Transformer Protection - Ametek UPLC CU44-VER04 Applications Manual

3.2.1 Transformer
Protection
A typical transformer protection
scheme is illustrated in Figure
3–13. A direct trip channel is keyed
to the trip state when the trans-
former protective relays operate.
The received trip signal will then
trip the remote end breaker and
lock out reclosing.
Although it is no longer widely
used, you may use a ground switch
operated by the transformer pro-
tective relays for transformer pro-
tection. In this technique, a ground
fault is initiated on the transmis-
sion line at G, providing adequate
fault current for the ground relays
at H to trip the breaker at H. This
system is slower but is widely used
on lower voltage systems and is
fairly simple and straightforward.
It does not require any secure com-
munication medium between G
and H. For this type of application,
the ground relays at H can be set to
operate for 100 percent of the line
and not overreach to bus G.
While a single switch on one phase
is normally applied, you may use a
double switch on two phases to ini-
tiate a double-phase-to-ground
fault. In the latter case, both phase
and ground relays can operate to
ensure redundancy. Fault ground-
ing is not applicable to all systems
because of high short-circuit
capacity.
May 2012
G
F
Local Positive
Local Negative
1
Local Positive
0
1
Local Negative
0
Trip Positive
Trip Negative
Trip Coincidence
Shaded Portion is
Remote Square Wave
Trip Coincidence
Internal Line Up
a) Normal Internal Fault
G
Local Positive
Local Negative
1
Local Positive
0
1
Local Negative
0
Trip Positive
Trip Negative
Trip Coincidence: None
Remote Square Wave
External Line Up
b) External Fault
G
F
Local Positive
Local Negative
1
Local Positive
0
1
Local Negative
0
Trip Positive
Trip Negative
Trip Coincidence
Remote Square Wave Shaded Portion is
Trip Coincidence
Internal Line Up
c) Internal Fault with Outfeed (Comparison at Strong Terminal)
Figure 3–11. Response of Segregated Phase Comparison
System with Offset Keying.
Chapter 3. Applications
H
Keying
Square
Trip Positive Wave
Trip Negative
Note:
Similar Comparison Occurs
at Terminal H.
H
F
I Key
Note:
Local Square
Waves "Nest" within
Remote Square Wave
Keying
to Provide Security
Square
Trip Positive
Wave
Trip Negative
Note:
Similar Comparison Occurs
at Terminal H.
H
I Key
Trip Positive
Trip Negative
Keying Square Wave is
Steady Trip Negative
Note:
Similar Comparison Occurs
at Terminal H.
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