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7UT51 v3
5
Motor or Generator Differential Protection (87M/G)
The motor/generator differential protection function
(87M/G) provides high speed, secure primary
protection for faults within a motor or generator. The
trip characteristic is configurable; as is restraint based
on detection that a through-fault is causing CT
saturation. The algorithm responds to instantaneous
current measurements, and so can often detect a
short circuit in less than one cycle.
The relay can also provide ground differential
protection (see Chapter 7).
5.1
Method of Operation
Differential protection is an application of Kirchoff's
current law—the sum of all the currents entering and
leaving a node must be zero. In a system consisting of
ideal components, if the sum of all the measured
currents (called the "differential current") is not zero,
then there must exist an unmeasured current — a
fault.
Actual applications do not use ideal components;
therefore, the differential protection must take into
account the non-ideal behavior of the protected
machine, the CTs monitoring it, and the rest of the
power distribution system. Specifically:
•
A small, constant differential current may exist
(due, for example, to a magnetizing current). To
prevent pickup due to this current, the relay has a
minimum 87 pickup level (see Section 5.4.1 on
page 72).
•
A fault outside the protected zone may cause
large currents to flow through the protected zone
that saturate the CTs. The pickup characteristic is
configurable to compensate for this (see
Section 5.4 on page 71), while separate settings
limit this restraint so that the relay can detect an
internal fault that occurs during an external fault
(see Section 5.5 on page 74).
PRIM-2330C
Motor or Generator Differential Protection (87M/G)
5.2
Describing the
Protected Motor or
Generator
The 7UT51 relay provides differential protection for
motors and generators with either longitudinal
windings or transverse windings (see Figure 5.1 on
page 70 and Figure 5.2 on page 70).
If the relay is used as differential protection for motors
or generators, all currents are measured as multiples
of the nominal current of the protected object. This
data is set during device configuration and includes
rated apparent power, rated voltage, and rated
currents of the current transformers.
The following information is needed for all types of
protected objects and the associated current
transformers:
•
What the protected object is
•
What the rated phase-to-phase voltage (Vn) of the
protected motor or generator is
•
What the rated frequency of the motor or
generator is
•
What the rated apparent power (VA) of the motor
or generator is
This section describes these choices and settings.
5
67
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