What Are Transformer Fault Types, Transformer Protection Types and Transformer Protection Device Types?

Transformers are critical power supply components in power systems. Various Transformer Fault Types may occur during operation, and these faults will severely impact power supply reliability and the normal operation of the power grid. Meanwhile, large-capacity transformers are also extremely costly equipment.

To achieve reliable protection of transformer, high-performance and stable protection devices in transformer shall be configured in accordance with each transformer’s capacity and operational importance level.

Transformer Fault Types

Internal Faults of Transformers

Faults inside the oil tank are extremely hazardous. The high-temperature electric arc will not only burn down windings and iron cores, but also decompose the insulating transformer oil to generate massive gas, which may lead to severe consequences such as explosion of the transformer oil tank.

Faults Inside the Oil Tank

Includes inter-phase short circuits, earth short circuits, inter-turn short circuits of windings, core burnout, etc.

Faults Outside the Oil Tank

Refers to inter-phase short circuits and earth short circuits occurring on bushings and outgoing leads.

Main Abnormal Operating Conditions of Transformers:

Overcurrent caused by external inter-phase short circuit of transformer (Inter-phase short circuit → Overcurrent)

Overcurrent and neutral point overvoltage caused by external earth short circuit of transformer (Earth short circuit → Overcurrent, Neutral point overvoltage)

Overload generated when load exceeds rated capacity (Overload)

Lower oil level caused by oil leakage and other faults (Lower oil level)

Transformer overexcitation under abnormal operating conditions such as overvoltage or low frequency (Overexcitation)

Consequences Caused by Abnormal Operating Conditions of Transformers:

1. Overheating of windings and iron cores.
2. For transformers with star connection and ungrounded neutral point, external earth short circuits may lead to neutral point overvoltage, which endangers the insulation of transformers.
3. Large-capacity transformers will suffer overexcitation under abnormal operating conditions such as overvoltage or low frequency, resulting in overheating of iron cores and other metal components.

Transformer Protection Types

Transformer Protection Relay Types mainly include main protection, backup protection and auxiliary non-electrical protection to cover all Transformer Fault Types comprehensively.

Transformer Internal Fault Scheme

Differential protection and Buchholz gas protection are the core components of transformer main protection to quickly isolate internal winding faults.

Gas Protection (Buchholz Protection)

Characteristics

Fast operation, high sensitivity, simple installation and wiring.

Buchholz relay transformer protection is the core non-electrical main protection for oil-immersed transformers to detect internal faults via generated gas.

Application Principle

Gas protection shall be equipped to cover all faults inside the transformer oil tank. It operates in response to gas or oil flow generated within the tank.

• Light gas protection: triggers alarm signals
• Heavy gas protection: trips circuit breakers on all power sides of the transformer

Installation Scope

Oil-immersed transformers with rated capacity of 800kVA and above;

Oil-immersed transformers installed inside workshops with rated capacity of 400kVA and above.

Longitudinal Differential Protection or Instantaneous Overcurrent Protection

Characteristics

It detects faults both inside and outside the transformer oil tank.

Differential protection or instantaneous overcurrent protection serves as the 3 phase transformer primary protection to cover all types of faults on transformer windings, bushings and outgoing leads.

Application Scope of Longitudinal Differential Protection

Transformers operated in parallel with capacity above 6300kVA;

Standalone transformers with capacity above 10000kVA;

Auxiliary transformers in power plants and critical transformers in industrial enterprises with capacity above 6300kVA.

Applicable Scenarios for Instantaneous Overcurrent Protection

Transformers with capacity below 10000kVA, where the time delay of their overcurrent protection exceeds 0.5s.

Once either longitudinal differential protection or instantaneous overcurrent protection operates, all circuit breakers on each power side of the transformer shall be tripped.

Protections to be Adopted for External Inter-phase Short Circuits

For transformer overcurrent resulting from external inter-phase short circuits, the following protections shall be equipped:

Over current protection in transformer

3 phase transformer overcurrent protection is applicable to step-down transformers. Its protection setting value shall take overload current that may occur under fault conditions into consideration.

