This example shows the operation of a +100 Mvar/-100 Mvar 48-pulse GTO STATCOM.
P. Giroux ; G. Sybille (Hydro-Quebec)
A 100-Mvar STATCOM regulates voltage on a three-bus 500-kV system. The 48-pulse STATCOM uses a Voltage-Sourced Converter (VSC) built of four 12-pulse three-level GTO inverters. Look inside the STATCOM block to see how the VSC inverter is built. The four sets of three-phase voltages obtained at the output of the four three-level inverters are applied to the secondary windings of four phase-shifting transformers (-15 deg., -7.5 deg., 7.5 deg., +7.5 deg. phase shifts). The fundamental components of voltages obtained on the 500 kV side of the transformers are added in phase by the serial connection of primary windings. Please refer to the "power_48pulsegtoconverter" example to get details on the operation of the VSC.
During steady-state operation the STATCOM control system keeps the fundamental component of the VSC voltage in phase with the system voltage. If the voltage generated by the VSC is higher (or lower) than the system voltage, the STATCOM generates (or absorbs) reactive power. The amount of reactive power depends on the VSC voltage magnitude and on the transformer leakage reactances. The fundamental component of VSC voltage is controlled by varying the DC bus voltage. In order to vary the DC voltage, and therefore the reactive power, the VSC voltage angle (alpha) which is normally kept close to zero is temporarily phase shifted. This VSC voltage lag or lead produces a temporary flow of active power which results in an increase or decrease of capacitor voltages.
One of the three voltage sources used in the 500 kV system equivalents can be varied in order to observe the STATCOM dynamic response to changes in system voltage. Open the "Programmable Voltage Source" menu and look at the sequence of voltage steps which are programmed.
Dynamic response of the STATCOM
Run the simulation and observe waveforms on the STATCOM scope block. The STATCOM is in voltage control mode and its reference voltage is set to Vref=1.0 pu. The voltage droop of the regulator is 0.03 pu/100 VA.Therefore when the STATCOM operating point changes from fully capacitive (+100 Mvar) to fully inductive (-100 Mvar) the STATCOM voltage varies between 1-0.03=0.97 pu and 1+0.03=1.03 pu.
Initially the programmable voltage source is set at 1.0491 pu, resulting in a 1.0 pu voltage at SVC terminals when the STATCOM is out of service. As the reference voltage Vref is set to 1.0 pu, the STATCOM is initially floating (zero current). The DC voltage is 19.3 kV. At t=0.1s, voltage is suddenly decreased by 4.5 % (0.955 pu of nominal voltage). The SVC reacts by generating reactive power (Q=+70 Mvar) in order to keep voltage at 0.979 pu. The 95% settling time is approximately 47 ms. At this point the DC voltage has increased to 20.4 kV. Then, at t=0.2 s the source voltage is increased to1.045 pu of its nominal value. The SVC reacts by changing its operating point from capacitive to inductive in order to keep voltage at 1.021 pu. At this point the STATCOM absorbs 72 Mvar and the DC voltage has been lowered to 18.2 kV. Observe on the first trace showing the STATCOM primary voltage and current that the current is changing from capacitive to inductive in approximately one cycle. Finally, at t=0.3 s the source voltage in set back to its nominal value and the STATCOM operating point comes back to zero Mvar.
If you look inside the "Signals and Scopes" subsystem you will have access to other control signals. Notice the transient changes on alpha angle when the DC voltage is increased or decreased in order to vary reactive power. The steady state value of alpha (0.5 degrees) is the phase shift required to maintain a small active power flow compensating transformer and converter losses.
The initial states required to start this example in steady state have been saved in the "power_statcom_gto48p.mat" file. When you open this example, the InitFcn callback (in the Model Properties/Callbacks) automatically loads into your workspace the contents of this .mat file ("xInitial" variable).
If you modify this model, or change parameter values of power components, the initial conditions stored in the "xInitial" variable will no longer be valid and Simulink® will issue an error message. To regenerate the initial conditions for your modified model, follow the steps listed below:
1. In the Simulation/Configuration Parameters/Data Import/Export Parameters menu, uncheck the "Initial state" parameter and check the "Final states" parameter.
2. In the Programmable Voltage Source menu, disable the source voltage steps by setting the "Time variation of " parameter to "none".
3. Make sure that the Simulation Stop Time is 0.4 second. Note that in order to generate initial conditions coherent with the 60 Hz voltage source phase angles, the Stop Time must an integer number of 60 Hz cycles.
4. Start simulation. When simulation is completed, verify that steady state has been reached by looking at waveforms displayed on the scope. The final states which have been saved in the "xFinal" structure with time can be used as initial states for future simulations. Executing the next two commands copies these final conditions in "xInitial" and saves this variable in a new file (myModel_init.mat).
>> save myModel_init xInitial
5. In the File/Model Properties/Callbacks/InitFcn window, change the line "xInitial = init_power_statcom_gto48p" to "load myModel_init.mat". Next time you open this model, the variable xInitial saved in the myModel_init.mat file will be loaded in your workspace.
6. In the Simulation/Configuration Parameters menu, check "Initial state".
7. Start simulation and verify that your model starts in steady-state.
8. In the Programmable Voltage Source menu, set the "Time variation of" parameter back to "Amplitude".
9. Save your model.