Design and performance of a fast divider for puncture testing

Impulse voltage puncture testing in air, as per IEC 61211, is used to assess the withstand strength of class B ceramic/glass insulators primarily against very fast front voltage transients in power systems. During this test, a fast rising voltage is applied to the disk insulator, which should not lead to puncture, but to an external flashover. Typical test voltage is c. 300 kV, with time to flashover of some hundred nanoseconds. The measurement system is affected by many factors, such as interference from the generating circuit, grounding of the test setup and stray capacitances and inductances of the measurement circuit. This paper focuses on the design, construction and performance of a new ceramic divider for puncture testing. In addition, the requirements for proper grounding of the generation and testing circuit are discussed. The design of the divider is based on using ceramic disk resistors. The advantages of this approach are the low inductance, high energy withstand and mechanical strength of the structure. The main disadvantages are the temperature and voltage dependences of the scale factor of the divider. However, these dependences can partly be compensated by using a ceramic resistor also in the low voltage arm. A low loss coaxial cable with high shielding factor is used to feed the signal from the divider to a high bandwidth pulse attenuator connected to the input of a recording instrument. The nominal scale factor of the system is 100000:1. The scale factor is selected to provide an output signal, which has a level suitable for connection to the input of a general-purpose wideband oscilloscope with 50 O input. The divider has a nominal maximum voltage of 600 kV for measurement of transients shorter than 0.5 µs; and the full lighting impulse voltage withstand is 400 kV. The divider has an input resistance of c. 5 kO, it is c. 1 m long, and it weighs about 10 kg. The measured rise time of the complete divider is less than 5 ns. The divider is without base, to enable compact measurement circuit with the high voltage electrode of the divider connected directly onto the insulator under test.