models of the Amrad SVC, the Eddy County HVDC, and the proposed wind turbines assuming GE equipment. Disturbance scenarios were evaluated appropriate for the study objective. Results are summarized by topic in the following sections. SSTI SSTI refers to the potential of grid-connected equipment having fast regulating controls to affect damping of torsional vibrations on generator shafts. These torsional modes exist on all machines, and have very little inherent damping. The fast regulators of transmission equipment such as HVDC can cause a negative damping effect, which may overcome the inherent positive damping and lead to growing torsional vibrations. Similar effects are possible with any large power-electronic device. Since the wind turbines employ power electronics to regulate power, and since the proposed new windfarm is so large, this study includes assessment of the wind turbine equipment on the existing generation in the El Paso area. The analysis method developed over the past 30 years of experience consists of several stages. The first stage is a screening study to identify conditions leading to highest potential interaction. The second stage is to perform an assessment of the electrical damping imposed by the transmission system on the generator in this scenario of highest interaction. The screening evaluation defined a scenario where the Newman units 1, 2, and 3, and the Rio Grande units 6 and 7, are out of service. In addition, the Newman-Afton 345kV line is out. This case yields the highest interaction between the remaining generator units and the wind farm. EPE_TechSIS_Final.doc 6

7 Detailed analysis of this case shows that the electrical damping on generator torsional vibrations is positive for the full range of torsional frequencies for all generators. The effect of the windfarm is small relative to pre-existing condition. Since these were evaluated for the network and generation condition yielding highest interaction, these represent the largest possible impact of the windfarm. Therefore the system is expected to be benign with respect to SSTI and no further study is deemed necessary. Control Interactions The purpose of this evaluation is to determine if there is a risk of adverse interactions between the control systems of the Amrad SVC, the Eddy County HVDC, and the wind farm. The scope in this study relates to high-frequency interactions, i.e. faster than approximately 2Hz. Slower interactions would be seen in conventional power-system stability dynamic evaluations that are being performed in other studies. The evaluation includes both small-signal and large-signal behavior. A network configuration is used that maximizes coupling between the controlled elements. For this grid, such a scenario is associated with the highest practical 345kV grid impedance to the west of Amrad. The case used for SSTI evaluation exhibits this characteristic so is used for the control interaction evaluation. Results show well-behaved responses, with no indication of potential adverse control interactions between the windfarm, HVDC, or SVC. Temporary Overvoltages The worst overvoltages at and beyond Caliente to the east occur for a 3ph fault near Picante, clearing the Picante-Newman 345kV line and blocking the HVDC during the fault. For this worst-case event, there is only a small difference due to adding the windfarm. At Artesia, the TOV increases from 2.05pu to At Caliente, the TOV increases from 1.8pu to 1.9. At Amrad there is not much difference. The actual overvoltages at these locations will be lower than indicated above due to surge arresters at the stations that were not modeled for this analysis. These results give an indication that energy duty in existing arresters may be increased, which should be evaluated in the facilities study to determine if higher-energy arresters are warranted. Similarly, the arresters applied at the windfarm will be subjected to higher than typical duty and this factor should be considered in the design of the new substation. At Newman, the impact is greatest for a 3ph fault on the Amrad-Caliente 345kV line and blocking the HVDC during the fault. In this case, the voltage is approximately 140% with 500MW from the windfarm. However, this overvoltage is of very short duration and is unlikely to pose risk to equipment. There is a risk of tripping the windfarm for events causing high overvoltage. This risk is low for single-phase faults or for the early stages of the windfarm development. In the final stage with 500MW of turbines, multi-phase faults close to the windfarm have a potential to create a post-fault voltage that would trip most or all of the wind turbines. This risk can be mitigated by remedial action systems that would deliberately trip part of EPE_TechSIS_Final.doc 7

8 the windfarm with the same timing as clearing the faulted line at the 345kV windfarm substation. It is also possible that using lower-impedance transformers in the windfarm or adding special functions to the wind turbine controls would mitigate the risk. Breaker Transient Recovery Voltage The existing EPE breakers, and those used with the new 345kV substation at the windfarm connection, will be exposed to potentially high overvoltages when they clear. The transient recovery voltage (TRV) across the breaker is affected by both local characteristics of the substation, in the micro-second time frame, and external overvoltages in the millisecond time frame. For this project, the unique aspect for breaker TRV is the longer-term overvoltage caused by the grid. This aspect was evaluated for 345kV and 115kV breakers specified by EPE, as indicated in the following tables: Table E-1: 345 kv Breakers List Case No. Breaker Name Side Name 1 4R05 Eddy County 2 4R10 Reactor Eddy County B Reactor_Amrad B(side1) Line Amrad-Wind Farm B(side2) Line Amrad-Caliente B(side1) Line Amrad-Caliente B(side2) Amrad Substation B(side1) Line Amrad-Wind Farm B(side2) Amrad Substation B(side1) Line Amrad-Caliente B(side2) Breaker 0428B B(side1) Breaker 3928B B(side2) Auto Transformer B(side1) Breaker 0428B B(side2) Breaker 4178B B(side1) Breaker 4688B B(side2) Line Amrad-Caliente B Reactor_Caliente EPE_TechSIS_Final.doc 8

