SECOND QUARTER FY 97
JANUARY / MARCH 1997
Utility Interconnection Protection Requirements for Dispersed Photovoltaic Systems
by Doug Dawson and John Stevens
Systems Performance and Reliability Workshop
Introduction
The work for this report was sponsored by the US Department of Energy through Sandia National Laboratories. Its goal was to develop a broad sense of the requirements of US electric utilities for the interconnection of small, dispersed photovoltaic systems and to use this information to plan the development of a national standard for photovoltaic systems that will make these systems acceptable to most US utilities.
For this work, knowledgeable engineers at more than 25 medium and large US electric utilities were interviewed by telephone. Many also provided copies of their utility's interconnection policy requirements, and several member companies of the Utility Photovoltaic Group also provided their written interconnection policy documents. Altogether, more than 30 utilities provided information. Of these, 20 utilities had sufficient experience either with photovoltaic systems or with comparable small power installations to have definite policies and requirements. The information from these 20 utilities and comments from knowledgeable engineers at the other utilities form the basis of this report.
Doug Dawson prepared the report in conjunction with Sandia's
John Stevens.
Background
At present, there is no national standard providing detailed interconnection
protection requirements for photovoltaic systems or other types
of dispersed generation.The IEEE Recommended Practice for Utility
Interface of Residential and Intermediate Photovoltaic Systems,
provides general guidance in the interconnection protection area,
but no definite requirements. IEEE Recommended Practice: Test
Procedure for Utility-Interconnected Static Power Converters
contains test procedures to verify inverter performance, including
protective functions, but does not define required protective
functions. The IEEE Guide for Interfacing Dispersed Storage
and Generation Facilities with Electric Utility Systems provides
information and guidance on many aspects of interconnecting dispersed
generation sources, but does not set requirements for protective
functions.
Utility Interconnection Requirements
In response to the interest in dispersed generation created by the Public Utility Regulatory Policy Act (PURPA) of 1978, many US utilities developed interconnection guidelines and requirements to define what measures they considered necessary for the safe and effective interconnection of dispersed generation systems to the utility's transmission or distribution system. Requirements for electrical protection of the utility's facilities are prominent in these interconnection requirements, which typically have two goals:
Islanding may occur either as a result of a fault that is detected by the utility, but not by the dispersed generator, or as a result of accidental opening of the normal utility supply by equipment failure, human error, or malicious mischief. Utilities feel that islanding must be avoided for several reasons:
The accompanying table titled "Utility Protection Requirements - Photovoltaic Inverters 50 kW or Less" lists the interconnection protection requirements of the 20 utilities contacted that had definite policies and requirements. A blank entry means the utility did not mention or does not have the particular requirement. The table represents general requirements and should not be assumed to show what a particular utility would do in every circumstance.
Utility Protection Requirements - Photovoltaic Inverters 50 kW or Less
Summary and Discussion of Findings
Voltage and Frequency Protection
Eighteen of the utilities surveyed said they require under/over voltage and frequency protection, including Georgia Power Co. and the New England Electric System, which accept the internal protection of qualified inverters. Over/under voltage and frequency protection is essentially an islanding protection measure. It can be viewed in either of two ways:
1. If the voltage and frequency "window" in which the generator is allowed to operate is sufficiently narrow, then the probability becomes extremely small that an island will be created in which the load and generation adequately match. Those holding this view prefer tight frequency tolerances on the order of 0.5 Hz.
2. If the voltage and frequency window defines values that will not cause damage to equipment, then even if an island is created, it will not be hazardous to the other customers. Those holding this view specify frequency tolerances of the order of 2-3 Hz.
Interest in the low-frequency capability of dispersed generation
sources has prompted utilities and regional reliability councils
to review and set requirements for low-frequency operation of
utility and non-utility generation resources. New requirements
typically specify continued low-frequency operation of generators
for seconds or minutes at frequencies in the area of 56-58 Hz.
