impacts of varying penetration of distributed resources...
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Managed by UT-Battelle for the Department of Energy
Impacts of Varying Penetration of Distributed
Resources with & without Volt/Var Control:
Case Study of Varying Load Types
D. Tom Rizy1, Senior Member,
Huijuan Li2, Member,
Fangxing Li1,3, Senior Member,
Yan Xu1, Member,
Sarina Adhikari3, Student Member,
Phil Irminger2, Student Member
2011 IEEE PESGM
July 26, 2011
1ORNL, Power & Energy Systems Group 2Oak Ridge Associated Universities 3University of Tennessee, Knoxville
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Background
Follow-up to 2010 paper “Properly Understanding the Impacts of Distributed Resources (DR) on Distribution Systems”
Addresses how DR impacts vary in regards to both DR voltage regulation capability and load mix
Focuses on impacts to distribution capacity, losses and voltage regulation with DR penetration
Comparison of DR with and without volt/var control on 10MVA feeder example with two DRs
Inverter-based volt/var controls based on ORNL R&D work at the DECC Lab.
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Impacts Of Distributed Resources (DR)
On Distribution System
The connection of DR systems to the distribution system
will have an impact on
Feeder Capacity
Line Losses
Voltage Regulation
System Protection
Safety
A steady-state analysis of DR impacts may not be
adequate for addressing the full impacts of DR in a
distribution system.
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Dynamic versus Steady-State Analysis
of DR Impacts
Standard approach is to use a power-flow-based program to calculate the network voltages with different DR sizes, penetration and feeder loadings.
Used to be quite difficult to dynamically model a small system not to mention a large distribution system.
However, new tools such as EMTP-RV make the modeling of large systems possible along with their dynamic behavior.
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Issues that Merit Consideration for DR
Impact Assessment
Voltage Sensitivity of the Feeder Loads
Impacts on feeder capacity, losses and voltage regulation depend on feeder load voltage sensitivity.
To what degree?
Load mix and distribution for each phase may be important.
Variable Distributed Resource (DR) Output
Concern with the variability of renewable DR (i.e., wind and PV) that does not have an energy storage component.
May not be a concern with penetration level lower than 10%
Penetration level of 20% or greater, intermittent DR may quickly change the feeder voltage profile.
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Issues that Merit Consideration for a
DR Impacts Assessment (cont.)
Representation of Multiple DR
A big question is a usable and accurate aggregation model.
Protection Changes with DR
Not expected to be an impact at low DR penetration (i.e. 10% or less)
Becomes a concern at higher DR penetration especially if the DR type is a generator-based system.
Inverter-based DRs are inherently current limited; but may need new modeling and protection methods for high penetration.
DR with Reactive Power Capability
DRs not allowed (i.e., per 1547) to regulate voltage on the distribution system unless authorized by the utility.
Being amended by both IEEE Standards (1547.8) and NIST (P2030).
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Advantages of Allowing DR to Provide
Volt/Var Control
Provides reactive power locally instead of delivery over transmission lines from central power plants.
Provides reactive power needed to maintain a steady voltage profile at the load.
Respond to voltage transients (i.e., motor starts or load step changes) to maintain voltage.
Improves feeder capacity by reducing reactive current flow from substation to load.
Reduces line losses along with line flows due to local reactive power injection for voltage regulation.
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Impacts Study Approach
DR with and without voltage regulation capability
No regulation – only active power injection from DR
Regulation – both active and reactive power injection (to maintain voltage reference)
Distribution feeder impacts with increasing DR levels
Total line flow (used capacity)
Line losses
Voltage profile
Repeated for different feeder load compositions
Constant Power – served as the benchmark
Constant Impedance
Constant Current
ZIP – equal combination of the previous ones
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Example Used to Evaluate DR
Impacts on Distribution System
Inverter-based DR controls1 used to compare impacts of DR with and without voltage regulation capability.
DR penetration level (% of total DR output to feeder capacity) varied to analyze impacts.
