All cable systems have a resonance frequency, with short cable lengths this is is not considered to be a problem as there is no power being transmitted on the line at that frequency to resonate. Symptoms of this resonance are quite easy to spot, there will be heating of transformers, mysterious cable pin holing & failures, cable joints & plugs will fail, and generally the system will run hotter for no apparent reason. The cure has been to install low pass “sine wave” filters on the output of the VFD to block the carrier frequency from the cable system or to use capacitors to tune the resonance point of the loaded system. SUBCOE manufactures both types of filter system with the capacitive tuning filter normally applied to the medium voltage systems up to 4750V.
With the widespread adoption of Pulse Width Modulated (PWM) Variable Frequency Drive (VFD) power supply systems this has become more of an issue as the PWM system works by switching transistors on/off at high frequency and they generate high frequency harmonics at multiples of the fundamental carrier frequency. The problems is that if a multiple of the carrier frequency coincides with the cable/load system resonance then there will be significant amplification of the relatively low harmonic current from the VFD output.
The SUBCOE Resonance Analyzer is a high resolution high frequency signal generator and measurement device, it is used to inject the loaded system with a wide frequency signal sweep that is then monitored to establish the system amplification. When the system includes a step-up transformer, the resonance analyzer can then be used in conjunction with our MV capacitive filter simulator to establish the capacitance that can be added to the system to shift the resonance point away from harmonics of the PWM carrier frequency. The addition of a MV capacitive filter to the system is a very cost effective method of waveform and system performance improvement.