A 300-Mvar Static Var Compensator (SVC) regulates voltage on a 6000-MVA 735-kV system. The SVC consists of a 735kV/16-kV 333-MVA coupling transformer, one 109-Mvar thyristor-controlled reactor bank (TCR) and three 94-Mvar thyristor-switched capacitor banks (TSC1 TSC2 TSC3) connected on the secondary side of the transformer.

Switching the TSCs in and out allows a discrete variation of the secondary reactive power from zero to 282 Mvar capacitive (at 16 kV) by steps of 94 Mvar, whereas phase control of the TCR allows a continuous variation from zero to 109 Mvar inductive.

Taking into account the leakage reactance of the transformer (15%), the SVC equivalent susceptance seen from the primary side can be varied continuously from -1.04 pu/100 MVA (fully inductive) to +3.23 pu/100 Mvar (fully capacitive). The SVC controller monitors the primary voltage and sends appropriate pulses to the 24 thyristors (6 thyristors per three-phase bank) in order to obtain the susceptance required by the voltage regulator.

Each three-phase bank is connected in delta so that, during normal balanced operation, the zero-sequence triplen harmonics (3rd, 9th...) remain trapped inside the delta, thus reducing harmonic injection into the power system.

The power system is represented by an inductive equivalent (6000 MVA short circuit level) and a 200-MW load. The internal voltage of the equivalent can be varied by means of programmable source in order to observe the SVC dynamic response to system voltage sags.

With the SSN solver, the natural way to decouple the system is to use the common connection point of the TCR and the 3 TSCs, resulting in 4 groups of 6 switches each and nodal matrix of size 3 only, efficient in computational terms. The TCS groups are interfaced with I-type SSN Nodal Interface Blocks while the TCR and network group are interfaced with a V-type block (hint: it is clearly inductive so it must be driven by a Voltage source for causality reasons >V-type).

With the ITVC compensation of thyristor firing, accurate simulation is achieved. The above figure shows the simulation results for a slow scan of the TCR bank firing angle. The figure below shows a typical effect of thyristor-based systems in fixed-step simulation.

In that case, a kind of quantization effect occurs on the system output reactive power, as it shows some discrete step effects. With the ITVC compensation of ARTEMiS and SSN, the reactive output of the systems is smooth with regards to the firing angle.