What do fuel cells, photovoltaics, and some
wind turbines have in common? If they are supplying
electricity to a facility and are also capable of supplying
excess energy back to the local utility, it is likely they
contain a utility-interactive static power inverter. In fact,
the static power inverter is one of the major keys to the
future success and growth of all alternative energy
technologies (see figure
1).
Fundamentally, all inverters transform
incoming direct-current (dc) electricity, into some
well-defined and desirable alternating-current (ac) output.
The static power inverter is so named to differentiate it from
rotating alternator/generator sets. An inverter uses power
transistors that are switched on and off in a controlled
manner to synthesize an ac output voltage and current waveform
from its dc input. This article provides some background on
static inverters with a particular emphasis on the
utility-interactive inverter. Utility-interactive inverters
are unusual in that they require not only electrical inspector
approval, but the local utility’s as well. It is this
blending of electrical safety issues and utility performance
concerns that makes this equipment unique from both an
inspector approval and testing lab evaluation standpoint. This
article explains some of Underwriters Laboratories Inc. (UL)
requirements for evaluating these products, the utility
performance concerns unique to utility-interactive static
inverters, and the relevant 2002 National Electrical Code (NEC) requirements.
State
of Alternative Energy Technologies
To
understand the approximate number of inverters in operation,
it is important to quantify the present number of
utility-interactive alternative energy facilities in the U.S.
This information is also intended to help put some perspective
on the number of projects that likely required electrical and
utility inspection. The short story is that the number of
U.S.-installed utility-interactive alternative energy systems
is currently small, less than approximately 5,000 units, but
growing at double-digit rates based on data from 1998 through
2002. This means that although this equipment may not be
commonplace now, it has the potential to be within a few
years.
A recent study by the National Renewable
Energy Laboratory (NREL) concluded that there were
approximately 2,149 on-grid (or utility-interactive) PV
installations in the U.S. through 2000.1 Through 2002, the number of U.S. installations has increased
to approximately 2,500 with a 4-year compound annual growth
rate of 34 percent; however, in the past two years the rate
has declined to around 6 percent.2
There are fewer installed fuel cells, with worldwide numbers roughly equal to the U.S. PV installations. According
to a Fuel Cell Today survey, there are some 1,900
stationary fuel cells, ranging in power from 500 W to 10 kW 3 and some 650 units, ranging in size from 10 kW to
11 MW that have been installed worldwide.4 The four-year compound annual growth rate for the 500 W – 10
kW group is 155 percent but only 9 percent for the 10 kW to 11
MW sizes. These studies indicate that potentially 1,800 total
units are installed in North America with the majority located
in the U.S.
Wind installations have experienced a
substantial surge in global demand as their electric
production costs have become competitive with conventional
centralized electric generation. The NREL database2 indicates that there are more than 550 completed wind projects
in the U.S. totaling over 5 gigawatts of installed
capacity. The four-year compound annual growth rate is
approximately 10-percent in the U.S. but is over 30 percent on
a worldwide basis.
One of the chief factors influencing the
number of installed alternative energy systems is strictly an
economic one. Alternative energy technologies are typically
more expensive than a conventional generator. Recently, many
utilities have begun programs to buy back any excess
electricity that an alternative energy system can provide.
This added financial incentive has a significant impact on the
number of U.S. alternative energy projects. However, many of
these programs require the use of a listed utility-interactive
inverter in order to participate, which makes this equipment
an important key to the alternative energy industries.
Inverter
Classes
There
are currently two different broad classes of inverters
available for alternative energy equipment: (1) the
stand-alone, and (2) the utility-interactive inverter. The
stand-alone inverter is arguably the oldest with origins
tracing back to the 1960s with the advent of suitable
thyristors.5 The stand-alone inverter is also the most widely available
inverter today, but it is intended to be used by itself
without a connection to the local utility. These devices are
said to be voltage sources, because their main mission
is to provide the user with a consistent ac voltage.
The level of electronic sophistication of a
typical utility-interactive inverter is considerably higher
than a stand-alone unit because these devices not only convert
dc electricity into ac, but they also have the capability to
supply their ac energy in a form suitable for back feeding the
local utility grid. In order for this equipment to work
properly, it must conform to and synchronize with the utility’s
voltage and frequency, respectively. It is the
utility-performance issues that make this equipment bridge two
worlds that are often separated, i.e., the electrical
inspector and the utility system protection engineer.
