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Distribution System Voltage Performance Analysis for High-Penetration PV

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Abstract and Figures

Distributed generation can have an impact on distribution feeder voltage regulation, and distributed solar photovoltaics (PV) are no exception. As the penetration level of solar PV rises over the coming decades, reverse power flow on the distribution feeder will happen more frequently and the associated voltage rise might lead to violations of voltage boundaries defined by ANSI C84.1. The severity of possible voltage problems depends on the relative size and location of distributed PV generation and loads, distribution feeder topology, and method of voltage regulation. In this paper, an illustrative distribution system feeder is assumed, and various case studies are conducted. The performance of the commonly used distribution voltage regulation methods under reverse power flow are investigated and presented. Voltage performance of the feeder, and the flow of active and reactive power are studied under different loading assumptions, and different assumptions of PV inverters' participation. The paper also explores the system performance using coordinated controls of inverters and utility equipment.
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A
national laboratory of the U.S. Department of Energ
y
Office of Energy Efficiency & Renewable Energ
y
National Renewable Energy Laboratory
Innovation for Our Energy Future
Distribution System Voltage
Performance Analysis for
High-Penetration Photovoltaics
E. Liu and J. Bebic
GE Global Research
Niskayuna, New York
Subcontract Report
NREL/SR-581-42298
February 2008
NREL is operated by Midwest Research Institute Battelle Contract No. DE-AC36-99-GO10337
National Renewable Energy Laborator
y
1617 Cole Boulevard, Golden, Colorado 80401-3393
303-275-3000 www.nrel.gov
Operated for the U.S. Department of Energy
Office of Energy Efficiency and Renewable Energy
by Midwest Research Institute Battelle
Contract No. DE-AC36-99-GO10337
Subcontract Report
NREL/SR-581-42298
February 2008
Distribution System Voltage
Performance Analysis for
High-Penetration Photovoltaics
E. Liu and J. Bebic
GE Global Research
Niskayuna, New York
NREL Technical Monitor: Ben Kroposki
Prepared under Subcontract No. ADC-7-77032-01
NOTICE
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Preface
Now is the time to plan for the integration of significant quantities of distributed renewable
energy into the electricity grid. Concerns about climate change, the adoption of state-level
renewable portfolio standards and incentives, and accelerated cost reductions are driving steep
growth in U.S. renewable energy technologies. The number of distributed solar photovoltaic
(PV) installations, in particular, is growing rapidly. As distributed PV and other renewable
energy technologies mature, they can provide a significant share of our nation’s electricity
demand. However, as their market share grows, concerns about potential impacts on the
stability and operation of the electricity grid may create barriers to their future expansion.
To facilitate more extensive adoption of renewable distributed electric generation, the U.S.
Department of Energy launched the Renewable Systems Interconnection (RSI) study during
the spring of 2007. This study addresses the technical and analytical challenges that must be
addressed to enable high penetration levels of distributed renewable energy technologies.
Because integration-related issues at the distribution system are likely to emerge first for PV
technology, the RSI study focuses on this area. A key goal of the RSI study is to identify the
research and development needed to build the foundation for a high-penetration renewable
energy future while enhancing the operation of the electricity grid.
The RSI study consists of 15 reports that address a variety of issues related to distributed
systems technology development; advanced distribution systems integration; system-level
tests and demonstrations; technical and market analysis; resource assessment; and codes,
standards, and regulatory implementation. The RSI reports are:
Renewable Systems Interconnection: Executive Summary
Distributed Photovoltaic Systems Design and Technology Requirements
Advanced Grid Planning and Operation
Utility Models, Analysis, and Simulation Tools
Cyber Security Analysis
Power System Planning: Emerging Practices Suitable for Evaluating the Impact of
High-Penetration Photovoltaics
Distribution System Voltage Performance Analysis for High-Penetration
Photovoltaics
Enhanced Reliability of Photovoltaic Systems with Energy Storage and Controls
Transmission System Performance Analysis for High-Penetration Photovoltaics
Solar Resource Assessment
Test and Demonstration Program Definition
Photovoltaics Value Analysis
Photovoltaics Business Models
iii
Production Cost Modeling for High Levels of Photovoltaic Penetration
Rooftop Photovoltaics Market Penetration Scenarios.
Addressing grid-integration issues is a necessary prerequisite for the long-term viability of the
distributed renewable energy industry, in general, and the distributed PV industry, in particular.
The RSI study is one step on this path. The Department of Energy is also working with
stakeholders to develop a research and development plan aimed at making this vision a reality.
iv
Acknowledgments
Reigh Walling of Power Systems Energy Consulting pointed out a significant deficiency
in the earlier version of this work. His firm grounding in reality and candid criticism are
gratefully acknowledged.
v
Executive Summary
Currently, electrical distribution systems are designed and operated based on the
assumption of centralized generation, with the corollary that the power always flows
from the distribution substation to the end-use customers. With the increasing penetration
of residential and commercial PV, the PV power generation could not only offset the
load, but could also cause reverse power flow through the distribution system. Significant
reverse power flow may cause operational issues for the traditional distribution system,
including:
Over-voltage on the distribution feeder (loss of voltage regulation).
Increased short circuit currents, potentially reaching damaging levels.
Protection desensitization and potential breach of protection coordination.
Incorrect operation of control equipment that may lead to an increase in the
number of operations and related equipment wear, or to further aggravation of
problems that affect more equipment and more customers.
Among all the potential problems that may be caused by the high penetration of PV,
voltage regulation is the most likely one, because it is directly correlated to the amount of
reverse power flow. This study was carried out to investigate the impact of different
penetration levels of PV on the feeder voltage profile and on the equipment commonly
used for feeder voltage regulation. The flow of reactive and active power on the feeder
was investigated with different assumptions of inverter participation, and with various
assumptions about the coordinated control of inverters and utility equipment.
A representative distribution feeder with a selection of typically used equipment was
selected from a previous NREL study4. This feeder included commercial and residential
loads. Tap-changing transformers and switched capacitors were applied at the substation
and along the feeder. The model was further refined by explicitly representing the low
voltage service transformers and the secondary circuits to which distributed PV
generation is connected.
A series of case studies was conducted with different penetrations of PV, assuming
several commonly used voltages regulation methods. The study results show:
Reverse power flow at all studied PV penetration levels can be accommodated
using traditional utility equipment with, perhaps, modified controls.
Voltage rise on the secondary circuits is significant, and it should be included in
the analysis. Establishing a communication link between service points (customer
meter connections) and the utility equipment is helpful as it enables explicit
control over the worst-case voltage.
In all the cases studied, PV inverters can positively contribute to feeder voltage regulation
and result in an improved voltage profile. At a high enough penetration, PV inverters may
be able to replace all voltage regulation equipment on a feeder. The study results show:
vi
At a low PV penetration level (5%), inverters do not make a significant impact on
the feeder’s voltage regulation during peak load.
At a medium PV penetration level (10%), inverter voltage support can help
reduce the size of the voltage support capacitors by nearly 40%.
At high PV penetration levels (30% – 50%), PV inverters might entirely displace
voltage support capacitors.
At higher penetration levels, inverter-coupled PV generation displaced some of the
conventional generation. In order to match the performance of the inverters to that of
conventional generators, inverters have to be able to exchange reactive power with the
system. To allow for this, the following is needed:
Current interconnection requirements need to be evolved.
Inverter ratings need to be increased to allow for reactive power capability at all
levels of power output. This can be stipulated by interconnection requirements or
by carefully designed incentive programs.
The operation of inverters has to be coordinated with the control of traditional
voltage control equipment to take full advantage of the available reactive power
capabilities of the inverters.
Recommendations for future research in this field are:
Develop a set of recommended practices to reconcile existing feeder voltage
control techniques with high penetrations of distributed PV. This work illustrates
several such cases, but it is limited to one feeder topology. A more comprehensive
coverage of feeder topologies would be beneficial.