9 Table E-2: 115 kv Breakers List Case No. Breaker Name Side Name B Line AMRAD-ALAMOGCP B(side1) Line AMRAD-ALAMOGCP B(side2) Line AMRAD-LARGO B(side1) Line AMRAD-LARGO B(side2) Transformer T B Line AMRAD-ORO GRANDE B ORO GRANDE Substation B Line ORO GRANDE-WHITE SAND B Line ORO GRANDE-AMRAD Line LARGO-MAR B(side1) Transformer T B(side2) Breaker 8616B B(side1) Breaker 6206B B(side2) Breaker 0756B B(side1) Breaker 8616B B(side2) Transformer T B(side1) Transformer T B(side2) Breaker 1536B B(side1) Breaker 9406B B(side2) Breaker 6316B B(side1) Breaker 1536B B(side2) Transformer T B(side1) Transformer T B(side2) Breaker 6166B B(side1) Breaker 6166B B(side2) Transformer T B ShuntCap "CALIENTE" Results were evaluated with respect to IEEE standard C37.011, both 1997 and The conclusion is that the addition of the windfarm will not change the TRV on these breakers such that the duty exceeds the IEEE standard. EPE_TechSIS_Final.doc 9

10 1 Introduction An independent developer is proposing to construct a 500 MW wind plant in New Mexico that will be interconnected to the 345kV transmission system of El Paso Electric (EPE) in the vicinity of the Eddy County HVDC converter station. Also in this area is the AMRAD static reactive power compensator (SVC) installation. The proposed wind plant is large relative to the strength of the EPE transmission system at the point of interconnection. Interactions with the existing HVDC, SVC, and conventional generation are possible. Also, this system experiences high temporary overvoltages on certain contingencies and the windfarm will change the character of these overvoltages. Due to the large size of the windfarm, and unique aspects of the grid, specific issues were identified as requiring evaluation as part of the system impact study process. This study addresses the following points: Sub-synchronous torsional interaction (SSTI) with existing generation Control interaction and stability Temporary overvoltages Circuit breaker transient recovery voltages EPE_TechSIS_Final.doc 10

11 2 System Model and Method of Analysis In general, the analysis is based on comparing performance with the windfarm versus the comparable situation in the grid, as it exists today. Comparisons are also made to industry standards where available, e.g. switchgear TRV capability. Several GE proprietary program packages were used to allow correct and detailed representation of the SVC, HVDC, generators, and wind turbines. All these tools are compiled into single transients program with appropriate interfaces connecting to the common electrical grid model. 2.1 Electrical Grid Existing Network Condition Starting point for the electrical network model was year 2010 power flow and stability data provided by EPE. The El Paso Electric system was retained and surrounding areas were reduced to 22 equivalent Thevenin sources behind frequency-dependent impedances. Frequency-dependent impedances were developed using full system frequency responses and fitting process described in [1]. Model validation was performed by comparison of frequency responses of driving point impedances at Caliente 345 kv, Amrad 345 kv and Artesia 345 kv for full and reduced system model. Agreement is excellent as shown in Figure 2.1. The other significant characteristics of the network model are listed in Table 2.1. Table 2.1 Number of Transformers Number of Transmission Lines Number of Generators Number of SVCs HVDC (MW) Number of Wind Turbines Wind farm Power Output (MW) For this network equivalence task, the original power flow was modified to have all generators at Newman and Rio Grande plants in service. EPE_TechSIS_Final.doc 11

12 Impedance Magnitude at Artesia ohms full system (FLICUP) boundary reduced system (EMTP) frequency (Hz) 100 Impedance Angle at Artesia 345 degrees full system (FLICUP) boundary reduced system (EMTP) frequency (Hz) Impedance Magnitude at Amrad ohms full system (FLICUP) boundary reduced system (EMTP) frequency (Hz) 100 Impedance Angle at Amrad 345 degrees full system (FLICUP) boundary reduced system (EMTP) frequency (Hz) EPE_TechSIS_Final.doc 12

13 Impedance Magnitude at Caliente 345 ohms full system (FLICUP) boundary reduced system (EMTP) frequency (Hz) 100 Impedance Angle at Caliente 345 degrees full system (FLICUP) boundary reduced system (EMTP) frequency (Hz) Figure 2.1 EPE_TechSIS_Final.doc 13

