What are the main transformer protection devices and monitoring systems?

I. Core monitoring and protection implementation module

1. Gas Relay (Gas relay)

   • principle: Gases are generated in the internal insulation of the transformer when it overheats or fails. Gas is lighter than oil and accumulates in the piping between the transformer tank and the oil conservator. The gas relay detects the amount of gas accumulation by means of an internal float. When the gas accumulates to a certain level, the float drops and triggers an alarm contact (light gas protection). Severe internal faults can also cause a shock to the oil flow in the tank, which strikes the baffle in the relay and trips the trip contacts (heavy gas protection).

   • functionality: Detects early overheating of the transformer's internal insulating parts or gases generated by short-circuit faults, as well as oil flow impacts generated in the event of a fault, and issues an alarm or performs a trip.

2. Oil Temperature Indicator (Top Oil Temperature Gauge)

   • principle: Based on the principle of thermal expansion and contraction of liquid (transformer oil). The probe is placed in the top layer of oil, and changes in oil temperature cause the medium in the probe to expand or contract, which drives the dial pointer to indicate the oil temperature through mechanical transmission. Usually with adjustable contacts, used to issue an alarm or control cooling equipment.

   • functionality: Monitors the temperature of the top layer of transformer oil, which is a direct reflection of the heat dissipation of the transformer. Used for overheating alarms and to control the start and stop of the cooling fan/oil pump.

3. Winding Temperature Indicator (Analog Winding Thermometer)

   • principleThe instrument combines the temperature rise of the top oil and the winding itself due to the load current. It estimates the actual temperature of the winding by means of a sensor that simulates the temperature of the winding (with an internal heating element controlled by the secondary current). When the analog winding temperature exceeds the set value, an alarm or trip contact is triggered.

   • functionality: Reflects the actual operating temperature of the transformer windings more accurately and prevents damage to the winding insulation due to overloading or internal faults.

4. Pressure Relief Device (Blowout Valve / Pressure Relief Valve / Rupture Disc)

   • principle: In the event of a serious failure inside the transformer, resulting in a sharp rise in pressure inside the tank beyond the safety limits for which it was designed, the device (which may be a spring-loaded valve or a fragile rupture disc) opens automatically to quickly release the excessive internal pressure to the external environment and prevent the tank from exploding. A contact switch is usually integrated to send a trip signal to the protective device when the pressure is released.

   • functionality: Serves as the last line of safety in the event of a serious failure inside the transformer, preventing the box from exploding due to excessive internal pressure.

5. Oil level indicator (oil level meter)

   • principle: Usually mounted on the oil reservoir (oil pillow), the oil level is indicated by an internal float and magnetic coupling mechanism. The float changes with the oil level and actuates an external pointer or contact. An alarm contact is provided to signal when the oil level is too high or too low.

   • functionality: Monitor the oil level of the transformer tank (especially the oil conservator) to ensure that the oil level is within the normal range to ensure good insulation and cooling effect, and to detect whether there is any oil leakage in the transformer.

6. current transformer

   • principle: Proportional conversion of high currents on the primary side to low currents on the secondary side (e.g. ratio 1000/5A). Based on the principle of electromagnetic induction, the current value is reduced by means of a secondary winding with a high turns ratio. The secondary side is connected to an ammeter or protective relay.

   • functionality: Provides precise current signals for transformer protection and monitoring. It is the basis for the realization of differential protection, overcurrent protection, zero sequence protection and other electrical quantity protection.

7. voltage transformer

   • principle: Proportional conversion of a high voltage on the primary side into a low voltage on the secondary side (e.g., ratio 110kV/100V). Based on the principle of electromagnetic induction, the voltage value is reduced by means of a secondary winding with a high turns ratio. The secondary side is connected to a voltmeter or protective relay.

   • functionality: Provides precise voltage signals for transformer protection and monitoring, for overvoltage and undervoltage protection and as a reference for phase comparison.

8. Microcomputer protection devices (numerical protection relays / intelligent protection devices)

   • principle: Utilizing microprocessor and digital signal processing technology, it carries out high-speed arithmetic and logic judgment by sampling the current and voltage signals sent from the transformer. Built-in a variety of protection algorithms, such as differential, overcurrent, zero sequence, overvoltage, undervoltage, grounding and so on. When a fault is detected, the circuit breaker is controlled to trip, issue an alarm or execute other control commands through the output circuit according to the preset protection value and logic.

   • functionality: It integrates a variety of protection functions to realize comprehensive protection of transformer and its connected lines. It has the advantages of high precision, intelligentization, self-diagnosis, fault recording and remote communication.

