Page 331 - Handbook of Electrical Engineering

318 HANDBOOK OF ELECTRICAL ENGINEERING

12.2.2.4 Influence of the load characteristics

Occasionally a power system may contain motors that have a normal continuous rating, that is large
in comparison with any one of the main generators in the system. All the generators may not be of
the same rating.
Direct-on-line starting of these large motors can cause several problems with the performance
of the generator protection relays, for example:-

a) Voltage dip at the generator terminals during the first 100 milliseconds, or so, as the motor begins
to run up to speed. This is caused by the high reactive current drawn by the motor, which remains
nearly constant until the motor approaches its full-speed operation. Voltage dip can cause tripping
of downstream switchgear if its control circuit supplies are taken from the AC power system e.g.
switchgear voltage transformers. Prolonged voltage dip may occur if the generator excitation is
not provided with sufficient ceiling voltage capability.
b) Overvoltage can occur at the end of the motor run-up period if the generator excitation has been
forced to a high level. The sudden loss of the high reactive starting current will raise the generator
terminal voltage significantly, which may take a second, or so, of time to recover. For high
voltage systems this rise in voltage may be unacceptable for the insulation limits of equipment in
the system e.g. motors, transformers, cables. This problem occurs particularly when motor run-up
times exceed about 5 seconds, e.g. large high speed gas compressors.
Undervoltage and overvoltage relays are often used on the generators to protect against
prolonged overload, seen as undervoltage; and excessive stress on insulation, seen as overvoltage.
These relays are usually chosen with adjustable definite time delays.
c) The high starting current of a single large motor may be sufficiently high to be seen by the genera-
tors as an overcurrent situation; particularly if a minimum number of the generators are running at
the time, and each one is already heavily loaded. This situation may influence the choice of Time
Multiplier Setting (TMS) or even the shape of the relay curve. This is illustrated in Figure 12.7.

High voltage generators rated above 2000 kW are usually provided with differential stator
current protection (87), which is very sensitive to internal winding faults. Generators have long
thermal withstand time constants and can therefore tolerate modest overcurrents for a relatively long
time. For these reasons the asymptotic current (current setting) is often set fairly high when compared
with other large items such as motors and transformers. Current settings up to 150% are often
acceptable, but advice should be taken from the manufacturer of the generator if a high setting is to be
used. The TMS of the overcurrent relay will often be set high when the kVA ratings of the downstream
transformers and motors are large compared with one of the parallel generators. Overcurrent protection
(51 V) of generators tends to be back-up protection to other facilities such as stator differential
protection (87). It will be the last protection to operate if all the other facilities fail to respond.

12.2.3 Differential Stator Current Relay

Differential current protection (87) is used for generators to detect internal winding faults, which
may develop between phase windings or between a phase and the steel core. Sensitive high-speed
action is required in order to minimise the possibility of damage to the stator core laminations in
particular. A current as low as 20 amps can cause significant damage if it is allowed to pass to the core