The Bottom Line: Three Devices Are Non-Negotiable — Buchholz Relay, Pressure Relief Device and Winding Temperature Indicator
Here is the answer up front. Every oil-immersed distribution transformer with a conservator must be fitted with at least three protection devices: a Buchholz relay to detect internal faults, a pressure relief device (PRD) to vent a dangerous overpressure, and a winding temperature indicator (WTI) to trip the unit before the insulation is damaged by heat. Together these three catch the failure modes that destroy transformers — winding short circuits, gas evolution from arcing, tank overpressure, and thermal ageing. On top of them, a magnetic oil level gauge, a silica-gel breather, and external overcurrent / earth-fault relays complete a package that most utilities and IEC 60076–based specifications treat as standard.
Key Takeaways
- Buchholz relay is the primary internal-fault detector for oil-immersed transformers with a conservator — it gives an alarm on gas and a trip on oil surge.
- Pressure relief device (PRD) vents at a set overpressure (typically ~0.5 bar above normal) so the tank never ruptures.
- Winding temperature indicator (WTI) simulates the hot-spot and trips the breaker before insulation life is consumed — the difference between a shutdown and a burn-out.
- Dry-type transformers have no oil, so they skip the Buchholz and PRD and rely on embedded winding temperature sensors instead.
- Specify the protection set in the PO — it is cheaper to confirm these devices on the order than to retrofit or discover their absence at commissioning.
Why Transformer Protection Devices Matter (and What They Protect Against)
A transformer is a sealed tank of oil, copper, and steel that can carry hundreds of amps at thousands of volts. When something goes wrong inside, the first signs are usually invisible from the outside: a hot spot on a winding, a small arc that slowly decomposes oil into combustible gas, or a build-up of pressure. Protection devices exist to convert those invisible signs into an alarm (tell the operator to investigate) or a trip (disconnect the transformer automatically) before the fault escalates into a fire, a ruptured tank, or a multi-week outage. The devices below are the standard set specified under IEC 60076 and GB 1094 for oil-immersed distribution units.
The Buchholz Relay: Your Transformer's Internal-Fault Smoke Detector
The Buchholz relay sits in the pipe connecting the main tank to the conservator (the oil-expansion vessel on top). It is a gas-and-oil-flow actuated relay with two stages. Stage one — gas accumulation: a slow fault (a loose connection, an overheating core bolt, or partial discharge) decomposes oil into gas; the gas rises and collects inside the relay chamber, displacing oil, lowering a float, and closing an alarm contact. Stage two — oil surge: a severe fault such as a winding short circuit vaporises oil almost instantly; the surge of oil rushing toward the conservator trips a flap and closes the trip contact, opening the transformer breaker. The alarm stage is your early warning that lets you take the unit offline on your schedule; the trip stage is the last line of defence that stops a fault from becoming a fire.
- Applies only to oil-immersed transformers with a conservator. Hermetically sealed or gas-filled units use a different gas-detection device instead.
- Two independent contacts (alarm + trip) wired to separate annunciation and protection circuits.
- Test it at commissioning — the float and flap can be exercised with air injection; our commissioning tests checklist covers this step.
The Pressure Relief Device: The Safety Valve That Prevents a Tank Rupture
If an internal fault builds pressure faster than the Buchholz relay can clear the fault, the tank itself becomes the weak point. The pressure relief device (PRD) is a spring-loaded valve on the tank cover (or a side wall) that opens automatically at a preset overpressure — typically around 0.5 bar above normal operating pressure — and vents the gas and oil to atmosphere through a discharge pipe routed toward the ground. Once the pressure drops, the spring reseats the valve. A contact on the PRD signals an alarm/trip so the operator knows the device operated. Without a PRD, a violent internal fault can weld a lid shut and then rupture the tank, spraying hot oil and creating a serious fire hazard.
Winding Temperature Indicator (WTI): The Device That Stops Thermal Ageing
Insulation life is the transformer's most valuable asset, and heat is what consumes it. The winding temperature indicator (WTI) measures the top-oil temperature and adds a heating element fed by a current proportional to the winding load, so the reading tracks the hot-spot temperature rather than just the oil. It typically provides two sets of contacts: an alarm (commonly set around 95 °C) and a trip set at the hot-spot limit that protects the insulation — around 98 °C for Class A insulation, the same limit discussed in our cooling methods guide. The oil temperature indicator (OTI) is the simpler sibling that reads top-oil temperature directly and is often specified together with the WTI for a complete thermal picture. When the trip contact operates, it opens the transformer breaker and protects the insulation from the thermal runaway that shortens service life by years.
Oil Level, Breather and the Rest of the Standard Package
Three more devices complete the standard oil-immersed set:
- Magnetic oil level gauge (MOG) — a float-driven indicator on the conservator that shows oil level and closes a low/high-level alarm contact, catching the slow oil leaks that no other device notices.
- Silica-gel breather — a desiccant canister on the conservator's air inlet that dries the air drawn in as oil contracts; the silica changes colour (typically blue to pink) when saturated and must be recharged, so it doubles as a moisture-health indicator.
- Sudden pressure relay — an optional faster-acting companion to the Buchholz for sealed or larger units where rapid pressure rise is the dominant risk.
How Dry-Type Transformers Are Protected Instead
Dry-type (cast-resin) transformers have no oil, so the Buchholz relay, PRD, and MOG simply do not apply. Instead, protection is built around embedded temperature sensors — PT100 resistance sensors cast into or placed on the high-voltage and low-voltage windings — wired to an electronic temperature controller that provides winding alarm and trip contacts plus fan control. The same logic holds: alarm early, trip before the insulation class limit (Class F is 155 °C, Class H is 180 °C) is breached. For a full breakdown of when dry-type makes sense at all, see our oil-immersed vs dry-type selection guide.
External Protection: Overcurrent, Earth Fault and Surge Arresters
The devices above live on the transformer itself; a complete protection scheme also includes devices on the switchgear side. Overcurrent and earth-fault relays (IEC 60255 / IEC 60947) protect the transformer from external short circuits and overloads, while surge arresters limit lightning and switching overvoltages to a level the insulation can withstand. The two groups work together: the relays clear external faults and the transformer-mounted devices clear internal ones. Lightning protection in particular deserves its own plan — see our transformer lightning protection guide for the BIL and arrester margin that keeps a stroke from reaching the winding.
What to Specify in Your Purchase Order
Protection devices are not always itemised on a standard quotation, which is exactly why a buyer should pin them down in writing. For an oil-immersed distribution transformer, confirm at minimum: Buchholz relay with alarm and trip contacts, a pressure relief device, a winding temperature indicator, an oil level gauge, and a silica-gel breather — plus the external overcurrent / earth-fault protection and surge arresters on the switchgear side. For a dry-type unit, confirm the embedded winding temperature sensors and controller. These are cheap insurance relative to the unit cost, and most are the kind of accessory a factory will happily include if you ask — the risk is only in not asking. When you are ready to specify the full unit, use the engineering toolbox to size it and generate a first-pass equipment list, then hand QDTB the specification with the protection set written in.
Sources / 资料来源
- Source: IEC 60076 — Power transformers (protection and accessory requirements for oil-immersed units).
- Source: IEC 60255 — Measuring relays and protection equipment (external overcurrent / earth-fault protection).
- Source: GB 1094 — Power transformers (Chinese national standard, harmonised with IEC 60076).
- Source: Industry experience — QDTB standard accessory set for oil-immersed and dry-type distribution transformers (as of 2026).