Page Contents

I. Preliminary Theoretical Calculations

A 3-phase Static VAR Compensator (SVC) rated (150 MVA capacitive, 60 MVA inductive) consists of the following components, rated on a per phase basis: (1) a 10 MVA Fixed Capacitor (FC) bank; (2) two Thyristor Switched Capacitor (TSC) banks rated 20 MVA each; (3) a 20 MVA Thyristor Controlled Reactor (TCR). The components are connected to a common bus fed from the transformer secondary at 13.8 kV (Shown in Figure 26). Solve the problem on a single-phase basis.

Open

Figure 20: Single line diagram of a Static VAR Compensator

Steady-state operation of the TCR independently

1. Compute the value of the reactance.

2. The TCR delay angle is set to 90^o. Draw the waveform of the voltage and current across the inductor. Indicate maximum and minimum instantaneous values of voltage and current. Draw the waveform of the current flowing in the thyristors and in ac the line. Give the rms value of the TCR current.

3. For delay angles of 120^o and 150^o, repeat Question 2.

4. Draw the approximate harmonic spectrum of the inductor current up to the 20^th harmonic, indicating harmonic order and approximate amplitude for all 3 operating conditions.

5. Draw the transfer characteristics or control law of the TCR, rms line current, effective reactance and reactive power as a function of delay angle α.

6. Draw voltage characteristics of the TCR as a function of reactive power and delay angle.

Operation of the TSC independently

1. Indicate the modes of operation of the TSC. Indicate when the thyristors can be gated and what additional element needs to be included to limit the transient turn-on current.

2. Draw the operating characteristics of the TSC, reactive power as a function of line voltage.

3. Tune the TSC-1 to act as a 5^th harmonic filter and compute the reactor value. Indicate the reactance of the TSC-1 branch at fundamental frequency. Compute the value of resistance to be added in series such that the LC filter has a quality factor of 50.

Operation of the SVC - all components in operation

1. Show the operation of the SVC elements (FC, TSC, and TCR) required to cover the complete reactive power range from capacitive to inductive.

2. Draw the operating characteristics of the SVC, reactive power as a function of line voltage, over the complete range of reactive powers.

II. Simulation Procedure

Steady-state operation of the TCR independently

By activating the TCR switch and based on the values calculated in section I proceed with the questions below:

1. Set the TCR delay angle to 90^o and set the reactance to the value computed in section I, question 1. Plot the waveform of the voltage and current across the inductor. Indicate maximum and minimum instantaneous values of voltage and current. Plot the waveform of the current flowing in the thyristors and in ac the line. Plot and record the rms value of the TCR current.

2. For delay angles of 120^o and 150^o, repeat Question 1.

3. Plot the harmonic spectrum of the inductor current up the 20^th harmonic, indicating harmonic order and amplitude for all 3 operating conditions (α = 90°, 120°, 150°). (Note: refer to Harmonics analysis using ScopeView for more details on how to plot the harmonic spectrum)

4. Record and plot the transfer characteristics of the TCR, rms line current and reactive power as a function of delay angle α (α = 90°, 120°, 150°, 180°).

5. Record and plot the voltage characteristics of the TCR as a function of reactive power and delay angle. Plot for values of α equal to (90°, 120°, 150°, 80°) and voltage equal to (0.2, 0.9, 1.0 pu). Note: Adjust the 345 kV AC source voltage such that the transformer secondary voltage is approximately (0.2, 0.9, 1.0 pu) for each value of α.

Table 2: Steady-state operation of the TCR independently Question 4 recorded values

Table 3: Steady-state operation of the TCR independently Question 5 recorded values

Operation of the TSC independently

1. Disconnect the TCR and connect TSC-1 to the circuit by clicking on the corresponding switches. Tune TSC-1 to act as a 5^th harmonic filter with a quality factor equal to 50, using the reactor, capacitor resistance value computed in section I. Plot the reactive power, source phase voltage and capacitor current for both modes of operation of the TSC. Indicate when the thyristors are gated to limit the transient turn-on current.

2. Record and plot the operating characteristics of the TSC, reactive power as a function of line voltage (0.2, 0.9, 1.0 pu). Note: Adjust the 345 kV AC source voltage such that the transformer secondary voltage is approximately (0.2, 0.9, 1.0 pu) when the TSC operates. 

3. Adjust the AC source voltage back to 345 kV. Reconnect the TCR to the same bus as TSC-1. Plot the harmonic spectrum of the line current up the 20^th harmonic, indicating harmonic order and amplitude for α = 120° and 150°.

Table 4: Operation of the TSC independently Question 2 recorded values

Operation of the SVC - all components in operation

1. Show the operation of the (FC, TSC, and TCR) and plot the operating characteristics of the SVC, reactive power as a function of line voltage (0.2, 0.9, 1.0 pu), over the complete range of reactive powers. Indicate which elements are connected to the system. Only consider the delay angles α equal to 90° and 180° for the TCR. Note: Adjust the 345 kV AC source voltage such that the transformer secondary voltage is approximately (0.2, 0.9, 1.0 pu) when the elements comprising the SVC are operating. 

Table 5: Operation of the SVC - all components in operation Question 1 recorded values

III. Questions

1. Tabulate the theoretical results calculated in section I and the simulated results obtained in section II for the operation of the TCR, TSC and the SVC. Compare the results and comment on the differences.

2. Explain the impact a transformer leakage inductance of 15% would have on the reactive power capabilities of the static VAR compensator.

3. Explain the ability and limits for the static VAR compensator to provide reactive power compensation for a voltage sag on an electric utility transmission network.

4. Explain the three-phase circuit configuration the TCR assumes to eliminate the triple harmonics.