14 2.1.2 Generation and Network Contingencies The study considers pre-existing contingencies for several issues, as a means to identify extreme situations. Generation scenarios were selected to correspond to typical unit commitment schedules. The generation dispatches evaluated are indicated in Table Line outage scenarios are listed in Table Table Evaluated Generation Dispatches ID Units Out of Service 0 All Generators In 1 Rio Grande G7 Out 2 Rio Grande G7 and G6 Ou 3 Rio Grande G7, G6 and Newman G2 Out 4 Rio Grande G7, G6 and Newman G2 and G1 Out 5 Rio Grande G7, G6 and Newman G2,G1 and G3 Out Table Evaluated Line Outages ID Lines out of Service 0 All Branches In 1 FWE345 - Amrad 345 kv Line Out 2 FWE345 - PICANTE 345 kv Line Out 3 CALIENTE PICANTE 345 kv Line Out 4 PICANTE NEWMAN kv Line Out 5 NEWMAN AFTON kv Line Out 6 NEWMAN ARROYO kv Line Out 7 CALIENTE AMRAD kv Line Out Short-Circuit Capacity at Windfarm Connection Approximate short circuit capacity at the windfarm point of interconnection 345 kv was calculated using power flow data and GE PSLF short circuit program. Results for several relevant network conditions are shown in Table 1. Table 1 Short circuit strength at the Wind Farm Point of Interconnection 345 kv Three-phase Short Circuit at the WindFarm 345kV PCC Branch Out of Service Ssc (MVA@345kV) All in 2700 Amrad - Windfarm 345 kv 2240 Picante - Windfarm 345 kv 1630 Windfarm Tx. 345/138 kv 2430 EPE_TechSIS_Final.doc 14

15 2.2 Power Electronics Equipment and Adjacent Generators The Eddy County HVDC and Amrad SVC are represented in detail with relevant control functions, switching logic and associated harmonic filters. The HVDC system includes a fast overvoltage protection function that blocks the inverter when voltage exceeds 130%. Some of the cases were done with this protection off to understand the effect of continued operation through a transient, but it is included unless otherwise noted. The Newman and Rio Grande generators are modeled using stability data from the power flow and stability files provided by EPE. A simplified excitation system is used, which is adequate for the analysis being performed since the phenomena of interest occur in a time frame of only a fraction of a second. 2.3 Proposed Wind Farm Grid Changes due to Proposed Wind Farm The proposed wind farm project will be developed in three phases. Phase one will have cumulative rated wind farm power output of 200 MW, phase two 350 MW and phase three 500 MW. The following network changes will be common for all of the three phases: Two new 345 kv substations will be added to allow new wind farm operation: o Substation at the wind farm point of interconnection. The substation will be located 53 miles west from Amrad 345 kv splitting the 345 kv Amrad- Artesia line. o Picante 345 kv substation located 7 miles from Caliente 345 kv splitting the 345 kv Newman-Caliente line. New 345 kv transmission line will be added from the wind farm point of interconnect to the new Picante 345 kv substation. Line length will be approximately 55 miles. New 345/115 kv transformer will be added at the Picante 345/115 kv substation. The following network changes will be made due to development of wind farm phase three: New 345/115 kv transformer will be added at the Calient 345/115 kv substation. This transformer will be of same size and ratings as the existing Caliente transformer 2. New 345/115 kv transformer will be added at the Arroyo 345/115 kv substation. This transformer will be of same size and ratings as the existing Arroyo transformer. EPE_TechSIS_Final.doc 15

16 2.3.2 Wind Farm Network The representation of the proposed wind farm is developed using network power flow data provided by EPE. The wind farm connects to the 345 kv grid approximately at 53 miles from Amrad 345 kv on the Amrad 345 kv Artesia 345 kv line. Interconnect consist of one 300 MVA 345/138 kv/kv autotransformer and four 75 MVA 138/34.5 kv/kv transformers leading to each of four 34.5 kv feeders. Each wind turbine is connected at a particular point along the feeders via 1.75 MVA 34.5/0.575 kv/kv transformer. The following three wind farm ratings, associated with the three development phases, are evaluated: 200 MW, 350 MW and 500 MW. The number of turbines will be 134, 236 and 334 in phases one, two and three respectively. Due to computing complexity, the modeling approach was to represent one equivalent turbine with associated padmount transformer per feeder with scaled power rating to reflect the aggregated number of turbines on that feeder Wind Turbines The wind turbines are assumed to be GE 1.5MW units. The representation includes full details of the control and protective systems used in the current generation of these products. These systems include a fast overvoltage protective function that trips the turbine should voltage at its low-voltage bus exceed 142%. EPE_TechSIS_Final.doc 16

