A power transformer is one of the most expensive and critical assets in any substation, and when an internal fault develops, the margin between a minor issue and a catastrophic failure can be measured in seconds. Protection relays are the devices that watch for those faults and act on them isolating, alarming, or relieving pressure before damage escalates, following the principles in guides such as IEEE C37.91.
But “protection relay” covers several different devices, each watching for a different kind of problem. This guide explains the main transformer protection relays the Buchholz relay, pressure relief valve, oil temperature indicator, and winding temperature indicator what each one detects, and how they work together to keep a transformer safe. (For a plain-language primer on how a protection relay works at all, see our transformer protection relay beginner guide.)
Why one relay isn’t enough
Transformer faults show up in different physical signals. A slow insulation breakdown releases gas. A sudden internal fault spikes the pressure. An overload shows up as rising temperature. No single device can sense all of these well, so transformers use a set of complementary relays each specialised for one symptom. Think of them as different senses guarding the same asset.
The Buchholz relay: catching faults through gas and oil movement
The Buchholz relay sits in the pipe between the main tank and the conservator, and it is one of the most important protective devices on oil-filled transformers with a conservator.
What it detects: gas accumulation and abnormal oil flow caused by internal faults.
How it works: Many internal faults — insulation breakdown, overheating, minor arcing generate gas as they develop. That gas rises and collects in the Buchholz relay, operating an alarm. A more serious fault produces a sudden surge of oil and gas toward the conservator; the relay senses this rush and trips the transformer offline.
The real value of the Buchholz relay is early warning: it can flag a slowly developing fault through gas accumulation long before it becomes severe, giving maintenance teams time to investigate.
The pressure relief valve (PRV): preventing tank rupture
When a major internal fault occurs, it can vaporise oil and spike the internal pressure almost instantly, fast enough to rupture the tank.
What it does: the pressure relief valve (or device) releases excess pressure rapidly to protect the tank from bursting.
How it works: the PRV is a spring-loaded valve that stays sealed under normal operating pressure. If pressure exceeds a set threshold, it opens quickly to vent, then reseals once pressure normalises, protecting the tank while keeping it sealed in normal operation. Many PRVs include a signal contact to alarm or trip when they operate.
The PRV is a last line of mechanical defence: it doesn’t prevent the fault, but it can prevent the fault from turning into a tank rupture and oil fire.
Oil Temperature Indicator (OTI) and Winding Temperature Indicator (WTI): guarding against thermal stress
Heat is the enemy of insulation, and sustained overtemperature shortens transformer life. Two indicators watch for it.
Oil Temperature Indicator (OTI): measures the temperature of the top oil in the tank. Rising oil temperature signals overloading or a cooling problem.
Winding Temperature Indicator (WTI): estimates the temperature of the windings themselves, typically the hottest part of the transformer and the true limit on loading. Because you can’t easily place a sensor directly in the windings, the WTI derives winding temperature using the oil temperature plus a thermal model of the load current.
What they do: both provide staged responses, starting cooling fans or pumps as temperature rises, raising an alarm if it climbs further, and tripping the transformer if it reaches a dangerous level. Together they protect the insulation from thermal ageing and prevent overload damage.
How the relays work together
| Relay / device | What it senses | Typical action |
| Buchholz relay | Gas accumulation, oil surge from internal faults | Alarm (gas), trip (surge) |
| Pressure Relief Valve | Sudden internal pressure rise | Vents pressure; alarm/trip |
| Oil Temperature Indicator (OTI) | Top-oil temperature | Start cooling, alarm, trip |
| Winding Temperature Indicator (WTI) | Winding hot-spot temperature | Start cooling, alarm, trip |
Layered this way, the relays cover the main fault types developing internal faults, sudden pressure events, and thermal overload. A complete protection scheme often also integrates with systems like a nitrogen injection fire protection system, where relay operation can be one of the triggers that activates fire prevention.
Keeping protection relays reliable
Protection relays only help if they work when needed. That means:
- Periodic testing of trip and alarm functions
- Calibration of temperature indicators
- Checking signal contacts and wiring to control and tripping circuits
- Inspecting the Buchholz relay and PRV for correct operation and seal integrity
Including relay testing in routine transformer service ensures the protection scheme is ready for the fault it was installed to catch a point reinforced across industry operation and protection guidance.
The bottom line
No single relay protects a transformer a layered scheme does. The Buchholz relay gives early warning of developing internal faults, the pressure relief valve guards against tank rupture, and the oil and winding temperature indicators protect against thermal overload. Together they form a defence system that catches different faults at different stages. Specifying the right relays and keeping them tested and calibrated is one of the most cost-effective investments in transformer reliability.
EMR designs and supplies protection relay systems for power and distribution transformers, and can help you specify, install, or upgrade a complete protection scheme.
Frequently Asked Questions
What are the main types of transformer protection relays?
The key devices are the Buchholz relay (detects gas and oil surge from internal faults), the pressure relief valve (vents sudden pressure rise), and the oil and winding temperature indicators (OTI/WTI) that guard against thermal overload. Together they form a layered protection scheme.
What does a Buchholz relay do?
It sits between the tank and conservator and detects internal faults two ways: gas from a slowly developing fault collects and triggers an alarm, while a sudden surge of oil and gas from a major fault trips the transformer offline.
What is the difference between OTI and WTI?
The Oil Temperature Indicator measures top-oil temperature, while the Winding Temperature Indicator estimates the hotter winding temperature using oil temperature plus a thermal model of load current. The winding hot-spot is usually what limits safe loading.
Why does a transformer need a pressure relief valve?
A major internal fault can spike tank pressure almost instantly, risking rupture. The pressure relief valve opens rapidly to vent excess pressure and then reseals, protecting the tank from bursting and helping prevent an oil fire.
How often should transformer protection relays be tested?
They should be tested and calibrated as part of routine transformer maintenance, checking trip and alarm functions, temperature indicator calibration, and signal wiring, so the protection scheme is guaranteed to operate when a real fault occurs.