Develop a set of recommended practices for modeling PV inverters for load flow
analysis and for other relevant planning purposes, such as short circuit current
calculations. At the same time, expand the number of possible control options for
traditional equipment in the analysis software. This would result in a more
consistent understanding of the issues in the industry, and would simplify the test
case setup so that it is possible to evaluate any specific situation.
Create a set of benchmark cases to facilitate testing the models and the associated
software. Some of the confusion and unwarranted concerns about the impact of
PV generation may be a result of inconsistent and incorrect modeling.
Develop automated screening tools that will enable evaluation of the impact of
PV on the distribution system; all prospective installations could then be screened
and only the ones requiring more detailed assessment would be floated up to the
utilities. This would help preserve low installation costs while allowing for the
more detailed assessment necessary at higher penetration levels of distributed PV.
vii
Develop functional requirements for communication infrastructure that will
enable the coordinated operation of all equipment on the distribution feeder. The
same infrastructure can be used to enable demand side management,
implementation of flexible metering tariffs, and enhanced distribution system
management.
Fund demonstration opportunities that illustrate feeder operation with significant
PV penetration.
viii
Table of Contents
1.0 Introduction ...........................................................................................................................................1
2.0 Current Practice ....................................................................................................................................2
2.1 Distribution System Voltage Control Requirements ........................................................2
2.2 Voltage Regulation Methods ............................................................................................3
2.2.1 On-Load Tap-Changing (OLTC) Transformers / Voltage Regulators .......................4
2.2.2 Switched Capacitor (SC).............................................................................................4
2.3 Inverters’ Reactive Power Support ...................................................................................5
3.0 Project Approach ..................................................................................................................................6
3.1 Analysis Approach............................................................................................................6
3.2 Model Development..........................................................................................................6
3.2.1 Distribution Feeder Model..........................................................................................6
3.2.2 Component Models.....................................................................................................8
3.2.2.1 Primary Circuit.........................................................................................................8
3.2.2.2 Load .........................................................................................................................9
3.2.2.3 Secondary Circuit.....................................................................................................9
3.2.2.4 Photovoltaic .............................................................................................................9
3.2.2.5 PV inverter Reactive Power (VAR) Support ...........................................................9
3.2.2.6 OLTC Transformer and SVR...................................................................................9
4.0 Project Results ....................................................................................................................................10
4.1 Baseline...........................................................................................................................10
4.1.1 First Baseline Configuration.....................................................................................10
4.1.2 Second Baseline Configuration.................................................................................12
4.2 Description of the Issue ..................................................................................................14
4.3 Results of Research.........................................................................................................14
4.3.1 Assumptions About PV Inverter Capabilities...........................................................15
4.3.2 Peak Load, 5%, 10%, 30% and 50% Penetration, OLTC + SVR, Inverters
Supplying Reactive Power........................................................................................16
4.3.3 Peak Load, 50% Penetration, OLTC, Inverters Supplying Reactive Power.............21
4.3.4 Power Export, 50% Penetration, OLTC, IEEE1547 Inverters..................................22
4.3.5 Power Export, 50% Penetration, OLTC, Inverters Controlling Feeder
Voltage......................................................................................................................24
4.3.6 Power Export, 50% Penetration, OLTC, Inverters Supplying Capacitive
Reactive Power .........................................................................................................25
4.3.7 Power Export, 50% Penetration, OLTC + SVR, Inverters Supplying
Capacitive Reactive Power .......................................................................................27
4.3.8 Power Export, 50% Penetration, OLTC, Inverters Controlling Total Service
Power Factor .............................................................................................................20
5.0 Conclusions and Recommendations................................................................................................31
6.0 References ...........................................................................................................................................33
7.0 Glossary ...............................................................................................................................................34
ix
List of Figures
Figure 1. Voltage range limits used in this study according to Willis3.........................................3
Figure 2. Determining the inverter’s reactive power limits ..........................................................5
Figure 3. PSLF single line diagram of the distribution system model..........................................7
Figure 4. Illustration of Feeder 2 in the distribution system model ..............................................8
Figure 5. Baseline 1: voltage profile at peak load with the switched capacitor ..........................11
Figure 6. Baseline 1: power flow at peak load with the switched capacitor ...............................11
Figure 7. Baseline 2: voltage profile at peak load with SVR ......................................................12
Figure 8. Baseline 2: power flow at peak load with SVR ...........................................................13
Figure 9. Relationship between inverter size and its reactive power capability .........................16
Figure 10. 5% PV penetration voltage profile: peak load with SVR ............................................17
Figure 11. 5% PV penetration power flow: peak load with SVR .................................................17
Figure 12. 10% PV penetration voltage profile: peak load with SVR ..........................................18
Figure 13. 10% PV penetration power flow: peak load with SVR ...............................................18
Figure 14. 30% PV penetration voltage profile: peak load with SVR ..........................................19
Figure 15. 30% PV penetration power flow: peak load with SVR ...............................................19
Figure 16. 50% PV penetration voltage profile: peak load with SVR ..........................................20
Figure 17. 50% PV penetration power flow: peak load with SVR ...............................................20
Figure 18. 50% PV penetration voltage profile: peak load with inverter only .............................21
Figure 19. 50% PV penetration power flow: peak load with inverters only .................................22
Figure 20. 50% PV penetration: voltage profile at max reverse power flow with OLTC ............23
Figure 21. 50% PV penetration: power flow at max reverse power flow with OLTC .................23
Figure 22. 50% PV penetration voltage profile: inverter voltage support ....................................24
Figure 23. 50% PV penetration power flow: inverter voltage support .........................................25
Figure 24. 50% PV penetration voltage profile: inverter VAR support........................................26
Figure 25. 50% PV penetration power flow: inverter VAR support.............................................27
Figure 26. 50% PV penetration voltage profile: inverter VAR support with SVR.......................28
Figure 27. 50% PV penetration power flow: inverter VAR support with SVR............................28
Figure 28. 50% PV penetration voltage profile: inverter power factor correction........................29
Figure 29. 50% PV penetration power flow: inverter power factor correction.............................30
List of Tables
Table 1. ANSI C84.1 Voltage Range for 120V voltage level 2......................................................2
Table 2. Feeder Line Impedance.....................................................................................................8
Table 3. Feeder Conductor Selection..............................................................................................8
Table 4. Feeder Voltage Equipment ...............................................................................................8
Table 5. Variational space for case studies ...................................................................................15
Table 6. Reduction in feeder losses due to inverter Q support relative to baseline
configuration 2 ................................................................................................................16
x
1.0 Introduction
Solar photovoltaics (PV) are among the fastest growing energy sources in the world, with
annual growth rates of 25-35% over the last ten years. The markets for solar PV have
undergone a dramatic shift in the last five years. Prior to 1999, the primary market for PV
was in off-grid applications, such as rural electrification, water pumping, and
telecommunications. However, now over 78% of the global market is for grid-connected
applications where the power is fed into the electrical network1. Furthermore, most of the
new PV capacity has been installed in the distribution grid as distributed generation. As
the use of solar photovoltaics continues to expand, concern about its potential impact on
the stability and operation of the electricity grid grow as well. Utilities and power system
operators are preparing to integrate and manage more of this renewable electricity source
in their systems.
This study assesses the effects of a high penetration of distributed PV on the distribution
system voltage control, and on the associated reactive power flow through the
distribution system. A representative distribution system feeder that includes both
residential and commercial content was selected from a previous study4. The selected
feeder also includes voltage control equipment such as switched capacitors and on-load
tap-changing transformers. The model was further expanded to represent service
transformers and secondary circuits at the point of service entrance (the customer meter)
– the likely connection point of the PV inverter.
