Summary of basic knowledge on power grid transformer maintenance

The role of transformers in power is to transform voltage to facilitate power transmission. After the voltage is stepped up by a step-up transformer , it can reduce line losses, improve the economy of power transmission, and achieve the purpose of long-distance power transmission. The step-down transformer can convert high voltage into the voltage of each level required by the user to meet the needs of the user.

(001): What are the normal inspection items for transformers ?

Answer: (1) Is the sound of the transformer operating normally?

(2) Whether the oil color and oil level in the oil pillow and oil-filled casing are normal, and whether there is any oil leakage;

(3) Check whether the bushings on each side are damaged, whether there are discharge marks or other abnormal phenomena;

(4) Whether the cooling device operates normally;

(5) Whether the upper oil temperature gauge indicates correctly and whether there are any abnormal conditions;

(6) Whether the diaphragm of the explosion-proof pipe is intact and whether there is any liquid accumulation;

(7) The degree of color change of the respirator's color-changing silicone;

(8) Is the gas relay full of oil?

(9) Check whether the main body and accessories have seepage or oil leakage;

(10) Check whether the pile heads and connecting wires on each side are hot or discolored;

(11) Whether the surrounding environment and stacked materials near the transformer may pose a threat to the safe operation of the transformer.

(002): What are the special inspection items for transformers?

Answer: (1) In strong winds, check whether there are any debris that can be blown up near the transformer to prevent it from falling onto the live parts, and pay attention to the swing of the lead wires;

(2) Check whether the bushing has flashover or discharge in foggy weather;

(3) On snowy days, check whether there is snow or ice between the transformer top cover and the bushing connection, and whether the oil level gauge, thermometer, and gas relay are covered with snow;

(4) After a thunderstorm, check the operation of the lightning arrester counters on each side of the transformer and check whether the bushing has any damage, cracks or discharge marks.

(5) When the temperature changes suddenly, check the changes in oil level and oil temperature.

(003): Based on the transformer oil temperature, how to judge whether the transformer is normal?

Answer: When the transformer is running under rated conditions, the heat loss of the core and windings causes the temperature of each part to rise. When the heat generation and heat dissipation reach a balance, the temperature of each part tends to be stable. During the inspection, attention should be paid to the ambient temperature, upper oil temperature, load size and oil level, and compared with previous values ​​for comparison and analysis. If under the same conditions, the upper oil temperature is 10°C higher than usual, or the load remains unchanged, but the oil temperature continues to rise, and the cooling device operates normally, and the temperature gauge is not malfunctioning, it can be considered that there is an abnormality and failure inside the transformer.

(004): What are the factors that affect the oil level and oil temperature of the transformer?

Answer: The factors that affect the oil level and oil temperature of the transformer are mainly: ① It rises with the increase of load current; ② As the ambient temperature increases and the heat dissipation conditions are poor, the oil level and oil temperature rise; ③ When the power supply voltage increases, the core magnetic flux is saturated, and the core overheats, the oil temperature will also be higher; ④ When the cooling device operates poorly or abnormally, the oil level and oil temperature will also rise; ⑤ Internal faults of the transformer (such as partial short circuit of the coil, partial looseness of the core, overheating, short circuit, etc.) will cause the oil temperature to rise.

(005): What are the reasons for false oil level in transformers?

Answer: Possible reasons for false oil level in transformer are: ① Oil gauge tube is clogged; ② Breathing device is clogged; ③ Explosion-proof pipe vent is clogged; ④ Air is not exhausted when refueling with film-protected oil pillow.

(006): What do the three scale lines +40℃, +20℃ and -30℃ on the transformer oil level mark mean?

Answer: The +40℃ on the oil level mark indicates the maximum limit of the oil level when the transformer is running at full load when the maximum ambient temperature at the installation location is +40℃. The oil level must not exceed this line. +20℃ indicates the oil level when the transformer is running at full load when the annual average temperature is +20℃. -30℃ indicates the minimum oil level of the no-load transformer when the environment is -30℃, which must not be lower than this line. If the oil level is too low, add oil.

(007): What is the main function of transformer oil in a transformer?

Answer: The oil in the transformer mainly plays a role in heat dissipation and cooling during operation; it plays a role in insulation and insulation maintenance for windings, etc. (maintaining good insulation status); the oil plays an arc extinguishing role at the high-voltage leads and the tap changer contact points, preventing the generation of corona and arc discharge.

(008): What factors are related to the deterioration of transformer oil quality?

Answer: The main factors that affect the deterioration of transformer oil are: high temperature, oxygen and moisture in the air. ① High temperature accelerates the deterioration of oil. When the oil temperature is above 70℃, the oxidation rate of the oil increases by 1.5-2 times for every 10℃ increase;

② When transformer oil is exposed to oxygen in the air for a long time and heated, it will produce acid, resin, and sediment, causing serious deterioration of the insulation material;

③ When water and moisture enter the oil, its electrical insulation performance is significantly reduced, making it prone to breakdown.

(009): How to easily judge the quality of oil using empirical methods?

Answer: The quality of oil can be easily judged by the empirical method intuitively, mainly based on: ① the color of the oil, new oil, good oil is light yellow, and poor quality oil is dark brown; ② the transparency of the oil, high-quality oil is transparent, poor quality oil is turbid, contains mechanical impurities, free carbon, etc.; ③ the difference in the smell of the oil: new oil, good quality oil is odorless or has a slight kerosene smell, poor quality oil has burnt smell (overheating), sour smell, acetylene smell (arcing) and other odors.

(010): What harm will the transformer operation suffer if the oil level is too low?