If applied to small, dispersed generation systems like photovoltaics,
these requirements may reduce the ability of voltage/frequency
windows to reliably detect islanding.
Specified Settings
About half of the 20 utilities specify particular settings for
the protective functions that they require. This means that if
these protective functions are to be included in photovoltaic
inverter controls, some means will have to be available to tailor
the settings to meet the requirements of the local utility. At
the same time, the settings would probably need some security
features to prevent tampering by unauthorized persons.
Reverse Power
Reverse power sensing on the main service to the site of the photovoltaic system is another means of islanding protection required by some utilities. It can take several forms including tripping for excessive power out of the site or insufficient power into the site. As an islanding protection, it works best when the on-site load will always exceed the generation output and thus any power flow out is an indication that an island has been created.
Reverse power appeals to utility protection engineers as a simple,
one-function protection that reliably detects islanding. To incorporate
it into a photovoltaic inverter, however, is complicated by the
fact that the reverse power function must monitor the total current
entering or leaving the site, not just the inverter current. Current
transformers to measure the main breaker current are readily installable
in most industrial switchgear, but the cost may be prohibitive
in a residential or small commercial service.
Overcurrent Protection
A few utilities require overcurrent protection, which is intended
to operate for utility system faults. This appears to be due to
lack of information about the fault current characteristics of
inverters or because inverters are not specifically addressed
in the utility's requirements. In general, inverters do not deliver
fault currents of much greater magnitude than their rated load
current and an overcurrent protection for utility system faults
will not operate. A requirement for overcurrent with voltage control
or voltage restraint essentially duplicates undervoltage protection
if applied to an inverter. Note that this form of overcurrent
protection requirement should not be confused with overcurrent
protection for the inverter branch circuit. The latter is a requirement
of the National Electrical Code and is not expected to operate
for utility system faults.
Primary Ground Fault Protection
Four of the utilities require, or may require, a separate primary
ground fault sensing scheme for larger installations, with the
thresholds ranging from 15 to 40 kW. One utility's published guidelines
require a ground fault sensing scheme for all forced-commutated
inverters. Strict adherence to such a requirement could, in many
cases, require the installation of a special distribution transformer
or primary voltage-measuring transformers. The ground-fault protection
requirement is basically another islanding issue, and if islanding
itself can be prevented, there seems to be little remaining argument
for the separate primary ground fault detection system.
Reclosure Blocking
This is yet another islanding issue. Two utilities indicate that
they may require installation of reclosure blocking schemes on
their feeder breakers or pole-top automatic reclosers. These schemes
prevent automatic or manual closing of the associated circuit
breaker if an island has formed. As with primary ground fault
detection, reclosure blocking is unnecessary if the formation
of islands can be prevented.
Inverter Internal Protection
Only a few utilities address using internal protection functions
in an inverter in lieu of discrete, external protective relays.
About half the utilities indicate that they would accept such
protection in some circumstances. Typically the reservations about
the concept are lack of field experience with it and lack of means
to prove that it will operate reliably. Thus the area of acceptance
is typically limited to small installations and line-commutated
inverters where the probability of islanding is perceived to be
small.
Utility-Grade Relays
One utility specified utility-grade relays for all installations
over 15 kW. One other required utility-grade relays for all installations,
but that utility had no experience with PV and not much with other
small power producers. Eighteen of the 20 had no requirement for
utility-grade relays in the size class under consideration.
Testable Relays
Six utilities have requirements that the protective relaying functions be tested at regular intervals, ranging from one to four years in accordance with their own practices for insuring that their protective equipment is always in good operating condition. The significance is that, if built-in inverter protection is to be acceptable to these utilities and others like them, then some means will have to be provided to perform periodic testing on the inverter protective functions.
Utility practice for periodic testing of protective relays developed
based on the needs of electromechanical relays. These devices
need periodic maintenance (cleaning, burnishing contacts, etc.)
as well as sometimes needing recalibration to combat setting drift.
