Protecting High-Voltage Supplies
A number of circuit configurations can be used for a high-voltage power supply. The most common incorporates a three-phase delta-to-wye transformer feeding a full-wave rectifier bridge, as shown in Figure 17.2. This arrangement provides high efficiency and low ripple content. With a well-balanced ac input line, the ripple component of the dc output is 4.2%, at a frequency six times the ac input frequency (360 Hz for a 60 Hz input). The dc output voltage is approximately 25% higher than the phase voltage, and each arm of the six-element rectifier must block only the phase voltage. The rms current through each rectifier element is 57% of the total average dc current of the load. The rectifier peak current is approximately equal to the value of the average dc output current. The typical power factor presented to the ac line is 95%.
Figure 17.3 illustrates the application of device staging to protect a high-voltage power supply from transient disturbances. As detailed previously for ac distribution systems, staging takes advantage of the series resistance and inductance of interconnecting wiring to assist in suppressing transient disturbances. The circuit includes two sets of varistors: primary and secondary units. The secondary set is rated for a lower clamp voltage and, together with the primary varistor groups, exercises tight control over disturbances entering the supply from the ac utility input.
Additional transient-suppression devices (CR1 to CR3) and three sets of RC snubbers (R1/C1 to R3/ C3) clip transients generated by the power transformer during retarded-phase operation. On the trans- former secondary, three groups of RC snubbers (R4/C4a-b to R6/C6a-b) provide additional protection to the load from turn-on/turn-off spikes and transient disturbances on the utility line.
Figure 17.4 shows the secondary side of a high-voltage power supply incorporating transient-over- voltage protection. RC networks are placed across the secondary windings of the high-voltage trans- former, and a selenium thyrector (CR7) is placed across the choke. CR7 is essentially inactive until the voltage across the device exceeds a predetermined level. At the trip point, the device will break over into a conducting state, shunting the transient overvoltage. CR7 is placed in parallel with L1 to prevent damage to other components in the system in the event of a sudden decrease in current drawn by the load. A sudden drop in load current will cause the stored energy of L1 to be discharged into the power supply and load circuits in the form of a high potential pulse. Such a transient can damage or destroy filter, feedthrough, or bypass capacitors. It also can damage wiring or cause arcing. CR7 prevents these problems by dissipating the stored energy in L1 as heat. Figure 17.5 shows an internal view of the high-power transmitter supply illustrated in Figure 17.4.