9. Inactivation Resistor Throwing Switch

   • principle: In some special transformers (e.g. extra-high voltage transformers operating in parallel) or after a system fault, it is necessary to quickly release the magnetic energy stored inside the transformer in order to prevent abnormal voltages from being generated. This type of switch is responsible for connecting a specialized demagnetizing resistor to one of the transformer's windings to dissipate and release the magnetic energy.

   • functionality: Used to protect transformer insulation against abnormally high voltages caused by saturation of the yoke or specific faults.

10. Grounding Resistors

    • principle: When the neutral point of a transformer needs to be grounded for operation, the grounding current in the event of a single-phase ground fault is limited by connecting a resistor with a fixed resistance value. The resistance value of the resistor affects the magnitude of the grounding current as well as the component of the zero sequence voltage.

    • functionality: Limits ground fault currents, reduces the effects of ground faults on other equipment, and may affect the sensitivity and selectivity of some zero sequence protections.

11. cable connector

    • principle: It is an overvoltage protection device, when the line or transformer is subjected to transient overvoltage caused by lightning strike or other reasons, the resistance value of the zinc oxide (ZnO) varistor on the lightning arrester will drop sharply, and the energy of the overvoltage will be drained into the earth, thus protecting the insulation of the transformer from damage. Under normal voltage, its resistance is so high that it hardly conducts.

    • functionality: Protects the transformer and its insulation from lightning strikes and operational overvoltages.

12. Electromagnetic Current Relay

    • principle: Type of relay used in the early days. Electromagnetic suction is generated by the current flowing in the coil to drive the moving contacts to close. Overcurrent or quick-break protection is realized according to the amount of current and delay characteristics of the contact action.

    • functionality: Provide basic protection functions such as overcurrent, quick disconnect, etc., which are mostly replaced by microcomputer protection devices nowadays.

13. Electromagnetic Voltage Relay

    • principle: Similar to current relays, electromagnetic suction is generated by the voltage in the coil to drive the contact action. Used for overvoltage, undervoltage and other protection.

    • functionality: Provides basic protection functions such as overvoltage and undervoltage.

14. thermal relay

    • principle: Utilizes the principle of bimetal bending by heat. When the current passes through the heating element, the bimetal is heated. When the current and the heating time cause the bimetal to bend to a certain degree, the contact operates. Usually used for overload protection.

    • functionality: Monitoring of long-term overloading of transformer windings to prevent overheating and damage to insulation.

15. Signal light indicator

    • principle: A simple indicator that lights up or flashes when a contact signal (e.g. alarm, trip, device status) is received.

    • functionality: Visualization of the operating status of the transformer, fault signals or the operation of protective devices.

16. operating mechanism

    • principle: A mechanical device attached to a circuit breaker (air switch, oil switch, vacuum switch, etc.) that is responsible for performing closing and opening operations. It can be electrically operated, spring operated or manually operated.

    • functionality: Receives trip commands from the protective device or closing commands from the operator, thus controlling the circuit breaker action to connect or disconnect the power supply to the transformer.

17. interrupter

    • principle: Switching appliances capable of connecting, carrying and disconnecting normal load currents as well as fault currents for a specified period of time under specified conditions. Its arc extinguishing medium can be air, vacuum, SF6 gas or oil.

    • functionality: Safely and reliably cuts off the power supply to the transformer and isolates the fault after the protective device has issued a trip command.

18. disconnect switch

    • principle: It is primarily used to provide a visible isolation break in the disconnected condition to ensure the safety of service personnel. It cannot be operated in the presence of load current, but is usually operated after the circuit breaker has been disconnected.

    • functionality: Provide a safe and reliable disconnect point when servicing transformers to ensure complete isolation between service personnel and energized equipment.

19. Grounding knife gate

    • principle: Installed on the high-voltage side or low-voltage side lead-in line of the transformer. Before the equipment maintenance, by manual or automatic operation to close the grounding knife gate, the equipment will be reliably grounded.

    • functionality: Reliably ground the high-voltage portion of the equipment during maintenance to prevent induced voltages or residual charges from posing a hazard to maintenance personnel.

20. Temperature transmitter

    • principle: Similar to a thermometer, but it converts the measured temperature signal into a standard (e.g., 4-20 mA) or digital signal that can be remotely transmitted over a communication line to a distributed control system (DCS) or monitoring system.

    • functionality: Digitize transformer temperature information for remote monitoring, data logging and analysis.

21. Pressure Transmitter

    • principle: Measures the pressure in the tank and converts the pressure signal into a standard analog or digital signal output for remote monitoring and control.

    • functionality: Monitoring pressure changes in the transformer tank can be used as an aid in determining internal transformer anomalies (e.g., oil leaks, faulty gas generation resulting in increased internal pressure).

22. Oil Leak Detection Device

    • principle: Usually installed at the bottom of the transformer or in the surrounding area, it utilizes sensors (e.g., conductive sensors, photoelectric sensors) to detect the presence or absence of transformer oil leakage.

