Digital Signal Processing Reference
In-Depth Information
Table 2.1
Selected phenomena and possible protection reaction
Exposure
Fault
Damage
Operation
Task
Short-circuit
Yes
Yes
Response with trip
command
Limitation of damage range
Ground fault
Yes
No
Response with signaling
Indication and location of
fault
Overload
Unbalance
No
No
Acting with command/
signal
Avoiding of possible damage
Overvoltage
Undervoltage
Underfrequency
No
No
Acting with command/
signal
Avoiding of possible damage
abnormal phenomena have to be promptly detected and classified, which is
performed by measuring of certain criteria values. In the following subsections the
dangerous phenomena for most important power system components are outlined,
the protection criteria for detecting faults are enumerated and basic requirements
for protective devices are discussed.
2.2 Faults and Abnormal Phenomena in Power Networks
The protection relays should differentiate the normal operating conditions from the
abnormal ones, including short-circuits and other impacts that can be dangerous to
protected equipment itself and may impair safe operation of the power system.
Under normal operating conditions the power system is characterized by:
• almost symmetrical three-phase voltages and currents,
• operational
currents
remaining
below
the
pre-set
levels
(including
some
permissible overload),
• voltage level within permissible range around the nominal value,
• frequency of the signals equal or very close to the nominal 50/60 Hz,
• harmonics content within permissible limits,
• power quality satisfying customer and standard requirements.
Of course, the power system is rather seldom in an ideal state, thus voltage
and current signals may vary due to variations of load and generation as well as
switching operations.
It must be noted here that some phenomena occurring in power system under
normal conditions may also be seen as abnormal for the sake of current/voltage
signal parameters being far distant from the nominal values. However, their reason
is not always a fault and the protection should not operate in such cases. Examples
of such situations are:
• charging of the transmission line after switching in,
• starting of large induction motors,
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