TABLE OF CONTENTS No matter how large SCRs and their relatives may become, there will always be a need for operating devices in series and/or parallel to meet the requirements of high-voltage DC (HVDC) systems, var compensators, and flexible AC transmission system (FACTS) devices. The design objective is to provide good sharing of the voltage in series operation and the current in parallel operation. Neither is trivial. SCRs have been chosen to illustrate the techniques, but other semiconductors will have similar requirements.
Series operation of SCRs requires the SCRs to share equally the applied voltages under all conditions of use. There are four distinct regions of operation for an SCR: turn-on, forward conduction, turn- off, and quiescent operation with either forward or reverse voltage. In a series stack of SCRs, the slowest device will have a disproportionate share of the overall voltage prior to its turn-on, since its partners will already be on. Therefore, it is necessary to turn on each SCR as quickly as possible with simultaneous gating signals having fast rise times to some 3 to 5 A in less than 500 ns. In nearly all situations of SCR gating currents, more, higher, and faster are all better. The driving voltage is typically 20 to 40 V open circuit with current limiting.
Figure 11.1 repeats Fig. 10.2 to show a typical gate drive suitable for series operation. If the series gates are fired from fiber optics, the dispersion times of the various emitters and detectors in the...
