Advertisement
Resistors (R9, R10, R11), shown in Figure 5, provide
a return path for the operating current.
The sharp turn-on characteristic is obtained by use of
special voltage reference circuit that consists of R33,
R34, R35 and TH2 (for temperature compensation).
The TP2 potential derived from this reference circuit
provides base drive for Q8 transistor that is pre-
vented from turning on as long as Q2 is turned on by
TP1 potential. Whenever operating voltage V
exceeds one of the restraint voltage (V
V
) transistor Q1 is turned on lowering the TP1
ZN
potential below TP2 potential thus making it possible
for Q8 to conduct, thus turning on Q9 and after time
delay controlled by R45-C9 time constant (or R76-C9
for SDG-1T, -2T or R77-C9 for SDG-4T relays), pro-
ducing an output.
2.8.
Zero Sequence Current Detector
(SDG-1T Only)
To prevent operation of the magnitude comparator
during a blown potential fuse or unequal pole closing,
a zero sequence current detector supervises the
operation of the triggering network by preventing
capacitor C10 in the triggering circuit from charging,
keeping the input to D51 at low potential through the
diode D59.
With no or very low zero sequence current Q12 tran-
sistor is conducting.
In the presence of the residual current a current
derived negative voltage through transformer I
appears across resistor R60, turns transistor Q12 off
and thus raising its collector close to the positive dc
voltage supply level, blocking D59 from discharging
the C10 capacitor allowing Q13 to operate.
2.9.
Frequency Verification
During certain switching conditions, such as energi-
zation of a transmission line, residual currents and
voltages may exist of higher frequencies than 60
hertz per second. The frequency verifier prevents
relay operation when the operating voltage period is
less than 5 ms. The frequency verification circuit con-
sists of two functional parts: zero-crossing and timing
circuit. The zero-crossing circuit consists of transis-
tors Q3, Q4, Q5 and Q6. The zero-crossing circuit is
used to allow operation in the presence of higher fre-
quencies of small magnitude superimposed on a fun-
damental of 60 Hz. During the positive or negative
half cycles of the operating voltage V
transistors are driven into saturation by the output of
the T
transformer. Transistor Q5 conducts until
FV
capacitors C6 or C7 respectively are fully charged.
While either capacitor is charging through R29, tran-
sistor Q5 drives transistor Q6 to discharge timing
capacitor C9, thus starting the timing cycle with close
to zero charge on the capacitor. The function of the
timing capacitor is to delay the operation of the relay
for 5 milliseconds. The delay is obtained by delaying
the firing time of Z2 Zener diode. If a next zero-cross-
ing should occur within the preset delay time, C9
WO
capacitor is discharged again and the timing cycle is
, V
or
XN
YN
repeated. In case of presence of predominant higher
frequencies (over 100-140 Hz), several zero-crossing
pulses will occur within the preset time delay, thus
keeping the C9 capacitor from charging up to the
Zener firing voltage.
The transient blocking is accomplished through the
operation of Q7 transistor that is driven by a short
pulse formed through R46 and C8 capacitor to clear
C9 capacitor of any charge before initiating 5 milli-
second delay. Any tripping signal coming off Q9 tran-
sistor of a duration less than 5 milliseconds output
circuit will not produce a relay output.
2.9.1. For SDG-1T and SDG-4T
The coil of the telephone relay is in series with the
collector of transistor Q16 so that, when Q16 is
turned on, the telephone relay is energized causing
the contacts to close. A Zener diode is connected
across the collector of Q16 to limit the inductive kick
voltage appearing at the collector during the pickup
and dropout of the telephone relay.
0
2.9.2. For SDG-2T
The coil of the telephone relay is in series with the
collector of transistor Q14 so that, when Q14 is
turned on, the telephone relay is energized causing
the contacts to close. A silicon diode is connected
across the coil of the telephone relay to limit the
inductive kick voltage appearing at the collector dur-
ing the pickup and dropout of the telephone relay.
2.10. Push-button Check Circuit
2.10.1. SDG-2T Only
For the SDG-2T relay the push-button check circuit is
used for in-service operational check-out of the out-
put relay is used on S = 1 setting. Depressing the
push-button operates the overcurrent circuit. This
action must be preceded by the opening of the relay
trip circuit (red handle) and relay voltage to neutral
, Q3 or Q4
WO
switch 7. The opening of the voltage switch produces
operating voltage conditions in the magnitude com-
parator that gates the thyristor switch. Operation of
I.L. 41-496.52D
5
Advertisement
