How do I Model a Series Capacitor in the Power Flow and Harmonics modules in EasyPower?

EasyPower does not have a symbol for the series capacitor. Instead, you can use a Current Limiting (CL) Reactor. By using a negative impedance, EasyPower recognizes this symbol as a capacitor. In this topic, we demonstrate how the Power Flow and Harmonics modules treat a CL reactor with negative impedance as a series capacitor.

Below is the dialog box of a shunt capacitor with the ratings of 3 MVAR and 12 kV (LL).

The reactance, Xc, is calculated to be 122(kV)/3 (MVAR) = 48 ohms.


Power Flow

To model this as a series capacitor, connect a current limiting reactor to the bus and enter the calculated reactance of 48 ohms as a negative as shown below. The X/R of 1000 keeps the resistance negligible. For this example, we chose a constant impedance load with a small value. This almost shorts the CL reactor (or series capacitor) and makes it comparable to a shunt capacitor. A constant impedance static load of 10000MW and zero MVAR is connected to the CL reactor.

          

Taking the reactance information from the shunt capacitor and adding it as a negative value with high X/R causes EasyPower to model the CL reactor as shunt capacitor. To run power flow, connect a small resistive load (with zero reactance) to the CL reactor. The full reactive power capabilities of the series capacitor is seen at the bus with no reactive power absorbed by the load.

When you solve Power Flow, both the shunt capacitor and the “series capacitor” inject 2.959884 MVAR.

Harmonics

In EasyPower, the resistance factor in the load data is set to 0.5 by default to account for the eddy currents. As the frequency increases, the resistance also increases. For the purpose of modelling the series capacitor and keeping the resistance fixed with regard to the frequency changes, we set this factor to 0.

When the frequency scan is conducted at the bus with only the shunt capacitor in one case and with only the series capacitor in another, observe from the plot below that the curves are identical in both the cases. The resistance in the load is constant for all the frequencies and the reactance in the load is zero. The frequency scan is only affected by the CL reactor with the connected load playing no part. Hence, the results are identical to that of a shunt capacitor.

Frequency Scan with Only the Shunt Capacitor

Frequency Scan with Only the CL Reactor (Series Capacitor)

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