The report is organized as follows. In Section 2, the current practices used for distribution
voltage regulation are summarized, and the technical capabilities of inverters relevant to
feeder voltage control are presented. Section 3 introduces the analysis approach,
discusses modeling requirements, and presents the models used. Section 4 presents the
study results; it includes establishing the baseline and analyzes the impact of the different
PV penetration scenarios (i.e., 5%, 10%, 30%, and 50%) on the feeder voltage profile. It
also illustrates how the reverse power flow on the distribution feeder impacts the
operation of voltage regulation devices such as the on-load tap-changing transformer.
Case results exploring different voltage control options are presented and discussed.
Reactive power flow through the feeder is also analyzed using different assumptions for
inverter participation and for the coordination of inverter and feeder control. The study is
summarized in Section 5. Future research needs to explore the opportunity for optimal
distribution system design for high penetration of PV are also identified.
1
2.0 Current Practice
2.1 Distribution System Voltage Control Requirements
Voltage regulation is an important subject in electrical distribution engineering, because
it is the utility’s responsibility to keep the customers’ service voltage (the voltage at the
customer’s meter, or the load side of the point of common coupling (PCC)) within the
acceptable range. ANSI C84.1 specifies a guideline for this range, but the utilities have
the freedom to specify it differently based on their specific circumstances. ANSI C84.1
also specifies utilization voltage, which refers to the voltage at the point of use where the
outlet equipment is plugged in. Furthermore, two ranges are defined: Range A is
recommended for normal operating conditions, while Range B corresponds to unusual
conditions, during which the occurrence has to be limited in time duration and frequency.
Recommended service and utilization voltage limits according to ANSI C84.1 are shown
in Table 1. Utilities are generally concerned with maintaining the service voltage within
acceptable limits; the utilization voltage then follows automatically, provided that the
house wiring is done according to building codes.
Table 1. ANSI C84.1 Voltage Range for 120V voltage level 2
Service Utilization
Min Max Min Max
Range A
(Normal)
-5% +5% -8.3% +4.2%
Range B
(Emergency)
-8.3% +5.8% -11.7% +5.8%
Irrespective of actual adopted voltage limits (by ANSI C84.1 or by the individual utility),
most utilities control the service voltage indirectly by controlling the voltage on the
primary circuit, the feeder. Service voltage is directly dependent on feeder voltage; when
considered on the same voltage base, service voltage is equal to the feeder voltage minus
the voltage drop across the service transformer and the secondary circuit connection.
Consequently, it is possible to predict the service voltage based on the feeder voltage as
long as the service transformer and service runs have consistent parameters for all loads.
Utilities capitalize on this fact and develop internal design guidelines for sizing service
transformers and for deciding the size and length of a service connection. Following the
guidelines then enables them to eliminate the need to record the data related to the
secondary circuits, resulting in a substantial saving in database size. (Admittedly,
database size is not a significant factor nowadays, but these practices were developed
more than 50 years ago when computer resources were scarce.) Hence, with design
guidelines in place, service voltage is controlled indirectly by controlling the feeder
voltage.
2
Figure 1 shows an example of voltage limits for the primary circuit, the service entrance,
and utilization based on one utility’s guidelines3. It reflects the adjustment for
assumptions about additional voltage drop in the secondary circuit and allows for the
necessary margin. In this study, the primary voltage and service entrance voltage limits
shown in Figure 1 were used as target limits.
1.04
1.04
1.05
0.92
0.94
0.97
0.90
0.95
1.00
1.05
1.10 Primary Service Entrance Utilization
Figure 1. Voltage range limits used in this study according to Willis3
2.2 Voltage Regulation Methods
The voltage regulation practice applied to distribution systems is based on the radial
power flow from the substation to the load. Voltage drop along the distribution feeder is
inevitable and it is, in fact, required in order to move power from the substation to the
customers. Typically, two (boundary) operating conditions of the feeder are considered:
feeder voltage should not drop below the minimum during peak load condition, and it
should not exceed the maximum during light load condition.
Voltage drop on the feeder is a consequence of current flow and the impedance
(resistance and reactance) of the feeder conductor, transformer, and load. Loads require
active and reactive power, and the related current that supplies the active and reactive
power causes the voltage drop on feeder conductors. Feeder conductors are a given (they
are selected first, based on economic considerations*). With conductor sizes known (i.e.,
their circuit parameters are fixed), there are two fundamental ways to control the voltage
on the feeder: by using on-load tap-changing transformers, or by installing fixed or
switched capacitors to offset the reactive power demand from the load and thus reduce
* Larger conductors have lower voltage drop and lower power losses, but they also cost more, so there is a
tradeoff between savings due to lowered losses and increased costs of the conductors. Utilities commonly
have design guidelines that are based on underlying economic consideration, practicality of carrying
varying sizes of conductors in stock, and other considerations such as feeder pick up from an adjacent
substation for increased reliability.
3
the current flow through the feeder and the related voltage drop. These two methods are
discussed in more detail next.
2.2.1 On-Load Tap-Changing (OLTC) Transformers / Voltage Regulators
The on-load tap-changing transformer (OLTC), or voltage regulator, is an essential part
of a distribution network. Automatically adjustable OLTCs are commonly used at
distribution substations to raise the starting voltage for a feeder under load, so that some
point along the feeder has a desired voltage. The adjustment is proportional to the load,
so this practice works well for all anticipated loading conditions. This control strategy is
referred to as the “line drop compensation.” The amount of permissible voltage increase
is limited if there is a load (customer) near the voltage regulator, so in some cases
additional voltage regulators along the feeder run might be necessary.
Voltage regulators, or OLTCs, are typically constructed as autotransformers with
automatically adjusting taps. The controls measure the voltage and load current, estimate
the voltage at the remote (controlled voltage) point, and trigger the tap change when the
estimated voltage is out of bounds. Multiple tap change actions may be performed until
the voltage is brought within bounds. The taps typically provide a range of ±10% of
transformer rated voltage with 32 steps. Each step of voltage is therefore 0.625% of the
rated voltage.2
2.2.2 Switched Capacitor
It was already explained that loads require active and reactive power, and that the related
current causes the voltage drop on feeder conductors. The load's reactive power demand
can either be supplied from the substation or by inserting capacitor banks along the
feeder. The reactive power supplied by the capacitor banks offsets the reactive power of
the load and consequently reduces the amount that needs to come from the substation and
the associated voltage drop. The capacitor banks can be fixed (permanently connected) or
switched (connected when needed), so that their supplied reactive power matches the
need of the load. In practical installations this matching is seldom perfect, because the
load and its reactive power demand vary continuously while the capacitor banks are
switched in chunks. Moreover, the reactive power from capacitors varies with voltage
squared, and drops off at low voltages when it is most needed. Overcompensation of the
feeder (associated with too much capacitance) leads to voltage rise on the feeder, and it
might require the voltage regulator in the substation to take action to lower the voltage to
accommodate the rise due to overcompensation by the capacitors.
The controls used for switching capacitor banks can be based on: a time clock (load is
correlated with time of day); the temperature (heavy load such as air-conditioning is
correlated with ambient temperature); the voltage (low feeder voltage is an indication of
the heavy load); the reactive power flow (to balance the reactive power actually drawn by
the load); or the feeder current (similar to reactive power control, but less expensive to
implement)2.
4
2.3 Inverters’ Reactive Power Support
Currently, standards such as IEEE 1547 and UL1741 state that the PV inverter “shall not
actively regulate the voltage at the PCC.” Therefore, PV systems are designed to operate
at unity power factor (i.e., they provide only active power) because this condition will
produce the most real power and energy. This limitation is a matter of agreement, not a
technical one; many inverters have the capability of providing reactive power to the grid
in addition to the active power generated by their PV cells. This is illustrated in Figure 2.
The inverter's ratings are represented by a vector with magnitude S; the semicircle with
radius S denotes the boundary of the inverter’s feasible operating range in PQ space.
