MONITORING CONSIDERATION
The
monitoring objectives often determine the choice of monitoring equipment,
triggering thresholds, methods for data acquisition and storage, and analysis
and interpretation requirements. Several common objectives of power quality
monitoring are summarized here.
Monitoring to characterize system performance:
This is
the most general requirement. A power producer may find this objective
important if it has the need to understand its system performance and then
match that system performance with the needs of customers. System
characterization is a proactive approach to power quality monitoring. By
understanding the normal power quality performance of a system, a provider can
quickly identify problems and can offer information to its customers to help
them match their sensitive equipment’s characteristics with realistic power
quality characteristics.
Monitoring to characterize
specific problems:
Many
power quality service departments or plant managers solve problems by performing
short-term monitoring at specific customer sites or at difficult loads. This is
a reactive mode of power quality monitoring, but it frequently identifies the
cause of equipment incompatibility, which is the first step to a solution.
Monitoring as part of an enhanced
power quality service:
Many
power producers are currently considering additional services to offer
customers. One of these services would be to offer differentiated levels of
power quality to match the needs of specific customers. A provider and customer
can together achieve this goal by modifying the power system or by installing
equipment within the customer’s premises. In either case, monitoring becomes
essential to establish the benchmarks for the differentiated service and to
verify that the utility achieves contracted levels of power quality.
Monitoring as part of predictive
or just-in-time maintenance:
Power
quality data gathered over time can be analyzed to provide information relating
to specific equipment performance. For example, a repetitive arcing fault from
an underground cable may signify impending cable failure, or repetitive
capacitor-switching restrikes may signify impending failure on the
capacitor-switching device. Equipment maintenance can be quickly ordered to
avoid catastrophic failure, thus preventing major power quality disturbances
which ultimately will impact overall power quality performance.
The
monitoring program must be designed based on the appropriate objectives, and it
must make the information available in a convenient form and in a timely manner
(i.e., immediately). The most comprehensive monitoring approach will be a
permanently installed monitoring system with automatic collection of
information about steady-state power quality conditions and energy use as well
as disturbances.
1. Monitoring as part of a
facility site survey
Site
surveys are performed to evaluate concerns for power quality and equipment
performance throughout a facility. The survey will include inspection of wiring
and grounding concerns, equipment connections, and the voltage and current
characteristics throughout the facility. Power quality monitoring, along with
infrared scans and visual inspections, is an important part of the overall
survey. The initial site survey should be designed to obtain as much
information as possible about the customer facility. This information is
especially important when the monitoring objective is intended to address
specific power quality problems. This information is summarized here.
1. Nature of
the problems (data loss, nuisance trips, component failures, control system
malfunctions, etc.)
2. Characteristics
of the sensitive equipment experiencing problems (equipment design information
or at least application guide information)
3. The times
at which problems occur
4. Coincident
problems or known operations (e.g., capacitor switching) that occur at the same
time
5. Possible
sources of power quality variations within the facility (motor starting,
capacitor switching, power electronic equipment operation, arcing equipment,
etc.)
6. Existing
power conditioning equipment being used
7. Electrical
system data (one-line diagrams, transformer sizes and impedances, load
information, capacitor information, cable data, etc.)
2. Determining what to monitor
Power
quality encompasses a wide variety of conditions on the power system. Important
disturbances can range from very high frequency impulses caused by lightning
strokes or current chopping during circuit interruptions to long-term
overvoltages caused by a regulator tap switching problem. The range of
conditions that must be characterized creates challenges both in terms of the
monitoring equipment performance specifications and in the data-collection
requirements. The methods for characterizing the quality of ac power are
important for the monitoring requirements. For instance, characterizing most
transients requires high-frequency sampling of the actual waveform. Voltage
sags can be characterized with a plot of the rms voltage versus time. Outages
can be defined simply by a time duration. Monitoring to characterize harmonic
distortion levels and normal voltage variations requires steady-state sampling
with results analysis of trends over time. Extensive monitoring of all the
different types of power quality variations at many locations may be rather
costly in terms of hardware, communications charges, data management, and
report preparation. Hence, the priorities for monitoring should be determined
based on the objectives of the effort. Projects to benchmark system performance
should involve a reasonably complete monitoring effort. Projects designed to
evaluate compliance with IEEE Standard 519-1992 for harmonic distortion levels
may only require steady-state monitoring of harmonic levels. Other projects
focused on specific industrial problems may only require monitoring of rms
variations, such as voltage sags.
