Transformer Protection Knowledge

Transformers are stationary equipment that operate continuously, with relatively reliable operation and less chance of failure. However, since most transformers are installed outdoors and are affected by the load they bear during operation and the short-circuit faults of the power system, various faults and abnormalities inevitably occur during operation.

1. Common faults and abnormalities of transformers

Transformer faults can be divided into internal faults and external faults.

Internal faults refer to faults that occur inside the box, including phase-to-phase short-circuit faults in windings, inter-turn short-circuit faults in one-phase windings, short-circuit faults between windings and iron cores, winding disconnection faults, etc.

External faults refer to various phase-to-phase short circuit faults between the external lead wires of the transformer, and single-phase ground faults that occur through the box shell due to flashover of the lead wire insulation bushings.

Transformer failure is very harmful. Especially when an internal fault occurs, the high-temperature arc generated by the short-circuit current will not only burn the insulation and iron core of the transformer winding, but also cause the transformer oil to thermally decompose to produce a large amount of gas, causing the transformer shell to deform or even explode. Therefore, the transformer must be removed when it fails.

Abnormal conditions of the transformer mainly include overload, reduced oil level, overcurrent caused by external short circuit, excessive oil temperature of the transformer during operation, excessive winding temperature, excessive transformer pressure, and cooling system failure, etc. When the transformer is in abnormal operation, an alarm signal should be given.

2. Configuration of transformer protection

Main protection for short circuit faults: mainly longitudinal difference protection, heavy gas protection, etc.

Backup protection for short circuit faults: mainly including composite voltage blocking overcurrent protection, zero sequence (directional) overcurrent protection, low impedance protection, etc.

Abnormal operation protection: mainly includes overload protection, over-excitation protection, light gas protection, neutral point gap protection, temperature oil level and cooling system fault protection, etc.

3. Non-power protection

Transformer protection that utilizes non-electrical quantities such as oil, gas, and temperature of the transformer is called non-electrical protection. Mainly include gas protection, pressure protection, temperature protection, oil level protection and cooler full stop protection. Non-electricity protection operates in tripping or signaling according to on-site needs.

1. Gas protection

When a fault occurs inside the transformer, due to the action of short-circuit current and short-circuit point arc, a large amount of gas will be generated inside the transformer, and the oil flow of the transformer will accelerate. The protection achieved by using gas and oil flow is called gas protection.

(1) Light gas protection: When a minor fault or abnormality occurs inside the transformer, the fault point is locally overheated, causing part of the oil to expand. The gas in the oil forms bubbles and enters the gas relay. The light gas protection operates and sends out a light gas signal.

(2) Heavy gas protection: When a serious fault occurs in the transformer oil tank, the fault current is large, and the arc causes the transformer oil to decompose in large quantities, generating a large amount of gas and oil flow. The impact of the baffle causes the heavy gas relay protection to act, sending out a heavy gas signal and The outlet tripped and the transformer was cut off.

(3) Heavy gas protection is the main protection for internal faults in the oil tank. It can reflect various faults inside the transformer. When a few inter-turn short circuits occur in the transformer, although the fault current is large, the differential current generated in the differential protection may not be large, and the differential protection may refuse to operate. Therefore, for internal faults in the transformer, it is necessary to rely on heavy gas protection to remove the fault.

2. Pressure protection

Pressure protection is also the main protection for internal faults in the transformer tank. Contains pressure relief and pressure sudden change protection to react to transformer oil pressure.

3. Temperature and oil level protection

When the temperature of the transformer rises and reaches the pre-warning value, the temperature protection sends an alarm signal and starts the backup cooler.

When the transformer leaks oil or the oil level drops due to other reasons, the oil level protection will act and send an alarm signal.

4. Cooler full stop protection

When the operating transformer cooler is completely stopped, the temperature of the transformer will rise. If not handled in time, the insulation of the transformer winding may be damaged. Therefore, when the cooler is completely stopped during the operation of the transformer, the protection will send out an alarm signal and cut off the transformer after a long delay.

4. Differential protection

Transformer differential protection is the main protection of the electrical quantities of the transformer, and its protection range is the part surrounded by the current transformers on each side. When faults such as winding phase-to-phase short circuit and inter-turn short circuit occur within this range, the differential protection must operate.

1. Excitation inrush current of transformer

The excitation current generated when the transformer is air-dropped is called excitation inrush current. The size of the excitation inrush current is related to factors such as the structure of the transformer, closing angle, capacity, residual magnetism before closing, etc. Measurements show that when the transformer is airdropped, the core saturation excitation inrush current is very large, usually 2 to 6 times the rated current, and the maximum can reach more than 8 times. Since the excitation inrush current only flows into the transformer on the charging side, a large differential current will be generated in the differential circuit, causing the differential protection to malfunction.

The excitation inrush current has the following characteristics: a. The inrush current value is very large and contains obvious non-periodic components; b. The waveform is peak-shaped and intermittent; c. It contains obvious high-order harmonic components, especially the second harmonic component. Obviously; d. The excitation inrush current is attenuated.

According to the above characteristics of the excitation inrush current, in order to prevent the misoperation of the transformer differential protection caused by the excitation inrush current, three principles are used in the project: high second harmonic content, asymmetric waveform, and large waveform discontinuity angle to realize the blocking of the differential protection.

2. Second harmonic braking principle

The essence of second harmonic braking is to use the second harmonic component in the differential current to determine whether the differential current is a fault current or an excitation inrush current. When the percentage of the second harmonic component and the fundamental component is greater than a certain value (usually 20%), it is judged that the differential current is caused by the excitation inrush current, and the differential protection is blocked.

Therefore, the larger the second harmonic braking ratio is, the more second harmonic current is allowed to be included in the fundamental wave, and the worse the braking effect is.

