Power System Blockset

Frequency Response of the AC and DC Systems

You will now measure the frequency response of the AC systems (rectifier and inverter sides) and of the DC line.

We have already explained in Session 2 in the Tutorial chapter how the Impedance Measurement block allows you to compute the impedance of a linear system from its state-space model. As the thyristor valves of the converters are nonlinear blocks, they will be ignored in the impedance calculation and you will get the impedances with the valves open.

Open the Measurements library and copy three Impedance Measurement blocks in your model and rename them Zrec, Zinv, and ZDC. Connect the two inputs of Zrec and Zinv between phase A and phase B of the AC system on the rectifier and inverter sides. Measuring the impedance between two phases will give two times the positive-sequence impedance. Therefore you must apply a factor of 1/2 on the impedance in order to obtain the correct impedance value.Open the two Impedance Measurement blocks and set the Multiplication factor to 0.5. Finally, connect input 1 of the ZDC block between the DC line terminal and the rectifier smoothing reactor and input 2 to ground. Save your modified circuit as case5Zf.

Now open the powergui. In the Tools menu select Impedance vs Frequency Measurement. A new window opens showing the three Zmeter block names. Fill in the Frequency range by typing 10:2:1500. Select the lin scale to display the Z magnitude and lin scale for the frequency axis. Check the Save data to workspace button and enter Zcase5 as the variable name that will contain the impedance vs frequency. Click on the Display button.

When the calculation is finished, three graphic windows appear with the magnitude and phase as function of frequency measured by the three Impedance Measurement blocks. If you check in your workspace, you should have a variable named Zcase5. It is a four-column matrix containing frequency in column 1 and the three complex impedances in columns 2, 3, and 4 with the same the same order as in the window displaying the block names.

The magnitudes of the three impedances as function of frequency are shown in Figure 2-29.

Figure 2-29: Positive-Sequence Impedances of the Two AC Networks and of the DC Line

Notice the two minimum impedances on the Z magnitudes of the AC systems. These series resonances are created by the 11th and 13th harmonic filters. They occur at 660 Hz and 780 Hz on the 60 Hz system. Notice also that the addition of 600 Mvar capacitive filters on the inductive systems creates resonances (around 188 Hz on the rectifier side and 220 Hz on the inverter side). Zoom in on the impedance magnitude in the 60 Hz region. You should find a magnitude of 56.75 for the 60 Hz system, corresponding to an effective short circuit level of 500²/56.75= 4405 MVA on the rectifier side (5000 MVA - 600 Mvar of filters).

For the DC line, note the series resonance at 240 Hz, which corresponds to the main mode likely to be excited on the DC side, under large disturbances.

Description of the HVDC Transmission System

Description of the Control System