Primary Side Transformer Protection

Primary-Side Transformer Protection Transformer is an electrical equipment that converts energy from one voltage level to another. Due to its importance in power network and considering the fact that it is very expensive, it has to be well protected to increase its effectiveness and to ensure continuity of service. The conventional way of protecting transformer is by using motor operated disconnect switch to initiate faults, but this method has been found not reliable, most especially in large distribution ring circuit, in which all transformers connected to the transmission line have to be off-line when one of the transformer is faulty. The advance method is for each of the transformer to be self protected, that is, local protective device should be individually provided to each transformer, which makes unnecessary interruption to service to be eliminated. This protective device is usually provided at the primary- side of the transformer, being the high voltage side and it has higher fault interrupting rating. In distribution transformer, secondary-side bus faults are mostly common, these are the major faults that a primary-side protective device must interrupt. Although, faults occurring at the secondary-side bus are of low magnitude compare to the primary-side, but the rate of rise of transient recovery voltage (TRV) is very high and hence, the Primary protective device must have the capacity to withstand this fast-rise voltage (TRV). “Transient recovery voltage (TRV) can be defined as the voltage that exists between the contacts of the protective device (e.g. Circuit breaker) after fault interruption. In this wise, primary -side protective device must be designed and specifically tested to determine its ability to withstand fast-rise transient recovery voltage. For instance, a device with 40KA primary-fault interrupting rating may not necessary be able to interrupt 4KA secondary fault with high rate of rise of transient voltage. Secondary- side interrupting rating is critical for primary side protective device, hence selecting the right primary protective device that is capable of interrupting secondary side bus faults is a very important issue which the buyer of the transformer must agree with the manufacturer, because it as much as a economic decision as a technical decision.The type of protection scheme equipped with a transformer depends on its rating. A smaller and less expensive transformer are often protected with power fuses, while a large transformer are protected with devices that use relaying scheme from simple overcurrent relays to sophisticated combinations of differential,buchholz,overcurrent and instantaneous relays. Once the primary- side protective device with the appropriate secondary side interrupting rating requirements has been identified, other factors that need to be considered during selection process are; the available substation location, bus layout, seismic requirements, structure height requirement and system rating. With these you will be able to choose whether the bus arrangement should be open- type or folded-type with one-half breaker scheme. In view of the above, to eliminate unnecessary disturbances on the transmission line, each of the distribution transformer in the network should be provided with primary- side protective device and this protective device must be capable of interrupting secondary- side bus faults, being the most common and stubborn faults that is difficult to interrupt in the distribution network.
Relevant Links:
www.sandc.com