Differential Protection (87T/87Hs, 87M/G, 87B); Overcurrent Protection (50/51, 50Hs) - Siemens 7UT51 Instruction Manual

2
Description of Relay
Pow er
Bloc k ed
Figure 2.1 Model 7UT512 Relay (left) and Model
7UT513 Relay (right)
14
87
8 7
87 H S
4 9
50 /5 1
87
8 7
7
8
9
1
1 0
2
87 H S
1
3 1
4 1 2
1 3
5
1
6 4
4 9
50 /5 1
87 N
64 T
2.2.2 Differential Protection
(87T/87HS, 87M/G, 87B)
Differential protection works differently for
transformers and rotating machines, but in each case,
the relay uses its user-programmable settings to
recognize a differential current condition and respond
to the condition. The variable percentage differential
characteristic of the relay provides fast, sensitive, and
reliable tripping for internal faults, and security against
operation on external faults. The relay develops
restraint and differential operating quantities to
accomplish this level of protection.
Transformer differential protection requires special
considerations for inrush and overexcitation. Second
harmonic current is used as additional restraint during
inrush conditions, and the fifth harmonic current is
used as additional restraint during overexcitation
conditions. These restraints prevent erroneous
tripping.
Differential protection for transformers can be further
complicated by transformation ratios and phase shift.
The 7UT51 relay will automatically accommodate
these factors based on the system settings, relay
configuration settings, and protection settings
programmed into the relay.
2.2.3 Overcurrent Protection
(50/51, 50HS)
O R
Overcurrent protection can be used as backup
protection for any selected winding or bus-branch, or
be applied to a separate additional protection zone
O R
(7UT513 only). The overcurrent protection can
operate as definite time or inverse time overcurrent
protection with a selectable characteristic. It can
operate with an independent high-set overcurrent
stage.
7UT51 v3
PRIM-2330C