17 Figure 2.2 EPE System with Proposed Wind Farm EPE_TechSIS_Final.doc 17

18 3 Sub-Synchronous Torsional Interaction (SSTI) SSTI refers to the potential of grid-connected equipment having fast regulating controls to affect damping of torsional vibrations on generator shafts. These torsional modes exist on all machines, and have very little inherent damping. The fast regulators of transmission equipment such as HVDC can cause a negative damping effect, which may overcome the inherent positive damping and lead to growing torsional vibrations. Similar effects are possible with any large power-electronic device. Since the wind turbines employ power electronics to regulate power, and since the proposed new windfarm is so large, this study includes assessment of the wind turbine equipment on the existing generation in the El Paso area. The analysis method developed over the past 30 years of experience consists of several stages. The first stage is a screening study to identify conditions leading to highest potential interaction. The second stage is to perform an assessment of the electrical damping imposed by the transmission system on the generator in the scenario identified to have the highest interaction. 3.1 SSTI Screening SSTI screening uses an evaluation of Unit Interaction Factor (UIF). This incorporates, in an approximate manner, the effect of a specific grid element on a specific generator (or group of identical generators). For this study, the grid element of interest is the windfarm. The values of UIF for this situation indicate only the relative interaction magnitude between cases, and have no meaning in an absolute sense. The UIF was calculated using the EPE power flow and dynamics system planning data for the year Portion of the system of interest including the proposed wind farm as well as the Newman and Rio Grande plant generators is shown in Figure 1. Method used for UIF calculation, for each generator at the Newman and Rio Grande station, was based on the following calculation: MVABase _ wf UIFi = Gvi Gci where, MVA Base _ gi i - a generator index G vi - voltage gain calculated as a ratio of voltage change at the wind farm point of interconnection versus the generator i internal voltage change G ci - current gain calculated as a ratio of current change at the generator i terminal versus the wind farm point of interconnection current injection MVA Base_wf - wind farm MVA base (500 MW) MVA Base_gi - individual generator i MVA base The voltage and current gains were determined using time domain PSLF dynamics simulation. UIF was evaluated on all combinations of generation status and line outages as defined in section The 48 calculated UIF indicators are listed in table 3-1. EPE_TechSIS_Final.doc 18

19 Table 3-1 UIF Results Dispatch Outage 5G12 G1 G2 G3 4G12 4S1 RG6 RG7 RG8 5S na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na na EPE_TechSIS_Final.doc 19

20 According to Table 3-1 the highest UIF is obtained for dispatch number five where the Newman unit 1, unit 2, unit 3 and Rio Grande unit 6 and unit 7 are out of service. The critical identified contingency is contingency number five described as NEWMAN AFTON kv line out of service. Figure 3.1. Critical contingency and generation dispatch for SSTI analyses EPE_TechSIS_Final.doc 20

21 3.2 SSTI Electrical Damping Calculation Electrical damping refers to the change in generator air gap torque due a change in generator shaft speed. If this is in a direction to oppose the speed change, then it is a positive damping. If its direction reinforces the speed change then it is a negative damping. The damping is important only at torsional mode frequencies of the particular machine. However, the analysis approach is to compute electrical damping for a wide range of frequencies. If electrical damping is positive over the range of typical torsional modes affected by fast-regulating grid equipment, i.e. from about 10Hz to about 40Hz, then there is no problem and no further analysis is warranted. The critical dispatch and the critical contingency determined in the screening step are used for this portion of SSTI analyses. Any impact of the windfarm will be largest in this condition, so if acceptable here then it will be acceptable in all other cases. The approach is to modulate speed of generators at the Newman four, Newman five as well as the generator eight at the Rio Grande plant and to monitor electrical torque for later correlation analysis with the speed input. The detailed transients program of the full EPE network is used for this analysis, including the Amrad SVC, the HVDC, and the windfarm. This is repeated for the system with proposed wind farm disconnected from the electrical grid and with the proposed wind farm connected to the electrical grid with variation of three different power outputs. Wind farm power outputs are, according to developing phases, 200 MW, 350 MW and 500 MW. Electrical torque and generator speed are correlated for each frequency, which yields both a synchronous torque coefficient (K e ) and a damping torque coefficient (D e ). These quantities are plotted together for corresponding cases without and with proposed wind farm. This is shown in Figures 3.2 to Each figure title describes which generator speed was modulated. The damping component is the important part for SSTI considerations. 3.3 SSTI Conclusions The results show that the electrical damping is positive for the full range of torsional frequencies for all generators. The effect of the windfarm is small relative to pre-existing condition. Since these were evaluated for the network and generation condition yielding highest UIF, these represent the largest possible impact of the windfarm. Therefore the system is expected to be benign with respect to SSTI and no further study is deemed necessary. EPE_TechSIS_Final.doc 21

22 Figure 3.2. Figure 3.3 EPE_TechSIS_Final.doc 22

23 Figure 3.4 Figure 3.5 EPE_TechSIS_Final.doc 23

24 Figure 3.6 Figure 3.7 EPE_TechSIS_Final.doc 24

25 Figure 3.8 Figure 3.9 EPE_TechSIS_Final.doc 25

26 Figure 3.10 Figure 3.11 EPE_TechSIS_Final.doc 26

27 Figure 3.12 Figure 3.13 EPE_TechSIS_Final.doc 27

28 Figure 3.14 Figure 3.15 EPE_TechSIS_Final.doc 28

29 Figure 3.16 EPE_TechSIS_Final.doc 29

30 4 Equipment Control Interaction The purpose of this evaluation is to determine if there is a risk of adverse interactions between the control systems of the Amrad SVC, the Eddy County HVDC, and the wind farm. The scope in this study relates to high-frequency interactions, i.e. faster than approximately 2Hz. Slower interactions would be seen in conventional power-system stability dynamic evaluations that are being performed in other studies. The evaluation includes both small-signal and large-signal behavior. A network configuration is used that maximizes coupling between the controlled elements. For this grid, such a scenario is associated with the highest practical 345kV grid impedance to the west of Amrad. The case used for SSTI evaluation exhibits this characteristic so is used for the control interaction evaluation. For all cases, the HVDC is importing to EPE at 200MW, with inverter gamma at 20deg. The Amrad SVC is operating at zero net, so the TCR has room to move in both directions. All cases are performed for four conditions related to the proposed wind farm: before wind farm, wind farm with 40% turbines installed and operating at full load (200 MW total), with 70% turbines installed and operating at full load (350 MW total) and wind farm with all turbines installed and operating at full load (500 MW total). From each simulation selected variables are plotted and used to determine if monitored control devices approach unstable condition or interact adversely among each other. Full results are shown in Appendix A. EPE_TechSIS_Final.doc 30

