Page 592 - Industrial Power Engineering and Applications Handbook

Voltage surges-causes, effects and remedies 171557
17.1 Introduction Ground fault (Section 20.1)
Sudden change of load (Section 24.6.2(ii))
Voltage surges are generally a phenomenon of high voltage Resonance and ferro-resonance effects (Sections
(HV) power systems and can be considered as the most 20.2.1(2) and 24.4.1 and 24.4.2)).
severe pollutant to the insulation of the power system and
the terminal equipment. In this chapter we analyse the 17.2.1 Ground fault
likely amplitude and steepness of surges that may arise
under different system conditions and the most appropriate High overvoltages occur in the healthy phases on a ground
insulation coor-dination between the equipment connected fault:
on the same system. Insulation coordination provides a
critcrion in selecting the right equipment with a more When the system is grounded through an arc suppression
economical insulation level for different applications and coil and is undercompensated whereas the arcing
locations. Generally, locations away from the source of grounds give rise to voltage surges.
voltage surges, i.e. equipment installed in the downstream When the system has an isolated neutral.
of a powcr system is subject to diminishing surge effects. When the system is solidly grounded.
For example, a rotating machine, which may be a motor
or a generator, would rarely be subject to a direct lightning For more details on grounding systems and the extent of
strike as it would seldom be connected on a bus exposed overvoltages refer to Chapter 20.
to direct strikes. It is usually connected through a bus or
a cable which is fed through a transformer. All these 17.2.2 Sudden change of load
interconnecting devices would withstand most of the effects
of a lightning strike and it would be only somewhat This is more pronounced on high-voltage and extra-high-
attenuated and dampened surge to which the terminal voltage systems (66 kV and above) when:
equipment would be subject.
This concept of diminishing value of voltage surges is Carrying large powers and where there may be wide
a logical parameter to economize on the cost of insulation variations in the load demand
as far as pcrrnissible, without jeopardising the adequacy Load rejection: The load side interrupter, feeding a
of protection to the system or the associated equipment. large load at the far end, trips
Different equipment installed at different locations on The load demand falls sharply as a result of substantial
the same power system may thus have varying degree of load rejection, when the generator feeding the system
basic insulation level (BIL), as discussed in Section 18.3. is suddenly underloaded and tends to overspeed. raising
One may notice the variation in BIL from Tables 11.6, its terminal voltage. While the field control system
13.2, or 14.1, and 32.1(A), for motors, switchgears and and the turbine governor will act immediately to regulate
bus systems respectively when installed in the same power the system, the time to normalize the situation may be
system. Similar variations would apply for other equip- a few seconds, hence the necessity to protect the system
ment connected on the same system. The aim here is to against such overvoltages.
cover the subject as widely as necessary for a proper
understanding without going into extensive details.
The type of a surge is identified by its shape, and its 17.2.3 Resonance and ferro-resonance effects
severity by its amplitude (V,) and time (t,) to reach this Such a phenomenon may occur when a circuit comprising
amplitude. All overvoltages discussed are termed surges, a capacitance C and inductance L is switched ON or
since their severity would last only for a few microseconds OFF and when such circuit parameters undergo a change
(ps). In our discussions here and elsewhere in this book,
we classify the overvoltages into two categories for easy during normal operation, as a result of a sudden change
in load. A power circuit will invariably possess such
identification. One is the temporary or dynamic parameters. For example, leakage capacitances between
overvoltages discussed in Chapters 20 and 24 existing in phase to phase or phase to ground are present in a cable
the system for a slightly longer duration say, one half of or a conductor and these would rise when series capacitor
a cycle to two to three cycles at the power frequency (50 banks are connected on the same system, say, to improve
or 60 Hz), and the other as voltage surges, appearing for the system regulation. Similarly, leakage inductance is
just for a few ps, at transient frequencies of a few kHz. also present in a cable or a conductor and that will also
rise when a transformer or a shunt reactor, having non-
17.2 Temporary overvoltages linear magnetizing characteristics (Figures 27.2b, and b,)
is also connected on the same system. According to the
These are of a relatively longer duration, and may have field data on this phenomenon it has been observed that
several successive peaks, lasting from one-half of a cycle it is more pronounced on HV and EHV systems (66 kV
to a few cycles at the power frequency, depending upon and above) particularly under the following conditions:
the time constant (= R/X,) of the circuit that gives rise to
such overvoltages. The likely causes of such overvoltages I When switching a lightly loaded circuit, having a
are discussed in Chapters 20 and 24. It has, however, transformer, and the natural frequency of the linear
been felt necessary to give a brief review of the same for part of the system corresponds to one of the harmonics
greater clarity of the present topic: of the magnetizing current.