Bus Differential Protection (87B); Method Of Operation; Describing The Protected Bus; Type Of Protected Object - Siemens 7UT51 Instruction Manual

7UT51 v3
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Bus Differential Protection (87B)

The bus differential protection function (87B) provides
high speed, secure primary protection for faults on a
two-branch or three-branch bus. The trip characteristic
is configurable; as is restraint based on detection that
a through-fault is causing CT saturation, or that a CT
circuit is apparently open. 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).
6.1

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.
Since actual applications do not use ideal components
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 87B pickup level (see Section 6.4 on
page 83).
• 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 6.4 on page 83), while separate settings
limit this restraint so that the relay can detect an
internal fault that occurs during an external fault
(see Section 6.5 on page 85).
If the 7UT51 relay is used as differential protection for
busses, all currents are referred to the nominal current
of the protected bus. This data is set during device
configuration. Thus, if the CTs at the ends of the bus
have different primary current, no external matching
devices are necessary.
PRIM-2330C
Bus Differential Protection (87B)
6.2 Describing the
Protected Bus
The following information is needed for all types of
protected objects and the associated CTs:
• What the protected object is,
• What the nominal secondary rating of the CT is
• What the rated frequency of the protected object is
• The manner in which the ground-current inputs
are used (7UT513 only).
This section describes these choices and settings.
6.2.1

Type of Protected Object

The relay uses different settings and algorithms for
different types of protected objects. Address 7801
specifies what type of device or equipment is being
protected.
7801 PROT.OBJ
Type of device or equipment the relay is protecting.
Option Description
- -
2 WIND TRANSF two
(see Chapter 4)
- -
3 WIND TRANSF three
(see Chapter 4)
MOTOR/ motor or generator
GENERATOR (see Chapter 5)
2 ENDED TIE PT. two
3 ENDEDTIE PT. three
-
1Ph. TRANSF single
(see Chapter 4)
-
winding transformer
-
winding transformer
-
branch bus
-
branch bus
-
phase transformer
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