31 4.1 Small Signal Disturbances The small-signal behavior is stimulated by switching reactors or capacitors onto the grid. These are not existing or planned future components, but are used solely to create an appropriate small disturbance to observe control response. Both balanced and unbalanced disturbances are performed. The small-signal disturbance cases are defined in Table 4-1. Table 4-1 Cases for Small-Signal Control Interaction Study Case Disturbance Purpose 1.1 Insert 10MVAr capacitor at Amrad 345, 3-phase 1.2 Insert 10MVAr capacitor at Amrad 345, 1-phase 1.3 Insert 10MVAr reactor at Amrad 345, 3-phase 1.4 Insert 10MVAr reactor at Amrad 345, 1-phase 2.1 Step Vthev of Xcel side of HVDC +5% 2.2 Step Vthev of Xcel side of HVDC -10% Force SVC to move more inductive to regulate voltage, with balanced grid Force SVC to move more inductive to regulate voltage, with unbalanced grid Force SVC to move more capacitive to regulate voltage, with balanced grid Force SVC to move more capacitive to regulate voltage, with unbalanced grid Xcel-side disturbance forcing fast response from HVDC, no control mode change Xcel-side disturbance forcing fast response from HVDC, with control mode change EPE_TechSIS_Final.doc 31

32 4.2 Large-Signal Disturbances Large-signal behavior is considered when interpreting the fault cases presented in the section on temporary overvoltages. List of contingencies consists of single-phase-ground faults on transmission lines and bus faults, all cleared in five cycles. The list is shown in Table 4-2. Table 4-2 Cases for large-signal Control Interaction Study Case Single-phase-ground Fault cleared in 5 cycles 3.1 Arroyo side of Arroyo - W.Mesa 345 kv transmission line 3.2 Caliente side of Newman-Caliente 345 kv transmission line 3.3 Luna side of Luna - Afton 345 kv transmission line 3.4 Hidalgo side of Greenlee - Hidalgo 345 kv transmission line 3.5 Luna side of Springerville - Luna 345 kv transmission line 3.6 Amrad 115 kv 3.7 Caliente 115 kv 3.8 Amrad side of Amrad - Caliente 345 kv transmission line 3.9 Wind Farm PCC side of Wind Farm PCC -Artesia 345 kv transmission line 4.3 Control Interaction Conclusions All cases exhibit well-behaved responses. No indications of potential problems are indicated in these results. EPE_TechSIS_Final.doc 32

33 5 System Temporary Overvoltage (TOV) 5.1 Scenario Selection for TOV Evaluation Initial screening of faults in vicinity of Amrad 345 kv, Caliente 345 kv and Artesia 345 kv resulted in a number of faults that could be followed by noticeable temporary overvoltages. Surge arresters are not included in the analysis. This will give pessimistic results without affecting the relative evaluation of TOVs due to addition of the proposed wind farm. The selected list of cases with TOVs is the following: 1. SLG Fault on Picante side of Picante - Newman 345 kv Line. Fault cleared in 5 cycles. 2. SLG Fault on Picante side of Picante - Newman 345 kv Line. Fault cleared in 5 cycles. HVDC blocked during fault. 3. Three-Phase Fault on Picante side of Picante - Newman 345 kv Line. Fault cleared in 5 cycles. 4. Three-Phase Fault on Picante side of Picante - Newman 345 kv Line. Fault cleared in 5 cycles. HVDC blocked during fault. 5. Three-Phase Fault on Amrad side of Amrad - Caliente 345 kv Line. Fault cleared in 5 cycles. 6. Three-Phase Fault on Amrad side of Amrad - Caliente 345 kv Line. Fault cleared in 5 cycles. HVDC blocked during fault. All cases above are repeated for the following scenarios: a) Wind farm out of service. b) Wind farm 200MW output. c) Wind farm 350MW output. d) Wind farm 500MW output. 5.2 Simulation Results Voltages were monitored and plotted at the following five locations: FWE 345 kv bus. Artesia 345 kv bus. Amrad 345 kv bus. Caliente 345 kv bus. Newman 345 kv bus. Full results are included in Appendix B. Maximum TOV of all three phases at each monitored location for each of the simulated contingencies is summarized in Figures 5-1 to /19/08 EPE_TechSIS_Final.doc, 33