Assuming that the power produced by PV array is Ppv, the feasible operating space
reduces to the red line denoted by Ppv or, more precisely, to the segment of the red line
delimited by its intersection points with the semicircle. Reactive power (Q) limits are
then found by projecting the end points of the segment down to the Q axis; the values are
labeled -Qlimit and Qlimit. It follows that the inverter can supply positive and negative
reactive power, that is, it can behave as both an inductor and a capacitor. The advantage
of an inverter relative to a fixed capacitor is that it can vary the supplied reactive power
continuously. The amount of Q available from the inverter depends on its ratings (S) and
the active power supplied by the PV array. Consequently, the inverter can use its entire
rating to supply Q if Ppv equals zero (there is no sun), and at the other extreme, it has no
Q capability if Ppv equals S. Some Q capability can always be retained by over-sizing the
inverter; this will be discussed in a later section. Note that this is for a unidirectional grid-
connected PV inverter. Inverters connected to energy storage may allow for full four-
quadrant charging and discharging of real and reactive power. In addition to the
continuous reactive power support, inverters can operate very fast (milliseconds to
microseconds with high switching frequency inverters) in comparison with capacitors,
which can cause switching transients.
P
pv
S
-Q
limit
Q
limit
Figure 2. Determining the inverter’s reactive power limits
A number of publications are available that address the benefits of using inverter-based
distributed generation for voltage support, their challenges, and their potential solutions. 8,9
IEEE 1547 Standard for Interconnecting Distributed Resources with Electric Power Systems, 2003.
5
3.0 Project Approach
3.1 Analysis Approach
The approach used in this study involves the following procedure:
1. Select and model a representative distribution feeder with various typical voltage
support equipment.
2. Refine the model by adding a low-voltage service transformer and a secondary circuit.
3. Conduct load flow simulation of various scenarios, including peak load and various
PV penetrations with maximum reverse power flow.
4. Review feeder voltage profile, the active and reactive power flows, and the impact of
various control configurations.
3.2 Model Development
A representative distribution system was based on a previous distributed generation study
conducted by General Electric under the contract with NREL 4. The selected system
includes the most commonly used distribution system components that are important for
investigation of voltage regulation; there are voltage regulators at the substation and
along the feeder, switched capacitors, and distribution transformers. In order to explicitly
investigate the voltage at the customer service entrance service transformers and
secondary circuits were added to the original model. The model is suitable for
examination of equipment interaction and the impact on the feeder voltage profile at
different PV penetration levels. The details of the feeder and the component models are
introduced in the following sections.
3.2.1 Distribution Feeder Model
The selected distribution system was modeled in PSLF 10. A one-line diagram of the
feeder as modeled in PSLF is shown in Figure 3. The assumptions are:
The considered distribution system is radial and supplied by a medium-voltage
transformer equipped with a tap changer;
The distribution system includes two main feeders with laterals and distributed
loads;
Distribution substation protections allow active and reactive power back feed;
Each load bus has a PV connected to it, with the size relative to the size of the
load on the same bus;
All the load buses are modeled as PQ buses;
The system base is 10 MVA;
Bus 999 represents the infinite bus and is the slack bus in the power flow model;
A loop, shown as a dashed connection between buses 108 and 208, can be closed
to link the two feeders. However, for this study, the loop is not connected.
6
Figure 3. PSLF single line diagram of the distribution system model
Although the entire system was modeled and analyzed, only the results from Feeder 2
(circled in red in Figure 3), which includes all the interested voltage regulation devices,
are illustrated in this report. Feeder 2 is described below.
It is about six miles in length.
Seven aggregated loads represent a mixture of residential load and commercial
loads ranging from 0.3 MW to 5 MW. The total load is 11 MVA.
The primary feeder voltage is 12.5 kV. The secondary voltages are 240 V for
residential loads and 600 V for commercial loads.
A switched capacitor is located at about 4.6 miles from the substation, at bus 205.
Two voltage regulators are employed – one in the substation and another at 2.6
miles from the substation, between buses 202 and 203. These two devices have
similar characteristics, but they are purposely labeled differently to simplify
notation in the following figures and text. The voltage regulator at the substation
is referred to as the on-load tap changer, abbreviated as OLTC, and the one along
the feeder is called the step voltage regulator, abbreviated as SVR.
Figure 4 shows an illustrative one-line diagram of Feeder 2.
7
Primary (main feeder)
Secondary
OLTC
201 202 203 204 205 206 207
SC
011 22 332 44 55 66
SV R
miles
OLTC: On-Load Tap Changer
SVR: St ep Vo l t a g e Re gu l a t or
SC: Switched Capacitor
Figure 4. Illustration of Feeder 2 in the distribution system model
3.2.2 Component Models
In this section, the models of the major components used in the study feeder are
introduced.
3.2.2.1 Primary Circuit
Table 2. Feeder Line Impedance
From To Length (mile) Conductor X/R
100 201 1.3 Z1 2
201 202 0.65 Z1 2
203 204 0.65 Z1 2
204 205 0.97 Z1 2
205 206 1.1 Z2 2
206 207 1.1 Z2 2
Table 3. Feeder Conductor Selection
Z1 0.648 Ohm/mile ACSR 556.6 18/1
Z2 0.768 Ohm/mile ACSR 266.8 18/2
Table 4. Feeder Voltage Equipment
Type From To Rating
Transformer 5 100 20 MVA
SVR 202 203 10 MVA
switched capacitor
(baseline 1)
205 6.2 MVAr
switched capacitor
(baseline 2)
205 2.5 MVAr
8
3.2.2.2 Load
Loads were modeled as a combination of 40% constant power (active and reactive) load,
and 60% of constant impedance load 3. The loads in the study feeder have the power
factor of 0.92, which is representative of the mixture of residential and commercial loads.
3.2.2.3 Secondary Circuit
In order to investigate the voltage performance at the customer service entrance the
secondary circuit, including the low voltage service transformers and the service feeder,
were added to the original feeder model.
The service transformers are rated at 1.5 pu relative to the load they serve, with an
impedance of 2.5% 2 and an X/R ratio of 1.5. The service feeders are typically 50 feet to
600 feet in length3. For this study, an average 200-foot feeder length was selected. The
impedance for the secondary feeder is calculated based on the conductor with 200 amps
of thermal capacity. To simplify the model the impedance of the service transformer and
the secondary feeder are added together and represented by the transformer impedance.
3.2.2.4 Photovoltaics
Grid-connected PV systems are designed to inject all of the real power produced by PV
modules; they control the power precisely regardless of the voltage level, so they are best
represented as negative constant power loads. The size of the negative PV load is defined
proportional to the actual load connected at the same bus, based on the penetration level.
3.2.2.5 PV inverter Reactive Power (VAR) Support
The PV inverter model is not readily available in PSLF. To represent the reactive power
capability of the inverters additional devices called Static VAR Devices (SVD), which
are available in the PSLF standard library, were used. In PSLF, SVDs can be configured
as switched or continuously controlled shunt elements whose admittance is adjusted in
order to regulate the voltage at a specified bus. For the purpose of this analysis, the
continuously controlled SVD model connected at each PV bus was used to simulate the
VAR support feature of the PV inverter.
Each SVD is given the reactive power limits to represent the VAR support capability of
the PV inverter at a studied condition. As was discussed in reference to Figure 2, the
reactive power capability is decided by the size of the PV inverter S, and the active power
output Ppv.
3.2.2.6 OLTC Transformer and SVR
The OLTC transformer and the SVR are modeled as tap-changing transformers that
monitor the voltage at a remote bus and regulate the voltage to the defined limits by
changing the turn ratio of the transformer. The tap range is ±10% of rated and the step
voltage is 0.625%. The controlled bus depends on the case studied and will be defined in
the specific case discussion.