Repeated for the four different loading cases.
1Developed and tested by ORNL
Substation
bus
1 32 4
INVERTER ICONTRO
LLER
DE
INVERTER CONTROLLER
65
INVERTER ICONTRO
LLER
DE
INVERTER CONTROLLER
DR1 DR2
10MVA Feeder
Total Load:
5.1MW, 3.7MVar, 0.8pf
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Inverter-Based DR Voltage Control
Power System
(Controlled System)
Voltage
ReferenceControllerCompare
Error
Voltage
(Controlled Variable)
Measure
vc
DR: Distributed Energy Resource
Control variable: the PCC voltage
Reference: the desired value of the PCC voltage
Error: difference between reference and measured PCC voltage
Fixed control:
PI control with Kp and Ki fixed
Kp and Ki typically by trial & error
Incorrect gains result in under-performance, oscillation, or instability
Adaptive control:
Kp and Ki values are initially conservative but adjusted in real-time to achieve desired system response time
Voltage stability is ensured
Load
Controller
ic
vdc
switching
signals
vt
(PCC)is i
l
Lc
ic
vc
is
vt
vdc
il
Ls R
sv
s
DEDR
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DECC Lab interfaced with Actual Distribution
System Supports Volt/Var Control
Development and Testing.
DECC Lab
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Voltage Regulation with both
Fixed and Adaptive Gain Control
(b) Voltage regulation with fixed gains. (a) No voltage regulation.
(c) Voltage regulation with adaptive gains.
Response to two- volt (2V) local voltage transient.
Tested on ORNL distribution system at DECC Lab.
Faster voltage regulation achieved with adaptive gains.
The voltage scales are different since under different distribution
system operating conditions on different days.
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0%
5%
10%
15%
20%
25%
30%
35%
5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55%
% O
utp
ut
by
Each
DR
Total DR Penetration Level (%)
DR1 %
DR 2 %
Study Assumptions
10MW Radial Feeder with 63% used capacity
DR penetration from 5% to 55% in 5% increments
Feeder load is fixed for each loading case
DRs provide -1.5MVAr to 1.5MVAr to regulate voltage
0.980pu for DR at bus 4
0.975pu for DR at bus 6
Capacitors both at substation and on circuit assumed to be fixed
DR active power scaled up instead of adding more DRs
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Voltage Feeder Profiles for the
Constant Power Load Case
Voltage profiles with increasing DR without voltage regulation
Voltage profiles with increasing DR with voltage regulation
0.960
0.965
0.970
0.975
0.980
0.985
0.990
0.995
1.000
1.005
Bus 1 Bus 2 Bus 3 Bus 4 Bus 5 Bus 6
Vo
lta
ge (p
er
un
it)
Substation to End of Feeder
0%5%10%20%25%30%35%40%45%50%55%
DR
Pen
etra
tio
n L
evel
s
0.960
0.965
0.970
0.975
0.980
0.985
0.990
0.995
1.000
1.005
Bus 1 Bus 2 Bus 3 Bus 4 Bus 5 Bus 6
Vo
lta
ge (p
er
un
it)
Substation to End of Feeder
0%5%10%20%25%30%35%40%45%50%55%
DR
Pen
etra
tio
n L
evel
s
Direction of Increasing DR
Penetration
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DR Impacts on Distribution
Losses for Constant Power load
Active power losses with and without DR voltage regulation
Reactive power losses with and without DR voltage regulation
40
50
60
70
80
90
100
110
120
130
0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%
Act
ive
Po
we
r Lo
sse
s (k
W)
DR Penetration Level (%)
No Regulation
Regulation
40
50
60
70
80
90
100
110
120
130
0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%
Re
acti
ve P
ow
er
Loss
es
(kV
Ar)
DR Penetration Level (%)
No Regulation
Regulation
Dashed line shows where DR with voltage regulation compared to DR
without voltage regulation no longer provides benefit. Losses increase.
