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

8.4.4.2 Analysis tips
Sudden changes in current will produce changes in the L-N or the N-G voltage for similar periods of time.
For example, if a load is energized that has a 1.5-s in-rush current, an L-N sag and an increase in N-G volt-
age will be generated for the same 1.5 s. In fact, the N-G voltage rise will be about one-half the magnitude of
the L-N sag. The sag is a product of the voltage drops of the line and neutral conductors, while the N-G volt-
age is only a product of the neutral (see Þgures 6 and 7).
Recognizing that most sag or swell conditions result from changes in current can help determine the cause of
most of these types of disturbances. Whenever both voltage and current are known, it is even easier to iden-
tify the possible causes. Depending on the monitoring location with respect to the entire power system, elec-
trical inertia may also contribute to sags and swells.
8.4.5 High frequency analysis
8.4.5.1 Scope
Many disturbances other than those at power frequencies exist in the power system. Some of these distur-
bances are continuous, low-voltage, high-frequency signals conducted on the power lines. Others are very
brief medium- to high-voltage signals known as transients. When these disturbances are injected into the
power system, it responds differently than it would at low frequency.
8.4.5.2 Analysis tips
At higher frequencies, the power system is subject to capacitive coupling, and other phenomena not signiÞ-
cant in low-frequency analysis. High-frequency models are used when examining transients and other distur-
bances with frequency components above about 20 times the fundamental frequency of the power system.
For example, above about 1.2 kHz on 60 Hz systems the high frequency effects cannot be ignored.
Field data has shown that transients can travel from one wire to another, even if the wires are not connected
(presumably by capacitive coupling). They can travel through open-circuit breakers, and can appear across
what appears to be an open circuit at lower frequencies. The high-frequency characteristics of the power sys-
tem need to be considered in the frequency range discussed earlier. Reßections at high frequencies can also
occur (remember the half-wavelength of 1 MHz is only 150 m), although they are generally damped out rap-
idly by capacitive loads on the line.
Transients are generally caused by adding or removing reactive loads from the line. Obviously environmen-
tal causes such as lightning occur, but far less often than load induced transients (see Þgures 1 and 4). A very
simplistic model of a capacitor and an inductor are shown in Þgure 20. 
A capacitor being added to a power system is typically in its discharged state. When it is turned on, it draws
up to 1000% of its nominal current for 1 to 5 cycles. This causes a switching transient. The transient reßects
the energy draw of the capacitor. This means that the transientÕs leading edge will be opposite in polarity
from the ac waveform since energy is being drawn from the source. If the capacitive load is turned on at the
positive half cycle of the ac, then the transientÕs leading edge will be negative.
As a capacitive load is introduced into the inductive power system, it may also alter the frequency response
of the system. An LC system has resonant frequencies that may be excited by the capacitive transient, lead-
ing to a damped oscillatory transient.
On the other hand, when an inductor is applied to the power system, not much happens in the transient
realm. The inductor will draw current and generate its magnetic Þeld. The inductor, however, causes a tran-
sient at de-energization. If the switch controlling the inductive load is opened, three things happen. First, the

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IEEE
Std 1159-1995
IEEE RECOMMENDED PRACTICE FOR
56
magnetic Þeld collapses creating a transient. This is called inductive kickback. Since this transient is adding
energy back into the system, its position on the ac waveform will be in the same polarity.
Second, the switch trying to break the ßow of current may arc slightly. Arcing is seen as very fast ÒnoiseÓ
superimposed on the inductive transient. The degree of arcing can also indicate proximity to the source of
the transient.
Third, depending on the amount of current being interrupted, the switch may bounce. Switch bounce pro-
duces a second, smaller transient immediately following the Þrst.