Transient overvoltages can arise from a number of sources. Power disturbances result from lightning strokes or switching operations on transmission and distribution lines. Switching of power factor correction capacitors for voltage control is a major cause of switching transients.
Surge capacitors can be used to slow the dv/dt and minimize the overvoltages on the winding ends. These are generally in the range of 0.5 to 1.0 μF for medium-voltage service. Some care should be exercised when these are used with SCR circuits because of the possibility of serious overvoltages from ringing. Damping resistors may be required.
All utility lines are designed for a certain basic insulation level (BIL) that defines the maximum surge voltage that will not damage the utility equipment but which may be passed on to the customer. Some consideration should be given to the supply system BIL in highpower electronics with direct exposure to medium-voltage utility lines.
Such information is generally available from the utility representative. The standard test waveform for establishing BIL capability is a voltage that rises to the instantaneous BIL value in 1.2 μs and decays to half that value in another 50 μs.
Other sources of transient overvoltages may lie within power electronics equipment itself. Interrupting contactor coils has already been mentioned. Diode and SCR reverse recovery current transients can also propagate within equipment. Arcing loads may require shielding of control circuits. In general, a solid grounding system will minimize problems.
Apparatus for surge protection covers the range from the little discs in 120-V power strips for computers to the giant lightning arresters on 765-kV transmission lines. Many types now utilize the nonlinear characteristics of 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.
The operating voltage is controlled by the thickness of the ceramic disk and the processing. MOVs may be stacked in series for higher voltages and in parallel for higher currents. Lightning arresters are classified by their current rating at a given clamping voltage. Station-class arresters can handle the highest currents and are the type used by utilities on transmission and subtransmission 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 used on distribution feeders and the smaller power electronics equipment. The cost, of course, is related to the clamping current. Arresters are rated for their clamping voltage by class and for their maximum continuous operating voltage, MCOV.
They are typically connected line-to-ground. Lightning arresters are often used to protect dry-type transformers in power electronic equipment, because such transformers may have a lower BIL rating than the supply switchgear. In 15-kV-class equipment, for example, the switchgear may be rated for 95 or 110 kV BIL, whereas the transformer may be rated for only 60 kV.
As a design rule, MOVs used for the 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.
On the other hand, the line-to-ground connection that protects against common-mode transients does not do as good a job on applied line transients. For optimum protection in equipments with exposure to severe lightning or switching transients, both may be appropriate. The volt-ampere curves for a MOV should be checked to be sure the device can sink sufficient current at the maximum tolerable 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 need. Small units are supplied with wire leads, whereas the larger units are packaged in molded cases with mounting feet and screw terminals for connections.
Another device in the protection arsenal is the surge capacitor. Transient voltages with fast rise times, high dv/dt, may not distribute the voltage evenly among the turns on a transformer or motor winding. This effect arises because of the turn-to-turn and turn-to-ground capacitance distributions in the winding.