Answer: If the oil level of the transformer is too low, the light gas protection will be activated. When there is a serious lack of oil, the iron core coil inside the transformer is exposed to the air, which makes it easy for the insulation to get damp (and affect the heat dissipation under load), resulting in lead discharge and insulation breakdown accidents. (011): What are the hazards of running the transformer at extreme temperatures for a long time?

Answer: Most oil-immersed transformers have Class A insulation, and their maximum heat-resistant temperature is allowed to be 105°C. During transformer operation, the winding temperature must be 10-15°C higher than the average temperature of the upper oil. That is, when the upper oil temperature reaches 85-95°C during operation, the winding has actually reached about 105°C. If it operates at this extreme temperature for a long time, the winding insulation will seriously age and accelerate the deterioration of the insulating oil, affecting its service life.

(012): What are the main advantages and disadvantages of autotransformer operation?

Answer: The main advantages of autotransformer operation are: ① less power loss and high efficiency;

②It can be made into a single large-capacity transformer;

③ Under the same capacity, it has small size, light weight, convenient transportation, saves materials and has low cost.

Main disadvantages: ①The impedance percentage is small, so the system short-circuit current is large;

② Low-voltage windings are more susceptible to overvoltage, so the neutral point must be directly grounded;

③The voltage regulation problem is difficult to handle.

(013): What are the conditions for transformer parallel operation? Why?

Answer: Conditions for parallel operation of transformers:

① The transformers participating in parallel operation must have the same wiring group. Otherwise, the voltage difference on the secondary side will be large, and the circulating current generated will be large or even like a short-circuit current, which will damage the transformer;

② The primary voltage of each transformer should be equal, and the secondary voltage should also be equal. Otherwise, the secondary side will generate circulating current, causing overload, heating, affecting load carrying, increasing power loss, and reducing efficiency;

③ The impedance voltage (short-circuit voltage) percentage of each transformer should be equal, otherwise the load distribution will be unreasonable after loading. Because the short-circuit voltage percentage of a large-capacity transformer is large, and the short-circuit voltage percentage of a small-capacity transformer is small, and the load distribution is inversely proportional to the short-circuit voltage percentage, this will cause the large transformer to have a small load and the equipment is not fully utilized; while the small transformer has a large load and is prone to overload, which limits the load operation of the parallel transformer.

(014): Under what circumstances does a single transformer operate with the highest efficiency? What is the economic operation mode of the transformer?

Answer: The highest operating efficiency of a single transformer is achieved when the variable loss (coil copper loss) is equal to the constant loss (core loss). The general load factor β=0.6 is about 60% of the rated load, which is the highest efficiency point.

When several transformers are operated in parallel, since the iron loss of each transformer remains basically unchanged, while the copper loss changes with the load, the number and capacity of the operating transformers need to be adjusted according to the load size to minimize the total power loss of the transformer. This operating mode is called the transformer economic operation mode.

(015): What should the operator pay attention to when performing on-load voltage regulation?

Answer: When the on-duty personnel are performing on-load voltage regulation, they should pay attention to whether the indication of the voltmeter is within the voltage regulation range, whether the position indicator and counter correspond correctly, and check whether the gas relay and oil level and oil color are normal, and keep records. When the load is greater than 80% of the rated value, it is forbidden to operate the on-load voltage regulation switch.

(016): What items should be inspected after the transformer is overhauled?

Answer: (1) Whether the inspection items are complete;

(2) Whether the maintenance quality meets the requirements;

(3) Whether all existing defects have been eliminated;

(4) Whether the electrical test and oil test items are complete and the results are qualified;

(5) Whether the maintenance, test and technical improvement information is complete and filled in correctly;

(6) Whether the on-load voltage regulator is normal and the indication is correct;

(7) Check whether the cooling fan and circulating oil pump are running normally;

(8) The gas protection transmission test operates correctly;

(9) Whether the voltage tap is at the gear required by the dispatcher and the three phases are consistent;

(10) Whether the transformer surface, bushing and maintenance site are clean.

(017) What preparations should be made before the transformer is put back into service after maintenance?

Answer: (1) Collect and terminate the relevant work tickets, remove or disconnect the relevant grounding wires and grounding switches; remove barriers and signs, and keep records of equipment repair and testing.

(2) Check in detail the primary equipment, secondary circuit and protective pressure plate to ensure they meet the operating requirements.

(3) Before putting the forced oil circulation transformer into operation, start all coolers and run them for a period of time to allow residual air to escape.

(018): What should be paid attention to when replacing the silicone inside the respirator of a transformer in operation?

Answer: (1) The heavy gas protection should be changed to signal.

(2) When removing the respirator, the connecting tube should be blocked to prevent air from being recycled.

(3) After replacing with dry silicone, the oil in the oil seal should cover the exhalation nozzle and seal the respirator.

(019): Under what circumstances is it not allowed to adjust the transformer's on-load tap changer?

Answer: (1) When the transformer is overloaded (except in special circumstances);

(2) When the light gas alarm of the on-load voltage regulating device is activated;

(3) When the oil pressure resistance of the on-load pressure regulating device is unqualified or there is no oil in the oil mark;

(4) When the number of voltage adjustments exceeds the prescribed number;

(5) When an abnormality occurs in the pressure regulating device.

(020): What are the preventive test items for transformers?

Answer: (1) Measure the insulation resistance and absorption ratio of the transformer winding.

(2) Measure the DC resistance of the winding.

(3) Measure the leakage current of the winding together with the bushing.

(4) Measure the dielectric loss of the winding and bushing.

(5) Electrical strength test and performance test of insulating oil. (021): What are the factors that produce unbalanced current in the transformer differential protection circuit?

Answer: (1) The influence of transformer excitation surge current.

