Use of static inverters for performing field testing on HVAC cables

The use of insulated medium and high voltage cables for the transmission of AC power over longer and longer distances has presented serious challenges to the conventional methods and equipment utilized to perform final tests on laid cables prior to grid energization. EHV cable circuits are now commonly ranging up to some tens of kilometers in length, resulting in very large capacitive charging currents when energized up to the AC test voltage levels specified in IEEE and IEC standards. A number of competitive testing technologies have arisen in recent years, such as Very Low Frequency (VLF) AC testing and Damped Alternating Voltages (DAV) which when combined with a partial discharge measurement all claim to be able to locate defects in the cable and its accessories. Although such testing technologies may appear to offer some relative advantages in terms of reduced weight and dimensions (improved transportability) and lower initial equipment cost, there continues to be controversy throughout the industry as to the effectiveness of these techniques in detecting defects. Indeed, in the case of the VLF technique, serious questions persist as to whether or not this method may actually be inflicting damage to the cable. It is for this reason that High Voltage AC testing, either with or without a simultaneous partial discharge measurement, remains the preferred test method. Modern insulated gate bipolar transistors (IGBT) may be used to construct relatively low cost, simple and robust frequency converters capable of supplying power in the range of some hundreds of kVA. The AC response of a resonant circuit comprised of inductive and capacitive energy storage elements is governed by the quality factor of the circuit, or the ratio of reactive power to real power. Since the high voltage cable appears to the source as a nearly purely capacitive load in the frequency range of interest, the quality factor of the test circuit can be quite high. It is not uncommon in such circuit- to achieve quality factors as high as 200. When combined with an IGBT based frequency converter capable of supplying a variable frequency AC excitation source rated up to several hundreds of kVA, an effective AC testing power in the range of tens of MVA is achievable. In conclusion, the first part of this paper reviews research results on the comparison of different voltage sources and frequencies with regard to 50-60 Hz testing. The second part is an overview of the extremely high power resonance test system capabilities. Variable frequency resonance test systems utilize modern IGBT based frequency converters as the excitation source in a controlled high voltage resonant circuit whose capacitance component is comprised of the cable under test. Techniques for properly calculating the resonant frequency, the required testing power capacity and the resulting sizing of the frequency converter are covered in detail.