Microprocessor-based protective relays were introduced in the
1980's and are rapidly growing in popularity. These relays do
not require maintenance and the settings do not drift. Nevertheless,
most utilities believe that microprocessor-based relays still
need to be tested periodically to be sure that they are functioning
as intended., but it is still unrealistic to expect many utilities
to accept built-in protective functions in inverters without some
means to verify their performance.
Dedicated Distribution Transformer
Three utilities require a dedicated distribution transformer to
serve the photovoltaic facility. Three others have the same requirement
for larger installations or special circumstances. As used here,
the term dedicated distribution transformer means that
the distribution transformer serving the photovoltaic facility
serves no other utility customers; it may serve loads within the
owner's own house or business.
In a rural area, the dedicated transformer requirement may be insignificant because each house or farm is already served by a dedicated transformer. In an urban or suburban area, however, it is typical to serve 5-10 homes from a single transformer. Apartment buildings with dozens of different utility customers are often served from a single large distribution transformer. The cost for the installation of a dedicated transformer may be comparable to the cost of the photovoltaic system itself.
One utility with extensive experience with photovoltaics requires
an isolation transformer, but not a dedicated distribution
transformer, for all SPP's greater than 10 kW. The isolation transformer
is defined as "…a transformer which serves only the
customer [the SPP site] and no other customers…" Thus
a dedicated distribution transformer will meet the isolation transformer
requirement, but the requirement can also be met by a transformer
at the output of the generator.
Summary
As can be observed from the number of utility requirements that
are derived from concerns about islanding, a reliable and quick
islanding detection scheme can meet the goals of almost all the
utility requirements. If the scheme does not rely upon tight frequency
tolerances, then it will also meet the demands for short-term
low frequency operation which are likely to be made on non-utility
generators. This situation strongly suggests that a good dedicated
islanding detection scheme could be a part of any effort to gain
acceptance for inverters with built-in protection functions.
The Ideal Inverter
From the information on utility requirements discussed above, it appears reasonable to define an ideal inverter protection system that would meet most, if not all of the utility's needs. This is not intended to define what an inverter must have to be useful, but to define an ideal which can be compared with what is practical to accomplish. The following features define the control scheme.
Tutorial Information
Several of the utility engineers who are in a position to make or apply utility interconnection policy said they needed to know more about the pertinent characteristics of inverters, including fault current delivery, tendency to run on, harmonic content, and the results of failures in the unit. Surveyers noted that the utility engineers most familiar with photovoltaic systems and inverters had the fewest and the most reasonable interconnection requirements.
It was apparent from the survey that much good could be done by
incorporating some tutorial material into an inverter interconnection
standard. The section should try to summarize the pertinent characteristics
of different inverter types and should contain readily-obtainable
references, where points in the summary could be verified by explanatory
papers, books, or test results.
Standards Development
The plan by Standards Coordinating Committee 21 to revise IEEE Std. 929 to include sufficient protective requirements to meet the needs of most utilities appears to be good. Active involvement by the IEEE Power System Relaying Committee is essential if the resulting document is to be acceptable to them. The designated coordinator from the committee should be kept up to date with drafts throughout the process.
Active participation on the standards committee by utility representatives
is also very necessary if the finished standard is to have any
impact. The difficulty with IEEE standards is that they are voluntary,
and no one has to pay any attention to them unless they wish to.
Utilities will only adopt standards as their own if they perceive
that doing so will solve problems which they are experiencing.---and
photovoltaics is not a pressing problem to most of them. At the
same time, there is a sense from some that the PURPA interconnection
problem has been around for some time and it would be nice if
someone would solve it and standardize what is required.
Certification
An area that requires some thought is whether an independent certification of compliance with the standard is desirable. Protective relaying products are ordinarily tested by their manufacturer, who then declares that the product meets the IEEE test requirements. In contrast, most commercial electrical equipment is tested, approved or labeled in some way by Underwriters Laboratories.
