Winter transformer problems rarely have a single cause. Low temperatures shrink insulating oil and reduce the elasticity of seals. The day-night temperature swing drives moisture into boxes and conservators. Add ice and snow, and oil level, breathers, sealing and outdoor insulation all come under pressure at the same time. Pre-winter inspection and protection is about dealing with these weak points before they cause trouble.
Oil level and oil temperature should be checked for trend first, then for the actual reading. Transformer oil level changes with load and ambient temperature, which is normal, but a sudden cold snap can push the level into the range where the gas relay operates. During a cold wave, several in-service and spare transformers sent light gas signals one after another. Inspection showed the direct cause was low oil level, with gas accumulated inside the relay. A gas relay should be full of oil. If about half the sight glass is gas, it needs attention. Before winter, check the oil level against the current oil temperature and confirm it is not below the manufacturer's minimum allowable value. Topping up is not simply filling to the mark. At the same time, check whether the conservator oil level indicator is working properly and whether the oil passage is blocked. If the relay has already collected gas, bleed it slowly according to procedure until the relay is full of oil. If the oil level keeps dropping and temperature does not explain it, look for seal leaks rather than repeatedly topping up.
The breather is the entry point to the oil system, and silica gel discoloration is a signal. The conservator breathes through the breather, and air passing in and out is dried by the silica gel. The gel is normally blue and turns pink or white when saturated. Once more than two-thirds has changed color, replace or regenerate it. If discoloration speeds up noticeably, the air entering the conservator is too humid, or the breather gasket or oil cup seal is faulty. Check these together rather than just replacing the gel. The consequences of a blocked breather are easy to underestimate. When oil temperature changes, pressure inside and outside the conservator cannot equalize, which can create negative or positive pressure. In severe cases this distorts the oil level gauge reading or even causes the gas relay to trip incorrectly. Checking the breather's overall seal, oil cup level and silica gel condition before winter is low-cost work with direct benefits.
Sealing and condensation control are about temperature difference, not just low temperature. Condensation risk on transformer bodies and accessories in winter comes mainly from the day-night temperature swing. The temperature inside mechanism boxes and terminal boxes fluctuates with the outside air, and moisture condenses on box walls and component surfaces. Damp secondary circuits then suffer insulation decline and signal abnormalities, often appearing before main equipment faults. Before winter, check the seals and cable hole sealing on terminal boxes and mechanism boxes one by one. Reinforce poor sealing with firestop putty. Confirm that heating and dehumidification devices inside the boxes operate normally, with reasonable heater capacity and placement, so they are not simply installed but unused, or heating adjacent wiring. On the transformer itself, check sealing at bushings, conservator connection flanges and the gas relay terminal box. Meltwater can seep in through weak seals and cause gas protection misoperation or bushing insulation decline. The relay terminals should not leak oil, and secondary circuits need water, oil and frost protection.
Outdoor insulation checks should not stop at the transformer body. The incoming and outgoing sides connect to disconnectors, surge arresters and busbars, and outdoor insulation should be checked as a whole in winter. Before winter, use an infrared thermometer to measure the main transformer bushings, disconnector contacts, busbar joints and cable terminations point by point, and compare with historical data, focusing on poor contact and local overheating. Disconnector bearings and transmission parts tend to stiffen in low temperatures. Check the sealing and heating of the operating mechanism box as well, so the disconnector does not stick or refuse to operate when needed. For surge arresters, focus on cracks, snow or ice on external insulation, whether the monitoring meter reads correctly, and whether the grounding down-lead connection is reliable. Ice formed by melting snow on insulator strings can bridge sheds and cause flashover, so clear it during special patrols after snow. These checks are not complicated, but they need to be done before the cold wave arrives. Fixing them after snow often means missing the window.
Coolers and on-load tap changers also need advance testing. For forced-oil circulating air-cooled transformers, cooler switching in winter should follow oil temperature and load. They should not stay off for long periods, nor run continuously. Damp control circuits or oxidized contactor contacts can prevent the cooler from starting when it should. Before winter, perform a manual and automatic switching test. Also check the drive mechanism and oil chamber of the on-load tap changer. Thickened grease at low temperatures can cause tap change failure, so replace it with low-temperature lubricant if necessary.
Run through pre-winter inspection and protection in terms of oil level and temperature, breathers, sealing and condensation, outdoor insulation, and cooling and tap changing. Most winter light gas alarms, secondary circuit abnormalities and disconnector operating problems can be avoided in advance. The key to winter transformer maintenance is not emergency repair, but handling the details that only show up when the weather turns cold before the cold wave arrives.