SimPowerSystems

Frequency Response of the AC and DC Systems

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

Session 2 in the Tutorial chapter already explained 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 are ignored in the impedance calculation and you get the impedances with the valves open.

Open the Measurements library, copy three Impedance Measurement blocks into 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 gives two times the positive-sequence impedance. Therefore you must apply a factor of 1/2 to 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 connect input 2 to ground. Save your modified model as case5Zf.

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

When the calculation is finished, the magnitude and phase as functions of frequency measured by the three Impedance Measurement blocks are displayed in the window. 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 order as in the window displaying the block names.

The magnitudes of the three impedances as a function of frequency are shown here:

Figure 2-28: Positive-Sequence Impedances of the Two AC Networks and of the DC Line

Note 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. Note 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.

HVDC System

Description of the Control System