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DC Tests

DC tests are used mainly to do pressure tests” on high voltage cables. Although the cables operate with AC, AC testing is not practical.

High voltage testing - Direct current (DC) - Photo by
High voltage testing – Direct current (DC) – Photo by

The high capacitance of the cables necessitates AC test sets with a high kVA rating to be able to supply the capacitive current. In the case of DC, once the cable is charged, only the losses have to be supplied.

DC test sets usually consist of half wave rectification, using HV selenium rectifiers.

Typical DC test set-up is shown in Figure 1.

Typical circuit for DC tests
Figure 1 – Typical circuit for DC tests

An AC high voltage test transformer is again supplied via a variac and a rectifier is used together with a filter capacitor C to limit the ripple to acceptable values. The earthing switch ES is a safety feature and closes automatically when the power is switched off to discharge the capacitor C.

Note that the peak inverse voltage required of the rectifier is 2 Vm.

Typical doubling circuit for DC tests
Figure 2 – Typical doubling circuit for DC tests

Doubling and multiplier circuits (as used in TV’s and household appliances) are also used to obtain an even higher voltage. A typical Cockcroft-Walton (in Germany: Greinacher) doubling circuit is shown in Figure 2.

Typical waveforms and a typical doubling circuit DC test source
Figure 3 – Typical waveforms and a typical doubling circuit DC test source

Resource: High Voltage Engineering Practice and Theory – Dr JP Holtzhausen; Dr WL Vosloo

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


    Nov 28, 2022

    Thanks for providing the complete detail about High voltage testing. Keep it up and keep doing this great work.

  2. kenneth
    Jan 06, 2022

    1. Why do you double the voltage when you pressure test?
    2. 3 Ansi Codes and their meanings.
    3. What is the name of the test that checks the tension between clusters and contacts?
    4. How do you connect your undervoltage coil to the UPS?

  3. Shubham Wadgaonkar
    Jan 24, 2018

    I want know about thermal test and DC high voltage test of PVC cable..

  4. turbold enkhtuvshin
    Nov 06, 2013

    thank you for your information

  5. mrahsanali
    Oct 08, 2013

    Love your work @Edvard. Every article is worth spending time on it. I’m final year undergraduate student and my final year project is “Simulation Based Impulse Testing of a Power Equipment”. Do you have anything in store for me regarding my project? It would be of great help. _Thanks

    • mrahsanali
      Oct 08, 2013

      “Power Equipment” can either be transformers, surge arrestors, isolators, Circuit Breaker or switchgear.

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