Transformer mechanical protections detect internal faults through their physical effects: gas and oil flow (Buchholz relay, ANSI 63), a sudden rise in pressure (sudden pressure relay and pressure relief device) and the oil and winding temperature (ANSI 26 and 49). They complement the 87T differential: they see incipient and turn-to-turn faults the differential cannot reach, and they protect the insulation against overheating.
What transformer mechanical protections are
A fault inside an oil-immersed transformer produces physical effects before, or in addition to, changing the currents: the arc decomposes the oil and generates gas, pushes the oil toward the conservator and raises the tank pressure. An overload or a cooling failure, on the other hand, shows up as temperature. Mechanical protections (also called transformer-mounted protections) monitor those effects.
They are identified as follows according to ANSI/IEEE C37.2:
- 63, pressure switch (by value or by rate of change); the flow switch is device 80. By custom it is the number usually assigned to the Buchholz relay and the sudden pressure relay. The pressure relief device is also often marked 63, with suffixes that vary between users.
- 26, apparatus thermal device: the oil temperature indicator.
- 49, machine or transformer thermal relay: the winding temperature indicator or thermal image, and also the thermal function calculated in the numerical relay (logical node PTTR in IEC 61850).
- 71, liquid level: the conservator oil level indicator, which usually goes with them.
These protections complement the transformer differential 87T: the Buchholz detects turn-to-turn faults and low-energy incipient defects that do not produce enough differential current.
Where they are applied
- Oil-immersed power transformers and reactors with a conservator, at medium, high and extra-high voltage.
- On-load tap changers, which have their own oil surge relay.
- Sealed transformers without a conservator, where there is no Buchholz and pressure relays and gas detectors specific to that design are used.
- Dry-type transformers, where only winding temperature monitoring with embedded sensors is used.
How they work
Buchholz relay (63). It is installed in the pipe between the tank and the conservator. It has two elements:
- Alarm (first stage): a float drops when gas accumulates in the relay chamber. It indicates a slow fault, such as a hot spot, partial discharge or air.
- Trip (second stage): a flap or a second float operates on a sudden surge of oil toward the conservator, typical of an arc, or on a large loss of oil.
The accumulated gas can be sampled through the relay valve and analyzed. Its composition tells air apart from the gases of an electrical or thermal fault.
Sudden pressure relay. It responds to the rate of pressure rise, not to its absolute value, so it ignores slow changes due to temperature and operates on internal faults that produce a pressure wave.
Pressure relief device. It opens mechanically to release pressure from the tank and prevent rupture. It has contacts that signal its operation, and its indicator stays visible until it is reset.
Oil temperature (26). A bulb at the top of the tank measures the hottest oil. The indicator has several adjustable contacts to start fans and pumps, raise an alarm and trip.
Winding temperature (49). In most transformers the winding hot spot is not measured directly. The thermal image estimates it: a heater fed by a CT proportional to load adds the winding-to-oil gradient to the oil bulb. Some transformers also have fiber-optic sensors in the winding. Numerical relays calculate an equivalent thermal function from the current; see thermal overload 49.
| Device | What it detects | How it responds |
|---|---|---|
| Buchholz 63, first stage | Accumulated gas: slow fault, hot spot, partial discharge or air | Alarm |
| Buchholz 63, second stage | Sudden oil surge toward the conservator or large loss of oil | Trip |
| Sudden pressure relay | Rate of pressure rise, not its absolute value | Operates on the pressure wave of an internal fault |
| Pressure relief device | Tank pressure | Opens, signals through contacts and leaves its indicator visible |
| Oil temperature 26 | Hottest oil, at the top of the tank | Fans, pumps, alarm and trip |
| Winding temperature 49 | Winding hot spot, estimated by thermal image | Thermal function or fiber-optic sensors |
The trips usually go to the lockout relay 86, which opens every transformer breaker and prevents closing until the transformer is inspected.
