Superimposed impulse voltage tests on HVDC cables using coupling capacitor

One electrical test that is characteristic for HVDC cables is superimposed impulse voltage test. Different test circuits that are used to perform this test impose technical challenges and are discussed in this paper. Superimposed impulse voltage test is quite challenging because test object is exposed to composite voltage consisting of DC with superimposed impulse voltage. Adequate coupling/blocking elements are inserted between the test object and test equipment to ensure the proper operation. To couple an impulse generator there are two commonly used options, coupling capacitor or sphere gap. Disadvantage of using the sphere gap during superimposed impulse voltage testing is related to the possible extinction of the arc caused by low voltage gradient and uncontrolled discharge of the test object when arc persists both can cause inadequate test voltage waveshape. Coupling capacitor is a more reliable solution which can be effectively realised with spare upper stages of the impulse generator. While unipolar charging generator capacitors are easily arranged to form a coupling capacitor, symmetrical charging generators are different and proposed alternative connection scheme with jumpers bridging sphere gaps offers full capacitor utilisation and improves voltage stress between stages.