Composite Voltage (Negative-sequence Voltage & Line Voltage) Initiated Overcurrent Protection

Applied to step-up transformers and step-down transformers where the sensitivity of conventional overcurrent protection fails to meet design requirements.

Negative-sequence Current & Single-phase Undervoltage Initiated Overcurrent Protection

Used for large-capacity step-up transformers and system interconnection transformers.

Impedance Protection

For step-up transformers and system interconnection transformers, impedance protection shall be adopted if Protection b and Protection c cannot satisfy the criteria of sensitivity and selectivity.

Protections for External Earth Short Circuits

Zero-sequence current protection shall be installed in power grids with directly grounded neutral points.

For autotransformers and three-winding transformers whose neutral points on both high-voltage and medium-voltage sides are directly grounded, zero-sequence directional elements shall be additionally equipped if selectivity is required.

Overload Protection

For transformers with capacity above 400kVA, whether operated in parallel or operated alone as backup power supply for other loads, overload protection shall be installed according to potential overload conditions.

Overexcitation Protection

For transformers with a high-side voltage of 500kV and above, overexcitation protection shall be installed to respond to the rise of transformer magnetizing current caused by frequency drop and voltage increase.

(Within the allowable overexcitation range of the transformer, the protection only sends out alarm signals; when the limit value is exceeded, the protection will trip the circuit breakers.)

Transformer Auxiliary Protection

Transformer Fire Protection

Fire protection for transformer serves as this auxiliary protection, which detects open flames, abnormal smoke and overheated oil of oil-immersed transformers, sending alarms or tripping breakers to suppress fire risks in time.

Transformer blast protection

Specially designed for oil-immersed transformers, this auxiliary protection monitors sudden tank pressure spikes from winding short-circuit gas buildup. It triggers alarms or trips all-side breakers instantly to avoid tank rupture, oil leakage and fires.

51G Transformer Ground Overcurrent Protection

51G protection for transformer serves as the backup ground fault protection to detect earth faults on transformer windings and neutral points.

51N Transformer Protection

51N transformer protection monitors zero-sequence current to identify earth faults on transformer secondary windings and neutral circuits.

transformer protection relay list

English Term	Transformer Protection Ansi Codes
Differential Protection	87T
Restricted Earth Fault (REF) Protection	REF
Overcurrent Protection	51
Earth Fault Protection	51N/59N
Overvoltage Protection	59
Undervoltage Protection	27
Overflux Protection	24
Winding Temperature Protection	49
Oil Temperature Protection	49
Oil Pressure / Blast Protection	63
Gas Protection (Buchholz)	63

Types of transformer protection devices

Two-winding Transformer Differential Protection Device (ANSI 87T)

Core Features

1. Designed exclusively for double-winding transformers (high voltage + low voltage), it calculates differential current by collecting CT currents from the two sides.
2. Equipped with ratio restraint, second harmonic restraint and intermittent angle blocking to avoid misoperation caused by magnetizing inrush and unbalanced currents during external short circuits.
3. Its protection scope covers all internal short-circuit faults of bushings, windings and leads on both sides. As the primary protection, it trips circuit breakers on all sides without time delay.
4. Simple logic and convenient setting commissioning, without calculation modules for the tertiary balancing winding.

Application Scenarios

35kV / 110kV double-winding main transformers, ordinary factory two-winding distribution main transformers, and double-winding power transformers in substations.

Three-winding Transformer Differential Protection Device (ANSI 87T)

Core Features

1. Adapted to transformers with high, medium and low voltage windings, it accesses three groups of CT currents simultaneously and embeds algorithms for three-side balance coefficients.
2. Added three-side current balance compensation logic to eliminate differential current induced by inconsistent capacities and transformation ratios of the three windings.
3. Retains inrush current blocking and ratio restraint functions, and allows independent switching of CT circuits on any single side.
4. Able to distinguish internal faults and through external faults, and provides protection against short circuits of the common winding.

Application Scenarios

110kV / 220kV three-winding main transformers, hub substation main transformers with three tiers of outgoing feeders (HV, MV, LV), and three-winding step-down transformers for mines and factories.

