Types of Transformer Protection & Function Introduction

Common Abnormal Conditions and Fault Types of Transformers

Common abnormal operating conditions and faults during transformer operation fall into four main categories: external short-circuit faults, neutral point overvoltage induced by external earth short-circuits, reduced oil level due to tank leakage, and equipment temperature rise caused by cooling system malfunctions.

In addition, large-capacity transformers are designed with a high rated core magnetic flux density to save materials, lower construction costs and reduce transportation weight. Since the transformer operating magnetic flux is proportional to operating voltage and inversely proportional to frequency, the equipment is highly susceptible to overexcitation faults under overvoltage or low-frequency operating conditions.

To address all the above faults and abnormal conditions, six standardized devices are included in the Types of Transformer Protection for large transformers, namely gas protection, biased differential protection of transformer & instantaneous overcurrent protection, overcurrent protection, zero-sequence current protection, overload protection and overexcitation protection.

Six Types of Transformer Protection

Buchholz protection for transformers

Guards against internal short-circuit faults inside the transformer and low oil level faults of the oil tank.

Differential Protection & Instantaneous Overcurrent Protection

Protects against phase-to-phase short circuits in transformer windings and outgoing leads, earth short circuits in high-current earthing systems, as well as winding turn-to-turn short-circuit faults.

Percentage differential protection of transformer diagram

Overcurrent Protection for transformer

Overcurrent protection in transformer detects external phase-to-phase short-circuit faults of transformers and acts as backup protection for gas protection and differential protection, including instantaneous overcurrent protection.

Zero-Sequence Current Protection

Guards against external single-phase earth short-circuit faults of transformers in high-current earthing systems.

Overload Protection

Monitors three-phase symmetrical overload conditions of transformers and only triggers alarm signals.

Overexcitation Protection

Real-timely limits the overexcitation level of transformers and keeps it within specified allowable thresholds.

Detailed Configuration and Operating Principle of Each Protection Device

Gas Protection

Gas protection is mainly used to respond to various internal faults inside oil-immersed transformer tanks and abnormal oil level drop, serving as the main protection against transformer internal faults.

Configuration Specifications

Gas protection shall be installed for all oil-immersed transformers with a capacity of 0.8 MVA and above, as well as workshop oil-immersed transformers of 0.4 MVA and above. On-load tap-changers of oil-immersed on-load regulating transformers shall be equipped with independent gas protection.

Operating Logic

When a small amount of gas accumulates in the tank or the oil level drops slightly, the protection instantaneously sends out an alarm signal. In case of massive gas generation or severe faults inside the tank, the protection trips all circuit breakers on each side of the transformer to isolate faulty equipment.

Differential Protection and Instantaneous Overcurrent Protection (Main Protection)

This set of protections acts as the main transformer protection, responding to all short-circuit faults on transformer outgoing leads, bushings and internal windings. It instantaneously trips all circuit breakers on each side of the transformer upon fault occurrence.

Configuration Requirements for Instantaneous Overcurrent Protection

Instantaneous overcurrent protection shall be installed for auxiliary transformers below 6.3 MVA, parallel-operated transformers, standby auxiliary transformers below 10 MVA and independently operated transformers, if the operating time limit of backup protection exceeds 0.5 s.

Configuration Requirements for Differential Protection

Transformer differential protection relay for transformer is mandatory for operating auxiliary transformers of 6.3 MVA and above, parallel transformers, standby auxiliary transformers of 10 MVA and above, and independently operated transformers.

It shall also be equipped for transformers of 2 MVA and above where the sensitivity of instantaneous overcurrent protection fails to meet operational requirements.

Special Configuration

Transformers with a high-side rated voltage of 330 kV and above may be fitted with dual sets of HT transformer protection to improve protection reliability.

Configuration Specifications for Generator Transformer Unit Protection

1. Where a circuit breaker is installed between the generator and transformer: the generator shall be equipped with an independent transformer differential protection.
2. Where no circuit breaker is installed between the generator and transformer: For generator-transformer units of 100 MVA and below, one shared differential protection for the whole unit shall be adopted; for units above 100 MVA, an independent generator differential protection shall be added on the basis of the shared generator-transformer unit differential protection.
3. For 200–300 MVA generator-transformer units: An additional independent transformer differential protection may be installed to form dual sets of high-speed main protection for unit safety operation.

Backup Protection against Phase Faults

This type of protection serves as backup protection for gas protection and differential protection (instantaneous overcurrent protection), responding to external phase-to-phase transformer faults and tripping with a time delay upon fault detection. It includes four protection types applicable to different operating conditions:

1. Overcurrent Protection: Suitable for all types of step-down transformers.
2. Composite Voltage Blocking Overcurrent Protection: Applied to step-up transformers, system interconnecting transformers, and step-down transformers where the sensitivity of conventional overcurrent protection is inadequate.
3. Negative-Sequence Current Protection & Single-Phase Low-Voltage Blocking Overcurrent Protection: Designed for step-up transformers of 63 MVA and above.
4. Impedance Protection: Installed as supplementary backup protection when the above two protections fail to meet sensitivity and selectivity criteria.

