To enhance the operational reliability and ensure the safety of equipment, large - scale power transformers are equipped with both electrical and non - electrical quantity protection. When an internal fault occurs in a transformer, if these protections operate correctly and cut off the power supply in a timely manner, it can limit the conversion of electrical energy into heat and chemical energy, as well as the rapid expansion of oil volume and the decomposition of insulating paper and oil into gases. In this way, the fault can be controlled within an allowable range, effectively protecting the main transformer, preventing the fault from spreading and reducing losses. Due to the inherent characteristics of electrical protection, when a fault is beyond the sensitivity range or fault types covered by electrical protection, non - electrical quantity protection must be relied on to ensure the safety of the main transformer. Table 1 shows several types of non - electrical quantity protection classified according to different physical quantities they respond to.
The electrical relay protection of power transformers, such as differential protection, over - current instantaneous protection, zero - sequence current protection, etc., is not sensitive to internal transformer faults. This is mainly because internal faults usually start with inter - turn short - circuits. Although the fault current inside the short - circuited turns is very large, the line current does not change significantly. Only when the fault develops into multi - turn short - circuits or ground short - circuits can the power supply be cut off. The main protection against internal transformer faults is gas protection, which can cut off the faulty equipment instantaneously. However, the sensitivity of the gas relay depends on the setting value (flow velocity).
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The relay should repeatedly discharge the gas inside the transformer body through the air plug.
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During the operation of the transformer, the gas protection device should be connected to both the signal and trip circuits. The heavy gas protection of the on - load tap - changer should be connected to the trip circuit. At the same time, the insulation performance of the signal and trip circuits should be ensured to be good.
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The gas relay should be subjected to secondary - circuit electrical insulation tests and light - gas operation accuracy verification during the transformer outage. When the transformer is under maintenance or conditions permit, the relay should be removed for operation characteristic verification, and records should be kept.
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The relay should have anti - vibration, rain - proof and moisture - proof functions.
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During the operation of the transformer, the protection mode of the relay should be adjusted in a timely manner according to different operation and maintenance modes and restored to the original state as soon as possible.
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When filtering oil, replenishing oil, replacing the submersible pump or the adsorbent of the oil purifier during the operation of the transformer, the heavy gas protection should be re - connected to the signal circuit, while other protection devices should still be connected to the trip circuit.
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When the oil level in the oil level gauge rises abnormally or there are abnormal phenomena in the breathing system and it is necessary to open the air - release or oil - drain valve, the heavy gas protection should be re - connected to the signal circuit first.
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During the earthquake prediction period, the operation mode of the heavy gas protection should be determined according to the specific situation of the transformer and the seismic performance of the gas relay. For transformers that have been shut down due to heavy gas protection operation caused by an earthquake, the transformer and the gas protection should be inspected and tested before being put back into operation, and they can be put into operation only after no abnormalities are confirmed.
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When the gas relay sends a signal or trips, corresponding electrical tests should be carried out, and gas samples should be taken for necessary analysis. The nature of the transformer fault should be comprehensively judged to decide whether to put the transformer back into operation.
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When gas accumulates in the gas relay of the on - load tap - changer, timely inspection and analysis should be carried out. Attention should be paid to the accumulation of free carbon in the relay, which may cause the insulation of the connection terminals to decline or cause grounding problems, and it should be discharged in time.
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When it is necessary to take a gas sample or discharge gas from the relay, first loosen the oil - drain plug at the bottom of the gas - guiding box to drain some transformer oil. The gas in the relay will enter the gas - guiding box through the copper tube as the oil level drops, and then gas can be discharged or sampled through the air - release plug on the gas - guiding box.
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During operation, attention must be paid to the sealing condition of each joint of the gas - guiding box.
To protect the safe operation of transformers, the temperature of the cooling medium and windings should be controlled within the specified range. This requires temperature controllers to provide functions such as temperature measurement and cooling control. When the temperature exceeds the allowable range, an alarm or trip signal is provided to ensure the service life of the equipment. Temperature controllers include oil - surface temperature controllers and winding temperature controllers.
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Principle of Oil - surface Temperature Measurement by the Temperature Controller
: The temperature controller is mainly composed of an elastic element, a capillary tube and a temperature bulb. The closed system formed by these three parts is filled with temperature - sensitive liquid. When the measured temperature changes, due to the "thermal expansion and contraction" effect of the liquid, the volume of the temperature - sensitive liquid in the temperature bulb changes linearly. This volume change is transmitted remotely through the capillary tube to the elastic element in the meter, causing it to displace correspondingly. After being amplified by the gear mechanism, this displacement can indicate the measured temperature. At the same time, a micro - switch is triggered to output an electrical signal to drive the cooling system, achieving the purpose of controlling the temperature rise of the transformer.
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Principle of Remote Display of Measured Values
: To transmit the measured values to the control room for remote indication, the temperature controller converts the change in the resistance value of the copper or platinum resistance sensor or the mechanical displacement caused by temperature change into a change in the resistance value of a slide - wire resistor. The analog output is a 4 - 20mA electrical signal, which is converted into a digital or analog display at the remote end. The advantage of using a slide - wire resistor is that the wiring is relatively simple. For long - distance transmission, no compensation circuit is required, and the current signal is not sensitive to stray magnetic fields and temperature interference.
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Setting Principles
: Large - scale power transformers should be equipped with oil - surface temperature controllers and winding temperature controllers with the function of remote temperature transmission. To fully reflect the temperature change of the transformer, the oil - surface temperature controller is generally configured in a dual - redundant manner, that is, oil - surface temperature controllers are installed on both sides of the main transformer.
To prevent tripping accidents caused by the malfunction of non - electrical quantity protection, many units stipulate that the contacts of the temperature controller should not be connected to the trip circuit. In fact, whether to connect to the trip circuit should consider the structural form of the transformer and the duty mode of the substation. For example, due to the special structure of shell - type transformers, when the substation is unmanned, the trip contacts of the oil - surface temperature controller should be strictly connected to the trip circuit as specified by the manufacturer. For transformers with forced - oil - circulation air - cooling, the trip circuit is generally connected, while for transformers with natural - oil - immersed air - cooling, only a signal may be sent. The high - temperature trip signal of the transformer must use the hard contacts of the temperature controller, and the remotely transmitted temperature to the control room should not be used to trigger the trip. In a 220kV substation, because the remotely transmitted temperature was used to trigger the trip, when the resistance thermometer circuit was broken or the contact resistance increased, the temperature reflected in the control room rose sharply, exceeding 150%, causing a malfunction and tripping.