Generator-transformer Unit Protection Device (Including 87G Unit Differential Protection)

Core Features

1. Integrated protection for generators and main transformers, incorporating full logics such as 87G generator-transformer unit differential protection, generator differential protection, main transformer differential protection, rotor earth fault protection, stator earth fault protection, loss-of-excitation protection, overfrequency & underfrequency protection.
2. Specially matched to the generator-transformer unit connection mode, and automatically compensates current difference of connecting leads between generator and main transformer.
3. Equipped with inter-tripping logic for steam turbines, de-excitation and auxiliary power switching interlocks, to realize coordinated tripping of the whole system upon faults.
4. Multiple fault criteria are configured to identify terminal faults, internal main transformer faults and excitation system faults.

Application Scenarios

Thermal & hydropower generating units, gas turbine power stations, and generator-transformer unit connections where no circuit breaker is installed between the generator and step-up transformer.

Transformer Backup Protection Device (ANSI 51, 51N, 51G)

Core Features

1. Acts as backup for differential primary protection; it trips with time delay if the primary protection fails to operate, or for busbar and outgoing feeder faults.
2. Integrates phase overcurrent (51), zero-sequence earth overcurrent (51N neutral zero-sequence, 51G neutral overcurrent), overload protection and voltage-restricted overcurrent protection.
3. Multi-stage time-lapse graded tripping: trip the LV side first, then the MV side, and finally the HV side to isolate faults step by step.
4. Supports alarm-only mode without tripping to monitor hidden risks such as long-term overload and unbalanced earth faults continuously.

Application Scenarios

Matched with all oil-immersed power transformers to form a complete protection system together with differential protection, providing defense against external short circuits, winding earth faults and long-term overload.

Transformer Non-electrical Protection Device (Auxiliary Protection, ANSI 63)

Core Features

1. Instead of collecting electrical current and voltage signals, it operates based on signals from mechanical, temperature and pressure sensors.
2. Embeds gas protection, transformer blast protection, winding & oil overtemperature protection, cooling system fault monitoring, low oil level alarm and transformer fire protection.
3. Separate alarm and tripping outputs: light gas and slight overtemperature only trigger alarms; heavy gas, sudden pressure surge and fire directly initiate tripping.
4. Independent hardware circuit free from impacts of CT disconnection and DC faults, serving as one of the critical primary protections against internal tank faults.

Application Scenarios

All oil-immersed main transformers and oil distribution transformers; dry-type transformers are only fitted with temperature monitoring modules.

Distribution Transformer Comprehensive Protection Device

Core Features

1. Compact all-in-one device integrating simplified logics including overcurrent, instantaneous overcurrent, zero-sequence earth fault and overtemperature overload protection, with low cost and small footprint.
2. Differential protection is omitted for most small-capacity distribution transformers; instantaneous overcurrent and zero-sequence protection act as the primary protection instead.
3. Built-in energy metering, zero-sequence lightning protection and phase-loss protection to adapt to unbalanced three-phase low-voltage loads.
4. On-site installation for box-type substations and power distribution rooms, supporting remote alarm signal transmission via 4G.

Application Scenarios

10kV/0.4kV box transformers for residential districts, factory distribution transformers, rural small step-down distribution transformers, and distribution transformers with capacity ≤ 2000 kVA.

FAQ

What is the difference between measuring and protection current transformers?

Measuring CTs maintain high precision under rated load and saturate quickly with excessive current to shield meters; protection CTs avoid saturation during short-circuit faults to supply accurate current signals for relay protection devices.

How to protect distribution transformer?

Distribution transformers adopt instantaneous overcurrent, overcurrent, zero-sequence and temperature protection; gas or comprehensive relays are added as needed to clear faults and avoid overload & overheating damage.

How to size transformer primary protection

Size transformer primary protection based on rated current, inrush and short-circuit current to set proper differential and instantaneous thresholds.

How to protect high voltage transformers?

Protect high voltage transformers with longitudinal differential, overexcitation, Buchholz, overcurrent, zero-sequence and temperature protection to cover electrical and mechanical faults.

How to protect isolation transformers from lightning?

Protect isolation transformers against lightning via surge arresters, shielded wiring and lightning grounding grids.