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LOAD FLOW STUDIES ON THE GUYANA POWER & LIGHT’S (GPL) DEMERARA SYSTEM

LOAD FLOW STUDIES ON THE GUYANA POWER & LIGHT’S (GPL) DEMERARA SYSTEM. Verlyn Klass Senior Lecturer & Head Department of Electrical Engineering University of Guyana October 2006. What is a Load Flow Study. A load flow study is done on a power system to ensure that

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LOAD FLOW STUDIES ON THE GUYANA POWER & LIGHT’S (GPL) DEMERARA SYSTEM

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  1. LOAD FLOW STUDIES ON THEGUYANA POWER & LIGHT’S (GPL) DEMERARA SYSTEM Verlyn Klass Senior Lecturer & Head Department of Electrical Engineering University of Guyana October 2006

  2. What is a Load Flow Study • A load flow study is done on a power system to ensure that • Generation supplies the demand (load) plus losses. • Bus voltage magnitudes remain close to rated values • Generation operates within specified real and reactive power limits • Transmission lines and transformers are not overloaded.

  3. A Load Flow Study Specifically Investigates the Following • Busbar voltages • Effect of rearranging circuits and incorporating new circuits on system loading. • Effect of injecting in-phase and quadrature boost voltages on system loading. • Optimum system running conditions and load distribution. • Optimum system losses. • Optimum rating and tap range of transformers.

  4. The Load Flow Problem • The starting point of a load flow problem is a single line diagram of the power system, from which input data for computer solutions can be obtained. Input data consist of bus data, transmission line data and transformer data.

  5. The Load Flow Problem • The following four variables are associated with each bus k - voltage magnitude Vk, phase angle dk, net real power Pk and reactive power Qk supplied to the bus.

  6. The Load Flow Problem • Each bus k is categorized into one of the following bus types: • Swing bus - There is only one swing bus which for convenience is normally numbered as bus 1, and is a reference bus for which V1 and d1 are 1 and 0o respectively • Load Bus or PQ bus- Most buses in a typical load flow program are load buses. Pk and Qk are specified and the program computes Vk and dk. • Voltage Controlled bus or PV bus - These are generally generator buses where Pk and Vk are specified and Qk and dk are computed.

  7. The Load Flow Problem • There are two methods of solving the load flow problem. • A) The Gauss Seidel Method • B) The Newton Raphson Method

  8. The Gauss-Seidel Method • This method solves, by an iterative process, the following equation that represents a power system having N buses

  9. GPL’S POWER SYSTEM • GPL’S power system, with an installed capacity of 105 MW, consists of the following: • Demerara Interconnected System • Berbice Interconnected System • Anna Regina System • Bartica System • Wakenaam System • Leguan System

  10. THE OBJECTIVES OF THIS STUDY WERE AS FOLLOWS: • To model the (GPL’S) Demerara system for load flow studies. • To perform load flow studies on GPL’s present Demerara 60 Hz system. • To use a static model of the frequency converters and perform studies on the Demerara 50 and 60 Hz system. • To perform load flow studies on GPL’s future Demerara power system (all load converted to 60 Hz.). • To analyse the results of the load flow studies.

  11. Demerara Interconnected System Data • Installed Capacity – 76 MW • Peak Load - 67 MW • Three power stations, two at Garden of Eden and one at Versailles, generating at 13.8 kV, 60Hz • Two power stations at Kingston generating at 11 kV, 50 Hz

  12. Demerara Interconnected System Data • Demerara Power, an Independent Power Producer, owns and operates two power stations at Garden of Eden and Kingston • All other power stations are owned by GPL

  13. Demerara Interconnected System Data • The 25 MVA rotary frequency converter station at Sophia has machines rated at 13.8 kV, 60 Hz and 11 kV, 50 Hz which operate as motors or generators depending on the flow of power

  14. Demerara Interconnected System Data • A 69 kV transmission system connects the Garden of Eden stations and the Sophia frequency converter station

  15. The Newton-Raphson Method • The Newton-Raphson method solves the nonlinear equation y = f(x) where the x, y and f vectors for the power flow problem are defined as

  16. Single Line Diagram of the Demerara Interconnected System

  17. Methodology of Study Data Collection Data Analysis Load Flow Study

  18. Data Collection • The following data was collected: • Single line diagram of the GPL Demerara system. • Reactances of all generators at the Demerara Power stations, and GPL’s Garden of Eden and Versailles power stations and the Sophia frequency converters. • Impedances of all transmission and distribution lines and transformers.