9
4.0 Project Results
In this section, the results obtained for different steady state conditions on the feeder are
presented. Cases include different PV penetration scenarios combined with different control
configurations. First is a review of the baseline conditions – a feeder under the peak load.
4.1 Baseline
In normal operating conditions the feeder voltage decreases as the distance from the
distribution substation increases, and it may become lower than the voltage specified by
the utility’s guidelines. Two feeder configurations are reviewed for peak load, which
were chosen because of the different Q (reactive power) profiles on the distribution
feeder, with different voltage regulation devices and locations.
4.1.1 First Baseline Configuration
In the first baseline configuration, the OLTC regulates the service voltage of the last bus
on the feeder to the voltage limits. A capacitor bank is used during peak load conditions
to inject capacitive reactive power and to boost the voltage along the feeder. As shown in
Figure 5, the combined action of the OLTC and the switched capacitor ensures that the
voltages of all the buses at the primary feeder and the service entrances are within the
specified limits. OLTC action shifts the entire curve up or down, while the switched
capacitor raises the voltage at its bus.
The corresponding P and Q flow on the feeder is shown in Figure 6. P is fed from the
substation to the load; the step curve begins at approximately 11 MW, then steps down to
~9 MW at the first load bus, indicating that the load at this bus is ~2 MW. The P curve is
characterized by a soft negative derivative between steps; this is representative of losses
on the corresponding feeder segment. The Q curve, on the other hand, starts negative at
the substation, indicating that the feeder is overcompensated by the switched capacitor;
capacitive reactive power is delivered to the grid from this feeder. This is not an
indication of a problem. It is a peculiar feature of this operating point. The steps in the Q
curve are representative of reactive power consumption by the loads, which are generally
inductive, resulting in negative steps. The sudden jump in Q at the switched capacitor bus
shows the contribution of the switched capacitor. It overpowers the inductive
consumption of the load at the switched capacitor bus and results in a net Q increase of
more than 5 MVAr. Past this point, Q is consumed at load nodes; approximately 2 MVAr
are drawn by the load at the end of the feeder.
10
0.9
0.92
0.94
0.96
0.98
1
1.02
1.04
1.06
1.08
1.1
1.12
012345
Feeder Le ngth (mile)
Voltage (pu)
6
Sw i t c h e d
Capacitor
OLTC
Primary Feeder Voltage
Service Entrance Voltage
Primary Feeder Voltage Limits
Service Entrance Voltage Limits
Primary Feeder Voltage
Service Entrance Voltage
Primary Feeder Voltage Limits
Service Entrance Voltage Limits
Figure 5. Baseline 1: voltage profile at peak load with the switched capacitor
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
012345
Feeder Length (mile)
P Flow (MW) or Q Flow (MVar)
6
P
Q
Figure 6. Baseline 1: power flow at peak load with the switched capacitor
11
4.1.2 Second Baseline Configuration
The second baseline configuration demonstrates the operation of SVR. The voltage
regulation capability of SVR is constrained by the autotransformer tap limits; ± 10% was
used. These limits may result in the need to combine SVR with other equipment, which
was the case in this example. Hence, SVR is used along with the OLTC and the switched
capacitor as follows. The OLTC regulates the primary side of the SVR, the SVR
regulates the service voltage at the last load bus, and the switched capacitor is switched in
to offset the reactive power consumption of the load along the feeder. The corresponding
voltage profile is shown in Figure 7. At the discontinuity, where a step change in voltage
occurs at the node with the SVR, the voltage is “stepped up.” The contribution of the
switched capacitor is analogous to the one in the first baseline configuration, but the
capacitor is smaller, as will be discussed next.
The P and Q flows corresponding to the second configuration are shown in Figure 8. The
P flow matches the one before, so it does not need additional discussion. However, the Q
flow is markedly different. In this configuration, reactive power flows from the substation
into the feeder, and it gets used up gradually at each load point. There is also a noticeable
Q loss across the SVR. The SVR is the transformer, and reactive power is consumed on
its leakage impedance. Reactive power injection by the switched capacitor is evident
from the Q jump at the switched capacitor bus, but in this case net Q injection is less than
3 MVAr, compared with over 5 MVAr in the first configuration.
Both baseline configurations result in the feeder and service voltages being within the
limits, but the underlying circuit behavior is quite different as is apparent from the above
discussion.
12
0.9
0.92
0.94
0.96
0.98
1
1.02
1.04
1.06
1.08
1.1
1.12
012345
Feeder Len
6
g
th
(
mil e s
)
Voltage (pu)
Sw i t c h e d
Cap a c ito r
OLTC
Step Volt age
Reg u l a t o r
Primary Feede r Voltage
Service Entrance Voltage
Primary Feede r Voltage Limits
Service Entrance Voltage Limits
Primary Feede r Voltage
Service Entrance Voltage
Primary Feede r Voltage Limits
Service Entrance Voltage Limits
Figure 7. Baseline 2: voltage profile at peak load with SVR
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0123456
Fee der Length (mile)
P Flow (MW) or Q Flow (MVar)
P
Q
Figure 8. Baseline 2: power flow at peak load with SVR
13
4.2 Description of the Issue
Introducing PV at the load side reduces the load demand and in turn leads to reduced
losses and improved voltage profiles on the feeder. This is a fair observation as long as
the PV generation coincides with the substantial load demand so that the net power flow
remains from the substation to the load. As the penetration levels of PV rise, there may be
time periods during the day when the net power flow is from the load (distributed PV)
towards the substation a situation not normally anticipated in the distribution system
design. To illustrate this condition, consider a predominantly residential area with a
significant penetration of PV. PV production is generally at its optimum at approximately
11 a.m., which is generally a period of light load condition, so power export through the
distribution feeder and the substation back to the system becomes possible.
The associated reverse power flow tends to raise the voltage on the distribution feeder. The
two presented base cases demonstrate that voltage drop drives power flow, so if the power
flow reverses, the voltage slope will reverse as well. The obvious questions then are: will this
situation cause problems, and how will the distribution circuit behave under these conditions?
These issues are discussed at various levels of PV penetration, combined with different
assumptions about feeder equipment and different roles for inverters in feeder voltage
control. To reduce data requirements the assumption was made that feeder load is zero
during PV generation. This may appear unrealistic, but it is a matter of defining
penetration. Assuming that 5% penetration causes 5% power export is equivalent to
assuming that 10% penetration, combined with 5% load, results in a net export of 5%.
Although estimating feeder load during optimal insolation (peak PV production) would
require significant data gathering, this assumption, while arguably incorrect, is
pessimistic relative to the study of the feeder voltage profile. It results in worse voltage
conditions.
4.3 Results of the Research
As indicated in Table 5, there are many possible variations of options for analyzing
feeder voltage control. The full set of variations includes 192 cases, but not all
combinations are meaningful. For example, it does not make sense to combine reverse
power flow with reactive compensation using the switched capacitor, as it would lead to
voltage problems. The number of options was carefully reduced to include “only” 56
cases that were then set up and evaluated. However, presenting all these results does not
add value, as the conclusions often repeat between similar cases. The report was therefore
reduced to the following cases.
Peak load cases are presented first under assumptions of different PV penetration using
the inverters for reactive power support – beyond their current role prescribed by IEEE
1547. After that, reverse power flow on the feeder for 50% PV penetration is presented
by combining two options of voltage regulators with four options of inverter
participation.
Since the role of the inverter is crucial for this discussion, assumptions about inverter
capabilities are defined first in the next section.
