When the weather turns cold, circuit breakers and transformers are most likely to run into problems in the details. Low temperatures alone don't necessarily cause immediate trouble. The real headache is the day-night temperature swing, condensation, icing and melting snow. Many faults aren't caused by the cold itself, but by the freeze-thaw cycle - melting during the day, freezing at night, over and over. Winter maintenance is mainly about watching the mechanisms, oil circuits, gas circuits and seals.
The first thing to check on a circuit breaker is the operating mechanism. In outdoor breaker mechanism boxes, trip and close coils, auxiliary switches and terminal blocks are all vulnerable to moisture. Turn on the heaters before winter arrives and check the temperature and humidity controllers. Don't wait until it snows to find out the heater is dead. Heater power doesn't need to be high - a few dozen watts to just over 100 W is enough for small terminal boxes, while switchgear and mechanism boxes generally need 200 to 500 W. The key is to pair them with temperature and humidity controllers for automatic start-stop. Also check the sealing of the box: cable entry holes, cable trench entrances, door gaskets. Fix any gap. If meltwater seeps in, the best case is DC grounding; the worst case is false alarms.
Low temperatures also affect operating mechanisms directly. Hydraulic mechanism oil gets thicker in cold weather, so pumping and operation may slow down. Spring mechanism grease stiffens, which affects opening and closing speed. Rotating parts and interlocks tend to stick. Before winter, check fasteners and apply low-temperature lubricant to load break switches, earthing switches and breaker transmission parts. Don't use ordinary grease - it hardens as soon as it gets cold. For pneumatic mechanisms, also check for water in the gas circuit. Drain the drain valve when needed to prevent ice blockage.
For SF6 circuit breakers, pay special attention to gas pressure. When the temperature drops, the SF6 pressure gauge reading falls - that's normal physics. But if the density relay alarms or locks out, take it seriously. First determine whether the pressure drop is caused by temperature or an actual leak. A temperature-compensated density relay generally reflects the equivalent pressure at 20°C. If it alarms, it's probably not just the cold. If the gas compartment pressure is below the manufacturer's specified value, top up the gas in time. Before filling, confirm gas quality. Don't use cylinders of unknown origin. Also clear snow and ice from outdoor breaker bushings. When snow melts, a conductive water film forms and flashover risk rises.
For transformers in winter, the focus is on the oil system and breather. Oil level drops as temperature falls. That's normal, but it must not go below the manufacturer's minimum allowable level. During inspections, compare the oil level with the current oil temperature. If the level is too low, the gas relay may trip incorrectly. Also check the conservator, bladder and breather. Replace the breather silica gel when color change exceeds two-thirds. If it discolors too quickly, moisture may have entered the oil system - check the seals. Clear snow from the transformer body and cooling fins promptly. Don't scrape with sharp tools; damaged anti-corrosion coating on cooling fins causes more trouble. Remove ice from bushings to prevent flashover during melting.
Coolers also need attention in winter. For forced-oil circulating air-cooled transformers, fans and oil pumps shouldn't run all the time, nor stay off all the time. Switch them based on oil temperature and load. Moisture in the control circuit or oxidized contactor contacts can prevent the cooler from starting when it should. Also check the oil chamber and drive mechanism of the on-load tap changer. In low temperatures, mechanism sticking can cause tap change failure. Don't overlook the gas relay rain cover and terminal box sealing either. Meltwater seeping in can cause false trips.
During and after snowfall, a few things need immediate action. Sweep snow from the tops of breaker mechanism boxes and terminal boxes so meltwater doesn't seep in. Check outdoor contacts, clamps and bushing connection plates for snow cover or steam. Steam means the contact may be overheating - snow landing on it evaporates immediately. Measure temperature on these points closely. Inspect cable trench covers and hole sealing again. Small animals look for warmth in cold weather and may enter trenches through unsealed openings. Surge arresters aren't the main focus, but glance at their monitoring meters and grounding down-leads. Winter overvoltage and lightning are less frequent, but pollution flashover and switching overvoltage still occur during snowmelt.
Don't skimp on winter supplies. Heaters, temperature and humidity controllers, dehumidifiers, sealing strips, industrial petroleum jelly, silica gel breathers, SF6 gas, insulating oil and spare trip/close coils may all come in handy. If insulators inside cabinets are dusty, wipe them with a dry cloth before winter and apply a thin layer of industrial petroleum jelly. It helps with moisture resistance. Repair any poor sealing before the cold sets in. Once heavy snow blocks access, the work becomes time-consuming, labor-intensive and dangerous.
Most winter problems with circuit breakers and transformers aren't major faults. They come from small issues piling up: condensation, sticking, low oil level, low gas pressure, poor sealing. During inspections, take one more look, feel one more heater, check one more seal - and you can often avoid a trip.