  19. Data Collection Hourly operations data for the system for weekdays (2) and Saturday and Sunday • Data from recent power analyser recordings giving feeders power factor and voltages

  20. Analysis of Data • The loads (MW and MVar) for the various busbars were calculated using hourly feeder current and voltages from the log sheets and the corresponding hourly power factor data recorded on a power demand analyser.

  21. Analysis of Data • Sophia was found to be the major load centre for the Demerara system with an evening peak of nearly 30 MW • The peak 60 Hz load is about 45 MW and is primarily residential • The 50 Hz load is mainly industrial /commercial and has a day peak of around 20 MW.

  22. Analysis of Data • 50 Hz system hourly power factors range from 0.79 to 0.89 and the 60 Hz system power factors are from 0.81 to 0.85. • The frequency converters produce between 20 to 30 % of the MVar requirement of the Demerara system.

  23. Analysis of Data • Comparison of power analyzer data and station logs revealed that the Sophia panel meters were overstating the Sophia 13.8 kV voltages.

  24. Analysis of Data • The Demerara Power generators are used as the base load generators for the system, with GPL’s Garden of Eden and Versailles stations being used to maintain bus voltages levels and for peaking purposes.

  25. Load Flow Studies • The 60 Hz machine of the frequency converters were modeled as generators and when they operated as motors the generators were deemed to be supplying negative power.

  26. Load Flow Studies • The various busbars were designated as follows: • Garden of Eden 13.8 kV busbar - Slack Bus • Demerara Power GOE busbars - PV Bus • Sophia Station 13.8 kV busbar - PV bus • Versailles Power Station busbar - PV bus

  27. Load Flow Studies • Transformer taps were set as per system • 1.0 for Garden of Eden transformers • 0.955 for Sophia transformers.

  28. Load Flow Studies • Hourly load flow runs were carried out for three of the days from which hourly data had been collected, that is, two weekdays and Saturday. • Transformer taps were changed to determine the best tap position

  29. Load Flow Studies • The frequency converters were represented as an 13.8/11 kV autotransformer in combination with a capacitor. Load flow runs were carried out on the combined Demerara 50 and 60 Hz systems for system peak load.

  30. Load Flow Studies • The frequency converters were removed from the system and the 60Hz system was extended to cater for the present 50 Hz load. • Load flow runs were carried out for day and night peaks.

  31. Analysis of Results • The following abbreviations apply: • DPGOE - Demerara Power station at Garden of Eden • GPLGOE - GPL’s Garden of Eden Station • LFR - The hourly load flow run

  32. Analysis of Results • As GPLGOE was the slack busbar comparison was made between its generation during GPL operations and the load flow runs. For the GPL operations GPLGOE generation was higher than that of the LFR by 70% during the off peak periods and up to 120% during evening peaks.

  33. Analysis of Results • The LFR showed an average of 2% system losses • GPL operations showed losses as much as 18% and averaged 11% over the period of analysis.

  34. Analysis of Results • The LFR consistently generated around 3 MVars at DPGOE. GPL operations show MVAR generation between 8 and 13 MVars at this location.

  35. Analysis of Results • The LFR generation of MVars at Sophia was consistently higher than that of GPL operations. This was as high as 15 MVar at peak load whereas GPL operations generate just around 10 MVars at the same period

  36. Analysis of Results • MVar generation at GPLGOE and Versailles were quite similar for both GPL operations and the LFR

  37. Analysis of Results • Changing of transformer taps • The present tap positions of the GPL transformers proved to be the optimum positions to maintain bus voltages and minimise losses.

  38. Analysis of Results • Static Representation of the Frequency Converters • The results achieved from the load flow run for system peak suggest that this model could be acceptable if separate representation is made for the mechanical losses of the frequency converters.

  39. Analysis of Results • Total Demerara Load at 60 Hz • The frequency converters would not be required so capacitors would be needed at all locations to provide the MVar injection presently done by the frequency converters • Switched capacitors would have to be used as different values would be required for the day and night peaks

  40. Conclusions • The usefulness of load flow studies in the investigation of the following were demonstrated • Optimum system running conditions and load distribution. • Optimum system losses. • Optimum tap range of transformers. • Effect of incorporating new circuits on system loading.

  41. Conclusions • The data collection and analysis highlighted problems with GPL’s system operations which were confirmed by the load flow study. • The difference in GPL’s calculated loads and generation show a high level of losses in GPL’s generation and transmission system which require further investigation. • The Sophia 13.8 kV bus voltages are lower than the other bus voltages and need to be increased for proper system operation.

  42. Conclusions • The static representation of the frequency converters by a transformer and a variable capacitor is an adequate model for load flow studies. Converter mechanical losses can be added subsequently to the total system losses.

  43. Conclusions • The availability of load flow studies would be helpful to small utilities as they seek to integrate their power system with different types of generation

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