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The performance of the Clapham SVC after a control retrofit is described. The replacement of an analogue with digital control was completed in 1995. The retrofit was accomplished well within the ten days of allotted shut down time. With the original control system ac system voltage unbalance caused by untransposed lines had to be balanced by manual adjustment. The primary improvement provided with the new control is automatic and continuous voltage balancing. Key Words: control retrofit, untransposed lines, negative sequence motor currents.

This paper develops an accurate and general stability model for the ASC. The model is valid for both the capacitive and inductive regions of the ASC operation. It is accurate in that the instantaneous voltage and current waveforms and also the fundamental component of the capacitor voltage (needed for stability) are determined every half-cycle by solving the differential equations of the ASC circuit. The model is capable of incorporating any control algorithms. The validity of the model is demonstrated by comparing the model with the Electro-Magnetic Transient Program (EMTP) digital simulations using step time of 10[[micro]seconds]. Keywords: Thyristor Controlled Reactor (TCR), Electro-Magnetic Transient Program (EMTP), Power System Simulator (PSS/E).