UL’s
Utility-interactive Static Inverter History
Starting
in 1983, Underwriters Laboratories Inc. (UL) began a standards
development effort to develop the requirements necessary for
the safe creation of utility-quality power from PV static
inverters. In 1985, UL issued an outline of investigation,
which set forth the fundamental requirements for the
construction and performance testing of this equipment. In May
1999, the initial standard, UL 1741, Static Inverters and
Charge Controllers for Use in Photovoltaic Power Systems, was published with input from alternative energy and inverter
manufacturers, the NEC code-making panels and
authorities having jurisdiction (AHJs) from the electrical
inspection community as well as from utilities across the
United States. Although it was developed alongside NEC 690, Solar Photovoltaic Systems, the scope was modified in a
January 2001 revision to also include "power systems
which combine other alternative energy sources with
inverters." Also in this revision, UL 1741 was
re-titled, Inverters, Converters and Controllers for Use in
Independent Power Systems.
Presently, there are over 600 individual
inverter models listed under the product category Static
Inverters and Converters for Use in Independent Power Systems,
UL category code (QIKH). Power output for these devices range
from a few watts to over 400 kW. You may access the UL guide
information for this category as well as the listings online
at www.ul.com/database and entering QIKH at the category code
search. This information can also be located in UL’s
Electrical Construction Equipment Directory (Green Book) or UL’s
General Information Directory for Electrical Equipment
Directory (the White Book).
The
Importance of the Utilities
Historically,
the utilities have played a disproportionate role in
determining the fate of utility-interactive alternative energy
projects. In fact, in 1998, while UL 1741 was still under
construction, NREL surveyed some 90 alternative energy
projects and developed 65 case studies that followed progress
from design through commissioning.7 The results showed that only 7 of the 65 projects did not
encounter a major utility barrier. The remaining 58 were
either significantly delayed, encountered significant
additional cost, or in some cases were abandoned. In fairness,
the current electric distribution systems were not designed
for two-way flow of power. As the utilities are ultimately
liable for outages and power quality problems, their
conservative approach to allowing the use of interactive
inverters is understandable. This is especially true when it
is considered that power reliability problems will not be
evident until some critical number of interactive inverters
are operating on the same distribution branch. This subject of
"penetration," or how many interactive-systems are
too many, is a debate that continues.
UL recognized early on that inclusion of
utility performance concerns was key to the development of a
successful product standard. This task was challenging
considering that there are over 3,138 electric utilities in
operation in the U.S.8 and many dictated their own performance requirements for
utility-interactive equipment. At present, UL 1741 represents
the best compromise in utility concerns.
Utility
Performance Concerns
Fundamentally,
the interactive inverter allows the utility to set the voltage
and frequency at which it will operate. The ac energy is then
delivered back to connected loads or the utility in the form
of current. From a practical standpoint, when the inverter is
operating, its voltage at the output terminals is typically
higher than the utility source. This can be the result of line
impedance between the inverter and the supply connection point
or due to a particular unit’s control scheme. Regardless,
interactive inverters are not allowed to influence the local
utility’s voltage and frequency. As a result, UL 1741
requires that units cease their power production if the output
voltage goes out of a consensus-based range of operation.
Connecting the inverter to an arbitrary waveform generator, or
"simulated utility" tests compliance with this
requirement. The interactive inverter’s response is measured
by varying the voltage and frequency of the simulated utility
in a known and repeatable way. Currently, interactive
inverters are only allowed to operate continuously in the
range given in table 1.
Table 1. Range of interactive inverter
operation
Voltage
Frequency
88%Vnormal110%
59.3Hz60.5
Outside of the table 1 limits, the unit
must cease exporting power in a prescribed number of cycles.
For example, if the utility voltage drops to less than 50
percent, the inverter must respond within 6 cycles. If the
voltage rises to more than 137 percent of normal range, the
inverter must respond within 2 cycles. The inverter is given
120 cycles, or 2 seconds, for utility voltage deviations
between the ranges of 50% to 88% and 110% to 137%. In a
similar fashion, frequency shifts outside of the table 1
values must be sensed by the inverter and power export ceased
within 6 cycles. By all accounts, these are fairly stringent
requirements. But, there is a solid rationale behind these
limits.