14
Table 5. Variational space for case studies
Variable Range Number of
options
PV penetration 5, 10, 30, 50% 4
Feeder load 0, 100% 2
PV inverter participation IEEE 1547, voltage control, max Q, PF
control
4
Voltage regulators OLTC, OLTC + SVR, 2
Switched capacitor 0, base line 1, base line 2 3
4.3.1 Assumptions About PV Inverter Capabilities
As discussed in Section 2.3, inverters have the capability to supply inductive and
capacitive reactive power to the grid, and this ability is limited only by their ratings. At
the present time, inverters are not allowed to provide reactive power, so the
manufacturers select their ratings to be equal to the maximum production of the
connected PV modules. In terms of our previous discussion, S = PPVmax. On the other
hand, when PPV = 0 (no sun) the entire inverter capacity can be dedicated to reactive
power support at essentially no extra cost, so it is often argued that the standards should
evolve to allow reactive power support. Evolution of standards will be required at higher
levels of PV penetration, so it is logical to assume that inverters could be allowed to
provide reactive power to regulate voltage. Other questions also emerge. Most notably,
how should this capability be used during maximum power production, and what is the
reasonable increase in ratings to provide reactive power support?
To provide reactive power injection while supplying maximum active power from PV
modules, it is necessary to increase the inverter size. Figure 9 illustrates the active and
reactive power capability of the inverter versus the size. As shown in the figure, by
increasing the inverter size by 10%, making S = 1.1PPVmax, the reactive power capability
can be increased from zero to nearly 46% in the maximum PV power generation
condition. This will give the power factor range of unity to 0.91 leading/lagging. The Q
capacity during no sun condition is then 110%.
In all the studied cases, inverters with 10% increased ratings were used, to allow for
ample reactive power capability.
* Some inverters are highly optimized for efficiency, and are not capable of Q injection, but this beyond the
scope of this discussion.
15
Figure 9. Relationship between inverter size and its reactive power capability
4.3.2 Peak load, 5%, 10%, 30% and 50% Penetration, OLTC + SVR, Inverters
Supplying Reactive Power
The reactive power capabilities of PV inverters can be used to offset the reactive load.
This reduces the reactive power flow on the distribution feeder, and in turn reduces the
voltage drop along the feeder. Inverters are configured to supply reactive power, and their
output capacity Q is equal to their ratings. As in baseline configuration 2, OLTC controls
the primary side of SVR, and SVR controls the service voltage at the last customer. The
feeder’s voltage profile and the associated active and reactive power flows for penetration
levels from 5% to 50% are shown from Figure 10 through Figure 17.
The performance is similar to the performance discussed for baseline configuration 2, so
only the differences will be highlighted. Notice that the amount of required reactive
compensation by the switched capacitor progressively reduces as the penetration level
increases. At 30% penetration, reactive power injected by the switched capacitor almost
completely diminishes, and none is necessary for 50% PV penetration. The amount of
reactive power supplied by the substation is also progressively lower with increased
levels of PV penetration; this results in lower current through the feeder and,
consequently, in lower I2R and I2X power losses. Compared with baseline configuration
2, losses are reduced as summarized in Table 6. Note that since the inverter losses were
neglected, these results are somewhat optimistic. A more detailed evaluation of losses
will be a topic of future research.
Table 6. Reduction in feeder losses due to inverter Q support relative
to baseline configuration 2
PV penetration [%] Loss reduction relative to
baseline configuration 2 [%]
5 2
10 2.5
30 3.5
50 7
16
0.9
0.92
0.94
0.96
0.98
1
1.02
1.04
1.06
1.08
1.1
1.12
012345
Feeder Length (mile)
Voltage (pu)
Prim ary Feeder Voltage
Service Entrance Voltage
Prim ary Feeder Voltage Lim its
Ser vice Ent rance Volt age Li mits
6
Prim ary Feeder Voltage
Service Entrance Voltage
Prim ary Feeder Voltage Lim its
Ser vice Ent rance Volt age Li mits
Sw i t c h e d
Ca p a c it o r
St ep Volt age
Re g u la t o r
OLTC
Figure 10. 5% PV penetration voltage profile: peak load with SVR
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
01234 56
Feeder Length (mile)
P Flow (MW) or Q Flow (MVar)
P
Q
Figure 11. 5% PV penetration power flow: peak load with SVR
17
0.9
0.92
0.94
0.96
0.98
1
1.02
1.04
1.06
1.08
1.1
1.12
012345
Feeder Length (mile)
Voltage (pu)
6
Sw i t c h e d
Cap a c it o r
Primary Feeder V oltage
Ser vice Entranc e Voltage
Primary Fee de r Voltage Limits
Ser vice Entranc e V oltage Li mits
Primary Feeder V oltage
Ser vice Entranc e Voltage
Primary Fee de r Voltage Limits
Ser vice Entranc e V oltage Li mits
OLTC
Step Volt ag e
Re g u la t o r
Figure 12. 10% PV penetration voltage profile: peak load with SVR
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0123456
Fee de r Le ngth (mile)
P Flow (MW) or Q Flow (MVar)
P
Q
Figure 13. 10% PV penetration power flow: peak load with SVR
18
0.9
0.92
0.94
0.96
0.98
1
1.02
1.04
1.06
1.08
1.1
1.12
01234 5
Feeder Le ngth (mile)
Voltage (pu)
6
St ep Voltage
Regulator
Primary Feeder Voltage
Service Entrance Voltage
Primary Feeder Voltage Limits
Ser vice Ent rance V oltage Li mits
Primary Feeder Voltage
Service Entrance Voltage
Primary Feeder Voltage Limits
Ser vice Ent rance V oltage Li mits
OLTC
Figure 14. 30% PV penetration voltage profile: peak load with SVR
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0123456
Feeder Length (mile)
P Flow (MW) or Q Flow (MVar)
P
Q
Figure 15. 30% PV penetration power flow: peak load with SVR
19
0.9
0.92
0.94
0.96
0.98
1
1.02
1.04
1.06
1.08
1.1
1.12
0123456
Feeder Length (mile)
Voltage (pu)
Primary Fee de r Volt age
Service E ntr ance Voltage
Primary Fee de r Volt age Limits
Service E ntr ance Voltage Limits
Primary Fee de r Volt age
Service E ntr ance Voltage
Primary Fee de r Volt age Limits
Service E ntr ance Voltage Limits
OLTC
St ep Vo lt a g e
Re g u la t o r
Figure 16. 50% PV penetration voltage profile: peak load with SVR
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0123456
Fee der Length (mile)
P Flow (M W) or Q Flow (MVar)
P
Q
Figure 17. 50% PV penetration power flow: peak load with SVR
20
4.3.3 Peak Load, 50% Penetration, OLTC, Inverters Supplying
Reactive Power
Similar performance is observed when the inverters are used to supply reactive power in
the circuit of baseline configuration 1. For brevity, only the 50% penetration case is
presented. Here too, the reactive power flow through the feeder is reduced significantly,
minimizing power losses on the feeder. In this case, the primary circuit voltage is slightly
below the limit at the last load – this is not important since the service voltage is still
maintained within the desired range, and the primary circuit limits are only a guideline.
The voltage profile on the feeder is show in Figure 18 and the corresponding P and Q
flows are shown in Figure 19.
0.9
0.92
0.94
0.96
0.98
1
1.02
1.04
1.06
1.08
1.1
1.12
012345
Feeder Length (mile)
Voltage (pu)
6
Primary Feeder Voltage
Service Entr ance Voltage
Primary Feeder Voltage Limits
Service Entr ance Voltage Limits
Primary Feeder Voltage
Service Entr ance Voltage
Primary Feeder Voltage Limits
Service Entr ance Voltage Limits
OLTC
Figure 18. 50% PV penetration voltage profile: peak load with inverter only
21
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
012345
Feeder Length (mile)
P Flow (MW) or Q Flow (MVar)
6
P
Q
Figure 19. 50% PV penetration power flow: peak load with inverters only
4.3.4 Power Export, 50% Penetration, OLTC, IEEE 1547 Inverters
This is the second part of the case study, and deals with power export through the feeder.
Work begins with an inverter that is compliant with IEEE 1547, and combined with an
OLTC that is controlling the voltage at the service connection of the last load. The
voltage profile is shown in Figure 20, and the corresponding active and reactive power
flows are shown in Figure 21.
