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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) pluslosses.

Bus voltage magnitudes remain close to ratedvalues

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 oftransformers.


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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 datafor computer solutions can be obtained.Input data consist of bus data,

transmission line data and transformerdata.


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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 forconvenience is normally numbered as bus 1, and is a referencebus for which V1and d1are 1 and 0orespectively

Load Busor PQ bus- Most buses in a typical load flowprogram are load buses. Pkand Qkare specified and theprogram computes Vkand dk.

Voltage Controlled bus or PV bus- These aregenerally generator buses where Pkand Vkare specified andQkand dkare 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 powersystem having N buses

1

*

1 1

11 1

k

k Nk k

k kn n kn n

n n kkk

P jQV i Y V i Y V i

Y V i


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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 loadflow studies.

To perform load flow studies on GPLs presentDemerara 60 Hz system.

To use a static model of the frequency convertersand 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 60Hz.).

To analyse the results of the load flow studies.


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Demerara Interconnected System Data Installed Capacity76 MW

Peak Load - 67 MW

Three power stations, two at Garden ofEden and one at Versailles, generating at13.8 kV, 60Hz

Two power stations at Kingston generatingat 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 converterstation at Sophia has machines rated at

13.8 kV, 60 Hz and 11 kV, 50 Hz whichoperate as motors or generatorsdepending 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 Sophiafrequency converter station


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The Newton-Raphson Method The Newton-Raphson method solves the

nonlinear equationy= f(x) where thex,y

and fvectors for the power flow problem aredefined as

1

1

( ) cos( )

( ) sin( )

2,3,......

N

k k k k kn n k n kn

n

N

k N k k k kn n k n kn

n

Y P P x V Y V

Y Q Q x V Y V

k N

d d

d d


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Single Line Diagram of theDemerara Interconnected System

KINGSTON

B

11 KV

SOPHIA

69KV

SOPHIA

13.8KV

SOPHIA

11KV

DEMERARAPOWER

GOE

DEMERARA

POWER

KINGSTON

VERSAILES

GOE

11 KV

11 KV

69 KV

13.8 KV

13.8 KV

69 KV

11 KV/4KV

69/13.8KV

13.8/69KV

11(50Hz)/

13.8(60Hz)

KV

DATEORIGINAL

LATESTREVISION

SCALE

JOBNO.

REVISIONS

N O. D A TE D ES CR IP TI ON

DRAWNCHECKED

GUYANA POWER& LIGHT

DEMERARA INTERCONNECTED

SYSTEM AS OF 2006 -

DEMERARA INTER-CONNECTED

SYSTEM SINGLE LINE DIAGRAM

11 KV/4KV

3X12.5MVA

2X6.87MVA 2X6.87 MVA

2X6.87 MVA

2X6.87 MVA

2X6.6257 MVA

2X2.5 MVA

12.5MVA

16.7 MVA

12.5MVA

16.7 MVA16.7 MVA

16.7 MVA 16.7 MVA 16.7 MVA

6.25 MVA

NOTTOSCALE


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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 DemeraraPower stations, and GPLs Garden of Eden andVersailles power stations and the Sophiafrequency converters.

Impedances of all transmission and distributionlines 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 andvoltages


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Analysis of Data

The loads (MW and MVar) for the variousbusbars were calculated using hourly feedercurrent and voltages from the log sheets

and the corresponding hourly power factordata recorded on a power demand analyser.


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Analysis of Data

Sophia was found to be the major loadcentre 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 ofaround 20 MW.


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Demerara 50 AND 60 HZ System Loads (Weekday)

0.0

10.0

20.0

30.0

40.0

50.0

60.0

1:00

2:00

3:00

4:00

5:00

6:00

7:00

8:00

9:00

10:00

11:00

12:00

13:00

14:00

15:00

16:00

17:00

18:00

19:00

20:00

21:00

22:00

23:00

24:00

TIME (HRS)

LOAD

(MW)

50 Hz System

60 Hz System

Total System


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Analysis of Data

50 Hz system hourly power factorsrange from 0.79 to 0.89 and the 60 Hz

system power factors are from 0.81 to0.85.

The frequency converters produce

between 20 to 30 % of the MVarrequirement of the Demerara system.


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Analysis of Data

Comparison of power analyzer data andstation logs revealed that the Sophia

panel meters were overstating theSophia 13.8 kV voltages.


