Part 9 – Protective Systems on Transformers
Large oil filled transformers
Large oil filled transformers are equipped with sensors that monitor safe levels of current, voltage, temperature and phase sequence. They are monitored for oil pressure, temperature, and a sudden or slow increase in gas bubbles.
Safety controls used on transformers include the following:
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Fast and Slow Gas Detection
Monitors the gas is produced from the breakdown of cooling oil and insulation as it ages. Gas is entrained, or dissolved, in the oil and is released from the oil in the expansion tank. Top of the tank is a convenient location for gas monitoring devices.
Slow gas monitoring device
Diaphragm chamber on top of the expansion tank will expand and trigger an alarm of high gas pressure. Frequency of venting required is also recorded to monitor the long-term rate of gas accumulation.
Fast Gas Monitoring Device
Detects sudden accumulations of gas and trips the transformer breaker. It prevents failure of transformer components due to overheating, release of oil from the cooling system due to overpressurization, and catastrophic explosion of the transformer.
Fast gas devices operate using one of the following methods:
- A diaphragm device, similar to a slow gas device, but sized and engineered to be sensitive to large, sudden quantities of gas.
- A float within a chamber connected to the transformer and to the expansion tank, which is activated as gas accumulates and displaces the oil within the chamber.
- A flow switch in the piping between the transformer and the expansion tank, which is calibrated to activate when the flow exceeds a predetermined value due to an increase in pressure.
Oil Temperature Alarms
Oil temperature alarms are sensors monitor cooling oil temperature. The will trip the transformer on high oil temperature
Low Oil Level Protection
Oil level detectors alarm if the level is low, and generate a trip signal if the level continues to fall. Oil level gauges are visible on the transformer enclosure.
Winding Temperature Alarms
Temperature sensors are embedded in either or both the primary and secondary windings.
Overcurrent and Undervoltage Protection
The transformer must be protected by a fuse or breaker, or combination of both. Must be able to protect above the maximum voltage rating of the transformer.
Time/Current Characteristic
Breakers and fuses operate on a time/current characteristic. The higher the current, the less time required to trip the breaker. If the current reaches short circuit levels, the breaker trip extremely quickly, thereby protecting equipment.
Breakers
Breakers use a thermal/magnetic principle. For current overloads of small magnitude, a bimetal strip heats over time and trips the breaker. For larger overloads and short-circuit conditions, the breaker uses a fast-acting electromagnet to trip the breaker.
Fuses
Fuses allow temporary overloads but provide fast short circuit protection. They use a metal alloy that heats up in proportion to the amount of current flowing through the fuse. The more current, the less time it takes for the alloy to heat and trip. They need to be replaced after they operate. Breakers can be manually reset.
Large power transformers
Overcurrent protection is managed with a dedicated overcurrent relay which causes the transformer breaker to trip when it is activated.
Overvoltage Protection
Large transformers commonly use a backup to overcurrent protection which monitors secondary winding voltage. A predetermined undervoltage level, if detected, causes the transformer breaker to trip, ensuring that excessive current will not result from an undervoltage situation.
Synchronization Checks
Phase rotations on the primary and secondary sides are synchronized to avoid damage to the transformer and its breaker. When the transformer trips or is taken out of service, the primary and secondary voltages decay because the supply voltage has been disconnected. Voltage on the load side of the transformer decays more slowly because of the back emf induced by induction motors located downstream. The two sets of windings will become out of synch. This occurs at approximately 40% of rated voltage, causing the synchronization check relay to activate and the transformer breaker to trip if it is not already open. It protects the transformer from electrical system upsets or loss of supply that cause a loss of synchronization while the transformer breaker is still closed.
Overexcitation
Overexcitation is a condition where the magnetizing current of the primary windings exceeds safe limits – can cause overheating in the transformer. Overexcitation protection monitors the saturation condition and will signal alarms and trip the transformer.
Ground Fault Protection
Ground fault protection is provided by extremely sensitive devices that monitor for any current that may be flowing through the grounded metal work. If such currents occur, the devices interrupt the flow of electricity to equipment.
Phase Sequence Relays
Used to detect the proper phase sequence in three-phase transformers. Trip the power supply to a transformer if the phase sequence changes.
Dissolved Gas Monitoring
Built-in gas detectors and relay devices which trigger a relay or alarm when dissolved hydrogen gas exceeds acceptable levels. Hydrogen is produced from cellulose breakdown in the transformer
Differential Protection
Differential protection is used to monitor internal faults. It involves using current transformers to monitor the primary and secondary currents of power transformers.