ACS AC Test Sets

The ground connection scheme is typically called “Star Point Ground” That means that all ground come back to a single location. In the case of the high voltage test system that is either the ground of the high voltage transformer or the coupling capacitor. The from the star point ground a connection is made to earth ground. Earth ground is usually facility or mains ground or laboratory ground tied to grounding rods.

Normal wire is perfectly suitable to provide safety grounds for 50/60Hz mains frequency operation. The issue with high voltage test equipment is that when a flashover occurs or partial discharge testing is performed a significant amount of high frequency signals flow over the ground connection. Therefore, flat copper foil or flat braid is a much better low impedance connection for the high frequencies and this work very well for the 50/60Hz mains frequency content also.

Insulation systems fail on peak stress, a waveform that is highly distorted may have significantly high peak stress of that of an undistorted sinewave which has a peak of 1.414 (√2) times higher than the RMS value. Due to mostly 3rd and 5th harmonics generated by the core excitation of the step-up transformers, distortion can occur well outside the 5% allowed by the standards.

The peak stress is more important with regard to insulation stress; therefore, standards require the Peak/ √2 as the measurement method as it more accurately displays peak stress even with slightly distorted waveforms than RMS. This helps with normalizing test results between different test systems or setups. Test systems supplied by HV Technologies Inc can display Peak/ √2 and RMS simultaneously. As long as the RMS value and the Peak /√2 is within 5% of each other the test complies with standard requirements.

Over voltage setpoint is a very useful feature. Most dielectric test systems have significantly higher ratings than some test objects require. Therefore, is desirable to set a maximum “do not exceed value” especially when operating in manual mode. Usually a maximum +10% is entered at the beginning of a test.

An isolation transformer provides multiple benefits. Of the main benefits is with regard to PD testing typically PD noise attenuation of 3 to 10 Db can be expected. A secondary benefit is to isolate test area ground from factory ground which is desirable for shielded rooms or dedicated high voltage test areas. Also, an isolation transformer may be needed when retrofitting AC systems with line filters, In order for the filters have their maximum performance.

High voltage filters, are typically used when PD testing is required. Filters are connected between the coupling capacitor and the high voltage source. Implementation may vary from a single inductive element or as a multiple L-C sections depending on load and noise filtering requirements.

Line filters suppress conducted noise from the mains. Line filters are connected on the low voltage side of the high voltage step up transformers and offer high attenuation of line noise. In addition, line filters may improve the waveform quality. Typically, line filers are used in test areas with PD measurement requirements.

Current flow in high voltage testing is usually quote low; therefore, it is tempting to make quick connection with small gage buss wire or clip leads. When testing voltages are low this is acceptable, however with higher voltages or when performing PD testing larger diameter connections are required. Corona free high voltage connections are typically of large diameter to reduce field stress, up to 100kV 0.75” diameter are recommended, for testing voltages above 200kV, 1” diameter per 100kV is recommended. Soft copper pipe or EMT thin wall conduit are easily manipulated, light weight, available and inexpensive.

Any high voltage test equipment is considered live unless visibly grounded with a ground stick. Ground sticks and visible ground switches are an essential part of any high voltage testing area or lab. It should be noted that the correct ground stick should be chosen for appropriate voltage rating and application.

95%+ of all test objects are capacitive, high voltage test apply to voltage to a conducting surface that has some capacitive coupling to a ground plain.