34 FEW345 kv Bus TOV (pu) Case WF OUT 200 MW 350 MW 500 MW a) 3 [pu] (f ile CI_wf _OUT_3_2T_BLK_c.pl4; x-v ar t) v :11960A v :11960B v :11960C factors: offsets: E E E-06 0 [s] 1.5 b) Figure 5-1 a) FEW345 kv Bus TOV for Different Faults and Wind Farm Dispatches b) Time simulations of the case with highest TOV at FEW345 kv (Wind farm out, fault 4) 2/19/08 EPE_TechSIS_Final.doc, 34

35 ARTESIA 345 kv Bus TOV (pu) WF OUT 200 MW 350 MW 500 MW Case 3 [pu] a) (f ile CI_wf _500MW_3_2T_BLK_c.pl4; x-v ar t) v :AR345A v :AR345B v :AR345C factors: offsets: E E E-06 0 b) Figure 5-2 a) ARTESIA 345 kv Bus TOV for Different Faults and Wind Farm Dispatches b) Time simulations of the case with highest TOV at ARTESIA 345 kv (wind farm 500 MW, fault 4) [s] 1.5 2/19/08 EPE_TechSIS_Final.doc, 35

36 AMRAD 345 kv Bus TOV (pu) WF OUT 200 MW 350 MW 500 MW Case a) 2.0 [pu] (f ile CI_wf _350MW_3_2T_BLK_c.pl4; x-v ar t) v :AM345A v :AM345B v :AM345C factors: offsets: E E E-06 0 [s] 1.5 b) Figure 5-3 a) AMRAD 345 kv Bus TOV for Different Faults and Wind Farm Dispatches b) Time simulations of the case with highest TOV at AMRAD 345 kv (Wind farm 350 MW, fault 4) 2/19/08 EPE_TechSIS_Final.doc, 36

37 CALIENTE 345 kv Bus TOV (pu) WF OUT 200 MW 350 MW 500 MW Case a) 2.0 [pu] (f ile CI_wf _350MW_3_2T_BLK_c.pl4; x-v ar t) v :CL345A v :CL345B v:cl345c factors: offsets: E E E-06 0 [s] 1.5 b) Figure 5-4 CALIENTE 345 kv Bus TOV for Different Faults and Wind Farm Dispatches b) Time simulations of the case with highest TOV at CALIENTE 345 kv (Wind farm 350 MW, fault 4) 2/19/08 EPE_TechSIS_Final.doc, 37

38 NEWMAN 345 kv Bus TOV (pu) WF OUT 200 MW 350 MW 500 MW Case a) 1.5 [pu] (f ile CI_wf _500MW_3_8T_BLK_c.pl4; x-v ar t) v :NE345A v :NE345B v :NE345C factors: offsets: E E E-06 0 [s] 1.5 b) Figure 5-5 a) NEWMAN 345 kv Bus TOV for Different Faults and Wind Farm Dispatches b) Time simulations of the case with highest TOV at NEWMAN 345 kv (Wind farm 500 MW, fault 6) 2/19/08 EPE_TechSIS_Final.doc, 38

39 5.3 Faults near Wind Farm Point of Interconnection The new 345 kv substation (power flow name - FEW345) will be developed to provide Point of Interconnection (POI) for the proposed wind farm. This substation is going to be located between Amrad 345 kv and Artesia 345 kv and will have links to three 345 kv transmission lines and one 345/138 kv transformer, connecting to the proposed wind farm power grid. System faults and outages of any of these transmission lines could have adverse impact on EPE system stability, subject to the wind farm power dispatch. The system condition during and immediately after a fault can be such that either Eddy County HVDC protection blocks converter or wind generator protection trips turbines. Dynamics of these events are illustrated in this section using several representative examples. Each of the simulated cases is accompanied with one plot page consists of the following quantities: Left side: Wind Farm POI 345kV L-G Voltage (pu) Wind Farm P, POI 345 kv (MW,MVAr) Wind Generator Terminal Voltages, Feeders: 1, 2, 3 and 4 (pu) Right side: Eddy 345kV L-G Voltage (pu) Eddy Inverter P, Q (MW,MVAr) Eddy DC Voltage (kv) Single Phase Fault near Wind Farm Point of Interconnection Simulation of five cycles single-phase-ground fault on Amrad - FEW345 kv line, at wind farm end, is simulated using the 500 MW power dispatch. Results are shown in Figure 5-6. This case recovers without protective action. 2/19/08 EPE_TechSIS_Final.doc, 39

40 Figure 5-6 Single-phase-ground fault on Amrad - FEW345 kv 345 kv line (wind farm POI side); Line protection trips line in 5 cycles 2/19/08 EPE_TechSIS_Final.doc, 40

