How To Protect The Transformers. the right way to protect the transformers can last the usage life of it.

transformer is static equipment running continuously, running more reliable, less chance of failure. But because the vast majority of transformers are installed outdoors, and are affected by the load during operation and the influence of short circuit fault of the power system, various faults and abnormal conditions are inevitable in the process of operation.

November 26, 2021

transformer is static equipment running continuously, running more reliable, less chance of failure. But because the vast majority of transformers are installed outdoors, and are affected by the load during operation and the influence of short circuit fault of the power system, various faults and abnormal conditions are inevitable in the process of operation.

1. Common faults and anomalies of transformers

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

Internal faults refer to faults occurring inside the case, such as inter-phase short-circuit faults of windings, inter-turn short-circuit faults of one-phase windings, short-circuit faults between windings and iron core, and windings disconnection faults, etc.

External faults refer to various phase short circuit faults between external lead lines of transformers and single-phase grounding faults that occur when the lead line insulation bushing flashover through the case.

Transformer failure does great harm. Especially when the 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 decompose and produce a lot of gas, resulting in transformer shell deformation and even explosion. Therefore, when the transformer fails, it must be removed.

Transformer anomalies are mainly overload, oil level reduction, external short circuit caused by overcurrent, running transformer oil temperature is too high, winding temperature is too high, transformer pressure is too high, and cooling system failure. When the transformer is abnormal, an alarm signal should be given.

Two, transformer protection configuration

The main protection of short circuit fault: longitudinal differential protection, heavy gas protection, etc.

Short circuit fault backup protection: mainly compound voltage locking overcurrent protection, zero sequence (direction) overcurrent protection, low impedance protection, etc.

Abnormal operation protection: mainly over-load protection, over-excitation protection, light gas protection, neutral gap protection, temperature oil level and cooling system failure protection.

Three, non-electric protection

The transformer protection composed of oil, gas, temperature and other non-electric volume is called non-electric protection. There are mainly gas protection, pressure protection, temperature protection, oil level protection and cooler stop protection. Non-electric protection acts as required by the site to trip or send signals.

1. Gas protection

When the transformer internal failure, due to the role of short-circuit current and short-circuit point arc, transformer internal will produce a lot of gas, at the same time the speed of transformer oil flow, the use of gas and oil flow to achieve protection is called gas protection.

(1) Light gas protection: when a slight fault or abnormality occurs inside the transformer, the local overheating of the fault point causes partial oil expansion, and the gas in the oil forms bubbles and enters the gas relay. Light gas protection action sends light gas signal.

(2) Heavy gas protection: when a serious fault occurs in the transformer tank, the fault current is large, and the arc causes the transformer oil to decompose in large quantities, resulting in a large amount of gas and oil flow. The impact baffle makes the heavy gas follow the protective action, sends out the heavy gas signal and the exit trip, and the transformer is cut off.

(3) Heavy gas protection is the main protection of the internal failure of the fuel tank, which can reflect various internal failures of the transformer. When the transformer occurs a few turn to turn short circuit, although the fault current is very large, but in the differential protection produced in the differential current may not be large, differential protection may refuse to operate. Therefore, for transformer internal faults, it is necessary to rely on heavy gas protection to remove the faults.

The picture

2, pressure protection

Pressure protection is also the main protection against internal faults of the transformer tank. With pressure release and pressure mutation protection, used to reflect the pressure of transformer oil.

3, temperature and oil level protection

When the transformer temperature rises to the warning value, the temperature protection sends an alarm signal and starts the standby cooler.

When the transformer leaks oil or lowers the oil level for other reasons, the oil level is protected and an alarm signal is sent.

4, the cooler all stop protection

When all transformer coolers in operation stop, the temperature of the transformer will rise. If not handled in time, the insulation of the transformer winding may be damaged. Therefore, when all the coolers stop in transformer operation, the protection sends out alarm signal and excises the transformer after a long time delay.

Four, differential protection

Transformer differential protection is the main protection of transformer electricity volume, and its protection scope is the part surrounded by each side current transformer. In this range occurs between windings short circuit, inter-turn short circuit and other faults, differential protection to action.

1, transformer inrush current

The field current produced when the transformer is dropped is called field inrush current. The size of excitation inrush current is related to the transformer structure, closing Angle, capacity, remanence before closing and other factors. The measurement shows that the inrush current is usually 2~6 times of the rated current, and the maximum is more than 8 times of the rated current. Because the inrush current only flows into the transformer at the charging side, it will produce a large differential current in the differential loop, resulting in the differential protection misoperation.

The inrush current has the following characteristics: a. The value of inrush current is very large and contains obvious aperiodic component; B. The waveform is pointed and discontinuous; C, it contains obvious high-order harmonic component, especially the second harmonic component; D. The excitation inrush current is attenuated.

According to the above characteristics of inrush current, in order to prevent inrush current from causing transformer differential protection misoperation, the project uses three principles: high second harmonic content, waveform asymmetry, waveform discontinuity Angle to realize the locking of differential protection.

2. Second harmonic braking principle

The essence of the second harmonic braking is to use the second harmonic component of the differential current to judge whether the differential current is fault current or inrush current. When the percentage of the second harmonic component to the fundamental component is greater than a certain value (usually 20%), the differential current is judged to be caused by the inrush current, and the differential protection is closed.

