In the fields of electrical engineering and applied physics, Different Types of High Voltages (d.c., a.c,, and impulse) are required for several applications. For example, electron microscopes and x-ray units require high d.c. voltages of the order of 100 kV or more. Electrostatic precipitators, particle accelerators in nuclear physics, etc. require high voltages (d.c. ) of several kilovolts and even megavolts.
High a.c. voltages of one million:volts or even more are required for testing power apparatus rated for extra high transmission voltages (400 kV system and above). High impulse voltages are required for testing purposes to simulate overvoltages that occur in power systems due to lightning or switching action.
For electrical engineers, the main concern of high voltages is for the insulation testing of various components in power systems for different types of voltages, namely, power frequency a.c., high frequency, switching or lightning impulses. Hence, generation of high voltages in laboratories for testing purposes is essential.
Different Different Types of High Voltages mentioned above are classified as
High d.c. voltages
High a.c. voltages of power frequency.
High a.c. voltages of high frequency.
High transient or impulse voltages of very shot over voltages, and
Transient voltages of longer duration such as switching surges.
Normally, in high voltage testing, the current under conditions of failure is limited to a small value (less than an ampere in the case of d.c. or a.c voltages and few amperes in the case of impulse or transient voltages).
But in certain cases, like the testing of surge diverters or the short circuit testing of switchgear, high current testing with several hundreds of amperes is of importance. Tests on surge diverters require high surge currents of the order of several kiloamperes