1. What are the conditions for juxtaposition of two transformers?
(1) the same ratio;
(2) The short circuit impedance is the same;
(3) The wiring group is the same;
(4) The phase sequence is the same;
2. What are the cooling methods of the transformer?
(1) oil immersion from cold;
(2) oil immersion and air cooling;
(3) Strong oil circulation air cooling;
(4) Strong oil-oriented air cooling.
3. What is the classification insulation? What should be paid attention to when operating the transformer with graded insulation?
The so-called grading insulation is the main insulation of the coil of the transformer close to the neutral point, and its insulation level is lower than the insulation level of the coil end.
Graded insulated transformers are generally specified to be put into operation only when the neutral point is directly grounded.
4. Why is there a magnetizing inrush current when the transformer is closed?
In the transformer coil, the relationship between the excitation current and the magnetic flux is determined by the magnetization characteristics. The more the iron core is saturated, the larger the excitation current required to generate a certain magnetic flux. Since the magnetic flux in the iron core is already full under normal conditions. If the switch is closed under unfavorable conditions, the maximum magnetic flux density in the core can reach twice the normal value, the core saturation will be very serious, the magnetic permeability will be reduced, the excitation reactance will be greatly reduced, and the excitation current will be The numerical value is greatly increased, and the current waveform determined by the magnetization characteristics is sharp. This inrush current can exceed 6-8 times of the rated current of the transformer. Therefore, due to the transformer electric, the conversion of magnetic energy, the phase angle of the closing voltage, the core The degree of saturation, etc., determines the magnetizing inrush current when the transformer is closed, and the magnitude of the magnetizing inrush current will be affected by the core remanence and closing voltage phase angle.
5. What is the danger of sudden short circuit on the transformer?
Sudden short circuit has two harmful effects on the transformer coil:
(1) The coil is subjected to a strong electromagnetic force and may be destroyed;
(2) The coil is severely heated.
6. What problems should be paid attention to when replenishing oil in transformer operation?
The oil filling work after the transformer is short of oil can be carried out without the power failure of the transformer. Pay attention to the following matters when replenishing the oil:
(1) Pay attention to prevent mixing of oil, and the newly added oil should pass the test.
(2) Before the oil is replenished, the heavy gas protection should be changed to the signal position to prevent the gas protection from malfunctioning and the transformer to trip.
(3) After replenishing the oil, check the gas relay and release the gas in time. After the air of the transformer is exhausted, the heavy gas protection can be put back into the trip position.
(4) The amount of oil should be suitable, and the oil level should be compatible with the oil temperature of the transformer at that time.
(5) It is forbidden to cut the oil from the lower part of the transformer to prevent the sediment at the bottom of the transformer from rushing into the coil, which will affect the insulation and heat dissipation of the transformer.
7. Under what circumstances should the transformer stop running immediately?
When one of the following occurs, the transformer should be shut down immediately:
(1) The internal sound of the transformer is very large, very abnormal, and there is a popping sound;
(2) Under normal load and cooling conditions, the oil temperature in the upper layer of the transformer is abnormal and continues to rise;
(3) oil pillow or explosion-proof cylinder injection;
(4) Severe oil leakage, causing the oil level to be lower than the limit of the oil level gauge;
(5) The oil color changes too much, and carbon is present in the oil;
(6) The casing has serious damage and discharge;
(7) Personal accidents within the scope of the transformer must be blacked out;
(8) The transformer is on fire;
(9) The casing joint and the lead are red, melted or blown.
8. The neutral point of the ungrounded system of the voltage transformer high-voltage side fuse one phase fuses the same point and different points of the system single-phase grounding phenomenon
The same point: both can send a ground signal.
The difference: the phenomenon when the high-voltage side is broken by one phase, the phase-off voltage is much lower, and the other two phases are the normal phase voltage.
The phenomenon of single-phase grounding is that the phase-off voltage indication is zero, and the other two phases are increased by three times.
9.When the main transformer after new or overhaul is put in, why do you need to do a full voltage impact test?
Before the new or overhauled main transformer is put into operation, a full voltage impact test should be performed. In addition, when the no-load transformer is put into the power grid, a magnetizing inrush current will occur. The magnetizing inrush current can generally reach 6-8 times the rated current, after 0.5– After 1 second, it may attenuate to 0.25–0.5 times of rated current, but all decay time is longer. Large-capacity transformer takes tens of seconds. Because the magnetizing inrush current can generate a lot of electric power, the impact test is also to evaluate the transformer. Mechanical strength and reliability of relay protection devices.
