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.
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.
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