41 5.3.2 Three Phase Fault near Wind Farm Point of Interconnection Wind Farm Power Output 500 MW A five cycle, three-phase fault on Amrad - FEW345 kv line, at the wind farm end, is simulated using the 500 MW power dispatch. Results are shown in Figure 5-7. Due to the high wind farm dispatch and severity of the fault recovery voltage, both the HVDC system and the wind farm generators trip, a loss of 700MW to EPE. One of possible remedies for this event could be to a remedial action system that would detect a multi-phase fault on one of the 345kV lines connected to the wind farm substation and use that signal to initiate a transfer trip of one or more wind farm feeders. The effect of such remedial action is shown for illustrative purposes in Figure 5-8, where tripping just one wind farm feeder is sufficient to prevent loss of the other 3 feeders in the windfarm Wind Farm Operation with Fewer Turbines The fault recovery voltage is driven in part by the total reactive current injected to the 345kV bus during the fault. The earlier phases of the project will have fewer turbines, and hence less tendency for high post-fault recovery voltage. Examples at the 350MW and 200MW stages are shown in figures 5-9 and At the 200MW stage all wind turbines ride through. At the 350MW stage, some turbines may trip off. 2/19/08 EPE_TechSIS_Final.doc, 41

42 Figure 5-7 Three-phase fault on Amrad - FEW345 kv 345 kv line (wind farm POI side); Line protection trips line in 5 cycles 2/19/08 EPE_TechSIS_Final.doc, 42

43 Figure 5-8 Three-phase fault on Amrad - FEW345 kv 345 kv line (wind farm POI side); Line protection trips line in 5 cycles; Transfer trip of Feeder 1 during the fault 2/19/08 EPE_TechSIS_Final.doc, 43

44 Figure 5-9 Three-phase fault on Amrad - FEW345 kv 345 kv line (wind farm POI side); Line protection trips line in 5 cycles; Wind farm power output 350 MW 2/19/08 EPE_TechSIS_Final.doc, 44

45 Figure 5-10 Three-phase fault on Amrad - FEW345 kv 345 kv line (wind farm POI side); Line protection trips line in 5 cycles; Wind farm power output 200 MW 2/19/08 EPE_TechSIS_Final.doc, 45

46 5.4 Conclusions from TOV Evaluation The worst overvoltages at and beyond Caliente to the east occur for a 3ph fault near Picante, clearing the Picante-Newman 345kV line and blocking the HVDC during the fault. For this worst-case event, there is only a small difference due to adding the windfarm. At Artesia, the TOV increases from 2.05pu to At Caliente, the TOV increases from 1.8 pu to 1.9. At Amrad there is not much difference. The actual overvoltages at these locations will be lower than indicated above due to surge arresters at the stations that were not modeled for this analysis. These results give an indication that energy duty in existing arresters may be increased, which should be evaluated in the facilities study to determine if higher-energy arresters are warranted. Similarly, the arresters applied at the windfarm will be subjected to higher than typical duty and this factor should be considered in the design of the new substation. At Newman, the impact is greatest for a 3ph fault on the Amrad-Caliente 345kV line and blocking the HVDC during the fault. In this case, the voltage is approximately 140% with 500MW from the windfarm. However, this overvoltage is of very short duration and is unlikely to pose risk to equipment. There is a risk of tripping the windfarm for events causing high overvoltage. This risk is low for single-phase faults or for the early stages of the windfarm development. In the final stage with 500MW of turbines, multi-phase faults close to the windfarm have a potential to create a post-fault voltage that would trip most or all of the wind turbines. This risk can be mitigated by remedial action systems that would deliberately trip part of the windfarm with the same timing as clearing the faulted line at the 345kV windfarm substation. It is also possible that using lower-impedance transformers in the windfarm or adding special functions to the wind turbine controls would mitigate the risk. 2/19/08 EPE_TechSIS_Final.doc, 46

47 6 Circuit Breaker Transient Recovery Voltage (TRV) This section explores relative impact of addition of the proposed wind farm on the existing EPE grid circuit breakers transient recovery voltage (TRV). TRVs appear across the contacts of a breaker, or other switchgear, on opening. The nature of the TRV is dependent on the circuit being interrupted, whether primarily resistive, capacitive or inductive, (or some combination). Additionally, distributed and lumped circuit elements will produce different TRV waveshapes. The breaking operation is successful if the circuit breaker is able to withstand the TRV. The analysis is valid only for evaluation of TRV envelopes and does not quantify initial TRV rate of rise. Ordinary TRV phenomena involve very fast transients, in the microsecond time span after interruption, that involve stray capacitance of all equipment connecting to the particular circuit breaker bus. Due to local nature, this high frequency phenomenon is not function of the addition of remote wind farm. However, there are some aspects of wind plant performance, which may produce slowrising TRV, particularly if a breaker, by opening, isolates a wind plant. 6.1 IEEE Standard for TRV Evaluation At its rated maximum voltage, each circuit breaker shall be capable of interrupting threephase grounded and ungrounded terminal faults at the rated short circuit current in any circuit in which the TRV does not exceed the rated TRV. For circuit breakers rated 100 kv and above, the TRV rating for a three-phase circuit breaker is determined by an envelope of required withstand capability. The rated TRV envelope waveshape is defined according to the IEEE standard by the higher of an exponential waveform and a 1-cosine waveform, as shown in Figure 6-1. The main parameters that define the TRV envelope are E1, T1, E2, T2, and R, where: E1 : the magnitude of the exponential component, T1 : the initial build-up of the TRV wave, E2 : the magnitude of the 1-cosine component, T2 : The rated times to peak of the 1-cosine component, R : the rate of rise of the exponential component. The time delay T1 is due to the capacitance of the circuit breaker, faulted bus, and any other connected equipment. 2/19/08 EPE_TechSIS_Final.doc 47