The regulations stipulate that the newly installed transformer impact test is 5 times, and the transformer impact test after overhaul is 3 times.
10. The busbar voltage transformer of the high voltage factory is stopped, what is the operating principle of power transmission?
(1) Principle of power outage operation:
a. When the high-voltage factory is running with the working power supply, the low-voltage tripping pressure plate of the high-voltage factory BZT circuit should be deactivated to prevent the high-voltage factory working power switch from tripping after the voltage transformer is powered off.
b. Pulling on the high voltage factory busbar low voltage protection DC lead wire, in order to prevent the voltage transformer from power failure after the power transformer is powered off, causing the high voltage factory bus low voltage protection to malfunction, so that the high voltage factory motor trips.
c. Pull open the secondary wire of the busbar voltage transformer of the high voltage factory.
d. Pull open the secondary plug of the busbar voltage transformer for the high voltage factory.
e. Pull out or pull open the high voltage factory bus voltage transformer trolley to open a high voltage factory bus voltage transformer.
f. Short circuit is used for the high voltage factory bus voltage monitoring relay contact of the low voltage factory BZT circuit, so that the corresponding low voltage factory BZT device is not invalid.
(2) Principle of power transmission operation: The power transmission operation is opposite to the power failure operation sequence.
11. The high voltage factory uses the bus voltage transformer to stop, what should pay attention to the power transmission operation?
Pay attention to the following items when the busbar voltage transformer of the high voltage factory is powered off:
(1) When the voltage transformer is deactivated, the relay protection and automatic device of the voltage transformer should be considered first. In order to prevent misoperation, the relay protection and automatic device or the DC power supply used can be deactivated.
(2) When the voltage transformer is deactivated, the secondary fuse should be removed.
(3) Then remove the primary fuse.
(4) When the trolley type or the tweezer type voltage transformer is powered off, the trolley or the twitch should also be pulled out, and the secondary plug-in should be pulled out at the same time.
Pay attention to the following items when powering the busbar voltage transformer of the high voltage factory:
(1) First check that the relay protection and automatic device of the voltage transformer are in the deactivated state.
(2) Put the primary side fuse of the voltage transformer into the inverter.
(3) Push the trolley or pumping voltage transformer to the working position.
(4) Put the secondary side fuse of the voltage transformer into the pole.
(5) Put the secondary plug of the trolley type or the tweezer type voltage transformer.
(6) Enable deactivated relay protection and automatic devices or their DC power supply.
(7) The voltage transformer itself should be inspected before the power transmission, and the insulation condition of the high and low voltage windings should be measured according to the regulations.
12. Under the circumstances, the factory transformer (working transformer and standby transformer) can be powered
(1) The factory transformer is tripped. If there is no linkage, the spare transformer can be forced into operation.
(2) The factory transformer has a time-limited over-current action. In the absence of a backup power supply, it can be forced once. If it is unsuccessful, it can not be sent again.
13. What should be paid attention to when adjusting the tap of the on-load tap changer during operation?
(1) Strengthen the monitoring of the oil level of the additional fuel tank.
(2) The number of operations of the on-load tapping device should be carefully checked and recorded.
(3) Remote electric adjustment and local manual adjustment cannot be performed at the same time.
(4) When adjusting, note that the tap position indicator is correct and the number is in the middle of the display hole.
(5) The adjustment operation requires the command of the leader, and is not allowed to proceed at will.
(6) The adjustment operation needs to be carried out by two people.
(7) The gas protection of the additional fuel tank of the transformer with load regulation needs to be invested frequently.
(8) When the remote electric adjustment is made, it should be carried out with a short ‘instantaneous moment’.
(9) The on-load tap-changer cannot be operated frequently when the transformer is overloaded.
(10) In-situ manual adjustment should be carried out according to the specific operation sequence.
14. Under what circumstances can a high-voltage factory transformer be used for strong power transmission?
High-voltage factory transformers can be powered strongly under the following conditions:
(1) When the high-voltage factory uses the working transformer to trip and the standby transformer is not connected, the on-duty personnel can force the standby transformer immediately without any inspection.
(2) When the automatic device is deactivated due to failure, the standby transformer is in the absence of standby, and the on-duty personnel can immediately apply the standby transformer without any inspection.
(3) When there is no standby transformer, when the working transformer is tripped by mistake or only the backup protection causes tripping (such as overcurrent protection), it can be sent without inspection.
