Use of a saturating choke in the series capacitor compensation of distribution lines

Series capacitors can increase the power carrying capacity of subtransmission and distribution lines by reducing voltage regulation. When considering series capacitor compensation of distribution lines and subtransmission lines careful consideration needs to be given to capacitor location, ferroresonance, ohmic reactive value, transient behaviour, short circuit withstand and capacitor protection. Conventional design approaches include shunt connected resistors, spark gaps, metal oxide varistors, thyristor controlled reactors and bypass switches. The use of a saturating choke and damping resistor to control ferroresonance, transients, and through-fault currents is described. A small-scale laboratory nonlinear single-phase ferroresonant circuit was constructed with realistic per unit component values. Both 3rd and 2nd subharmonic ferroresonance modes were predicted by modelling and generated in practice. Choke and damping resistor parameters were selected by modelling to eliminate all the unwanted ferroresonant states. Experimental work confirmed that all the unwanted ferroresonant states were eliminated from the laboratory circuit. The transient and short circuit performance of the system is considered. The proposed arrangement offers an effective countermeasure to ferroresonance for series compensated distribution lines. The system also allows some control of system fault levels and transient circuit behaviour. The technique is simple, effective and requires no sophisticated control, protection or bypass switch systems