(2) The impact of the difference between the actual transformation ratio of the current transformer and the calculated transformation ratio.

(3) The impact caused by the different types of current transformers on the high and low voltage sides.

(4) The influence of transformer on-load voltage regulation.

(022): What are the regulations for the upper oil temperature and temperature rise of a transformer in operation?

Answer: For forced oil circulation air-cooled transformers, the upper oil temperature is 75℃ and the temperature rise is 35℃. For oil-immersed natural circulation, self-cooling and air-cooled transformers, the upper oil temperature should generally not exceed 85℃, and the maximum should not exceed 95℃ and the temperature rise should not exceed 55℃. If it is found that one of the limit values ​​exceeds the regulations during operation, it should be reported to the dispatcher immediately and load limiting measures should be taken.

(023) What are the regulations for the operation of a forced oil circulation air-cooled transformer when the cooling device is completely stopped?

Answer: When a forced oil circulation air-cooled transformer is in operation and all cooling devices stop working, it is allowed to run at rated load for 20 minutes. After 20 minutes, if the upper oil temperature has not reached 75°C, it is allowed to continue running until the upper oil temperature rises to 75°C. However, the maximum operating time after cutting off all cooling devices shall not exceed 1 hour under any circumstances.

(024): What work should be done during transformer operation to change the heavy gas protection from tripping to signaling?

Answer: (1) Filter or refuel the oil with electricity;

(2) The gas relay is inspected and tested and its protection circuit is working or DC grounding occurs;

(3) Treatment of defects in the oil circuit system for strong oil circulation and replacement of the submersible oil pump;

(4) Open the relevant air and oil drain plugs to find out the cause of the abnormal increase in oil level.

(025): How is the number of on-load voltage regulation for transformers regulated?

Answer: The voltage regulation operation of the on-load voltage regulating device is performed by the operating personnel according to the voltage curve determined by the competent dispatching department. The number of adjustments per day for 35kV main transformers generally does not exceed 20 times, and for 110￣220kV main transformers generally does not exceed 10 times (each adjustment of a branch is counted as one time). The reverse regulation method is used to control the power supply voltage at the optimal level as much as possible.

(026): What is the short-circuit voltage percentage of a transformer? How does it affect the transformer voltage change rate?

Answer: The short-circuit voltage percentage of a transformer is the voltage when one side of the transformer is short-circuited and the other side is connected to the rated current. This voltage accounts for the percentage of its rated voltage. In fact, this voltage is the voltage drop of the leakage reactance of the energized side and the short-circuit side of the transformer under the rated current. For transformers of the same capacity, the larger the reactance, the larger the short-circuit voltage percentage. For the same current, the transformer with large reactance will produce greater voltage loss. Therefore, the transformer with a large short-circuit voltage percentage has a greater reactance change rate.

(027): Why does cutting off an unloaded transformer generate overvoltage? What measures are generally taken to protect the transformer?

Answer: The transformer is a large inductive component. When it is running, electrical energy is stored in the windings. When the no-load transformer is cut off, the electrical energy in the transformer will generate an overvoltage on the circuit breaker. In a power grid with direct grounding at the neutral point, when the 110~330 kV no-load transformer is disconnected, the overvoltage multiple generally does not exceed 3.0Uxg. In a 35 kV power grid with indirect neutral point grounding, it generally does not exceed 4.0Uxg. At this time, a valve type lightning arrester should be installed between the high-voltage side of the transformer and the circuit breaker. Since the magnetic energy of the no-load transformer winding is much smaller than the energy allowed to pass by the valve type lightning arrester, this protection is reliable and should not be withdrawn in the non-thunder season.

(028): What are the main reasons for the increase in transformer no-load loss and no-load current?

A: There are mainly the following reasons:

1. Poor insulation between silicon steel sheets;

2. A short circuit occurs between some silicon steel sheets in the magnetic circuit;

3. The insulation of the through-core bolts or pressure plates, upper yokes and other parts is damaged, resulting in a short circuit;

4. The silicon steel sheet in the magnetic circuit is loose and an air gap appears, increasing the magnetic resistance;

5. The coil has a short circuit between turns or parallel branches;

6. The number of turns in each parallel branch is different;

7. The winding ampere-turns are incorrect.

(029): What is the scope of protection of gas protection?

Answer: 1) Multi-phase short circuit inside the transformer.

2) Turn-to-turn short circuit, winding short circuit with core or casing.

3) Core failure.

4) The oil level is below or leaking.

5) The tap changer has poor contact or the wire welding is not firm.

(030): What are the conditions for the main transformer differential protection to operate?

Answer: 1) The main transformer and bushing lead-out line are faulty.

2) Protect against secondary line faults.

3) The current transformer is open or short-circuited.

4) Internal fault of the main transformer. (031): What are the differences between the functions of the main transformer differential and gas protection?

Answer: 1) The main transformer differential protection is designed and manufactured based on the circulating current principle, while the gas protection is designed and manufactured based on the characteristic that gas will be generated or decomposed when there is an internal fault in the transformer.

2) Differential protection is the main protection for the transformer, and gas protection is the main protection when there is an internal fault in the transformer.

3) Different scope of protection:

A Differential protection:

1) Multi-phase short circuit occurs in the main transformer lead wire and transformer coil.

2) Single-phase serious short turn

3) Protect against ground faults on coils and lead wires in high current grounding systems.

B Gas protection:

1) Multi-phase short circuit inside the transformer

2) Turn-to-turn short circuit, turn-to-core or external short circuit

3) Core failure (heat and burn)

4) The oil level is below or leaking.

5) The tap changer has poor contact or the wire welding is poor.

(032): How to deal with the main transformer cooler failure?

