DECISIONS on operating transfer limits and transmission

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1 1144 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 19, NO. 2, MAY 2004 Hydro Turbine-Governor Model Validation in Pacific Northwest Dmitry Kosterev Abstract This paper presents an approach for generator model validation based on the measurements taken at the point of interconnection. The approach is initially applied for The Dalles powerhouse, where the simulated and actual governor responses were found different. Governor tests and monitoring were performed and resulted in model revisions. The approach also resulted in improvements of John Day turbine-governor models. Grand Coulee governor response was validated and found adequate. Based on the project experience, a process for generator model validation is proposed. Index Terms Disturbance monitoring, generator testing, model validation, power system dynamic modeling. I. INTRODUCTION DECISIONS on operating transfer limits and transmission network reinforcements are based on system studies. The studies are relying on models to correctly predict system response to disturbances specified in the reliability criteria. Therefore, having realistic models is very important for safe, reliable, and economic system operation. Under the existing process in Western Electricity Coordinating Council (WECC), formerly WSCC Western Systems Coordinating Council, generator owners are responsible for determining models of their generating equipment and providing data directly to WECC. WECC compiles generator models in a system database used in the planning and operational studies. On August 10, 1996 WSCC system experienced major collapse, while dynamic models predicted system stability for the same disturbance scenario, Fig. 1 [1]. Following the outage, WSCC approved a policy requiring that all generators greater than 10 MW should be tested and their dynamic models be validated every five years. On August 4, 2000, sustained oscillations were observed in the system for about one minute after losing a tie line between Alberta and British Columbia. Again, the model predicted a well-damped dynamic response [2], Fig. 2. Studies of underfrequency events showed that the governor response is over-represented by more than 50% system wide. Similar discrepancies between recorded and simulated governor responses were observed in the Eastern Interconnection [3]. It is evident that existing process of model development is not working. This is because control of data quality is very limited under the existing open loop approach. Basically, it is a good faith effort by equipment owners. There is an obvious Manuscript received April 23, The author is with the Transmission Operations and Planning, Bonneville Power Administration, Vancouver, WA USA. Digital Object Identifier /TPWRS Fig. 1. Comparison of simulated and actual California-Oregon Intertie (COI) power responses during August 10, 1996 WSCC system disturbance. Fig. 2. Comparison of simulated and actual COI power responses during August 4, 2000 WSCC system disturbance. need to close the loop by developing a mechanism for validating and, if necessary, correcting dynamic models. In 1999, Bonneville Power Administration (BPA) initiated an effort on developing techniques for validating generator models based on the measurements taken at the point of interconnection. The project resulted in significant governor model improvements for The Dalles, John Day, Bonneville and most recently model validation of Grand Coulee hydro power plants. II. BPA WIDE-AREA MEASUREMENT SYSTEM Bonneville Power Administration Transmission (BPAT) recognizes that generators have significant impact on system /04$ IEEE

2 KOSTEREV: HYDRO TURBINE-GOVERNOR MODEL VALIDATION IN PACIFIC NORTHWEST 1145 Fig. 3. Model validation process. Fig. 4. One-line diagram of The Dalles powerhouse. dynamic performance. BPAT installed several phasor measurement units (PMUs) to monitor dynamic performance of large generating stations connected to its network. Present PMU installations include The Dalles (1776-MW capacity), John Day (2576-MW capacity), Grand Coulee 3rd Powerhouse (4598-MW capacity) and Colstrip 3 and 4 (1526-MW capacity) generating stations. PMUs are installed at the point of interconnection, and measure bus frequency, voltage phasors and current phasors in each powerhouse line. Real and reactive power can be derived from the phasors. The PMU data was used for generator dynamic model validation. III. MODEL VALIDATION SETUP An infinite-bus model was developed and implemented in General Electric PSLF program. The model injects recorded bus voltage and frequency in dynamic simulations. The infinite bus is modeled with a large synchronous machine with fast-responding exciters and governors. Recorded bus voltage and frequency are input as references for the generator voltage regulator