Advertisement
41-348.1H
2.1. R
C
ESTRAINT
IRCUIT
The restraint circuit of each phase consists of a cen-
ter-tapped transformer, a resistor, and a full wave
rectifier bridge. The outputs of all the rectifiers are
connected in parallel. The parallel connection of rec-
tifiers is a maximum voltage network. Hence, the
voltage applied to the filter circuit is proportional to
the phase current with the largest magnitude.
2.2. O
C
PERATING
IRCUIT
The operating circuit consists of a transformer, a
resistor, and a full wave rectifier bridge. The outputs
of all the rectifiers are connected in parallel. This par-
allel connection of rectifiers is a maximum voltage
network. Hence, the voltage applied to the filter cir-
cuit is proportional to the phase current with the larg-
est magnitude.
2.3. S
C
ENSING
IRCUIT
The sensing circuit is connected to the output of the
restraint filter circuit, the operating filter circuit and
the input to the amplifier circuit.
2.4. A
C
MPLIFIER
IRCUIT
The amplifier circuit consists of a two-transistor
amplifier which controls the operation of a relaxation
oscillator.
The amplifier circuit is connected to the sensing cir-
cuit such that it receives the difference in output of
the restraint filter and the operating filter. Thus, the
polarity of the input voltage to the amplifier depends
upon the relative magnitude of the voltages appear-
ing on the restraint and operating filters. When the
voltage output of the operating filter is greater than
the output voltage of the restraint filter, a voltage of a
certain polarity appears across the input of the ampli-
fier. To trigger the amplifier requires that the output
voltage of the operating filter be greater than the out-
put voltage of the restraint filter.
2.5. T
C
RIP
IRCUIT
The trip circuit consists of a thyristor which has an
anode, cathode, and a gate. The anode of the thyris-
tor is connected to the positive side of the dc supply
and the cathode of the thyristor is connected to the
negative side of the dc supply through the trip coil of
a breaker. The gate of the thyristor is connected to
the output of the amplifier circuit through a pulse
transformer.
2
With no gate current flowing, the thyristor acts as an
open circuit to the breaker trip coil. When a gate cur-
rent is applied tot he thyristor the thyristor connects
the breaker trip coil to the dc supply.
2.6. I
C
NDICATING
IRCUIT
The indicating circuit is triggered by a signal from the
amplifier of the relay. Under normal or non-fault con-
ditions, the indicating circuit is turned off. When a
fault is applied to the relay, the amplifier will conduct
to cause a signal to flow into the indicator circuit.
When the indicator circuit is triggered, the lamp will
turn on. This lamp will remain lit until the indicator cir-
cuit is interrupted by resetting the micro-switch.
2.7. S
P
URGE
ROTECTION CIRCUIT
The surge protection circuit consists of two capaci-
tors (C10 and C11) and a R-C network which is con-
nected across the anode and cathode of the tripping
thyristor to prevent the SCR from firing by a surge of
voltage.
2.8. E
R
XTERNAL
EACTORS
Three reactors are mounted on a metal plate with a
separate terminal strip. The reactors are of the satu-
rable type.
3. OPERATION
The Type SA-1 relay is connected to the protected
apparatus as shown in Figure 9. On external faults,
current flows through the primary winding of the
restraint transformers to induce a voltage on the
restraint side of the sensing circuit. If the two sets of
main current transformers have different perfor-
mances, some current will flow out of the mid-tap of
the restraint transformers to the operating transform-
ers. This will produce a voltage on the operating side
of the sensing circuit. With the relay correctly applied,
sufficient restraint voltages will exist to prevent the
operating voltage from triggering the amplifier.
The percentage slope characteristic of the relay lim-
its the operating voltage on heavy external faults
where the performance of the two sets of current
transformers may be quite different.
On internal faults, the operating coil current is the
sum of the current flowing in each of the windings of
the restraint transformer and sufficient operating volt-
age is available to overcome the restraint voltage.
Type SA-1
Generator Differential Relay
Advertisement