Answer: 1) When the working power supply of cooler sections I and II is lost, the "#1, #2 power failure" signal is issued, and the main transformer cooler full stop tripping circuit is connected. The dispatcher should be reported immediately and the protection set should be disabled.

2) When the power supply switching between sections I and II fails during operation, the "Cooler Full Stop" indicator lights up. At this time, the main transformer cooler full stop trip circuit is connected. The dispatcher should be informed immediately to disable the protection set and perform manual switching quickly. If KM1 or KM2 fails, strong excitation cannot be performed.

3) When any one of the cooler circuits fails, the failed cooler circuit will be isolated.

(033): What is the working principle of the open cup baffle type gas relay?

A: Under normal circumstances, the open cup of the gas relay is filled with oil. Since the torque generated by the weight of the oil itself is smaller than the torque generated by the weight of the gas, the open cup makes the contacts in the disconnected position. When a minor fault occurs in the main transformer, gas will flow into the gas relay, forcing the oil level to drop, causing the open cup to drop with the oil level, making the contacts connected and sending out a "heavy gas action" signal.

(034): What are the consequences of parallel operation of transformers that do not meet the parallel operation conditions?

Answer: When transformers with different transformation ratios are operated in parallel, circulating current will be generated, affecting the output of the transformer. If the percentage impedance is inconsistent and the transformers are operated in parallel, the load cannot be distributed according to the capacity ratio of the transformer, which will also affect the output of the transformer. When transformers with different connection groups are operated in parallel, the transformer will short-circuit.

(035): What are the conditions that must be met for two transformers to operate in parallel?

Answer: The following conditions must be met for two transformers to operate in parallel: a) the winding connection groups are the same; b) the voltage ratio is equal; c) the impedance voltage is equal; d) the capacity ratio does not exceed 3:1.

(036): What are the four main data of a transformer besides the rated parameters?

Short circuit loss, no-load loss, impedance voltage, no-load current.

(037): Why must the neutral point of an autotransformer be grounded?

The neutral point of the autotransformer must be grounded during operation, because when a single-phase grounding fault occurs in the system, if the neutral point of the autotransformer is not grounded, the neutral point will be displaced, the voltage of the non-grounded phase will increase, and even reach or exceed the line voltage, and the medium voltage side coil will be over-voltage. In order to avoid the above phenomenon, the neutral point must be grounded. The neutral point potential after grounding is the ground potential, and the medium voltage side will not be over-voltage after a single-phase grounding fault occurs.

(038): For the gas protection of a substation in operation, when the following work is being carried out, the heavy gas should be changed from tripping to signal, and immediately changed to tripping after the work is completed?

Answer: 1. Oil filling and oil filtering of transformer

2. When the transformer's breather is being cleared

3. When the upper air vent valve of the transformer gas relay is vented

4. Switch the valve on the gas relay connecting pipe

5. When working on the secondary circuit of the gas relay

(039): After the light gas protection device is activated, the following items should be checked:

Answer: 1. Transformer oil level

2. Check whether the safety release valve is actuated, ruptured or sprayed with oil.

3. Is there any abnormal sound inside?

4. Report the dispatch in time and wait for processing orders

(040): When a transformer sends out an overload signal during operation, how should it be checked and handled?

Answer: When a transformer in operation sends out an overload signal, the on-duty personnel should check whether the current on each side of the transformer exceeds the specified value, and report the transformer overload quantity to the on-duty dispatcher, then check whether the oil level and oil temperature of the transformer are normal, and put all coolers into operation at the same time. The overload quantity and time shall be implemented according to the provisions of the on-site regulations, and patrol inspections shall be carried out at the specified time, and special patrols shall be added when necessary. (041): What is the function of the transformer oil pillow?

Answer: Transformer oil has the physical phenomenon of thermal expansion and contraction. Installing an oil pillow will prevent the oil from overflowing from the transformer due to thermal expansion, and will prevent the oil from being insufficient due to thermal contraction. At the same time, the contact area between the insulating oil and the air is greatly reduced with the oil pillow, making the transformer less susceptible to moisture intrusion and avoiding oil deterioration.

(042):What kind of faults can gas protection protect?

Answer: (1) Multi-phase short circuit inside the transformer.

(2) Turn-to-turn short circuit, winding short circuit with iron core or casing.

(3) Core failure.

(4) Oil level drops or oil leaks.

(5) The tap changer has poor contact or the wire welding is not firm.

(043): Abnormal operating status of the transformer?

1. Severe oil leakage

2. The oil level in the oil pillow cannot be seen or is too low

3. Abnormal oil level rise

4. Transformer oil carbonization

5. There is an abnormal sound inside the transformer

6. The porcelain parts have abnormal discharge sound or spark phenomenon

7. The transformer bushing is cracked or severely damaged

8. The lead wire clamps of the transformer high and low voltage bushings are overheated

9. Cooler device failure

10. Gas in the gas relay accumulates continuously and it continuously operates and sends signals.

11. Abnormal increase in oil temperature under normal load and cooling conditions

(044): What safety precautions should be taken when taking gas from a running transformer?

Answer: (1) Gas extraction must be done by two persons, one to operate and one to supervise;

(2) When climbing up the transformer to take out air, you should maintain a safe distance and not cross the dedicated fence.

(045): What might be the cause of abnormal sound from the transformer?

Answer: (1) Caused by overload;

(2) Discharge and sparking due to poor internal contact;

(3) Some parts are loose;

(4) The system is grounded or short-circuited;

(5) High power starting and large load changes;

(6) Ferromagnetic resonance.

(046): If one side of a three-coil transformer is shut down, can the other two phases continue to operate? What should be paid attention to?

