SVC Controller

Introduction

This model represents the power component and the control system of a static compensator. The power component consists of a thyristor controlled reactor (TCR inductive branch) and three thyristor switched capacitors (TSC capacitive branches). The transformer is not modeled internally and must be added by the user.
The control system includes measuring, synchronization, regulation, distribution and firing subsystems. Depending on the operation mode, the model allows users to study step responses either for a voltage reference or a susceptance reference. This feature can be used to optimize the regulator parameters.The thyristors can also be fired by signals generated from an external source.

Static Compensator Icon and Diagram

Static Compensator Model
Figure 11 - 19 shows the static compensator diagram.

Power Component

The models of the four branches of a static compensator are identical (see Figure 11 - 20). Resistances R and r respectively represent ohmic losses in the capacitor and the reactor. One of these elements can be ignored by assigning a zero value to it in the control panel. The two resistances, the capacitor and the reactor form a type of black box. Hence, it is not possible to measure the voltage across one of the resistances or the reactor. However, the voltage across the capacitor is calculated and available as a signal. This is also the case for the voltages across the thyristors.

Static Compensator Diagram

Static Compensator Branch

Control System Synchronization Unit
The synchronization unit consists of a phase-lock loop (PLL) applied to each voltage phase on the transformer primary. The PLL calculates the frequency and phase angle required to fire the thyristors. Figure 11 - 21 shows a simplified diagram of this unit.
This type of synchronization has the advantage of being insensitive to harmonics and stable in frequency.

Phase-Lock Loop (PLL)

Measuring Unit
Voltage measuring must be accurate, fast and insensitive to harmonics. To do this, the output of the Park conversion block is integrated and the voltage is measured by subtracting two consecutive samples of the integrator output with a delay of one cycle between them. The voltage measuring unit is shown in Figure 11 - 22.

Measuring Unit

Control Unit

The control unit consists of a proportional and integral (PI) controller. The latter compares the voltage measured and voltage reference to achieve.

EQ 11

The output of the controller is given by:

EQ 12

Bp is the required susceptance on the primary side for regulation. The current I is not measured but calculated using Bp and Umes. The response of the controller depends on the value of the gains. The integral gain determines the speed of the controller, while the proportional gain can be used to compensate for the delay in the firing system.

Distribution unit

The distribution unit receives the following input:

• The leakage inductor of the transformer 
• The signal Bp from the PI
• The states and vaolues of each TSC capacitive and TCR inductive branch
• And the value of the hysteresis to apply at transition points

From the primary susceptance Bp of the static compensator and the leakage inductance of the transformer, the susceptance Bs on the secondary side is calculated and then represented as a parallel combination of the TSC capacitive and TCR inductive branches.

EQ 13

The value of the capacitance Bcap produced by the parallel capacitive branches and the value of Bind is given by the non-linear function:

EQ 14

The calculation of the equivalent impedance of the parallel TSC capacitive branches take into account the availability of the branches. Therefore, it is possible to operate in downgraded or degradation mode.

In order to avoid oscillations when the capacitors are switched, hysteresis is used at transition points when the number of parallel TSC capacitive branches changes.

Firing Unit

The function of the firing unit is to send the firing orders to the thyristors of the different branches. To do this, it receives the following input:

• Phase angle (omega) of the synchronization voltage
• Firing angle alpha
• Firing order of the TSC capacitive branches

Since the TCR inductive branch is controlled, the firing unit sends the omega t degree pulses after the last zero-crossing of the synchronization voltage. Since the TSC capacitive branches are only switched and not controlled, their firing is always executed at the same time on the waveform, that is 90 degrees before the zero-crossing of the voltage.