High-voltage transmission is carried by means of AC overhead transmission lines and DC overhead transmission lines and cables. Ancillary equipment such as switch-gear, protective equipment and reactive power support equipment is needed for the correct functioning of the transmission system.
The thyristor-switched capacitor (TSC)
Thyristor switched capacitor is defined as 'a shunt-connected, thyristor-switched capacitor whose effective reactance is varied in a stepwise manner by full- or zero- conduction operation of the thyristor valve'.
Increasing the gating angle (reducing the conduction angle) has two other important effects. First, the power losses decrease in both the thyristor controller and the reactor. Second, the current waveform becomes less sinusoidal; in other words, the TCR generates harmonic currents. If the gating argles are balanced, (i.e. equal for both thyristors), all odd order harmonics are generated, and the rms value of the nth harmonic component is given by ...
Switching transients in the general case
Under practical conditions, it is necessary to consider inductance and resistance. First consider the addition of series inductance in Figure 6.16. In any practical TSC circuit, there must always be at least enough series inductance to keep di/dt within the capability of the thyristors. In some circuits there may be more than this minimum inductance. In the following, resistance will be neglected because it is generally small and its omission makes no significant difference to the…
Switching transients and the concept of transient-free switching
The simple case of a switched capacitor, with no other circuit elements than the voltage supply, is used first to describe the important concept of transient-free switching. Figure 6.16 shows the circuit.
The thyristor-controlled transformer (TCT)
Another variant of the TCR is the TCT (Figure 6.9). Instead of using a separate step- down transformer and linear reactors, the transformer is designed with very high leakage reactance, and the secondary windings are merely short-circuited through the thyristor controllers
Current trends in power semiconductor technology
For this converter bidirectional self- commutated devices are needed to build the converter. At the moment research efforts show some promising results (Heinke et al., 2000). However, a commercial product is probably not going to be available before the next decade or so.
Protection of semiconductors - snubber circuits
The relative magnitudes of the conduction and switching losses are greatly dependent on the type of the converter (i.e. resonant, quasi-resonant, PWM, etc.), the operating frequency, the type of the load (i.e. linear or non-linear, resistive or inductive), and certain characteristics of the switch itself (i.e. turn-on time, turn-off time, etc.). Off- state losses are generally a very small portion of the total losses and are considered negligible.
Many other components must be used to make a converter topology function properly to shape the voltage and current supplied from the source to the ones required by the load in a regulated manner and in many cases allow the power flow to be bidirectional. Components such as inductors, capacitors and resistors must be used not only as part of protection devices in the case of snubbers, but also as filter elements.
Desired characteristics of fully-controlled power semiconductors
High forward and reverse voltage blocking ratings. In order to achieve higher power ratings for a given converter, many switches are connected in series to build a valve especially for high and ultra-high power applications. lf new devices become available with higher voltage ratings, the number of the required switches connected in series to produce the same valve will be reduced. This will minimize the problems with the voltage sharing of the various switches in series, will increase the…
Power semiconductor devices and converter hardware issues
The advances of high voltage/current semiconductor technology directly affect the power electronics converter technology and its progress. The 'perfect' high-power semiconductor is yet to be fully developed and become commercially available.