Answer: No matter which of the three sides of the three-coil transformer, high, medium, or low voltage, stops running, the other two sides can continue to run. If the low voltage side is a delta connection, a lightning arrester should be put into operation when it stops running, and the operation mode and setting value of the relay protection should be considered according to the operation mode. Attention should also be paid to the capacity ratio, monitor the load situation, and the differential protection current transformer on the power outage side should be short-circuited.

(047): When the heavy gas protection of the transformer trips, how should it be checked and handled?

Answer: (1) Collect the gas in the gas relay for color spectrum analysis. If there is no gas, check whether the secondary circuit and the terminal and lead wires of the gas relay are in good condition;

(2) Check whether the oil level, oil temperature and oil color have changed;

(3) Check whether the explosion-proof pipe is broken and spraying oil;

(4) Check whether the transformer casing is deformed and whether the welds are cracked or sprayed with oil;

(5) If no abnormality is found after inspection, and the malfunction is indeed caused by a secondary circuit fault, the heavy gas protection can be withdrawn with the differential protection and overcurrent protection turned on, and the transformer can be tested and monitored more closely;

(6) The circuit breaker shall not be closed and power shall not be supplied until the cause of the gas protection operation is found out.

(048): What should be done when the oil level is too high or oil overflows from the oil pillow during transformer operation?

First, check whether the load and temperature of the transformer are normal. If the load and temperature are normal, it can be determined that the false oil level is caused by the blockage of the respirator or oil mark tube. At this time, after the consent of the on-duty dispatcher, the heavy gas protection should be changed to a signal, and then the respirator or oil mark tube should be unblocked. If the ambient temperature is too high and causes the oil pillow to overflow, the oil should be drained.

(049): What abnormal phenomena occur when a transformer has a short circuit between winding layers or turns? What protective actions are caused?

Answer: 1. The current increases;

2. The oil level rises and a "gurgling" sound is heard inside the transformer;

3. The side voltage is unstable, with high and low voltage;

4. Valve oil spraying will cause gas protection or differential protection to operate.

(050): When should the main transformer heavy gas protection be changed from "jumping" to "jumping"?

A: After the transformer oil is changed, the gas protection is calibrated, the cooler is overhauled, and the transformer oil is drained and filtered, the gas protection should be changed from tripping to signaling. (051): After the main transformer is filtered and the oil is changed, how long does it usually take for the heavy gas protection to be signaled?

Answer: Generally, the deposit period is 24 hours, and the minimum deposit period is 12 hours.

(052): What is the sign that the main transformer heavy gas protection is released?

Answer: The sign is oil leaking from the vent hole.

(053): What are the conditions for commissioning auxiliary coolers and standby coolers?

A: The auxiliary cooler is automatically put into operation when the transformer load reaches a set value or the oil temperature exceeds 55 degrees. The standby cooler is automatically put into operation when the operating cooler fails.

(054): Under what circumstances should transformers be monitored and inspected more frequently?

Answer: If the transformer has any of the following conditions, it should be monitored and inspected more closely:

1. There are abnormal sounds;

2. Flashing or flashing marks on the casing, discharge sounds, etc.

3. The lead-out wire pile head is hot;

4. Serious oil leakage, the oil level gradually drops or the oil changes color;

5. Light gas signal

(055): Under what circumstances is the transformer zero-sequence protection put into operation?

Answer: The transformer zero-sequence protection is installed on the directly grounded side of the transformer neutral point. It is used to protect the ground short circuit inside the winding and on the lead-out wire. It can also serve as a backup protection to prevent the corresponding busbar and line ground short circuit. Therefore, when the transformer neutral point is grounded, zero-sequence protection should be put into use.

(056): What is the role of transformer zero-sequence current protection?

Answer: Transformers operating in a neutral point directly grounded power grid are equipped with zero-sequence current protection. When the high-side or high-voltage side line of the transformer is grounded, zero-sequence current is generated and the zero-sequence current protection is activated. Here, the low-voltage side winding of the transformer is considered to be a triangle connection. It can be used as a protection for the transformer high-voltage winding lead-out line and busbar grounding short circuit, and can also be used as a backup protection for the adjacent lines and the main protection of the transformer itself.

(057): What are the items for regular transformer testing?

Answer: (1) Insulation resistance and absorption ratio; (2) Dielectric loss angle; (3) Leakage current; (4) DC resistance of the tap changer; (5) Electrical properties of the transformer (including insulation resistance, loss angle and breakdown voltage); (6) Oil chromatography analysis.

(058): How to judge whether there is a fault and the location of the fault based on the color, smell and flammability of the gas in the gas relay?

Answer: 1. Air is colorless and non-flammable;

2. Yellow. The flammable gas is generated by the essential fault;

3. The light grey, flammable and smelly gas is generated by paper failure;

4. Gray-black and flammable is the gas produced by the decomposition of insulating oil due to iron failure;

(059): What should be done after the forced oil circulation air-cooled transformer cooler is completely shut down?

Answer: When the "cooler stops completely" signal is sent during the operation of the transformer, the on-duty personnel should quickly check whether the AC power fuse and automatic switch of the transformer fuse are normal, and eliminate the fault as soon as possible, and put the cooler into operation. If the fault is still not eliminated after the specified time, the transformer should be shut down.

(060): How to deal with the three-phase voltage imbalance during transformer operation?

Answer: If the three-phase voltage is unbalanced, the three-phase load should be checked first. For a three-phase transformer with △/Y connection, if the three-phase voltage is unbalanced and the voltage exceeds 5V, it may be that the transformer has a turn-to-turn short circuit, and the power must be turned off for processing. For a transformer with Y/Y connection, a 10% difference in the three-phase voltage to ground is allowed under light load; under heavy load, the three-phase voltage should be balanced. (061): How to operate the transformer neutral point grounding switch?

