Failure Modes and Consequences of Conventional CVTs

 1. The failure modes for conventional CVTs are:

? Failure of one or more capacitor elements in the HV stack (C1), which is usually oil impregnated. The critical factor is the increase in voltage and stress upon healthy capacitors as each capacitor fails, e.g. 275kV CVT has about 160 capacitors in C1. This can lead to an avalanche failure mode and a possible explosion.

Failure of one or more capacitor elements in the LV grounding stack (C2), which is usually oil impregnated. The important factor is the decrease in secondary voltage. However, this failure mode can result in an explosion as experienced in New Zealand when C2 failed due to a faulty connection.

? Failure of the intermediate voltage transformer or the series reactor, which can result in changes in phase angle and/or voltage.

? Failure of the ferroresonance suppression circuit, which can produce waveform distortion, changes in phase angle and/ or voltage. It is possible for ferroresonance events to occur if the connected burden has too low a knee point voltage in 89 its transformer(s). Powerlink experienced an intermittent connection in the CVT’s ferroresonance damping circuit and this fault produced a reasonably stable voltage (64V compared to 67V on healthy phases) and fluctuating frequency in one phase between 47 to 53 Hz (50Hz nominal; measured with DMM) in the output voltage. Investigation revealed there was an open circuited choke in the ferroresonance damping circuit in the CVT basebox due to an imperfectly soldered joint in the wire, within a sleeve in the choke toroid. This open circuit had the effect of directly shunting portion of the VT primary winding with a capacitive impedance rather than the normal high 50Hz impedance and would have affected the voltage output.

? Failure of the filter circuit or spark gaps, which are used to minimize harmonic and transient voltages in the output voltage. Frequent overvoltage events can wear out the spark gap and the flashover voltage level increases. This will increase the stress on components in the VT circuit and these eventually fail.

? External flashover along the porcelain bushing due to pollution contamination of flashover clearance. The cause is incorrect CVT specification for the local environment when purchasing the CVT.

? Failure of expansion membrane, which results in contamination of oil and capacitor failure. Powerlink has experienced failure of the expansion membrane in a magnetic VT because the membrane was incompatible with the synthetic oil. This eventually resulted in the VT exploding.

? Failure in gasket seal which may allow high moisture content (>30 ppm) in oil which reduces the withstand voltage capability and increases stress in basebox items which use oil impregnated paper.

? Low oil condition due to prolonged oil leak which results in capacitor failure.

? The capacitor, series reactor and intermediate voltage transformer components can be degraded by high harmonic currents (e.g. AC-powered trains), lightning or prolonged ferroresonance conditions. 

2. The consequences of CVT failure could be:

? The CVT can explode if sufficient number of capacitor elements fail and arcing occurs within it. The explosion can rupture the porcelain shell and radiate porcelain fragments and hot synthetic oil within the local area. This debris is a real threat to safety of staff and to surrounding plant.

? The CVT is commonly located on the substation bus and bus protection will clear the fault. This can result in loss of supply to a large number of customers, or weakened system integrity (stability problems).

? The failure mechanism was due to a generic or age related fault. Thus the remaining CVTs could be deemed suspect and, without monitoring, result in constraints upon system operations and substation work.

? Progressive failure over a long period of time will cause incorrect revenue billing because one secondary voltage was incorrect. Microprocessor revenue meters will alarm if the voltage exceeds the typical limits of 80% to 115%. CVT monitoring can overcome this problem, eliminate the need to repay/recoup the amount of incorrect billing and maintain a company’s reputation. 

Reference: Content of this article are direct extract from a the paper "Monitoring Ageing CVTs by Bogdan Kasztenny, Ian Stevens"

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