Impact Studies of PV Integration in Distribution Systems

Le Xu, PhD, PE

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“Case Studies of Experiences with Distributed Resource Interconnections on Distribution Systems” PanelIEEE PES General Meeting, National Harbor, MD

July 29, 2014

Outline• Introduction• Methodology

– Cluster Analysis– Steady State Analysis– Dynamic Analysis– System Extrapolation

• Observations• Summary

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Introduction• Exponential growth of global solar PV installation

– 4,751MW of new PV capacity installed in 2013, up 41% over 2012.– 1,330MW of new PV capacity installed in Q1 2014, up 79% over Q1

2013

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Source: SEIA: Solar Energy Facts: 2013 Year in Review Source: SEIA: U.S. Solar Market Insight Report 2014 Q1

Introduction• PV has diverse impacts on distribution system planning and

operation.• Study requests are typically for individual projects.• There is a need for a system level study for both frontrunners

and beginners.

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Source: SEIA: Solar Energy Facts: 2013 Year in Review

Methodology• System level study vs. Individual project study

– Representative feeders selection to balance effort and accuracy– Long term impact estimate (i.e., penetration rates)– Stochastic PV sampling to model uncertainties (e.g., locations)– System extrapolation

• Medium and small scale PV vs. Utility scale PV

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Methodology• Components

– Statistical cluster analysis– Scenario generation– Steady state analysis– Dynamic analysis– System extrapolation

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Cluster Analysis• Cluster Analysis

– Feeder characteristics may differ significantly from each other.– To group the entire set of feeders into a small number of clusters so that

feeders are similar in the same cluster but are dissimilar among different clusters.

• Key components– Feeder features– Feature weights– Cluster number– Clustering algorithm

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Steady State Analyses• Some expected impacts are:

– Reverse power flow– Voltage variations– Interaction with voltage regulation devices (substation LTC, line

voltage regulators, capacitor banks, etc.)– Reactive power fluctuations– Voltage unbalance (due to single-phase PV)

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Steady State Analyses• Scenario setup

– Penetration rate– PV sizes– PV distribution

• Inputs– Feeder models (distribution analysis software)– Typical PV injection profile– Feeder daily/daytime load curves – Status of voltage control equipment

(voltage regulators and capacitor banks)

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Example 1: Power flow & Losses• Power flows and power losses as a function of PV penetrations

10

0

50

100

150

200

250

300

350

400

450

500

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

Active Po

wer Loss (kW)

Time (hour)

Feeder Power Loss (Minimum)

Base 5% PV 10% PV 20% PV 50% PV 100% PV

‐10

‐5

0

5

10

15

20

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

Active Po

wer Flow (M

W)

Time (hour)

Active Feeder Power Flow (Maximum)

Base 5% PV 10% PV 20% PV 50% PV 100% PV

Example 2: Voltage Profile

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118

120

122

124

126

128

130

132

0 10000 20000 30000 40000 50000 60000Vo

ltage (V)

Distance from Substation (ft)

Nodal Voltage at Noon (Minimum)

BASE %5 PV 10% PV 20% PV 50% PV 100% PV

120.5

121

121.5

122

122.5

123

123.5

124

124.5

125

125.5

0 5000 10000 15000 20000 25000

Voltage (V)

Distance from Substation (ft)

Nodal Voltage at Noon (Maximum)

BASE %5 PV 10% PV 20% PV 50% PV 100% PV

Example 3: Reactive power fluctuations

12

0

0.2

0.4

0.6

0.8

1

1.2

1.4

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

Reactive Po

wer Flow (M

Var)

Time (hour)

Reactive Feeder Power Flow (Maximum)

Base 5% PV 10% PV 20% PV 50% PV 100% PV

0.0

0.2

0.4

0.6

0.8

1.0

1.2

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

Normalized

 Solar PV Outpu

t

Hour

Dynamic Analyses13

0.0

0.5

1.0

1.5

2.0

2.5

3.0PV

‐DG ou

tput (MW)

Time

Dynamic Analyses• Some expected impacts are:

– Dynamic voltage fluctuations– Temporary over/under voltage and Transient overvoltage (TOV) – Interactions of intermittent PV with voltage regulating devices– Harmonics

• Inputs– Feeder models (transient and dynamic analysis software)– Feeder minimum daytime load– Inverter models– intermittent PV profile– Settings of voltage control equipment

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0 10 20 30 40 50 600

0.2

0.4

0.6

0.8

1

1.2

1.4

Pow

er (p

u)

time (min)

