Mention The Short Comings Of Merz Price Scheme Of Protection Applied To A Power Transformer

While the Merz-Price scheme of protection is widely used for protecting power transformers, it also has some shortcomings or limitations. These include:

1. Sensitivity to Saturation: The Merz-Price scheme relies on current transformers (CTs) to measure the differential current between the primary and secondary windings of the transformer. However, if the current through the transformer exceeds the saturation level of the CTs during a fault, accurate differential current measurement becomes challenging. CT saturation can lead to inaccurate operation of the protection scheme or failure to detect faults.

2. Inability to Detect Winding-to-Ground Faults: The Merz-Price scheme is primarily designed to detect internal faults such as turn-to-turn faults or phase-to-phase faults within the transformer windings. It may not be effective in detecting faults between winding turns or winding-to-ground faults, which are common types of faults in transformers. Additional protection schemes, such as overcurrent or earth fault protection, are needed to address these types of faults.

3. Complexity and Cost: Implementing the Merz-Price scheme requires multiple CTs, relays, wiring, and communication equipment, which can add complexity and cost to the protection system. The installation, testing, and maintenance of these components require specialized expertise and resources, making it potentially more expensive than simpler protection schemes.

4. Requirement for Accurate CT Matching: The differential protection scheme requires that the CTs used for measuring current on the primary and secondary sides of the transformer are accurately matched in terms of their characteristics, including accuracy class, saturation characteristics, and phase angle errors. Any mismatch between CTs can lead to inaccurate differential current measurements and compromise the effectiveness of the protection scheme.

5. Limited Fault Discrimination: The Merz-Price scheme may have limited ability to discriminate between different types of faults or to localize faults within the transformer winding. In cases where multiple transformers are interconnected or where the fault location is unclear, additional diagnostic tools or protection schemes may be needed to pinpoint the fault and initiate appropriate corrective actions.

Despite these limitations, the Merz-Price scheme remains a widely used and effective method for protecting power transformers against internal faults. When properly designed, installed, and maintained, it provides valuable protection against catastrophic failures and helps ensure the reliable operation of electrical power systems.