In the field of electrical engineering, 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 d.c. voltages 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 on power system due to lightning or switching action. For electrical engineers, the main concern of high voltage generation is for the insulation design, testing before commissioning and in service testing of various components in power system for different types of voltages namely :

1) high d.c. voltages
2) high a.c. voltages of power frequency
3) high a.c. voltages of high frequency
4) high transient or impulse voltages of very short duration such as lightning overvoltages, and
5) transient voltages of longer duration such as switching surges.