Applications of signal processing tools in a power systems course

International Journal of Electrical Engineering Education, Jan 2004 by Jurado, Francisco, Valverde, Manuel

Overvoltage: At the switching instant, the voltage in the capacitor cannot change instantaneously. The bus voltage is pulled down, and then rises as the capacitor begins to charge. During the process, the capacitor voltage may overshoot and ring at the natural frequency. The overvoltage in normal energising is usually between 1.1 and 1.4p.u.

Polarity and magnitude of step voltage: One of the most common identifying features of normal energising of utility capacitors is the 'polarity' of the step voltage. If the power quality monitor is located at or near capacitors that have no series reactor, a fast initial voltage step will be observed. The voltage step at the instant of closing cannot go beyond zero if the capacitor has no initial net charge at the closing instant or if the capacitor is grounded. If the power quality monitor is located farther away from the capacitor, the voltage step change may not be observed, or at least it is not as prominent. In any event, sudden changes of voltage never cross the zero line, i.e. they do not change polarity. This behaviour is exhibited in nearly all normal energising of utility capacitors.

Oscillation frequency of the phase voltage during the energising event: The oscillation frequency of the phase voltage during any kind of capacitor energising is generally between 300 and 1000 Hz. Thus, the frequency of oscillation is helpful in identifying capacitor energising in general, but it cannot be used to discriminate normal energising from other types of capacitor energising. Extracting the oscillation frequency from the capacitor energising transients is difficult.

Manified transient voltages: Energising a shunt capacitor from a predominantly inductive source creates an oscillatory transient voltage that can approach twice the normal peak voltage. This energising transient is important because it can excite an LC circuit, resulting in magnified transient voltages at remote locations. When customers apply low voltage capacitors for power factor correction, significantly higher transient voltage magnitudes can occur at the low voltage bus. ASDs that have large capacitors in the DC link to supply voltage source inverters are particularly sensitive to these capacitor switching transients. There are two reasons for this sensitivity:

1. The DC capacitors form part of an LC circuit (with the inductance between the drive and the switched capacitor) that can be excited by the capacitor switching transient. The result is a significant current surge into the DC capacitor, increasing the voltage on the DC link.

2. The drive controls are very sensitive to overvoltages on the DC link. In order to protect the DC capacitor and inverter components, controls are usually set to trip whenever the DC link voltage exceeds approximately 1.2 times the normal DC voltage.

The result is that small ASDs often trip when utilities switch capacitors on the primary distribution system.

Case studies

Two case studies are presented. For the purpose of harmonics education, an ASD with a six-pulse converter is designed and implemented to simulate a nonlinear load. In EMTP this can be done using MODELS. The simulation examples presented are based on the EMTP.