By

Mayura Sri-on

Assistant Chief of Power Quality Analysis Section

Customer Service Division

Engineering and Service Department

PEA Area1(Northeast) Udon Thani Province, Thailand 1

Distribution System

Voltage Rise Effect

Study System

Result

Conclusion

Loss Analysis

2

Distributed Generation

Connected to Grid

3

Traditional Distribution Network

The voltage decrease along the feeder, from substation to the feeder-end.

The power flow is in one direction only, from the transmission system to the HV substations and to the MV&LV distribution system.

Subtrasmission

Customer 69kV, 115kV

Distribution

Customer 11kV,22kV,33kV

Low Voltage

Customer 220 V, 380 V

Trasmission

Lines 230kV, 500kV

4

The power voltage at DG will increase voltage, which may cause the

voltage at its connection point to be higher that voltage at the substation.

Figure 1 Illustration of a DN with DG connected

This circuit represents the basic features of a DN in which DG is connected to a DN feeder.

5

V2 V1

𝑉2 ≈ 𝑉1 + 𝑅 𝑃𝐺 − 𝑃𝐿 + ±𝑄𝐺 − 𝑄𝐿 𝑋

This simple equation can be used to qualitatively analyze the relationship

Between the voltage at 22 kV Bus and the amount of generation that can be connected to the DN.

Normally, before allowing the installation of DG, the Distribution System

Operation will ensure that the voltage, line thermal characteristics and

substation transformer capacity will not be violated for the worst

operating scenarios:

1) No generation and maximum load

2) Maximum generation and maximum load 3) Maximum generation and minimum load

6

V2 V1

(Eq.1)

Source Load

r

G

A simple model of the distribution system long G km.

𝑃𝑜𝑤𝑒𝑟𝑆𝑜𝑢𝑟𝑐𝑒 = 𝐿𝑜𝑎𝑑 + 𝑃𝑜𝑤𝑒𝑟 𝐿𝑜𝑠𝑠

I

𝑃𝑜𝑤𝑒𝑟 𝐿𝑜𝑠𝑠 = 𝐼2𝑟𝐺

7

(Eq.2)

(Eq.3)

r : Resistance of line (/km)

Figure 3 Simple model of Distribution system

Source Load

r

G

DG

G1 G2

i1 I

𝑃𝑜𝑤𝑒𝑟 𝐿𝑜𝑠𝑠𝑤𝑖𝑡ℎ 𝐷𝐺 = 𝑖12𝑟𝐺1 + 𝐼2𝑟𝐺2

𝑃𝑜𝑤𝑒𝑟 𝐿𝑜𝑠𝑠𝑤𝑖𝑡ℎ 𝐷𝐺 = 𝑃𝑜𝑤𝑒𝑟𝑆𝑜𝑢𝑟𝑐𝑒 + 𝑃𝑜𝑤𝑒𝑟𝐷𝐺 − 𝐿𝑜𝑎𝑑

8

(Eq.5)

(Eq.4)

Figure 4 Simple model of Distribution system with DG installation

Voltage level

Voltage Normal Contingency

Max Min Max Min

Transmission ±5% ±10%

115 kV 120.7 109.2 126.5 103.5

69 kV 72.4 65.5 75.9 62.1

Distribution ±5% ±10%

33 kV 34.7 31.3 36.3 29.7

22 kV 23.1 20.9 24.2 19.8

Low Voltage ±10% Approx. ±10% Approx.

380 V 418 342 418 342

220 V 240 200 240 200

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Figure 4 Radial DN 22 kV of PEA in the northeast area of Thailand

The main feeder had been divided into 2 main branches at ‘B’ and each

branches protected by the line recloser.

‘C’ for connecting of DG2 (PV-DG 5.1 MW)

‘E’ for connecting of DG1 (Biomass-based DG 2.4 MW)

‘D’,’F’ the end of line.

A

B C D

E F

DG2

DG1

10

DigSILENT software

Geographic Information System (GIS)

SCADA system

Constant impedance load dispersed along line

Power quality measuring of

Substation, DG1 and DG2 follow

EN50160 std.

Dranetz model Power X

11

DG1

Biomass generation rating 2.4 MW

Sugarcane mill factory

22kV VSPP

Generation period during milling from Dec – Apr

DG2

PV generation rating 5.1 MW

22kV VSPP

Generation period during daytime

12

Rural area, small city

Commercial load

Small business load

Mining factory load

Residential load

Total average load 6.25 MW

Nominal Voltage: 22 kV

Sending Set Point Voltage: 22.51.25%

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0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

MW

Power Profile by hour DG1

DG2

DG1+DG2

SystemLoad

DG1 Biomass-based: generate power at 2.4 MW average

DG2 PV: generate power only day time vary by sunrise cycle, max at 5.1 MW

System load: dispersed along line, most are residential consumers.

Commercial & Business consumers: sugar factory and small business.

Peak load at night time(07.00-09.00 pm.) By day time total load was lower than the generation of DGs.

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Figure 5 Power Profile of load and DG generation rating 24 hrs.

Network Modeling

Scaling load follow measuring data

Newton Raphson (Power Equation Classical) used

Run voltage profile with load profile 24 hrs.

Run power flow single short for voltage level by location. With and without DG

Vary load from 25% - 150%

Run power flow single short for power loss With and without DG

Vary DG Ration by vary load from 0.8 to 4.8

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• DiSILENT Software

• Simulation data rechecked

with the result from

measuring data.

• Measuring data collected

at 3 points: substation,

DG1 and DG2.

• Collecting data period: 7 days

• Total Distribution Network

length: 45 km.