Therefore, the larger the second harmonic braking ratio is, the more the second harmonic current contained in the allowed fundamental wave is, 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 enough to lead to CT saturation, the CT secondary current also contains a large number of harmonic components. According to the above description, it is likely that the differential protection will be locked or delayed due to the second harmonic braking. This will seriously damage the transformer. To solve this problem, differential quick-break protection is usually set up.

Differential quick-break element is actually a high constant differential element of longitudinal differential protection. Different from the general differential element, it reflects the effective value of the differential flow. No matter how the waveform of the differential flow, how the size of the harmonic component, as long as the effective value of the differential flow exceeds the setting value of the differential speed break (usually higher than the setting value of the differential protection), it will immediately remove the transformer, without the excitation inrush current and other criteria locking.

Five, transformer backup protection

The main protection of the transformer is briefly introduced, and the backup protection of the transformer continues to be introduced. There are many kinds of backup protection configuration of transformers. Here, we briefly introduce the two types of backup protection of transformer, which are double voltage locking overcurrent protection and grounding protection.

1, double pressure locking over current protection

The compound voltage locking overcurrent protection is the backup protection for the fault of short circuit between large and medium transformers. Suitable for step-up transformers, system connection transformers and step-down transformers whose overcurrent protection cannot meet sensitivity requirements. The compound voltage composed of negative sequence voltage and low voltage can reflect various faults in the protection range, reduce the setting value of overcurrent protection, and improve the sensitivity.

Compound voltage overcurrent protection is composed of compound voltage element, overcurrent element and time element. The access current of protection is the secondary THREE-PHASE CT current at the local side of the transformer, and the access voltage is the secondary three-phase PT voltage at the local side or other sides of the transformer. For microcomputer protection, the voltage of this side can be provided to other sides through software, so as to ensure that any side of PT maintenance, can still use the complex voltage overcurrent protection. The action logic is shown below.

2, transformer grounding protection

The backup protection of grounding short circuit fault of large and medium-sized transformer is usually: zero sequence overcurrent protection, zero sequence overvoltage protection, gap protection and so on. The following is a brief introduction of three different grounding modes based on the neutral point.

(1) The neutral point is directly grounded

For transformers whose voltage is 110kV and above neutral point is directly grounded, zero-sequence current protection should be set at the side of high current grounding system to reflect grounding fault. For transformers that are directly grounded on both high and middle sides, the zero-sequence current protection should be in the direction of each side bus.

The principle of zero sequence current protection is similar to that of line zero sequence protection, refer to issue 30. The zero-sequence current can be derived from the secondary CT current of the neutral point, or from the secondary three-phase CT current of the local side. The zero sequence voltage connected to the directional element can be obtained from the PT opening triangle voltage of the local side or from the secondary three-phase voltage of the local side. In the microcomputer protection device, mainly adopt self - produced way.

For large three-winding transformers, the zero-sequence current protection can be three-stage. There is a direction in section I and II, and no direction in section III. Each section generally has a two-stage delay, with a shorter delay to reduce the fault range (jump bus or local switch), with a longer delay to remove the transformer (jump three-side switch). The specific protection configuration depends on the actual situation.

As shown in the figure, after the operation of the zero-sequence direction current protection in section I or II, t1 or T3 bus or local switch with a short delay should be jumped first to reduce the scope of influence of the fault. If the fault quantity is still there, the transformer should be cut off with t2 or T4 three-way switch with a long delay. Section III without direction, the transformer is directly removed by delay.

(2) the neutral point is not grounded

The zero sequence current passes through the neutral point of the transformer to form the zero sequence loop. However, if all the neutral points of the transformer are grounded, the short-circuit current at the grounding point will be distributed to each transformer, which will cause the sensitivity of zero-sequence overcurrent protection to be reduced. Therefore, in order to limit the zero-sequence current in a certain range, the number of transformers grounded to the neutral point is regulated.

For ungrounded transformer, zero-sequence voltage protection should be configured to prevent overvoltage damage to transformer caused by gap arc at fault point when grounding fault occurs.

Fully insulated transformer due to its high insulation level of neutral point, when the system occurs grounding fault, the first zero-sequence current protection to remove the neutral point grounding transformer, if the fault still exists, and then zero-sequence voltage protection to remove the neutral point ungrounded transformer.

(3) The neutral point is grounded through discharge gap

All hV transformers are semi-insulated, and the insulation of neutral coil to ground is weaker than other parts. Neutral insulation is prone to breakdown. Therefore, you need to configure clearance protection.

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

As shown, abreakdown gap is installed between the neutral point of the transformer and the ground. When the earth isolating switch is closed, the transformer is directly grounded and zero sequence overcurrent protection is put in. When the grounding isolation switch is disconnected, the transformer is grounded through the gap and put into the gap protection.

The gap protection is realized by using the gap current 3I0 flowing through the neutral point of the transformer and the opening triangle voltage 3U0 of the bus PT as the criterion.

If the neutral point to the location of the fault is elevated, the gap breakdown, resulting in a large gap current 3I0, at this time, the gap protection action, after a delay to remove the transformer. In addition, when the system has a grounding fault, the neutral point grounding operation transformer zero sequence protection action, first cut the neutral point grounding transformer. After the system loses the ground point, if the fault still exists, the open triangle voltage of bus PT 3U0 will be large, and the gap protection will also operate at this time.

Source: Power pulpit


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