Impedance monitor performance tests

An impedance monitor has been developed to determine the positive-sequence network impedance characteristics over a frequency range of 30 Hz to 400 Hz without disturbing the system. Its design incorporates (1) a 60 Hz cancellation filter based on a phase locked loop, (2) Clarke's transformation and Park's transformation, and (3) a transfer function approach for determination of the system impedance. This new approach to the system impedance measurement substantially improves the signal to noise ratio and reduces the effects of background noise. The paper presents the results of the performance tests conducted at a 77 kV SVC site in Japan and a 230 kV HVDC site in the USA.