3. Choosing monitoring locations
Obviously,
we would like to monitor conditions at virtually all locations throughout the
system to completely understand the overall power quality. However, such
monitoring may be prohibitively expensive and there are challenges in data
management, analysis, and interpretation. Fortunately, taking measurements from
all possible locations is usually not necessary since measurements taken from
several strategic locations can be used to determine characteristics of the
overall system. Thus, it is very important that the monitoring locations be
selected carefully based on the monitoring objectives.
4. Options for permanent power
quality monitoring equipment
Permanent
power quality monitoring systems, such as the system illustrated in Fig. 5.1,
should take advantage of the wide variety of equipment that may have the
capability to record power quality information. Some of the categories of
equipment that can be incorporated into an overall monitoring system include
the following:
ü Digital fault recorders (DFRs). These may
already be in place at many substations.
DFR manufacturers do not design the devices specifically for power quality
monitoring. However, a DFR will typically trigger on fault events and record
the voltage and current waveforms that characterize the event. This makes them
valuable for characterizing rms disturbances, such as voltage sags, during
power system faults. DFRs also offer periodic waveform capture for calculating
harmonic distortion levels.
ü Smart relays and other IEDs. Many
types of substation equipment may have the
capability to be an intelligent electronic device (IED) with monitoring
capability. Manufacturers of devices like relays and re closers that monitor
the current anyway are adding on the capability to record disturbances and make
the information available to an overall monitoring system controller. These
devices can be located on the feeder circuits as well as at the substation.
ü Voltage recorders. Power
providers use a variety of voltage recorders to monitor steady-state voltage
variations on distribution systems. We are encountering more and more
sophisticated models fully capable of characterizing momentary voltage sags and
even harmonic distortion levels. Typically, the voltage recorder provides a
trend that gives the maximum, minimum, and average voltage within a specified
sampling window. With this type of sampling, the recorder can characterize a
voltage sag magnitude adequately. However, it will not provide the duration
with a resolution less than 2 s.
ü In-plant power monitors. It is now
common for monitoring systems in industrial
facilities to have some power quality capabilities. These monitors,
particularly those located at the service entrance, can be used as part of a
utility monitoring program. Capabilities usually include wave shape capture for
evaluation of harmonic distortion levels, voltage profiles for steady-state rms
variations, and triggered waveshape captures for voltage sag conditions. It is
not common for these instruments to have transient monitoring capabilities.
5. Finding the source of a
disturbance
The first
step in identifying the source of a disturbance is to correlate the disturbance
waveform with possible causes. Once a category for the cause has been
determined (e.g., load switching, capacitor switching, remote fault condition,
recloser operation), the identification becomes more straightforward. The
following general guidelines can help:
■ High-frequency voltage variations will be limited
to locations close to the source of the disturbance. Low-voltage (600 V and
below) wiring often damps out high-frequency components very quickly due to
circuit resistance, so these frequency components will only appear when the
monitor is located close to the source of the disturbance.
■ Power interruptions close to the monitoring
location will cause a very abrupt change in the voltage. Power interruptions
remote from the monitoring location will result in a decaying voltage due to
stored energy in rotating equipment and capacitors.
■ The highest harmonic voltage distortion levels
will occur close to capacitors that are causing resonance problems. In these
cases, a single frequency will usually dominate the voltage harmonic spectrum.
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