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Section 1. DESCRIPTION
The MBCH is a range of high-speed biased differential relays suitable for protection
of two or three winding power transformers, auto-transformers or generator
transformer units.
The MBCH may also be regarded as an alternative to the high impedance relays for
the protection of reactors, motors and generators.
The relay is extremely stable during through faults and provides high speed operation
on internal faults, even when energized via line current transformers of only moderate
output. Immunity to false tripping due to large inrush currents on energization of the
power transformer, and during overfluxing conditions, is guaranteed without the use
of harmonic filter circuits, therefore eliminating their associated delay.
A tapped interposing transformer for ratio matching of the line current transformers is
available where required. The transformer taps are spaced at intervals of 4% and
better, allowing matching to well within 2% in most cases.
The relay models available are as follows:
Type No of bias Publication
designation circuits Application ref no
MBCH12 2 Two winding power transformer R6070
MBCH13 3 Generally 3 winding power
transformer, where bias is required R6070
for each of the 3 groups of CTs
MBCH16 6 For all applications requiring R6070
4, 5 or 6 bias circuits
Section 2. INSTALLATION
2.1 General
Protective relays, although generally of robust construction, require careful treatment
prior to installation and a wise selection of site. By observing a few simple rules the
possibility of premature failure is eliminated and a high degree of performance can
be expected.
The relays are either despatched individually or as part of a panel/rack mounted
assembly in cartons specifically designed to protect them from damage.
Relays should be examined immediately they are received to ensure that no
damage has been sustained in transit. If damage due to rough handling is evident,
a claim should be made to the transport company concerned immediately, and
ALSTOM T&D Protection & Control Ltd should be promptly notified.
Relays which are supplied unmounted and not intended for immediate installation
should be returned to their protective polythene bags.
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