1

Long-Duration Voltage Variations

2

LoadV2V1

V2

I

I

RI

jX IV1

At given pf at full load, nominal V2

R X

3

Voltage Regulation• Definition: Voltage regulation (at point x)

is the percent voltage rise caused by unloading a power system (at point x)– Assumption 1: The original power factor at

point x is given– Assumption 2: The original voltage is the

nominal value at point x, or a given value if not nominal; the source voltage is fixed

– Assumption 3: Original system is at full load, or a given value if not full load

4

V2,NL = V1

At no load, V2 normally rises to equal V1

V2FL

%100V

VV%100

VVV

RegR2

R2NL2

FL2

FL2NL2

Last equality assumes full-load voltage is nominal or rated value for the system

5

• The system inductive reactance usually causes voltage drops under normal loading

• If the load pf is leading or if very long transmission lines at EHV (345 kV and up, the line charging current may be very large), then regulation may be negative

6

Root Cause

• Most long-duration voltage variations are caused by too much impedance (Zth) in the power delivery system

• The power system is too weak for the load– voltage drops to a low value under heavy

loads (lagging pf)– voltage rises to a high value under light loads

(more leading or less lagging pf)

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Solutions to Improve Voltage Regulation

• Add shunt capacitors to increase the load power factor (not leading however) tending to decrease the load kVA by decreasing the load kVAr

• Add static var compensation or other dynamic reactive power compensation (same reason as shunt capacitor addition, but better control)

• Add series capacitors to lines to cancel part of the jXI voltage drop (long transmission lines and (rarely) short lines with impact loads)

8

Solutions to Improve Voltage Regulation

• Add voltage regulators to boost V under heavy load and buck voltage under light load

• Increase the size of conductors to reduce Z

9

Loads…

Step voltage regulators

RaiseLower

Load side

Source side

10

V sensing and gate control

Source side

…Load side

…Electronic tap-switching voltage regulator

11

Loads…

voltage regulator set at 105% without line-drop compensationV(x)

x120 V

126 V

114 V

voltage profile for light load

voltage profile for heavy load

12

Loads…

voltage regulator set at 100% with line-drop compensationV(x)

x120 V

126 V

114 V

voltage profile for light load

voltage profile for heavy load

13

V(x)

x120 V

126 V

114 V

14

V(x)

x120 V

126 V

114 V

Voltage profile after load rejection

Needs rapid runback controls

15

Flicker

Sources of flicker-Load change-Induction motor starting -Variable power generation

Observable flicker is dependent on the following:-Size (VA) of potential flicker-producing source -System impedance (stiffness of utility)-Frequency of resulting voltage fluctuations

Example of Flicker

Power system model at Ulleung Island of South Korea.

YDG

4.5[MW]/0.5[MW]

4.5[MVA]/0.5[MVA]3.3[kV]/6.6[kV]

DG

1.5[MW]

1.5[MVA]3.3[kV]/6.6[kV]

WG

0.6[MW]0.6[MVA]

0.48[kV]/6.6[kV]

1.53+j0.790 [Ω]

1.16+j0.600 [Ω]

SMESPs, Qs

C 0.305 MVAR

Load 6[MW]/2[MW]

6.0[MVA]/2.0[MVA]0.23[kV]/6.6[kV]

1.16+j0.599 [Ω]Y

0.6[MVA]/0.1[MVA]6.6[kV]/3.3[kV]

HG

HG

0.6[MW]/0.1[MW]

0.1[MW]

Y

Y

0.378+j0.195 [Ω]

PL, QL

VGPWG Unit

System Responses

Wind speed data.

0 10 20 30 40 50 606

8

10

12

Win

d sp

eed

[m/s

]

Time [sec]

0 10 20 30 40 50 6058

59

60

61

62

0 10 20 30 40 50 600.0

0.5

1.0

1.5

2.0

2.5

Freq

uenc

y [H

z]

Time [sec]

Without Wind generator With Wind generator

Activ

e pow

er [M

W]

Time [sec]

Wind generator Diesel generator 1&2 Hydraulic generator 1&2 Load

Responses of active power and system frequency.

System Responses with SMES

0 10 20 30 40 50 6058

59

60

61

62

0 10 20 30 40 50 600.0

0.5

1.0

1.5

2.0

2.5

Freq

uenc

y [H

z]

Time [sec]

Without SMES With SMES

Act

ive

pow

er [M

W]

Time [sec]

Wind generator Transmission line Diesel generator 1&2 Hydraulic generator 1&2 Load

Responses of active power and system frequency with SMES .

19

Source side Loa

d

Thyristor-controlled reactorOne type of static var compensator

3rd 5th 7th

capacitors configured as harmonic filters

Flicker Mitigation Techniques

-Adding series reactor

20

Source side Loa

d

Thyristor-switched capacitorAnother type of static var compensator

capacitors are gated fully on in sequence