Merz Price or Differential Protection of Power Transformer
Merz Price or differential protection is critical for safeguarding power transformers from faults and potential damages. As transformers play a vital role in electrical power systems, ensuring their reliable operation and protecting them from harmful conditions is crucial. The Merz Price differential protection scheme provides an effective solution by comparing the currents entering and exiting the transformer windings. Let’s delve into the specifics of Merz Price or differential protection in this article. We will discuss its working principle, components, and how it is used to protect power transformers. By understanding this essential protection mechanism, engineers and professionals in the electrical industry can ensure the safe and efficient operation of power transformers, contributing to the stability and reliability of electrical power systems.
Merz Price Protection of Power Transformer
Merz price circulating current principle is commonly used to protect power transformers of 5MVA and above against earth and phase faults. The system applied to transformers is fundamentally the same as that for generators but with certain complicated features not encountered in the generator protection system. They are explained below;
CT Ratios
In a power transformer, currents in the primary and secondary are to be compared. These currents will be different due to different primary and secondary voltages. Hence, the difference is compensated by CTs’ different turn ratios. If N is the turn ratio of the power transformer, then the turn ratio of the LV side’s CTs is N times the HV side’s CTs. In this condition, the secondaries of the two CTs will carry identical currents under normal conditions. Consequently, there will not be any differential current flowing through the relay, so it remains inoperative.
CT Connections
There is usually a phase difference between the primary and secondary currents of a 3-phase power transformer. Even if CTs of proper turns ratio are used, appropriate CT links affect a differential connection for phase difference. The CTs on one side of the transformer are connected so that the resultant currents fed into the pilot wires are displaced in phase from the individual phase currents in the same direction as and by an angle equal to the phase shift between the power transformer primary and secondary currents.
The power transformers are grouped according to the phase displacements below [as per IS 2026-1962].
Group 1: Star-star, phase displacement = 0o
Group 2: star-star, phase displacement = 180o
Group 3: Delta-star, Phase displacement = –30o
Group 4: Delta-star, phase displacement = +30o
The following rules are followed to make the currents in both side CTs (HV and LV side CTs) in phase.
Secondaries of CTs on the connected side of the power transformer are connected in delta as shown in the figure below:
Secondary CTs on the delta side of the power transformer are connected, as shown in the figure below.
With such an arrangement, the phase displacement between currents gets canceled with the phase displacement due to the secondary wind.
The table below shows the type of connections CTs use to compensate for the phase difference in the transformer’s primary and secondary currents.
| S. No | Power Transformer Connection | Current Transformer Connection | ||
| Primary | Secondary | Primary | Secondary | |
| 1 | Star (with Neutral Earthed) | Delta | Delta | Star |
| 2 | Delta | Delta | Star | Star |
| 3 | Star | Star (with Neutral Earthed) | Delta | Delta |
| 4 | Delta | Star (with Neutral Earthed) | Star | Delta |
Merz-price Differential Protection for Delta-Delta Power Transformer
For a delta/star power transformer, the CTs on the delta side should be connected to the star, and those connected in the delta are on the star side.
The figure shows Merz price differential protection for delta/delta 3-phase power transformer. The CTs on both sides are connected in star. This compensates for the phase difference between the power transformer’s primary and secondary currents. Pilot wires connect the CTs, and one relay is used for each pair.
During normal, i.e., no-fault conditions, the secondaries of CTs carry identical currents. Therefore, the winds entering and leaving the pilot wires at both ends are the same, and no current flows through (OC) of the relays. If a ground or phase-to-phase fault occurs, the currents in the CT secondary will no longer be the same, and the differential current flowing through the relay circuit will make the breaker on both sides of the power transformer trip.
This scheme also protects short circuits between turns on the same phase winding. When a short circuit occurs between the turns, the power transformer’s turn ratio is altered, causing an imbalance in current transformer pairs. When enough differential current flows through the relay, it acts and clears the fault.
Conclusion
In conclusion, The Merz Price or differential protection ensures power transformers’ safe and dependable functioning. By continuously monitoring the currents entering and exiting the transformer windings, this protection scheme can detect even the smallest fault or abnormality in the system. Quickly identifying and isolating faults helps prevent further damage to the transformer and minimize disruption to the power system. Due to its effectiveness and reliability, the Merz Price differential protection technique has been widely adopted in various power systems and industrial applications. As technology advances, further advancements in differential protection may offer enhanced features and capabilities. Nevertheless, the Merz Price method remains a cornerstone in protecting power transformers, contributing to the stability and efficiency of electrical power networks worldwide.