Manufacturer-certified testing works well for relaying products
because the relay manufacturers are well known to the utilities
and are trusted to conduct the tests properly and fairly. On the
other hand, certification by an independent laboratory might
have more stature than a manufacturer certification, particularly
if the manufacturer was a small, new, or unknown one.
Conclusions
Utilities Contacted 
SYSTEMS PERFORMANCE AND RELIABILITY WORKSHOP
The Southwest Technology Development Institute and New Mexico State University are hosting a two-day 1997 Systems Performance and Reliability Workshop August 5 and 6, 1997 at the Las Cruces Hilton in Las Cruces, New Mexico. A hosted social is planned from 5:30 to 7:30 p.m. the evenings of August 4 and 5. Lunch will be provided on August 5 and 6. The nominal registration fee of $75 will cover the cost of all materials, continental breakfast both mornings of the workshop, breaks, and the social hours.
The focus is to identify and discuss reliability issues that require maintenance and add to the overall cost of photovoltaic systems. The workshop is designed for the photovoltaic community that currently designs, installs, and services systems. The goal of the workshop is to agree on strategies to enhance performance through improved hardware and system designs and maintenance strategies.
Those attending will receive copies of the papers presented, a list of participants, and a summary of the findings in the workshop. Some hands-on operation and data collection of small photovoltaic and hybrid systems will be part of the workshop. Papers are invited and more than 60% will be given by industry, utilities, and institutional users.
The Hilton is holding rooms at $54 single or double for the workshop
(505)522-4300. Air transportation is through El Paso, Texas. Isabel
Martinez at Sandia National Laboratories is the meeting coordinator;
please contact her to register and with any questions about logistics
(505)844-0558 (imartin@sandia.gov). Mike Thomas (Sandia) and Dick
DeBlasio (National Renewable Energy Laboratory) are the technical
co-chairmen. Please call Mike Thomas with technical questions,
(505)844-1548.
Codes, Standards, Hardware Listing, and Guidelines for Photovoltaic Power Systems
The photovoltaic industry has been pro-active in writing standards,
codes and guidelines in the domestic and international arena.
Several key standards, codes and guidelines are now being orchestrated
to reach a 1999 crescendo. The 1999 National Electrical Code
(NEC) proposals, written and agreed on by the PV industry, have
been submitted and are well on their way to bringing Article 690 - Solar Photovoltaic Systems -
of the NEC up to a level commensurate
with today's PV technology. Underwriter Laboratories Standards
(UL1741 and UL1703) are being coordinated and reviewed to be compatible
with the new 1999 NEC. The UL1741 Standard for listing inverters
is being expanded to include charge controllers and AC modules,
and will emerge from its draft format late this year, with the
effective date placed at January 1, 1999. The UL1703 standard
for listing PV modules is being reviewed and will be changed to
reflect new industry needs and the needs of the 1999 NEC. The
Institute of Electrical and Electronic Engineers (IEEE) Standards
for PV related topics are being reviewed, re-certified or rewritten.
A very critical IEEE standard, now designated P929, Recommended
Practice for Utility Interface of Photovoltaic (PV) Systems, is
currently being written with a targeted publish date also late
this year. Another IEEE Guide, PAR 1374, IEEE Guide for Terrestrial
Photovoltaic Power Systems Safety is scheduled for re-ballot in
June and it is expected that this safety document written to harmonize
hardware availability and system design with the NEC will also
be published this year. Other critical IEEE Standards and Recommended
Practices for PV module qualification, battery applications for
PV systems, and field measurements for systems acceptance are
all completed or in process with target publish dated also before
1999. The International Electrotechnical Commission (IEC) standards
for PV applications are also being written in the international
arena with critical reviews and input being supplied by the US
Technical Advisory Group members with industry participation.