48 Figure 6-1: Rated TRV Envelope for Circuit Breakers Rated 100 kv and above. The rated TRV parameters are summarized in Table 6-1 according to IEEE standards C and C The rated maximum voltage for 115 kv and 345 kv breakers is 123 kv and 362 kv respectively. The value of T2 varies with circuit breaker rated voltage and it also depends on the breaker rated short circuit current, which is 40 ka for both breaker voltage levels under study. Table 6-1: Rated TRV Parameters for Breakers Rated 100 kv and above Breaker Standard Vmax Rated max voltage (kv) rms 115 kv IEEE Std IEEE Std kv IEEE Std IEEE Std E2 (kv) T2 (usec) R (kv/usec) E1 (kv) T1 (usec) Vmax Vmax Vmax Vmax Vmax Vmax Vmax Vmax 2 It is worth mentioning that IEEE Std. C is more conservative than IEEE Std. C for the steady state TRV in terms of lower value of E2, while it is a bit more relaxed for fast transients in terms of higher R. The circuit breaker can interrupt short-circuit currents that are less than the rated shortcircuit current. In this situation, the TRV parameters are modified according to Table 6-2. It can be inferred that as the interrupted current is decreased, E2 is increased and T2 is decreased. The rate of rise, R, first increases as the current decreases; then a point is reached where a further decrease in current causes a decrease in R, until at 30% of rated current and below, the exponential-cosine wave changes to a 1-cosine wave. Interpolation between the listed values is linear. 2/19/08 EPE_TechSIS_Final.doc 48

49 Table 6-2: Transient Recovery Voltage Capabilities of Circuit Breakers at Various Interrupting Fault Current levels Interrupting fault current rating (%) Multipliers for Rated Parameters E2 T2 R Circuit Breakers List for TRV Analysis A list of the rating information of the concerned 345 kv and 115 kv breakers for TRV analysis was provided by EPE. All system generation units and lines are in with HVDC providing 200 MW and SVC unit is in. Different wind farm sizes were considered for the study to examine the impact of different wind farm stages, namely 200, 350, and 500 MW, with respect to the case without the wind farm. 6.3 Fault Type, Duration, and Location To examine the system breakers TRV, a fault was initiated when the system is at steady state condition. The fault remained for four cycles before the concerned breakers open for fault clearing and isolating the faulted part of the system. The implemented fault was three phase ungrounded fault type since it would produce the worst circuit breaker TRV. The fault was applied at both sides of the tested circuit breaker whenever the fault current is affected by the wind farm, and to only one side of the breaker when the other side is not affected. EMTP model was developed for the TRV study with model reduction that has equivalent boundaries for system voltage below 115 kv. In some cases, this might not keep small loads at the medium voltage connected to the system. If a breaker at 115 kv is supplying only this load, it would be isolated. To analyze the TRV for such a breaker, as was the case for breaker 8766B in 115 kv Amrad Substation, a fault was applied to the 115 kv side of the associated transformer and the breaker opened for fault clearing. 6.4 Simulation Cases TRVs were evaluated according to both IEEE standards C and C The breaker TRV capability is determined according to the exponential cosine curve using the parameters listed in Table 6-1. Simulation cases were conducted for the concerned 345 kv and 115 kv breakers for different scenarios. As pointed out before, the main concern is the TRV result due to the wind farm impact and thus, the fast TRV 2/19/08 EPE_TechSIS_Final.doc 49

50 transients were ignored when determining if the TRV exceeds the breaker capability or not. The four scenarios considered in the study are listed in Table 6-3. Table 6-3: Considered Scenarios for TRV Analysis Scenario No. Description 1 Without wind farm MW wind farm MW wind farm MW wind farm Table 6-4 and Table 6-5 show the list of the breaker names and the side name where the fault was applied for 345 kv and 115 kv respectively. TRV plots and tables with results for different wind farm power dispatches and different faults are summarized in Appendix C. The TRV case names were assigned as follows: TRVXXXkV_SY_NZZ Where: XXX represents the system voltage level of the breakers in kv, i.e., 345 or 115 kv, Y represents the scenario number, ZZ represents the breaker case number. Table 6-4: 345 kv Breakers List Case No. Breaker Name Side Name 1 4R05 Eddy County 2 4R10 Reactor Eddy County B Reactor_Amrad B(side1) Line Amrad-Wind Farm B(side2) Line Amrad-Caliente B(side1) Line Amrad-Caliente B(side2) Amrad Substation B(side1) Line Amrad-Wind Farm B(side2) Amrad Substation B(side1) Line Amrad-Caliente B(side2) Breaker 0428B B(side1) Breaker 3928B B(side2) Auto Transformer B(side1) Breaker 0428B B(side2) Breaker 4178B B(side1) Breaker 4688B B(side2) Line Amrad-Caliente B Reactor_Caliente 2/19/08 EPE_TechSIS_Final.doc 50