Basic protection knowledge of transformers

I. Transformer Faults
1. Internal Faults
Internal transformer faults mainly include inter-turn short circuits, turn-to-turn short circuits, and ground faults in the neutral point grounding system windings. These faults are very dangerous because the high-temperature electric arc generated by the short-circuit current will not only burn the winding insulation and iron core, but also decompose the insulating materials and transformer oil, producing a large amount of gas, which may cause local deformation or rupture of the transformer oil tank, or even an oil tank explosion. Therefore, when an internal fault occurs in the transformer, the transformer must be quickly cut off. This must be remembered.
2. External Faults
External transformer faults mainly include inter-phase short circuits and ground faults that occur on the transformer bushings and leads. When such faults occur, the transformer should also be quickly cut off to minimize the impact of the short-circuit current on the transformer.
II. Abnormal Operating Conditions of Transformers
The main abnormal operating conditions of transformers are:
(1) Current caused by external short circuits.
(2) Overload.
(3) Oil level reduction caused by oil leakage from the tank.
(4) Overexcitation caused by increased neutral point voltage, excessive external voltage, or reduced frequency.
III. Protective Devices to be Installed on Transformers
(1) Gas protection that reflects internal faults and oil level reduction in the transformer tank.
(2) Differential protection or current-breaking protection that reflects inter-phase short circuits of transformer windings and leads, and single-phase ground faults of windings and leads in neutral point directly grounded systems.
(3) Overcurrent protection (or composite voltage-initiated overcurrent protection or negative sequence overcurrent protection) that reflects external inter-phase short circuits of the transformer and serves as a backup for gas protection and differential protection (or current-breaking protection).
(4) Zero-sequence current protection that reflects external and internal ground faults in neutral point directly grounded systems.
(5) Overload protection that reflects symmetrical overloads of the transformer.
(6) Protection that reflects overexcitation of the transformer.
IV. Main Protection of Transformers
(1) Gas Protection
(1) Basic Working Principle of Gas Protection
Protection that reflects the amount of gas and oil flow rate during a fault is called gas protection. When an internal fault occurs in a transformer, the local high temperature at the fault point causes the transformer oil temperature to rise and the volume to expand, and the air in the oil is expelled to form rising gas. If an arc is generated at the fault point, the transformer oil and insulating materials will decompose into a large amount of gas, and this gas flows from the oil tank to the oil reservoir.
The more serious the fault, the more gas is produced, and the faster the oil flow rate to the oil reservoir. Since the amount of gas and the oil flow rate can directly reflect the nature and severity of the transformer fault, a small amount of gas and a low gas flow rate will cause a light gas alarm; for a serious fault with a high oil flow rate, the heavy gas protection will instantly trip the circuit breaker.
The gas relay is the main component of the gas protection system. It is installed in the middle of the connecting pipe between the oil tank and the oil reservoir, so that the gas inside the oil tank must pass through the gas relay before it can flow to the oil reservoir. In order for the gas to flow smoothly to the oil reservoir, older transformers require a certain inclination of the oil tank and connecting pipe, with the oil tank requiring 1%-1.5% and the connecting pipe requiring 2%-4% inclination.
In new transformers, gas collecting branches are installed in places where gas is easily accumulated (such as the bushing riser). Each gas collecting branch is connected to a gas collecting main pipe, and then the gas collecting main pipe is connected to the connecting pipe at the front end of the gas relay. In this way, as long as the gas collecting pipe and the connecting pipe have a certain inclination, the gas can flow into the oil reservoir, so there is no requirement for the inclination of the oil tank.
Currently, domestic open-cup baffle-type gas relays are used, and their working principle is as follows:
1) During normal operation, the open cup is filled with oil. Because the torque generated by the self-weight of the open cup is less than the torque generated by the counterweight, the open cup is pushed upwards, and the reed switch is open.
2) When a minor fault occurs inside the transformer oil tank, a small amount of gas will accumulate at the top of the relay, causing the oil level inside the relay to drop, and the open cup will be exposed to the oil surface. Because the torque generated by the self-weight of the open cup plus the weight of the oil in the cup is greater than the torque generated by the counterweight, the open cup rotates downwards. When the magnet fixed on the open cup descends to near the reed switch, the switch closes and sends a light gas alarm signal.
3) When a serious fault occurs inside the oil tank, a large amount of gas will be generated, accompanied by oil flow impacting the baffle. When the oil flow rate reaches the set value of the relay, the baffle is pushed to a certain position, and the magnet fixed on the baffle is close to the reed switch, causing the switch to close, and this switch closure action trips the circuit breaker.
(2) Setting of Gas Protection
1) Setting of Light Gas Protection
The magnitude of the light gas action value is expressed by the amount of gas. Generally, the gas volume range for light gas protection is 20-300 cm3; the adjustment of the gas volume can be achieved by changing the length of the counterweight lever arm.
2) Setting of Heavy Gas Protection
The magnitude of the heavy gas protection action value is expressed by the oil flow rate. General requirements for oil flow: 0.8-1.0 m/s for self-cooled transformers, 1.0-1.2 m/s for forced oil circulation transformers, and 1.2-1.3 m/s for transformers above 120 MVA.
(3) Advantages and Disadvantages of Gas Protection
The main advantage of gas protection is its simple structure and its ability to comprehensively reflect various faults inside the transformer tank. Especially when a turn-to-turn short circuit occurs and the number of shorted turns is very small, although the current in the fault loop is large and may cause serious local overheating, the change in current reflected in the external circuit is very small, and even the differential protection with higher sensitivity may not operate. Therefore, gas protection is particularly important for reflecting this type of fault. In addition, gas protection is the only protection against core damage. Because of its simplicity, sensitivity, and economy, gas protection is widely used and should be installed in oil-immersed transformers of 800 kVA and above and indoor oil-immersed transformers of 400 kVA and above.