Other international activities includes the International Energy
Agency (IEA), while not in the standards writing arena, is reviewing
critical utility interconnection issues on an international scale,
providing reports on interconnect requirements and utility configurations
in member countries, is planning collaborative international testing
on critical "islanding" and interconnect parameters,
and is planning an international workshop in September of this
year involving inverter manufacturers and utilities to address
utility interconnection issues. The IEA activities are also scheduled
to be completed by late 1998 with a final meeting and review planned
to be held at Sandia in September 1998.
These standards, codes and guidelines activities have been in
process for many years. For example, the proposed changes for
the 1999 NEC were compiled through seven task group meetings over
a period of three years. The 1999 crescendo will mark a time
when the NEC, UL, IEEE, and IEC publications have converged to
better equip the PV industry with interconnect guidelines acceptable
to industry and utilities, the NEC, and listing standards The
following short descriptions of safety and interconnect-related
activities illustrated the convergence of several of these key
codes, standards and guidelines.
The National Electrical Code and the changes proposed for photovoltaic systems
Increasing numbers of photovoltaic systems are coming under the scrutiny of electrical inspectors throughout the country. Utility companies are now purchasing these systems in significant numbers, both grid-connected and stand-alone, and most new installations are required to comply with the National Electrical Code (NEC). In response, the Department of Energy's National Photovoltaic Program, collaborating with all sectors of the photovoltaic industry, recently completed a critical milestone in providing support through Sandia National Laboratories and the Solar Energy Industries Association related to the 1999 revision of the NEC.
The National Fire Protection Association, which is responsible for the NEC, established an ad hock task group to bring NEC Article 690 up to the level of today's photovoltaic technology. The group, which is chaired by Sandia's Ward Bower, collaborated with the photovoltaic industry and key organizations such as Underwriters Laboratories. Bower also represents SEIA's Technical Review Committee, which has provided valuable collaborative technical support.
The task group concentrated on issues related to safety and disablement of a photovoltaic arrays for installation and servicing, integration of photovoltaics in building electrical systems, point-of-connection for building integrated photovoltaic systems, the new AC photovoltaic module, hybrid systems, batteries and charge controllers, and other installation and safety related issues. The outcome of this collaborative work will be an Article 690 that has been thoroughly reviewed and changed by the photovoltaic industry for inclusion in the 1999 NEC. The new Article 690 will clearly address the installation issues for photovoltaic applications that are known today.
The coordination of the NEC changes and UL standards for listing inverters, charge controllers and AC modules will result in safe and more reliable photovoltaic systems that are better understood by electrical contractors and designers. The coordination of the NEC changes and IEEE standard for interconnection and photovoltaic safety will help integrate photovoltaic systems into utility, stand-alone, and hybrid applications nationwide. Good installation practices required by the NEC will also help to improve long-term system performance and reliability. Collaborative actions with other standards activities will also provide the needed bridge to solidify interconnection issues for the photovoltaic industry and the utilities.
The 1999 NEC is scheduled to be issued by the standards council
in July 1998, and the effective date for the 1999 NEC is January
1, 1999.
Underwriters Laboratories' standard for listing inverters, charge controllers and ac modules in photovoltaic power systems
Underwriters Laboratories, Inc. (UL) is currently reviewing the
proposed first edition of the "Standard for Inverters, Charge
Controllers and AC Modules for Use in Residential Photovoltaic
Power Systems, UL1741". The Underwriters Laboratories Industry
Advisory Group met in January 1997 to review the latest version
and to provide photovoltaic industry input during preparation
of the draft standard and before public review. The group consisted
of participants associated with photovoltaic module manufacturing,
inverter manufacturing, charge controller manufacturing, ac module
development, systems integration and the US DOE Photovoltaic program.
The draft is tentatively scheduled to be ready for public review
by June, 1997, and the UL goal for publishing the completed standard
is set for early December 1997. Dates were established to coincide
with completion of inputs to the 1999 National Electrical Code.