One of the biggest utility concerns has
been the potential for interactive inverters to continue to
export power after the utility has suffered an outage. This
so-called "islanding" problem can result in a
section of the distribution system being energized from only
the alternative energy source. From the utility perspective,
the consequences include:
-
shock hazards to
utility line crews,
-
current
contribution to the utility fault,
-
potential problems
in re-energizing the line, or,
-
serious damage to
equipment if the line is re-energized out of sync with the
interactive inverter.
Thus, if the utility experiences an event
that causes its voltage or frequency to go out of normal
range, it is important that the interactive inverter cease to
export power. On the other hand, sensing a voltage or
frequency change would by itself be insufficient if by some
chance the interactive inverter was connected to a resonating
load that mimicked a normally-operating utility.
In order to ensure that interactive
inverters will not island, a special test was devised that
represents a worst-case combination of possible connected
loads (see figure
2). By balancing the reactive contributions from
capacitors, inductors and a real load in the form of
resistors, a resonant circuit can be created. At present, UL
1741 requires that interactive inverters be capable of
detecting this scenario and ceasing their power export within
2 seconds.
The interactive inverter must incorporate
an active means to destabilize a resonant load within 2
seconds. This often requires than the unit periodically
"bump" its voltage or current output to cause the
resonant load to collapse to the point that the voltage or
frequency goes out of the normal range indicated in table 1.
This, however, leads to another leading utility concern—current
distortion.
An example waveform is shown on figure
3. Here it can be seen that the unit briefly surges the
output periodically to ensure compliance with the UL 1741
anti-islanding test. The impact of this surge and its
contribution to the total harmonic distortion output current
waveforms are analyzed according to table 2, which is
identical to the requirements of IEEE 519-1992,
"Recommended Practices and Requirements for Harmonic
Control in Electrical Power Systems."
Table 2. Harmonic distortion limits for
interactive inverters
Odd
Harmonics
Distortion Limit
3rd through 9th
< 4%
11th through 15th
< 2%
17th through 21st
< 1.5%
23rd through 33rd
< 0.6%
above 33rd
< 0.3%
Furthermore, the even order harmonics are
restricted to 25 percent of the above odd harmonic values.
Inverters that are listed must meet the above requirements
while operating at 100 percent of their rated power output.
As one might imagine, there are several
other testing requirements in UL 1741 to address utility
concerns. For instance, interactive inverters cannot inject
more than 0.5 percent dc into an ac circuit, must operate at a
power-factor of 0.85 or higher, and if the interactive
inverter ceases power export due to a utility fault, the unit
must remain off-line for 5 minutes after the utility has
returned to normal operation.
Currently, a few of these requirements are
in the process of change due to the publication of IEEE
1547-2003, "Standard for Interconnecting Distributed
Resources with Electric Power Systems." UL contributed to
this development effort and is now under a contract with the
Department of Energy to bring UL 1741 into alignment with the
IEEE 1547 standard. The IEEE standard represents the efforts
of more than 350 people that began work in 1999 to define a
utility, manufacturer and user-based consensus standard that
covers the requirements for acceptable utility
interconnection.
Fire,
Shock and Mechanical Evaluation
Although
much has been said of the utility performance aspects of UL
1741, it is important to know that interactive inverters must
also undergo all of the fire, shock and personal injury hazard
evaluations that effectively reduce these risks.
Other hazards that are evaluated include
installation and mechanical hazards. Units are intentionally
mis-wired to determine the safety issues that could occur from
improper wiring. Wire bending spaces are examined, as is the
general suitability of the enclosure for the intended
environment. Also hazards from mounting the unit or its
potential to tip over are evaluated. Concurrent with this
testing, installation instructions and marking details are
studied to ensure that appropriate and code-compliant
information is provided to the user. Among other important
information, "Utility-Interactive" must always
appear on the label of an interactive inverter.
The NEC
From
a code perspective, interactive inverters must comply with the
requirements of the 2002 NEC Article 705,
Interconnected Electric Power Production Sources. Specifically
of note are:
1. Disconnecting means in Section 705.21
and the marking requirement of Section 705.22, which is
required typically between the alternative energy source and
the inverter and between the inverter and the interconnection
panel, which is fed from a utility source.
2. Connection of the interactive inverter
must be made to the supply-side of installed ground-fault
protectors, per Section 705.32.
3. Section 705.40, which requires
disconnection of the interactive inverter from the utility
upon loss of utility power.
4. Also, Section 705.42 that requires a
similar disconnection of the interactive inverter if one phase
of a three-phase connection opens.
