The insulation on the windings of rotating machines, such as large or small motors, a-c generators and synchronous condensers, is held to a minimum because of limited space. Also, since the insulation is not immersed in oil, its surge strength is not much greater than the peak of the 60 –cycle voltage breakdown. Special measures are, therefore, necessary to protect such equipments when it is connected to a system subject to the hazards of lighting –surge voltages Likewise, the method of grounding effects the over voltages, during fault conditions and switching, which may be impressed on rotating machines.
The stress on the major insulation of any machine, that is, the insulation between the winding and frame, is determined mainly by the magnitude of the surge voltage to ground, whereas the stress on the turn insulation is more a function of the rate of rise of surge voltage as the surge penetrates the winding. Protection of a rotating machine, therefore, requires limiting the surge voltage magnitude at the machine terminals and sloping the wave front of the incoming surge.
In the ungrounded machines, because of the possibility of reflection from the neutral point, the voltage may double at the neutral. To limit the voltages at the neutral as recommended, it is necessary to hold the rate of rise of the surge entering the machines by connecting a Surge Protective Capacitor of small microfarad values. In table below are given the recommended capacitances for various voltage classes from 650 to 13,800 volts. For 11,500 and 13,800 volt classes, two standard ¼ microfarad units are recommended for ungrounded machines, whereas ¼ microfarad is sufficient for a grounded machine.Below 11,500volts,the standard unit contains ½ microfarad so no increase is required for ungrounded machines. In some applications it may be expedient to use an arrester from the neutral point of the machine to ground.
Recommended Capacitance Values for Line Surge Impedance Method of Protecting Rotating Machines