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.
1 comment:
splitz, is this you? - K
Post a Comment