Transformer protection requirements
The transmission transformer is a major component in the bulk power system and is normally held to a single contingency failure requirement. Based on types of failures, basic design of the transformer and the requirements of the power system, the protective relaying schemes must meet the following main requirements:
• Maintain transient stability of the power system.
• Prevent unwanted operation by load currents or by inrush currents or OLTC operation
• Maintain required quality of power
• Provide for maintenance of protective equipment with primary equipment energized by providing two groups of protections
The protection system must satisfy the following requirements specific to the transmission transformer:
• Protect for internal tank faults to ground or to alternate phases.
• Protect for external tank faults to ground or to alternate phases.
• Protect for internal partial winding or turn-to-turn faults.
• Limit through fault magnitude and duration within transformer mechanical and thermal ratings.
• Limit top oil temperature to maximum temperature limit
• Limit hot spot winding temperature to maximum temperature limit
• Prevent rupture of transformer tank during internal fault
• Provide for failure of a switching device
• Limit overload as per thermal capability of the transformer
• Provide for Tertiary winding protections over voltages caused due to transferred lightning surges or switching surges from high voltage sides
• Provide protection of winding insulation by suitably dimensioned and monitored surge arresters
• Provide protection from over-excitation of transformer core Most transmission transformers are located in critical areas of the power system where either utility voltage
stability or transient stability may be a problem. Providing high-speed fault clearing to meet generator voltage and stability limits is essential.
One of the key problems in security of a transmission transformer is tripping during a through fault current flow. In many cases, this means that the faulted element (usually a transmission line) will be properly tripped
by the protection provided for it, failing which the back-up protection of the transmission transformer will trip the transformer. This common mode failure where both line and transformer gets disconnected can be
disastrous to a single contingency designed power system. In many systems the single contingency limit only exists during the peak load of the power system, which greatly reduces the possibility of cascading
failures. It does identify a serious potential problem if through fault trips are allowed to happen.
Turn-to-turn faults challenge the dependability of the transformer protection. The fault current is high, and sometimes very high, but the changes in the phase currents may be less than 10% of the rated current.
Maintaining the quality of the power supply for sensitive industrial customers as well as for general customers is a concern of transformer protection. The transformer can withstand a very long duration of low magnitude through fault current but even this may cause severe problems for motor driven industrial processes, unless the fault is cleared in a short number of cycles with due care for additional reactive power support.
Another requirement is that a protection relay can be taken for maintenance while the transformer is energized for critical installations. Hence, it is required that the protection schemes meet redundancy requirements to provide protection during outage of any one relay for maintenance. Normally this is done by grouping the
protections in two groups A and B that are not exactly duplicated but provide redundant protection groups.
Further, It is not advisable to keep main and back up protection functions in a single relay hardware.
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