and governor respectively. Because of the large size of the infinite-bus machine and fast response controls, simulated bus voltage and frequency are forced to follow very closely the recorded voltage and frequency (Fig. 3). This setup was used for validating dynamic models of generators and their controls as seen from the point of interconnection. For example, if the turbine-governor model is correct, the simulated power should be very similar to the recorded power for events of system frequency excursions. IV. DALLES A. System Setup The Dalles Dam is located on lower Columbia River about 80 miles upstream from Portland, Oregon. Fig. 4 shows a one-line diagram of The Dalles powerhouse. Units #5 14 are 94 MVA Fig. 5. Comparison of simulated (old model) and recorded responses of The Dalles generators to system frequency excursion caused by the outage of Four Corners generators on December 25, units, and units #15 22 are 104-MVA units. There are four powerhouse lines connecting generators with Big Eddy 230-kV substation (point of interconnection). The Bonneville Power Administration installed a PMU at Big Eddy 230-kV bus. The Dalles powerhouse was modeled in General Electric PSLF program as shown in Fig. 4. The model represents all four powerhouse lines, stepup transformers and all eighteen generators. The infinite bus machine is attached to the Big Eddy 230-kV bus to represent the equivalent of the WSCC system. B. Model Validation BPA PMU has been recording powerhouse response to large generation outages in WSCC system. A large library of disturbance recordings allows elimination of single event abnormalities. Fig. 5 shows comparison between recorded and simulated responses for the 1975 MW Four Corners plant outage on December 25, IEEE mechanical-hydraulic governor model and nonlinear turbine model were used to represent the turbine-governor [3], Figures 2 and 13 in [4]. Since the recorded frequency was used to drive the model response, the simulated and recorded frequencies are nearly identical. However, there is an observable difference between the simulated and recorded power pick-up by the generators connected to the powerhouse line #6. The simulated power pickup is much faster in the first 15 seconds following the generation loss. (The power pick-up was measured at Big Eddy 230-kV bus). Most significantly, the actual governors have no response at the time of minimum frequency dip, while the simulated governors complete 50% of their response by the time of minimum frequency dip. Fig. 6 shows a comparison for Hoover trip during WSCC governor response test on May 18, The powerhouse line #6 had three units generating, two were lightly loaded and one was normally loaded. There is a significant difference in amount and speed of the recorded and simulated governor responses. Similar modeling errors were observed for other Dalles and John Day generators.

3 1146 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 19, NO. 2, MAY 2004 Fig. 6. Comparison of simulated (old data) and recorded responses of The Dalles generators to system frequency excursion caused by Hoover trip on May 18, Fig. 7. Comparison of simulated (new data) and recorded responses of The Dalles generators to system frequency excursion caused by the outage of Four Corners generators on December 25, C. Working With Generator Operators on Correcting Turbine-Governor Representation Bonneville Power Administration and US Army Corps of Engineers worked together on resolving modeling differences. The Dalles Dam (and many other low-head high-flow plants on Columbia River) have Kaplan turbines, in which power is controlled by coordinated adjustment of wicket gates and turbine blades. Approximately MW of hydro-generating capacity in Northwest have Kaplan turbines. Existing turbine models [4], [5] do not recognize blade angle response. Blade angle control is shown to have significant impact on the turbine power response by affecting water discharge through the turbine and turbine efficiency. A Kaplan turbine model was developed and applied to The Dalles project. Model details are given in the Appendix. Turbine-governor tests were conducted including online speed reference steps and partial load rejection tests. Monitoring equipment was left at the plant to record governor responses to under-frequency events. Based on the test and monitoring data, the model parameters were determined. Then, the overall response was validated using disturbance recordings at the point of interconnection, as described above. Simulations and actual recordings are compared in: Fig. 7 for 1975-MW Four Corners plant outage (response using previous model is shown in Fig. 5); Fig. 8 for 750-MW Hoover trip during governor response test (response using previous model is shown in Fig. 6); Fig. 9 for 2400-MW RAS trip on July 15, 2002; Fig. 10 for 2590-MW RAS trip on July 16, Governor droop and response time were