For more information regarding UL1741, please contact Jodi Smyth
at UL, 847-272-8800 X 42418.
Proposed Institute of Electrical and Electronic Engineers (IEEE) Guide for Terrestrial Photovoltaic Power Systems Safety
Fire safety and personnel safety are top priorities for designers, installers, inspectors and users of installed photovoltaic systems. The National Electrical Code spells out the requirements for installation of all electrical systems, but the 1069 pages are often unfamiliar to those involved with photovoltaic systems. The IEEE Guide for Terrestrial Photovoltaic Power Systems Safety is being written to provide an easily read safety document written specifically for photovoltaic systems that is correlated with the National Electrical Code and other ANSI/IEEE Recommended Practices and Standards.
The purpose of the guide is to describe photovoltaic-specific
topics or components relating to the design and installation of
photovoltaic power systems that affect safety and to suggest good
engineering safety practices for photovoltaic electrical balance-of-system
design, equipment selection and hardware installations. The area
emphasized in the guide are photovoltaic-unique electrical power
requirements. The guide describes system types and addresses
wiring for photovoltaic modules, balance-of-system, and battery.
Particular attention is given to temperature considerations required
for photovoltaic systems, voltage ratings, cable types, wiring
ampacity and calculations needed for safe and reliable design.
Other important topics such as overcurrent protection, disconnects,
grounding, surge and transient protection and instrumentation
are also described with examples and recommendations for selection
of the hardware. The guide is also cross-referenced to the applicable
articles and sections in the National Electrical Code.
International Energy Agency guidelines for photovoltaic system safety, interconnections and power quality
The International Energy Agency's Photovoltaic Power Systems (IEA PVPS) Implementing Agreement was established in 1993 as an effort by 20 countries to focus on the planning, design, construction, operation, performance, and promotion of photovoltaic power systems. The mission of the program is to enhance international collaboration efforts through which photovoltaic energy becomes a more significant energy option in the near future. The United States is currently active in five of the seven annexes of the implementing agreement as listed below.
Task I is the exchange and dissemination of information on PVPS. Task IV is focused on modeling of dispersed PVPS in support of the utility grid, but is limited to an ad hoc task group between the USA and Italy. Task V is for grid interconnection of building-integrated and other dispersed PVPS. Task VI focuses on the design and operation of modular photovoltaic plants for large scale power generation. Task VII has just begun and focuses on PVPS in the buildings environment.
Sandia's participation focuses primarily on Tasks I and V. Under Task I, a status survey report on photovoltaic power systems is to be updated every two years. The second of these survey reports will be published in the spring of 1997 and will be distributed to the US photovoltaics industry through the Edison Electric Institute and Sandia. The overall objective of Task V is to develop and verify technical requirements that will serve as technical guidelines for grid interconnections for building-integrated and other dispersed power systems. These guidelines focus on safety and reliable interties to the grid at the lowest cost. The work focuses on three categories; review, definition of guidelines, and collaborative testing to demonstrate technical issues such as islanding or control algorithms with solutions to identified problem areas. Under Task V reports have already been published on existing guidelines for photovoltaic power systems interconnections and on utility distribution systems. A report on interconnection equipment is under final revision. Nine technical topics are under investigation in Task V for new guidelines. A summary of the findings and proposed guidelines for each will be published as part of the final report for Task V and distributed through Sandia.
Another important milestone for Task V work is upcoming with an international workshop that will be held in Zurich Switzerland on September 15 and 16, 1997. The workshop will be designed to involve utilities, inverter manufacturers, photovoltaic system suppliers and R&D engineers in an international meeting between utility representatives and inverter manufacturers to discuss guidelines that may be used and an international level. Topics will include islanding, reclosing, external disconnect requirements, overvoltage protection, grounding, and dc injection.
For more information on any of the codes and
standards discussed here, please contact Ward Bower, (505) 844-5206;
wibower@sandia.gov.
