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
11
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
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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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