Intermittent PV Profile

Dynamic Analyses• Example study cases

– Incremental addition of PV penetration– Sudden change in solar radiations

• Step increase (0% to 100% PV)• Step decrease (100% to 0% PV)

– Feeder voltage variations• 20% voltage decrease on HV systems for 5 cycles• 10% voltage increase on HV systems for 5 cycles

– Islanding cases: disconnect feeder circuit breaker or recloser• Excess power: 100% PV generation• Power balance: PV generation = load

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Example 1: Power Flow & LTC Operation

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0 10 20 30 40 50 60-1.5

-1

-0.5

0

0.5

1

1.5

time (min)

Rea

l Pow

er (M

W)

Real power down feeder

100%50%20%10%5%

0 10 20 30 40 50 600.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

time (min)

Rea

ctiv

e Po

wer

(MVA

R)

Reactive power down feeder

100%50%20%10%5%

0 10 20 30 40 50 600.996

0.997

0.998

0.999

1

1.001

time (min)

Volta

ge (p

u)

Source Voltage

100%50%20%10%5%

0 10 20 30 40 50 601.04

1.045

1.05

1.055

time (min)

Volta

ge (p

u)

LTC Voltage (secondary)

100%50%20%10%5%

Example 2: Islanding

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Islanding

x 1.950 2.000 2.050 2.100 2.150 2.200

0.00 0.20 0.40 0.60 0.80 1.00 1.20

V (

pu

)

Vpv1 Vcap1 Vcap2 Vpv8

0.00 0.20 0.40 0.60 0.80 1.00 1.20

MW

/ M

VA

r

Pfeeder QLTCs

System Extrapolation• The analyses obtained from the representative feeders are

extrapolated to the system as a whole. – The analyses from a representative feeder are considered to be able to

represent the PV impact on the feeders in the same cluster; – A system level impact is a weighted sum of the impacts from different

representative feeders.

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Power Loss

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100.0%

89.4%

79.2%

61.6%

29.5%

42.5%

21.6%17.1%

13.4%8.8%

15.2%

87.5%

0%

20%

40%

60%

80%

100%

120%

Base 5% 10% 20% 50% 100%

Percen

tage

 of B

ase (M

ax) C

ase Po

wer Loss (%)

PV Penetration Rates

Trend of Active Power Loss

Maximum Noon Loading Day Minimum Noon Loading Day

Over-voltage

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0.0% 0.0% 0.0% 0.0% 0.6%

8.5%

0.0% 0.0% 0.1% 0.3%

6.2%

13.9%

0%

2%

4%

6%

8%

10%

12%

14%

16%

Base 5% 10% 20% 50% 100%PV Penetration Rates

Trend of Over‐Voltage (Peak Hour System Average)

Maximum Noon Loading Day Minimum Noon Loading Day

Observations• Low penetration level: no significant problems

– Overvoltage in one representative feeder– No overload issues– No transient nor rapid variation issues– No switching limitations incurred and no capacitor banks turned off

due to voltage rise issues– Not expected to sustain an island– No temporary over/under voltage condition nor TOV situation– Distribution feeder voltages would recover properly after system

transients and disconnection of PV inverters

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Observations• Medium penetration level: some problems

– Overvoltage violations in five representative feeders. – No overload issues – Transient voltage rises between 2-3% of nominal feeder voltage on

relatively weak 12 kV feeders– No issues due to islanding– Temporary over/under voltages or TOV for 4 kV feeders– A few cases of LTC or voltage regulator tap operations and change in

status of capacitor banks

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Observations• High penetration level: more frequent problems

– More prevalent overvoltage issues– Minor line overloading issues in one representative feeder– Transient voltage variations between 3-4% on strong feeders and large

transient voltage variations between 10-15% on weak feeders. – Some potential for sustained islanding and delayed tripping of PV

inverters. Under those conditions, TOV of up to 1.3 pu was observed. – Some capacitor banks turned off due to the voltage rise and turned on

again due to generation intermittency – Significant numbers of LTC and voltage regulator tap changers

operation

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Summary• PV impact studies are typically for individual projects, but we

have noticed a need for a system level study as well.• A system level study aims to capture the overall trends and

impact patterns as the PV penetration grows to assess the short, mid and long term challenges for the host utilities.

• As expected, the impact gets severe as the PV penetration grows, but finding the turning points can assist utilities to plan accordingly.

• The representative feeder approach can also help identify feeder clusters that need special attention.

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Contact: lxu@quanta-technology.com