In past decades, as the demand for power generation has increased at an ever faster rate, the distribution of power at higher voltage levels has also increased proportionately, due to economic considerations. With the advent of these higher voltages, stricter requirements have been imposed on manufacturers. Recently these test standards have swung from the common high voltage DC proof test to an actual over- voltage power frequency test in which the product is normally stressed in standard service. Three test methods are commonly used for producing this applied power frequency or close to applied power frequency, which are the conventional AC test transformer method, the variable inductance resonant test method, and the variable frequency resonant test system method.
ADVANTAGES OF RESONANT TEST SYSTEMS
When the samples under test are of capacitive nature, resonant test systems provide inherent advantages over the conventional AC test transformer approach. These advantages include:
1. Reduced input power requirements save costs in installation and service.
2. Since the circuit utilizes a tuned circuit at the power frequency, a pure sine wave, without harmonics or distortion, appears across the test sample.
3. No power follow-up occurs as a result of test failure. The impedance of a normal test transformer is approximately 10%. When a failure occurs at full voltage, the inrush current can be as high as ten times the rated current until the overload devices trip the unit off. During this trip-out period (approximately ten cycles), the test transformer and load may see peak voltages as high as twice the initial test voltage, in addition to the higher currents.
4. When partial discharge measurements are required, the reduced input kVA requirements result in proportionately less expensive line filter systems. In addition, the series impedance of the high voltage reactor represents a high ohmic source to associated line noises and transients (Series Resonance Mode Only).
5. Test equipment is reduced in both size and weight. The normal resonant system is designed for a Q of 40 that reduces the voltage regulator kVA requirement to 1/40 of the normal test transformer requirement. This reduction also applies to the input circuit breaker and main AC contactor. In addition, the high voltage reactor utilizes a single high voltage winding as compared to the conventional test transformer that requires a primary winding of the same kVA characteristics as the secondary winding. The result is a direct size reduction, leading to weight and cost savings that can be passed on to the customer. Other derived benefits include cost savings in the construction of high-voltage labs, shielded rooms, and so on.
ADVANTAGES OF VARIABLE FREQUENCY RESONANT TEST SYSTEMS
1. VRTS systems are equipped with three-phase input. This will ensure a more balanced loading of the input service. When performing onsite tests lower power generators can be used.
2. Since the reactor gap is non-moveable the weight of variable frequency modules can be up to 50% lighter than the conventional variable inductance modules.
3. By using a square wave the variable frequency systems can be used to make partial discharge measurements.
4. The reactor operating noise is significantly reduced due to the more mechanically rigid construction.