Answer: The switching principle is to ensure that the power grid does not lose its grounding point, and the operation method of closing first and then pulling is adopted:

(1) Close the isolating switch of the backup grounding point.

(2) Pull open the isolating switch at the working grounding point.

(3) Switch the zero-sequence protection to the transformer with grounded neutral point

(062): Under what circumstances is it necessary to disable the transformer differential protection in operation?

Answer: The differential protection should be disabled when one of the following situations occurs during transformer operation:

(1) When the differential protection secondary circuit and current transformer circuit are changed or checked.

(2) Relay protection personnel measure the current phase and differential pressure of the differential circuit.

(3) One phase of the differential protection transformer is disconnected or the circuit is open.

(4) Obvious abnormalities occur in the differential circuit.

(5) False tripping.

(063): What kind of fault will cause the gas protection to operate?

Answer: The types of faults that gas protection can protect are:

(1) Multi-phase short circuit inside the transformer.

(2) Turn-to-turn short circuit, winding short circuit with iron core or casing.

(3) Core failure.

(4) Oil level drops or oil leaks.

(5) The tap changer has poor contact or the wire welding is not firm.

(064): Why are overcurrent protection devices installed on all three sides of a three-winding transformer? What is their protection range?

A: When a short circuit occurs on the busbar on any side of the transformer, the overcurrent protection will be activated. Because all three sides are equipped with overcurrent protection, the fault can be selectively removed without shutting down the transformer. The overcurrent protection on each side can be used as backup protection for the busbar and line on this side, and the overcurrent protection on the main power supply side can be used as backup protection for the other two sides and the transformer.

(065) What are the reasons for transformer oil shortage?

Answer: The reasons for transformer oil shortage are:

(1) The transformer has been leaking oil for a long time or in large quantities.

(2) When repairing the transformer, the oil was not replenished in time after draining.

(3) The capacity of the oil pillow is small and cannot meet the operating requirements.

(4) The temperature is too low and the oil storage capacity of the oil pillow is insufficient.

(066): What should be paid attention to when replacing the desiccant in the transformer breather?

A: When replacing the desiccant in the breather, you should pay attention to:

(1) The heavy gas protection should be changed to signal.

(2) When removing the respirator, the connecting tube should be blocked to prevent back-inhalation of air.

(3) After replacing the dry desiccant, the oil in the oil seal should cover the exhalation nozzle to seal the respirator.

(067): What is the cause of the failure of the transformer's on-load voltage regulator?

Answer: The main reasons for the failure of the on-load voltage regulating device are:

(1) The operating power supply voltage disappears or is too low.

(2) The motor winding is broken and burned, causing the starting motor to lose pressure.

(3) The interlocking contacts are in poor contact.

(4) The rotating mechanism is disengaged and the pin falls off.

(068): What are the causes of failure of the on-load tap-changer of the tap-changer?

A: It is caused by the following reasons:

(1) The transition resistor in the auxiliary contact is broken down and burned out during the switching process.

(2) The tap changer is not sealed tightly, and water enters, causing a phase-to-phase short circuit.

(3) Due to the jamming of the contact roller, the tap changer stops in the transition position, causing a short circuit between turns and burning.

(4) The tap changer tank is short of oil.

(5) A through fault current is encountered during the voltage regulation process.

(069): Why do we need to measure the slope of the transformer cover and oil pillow connecting pipe after the transformer is newly installed or overhauled? What is the standard?

Answer: There are two slopes on the gas relay side of the transformer. One is the slope of the transformer cover along the direction of the gas relay, which should be 1% to 1.5%. The slope of the transformer cover requires padding from the bottom when installing the transformer. The other is the slope from the transformer oil tank to the oil pillow connecting pipe, which should be 2% to 4% (this slope is manufactured by the manufacturer). These two slopes are to prevent air from being stored in the transformer and to facilitate the rapid and reliable flow of gas into the gas relay in the event of a fault, ensuring the correct operation of the gas relay.

(070): What is the unbalanced current of a transformer? What are the requirements?

Answer: The unbalanced current of a transformer refers to the current difference between the windings of the three-phase transformer.

In a three-phase three-wire transformer, the unbalanced degree of load on each phase shall not exceed 20%. In a three-phase four-wire transformer, the neutral line current caused by the unbalanced current shall not exceed 25% of the rated current of the low-voltage winding. If the above regulations are not met, the load should be adjusted. (071): What is the function of the thermosyphon filter installed on one side of the transformer tank?

Answer: Transformer oil will gradually become dirty and oxidized during operation. In order to extend the service life of the oil and make the transformer operate under better conditions, it is necessary to maintain good oil quality.

Thermosyphon filters can keep transformer oil in good quality during operation without severe aging. In this way, the oil can be regenerated for many years without special treatment.

(072): What are the five main components of a resistor current limiting on-load voltage tap changer? What are their uses?

Answer: The composition and function of the resistor current limiting on-load voltage regulating tap changer are as follows:

(1) Switch: used to switch load current.

(2) Selector switch: used to pre-select the tap before switching.

(3) Range switch: used for reversing or coarse adjustment of tap changer.

(4) Operating mechanism: It is the power part of the tap changer and has functions such as interlocking, limiting, and counting.

(5) Rapid mechanism: Rapid switching according to a predetermined program.

(073): Why is the temperature rise of Class A insulation transformer windings specified at 65°C?

Answer: The transformer will produce iron loss and copper loss during operation. These two losses are all converted into heat, causing the iron core and winding to heat up and the insulation to age, affecting the service life of the transformer. Therefore, the national standard stipulates that the insulation of the transformer winding should mostly use Class A insulation, and the temperature rise of the winding is stipulated to be 65°C.