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Sophia Panel Meter and Power Analyser Voltage Readings

12.8

13.0

13.2

13.4

13.6

13.8

14.0

14.2

14.4

1:00

2:00

3:00

4:00

5:00

6:00

7:00

8:00

9:00

10:00

11:00

12:00

13:00

14:00

15:00

16:00

17:00

18:00

19:00

20:00

21:00

22:00

23:00

24:00

Time (Hrs)

Voltage(kV)

Panel Meter

Analyser Readings


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Analysis of Data

The Demerara Power generators areused as the base load generators for

the system, with GPLs Garden of Edenand Versailles stations being used tomaintain bus voltages levels and for

peaking purposes.


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Demerara System Generation by Power Station

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

1:00

2:00

3:00

4:00

5:00

6:00

7:00

8:00

9:00

10:00

11:00

12:00

13:00

14:00

15:00

16:00

17:00

18:00

19:00

20:00

21:00

22:00

23:00

24:00

Time (hrs)

Generation(MW)

VERSAILLES

GPLGOE

DPLK'STON

DPLGOE


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Load Flow Studies

The 60 Hz machine of the frequencyconverters were modeled as generators

and when they operated as motors thegenerators were deemed to besupplying 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

Hourly load flow runs were carried outfor three of the days from which hourly

data had been collected, that is, twoweekdays and Saturday.

Transformer taps were changed todetermine the best tap position


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Load Flow Studies

The frequency converters wererepresented as an 13.8/11 kV

autotransformer in combination with acapacitor. Load flow runs were carriedout on the combined Demerara 50 and

60 Hz systems for system peak load.


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Load Flow Studies

The frequency converters wereremoved from the system and the 60Hz

system was extended to cater for thepresent 50 Hz load.

Load flow runs were carried out for dayand 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 EdenStation

LFR - The hourly load flow run


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Analysis of Results

As GPLGOE was the slack busbarcomparison was made between its

generation during GPL operations andthe load flow runs. For the GPLoperations GPLGOE generation was

higher than that of the LFR by 70%during the off peak periods and up to120% during evening peaks.


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Analysis of Results

The LFR showed an average of 2%system losses

GPL operations showed losses as muchas 18% and averaged 11% over the

period of analysis.


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COMPARISON BETWEEN GPL AND LOAD FLOW GENERATION (MW)

25.0

30.0

35.0

40.0

45.0

50.0

55.0

1:00

2:00

3:00

4:00

5:00

6:00

7:00

8:00

9:00

10:00

11:00

12:00

13:00

14:00

15:00

16:00

17:00

18:00

19:00

20:00

21:00

22:00

23:00

24:00

TIME (HRS)

Generation

(MW)

System Load

GPL Generation

Load Flow Generation


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Analysis of Results

The LFR consistently generated around3 MVars at DPGOE. GPL operations

show MVAR generation between 8 and13 MVars at this location.


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Analysis of Results

The LFR generation of MVars at Sophiawas consistently higher than that of

GPL operations. This was as high as 15MVar at peak load whereas GPLoperations generate just around 10

MVars at the same period


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Analysis of Results

MVar generation at GPLGOE andVersailles were quite similar for both

GPL operations and the LFR


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Comparison Between GPL and Load Flow Generation (MVAR)

10.0

15.0

20.0

25.0

30.0

35.0

1:00

2:00

3:00

4:00

5:00

6:00

7:00

8:00

9:00

10:00

11:00

12:00

13:00

14:00

15:00

16:00

17:00

18:00

19:00

20:00

21:00

22:00

23:00

24:00

TIME (HRS)

MVAR

SYSTEM LOAD

GPL GENERATION

LOAD FLOWGENERATION


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Analysis of Results

Changing of transformer taps

The present tap positions of the GPL

transformers proved to be the optimumpositions to maintain bus voltages andminimise losses.


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Analysis of Results

Static Representation of theFrequency Converters

The results achieved from the load flowrun for system peak suggest that thismodel could be acceptable if separaterepresentation is made for the mechanicallosses 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 atall locations to provide the MVar injectionpresently done by the frequency converters

Switched capacitors would have to be usedas different values would be required forthe day and night peaks


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Conclusions

The usefulness of load flow studies in theinvestigation 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 highlightedproblems with GPLs system operations whichwere confirmed by the load flow study.

The difference in GPLs calculated loads andgeneration show a high level of losses inGPLs generation and transmission systemwhich require further investigation.

The Sophia 13.8 kV bus voltages are lowerthan the other bus voltages and need to beincreased for proper system operation.


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Conclusions

The static representation of thefrequency converters by a transformer

and a variable capacitor is an adequatemodel for load flow studies. Convertermechanical losses can be added

subsequently to the total system losses.


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Conclusions

The availability of load flow studieswould be helpful to small utilities as

they seek to integrate their powersystem with different types ofgeneration

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