measured on units 17, 19, 20 in the same powerhouse line. We found droop to be different among units, ranging from 5.5% to 6.5%. Response time was very similar, except unit #17 which was observably faster. No attempt was made to come up with exact numbers. The new model captures well The Dalles governor responses to system frequency excursions under various operating condi- Fig. 8. Comparison of simulated (new data) and recorded responses of The Dalles generators to system frequency excursion caused by the tripping Hoover generators during WSCC test on May 18, tions. A particularly good match is recorded for July 16 event when only tested unit #22 was generating. Similar work has been performed for John Day and Bonneville governors. V. GRAND COULEE Grand Coulee Dam is located in eastern Washington on upper Columbia River, and operated by the U.S. Bureau of Reclamations (USBR). The third powerhouse consists of three 615-MVA (units #19 21) and three 718-MVA (units #22 24) generators. The generators were tested by the USBR engineers as required by WECC. The governors are double-derivative, and the turbine type is Francis. The model validation was set up in PSLF program similarly to that at The Dalles. Grand Coulee is one of major load-following

4 KOSTEREV: HYDRO TURBINE-GOVERNOR MODEL VALIDATION IN PACIFIC NORTHWEST 1147 Fig. 9. Comparison of simulated (new data) and recorded responses of The Dalles generators to system frequency excursion caused by the AC RAS event on July 15, Fig. 11. Comparison of simulated and actual responses of Grand Coulee generator #20 to system frequency excursion caused by the AC RAS event on July 15, Fig. 10. Comparison of simulated (new data) and recorded responses of The Dalles generators to system frequency excursion caused by the AC RAS event on July 16, 2002 (only tested unit #22 was generating in powerhouse line #6). projects in BPA Control Area and is normally operated on Area Generating Control (AGC). To exclude AGC effects, we simulated only generation trips caused by Northwest Remedial Action Scheme (RAS), when BPA AGC is suspended. Figs. 11 and 12 show Grand Coulee #20 responses for RAS generation trips on July 15 and July 16, It is evident that Grand Coulee #20 governor response is modeled correctly. VI. CONCLUSION An approach is developed for generator model validation using measurements taken at the point interconnection. The approach is demonstrated for The Dalles, John Day, and Grand Coulee power plants. Lately, it has been used for thermal governor response validation in WECC. Based on the project experience, the following process for dynamic model validation is proposed. Fig. 12. Comparison of simulated and actual responses of Grand Coulee generator #20 to system frequency excursion caused by the AC RAS event on July 16, ) A generating company will provide a dynamic model of its generating plant. The model will characterize plant responses to system disturbances (voltage and frequency deviations at point of interconnection, oscillations) and control signals (power and voltage schedule). The model will be a part of the power system model used in system studies to determine operating transfer limits and network reinforcements. An incorrect model may result in incorrect transfer limits, which can either put system at risk of failure or unnecessarily restrict transmission use. 2) The model data are submitted to a corresponding control area. The control area will review generator model data, test, and validation report. 3) The control area will monitor performance of a generating plant at the point of interconnection. A control area

5 1148 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 19, NO. 2, MAY 2004 Fig. A1. Block-diagram of The Dalles turbine controls. Fig. A3. Power versus gate steady-state characteristic. Fig. A2. Block-diagram of Kaplan turbine model. will collect bus voltage and frequency, powerhouse line currents, and control signals sent to the generating plant. The data-acquisition rate will be fast enough to observe dynamic response of generator controls voltage regulators, power system stabilizers, and speed governors. 4) A control area will compare recorded power plant responses with those predicted by the model. If a severe discrepancy is observed, the transmission utility will require the generation equipment in question be tested, and that temporary monitoring equipment be installed at the plant site. Control area provides operational data to WECC, including which generators are expected to operate baseloaded and which generators are on AGC. This process will provide better control of generator model quality, and should be beneficial to the power industry. APPENDIX KAPLAN TURBINE MODEL Fig. A1 shows a block-diagram of The Dalles turbine controls. The turbine type is Kaplan with adjustable propeller blades. The speed governor and blade angle controller are mechanicalhydraulic. A