Ieee std 1159-1995, ieee recommended Practice for Monitoring Electric Power Quality

4.4.7 Power frequency variations
The power system frequency is directly related to the rotational speed of the generators on the system. At
any instant, the frequency depends on the balance between the load and the capacity of the available genera-
Figure 11ÑExample of voltage notching caused by converter operation

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IEEE
Std 1159-1995
IEEE RECOMMENDED PRACTICE FOR
24
tion. When this dynamic balance changes, small changes in frequency occur. The size of the frequency shift
and its duration depends on the load characteristics and the response of the generation system to load
changes.
Frequency variations that go outside of accepted limits for normal steady-state operation of the power
system are normally caused by faults on the bulk power transmission system, a large block of load being dis-
connected, or a large source of generation going off-line.
Frequency variations that affect the operation of rotating machinery, or processes that derive their timing
from the power frequency (clocks), are rare on modern interconnected power systems. Frequency variations
of consequence are much more likely to occur when such equipment is powered by a generator isolated from
the utility system. In such cases, governor response to abrupt load changes may not be adequate to regulate
within the narrow bandwidth required by frequency sensitive equipment.
NOTEÑVoltage notching can sometimes cause frequency or timing errors on power electronic machines that count zero
crossings to derive frequency or time. The voltage notch may produce additional zero crossings that can cause frequency
or timing errors.
5. Monitoring objectives
5.1 Introduction
Power quality monitoring is necessary to characterize electromagnetic phenomena at a particular location on
an electric power circuit. In some cases, the objective of the monitoring is to diagnose incompatibilities
between the electric power source and the load. In others, it is to evaluate the electrical environment at a par-
ticular location to reÞne modeling techniques or to develop a power quality baseline. In still others, monitor-
ing may be used to predict future performance of load equipment or power quality mitigating devices. In any
event, the most important task in any monitoring project is to deÞne clearly the objectives of monitoring.
The objectives of monitoring for a particular project will determine the choice of monitoring equipment, the
method of collecting data, the triggering thresholds needed, the data analysis technique to employ, and the