Surge Suppression in Power Systems

Causes of Power Surges

Transient overvoltages in power systems may arise from a number of sources. Power disturbances resulting from lightning strikes or switching operations on transmission and distribution lines.

The switching of power factor correction capacitors for voltage control is the main cause of switching transients. All utility lines are designed to certain basic insulation level (BIL) that defines the maximum surge voltage that will not damage the utility equipment, but which may be passed to the consumer. Other sources of transient overvoltages may lie within the power electronics equipment itself. Interrupting contactor coils can cause overvoltage transients. Diode and SCR reverse recovering current transients can also propagate within the equipment. Arcing loads may require shielding of control circuits. Generally, a good grounding system will minimize glitches.

Surge Protection Equipment

Equipment for surge protection range from the small discs in 120 V power strips for PCs to the giant lightning arresters in 765 kV transmission lines. Presently, many types utilize the nonlinear characteristics of metal oxide varistors (MOVs). These ZnO ceramic elements have a low leakage current as the applied voltage is increased until a threshold is reached at which the current will increase rapidly for higher voltages. This operating voltage is controlled by the thickness of the ceramic disk and the processing. MOVs can be stacked in series for higher voltages and in parallel for higher currents.

Lightning arresters are classified by their current rating at given clamping voltage. Station-class arresters can handle the highest currents and are the types employed by utilities on transmission and sub-transmission lines. Intermediate-class arresters have a lesser clamping ability and are used on substations and some power electronics that are directly connected to a substation. The lowest clamping currents are in distribution-class arresters that are employed on distribution feeders and the smaller power electronics apparatus. The cost of this equipment is related to the clamping current. Lightning arresters are rated for their voltage by class and for their maximum continuous operating voltage (MCOV). They are typically connected to the ground.

Lightning arresters are frequently used to protect dry-type transformers in power electronics equipment because such transformers may have a lower basic insulation level (BIL) rating than the supply switch gear. For example, in 15 kV class equipment the switchgear may be rated 95 or 110 kV BIL, while the transformer may be rated for only 60 kV.

As a design rule, metal oxide varistors (MOVs) used for protection of power electronics will limit peak voltage transients to 2-1/2 times their maximum continuous rated rms voltage. They may be connected either line to line or line to ground in three-phase circuits. Line to line connections limit switching voltage transients best but do not protect against common-mode (all three lines to ground) transients. Alternatively, the line to ground connection that protects against common-mode transients does not do well on applied line transients. For optimum protection in equipment with exposure to severe lightning or switching transients, both connections may applicable. The volt-ampere curves for a MOV should be checked to be sure the device can sink sufficient circuit voltage to handle the expected transient energies. This current will be a function of the MOV size, and a wide range of diameters is available to handle nearly any design requirement. The small units are supplied with wire leads whereas the larger units are packaged in moulded cases with mounting feet and screw terminals for connection.

Related: Switchgear

Surge capacitor is another device used for surge protection. Transient voltages with fast rise times, high dv/dt may not distribute the voltage evenly among the turns on the transformer or motor windings. This effect arises because of the turn-to-turn and turn-to-ground capacitance distributions in the winding. Surge capacitors may be employed to slow the dv/dt and minimize the overvoltages on the winding ends. These are usually in the range of 0.5 to 1.0 kF for medium-voltage service. Some care ought to be taken when these are used with the SCR circuits because of the possibility of serious overvoltages from ringing. Damping resistors may be needed in this case.