turbine model was developed to represent effect of blade angle on turbine power output, Fig. A2. Solid lines represent existing IEEE turbine model [3], [4], and dashed lines represent model additions to represent a Kaplan turbine. The intent of the model is to capture impact of blade angle adjustment on turbine power output. Blade angle affects water discharge through the turbine and turbine efficiency in coordination with gate opening. Ultimately, both affect turbine power output [4]. Turbine model inputs: gate servomotor stroke (per unit of full gate opening); blade servomotor stroke (per unit of full blade stroke). Turbine model output is turbine power (per unit of turbine megawatt capacity). The turbine megawatt capacity is defined at full gate opening at a specified head. Turbine model data for The Dalles #22: water inertia time constant (s); h0 steady-state head; no-load turbine flow; a gain representing blade angle impact on the turbine water flow (per unit flow/per unit of blade); gate time constant (s); blade time constant (s); steady-state Gate Power Blade characteristic, Figs. A3 and A4. Figs. A3 and A4 show power versus gate opening and blade versus gate opening steady-state characteristics. At no-load, wicket gates are opened about 18% and blades are flat. The blades remain flat until the gate opens about 55 60%. At 45% to 55% gate opening, turbine power output is saturating as a function of gate. When the gate opens above 60%, the blade angle starts adjusting, increasing turbine power output. In the normal operating range (60% 90% of gate opening), the blade angle adjustment allows nearly linear power-gate regulation. The blade angle can be adjusted up to about 15 degrees from flat to steep angle. Values for the flat angle are 18 degrees and for the steep angle are 33 degrees for The Dalles. Blade angle remain steep with the gate opening above 90%, and the turbine power output saturates as a function of gate. The blade angle controller model is developed based on inspection of actual controls and manufacturer s manual. It is similar to a model of a mechanical governor. The blade controller input is gate servomotor stroke (per unit of full gate opening). The blade controller output is blade servomotor stroke (per unit of full stroke) [Fig. A5].

6 KOSTEREV: HYDRO TURBINE-GOVERNOR MODEL VALIDATION IN PACIFIC NORTHWEST 1149 Blade versus gate cam characteristic, as shown in Fig. A4. The speed governor is adequately represented with existing model for mechanical-hydraulic governors, Fig. 13 in [5]. ACKNOWLEDGMENT The author is very thankful to B. Mittelstadt and G. Keenan at Bonneville Power Administration and L. Pereira at Northern California Power Agency for productive discussions and suggestions of the proposed disturbance monitoring policy. The author is very thankful to S. Wiese and K. Martin for their effort in installing PMUs at The Dalles, John Day, and Grand Coulee generating stations. Fig. A4. Blade versus gate steady-state characteristic (blade in on cam to get optimal efficiency). REFERENCES [1] D. N. Kosterev, C. W. Taylor, and W. A. Mittelstadt, Model validation for the August 10, 1996 WSCC system outage, IEEE Trans. Power Syst., vol. 14, pp , Aug [2] J. F. Hauer, M. J. Beshir, and W. A. Mittelstadt, Dynamic performance validation in the western power system, in APEx 2000 in Kananaskis, Alberta, Oct [3] R. P. Schultz, Modeling of governor response in the eastern interconnection, in Proc. IEEE Winter Power Meeting, Jan. 1999, pp [4] J. M. Undrill and J. L. Woodward, Non-linear hydro governing model and improved calculation for determining temporary droop, IEEE Trans. Power App. Syst., vol. PAS-86, pp , Apr [5] Hydraulic turbine and turbine control models for system dynamic studies, IEEE Trans. Power Syst., vol. 7, pp , Feb Fig. A5. Blade controller block diagram. The blade controller parameters: Tc blade cam time constant; Tv blade pilot valve servomotor time constant; Kb blade servomotor gain. Unintentional dead-band is used to represent overlap in distributing valve plunger Dmitry Kosterev received the Ph.D. degree in electrical engineering from Oregon State University, Corvallis, in Currently, he is a Network Planning Engineer at the Bonneville Power Administration, Vancouver, WA. He has been involved in many planning, operating, and generation integration studies. He was responsible for model validation studies for August 10, 1996 and August 4, 2000 WSCC system disturbances. He was actively involved in design, testing, and commissioning of Line Drop Compensation at John Day Powerhouse on the Columbia River. Recently, he has been leading a project on hydro turbine-governor model validation in the Pacific Northwest. He is involved in developing BPA generator interconnection standards. Dr. Kosterev is a member of WECC Model and Validation Work Group and is Chairman of the WECC Load Modeling Task Force.