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




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.
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.
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.
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.
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.
The Load Flow Problem

There are two methods of solving the
load flow problem.


A)
B)
The Gauss Seidel Method
The Newton Raphson Method
The Gauss-Seidel Method

This method solves, by an iterative process,
the following equation that represents a power
system having N buses
1
Vk  i  1 
Ykk
N
 Pk  jQk k 1

  YknVn  i  1   YknVn  i  
 *
n 1
n  k 1
 Vk  i 

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
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.
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
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
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
Demerara Interconnected System Data

A 69 kV transmission system connects the
Garden of Eden stations and the Sophia
frequency converter station
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
N
Yk  Pk  Pk ( x)  Vk  YknVn cos(d k  d n   kn )
n 1
N
Yk  N  Qk  Qk ( x)  Vk  YknVn sin(d k  d n   kn )
n 1
k  2,3,......N
Single Line Diagram of the
Demerara Interconnected System
KINGSTON
B
3X12.5MVA
SOPHIA
11KV
11 KV
6.25 MVA
11 KV
11 KV/4KV
SOPHIA
13.8KV
11 KV
12.5MVA
12.5MVA
16.7 MVA
SOPHIA
69KV
11(50Hz)/
13.8(60Hz)
KV
16.7 MVA 16.7 MVA
13.8/69KV
69 KV
2X6.87MVA
2X6.87 MVA
DEMERARA
POWER
KINGSTON
69 KV
16.7 MVA
16.7 MVA 16.7 MVA
69/13.8KV
2X6.87 MVA
DEMERARA
POWER
GOE
13.8 KV
13.8 KV
2X6.87 MVA
GOE
GUYANA POWER & LIGHT
DEMERARA INTERCONNECTED
SYSTEM AS OF 2006 DEMERARA INTER-CONNECTED
SYSTEM SINGLE LINE DIAGRAM
DATE ORIGINAL
SCALE
LATEST REVISION
JOB NO.
REVISIONS
2X6.6257 MVA
NO.
DATE
DESCRIPTION
VERSAILES
2X2.5 MVA
NOT TO SCALE
CHECKED
DRAWN
Methodology of Study
Data Collection
Data Analysis
Load Flow Study
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.
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
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.
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.
Demerara 50 AND 60 HZ System Loads (Weekday)
60.0
50.0
Total System
40.0
LO AD
(MW)
60 Hz System
30.0
50 Hz System
20.0
10.0
TIME (HRS)
24:00
23:00
22:00
21:00
20:00
19:00
18:00
17:00
16:00
15:00
14:00
13:00
12:00
11:00
10:00
9:00
8:00
7:00
6:00
5:00
4:00
3:00
2:00
1:00
0.0
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.
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.
Sophia Panel Meter and Power Analyser Voltage Readings
14.4
14.2
14.0
13.6
Panel Meter
Analyser Readings
13.4
13.2
13.0
Tim e (Hrs)
24:00
23:00
22:00
21:00
20:00
19:00
18:00
17:00
16:00
15:00
14:00
13:00
12:00
11:00
10:00
9:00
8:00
7:00
6:00
5:00
4:00
3:00
2:00
12.8
1:00
Voltage (kV)
13.8
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.
Tim e (hrs)
24:00
23:00
22:00
21:00
20:00
19:00
18:00
17:00
16:00
15:00
14:00
13:00
12:00
11:00
10:00
9:00
8:00
7:00
6:00
5:00
4:00
3:00
2:00
1:00
Generation (MW)
Demerara System Generation by Power Station
70.0
60.0
50.0
40.0
VERSAILLES
GPLGOE
DPLK'STON
30.0
DPLGOE
20.0
10.0
0.0
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.
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
Load Flow Studies

Transformer taps were set as per
system

1.0 for Garden of Eden transformers

0.955 for Sophia transformers.
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
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.
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.
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
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.
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.
COMPARISON BETWEEN GPL AND LOAD FLOW GENERATION (MW)
55.0
GPL Generation
50.0
45.0
Generation
40.0
(MW)
35.0
Load Flow Generation
System Load
30.0
TIME (HRS)
24:00
23:00
22:00
21:00
20:00
19:00
18:00
17:00
16:00
15:00
14:00
13:00
12:00
11:00
10:00
9:00
8:00
7:00
6:00
5:00
4:00
3:00
2:00
1:00
25.0
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.
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
Analysis of Results

MVar generation at GPLGOE and
Versailles were quite similar for both
GPL operations and the LFR
Comparison Between GPL and Load Flow Generation (MVAR)
35.0
GPL GENERATION
30.0
25.0
MVAR
20.0
SYSTEM LOAD
LOAD FLOW GENERATION
15.0
TIME (HRS)
24:00
23:00
22:00
21:00
20:00
19:00
18:00
17:00
16:00
15:00
14:00
13:00
12:00
11:00
10:00
9:00
8:00
7:00
6:00
5:00
4:00
3:00
2:00
1:00
10.0
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.
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.
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
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.
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.
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.
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