Digital Signal Processing Reference
In-Depth Information
- overexcitation,
- overload;
• synchronous generators:
- ground faults,
- phase faults between windings,
- inter-turn faults,
- duplicate ground faults,
- stator/rotor overload,
- loss of excitation,
- pole slipping,
- under-/overfrequency,
- overvoltage,
- asymmetric load,
- motoring;
• HV three-phase motors:
- phase faults between the stator windings or on the motor feeder,
- phase faults on a stator winding,
- stator inter-turn short-circuits,
- overload,
- phase imbalance,
- voltage dips,
- asynchronous operation (in case of synchronous machines).
Each of the above enumerated fault types can be recognized through deter-
mination (measurement) of certain features, called criteria, resulting from pro-
cessing of information contained in voltages, currents and other signals. Below a
list of most frequently used protection criteria are given. It is not the aim of this
topic to thoroughly discuss their application for particular power system elements
nor to discuss the rules for relay settings and their coordination. The main focus is
directed on the methods and detailed algorithms for criteria values measurement,
which is described in further chapters.
2.3 Criteria Signals
Overcurrent
An overcurrent is detected when the maximum load current permissible for an
item of an electrical plant is exceeded. Overcurrent protection devices monitor the
current being conducted by the protected unit and issue a tripping command for the
circuit-breaker when the current exceeds the setting (the so called relay pick-up
current). According to the operating speed one can distinguish instantaneous and
time-delayed (definite time or inverse time) overcurrent relays.
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