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DR Output with Voltage
Regulation for the Load Cases
DR active/reactive power with increasing penetration
DR power factor with increasing penetration
0.0
0.5
1.0
1.5
2.0
2.5
3.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%
Act
ive
Po
we
r O
utp
ut
(MW
)
Rea
ctiv
e P
ow
er O
utp
ut
(MV
ar)
DR Penetration Level (%)
Q_CP - VR
Q_CI - VR
Q_CZ - VR
Q_ZIP - VR
P_ALL
Injecting Reactive Power
Absorbing Reactive Power
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%
DR
Po
we
r Fa
cto
r
DR Penetration Level (%)
CP_pf
CI_pf
CZ_pf
ZIP_pf
Injecting Reactive Power
Absorbing Reactive Power
Dashed line shows where DR with voltage regulation absorbs reactive
power instead of injecting reactive power.
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2.0
2.5
3.0
3.5
4.0
4.5
5.0
0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%
Rea
ctiv
e P
ow
er L
ine
Flo
w (
MV
ar)
DR Penetration Level (%)
CP - No VRCP - VRCI - No VRCI - VRCZ - No VRCZ - VRZIP - No VRZIP -VR
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%
Act
ive
Po
wer
Flo
w (
MW
)
DR Penetration Level (%)
CP - No VR CP - VR
CI - No VR CI - VR
CZ - No VR CZ - VR
ZIP - No VR ZIP -VR
DR Impacts on Distribution Power
Flow for the Load Cases
Active power flow with and without DR voltage regulation
Reactive power flow with and without DR voltage regulation
Dashed line shows where DR with voltage regulation compared to without
voltage regulation no longer provides benefit. Reactive load increases.
VR
No VR
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Voltage Profiles for the Four
Different Load Cases
5% DR
Penetration
35% DR
Penetration
20% DR
Penetration
55% DR
Penetration
VR
No VR
VR
No VR
VR
No VR
VR
No VR
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Results
Observed slight differences in distribution impacts for the four load cases.
Most difference was in losses - maximum variation of 8.3% between load cases.
Maximum variation of 6.0% for line flows between load cases.
Similar voltage responses for the four load cases.
Reach a point of diminishing return using high penetration DR to provide local voltage regulation.
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Results (cont.)
At 35% or greater DR penetration:
Active power from DR causes mode change.
DR voltage regulation mode reverses from injecting to absorbing reactive power.
DR becomes additional reactive load.
Penetration trigger point could be higher if voltage references are allowed to be higher.
However at 30%, reactive power losses increase above no regulation case.
Analysis is based on our specific example and on fixed DR voltage references.
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Summary
A number of distribution system impacts merit consideration for DR interconnection (i.e., losses, feeder capacity, voltage regulation, protection and safety).
If some DRs can provide reactive power in addition to active power, they can provide voltage regulation and support.
If voltage regulation is dynamic, DRs can respond to a voltage transient in 0.5 or less and be transparent to system voltage controls.
Local voltage regulation done correctly can lower feeder line flows and losses and increase capacity.
Standards are being developed to determine when DR volt/var control is appropriate.
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Possible Future Considerations
Impacts as loading and distribution is varied (rather than fixed) with high DR penetration.
Impacts of intermittent DR (i.e., PV) penetration.
Impacts of air-conditioning stall and DR impacts together.
Impacts of multiple inverter-based DR control to address possible interaction.
More complex distribution network.
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Q&A
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OAK RIDGE NATIONAL LABORATORY Managed By UT-Battelle for the Department of Energy
D. Tom Rizy, Research Staff
Power & Energy Systems Group
Energy & Transportation Science Division
One Bethel Valley Road, MS-6070
Oak Ridge, Tennessee 37831-6070
(865) 574-5203 Voice, 575-7643 Fax
(865) 207-6769 Cell
Email: rizydt@ornl.gov
www.ornl.gov, www.ornl.gov/sci/decc
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