    • functionality: Timely detection of transformer oil leakage phenomenon, to prevent insufficient oil to affect the cooling and insulation, and can issue an alarm.

23. gas pressure gauge

    • principle: For displaying the gas pressure values inside the transformer tank or in the nitrogen protection system.

    • functionality: Monitoring of the pressure status of the gases inside the transformer, e.g. the operating pressure of the nitrogen protection system.

24. Insulation Monitor

    • principle: The health of transformer insulation is assessed by measuring parameters such as insulation resistance or dielectric loss angle tangent between the transformer windings to ground.

    • functionality: Monitor the degree of deterioration of transformer insulation in real time or at regular intervals to provide a basis for predictive maintenance.

25. Partial Discharge Detection Device

    • principle: Small electrical discharges (partial discharges) may be present in the internal insulation of a transformer, which generate electromagnetic waves, acoustic waves or chemical substances. Partial discharge detection devices (e.g. high-frequency current sensors, ultrasonic sensors, chromatographic analyzers) monitor these signals to determine whether there are early defects in the insulation.

    • functionality: Detecting weak discharges within the insulation material and recognizing insulation defects is an important means of early fault diagnosis in transformers.

26. Dissolved gas analyzer in oil

    • principle: The types and contents of dissolved gases in oil are analyzed by collecting transformer oil samples and using techniques such as gas chromatography. Different types of faults (e.g. overheating, discharges) produce characteristic gases and their combinations in the oil.

    • functionality: Diagnosing the type of faults occurring inside the transformer (e.g., overheating, partial discharge, high-energy discharge, etc.) and their severity by analyzing the dissolved gases in the oil is a key technology for transformer condition monitoring and fault diagnosis.

27. Transformer online monitoring system

    • principle: It is a comprehensive system that integrates a variety of sensors and instruments mentioned above (e.g., oil temperature, winding temperature, oil level, pressure, partial discharge, gas analysis, etc.), and transmits all the data to a central control room or a cloud platform for real-time monitoring, analysis, and early warning through data acquisition and communication networks.

    • functionality: Provide comprehensive, real-time monitoring of transformer operation status, realize condition maintenance and predictive maintenance, and improve the reliability and safety of operation.

28. grounding

    • principle: Connects the non-electrically charged parts of the transformer, such as the metal case and core, to earth. This is achieved by means of conductors and grounding electrodes.

    • functionality: When the insulation breakdown inside the transformer causes the shell to become energized, it is able to conduct the fault current to earth, protect personnel from electric shock, and provide an action circuit for the protective device.

29. Transformer body insulation resistance tester 

    • principle: The insulation of a transformer is tested using a high voltage, low current DC power supply to measure the insulation resistance value.

    • functionality: Periodically inspect the overall condition of the transformer insulation to assess the degree of moisture, deterioration or fouling.

30. Transformer Winding DC Resistance Tester

    • principle: Measurement of the DC resistance value of a transformer winding by applying a DC current.

    • functionality: Detect windings for broken strands, loose connectors, short circuits between turns, etc.

31. Transformer Ratio Tester

    • principle: Verify that the turns ratio of the transformer windings is accurate by measuring the ratio of voltage (or current) on the primary and secondary sides.

    • functionality: Test the transformer for turn-to-turn short circuits, broken circuits, or poor tap changer contact.

32. Tester for transformer no-load characteristics

    • principle: Measurement of no-load current, no-load loss and other parameters of a transformer under no-load conditions (primary side connected to voltage, secondary side open).

    • functionality: Evaluate the quality of the transformer core and the condition of the magnetic circuit.

33. Tester for transformer load characteristics

    • principle: Measurement of transformer parameters under load, such as short-circuit voltage and load loss.

    • functionality: Evaluate losses in transformer windings and connections.

34. Transformer oil chromatography analyzer

    • principle: Analyzes the composition and content of dissolved gases in transformer oil by gas chromatography to diagnose internal transformer faults. This is a more specific realization equipment of the dissolved gas analyzer in oil.

    • functionality: It is the core equipment of the dissolved gas analyzer in oil, which is used to finely analyze the composition of the gas in the oil and determine the nature of the fault.

35. thermal imaging camera

    • principle: Infrared radiation sensors are utilized to detect infrared thermal radiation from the surface of an object and convert it into a visible image. The higher the temperature of the object, the stronger the infrared radiation emitted.

    • functionality: Used for non-contact surface temperature detection during transformer operation, rapid detection of potential hot spots (e.g. loose contacts, blocked coolers, etc.), often used for inspection.

36. Ultrasonic detector

    • principle: Detection of ultrasonic signals generated during transformer operation. Partial discharges, corona discharges or arc discharges generate specific ultrasonic waves.

