08-02-2013, 09:35 AM
The Role of Distributed Generation in Power Quality and Reliability
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EXECUTIVE SUMMARY
The nature of business and power consumption has changed considerably over the last two
decades. Facilities of all kinds now make widespread use of sensitive electronic components,
computers and programmable logic controllers. There is also a growing need for reliable and
continuous communications with customers, suppliers, and financial institutions. Many
businesses suffer economic losses when electric power interruptions occur or even when there
are voltage or current abnormalities present in the power delivery. While the performance of the
U.S. and New York electric utility industries is extremely good, even this level of performance is
not sufficient to protect customers with highly sensitive loads from economic losses. These
customers must invest in on-site equipment to ensure higher levels of reliability and power
quality than is delivered from the electric grid. This report explores the power quality sensitivity
of the power market in New York and examines the value of integrating distributed generation
into an overall customer power quality and reliability solution. The basic premise for this study is
that distributed generation can be used to support customer’s power quality and reliability needs
and by so doing the value of distributed generation is increased.
Power Quality Issues and Frequency1
The ability of the electric system to deliver electric power without interruption is termed 100%
reliability. The ability to deliver a clean signal without variations in the nominal voltage or
current characteristics is termed high power quality. Higher or lower than normal voltage or
current can damage or shut down certain types of electrical equipment. Such variations in the
normal signal occur on the typical power delivery circuit multiple times per year. Figure ES1
shows the results of a prior study that monitored 300 sites on 100 distribution feeders at 24
utilities throughout the U.S. The average number of events per site, per year, is given in 18 bin
groupings (incremented by 5% from 0 to 85%) representing the reduction in voltage that
occurred at the site. Minor voltage excursions, within 10% are considered normal. There were
nearly 75 events per customer per year. Most of the recorded events were minor sags, though the
average site also experienced 8.5 momentary or longer service interruptions per year.
Power Quality Control Techniques and Costs
A range of technologies can be used to improve the power quality at a site. These technologies
can help to insulate the customer from variations in PQ in utility supplied power or to mitigate
PQ disturbances emanating from the customer’s own equipment. These technologies are often
used as individual components of an overall PQ control strategy and include: transient voltage
surge suppressors; VAR compensators; dynamic voltage restorers; isolation transformers; motoralternators
(motor-generators); and various types of UPS. A facility may choose to protect its
entire load (at the electric service entrance), sensitive sub-facilities (individual circuit
protection), or individual operations (controls or individual equipment protection.) The
protection level depends on the size and type of critical load.
Economics of Power Quality and DG
A framework for evaluating PQ investments was developed based on a hypothetical utility and
customer. The utility feed, typical of the type of service seen by a large commercial or industrial
customer had 20 voltage sags throughout the year lasting only a fraction of second each, 2
momentary power interruptions per year, and one extended outage lasting 60 minutes every other
year. The facility disruptions caused by these events, however, were assumed to be 50 minutes
for a sag, 1.4 hours for a momentary interruption, and 5 hours for the extended outage. In this
example the 22.5 disruptions per year (lasting a an average of only 30.2 minutes per year) causes
22 hours of facility downtime with only 16 hours mean time between forced outages.
Various combinations of mitigation measures were evaluated in terms of their cost and
effectiveness as shown in Table ES2. A UPS system with standby generator would have an
annual cost of $149/kW and reduce the facility downtime from 22.5 hours/year to only 15
minutes. Such an investment would be worthwhile for any customer whose annual outage costs
were greater than or equal to $6.90/kW. A standby system alone is comparatively less cost
effective because of its inability to deal with momentary sags and interruptions – the majority of
events during the year.
New York Customer Views of Power Quality and Utility Response
The project team spoke with nine facility managers in sensitive industries throughout New York
State about their power quality and reliability issues. While, the very dramatic extended
Northeast blackout of 2003 created problems for most of them, this once in 25 year event is not
as important as the more subtle and more frequent disturbances these facilities experience every
year, every summer, and, in some cases, nearly every day. Problems arise with computers,
microprocessors, fluorescent light ballasts, sensitive medical imaging equipment, variable speed
drives, computer directed design and manufacturing, critical communications equipment,
nuisance trips on circuit breakers, overheating of equipment, etc. Some facilities don’t know
what is causing their equipment to break, go off-line or lose data, they just go in and fix it to their
systems get back up and running. Other facilities have become very sophisticated in monitoring
their power signal and in diagnosing problems.
Costs Incurred Due to Power Quality and Reliability Problems
The diverse group of customers interviewed as part of this study experienced a variety of costs
due to power quality and reliability problems. The costs included the value of lost production,
increased labor costs, damage to work-in-process with resulting reduced value or costs of
reworking, value of lost materials, equipment damage, revenue (opportunity) loss due to failure
to perform contracts, transaction processing losses and the need to ration services to customers.
The costs of disruptions are not typically quantified, with a few exceptions. In one instance a
plastics business reported that when its machines were down due to power quality problems, it
suffered the indirect cost of lost production but also incurred the direct cost of paying all line
workers even though no product was being made. Another plastics business reported
documenting $16,500 in equipment damage costs in a single month. This figure was said to be
quite conservative as it only included the direct costs to repair damaged equipment that could be
directly linked to poor power quality. The company did not document and account for “softer,”
though no less real costs, such as the value of lost production, losses to work-in-progress and
other costs.
Mitigation Measures Taken to Reduce the Costs of Incidents
Measures taken by a financial services company, for which cost was not a major consideration,
included: all critical loads strictly connected to a UPS system sensitive to frequency range; each
generator has two separate power panel feeds, with three 750 kVA modules in each feed; more
than one control room; special specification for the generators, with the cooling capacity
increased so they are primary rated; weekly, monthly and annual testing of generators; ATS has
its own chillers, air conditioning and ventilation with a battery room; fuel tanks are physically
separated using two diverse paths; and UPS units are physically separated on separate floors,
with inherent fire barriers and separate “bathtubs” with drains to take water away from systems.
A paper mill also has UPS for its computers, a hospital for its computer room and medical
equipment; a university for its computer systems; a metal manufacturing plant for its boilers,
phone system and augmentation of its switchgear; a company making “superalloys” for its main
computer facility, satellite computer servers and computer controls in the manufacturing area: a
manufacturer of precision plastic parts for its microturbines controller and for its computer
servers; and a major network television and radio broadcasting center for all critical systems.