5. Note that for PV systems with
interactive inverters Article 690, Solar Photovoltaic Systems,
takes precedent over Article 705.
6. Similarly note that for fuel cell
systems, Article 692, Fuel Cell Systems, takes precedent over
Article 705.
With regard to PV and fuel cell
installations that use an interactive inverter, it is
important to note that the typical installation practice is to
connect the utility-interactive inverter through a back-fed
breaker into an existing load center. NEC Sections
690.64 for PV and 692.64 for fuel cells provide guidelines for
how to connect an interactive inverter onto a dedicated branch
circuit. The main code concern is to avoid a possible
overloading of the load center busbar or conductor rating with
the addition of the new alternative energy system supply.
Often this means that the main breaker is de-rated by the size
of the new supply branch overcurrent protector. Note, however,
that dwellings can allow an additional source which brings the
panel to 120 percent of its rating per Sections 690.64(B)(2)Exception and 692.65(B)(2)Exception.
The subtle mixing of utility performance
concerns into the language of the 2002 NEC 705.40 and
705.42 (above) is an interesting trend that will likely
continue as alternative energy projects, and the interactive
inverters that make them possible, become more prevalent
within the U.S. To help ensure that the trend continues, UL is
working hard to ensure that both electrical inspector and
utility engineer’s concerns are met with UL 1741 and UL’s
listing of utility interactive inverters.
Summary
The
utility-interactive inverter is an important key for unlocking
future U.S. growth of alternative energy projects. As this
equipment has the ability to export energy back to the local
utility, it must meet not only the requirements of the
electrical inspector, but the utility system protection
engineer as well. This bridging of jurisdictional concerns has
been incorporated into the standard used to evaluate this
equipment, UL 1741, Inverters, Converters and Controllers
for Use in Independent Power Systems, and is also evident
in the NEC Article 705, Interconnected Electric Power
Production Sources. Although the present number of
utility-interactive systems is approximately 5,000, growth in
wind, solar and fuel cell projects has been impressive. So,
the chances are that most inspectors will, at some point in
the future, have exposure to this equipment.
To make electrical inspections easier for
AHJs, UL has included a list of all distributed generation
(DG) product categories in the front of the General
Information for Electrical Equipment Directory (the White
Book) on page xix. Also, for more information on
utility-interactive inverters and distributed generation
equipment in general, be sure to visit us on the web at
http://www.ul.com/dge.
Endnotes
1 Price, S., Herig, C.,
Goldstein, H., Gillette, L., Boedecker, E., and Holihan,
J., "U.S. On-Grid Photovoltaic Capacity: A Baseline
for the National Energy Modelling System," National
Renewable Energy Laboratory report NREL/CP-620-32104
(May 2002).
2 Data obtained from the
National Renewable Energy Laboratory online database
available at http://analysis.nrel.gov/repis/online_access.asp.
3 Geiger, S., and Cropper, M.,
"Fuel Cell Market Survey: Small Stationary
Applications," Fuel Cell Today (July 2003): online
report available at www.fuelcelltoday.com.
4 Cropper, M., "Fuel Cell
Market Survey: Large Stationary Applications," Fuel
Cell Today (September 2003, online report available at
www.fuelcelltoday.com.
5 Qin, Y.C., Mohan, N., West,
R. and Bonn, R., "Status and Need of Power
Electronics for Photovoltaic Inverters, Sandia National
Laboratories," Sandia National Laboratories report
SAND2002-1535 (June 2002): 32.
6 Underwriters Laboratories
Inc. Standard for Safety for Inverters, Converters, and
Controllers for Use in Independent Power Systems, UL
1741, First Edition (May 7, 1999).
7 Alderfer, R., Eldrige, M.,
Starrs, T, and Nakarado, G., "Making Connections—
Case Studies of Interconnection Barriers and Their
Impact on Distributed Power Projects." National
Renewable Energy Laboratory report NREL/SR-200-28053
(May 2000).
8 American Public Power
Association. "U.S. Electric Utility Statistics,
2001." 2003 Annual Directory & Statistical
Report (2003): 13. Online report available at
www.APPAnet.org.
Kent Whitfield is a R&D manager
for Underwriters Laboratories at the corporate
headquarters located in Northbrook, Illinois. For the
past twelve years he has been actively involved in all
aspects of alternative energy systems from testing and
certification through manufacture and design. Mr.
Whitfield has a master’s degree in mechanical
engineering and is a registered professional engineer in
Illinois
|