3. Differential quick-break protection

When a serious fault occurs inside the transformer and the fault current is large causing CT saturation, the CT secondary current also contains a large amount of harmonic components. According to the above description, this is likely to cause differential protection due to second harmonic braking. Block or delay action. This will seriously damage the transformer. In order to solve this problem, differential quick-break protection is usually set up.

The differential quick-break element is actually a high-set value differential element for longitudinal difference protection. Different from general differential components, it reflects the effective value of the differential current. Regardless of the waveform of the differential current or the size of the harmonic components, as long as the effective value of the differential current exceeds the differential quick-break setting value (usually higher than the differential protection setting value), it will immediately cut off the transformer without excitation. Blocking of inrush and other criteria.

5. Backup protection of transformer

We will briefly introduce the main protection of the transformer and continue to introduce the backup protection of the transformer. There are many types of backup protection configurations for transformers. Here we briefly introduce the two types of backup protection: complex voltage blocking overcurrent protection and grounding protection of transformers.

1. Re-voltage blocking over-current protection

The complex voltage blocking overcurrent protection is the backup protection for phase-to-phase short circuit faults of large and medium-sized transformers. It is suitable for step-up transformers, system contact transformers and step-down transformers whose overcurrent protection cannot meet the sensitivity requirements. The composite voltage composed of negative sequence voltage and low voltage can reflect various faults within the protection range, reduce the setting value of over-current protection, and improve the sensitivity.

Composite voltage overcurrent protection consists of composite voltage components, overcurrent components, and time components. The access current of the protection is the secondary three-phase current of CT on the local side of the transformer, and the access voltage is the secondary three-phase voltage of PT on this side of the transformer or other sides. For microcomputer protection, the voltage on one side can be provided to other sides through software, thus ensuring that the re-voltage overcurrent protection can still be used during PT maintenance on any side. The action logic is shown in the figure below.

2. Ground protection of transformer

The backup protection for ground short-circuit faults of large and medium-sized transformers usually includes: zero sequence overcurrent protection, zero sequence overvoltage protection, gap protection, etc. The following is a brief introduction based on the three different grounding methods of the neutral point.

picture

(1) The neutral point is directly grounded

For transformers with a voltage of 110kV and above and a neutral point directly grounded, a zero-sequence current protection that responds to ground faults should be installed on the high-current grounding system side. For transformers with both high and middle sides directly grounded, the zero-sequence current protection should be oriented, and the direction should be directed towards the busbars on each side.

The principle of zero-sequence current protection is similar to the zero-sequence protection of lines. Please refer to Issue 30. The zero sequence current can be taken from the neutral point CT secondary current, or it can be self-produced by the local CT secondary three-phase current. The zero-sequence voltage connected to the directional element can be taken from the PT open delta voltage on this side, or it can be self-produced by the secondary three-phase voltage on this side. Among microcomputer protection devices, self-produced methods are mainly used.

For large three-winding transformers, the zero-sequence current protection can be three-stage. Among them, segment I and segment II have direction, and segment III does not have direction. Each section generally has two levels of delay. A shorter delay is used to reduce the fault scope (tripping the bus coupler or strip-side switch), and a longer delay is used to cut off the transformer (tripping the three-side switch). The specific protection configuration is determined based on the actual situation.

As shown in the figure, after the zero-sequence direction current protection section I or II is activated, the bus coupler or the switch on the local side is tripped first after a short delay t1 or t3 to reduce the scope of the fault. If the fault amount is still there, then after a longer delay Delay t2 or t4 to jump the three-side switch to cut off the transformer. Section III has no direction and directly cuts off the transformer through delay.

(2) The neutral point is not grounded

The zero-sequence current passes through the neutral point of the transformer to form a zero-sequence loop. However, if the neutral points of all transformers are grounded, the short-circuit current at the ground point will be shunted to each transformer, which will cause the zero-sequence overcurrent protection sensitivity to decrease. Therefore, in order to limit the zero-sequence current within a certain range, there are regulations on the number of transformers operating with neutral points grounded.

For transformers operating without grounding, in order to prevent overvoltage damage to the transformer caused by gap arcing at the fault point during a ground fault, zero sequence voltage protection should be configured.

Due to the high insulation level of the neutral point of a fully insulated transformer, when a ground fault occurs in the system, zero-sequence current protection is first used to remove the neutral point from being grounded. If the fault still exists, zero-sequence voltage protection is used to remove the neutral point from being grounded. of transformers.

(3) The neutral point is grounded through the discharge gap

Ultra-high voltage transformers are all semi-insulated transformers, and the insulation of the neutral point coil to ground is weaker than that of other parts. Neutral point insulation is prone to breakdown. Therefore, gap protection needs to be configured.

The function of gap protection is to protect the insulation safety of the neutral point of an ungrounded transformer.

Install a breakdown gap between the transformer neutral point and ground. When the grounding isolating switch is closed, the transformer is directly grounded and zero sequence overcurrent protection is enabled. When the grounding isolating switch is disconnected, the transformer is grounded through the gap and put into gap protection.

Gap protection is implemented using the gap current 3I0 flowing through the neutral point of the transformer and the busbar PT opening triangle voltage 3U0 as criteria.

If the fault neutral point rises relative to the ground point, the gap breaks down and a large gap current 3I0 is generated. At this time, the gap protection operates and the transformer is cut off after a delay. In addition, when a ground fault occurs in the system, the neutral point is grounded to run the transformer zero sequence protection action, and the transformer with the neutral point grounded is first removed. After the system loses the grounding point, if the fault still exists, the open delta voltage 3U0 of the busbar PT will be very large, and the gap protection will also act at this time.

HZ-2162 DC Ratio Integrated Tester