(074): Why does the secondary circuit of a 110kV voltage transformer pass through the auxiliary contacts of the primary-side disconnector?

A: The auxiliary contacts of the 110kV voltage transformer disconnector should correspond to the position of the disconnector, that is, when the voltage transformer is disabled (when the primary disconnector is opened), the secondary circuit should also be disconnected. This can prevent the voltage transformer on the double bus from charging the secondary of the voltage transformer that is powered off, causing the high-voltage side of the voltage transformer that is powered off to be powered on.

(075): Why is it not allowed that the secondary side of a current transformer is open circuited during operation?

Answer: The secondary side of the current transformer is in a short-circuit state during normal operation. If the secondary side is open, the following hazards will occur: ① The induced potential can generate high voltages of several thousand volts or more, endangering the safety of workers on the secondary circuit and damaging secondary equipment; ② Due to the high magnetic saturation and heat generation of the iron core, the insulation of the secondary winding of the current transformer may be damaged.

(076): Why is it not allowed to short-circuit the secondary side of a voltage transformer during operation?

Answer: During normal operation of the voltage transformer, the secondary side is close to an open circuit state. Generally, the secondary side voltage can reach 100 volts. If a short circuit occurs, a short circuit current will be generated, causing the fuse to blow, affecting the meter indication and causing false operation of the relay protection. If the fuse is not selected properly, the secondary winding of the voltage transformer may be damaged.

(077): Why are fuses not installed on the primary side of voltage transformers of 110 kV and above?

Answer: Because the structure of voltage transformers of 110kV and above adopts single-phase cascade type with high insulation strength, and because the 110kV system is a neutral point direct grounding system, each phase of the voltage transformer cannot withstand line voltage operation for a long time, so fuses are not installed on the primary side.

(078): What is the impact of voltage transformer failure on relay protection?

Answer: Common faults in the secondary circuit of the voltage transformer include: blown fuses, poor contact of the auxiliary contacts of the disconnector, loose secondary wiring, etc. The result of the fault is that the voltage of the relay protection device is reduced or disappears, which may cause false operation or refusal to operate for the protection relay that reflects the voltage reduction and the protection device that reflects the phase relationship between voltage and current, such as directional protection and impedance relay.

(079): What phenomena during the operation of the voltage transformer require immediate cessation of operation?

A: If the voltage transformer has the following phenomena, it must be stopped immediately:

(1) The high-voltage side fuse circuit blows two or three times in succession.

(2) Lead terminals are loose and overheated

(3) Abnormal discharge sound or noise occurs inside

(4) If discharge is seen, there is a risk of flashover.

(5) Emitting a foul odor or smoke

(6) Oil spill

(080): Why is it not allowed for current transformers to operate under overload for a long period of time?

Answer: If the current transformer is overloaded for a long time, the error will increase and the meter indication will be incorrect. In addition, due to the increase of primary and secondary currents, the core and winding will overheat, the insulation will age quickly, and even the current transformer will be damaged. (081): What are the main reasons for the high-voltage fuse of the voltage transformer?

Answer: 1. The system has a single intermittent arc grounding, causing the ferromagnetic resonance of the voltage transformer

2. The fuse has been in operation for a long time and has naturally aged and melted.

3. Single-phase grounding or phase-to-phase short circuit occurs inside the voltage transformer itself

4. If a short circuit occurs on the secondary side and the secondary side fuse does not blow, it may also cause the high-voltage fuse to blow.

(082): What should I pay attention to when the voltage of the voltage transformer disappears?

The starting element and measuring element of LH-11 distance protection are both connected with 10mA auxiliary magnetic current. When the voltage of the voltage transformer disappears, the actuator disappears due to the instantaneous braking torque. Under the action of the auxiliary magnetic current, the contact is closed and does not return. Therefore, once the voltage of the voltage transformer disappears, the protection is first withdrawn, and then the DC of this protection is released to return the actuator of the starting element and the measuring element. When the protection is put into operation, the voltage circuit of the protection must be put into operation first, and then the DC is put into operation.

(083): What harm does excessive voltage do to a transformer in operation?

The regulations stipulate that the voltage of the transformer in operation shall not exceed 5% of the rated voltage. Excessive voltage will increase the excitation current of the transformer core, sometimes saturating the core and generating harmonic flux, which will increase the loss of the core and cause the core to overheat. Excessive voltage will also accelerate the aging of the transformer and shorten the service life of the transformer, so the voltage of the transformer in operation cannot be too high.

(084): Why should the secondary winding of a voltage transformer be grounded during operation?

Answer: This grounding is protective grounding to prevent high voltage from being introduced into the secondary side when insulation breakdown occurs, endangering the safety of people and equipment.

(085): What are the causes of light gas protection action?

Answer: (1) There is a minor fault inside the transformer that produces gas.

(2) Air enters the transformer

(3) A through-circuit fault occurs

(4) The oil level drops seriously below the gas relay, causing the gas relay to operate.

(5) DC multi-point grounding, secondary circuit short circuit

(6) Affected by strong vibration

(7) There is a problem with the gas relay itself

(086): What are the symptoms of an open circuit or poor contact in the secondary circuit of a current transformer?

Answer: (1) The ammeter indicates an imbalance, with one phase (open circuit phase) being zero or smaller.

(2) The current transformer makes a buzzing sound

(3) The power meter indication is incorrect and the meter rotates slowly

(4) Current transformer heating

(087): What should be paid attention to when deactivating a voltage transformer?

answer:

(1) First of all, we should consider the possible malfunction of related protection (such as distance protection) and automatic devices (such as standby protection) caused by the deactivation of the voltage transformer, and we must first apply to deactivate the related protection and automatic devices.

(2) Disable the voltage transformer including the high-voltage side knife switch, secondary air switch or fuse to prevent reverse charging on the secondary side.

(088): How to operate the power supply of the low-frequency load reduction device when switching the voltage transformer?

Answer: When switching voltage transformers, the power supply of the low-frequency load shedding device should not be disconnected. Generally, two voltage transformers can be operated in parallel. Therefore, when switching voltage transformers, first use the low-voltage parallel switch to connect the two voltage transformers in parallel, and then disconnect the disabled voltage transformer to ensure that the low-frequency load shedding device does not lose power. When the voltage transformers cannot be connected in parallel, the DC power supply of the low-frequency load shedding device should be disabled before switching the voltage transformers.

(089): What serious consequences will occur when the secondary side of the current transformer is open-circuited?

Answer: The magnitude of the primary current of the current transformer has nothing to do with the magnitude of the current of the secondary load. When the transformer is working normally, due to the small impedance, close to the short-circuit state, most of the magnetizing force generated by the primary current is compensated by the secondary current, the total magnetic flux density is not large, and the secondary coil potential is not large. When the current transformer is open-circuited, the impedance increases infinitely (Z^Q2=∞), the secondary current is equal to zero, the magnetizing force is equal to zero, and the total magnetizing force is equal to the original winding magnetizing force (IoW1=I1W1). A very high potential is generated in the secondary coil, and its peak value can reach several thousand volts, threatening personal safety, or causing damage to the secondary insulation of the instrument, protection device, and transformer. On the other hand, the magnetizing force of the original winding causes the magnetic flux density of the core to increase excessively, which may cause the core to overheat and be damaged.

(090): What phenomena of voltage mutual inductance should cause the operation to be stopped immediately?

Answer: 1) The high-voltage side fuse blows twice in succession.

2) The lead terminal is loose and overheated.

3) Abnormal discharge sound or noise occurs inside.

4) If discharge is seen, there is a risk of flashover.

5) Emitting a foul odor or smoke.

6) Oil spill. (091): When a voltage transformer of 35 kV or below is found to have any abnormality during operation, an application should be made to deactivate it?

Answer: While strengthening monitoring, you should apply to the dispatcher to deactivate the voltage transformer.

1) The high-voltage side fuse blows continuously.

2) There is discharge and abnormal sound between the internal winding and the outer casing or between the lead wire and the outer casing.

3) The casing has serious cracks and discharge.

4) Serious oil leakage.

(092): What is the protection range of the primary and secondary fuses of the voltage transformer?

Answer: The protection scope of the primary fuse of the voltage transformer is: internal fault of the voltage transformer (the fuse may not blow due to short-circuit fault between turns), or short-circuit fault on the connection line between the voltage transformer and the grid.

The protection scope of the secondary fuse of the voltage transformer is: continuous short circuit fault caused by short circuit of the circuit below the secondary fuse (generally, the primary fuse will not blow when there is a fault in the circuit below the secondary fuse).

(093): What should be paid attention to when deactivating a voltage transformer?

A: The issues that should be noted are:

(1) Do not cause the automatic protection device to lose voltage.

(2) Voltage switching is required

(3) To prevent reverse charging, remove the secondary fuse (including capacitor)

(4) After all secondary loads are disconnected, disconnect the primary power supply of the transformer.

(094): What kind of protection is usually connected to the voltage transformer used in power substation?

Answer: (1) Impedance protection (distance protection);

(2) High frequency protection;

(3) Direction protection;

(4) Low frequency load shedding and low voltage load shedding;

(5) Low voltage lockout;

(6) Self-operating device;

(7) Synchronous reclosing.

(095)What causes the error in voltage transformer?

(1) The existence of excitation current;

(2) PT has internal resistance;

(3) Because the primary voltage U1 affects the excitation current, it also affects the size of the error;

(4) When the secondary load of the PT changes, it will affect the changes in the secondary and primary currents of the PT, so it will also have a corresponding impact on the error;

(5) The change of the power factor angle of the secondary load will change the phase of the secondary current and the secondary voltage, so it will have a certain impact on the error.

(096): What is the protection range of the primary and secondary fuses of the voltage transformer?

The protection scope of the primary fuse of the voltage transformer: internal fault of the PT (the fuse may not blow due to a short-circuit fault between turns), or a short-circuit fault on the connection line between the voltage transformer and the grid.

The protection scope of the secondary fuse of the voltage transformer: continuous short circuit fault caused by short circuit in the circuit below the secondary fuse (generally, the primary fuse will not blow when there is a fault in the circuit below the secondary fuse).

(097): What are the values ​​of the voltages on each side of a three-phase five-column voltage transformer?

Answer: The primary winding voltage is the line voltage of the access system, the main secondary winding phase voltage is 100/√3V, and the auxiliary secondary winding phase voltage is 100/3V.

(098): How to calibrate the transformer?

Answer: The phase of the voltage transformers on the two busbars should be calibrated with a running transformer first, and then the primary busbar should be charged with a newly added transformer, and then the phase should be calibrated. Generally, a phase meter or voltmeter is used. If the measured result is that the two voltages in the same phase are equal to zero and the non-in-phase voltages are line voltages, it means that the phase sequence of the two transformers is consistent.

(099): What are the wiring methods for current transformers?

Answer: The wiring methods of current transformers include: two-phase V-shaped wiring and two-phase current difference wiring using two current transformers, three-phase Y-shaped wiring, three-phase △-shaped wiring and zero-sequence wiring using three current transformers.