The main disadvantage of gas protection is that it cannot reflect faults in the transformer bushings and leads. Therefore, gas protection cannot be used as the sole main protection for the transformer; it acts as the main protection for the transformer together with differential protection.
(2) Differential Protection
1. Basic Principle of Differential Protection
Transformer differential protection is based on the circulating current principle. It can correctly distinguish between internal and external transformer faults and instantly remove faults within the protected area. Current transformers TA1 and TA2 are installed on both sides of the transformer, and their secondary windings are connected in series according to the circulating current principle. The differential relay is connected to the differential current circuit.
During normal operation or external faults, current flows through both sides of the transformer. If the turns ratio of the two current transformers is appropriately selected, the secondary currents I12 and I22 are equal in magnitude and the same in direction, but in the differential circuit, the directions of I12 and I22 are opposite. Therefore, the current in the differential relay KD is equal to the difference between the secondary currents of the current transformers on both sides, which is zero. Therefore, the relay will not operate during normal operation or external faults.
When an internal fault occurs in the transformer, the secondary currents Id12 and Id22 of the current transformers on both sides are in the same direction in the differential circuit, and the current flowing through the differential relay is the sum of the two currents, causing the differential relay to operate.
In practice, due to factors such as transformer magnetizing inrush current, wiring methods, and current transformer errors, unbalanced current will flow in the differential relay. The larger the unbalanced current, the larger the operating current of the relay, which reduces the sensitivity of the differential protection. Therefore, one of the main problems that differential protection needs to solve is to adopt various measures to avoid the influence of unbalanced current. Under the condition of ensuring selectivity, sufficient sensitivity and speed must also be ensured in case of internal faults.
2. Special Issues of Differential Protection
1) Influence of Magnetizing Current
The magnetizing current during normal operation of the transformer only flows through the power supply side and is reflected into the differential circuit through the current transformer, causing unbalanced current. However, under normal circumstances, the transformer magnetizing current is very small, generally not exceeding 1% of the rated current; during external faults, due to the voltage drop, the magnetizing current also decreases, so its influence is even smaller, so it is not considered during actual setting.
2) Influence of Magnetizing Inrush Current
When the transformer is energized under no-load conditions, a large magnetizing inrush current may occur, which can reach 6-8 times the rated current of the transformer. It is transmitted to the secondary side through the current transformer on the power supply side of the transformer. If it flows into the differential circuit, it will often cause the differential protection to malfunction.
Measures to prevent magnetizing inrush current from causing differential protection malfunction:
(1) Using differential instantaneous protection. Since differential instantaneous protection has an inherent operating time, the operating current does not need to avoid the maximum current. This scheme has low sensitivity and is only suitable for small transformers.
(2) Using a differential relay with an intermediate-speed saturated current transformer. The intermediate-speed saturated current transformer can suppress the transmission of magnetizing inrush current, thereby preventing the protection from malfunctioning. However, since the transient current during internal short circuits also contains an aperiodic component, the protection should operate with a delay. In addition, since there is often one phase without an aperiodic component in the three-phase inrush current, the fast saturated current transformer in that phase does not work, which requires the protection operating value to be increased, thus reducing the sensitivity of the protection. Because this method is slow and has poor sensitivity, it is only suitable for medium and small transformers.
(3) Using second harmonic restraint. In the magnetizing inrush current, in addition to the fundamental wave and aperiodic component current, the second harmonic current is the largest, which is the most obvious characteristic of the magnetizing inrush current, because in other operating conditions, there is rarely any second harmonic generated. This is the main measure for differential protection of large transformers to prevent magnetizing inrush current.
(4) Using the characteristic that the magnetizing inrush current waveform has a clear discontinuity angle to avoid the magnetizing inrush current. Currently, there are two schemes: one is to directly identify the magnitude of the discontinuity angle to determine whether it is magnetizing inrush current or an internal short circuit. The other is to compare the rate of change of the magnetizing inrush current and the secondary short-circuit current.
(5) Installing differential protection separately on the windings on each voltage side of the transformer, so that the magnetizing inrush current no longer enters the differential circuit.
3) Influence of Different Phase Angles of Currents on Each Side of the Transformer
Transformers often use the Y, d11 connection method. Therefore, the phase angles of the currents on both sides of the transformer are inconsistent. Under normal circumstances, the line current on the delta side of the transformer leads the corresponding current on the star side by 30°. If the same wiring method is used for the current transformers on both sides, the secondary currents also differ by 30°. Therefore, it is necessary to compensate for the unbalanced current caused by the different phase angles of the currents on both sides.
4) Influence of Different Errors of Current Transformers on Each Side
Due to the different magnetizing characteristics and secondary loads of the current transformers, a large unbalanced current will be caused in the differential circuit.
5) Influence of the Difference Between the Calculated Turns Ratio of the Current Transformer and the Selected Standard Turns Ratio
This difference in turns ratio will cause unbalanced current. When this unbalanced current is greater than 5% of the rated load current, compensation measures should be taken. Common compensation methods are to use an auxiliary autotransformer or to use the balancing coil of the differential relay for balancing.
6) Influence of Transformer Tap Changer
During operation, the transformer needs to be tapped according to the system voltage requirements, which actually changes the turns ratio of the transformer, thus generating unbalanced current. The magnitude of the unbalanced current is related to the range of tap changing. Since it is impossible to readjust the relay with the change of the transformer tap during operation, the unbalanced current caused by the transformer tap changer should be considered when setting the protection operating value.