Relative to baseline configuration 1, a switched capacitor is not used, and the OLTC
lowers the voltage at the substation to allow for the rise due to reverse power flow. As
expected, voltage rises with increased distance from the substation, since the power flow
is now towards the substation not away from it. This negative power flow is also
indicated in Figure 21. Starting from the feeder’s end, power flow is progressively more
negative as generation is collected by distributed PV sources. Notice that the slope of the
power curve between the nodes continues to be negative, because losses are still in the
“same direction,” as in the baseline cases.
The resulting Q flow is relatively low; Q supplied from the substation end covers only
feeder Q losses, since the inverters operate at unity power factor.
This is a simple and effective measure, and the performance of the feeder is acceptable.
22
0.9
0.92
0.94
0.96
0.98
1
1.02
1.04
1.06
1.08
1.1
1.12
012345
Feeder Length (mile)
Voltage (pu)
6
OLTC
Primary Feeder Voltage
Service Entrance Voltage
Primary Feeder Voltage Limits
Ser vice Entrance V oltage Limits
Primary Feeder Voltage
Service Entrance Voltage
Primary Feeder Voltage Limits
Ser vice Entrance V oltage Limits
Figure 20. 50% PV penetration: voltage profile at max reverse power flow with OLTC
-6.0
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
0123456
Feeder Length (mile)
P Flow (MW) or Q Flow (MVar)
P
Q
Figure 21. 50% PV penetration: power flow at max reverse power flow with OLTC
23
4.3.5 Power Export, 50% Penetration, OLTC, Inverters Controlling
Feeder Voltage
In the following cases, the role of inverters with capabilities beyond the IEEE 1547
requirements is reviewed and compared with cases studied in previous sections.
If the requirement for the unity power factor operation of inverters were removed, the
instinctive reaction would be to allow inverters to control the voltage at their terminals.
This case is representative of such a strategy. The voltage profile is shown in Figure 22
and the active and reactive power flow is shown in Figure 23. The resulting voltage
profile is nearly flat, as was desired, and active power flow is as it was in the previous
case. Reactive power flow is, however, interesting relative to the previous case. The
inverters are trying to reduce the voltage, and they accomplish this by absorbing reactive
power, i.e., by operating as inductors. The resulting inductive loading of the feeder is
substantial, as indicated by the approximately 1.5 MVAr supplied from the substation.
This is detrimental in two ways: first, feeder losses are increased due to unnecessary
reactive power flow, and second, the reactive power demand on the transmission system
is increased because of the use of distributed PV. Most traditional generators are based on
synchronous machines, and they normally supply reactive power to the system, and do
not absorb it. It would be beneficial to change the control strategy for the inverter and
combine it with feeder voltage control to allow for net export of reactive power. This is
discussed in the next section.
0.9
0.92
0.94
0.96
0.98
1
1.02
1.04
1.06
1.08
1.1
1.12
0123456
Feeder Length (mile)
Voltage (pu)
Primary Fee der Voltage
Service Entrance Voltage
Primary Fee der Voltage Limits
Service Entrance Voltage Li mits
Primary Fee der Voltage
Service Entrance Voltage
Primary Fee der Voltage Limits
Service Entrance Voltage Li mits
OLTC
Figure 22. 50% PV penetration voltage profile: inverter voltage support
24
-6.0
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
0123456
Feeder Length (mile)
P Flow (MW) or Q Flow (MVar)
P
Q
Figure 23. 50% PV penetration power flow: inverter voltage support
4.3.6 Power Export, 50% Penetration, OLTC, Inverters Supplying Capacitive
Reactive Power
In this case, the control of the inverters is combined with the control of the OLTC in
order to allow for Q export from the feeder.
Distributed PV generation is envisioned to displace some of the conventional generators
in the near future. An important feature of conventional generators is their ability to
provide reactive power to the system. Conventional generators can generate or absorb
reactive power within the limits of the exciter, and they are normally configured to
supply reactive power. The PV inverters have a similar reactive power capability. PV
inverters generally have analogous capabilities, but since they are connected to the
distribution system their operation has to be coordinated with the feeder voltage control
in order to allow net reactive power export.
This case illustrates a control strategy that allows net export; the OLTC now reduces the
output of the substation to the lower limit and the inverters raise the service voltage to the
upper limit. The resulting voltage profile is shown in Figure 24, and the active and
reactive power flows are shown in Figure 25.
As shown in Figure 25, net Q export is accomplished, but it is hampered by the voltage
constraints on the feeder. Specifically, the Q capability of the equivalent inverter at the last
node is above 1 MVAr and it supplies less than 0.5 MVAr. This is a consequence of voltage
25
limitations and feeder topology, and given the availability of the voltage control equipment,
nothing can be done to increase Q supply from the last node. An important conclusion is
that over-sizing the inverters to provide Q support does not always help, as it can be
restrained by the inverter placement on the feeder and available voltage control equipment.
In the next case, a way to remedy this limitation is illustrated. SVR is used to increase Q
flow through the feeder.
0.9
0.92
0.94
0.96
0.98
1
1.02
1.04
1.06
1.08
1.1
1.12
0.0 1.0 2.0 3.0 4.0 5.0 6.0
Feeder Length (miles)
Voltage (pu)
Primary Fee de r Volt age
Service E ntr ance Voltage
Primary Fee de r Volt age Limits
Service E ntr ance Voltage Limits
Primary Fee de r Volt age
Service E ntr ance Voltage
Primary Fee de r Volt age Limits
Service E ntr ance Voltage Limits
OLTC
Figure 24. 50% PV penetration voltage profile: inverter VAR support
26
-6.0
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
012345
Feeder Length (mile)
P Flow (MW) or Q Flow (MVar)
6
P
Q
Figure 25. 50% PV penetration power flow: inverter VAR support
4.3.7 Power Export, 50% Penetration, OLTC + SVR, Inverters Supplying
Capacitive Reactive Power
In this case, the coordinated action of the OLTC, the SVR, and the inverters is used to
maximize Q export from the feeder and use up all the available Q capacity from the
inverters. The resulting voltage profile is shown in Figure 26, and the corresponding P
and Q flows are shown in Figure 27.
The Q export to the feeder is now maximized. It is slightly over 2 MVAr at the
substation, and the feeder and service voltages still remain within the desired limits.
Conceptually, this is a simple modification of the circuit, and it can be generalized to
maximize export of Q on any feeder regardless of length. Using such a control strategy
enables the full utilization of the inverters’ Q capabilities, and enables distributed PV to
displace conventional generation without compromising performance.
27
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0.98
1
1.02
1.04
1.06
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1.1
1.12
012345
Feeder Length (miles)
Voltage (pu)
Primary Feeder Voltage
Service Entrance Voltage
Prim ary Feede r Voltage Lim its
Service Entrance Voltage Lim its
6
Primary Feeder Voltage
Service Entrance Voltage
Prim ary Feede r Voltage Lim its
Service Entrance Voltage Lim its
OLTC St ep Volt age
Re g u la t o r
Figure 26. 50% PV penetration voltage profile: inverter VAR support with SVR
-6.0
-5.0
-4.0
-3.0
-2.0
-1.0
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012345
Feeder Length (mile)
P Flow (M W) or Q Flow (MVar)
6
P
Q
Figure 27. 50% PV penetration power flow: inverter VAR support with SVR
28
4.3.8 Power Export, 50% Penetration, OLTC, Inverters Controlling Total
Service Power Factor
Finally, the inverters are used to control the power factor at their total service connection,
making the connection slightly capacitive in order to cover for the reactive drop through
the service transformer and the corresponding feeder impedance.