Setting criteria
- Buchholz: the sensitivity (gas volume and flow velocity) is set by the relay manufacturer according to the pipe diameter and the transformer size. It is not adjusted in the field; it is verified.
- Oil and winding temperatures: the cooling, alarm and trip thresholds depend on the insulation class, the cooling system and the owner’s loading policy. The loading guides IEC 60076-7 and IEEE C57.91 provide the framework; the values come from the transformer manufacturer.
- Thermal image: the heater current is adjusted to reproduce the gradient from the factory temperature-rise test.
- Destination of each contact: which ones alarm, which trip and which start cooling, according to the protection philosophy.
How each manufacturer implements it
Two manufacturers are involved here: the transformer manufacturer, which supplies the mechanical devices, and the relay manufacturer, which receives their contacts. In Siemens SIPROTEC, SEL, Relion (Hitachi Energy and ABB), GE Vernova (Multilin) and Schneider MiCOM/Easergy relays, the Buchholz, pressure and temperature signals usually come in as binary inputs assigned to trip, alarm or recording, each in its own software (DIGSI, AcSELerator QuickSet, PCM600, EnerVista, MiCOM S1 or Easergy Studio). Some relays accept temperature probes (RTDs) directly or through external modules, and offer the calculated 49 thermal function. How each input is named, whether it is filtered and how it ties into the 86 depends on the brand and the control design.
Why they trip or block
- Buchholz alarm with no fault: air after filling or maintenance, or air entering through a leak on the suction side of a pump.
- Buchholz trip on oil surge: starting of oil pumps that moves the oil abruptly, earthquakes or strong vibration, besides a genuine internal fault.
- Spurious trips from wiring: moisture in the relay terminal box, damaged insulation of the control cable, or a ground fault on the DC system that activates a binary input.
- Temperature trip: genuine overload, fans or pumps out of service, radiators with closed valves, or a badly calibrated thermal image.
- Pressure relief device operated: severe internal fault or, at times, a damaged device that must be inspected.
After a Buchholz trip, the transformer must not be re-energized without analyzing the gas and the oil. Oil sampling and dissolved gas analysis guide the decision.
How they are tested
- Buchholz: air is injected through the test valve to check the alarm, and the test mechanism is operated for the trip, following the manufacturer’s procedure. Each contact is confirmed to reach the relay and the 86.
- Thermometers: they are compared against a reference (for example, in a temperature bath) and the operation of each contact is checked.
- Thermal image: the heater current is injected with a secondary source (such as the Omicron CMC 356) and the gradient is checked.
- Complete circuit: from the contact on the transformer to the actual trip of the breakers, with an insulation measurement of the wiring.
These checks are part of power transformer testing during commissioning and maintenance.
Did your relay trip or block on this function and the cause is not clear? At Tiza Group we review the events and settings remotely. See remote relay configuration or contact us.
References
- IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms and Contact Designations.
- IEEE C37.91, Guide for Protecting Power Transformers.
- IEC 60076-7, Loading guide for mineral-oil-immersed power transformers.
- IEEE C57.91, Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators.
- IEC 60567, sampling and analysis of free and dissolved gases in oil-filled equipment.
- IEC 60599, interpretation of dissolved and free gas analysis.
- J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.
Frequently asked questions
Why did the Buchholz alarm after maintenance?
Most often it is air trapped in the oil coming out after filling or after work with the tank open. It is confirmed by sampling the accumulated gas: if it is air, it is vented; if it contains combustible gases, there is an internal fault that must be analyzed before the transformer stays in service.
What is the difference between oil temperature and winding temperature?
The oil thermometer measures the top-oil temperature in the tank. The winding thermometer estimates the hot spot by adding to that reading a gradient proportional to load, through a thermal image fed by a CT.
Should the Buchholz trip go through the protective relay?
It depends on the owner's philosophy. It is common to wire it directly to an auxiliary relay or to the lockout relay 86, and also to bring it into the relay as a binary input so the event is recorded with its time stamp.