Protection Riddle No.28 - Current transformer over voltage

#1
Tue, February 10, 2009 - 15:20
Although the rms magnitude of voltage induced in a CT secondary is limited by core saturation, very high voltage peaks can occur. Such high voltages are possible if the CT burden impedance is high ( secondary open circuit ) , and if the primary current is many times the CTs continuous rating. The peak voltage occurs when the rate-of-change of core flux is highest, which is approximately when the flux is passing through zero. The maximum flux density that may be reached does not affect the magnitude of the peak voltage. Therefore, the magnitude of the peak voltage is practically independent of the CT characteristics other than the nameplate ratio.
One series of tests on bushing CTs produced peak voltages whose magnitudes could be expressed empirically as follows:

e = 3.5 Z I 0.53

where e = peak voltage in volts.
Z = unsaturated magnitude of CT burden impedance in ohms.
I = primary current divided by the CTs nameplate ratio.

The value of Z should include the unsaturated magnetizing impedance of any idle CTs that may be in parallel with the useful burden. If a tap on the secondary winding is being used, as with a bushing CT, the peak voltage across the full winding will be the calculated value for the tap multiplied by the ratio of the turns on the full winding to the turns on the tapped portion being used; in other words, the CT will step up the voltage as an autotransformer. Because it is the practice to ground one side of the secondary winding, the voltage that is induced in the secondary will be impressed on the insulation to ground.
The standard switchgear high potential test to ground is 1500 volts rms, or 2121 volts peak; and the standard CT test voltage is 2475 volts rms or 3500 volts peak. The lower of these two should not be exceeded.
Another possible cause of overvoltage is the switching of a capacitor bank when it is very close to another energized capacitor bank.
The primary current of a CT in the circuit of a capacitor bank being energized or de-energized will contain transient high-frequency currents. With high-frequency primary and secondary currents, a CT burden reactance, which at normal frequency is moderately low, becomes very high, thereby contributing to CT saturation and high peak voltages across the secondary. Overvoltage protectors may be required to limit such voltages to safe values.