Figure 28 and Figure 29 show the resulting voltage profile and the P Q flow of the study
feeder. This case demonstrates that the inverters can achieve approximately unity PF at
the feeder connection to the substation – a slightly better performance relative to the
IEEE 1547 case, where reactive power flow was supplied from the substation.
Another very important characteristic of this control strategy, relative to most of the
others presented, is that it can be implemented using local measurements only. To
illustrate some of the control schemes, voltage magnitude was used at the service
entrance of the last load to control the OLTC. Such setup requires a point-to-point
communication link, or if a more general setup is desired, a communication link should
exist between the OLTC and all of the service voltage connections to allow for control
based on the worst-case voltage. Much work is needed to come up with an optimal
communication infrastructure to ensure effectiveness without excessive costs.
0.9
0.92
0.94
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1
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0123456
Feeder Length (mile)
Voltage (pu)
Primary Feeder Voltage
Service Entrance Voltage
Primary Fee de r Voltage Lim its
Service Entrance Voltage Li mits
Primary Feeder Voltage
Service Entrance Voltage
Primary Fee de r Voltage Lim its
Service Entrance Voltage Li mits
OLTC
Figure 28. 50% PV penetration voltage profile: inverter power factor correction
29
-6.0
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-4.0
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Feeder Length (mile)
P Flow (MW) or Q Flow (MVar)
6
P
Q
Figure 29. 50% PV penetration power flow: inverter power factor correction
30
5.0 Conclusions and Recommendations
In this study, a representative distribution feeder was selected for analyzing the impact of
different penetrations of PV on the feeder voltage regulation. Commonly used voltage
regulation equipment, such as a step voltage regulator and a switched capacitor, were
applied individually and in combinations to create different feeder configurations. Two
extreme scenarios, peak load and maximum reverse power flow, were simulated in order
to study the feeder’s voltage performance under different conditions.
A series of case studies was conducted with different penetrations of PV, each assuming
several commonly used voltage regulation methods. The study results show:
Reverse power flow at all studied PV penetration levels can be accommodated
using traditional utility equipment with perhaps modified controls.
Voltage rise on the secondary circuits is significant and it should be included in
the analysis. Establishing a communication link between service points (customer
meter connections) and the utility equipment is helpful as it enables explicit
control over the worst-case voltage.
In all the cases studied, PV inverters can positively contribute to the feeder voltage
regulation and result in an improved voltage profile. At a high enough penetration, PV
inverters may be able to provide feeder voltage support. The study results show:
At a low PV penetration level (5%), inverters do not make a significant impact on
the feeder’s voltage regulation during peak load.
At medium a PV penetration level (10%), inverter voltage support can help
reduce the size of the conventional voltage support capacitors by nearly 40%.
At high PV penetration levels (30% – 50%), PV inverters might be sufficient to
provide all of the feeder voltage support.
As the inverter-coupled PV sources displace conventional generation, they will also have
to match most of their performance characteristics. With respect to reactive power supply
to the system, PV inverters are disadvantaged because their reactive power injection may
be limited by the feeder voltage limits. This can be resolved by coordinated control of
utility equipment and inverters, and in some cases additional utility equipment might be
needed to take full advantage of the inverters’ reactive power capabilities.
Recommendations for future research in this field are:
Develop a set of recommended practices for reconciling existing feeder voltage
control techniques with high penetration of distributed PV. This work illustrates
several such cases, but it is limited to one feeder topology. A more comprehensive
coverage of feeder topologies would be beneficial.
Develop a set of recommended practices for modeling PV inverters for load flow
analysis and for other relevant planning purposes, such as short circuit current
31
calculations. At the same time, expand the number of possible control options for
traditional equipment in the analysis software. This would result in a more
consistent understanding of the issues across the industry and would simplify the
test case setup to evaluate any specific situation.
Create a set of benchmark cases to facilitate testing the models and the associated
software. Some of the confusion and unwarranted concerns about the impact of
PV generation may be a result of inconsistent and incorrect modeling.
Develop automated screening tools that will enable evaluation of the impact of
PV on the distribution system; all prospective installations could then be screened
and only the ones requiring more detailed assessment would be floated up to the
utilities. This would help preserve low installation costs while allowing for the
more detailed assessment that would be necessary at higher penetration levels of
distributed PV.
Develop functional requirements for a communication infrastructure that will
enable coordinated operation of all equipment on the distribution feeder. The
same infrastructure can be used to enable demand-side management, the
implementation of flexible metering tariffs, and enhanced distribution system
management.
Fund demonstration opportunities that illustrate feeder operation with significant
PV penetration.
32
6.0 References
1. “The Potential of Solar PV in Ontario”, Rob McMonagle, The Canadian Solar
Industries Association, January 30, 2006
2. “Electric Power Distribution Handbook”, Tom Short
3. “Power Distribution Planning Reference Book”, H. Lee Willis
4. “Reliable, Low Cost Distributed Generator/Utility System Interconnect, 2001 Annual
Report”, GE Corporate Research and Development, August 2003, NREL/SR-560-
5. “Study of the Impact of PV Generation on Voltage Profile in LV Distribution
Networks”, S. Conti, et. al., IEEE Porto Power Tech Conference, September, 2001, Porto,
Portugal
6. “IEEE Standard for Interconnecting Distributed Resources with Electric Power
Systems”, IEEE 1547 Standard, 2003.
7. “IEEE-1547 Comments”, Idaho Power Company, Oregon PUC Technical Workshop,
Oct. 2006,
http://www.puc.state.or.us/PUC/admin_rules/workshops/interconnection/ipcoct18.pdf
8. “Voltage regulation - Tapping Distributed Energy Resources”, John D. Kueck, et. al.,
Public Utilities Fortnightly, Sept. 2004
9. “Dynamic Voltage Regulation Using Distributed Energy Resources”, Yan Xu, et. al.,
19th International Conference on Electrical Distribution (CIRED), May 07
10. “PSLF - Power System Analysis Software”,
http://www.gepower.com/prod_serv/products/utility_software/en/downloads/pslf05.pdf
33
7.0 Glossary
PV Photovoltaic
ANSI American National Standard Institution
TC Tap Changer
SVR Step Voltage Regulator
OLTC On-Load Tap Changer
VAR Reactive Power
S Apparent Power (also known as Complex Power)
P Active Power (also known as Real Power)
Q Reactive Power (also known as Imaginary Power)
34
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Distribution System Voltage Performance Analysis for High-
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This report examines the performance of commonly used distribution voltage regulation methods under reverse
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distribution system; photovoltaics; PV; voltage regulation; inverters; renewable systems interconnection; GE Global
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The Potential of Solar PV in Ontario The Canadian Solar Industries Association
  • Rob Mcmonagle
" The Potential of Solar PV in Ontario ", Rob McMonagle, The Canadian Solar Industries Association, January 30, 2006
Electric Power Distribution Handbook Power Distribution Planning Reference Book Lee Willis 4 Reliable, Low Cost Distributed Generator/Utility System Interconnect
" Electric Power Distribution Handbook ", Tom Short 3. " Power Distribution Planning Reference Book ", H. Lee Willis 4. " Reliable, Low Cost Distributed Generator/Utility System Interconnect, 2001 Annual Report ", GE Corporate Research and Development, August 2003,
Reliable, Low Cost Distributed Generator/Utility System Interconnect
2. "Electric Power Distribution Handbook", Tom Short 3. "Power Distribution Planning Reference Book", H. Lee Willis 4. "Reliable, Low Cost Distributed Generator/Utility System Interconnect, 2001 Annual Report", GE Corporate Research and Development, August 2003, NREL/SR-560-
Study of the Impact of PV Generation on Voltage Profile in LV Distribution Networks
  • S Conti
"Study of the Impact of PV Generation on Voltage Profile in LV Distribution Networks", S. Conti, et. al., IEEE Porto Power Tech Conference, September, 2001, Porto, Portugal 6. "IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems", IEEE 1547 Standard, 2003.