A Mitigation Strategy to Address Voltage Flicker

due to the Cloud Cover Effect

Soonwook Hong

Solectria Renewables

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2

High Penetration Scenario Example:

1.7 MW site in Cedarville NJ

4.7 miles from substation 12kV feeder, 6MW mid-day load

Concerns of local overvoltage

Utility has closed circuit for more PV

0.5 MW1.7 MW

Courtesy of Pepco Holdings

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Overvoltage Concerns

3.0% of points

exceed +5% limit

Courtesy of Pepco Holdings

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Mitigating overvoltage issue with Power Factor control

< 0.1% of points

exceed 5% limit

• Feeder could be reopened for PV after PF adjustment to 0.97

• “Flicker” Mitigation (cloud induced voltage transients)

Courtesy of Pepco Holdings

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Overvoltage Issues

Utility concerns

• Feeder voltage can exceed the ANSI 84.1 Guideline

• The number of operations of tap changer, regulators and cap banks

increases with voltage fluctuation

• Conservation voltage reduction will not work well with DGs

Site/Project owner concerns

• Projects can be denied by utilities due to the overvoltage concerns.

• Due to the high AC voltage, inverters trip with the overvoltage protection.

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Topics

� How voltage changes with DG generation?

� How power factor control contribute to regulate the feeder voltage?

� More considerations to enhance the power factor control

� Benefits and applications of XR Compensation (power factor control)

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Voltage rise of PV plant

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Voltage profile with irradiation changes

1.01

1.015

1.02

1.025

1.03

1.035

1.04

1.045

1.05

5:27 8:19 11:12 14:05 16:58 19:51

Feeder Voltage

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Line Impedance Z = 0.1pu

Line Impedance Z = 0.2pu

• Low Impedance feeder experiences less voltage rise

• With a fixed impedance, high X/R ratio feeder experiences less voltage rise

Voltage rise with different feeder characteristics

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How power factor control regulates voltage?

(XR Compensation)

• If the X/R ratio of the feeder is known, the required power factor can

be calculated.

• Slight overcompensation can be expected with the high generation

level

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800

806 808 812 814

810

802 850

818

824 826

816

820

822

828 830 854 856

852

832

888 890

838

862

840

836860834

842

844

846

848

864

858

PV

Simulation with IEEE 34 Node Test Feeder

Regulator

Load

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XR Compensation (Power Factor Control)

Sunny Day Cloudy Day

0

10

20

30

40

No PV Generation Mode PF Control

0

10

20

30

40

No PV Generation Mode PF Control

Number of operations of the regulator at 850

1.01

1.015

1.02

1.025

1.03

1.035

1.04

1.045

1.05

5:27 8:19 11:12 14:05 16:58 19:51

With No PV

Generation mode

PF Control 1.01

1.015

1.02

1.025

1.03

1.035

1.04

1.045

1.05

5:27 8:19 11:12 14:05 16:58 19:51

With No PV

Generation mode

PF Control

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PF = 1 PF = 0.949 PF = 0.976

Compensation of “Over-Compensation”

Error is caused by using approximation! (usually not significant though)

Corrected power factor can be accurately calculated with feeder impedance (Z and X/R)

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Line Impedance Z = 0.2pu

• High X/R ratio requires smaller power factor angle, thus low reactive power

• The control is more effective with high X/R ratio

• For low X/R ratio feeder, adding reactor can be considered to increase energy harvest

Voltage Compensation with different X/R ratio

PF = cos(39°°°°)

Q = 0.777pu

PF = cos(12.7°°°°)

Q = 0.976

PF = cos (5.5°°°°)

Q = 0.995

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Typical X/R ratio of distribution feeders

The required power factor is usually higher than 0.95 when a PV plant

includes a MV transformer and larger than 1MVA.

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Generation Reduction with Power Factor Control (0.97)

0%

20%

40%

60%

80%

100%

97%0.4%

Gen

era

ted

Po

wer

time

Generation Profile on a sunny day

Still, 99.6% generation is expected with the power factor control.

Without it, the grid connection might not be possible!

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Uniqueness of the XR Compensation

XR Compensation Other regulation scheme

Reduce feeder overvoltage Regulate feeder voltage

(both over and under voltage)

Controlled by DG generation

(self induced overvoltage)Controlled by voltage variations

due to load, generation and fault

DG plant voltage compensation Distribution line voltage regulation

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XR Compensation Advantages

� Regulates the self-induced voltage fluctuation issue

� Steady state as well as dynamic voltage regulation

� Autonomous function without need for communication

� 0.4% generation reduction when pf = 0.97 on sunny days

� Overcompensation can be compensated properly

� Low X/R ratio feeder can use series inductor to effectively use power

factor control (or high impedance transformer)

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Thank you!

Soonwook Hong

soonwook.hong@solectria.com