Circuit switching

An electronic switching circuit can be on or ojf, and these positions are used to represent the two basic binary numbers 1 and 0, respectively. Decimal 2 is 10 in binary (switch settings on, off), and decimal 1 is 01 in binary (switch settings off, on). 

Generation of harmonics and switching surges in a static device switching circuit 6/129... 

The use of SCRs in an inverter circuit is intricate because of the absence of a natural commutation. Now only a forced commutation is possible, as it is connected to a d.c. source which provides no current zeros and hence facilitates no natural commutation. A forced commutation calls for a separate switching circuit, which is cumbersome, besides adding to the cost. As a result of this feature, they are also called forced commutated thyristors. 

This is Ihe most commonly used inverter for Ihe control of a.c. motors and is shown in Figure 6.28(a). The fixed d.c. voltage from the uncontrolled rectifier converter acts as a voltage source to the inverter. The voltage in Ihe inverter unit is varied to Ihe required level by using a pulse width modulation, as noted earlier. Through Ihe switching circuit of Ihe inverter Ihe frequency of the... 

This is applicable to thyristor (SCR) circuits to protect all the semiconductor devices used in the switching circuit, such as diodes (also power diodes) or IGBTs, in addition to SCRs. The same protection can be applied to all the semiconductor circuits likely to experience high dv/di. 

Such a relay is connected on the supply side as shown in Figure 12.23(a). The arrangement is such that unless the relay closes, the motor switching circuit will not energize and ihe motor will not start. The contact closes only when the supply voltage is normal and the phase sequence positive. Even for undervoltage conditions, ihe torque developed by the relay may not be adequate to close the circuit. Such relays are. therefore, effective against... 

By installing p.f. improvement capacitors to compensate the no-load magnetizing current of the circuit being switched (Section 23.1 3). These may be installed in the same switching circuit to switch together with the inductive load,... 

Consider the simple switching circuit of Figure IS.I9ic). If we w ish lo find Ihe current through the impedance Zi. on Ihe closure of Ihe sw itch. S. then ... 

The impedance of the capacitor switching circuit will determine the attenuation of the striking voltage and the titne of arc extinction. 

If/ . L of cable, transformer and load inductances) and C are the switching circuit parameters then... 

Here we have considered the fault level of the switching circuit to be same as that of the entire system, as they are the total capacitor banks connected in the system and switched together as a single unit. 

At the instant of switching the surge impedance, Z, (equation (23.12)), and the natural frequency of the switched circuit, i.e. the transient frequency, /i, (equation (2.2.14)). determines the amount of inrush current. 

Let us refer to equation (23.13) for the inrush current, / on parallel switching. If we can increase the impedance of the switching circuit by introducing a resistance R or inductance L, or both, we can easily control this current to a desired level. 

Figure 23.23 Inductance L introduced into the switching circuit...

Figure 23.26 Equivalent switching circuit for six capacitor units of 60 kVAr each...

However, in no case will the switching current exceed the short-circuit kA of the switching circuit. 

The equivalent switching circuit is shown in Figure 23.27(c) . -. Improved... 

By providing 12 turns of 150 mm mean diameter of the 50 mm flexible copper cable connecting each 60 kVAr capacitor bank a self-inductance of roughly 42.93 x 10 H can be introduced into each switching circuit, which will limit the switching inrush current to almost the permissible value of the making current (/, ) of the switching device. 

This will be the surge frequency of the switching circuit when the sixth capacitor is switched on a circuit that has five capacitor units already switched. The improved value of this switching frequency, /s, af si the introduction of inductance 42.93 uH in each capacitor circuit will become. 

The above discharge is also possible through only an extra resistance in each capacitor switching circuit. Using equations (25.5) and (25.6),... 

The switching currents must be limited to as low a value as possible. However, in general, capacitors are designed for switching cuirents up to lOO/f (r.rn.s.) for milliseconds. If necessary, a series resistance or indtictance can be introduced into Ihe switching circuit to limit them to a desired level (Seetion 23.1 I ). 

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