The timing for the total interruption duration is performed by three programmable counters.
One counter controls the drop-out time, the point on the supply voltage reference waveform
that the IGCTs open. A second counter controls the interruption duration, the length of time
that the IGCTs remain open. A third counter controls the pick-up time, the point on the
supply voltage reference waveform that the IGCTs close.
The 6kHz clock signal is used as the clock for the drop-out counter and the pick-up counter
and the 60Hz clock signal is used as the clock for the duration counter. This means that the
drop-out and pick-up counters count 100 times faster than the duration counter. The duration
counter counts in cycles, the drop-out and pick-up counter count in 0.01 cycles.
B.2 Solid State Switch Schematic
Fig 21 shows the solid state switch circuit diagram for one phase. All three phases are
configured identically. Momentary service interruptions are generated by using an IGCT as a
switch. The IGCT is controlled by a fiber optic line and is a forward conducting, forward
blocking device. Diodes D1, D2, D3, and D4 are therefore needed to allow the IGCT to be
used with alternating current. During the positive half cycle, current flows from the supply,
through D1, through IGCT1, through D2, and to the load. During the negative half cycle,
current flows from the load, through D3, through IGCT1, through D4, and to the supply.
Diode D5, resistor R1, and capacitor C1 make up the turn-off snubber circuit. A turn on
snubber is not needed because of the inherent inductance in the line and in the load.
Switch SW1 and resistor R2 are used to generate voltage sags. When a momentary service
interruption is desired, SW1 is left open, so no current flows through R2. However, if a
voltage sag is desired, SW1 is closed so that R2 is effectively inserted in series with the load
when IGCT1 is open. This creates a voltage divider between R2 and the load.
During voltage sag operation, the value of the resistor R2 shown in Fig 79 determines the sag
level. If the load is a balanced three-phase load, choosing identical resistor values for all
three phases will produce a balanced three-phase sag. However, different values may be
chosen to produce unbalanced sags.