Zero-Sequence Current Protection

Applicable Scenarios

Specially deployed for high-current earthing systems of 110 kV and above to protect transformers against external single-phase earth short-circuit faults.

Configuration Requirements

Zero-sequence current protection shall be installed as part of hv transformer protection for all two-winding and three-winding step-up/step-down transformers with neutral points available for earthing operation within the system.

This hv transformer protection provides dual backup functions: it acts as backup protection for the transformer’s main protections, as well as backup protection for adjacent electrical components in the power grid.

Operating Principle of Zero-Sequence Current Protection

Zero-sequence current protection is activated by zero-sequence current generated during earth faults. For cable circuits, a dedicated zero-sequence current transformer is sleeved on the exterior of the three-core cable, with the transformer secondary winding connected to a current relay.

Under normal operation or fault-free conditions, the vector sum of three-phase cable currents equals zero, and only tiny unbalanced current flows through the transformer secondary winding without triggering relay operation. When an earth fault occurs on the circuit, a large zero-sequence current is induced in the secondary winding to actuate the relay, realizing fault alarm or faulty circuit isolation.

Overload Protection

Protection Function

Monitors three-phase symmetrical transformer overload abnormal conditions, with early warning as its core function.

Configuration Scope

Overload protection shall be installed for transformers of 400 kVA and above, whether operated in parallel multiple units or as independent standby power supplies, based on potential overload risks. For autotransformers and multi-winding transformers, the protection shall simultaneously monitor overload conditions of the common winding and each side winding.

Operating Logic

Transformer overload mostly presents as three-phase symmetrical conditions, so the protection only accesses single-phase current and actuates a current relay with a fixed time delay, normally only sending alarm signals.

Special Configuration

For unattended substations, the overload protection may be set to trip mode or partial load shedding mode according to operational demands.

Overfluxing Protection of Transformer

Protection Function

Real-timely monitors and limits transformer overexcitation conditions to prevent overheating and permanent equipment damage.

Configuration Background

Large transformer cores feature a rated operating magnetic flux density of 1.7–1.8 T, close to the saturation flux density of 1.9–2 T, making them highly prone to overexcitation under overvoltage conditions.

The transformer magnetization curve has a steep gradient, leading to severe hazards once overexcitation occurs: core saturation sharply reduces excitation impedance and surges excitation current.

When the magnetic flux density reaches 1.3–1.4 times the rated value, the excitation current approaches the rated current. Meanwhile, the excitation current contains abundant high-order harmonics, and eddy current losses in the core and metal structural components increase proportionally to the square of frequency, causing severe overheating of equipment and insulating materials.

Prolonged severe overexcitation will result in permanent transformer damage. Therefore, the power transformer protection relay is equipped with mandatory overexcitation protection for transformers with a 500 kV high voltage side.

Protection Purpose

Real-timely monitor transformer overexcitation status, issue timely alarm or trip commands, confine the overexcitation magnitude within allowable limits, and eliminate transformer burnout faults.

Concluding remarks

Covering all Types of Transformer Protection, our comprehensive protection solutions stand ready for global cooperation. Overseas customers are warmly invited to send project inquiries.

FAQ

How to protect power transformer?

Power transformers are fully protected by differential protection, backup overcurrent, earth fault, thermal overload, Buchholz relay and surge arresters.

What is the Merz-Price differential protection of a transformer?

CTs are installed on all sides of the transformer to form a circulating current loop for Merz-Price protection of transformer. In the event of an internal short circuit of the transformer, the currents on both sides become unbalanced, generating a differential current to trigger the protection trip. This Merz-Price protection of transformer serves as the core main protection for transformers.

What functions does the pressure relief valve for transformer protection perform?

It serves as the primary non-electrical protection for oil-immersed transformers. When an internal short-circuit fault occurs inside the transformer, the oil temperature and oil pressure rise sharply. The pressure relief valve opens automatically to release pressure and prevent the transformer tank from bursting. Meanwhile, it can interlock with the protection device to send alarm or trip signals.

How to protect transformers against inrush current?

Transformers can be protected against inrush current by adopting source-side inrush suppression equipment and equipping differential relays with second harmonic restraint and short-time delay logic to avoid false tripping.

How to calculate transformer differential protection?

Calculate transformer differential protection by matching CT ratios, setting restraint current against inrush and through faults, and verifying operation current with second harmonic blocking logic.

How to protect a grounding transformer?

Grounding transformers are protected by overcurrent relays, zero-sequence protection, thermal overload protection and surge arresters against faults and overvoltage.

how to protect transformer from overload?

Install thermal overload relays, anti-overload current protection and temperature monitoring devices, and limit long-term load below transformer rated capacity.