Lab 2 - Introduction

AC controllers are AC-AC converters frequently used for controlling the magnitude of the fundamental component of an AC waveform, keeping the same frequency as of the input waveform, or for controlling the rms value of the voltage supplied to a given load. In electric utility applications that require either or control of the output voltage, such as in static VAR compensators, thyristor AC-AC converters are used.

The reactive power consumed by the TCR can be controlled continuously from rated value to zero by varying the firing angle of the thyristors (α) between 90° and 180°, respectively. In other words, the AC controller operates with phase control. In steady-state and at the grid frequency, the TCR can be replaced by an equivalent inductive reactance, whose value is determined by α. The TCR does, however, introduce current harmonics, which affect the power quality of the voltage of the AC bus to which the TCR is connected. There are various techniques to circumvent this problem, such as the use of multi-pulse topologies and harmonic traps, which are essentially LC shunt filters tuned at the frequencies of the main harmonics created by the TCR.

Figure 1: (a) Thyristor-Controlled Reactor (TCR, left) (b) Thyristor-Switched Capacitor (TSC, right)

Figure 2: (a) Thyristor-Controlled Reactor (TCR, left), and (b) TCR current waveforms (right)