Abnormal States in Power Systems and Criteria for Their Recognition - Digital Signal Processing in Power System Protection and Control - page 7

Over- and Undervoltage
Over- and undervoltage protections react when the voltages measured exceed
the permissible limits.
Frequency
The deviation of frequency from its rated value is an indication of power
imbalance in the system.
Differential Principle
Difference of current magnitudes or phase angles at the terminals of protected
plant is a clear sign of internal fault.
Impedance
Impedance is measured to detect faults on transmission systems or underexci-
tation or out-of-step conditions of generators.
Symmetrical Components
In some applications they are more suitable for protection purposes than the
phase values. The basic types of faults detected by monitoring of symmetrical
components: phase-to-phase and ground faults in solid and low-resistance
grounded systems, ground faults in medium voltage systems (ungrounded or
inductively grounded networks), asymmetric system configuration, asymmetric
load and open-circuit phase conductor.
Power Direction
Discrimination of power flow direction is used in combination with overcurrent
units when the overcurrent criterion on its own is insufficient to preserve selec-
tivity of tripping. Typical applications are for making the relays selective for ring
and parallel lines.
Other Criteria
Other criteria used by protective devices are:
• temperature (typical application: transformer oil temperature),
• rate of oil flow, accumulation of gas (for detecting internal transformer faults),
• harmonics in the neutral current and voltage (for detecting ground faults in
inductively grounded systems),
• harmonics in generator current (for detecting internal generator faults),
• transient current and voltage signals, traveling waves (e.g. in transmission lines
protection and fault location).
A very basic overview of how the protection criteria are applied for protecting
against the most frequent abnormal phenomena can be found in Table 2.2 . No further
discussion is provided here, for more specific rules of protecting particular power
system elements the reader should refer to some other topics, e.g. [ 1 - 6 ].
2.4 Requirements for Protective Devices
Protection devices/relays cannot unfortunately prevent the faults that happen,
they act only after fault appearance. The protection devices are designed and
expected to: