Merz Price Protection is used to protect transformers and generator. It is also known as Differential protection of transformer. Merz Price protection is a type of differential protection scheme used to safeguard electrical equipment like transformers, alternators, and busbars. Named after engineers Charles Merz and Bernard Price, it is designed to detect internal faults in transformers by comparing the currents entering and leaving the equipment. The protection of power transformers is very important to ensure safety and prevent losses during faults. One of the widely used differential protection schemes for transformers is Merz Price Protection. The protection of electrical equipment, especially transformers, is very important. The Merz Price protection of power transformers stands out as a significant method in ensuring the longevity and reliability of transformers.

Let us understand the working principle, advantages, limitations, and improvements of Merz price protection of transformer in detail.

What is Merz Price Protection of Transformers?

Merz Price protection is a percentage differential protection scheme used for the protection of power transformers from internal faults like short circuits, ground faults, and inter-turn faults. It works on the principle of circulating currents using current transformers (CTs) connected on both the primary and secondary sides of the transformer. Under normal load conditions, the vector sum of currents from the two sides is zero. However, during internal faults the currents become unequal, producing a voltage/current signal in the relay circuit to trip the circuit breaker.

This scheme is designed to detect discrepancies in the current entering and leaving the transformer, ensuring any internal faults are quickly identified and isolated. This form of protection is very important in preventing severe damage to transformers, which are essential components in power systems.

• Merz-Price protection is used to protect both transformer and alternator.
• Power transformer protection is done by Merz price protection.

• Merz price protection is also called differential protection.
• Merz Price protection scheme or differential circulating current protection scheme functions on the concept of comparing two currents in and out of stator coil.
• In normal condition, the two currents will be the same, and if a fault occurs there will be the same difference, and merz price protection scheme work by detecting this difference or differential currents.

The working principle of the Merz Price protection of transformer is based on the differential protection method. It involves comparing the currents at the primary and secondary sides of the transformer. 

• Current Transformers (CTs) Installation: Current transformers are installed on both sides of the power transformer.
• Relay Operation: The differential relay compares the current entering and leaving the transformer.
• Fault Detection: Under normal operating conditions, the current difference (differential current) is zero or very small. If an internal fault occurs, a significant differential current flows through the relay.
• Tripping Mechanism: If the differential current exceeds a predefined threshold, the relay operates, triggering the circuit breaker to isolate the faulty transformer.

Below is the Merze Price protection of transformer diagram shown:

Fig- Merz Price Protection of Transformer Diagram

The Merz-Price Circulating Current Scheme is widely used for protecting power transformers against earth and phase faults. While this system is fundamentally similar to the one used for generators, it introduces several complexities when applied to transformers. Below, we address these complicating features and their corresponding remedial measures:

Differences in Primary and Secondary Currents

In power transformers, the currents in the primary and secondary windings are typically different. Using identical current transformers (CTs) with the same turn ratio would result in differential current, causing the relay to operate even under no-load conditions.

Remedial Measure

To compensate for the difference in current magnitudes, different turn ratios of CTs are employed. If T represents the turn ratio of the power transformer, the turn ratio of the CTs on the low-voltage (LV) side should be T times that of the CTs on the high-voltage (HV) side. By fulfilling this condition, the secondary currents of the two CTs will be identical under normal load conditions. Consequently, no differential current will flow through the relay, keeping it inoperative during normal conditions.

Phase Difference Between Primary and Secondary Currents

In a three-phase power transformer, there is often a phase difference between the primary and secondary currents. Even with CTs having the correct turn ratio, this phase difference can cause a differential current to flow through the relay under normal conditions, leading to false relay operation.

Remedial Measure 

The phase difference is corrected by appropriate connections of CTs. The CTs on one side of the power transformer are connected in such a way that the resultant currents fed into the pilot wires are phase-displaced in the same direction and by an angle equal to the phase shift between the primary and secondary currents of the power transformer. 

The table below indicates the type of connections to be used for CTs to compensate for the phase difference:

These connections ensure that the phase shift introduced by the power transformer is countered by the CT connections, preventing any differential current from flowing through the relay under normal operating conditions. By addressing these complexities, the Merz-Price Circulating Current Scheme provides effective protection for power transformers, ensuring reliable operation and safeguarding against potential faults.

This method ensures that any internal faults within the transformer are quickly detected and mitigated, preventing extensive damage and ensuring system stability.

Merz Price Circulating Current Scheme

• Under the normal working condition, there will be no current difference in two sides of the protected zone, hence two CT’s in a phase will conduct same current \((I₁ = I₂)\) or \((I₁ − I₂ = 0)\).
• Thus the current is balanced and vectorially assumed up as zero.
• When the fault occurs, there will be a difference between the two currents if two CT’s of a phase\( (I₁ ≠ I₂) \)or \((I₁ − I₂ ≠ 0)\).

Merz price protection detects this difference and operates the relay.

Advantages of Merz Price Protection of Transformer

The Merz Price protection of the transformer offers several advantages, making it a preferred choice in many electrical systems:

• High Sensitivity: It can detect even minor internal faults, ensuring prompt isolation of the faulty section.
• Fast Operation: The scheme operates quickly, reducing the risk of severe damage to the transformer.
• Selective Protection: It ensures that only the faulty section of the transformer is isolated, maintaining the integrity of the rest of the system.
• Stability during External Faults: The scheme remains stable and does not operate for faults outside the protected zone, preventing unnecessary interruptions.
• Reduced Maintenance Costs: By preventing severe damage, the scheme helps in reducing maintenance and repair costs.

Limitations of Merz Price Protection of Transformer

While simple in design and operation, Merz price protection has limitations due to its working principle which need to be addressed:

• Tap changers on transformers produce unequal currents under normal operation.
• CT ratio mismatches due to transformer turns ratio and load current variations.
• Inrush currents during transformer energization cause operation.
• Effect of system earthing and transformer saturation impact fault detection.
• Interposing CTs is complex for multiple winding transformers.
• Phase angle shifts between windings lead to fault.
  Merz Price Protection of Transformecirculation current.
• Long pilot wires increase impedance uncertainties.

Overcoming Limitations of Merz Price Protection

To overcome the above limitations, modern percentage differential relays employ the following techniques:

Interposing CTs for ratio matching between sides.

• Second harmonic filtering for blocking inrush currents.
• Tap changer compensated characteristics.
• Zero sequence networks for external fault discrimination.
• Multiple restraint and high set characteristics.
• Phase angle and ratio matching logic controls.
• Fiber optic or numeric communication for relay-relay interfaces.

Merze Price Protection of Transformer Numerical PDF

Merz Price protection is a simple but economical percentage differential scheme for transformer protection. With advancements in digital relays and techniques to address limitations, it continues to be widely used for efficient protection of power transformers from internal faults in distribution and transmission applications. Proper engineering is needed to optimize settings.

This article summarises all the information related to Merz Price Protection of Transformer, which helps in propelling your preparation for various AE/JE and ESE examinations. You can visit the Testbook app to keep yourself updated with all the exam-oriented information related to the upcoming examination, like SSC JE, GATE, ESE, RRB JE, and state AE/JE Exams.