LOAD FLOW STUDIES ON THE GUYANA POWER

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LOAD FLOW STUDIES ON THEGUYANA POWER LIGHTS
(GPL) DEMERARA SYSTEM

• Verlyn Klass
• Senior Lecturer Head
• Department of Electrical Engineering
• University of Guyana
• October 2006

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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.

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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.

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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.

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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.

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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.

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The Load Flow Problem

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

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The Gauss-Seidel Method

• This method solves, by an iterative process, the
  following equation that represents a power system
  having N buses

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GPLS POWER SYSTEM

• GPLS 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

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THE OBJECTIVES OF THIS STUDY WERE AS FOLLOWS

• To model the (GPLS) Demerara system for load
  flow studies.
• To perform load flow studies on GPLs 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 GPLs future
  Demerara power system (all load converted to 60
  Hz.).
• To analyse the results of the load flow studies.

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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

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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

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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

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Demerara Interconnected System Data

• A 69 kV transmission system connects the Garden
  of Eden stations and the Sophia frequency
  converter station

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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

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Single Line Diagram of the Demerara
Interconnected System
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Methodology of Study

• Data Collection
• Data Analysis
• Load Flow Study

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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 GPLs Garden of Eden and
  Versailles power stations and the Sophia
  frequency converters.
• Impedances of all transmission and distribution
  lines and transformers.

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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

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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.

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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.

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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.

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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.

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Analysis of Data

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

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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.

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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

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Load Flow Studies

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

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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

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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.

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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.

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Analysis of Results

• The following abbreviations apply
• DPGOE - Demerara Power station at Garden of
  Eden
• GPLGOE - GPLs Garden of Eden Station
• LFR - The hourly load flow run

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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.

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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.

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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.

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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

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Analysis of Results

• MVar generation at GPLGOE and Versailles were
  quite similar for both GPL operations and the LFR
  

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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.

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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.

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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

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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.

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Conclusions

• The data collection and analysis highlighted
  problems with GPLs system operations which were
  confirmed by the load flow study.
• The difference in GPLs calculated loads and
  generation show a high level of losses in GPLs
  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.

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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.

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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