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D

F

B

C

E

A

Figure 6 Network modeling export from GIS

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20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

110.00%

1.00

1.01

1.01

1.02

1.02

1.03

1.03

1.04

1.04

1.05

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

p.u

.

Substation Voltage Profile Vsub withDG

Vsub WoDG

Load-ratio

Voltage profile at substation with and without DGs

The figure showed the voltage of system with DG was higher.

Especially during day time (09.00 am-4.00pm) with generation of the PV-DG

raised significantly.

Load ratio: load/peak load ( avg. of peak load in a week 6.25MW) At peak load (Load-ratio = 100%) voltage at the substation also raised by DG.

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Figure 7 Result of voltage profile 24 hrs. at Substation

Voltage

Level

pu. (22kV)

%Load

20%

40%

60%

80%

100%

120%

140%

0.92

0.94

0.96

0.98

1.00

1.02

1.04

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

p.u

.

Voltage Profile at Midline(B)

Vmid withDG Vmid WoDG Load-ratio

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

110.00%

0.950

0.960

0.970

0.980

0.990

1.000

1.010

1.020

1.030

1.040

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

p.u

.

Voltage Profile at Endline(D) Vend(D) withDG Vend(D) WoDG Load-ratio

19

Figure 8 Result of

voltage profile 24 hrs. at Midline (B)

Figure 9 Result of

voltage profile 24 hrs. at Endline (D)

19.500

20.000

20.500

21.000

21.500

22.000

22.500

23.000

A B C D E F

kV

Voltage by location

W/O DG DG1 DG2 DG1+DG2

At ‘E’ and ‘F’ end of line, voltage without DG was lower than limit of

Voltage level (20.9 kV). With support of DG the voltage level of ‘F’ rise from 20.6 kV to 21.8 kV.

Limit at 20.9 kV

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W/O DG DG1 DG2 DG1+DG2

DG Ratio (DG/Load) 0 0.39 0.81 1.20

Figure 10 Result of voltage level at each location

Location W/O DG DG1 DG2 DG1+DG2

DG Ratio DG/Load 0 0.4 0.8 1.2

o Substation A 22.339

0.40%

(22.428)

0.91%

(22.543)

1.20%

(22.606)

o Middle of line B 21.278

1.55%

(21.608)

3.03%

(21.923)

4.34%

(22.201)

o DG2(PV 5.085MW) C 21.164

1.40%

(21.461)

4.35%

(21.084)

5.50%

(22.327)

o EndlineDG2 D 21.094

1.33%

(21.375)

3.71%

(21.876)

4.80%

(22.106)

o DG1(Gen 2.414MW) E 20.882

2.88%

(21.483)

3.25%

(21.560)

5.84%

(22.102)

o EndlineDG1 F 20.603

2.78%

(21.176)

3.34%

(21.291)

5.83%

(21.804)

DG Ratio VS Voltage Rise %

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19.50

20.00

20.50

21.00

21.50

22.00

22.50

23.00

23.50

24.00

A B C D E F

kV

25%

50%

75%

100%

125%

150%

%Load

23.1 kV

DN with the support of voltage level by DG, voltage level in the range of

grid code event the high load without under voltage problem at end of line. Weak network, max DG with min load, voltage rise may occur.

20.9 kV

22

Voltage limit

Voltage limit

Figure 11 Result of voltage level with vary of load from 25% - 150%

20.50

21.00

21.50

22.00

22.50

23.00

23.50

24.00

24.50

25.00

A B C D E F

kV

Voltage by location with Cap

25%

50%

75%

100%

125%

150%

%Load

Traditional improving under voltage level is install a shunt capacitor

In the network. In this feeder, a 1500kVar shunt capacitor was used

to improve the voltage level before installing of DG.

This important, when DG integrated to DN, the voltage level was raised.

When switching shunt capacitor to DN, the Distribution operator should

consider carefully with load level.

23.1 kV

23

Voltage limit

Figure 12 Result of voltage level with vary of load from 25% - 150% with support of switching capacitor installation.

Effect on traditional improving under voltage level

DG Ratio (DG/load)

20.50

21.00

21.50

22.00

22.50

23.00

23.50

24.00

0.80 0.96 1.20 1.60 2.40 4.80

kV

Voltage level with DG

DG2(PV 5.1MW)

Midle

Sub

1.5

At DG Ratio higher than 1.5, voltage level along feeder inclined to be

higher than voltage at substation. With DG smaller than load the voltage may be raised by effect of DG but not higher than at substation.

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Figure 13 Result of voltage level with vary of DG Ratio

0

0.05

0.1

0.15

0.2

0.25

0.3

No DG DG1 DG2 DG1+DG2

MW Loss

MW LossMW

In this case, with DG installation the power loss was less than without DG.

Due to DG smaller than Load, the incorporation of DG into a radial distribution system could result in line loss reduction.

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Figure 14 Result of Power Loss with vary of DG installation rating

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0.000

0.050

0.100

0.150

0.200

0.250

80% 96% 120% 160% 240% 480%

Power Losses

MW loss

MW lossTrend of losses

MW

DG Ratio (DG/load)

When DG-ration increased beyond 1.5, loss essentially increases. This showed

that the integration of DG into a radial distribution system may result in higher line loss if DG power rating and load were not properly related.

Figure 15 Result of Power Loss with vary of DG Ratio

DG could raise voltage level of Distribution

Network.

DG Ratio (DG/Load): the higher DG Ratio ,

the higher voltage rise % especially end of

line.

Weak network, maximum DG with minimum

load, leaded to voltage rise problem.

DG could reduce power loss with the proper

generation rating.

DG Ratio: the higher than 1.5, the increasing

of power loss in the DN.

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