High-Frequency Transients and Overvoltages Phenomenon:
In recent years, dry-type distribution/power transformers such as vacuum-pressure impregnated, cast coil, and vacuum-pressure encapsulated has been widely utilized in the industry. Especially with the widespread application of vacuum breakers (VCB) for transformer switching, this phenomenon has been attributed to a significant number of transformer failures worldwide, as documented in various publications [Ref:1-3] and reports from insurance companies [Ref:4]. Depending on the characteristics of VCB or SF6 type circuit breaker and the power system parameters, the switching transients namely Transient Recovery Voltage (TRV) and Temporary Overvoltages (TOV) result when the breaker exhibits virtual current chop and multiple re-ignitions. Such transient voltages of significant magnitude and frequency can cause deterioration and insulation failure of the connected dry-type transformer and the circuit breaker itself (Fig-1 and Fig-2).The only remedy for such failures is the applications of RC Snubber circuits. In Fig-3 and Fig-4, the effect of an RC Snubber circuit during the switching of a dry-type transformer has been shown.
Engineering Services for RC Snubbers by Hilkar Electric:
Hilkar Electric is proud to offer specialized engineering services focused on the design of RC Snubbers, providing essential protection against the potential failures of dry-type transformers during switching operations. The power system is modelled in detail and the worst cases for circuit breaker switchings causing the highest magnitudes of TRVs and TOVs are found. The sophisticated parameter swapping algorithms are utilized to identify the optimal combinations of resistance (R) and capacitance (C) values for snubber circuits. All these comprehensive studies are conducted by some powerful simulation programs like EMTDC/PSCAD. When finding the optimum solution of RC Snubber, the limit values of TRV and TOV for circuit breaker and dry-type transformer are defined according to international standards like IEEE C37.06-2009 and IEEE C37.06.1-2017. At the end, we issue a very detailed design report to our clients with all necessary technical specifications of RC snubbers so that they can easily supply them from any manufacturers.
Other Application Areas of RC Snubbers:
Apart from dry-type transformers, we also design RC Snubbers for the following power system components:
- A) Shunt Reactors
- B) MV Motors
- C) Generators
- D) Arc/Ladle Furnace Transformers
References:
[1] Transformer Failure due to Circuit Breaker Induced Switching Transients, David D. Shipp etal., IEEE Transactions on Industry Applications, Vol.47,No.2, March/April 2011.[2] The Experience Acquired Sizing Snubbers to Mitigate Switching Transients in Industrial Power Systems, Claudio S. Mardegan etal., IEEE Transactions on Industry Applications, Vol. 52, No.5, September/October 2016.
[3] Field Experiments on Overvoltage Caused by 12-kV Vacuum Circuit Breakers Switching Shunt Reactors, Qing Yang etal., IEEE Transaction on Power Delivery, Vol.31, No.2, April 2016.
[4] Factory Mutual Insurance Co., “Arc furnace transformer installations”, Property Loss Prevention Data Sheets, pp. 1–13, Jan. 2002.
