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Distribution System Voltage Performance Analysis for High-Penetration Photovoltaics
A national laboratory of the U.S. Department of Energy
Office of Energy Efficiency & Renewable Energy
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
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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
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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 nations 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 study
4
. 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 th