    • functionality: Used to detect partial discharges or corona discharges within transformers and is a tool for early fault diagnosis.

37. Dielectric strength tester for insulating oil

    • principle: A gradually increasing voltage is applied to a standard oil sample until the oil breaks down. The breakdown voltage is measured to assess the insulating properties of the oil.

    • functionality: Detects the breakdown voltage of transformer insulating oil to determine whether the insulating ability of the oil is qualified and whether it needs to be filtered or replaced.

38. Dielectric loss tester for insulating oil 

    • principle: Measurement of the dielectric loss factor (tanδ) of transformer insulating oil at a certain frequency and voltage. The dielectric loss is related to the purity and moisture content of the oil.

    • functionality: Evaluate the cleanliness and moisture content of the insulating oil.

39. On-load tap-changer position indicator

    • principle: Mechanical or electronic device indicating the current position of the transformer on-load tap-changer.

    • functionality: Display the position of the transformer voltage regulator (on-load voltage regulator switch) to confirm that the voltage regulation is normal.

40. Automatic pressure regulator

    • principle: A separate system (sometimes integrated into the transformer control cabinet, but more often an external system) is responsible for monitoring the system voltage and automatically adjusting the tap position of the transformer to maintain the output voltage within the set range.

    • functionality: Maintains the stability of the transformer output voltage and adapts to grid voltage fluctuations.

41. Excitation inrush current suppressor

    • principle: A large excitation inrush current will be generated when the transformer is just closed, in order to avoid this inrush current false activation of the protection device, some technical means (such as temporarily dropping the voltage, changing the closing phase, use of inrush compensator, etc.) will be used to inhibit or compensate for this inrush current.

    • functionality: Prevents the excitation inrush at the moment of transformer closing from causing false protection action.

42. Ground Resistance Measuring Instrument

    • principle: Measurement of the resistance of the grounding system using specialized instruments.

    • functionality: Ensure that the grounding resistance of the transformer enclosure and its grounding circuit meets the requirements and that the grounding effect is guaranteed.

43. High-voltage circuit monitoring device

    • principle: It may be an integrated monitoring system that detects parameters such as voltage, current, power factor, frequency, etc. on the high voltage side of the transformer in real time and is linked to the protection device.

    • functionality: Provides comprehensive monitoring of the operation of the high voltage side and supports the decision making of protection devices.

44. Low-voltage circuit monitoring device

    • principle: Similar to the HV circuit monitoring device, but for monitoring parameters on the LV side of the transformer.

    • functionality: Provides comprehensive monitoring of low voltage side operation and supports decision making for protective devices.

45. High-voltage voltage dividers for insulation monitoring

    • principle: A device used to divide a high voltage into lower voltages in proportion to the voltage, so that the state of its insulation can be measured or monitored.

    • functionalityFor insulation monitor or other online monitoring equipment to connect to the high-voltage side for insulation parameter measurement.

46. Low-voltage voltage dividers for insulation monitoring

    • principle: Used when it is necessary to monitor the insulation of a low-voltage part of a transformer.

    • functionality: Provides signal access for low voltage side insulation monitoring.

47. High frequency coupling device for insulation monitoring

    • principle: By means of coupling capacitors, etc., the high-frequency monitoring signal is injected into the high-voltage circuit of the transformer, and at the same time, the local discharge signal of the monitored point is coupled out.

    • functionality: Enables on-line monitoring of partial discharges and isolation from the main protection.

48. Transformer online operation condition monitoring and diagnosis system

    • principle: This is a higher-level integrated system that combines data from the multiple monitoring instruments and sensors mentioned above, with advanced algorithms for data fusion, trending, troubleshooting and life prediction.

    • functionality: Provides in-depth analysis of transformer operating status, realizes predictive maintenance, and improves equipment health management.

49. Insulation withstand voltage test equipment

    • principle: Provides a high voltage (usually industrial frequency or direct current) for insulation withstand tests to verify the ability of the transformer insulation to withstand a specified voltage.

    • functionality: Preventive testing of transformers after manufacture, installation or overhaul to ensure insulation is satisfactory.

50. On-line measurement unit for transformer parameters

    • principle: A series of sensors and measurement circuits are integrated for real-time measurement of various operating parameters of the transformer, such as voltage, current, power, frequency, temperature, insulation impedance, etc., and the data are transmitted to the main control system.

    • functionality: Realization of continuous online measurement and data acquisition of transformer operating parameters

Please note that some of the devices in the above list may be interrelated or integrated. For example, many protection functions are now integrated in a single "micro-protection device", which internally utilizes signals from various sensors. In addition, some devices (e.g. earth resistors, surge arresters, disconnect switches) are important components of the transformer system, but they are more of a safety or maintenance function than a direct type of "protective device".