15. What are the advantages of split winding transformer compared to double winding transformer?
Has the following advantages:
(1) Limiting the effect of short-circuit current is significant;
(2) The self-starting condition of the motor is improved. Since the through-impedance of the split transformer is smaller than that of the double-winding transformer of the same capacity, the voltage drop of the transformer is smaller when the starting current flows, and the allowable starting capacity is Be bigger;
(3) When a short circuit fault occurs in one branch of the split winding, the bus voltage of the other branch of the split winding decreases little, so it can maintain normal operation.
16. Why should the secondary side of the voltage transformer not be short-circuited?
In normal operation, the primary side of the voltage transformer is connected to the grid voltage, and its secondary side is connected to the load, that is, the voltage coil of the meter and the relay. Their impedance is very large, so the working state of the voltage transformer is close to the transformer no-load condition.
If the secondary side of the voltage transformer is short-circuited, its impedance is reduced, and only the internal resistance of the secondary coil is left, so that a large current will be generated in the secondary coil, causing the voltage transformer to burn out. In the voltage transformer, the secondary side is connected with a fuse. The fuse will blow the fuse, the meter and protection will fail. Therefore, the secondary side of the voltage transformer is not allowed to short circuit.
17. Why can’t the current transformer be opened twice? How to deal with the situation with open circuit?
Due to the severe saturation of the core during the second open circuit, the following consequences occur:
(1) High voltage is generated, which is dangerous to equipment and operators;
(2) The core loss increases, it is severely hot, and there is a possibility of burnout;
(3) Residual magnetism is generated in the iron core, which increases the current transformer error. Therefore, the secondary open circuit of the current transformer is not allowed.
The method for finding the secondary open circuit phenomenon of the current transformer is:
(1) It is possible to transfer the power outage treatment of the load blackout process;
(2) If the power can not be cut off, if it is open at the current transformer, it is limited to the safe distance. People can’t approach the treatment, only reduce the load current, and then stop the power after the peak;
(3) If the screw on the terminal block is loose after the disk, stand on the insulating mat, with gloves, use the screwdriver with the insulating handle, and actuate the screw quickly and decisively.
18. What are the regulations on the insulation resistance value of the transformer?
After the new installation or overhaul and the shutdown of the transformer for more than half a month, the insulation resistance of the coil should be measured before putting into operation. When measuring the insulation resistance of the transformer, the coil operating voltage should be 1000–2500 volts if the operating voltage is above 500 volts. Table, 500 volts or less should use 500 volt shake table.
The insulation condition of the transformer is judged according to the following requirements:
(1) The ratio of the measured insulation resistance value when the transformer is used and the value measured before the transformer is put into operation after installation or overhaul is not less than 50%.
(2) The absorption ratio R60″/R15″ shall not be less than 1.3 times.
If the above conditions are met, the transformer is considered to be qualified.
Pay attention to the following issues when measuring transformer insulation:
(1) Must be carried out when the transformer is out of power, and each coil outlet has an obvious breaking point;
(2) Clean around the transformer, no grounding, no operator;
(3) Discharge the ground before measurement, and respond to ground discharge after measurement;
(4) The shaker used for measurement shall meet the voltage level requirements;
(5) For transformers with neutral grounding, the neutral point knife gate should be pulled open before measurement, and should be restored to the original position after measurement.
19. Under what circumstances do special inspections of running transformers? Check which items
(1) When overload occurs, the load should be monitored, the oil temperature and oil level should be changed, the joint contact should be good, and the cooling system should be running normally;
(2) When the wind is strong: check that the lead wires of each part should not be violently oscillated, there is no debris around, and there is no possibility of scraping the charged part;
(3) During thunderstorms: there should be no discharge marks in all parts;
(4) Heavy snow days: there is no overheating at each contact point, and there is no discharge and icing in each part;
(5) Heavy foggy days: there should be no serious sparks or discharges in all parts;
(6) Temperature change: check the oil level and oil temperature change of the oil pillow.
20. How should the transformer oil level rise or fall significantly?
The oil level rises and does not exceed the oil level specified in the oil pillow. The duty officer should:
(1) Check if there is a problem with the cooling device;
(2) Check the transformer load change;
(3) Check if the ambient temperature change around the cooler is too large;
(4) The oil level rises due to the temperature rise, and the maintenance personnel should be contacted to drain the oil.
If the oil level is found to drop or the oil level is not visible in the oil pillow, the duty officer should:
(1) Check if there is a large amount of oil leakage;
(2) Check if the load is reduced;
(3) Check if the cooling environment temperature is lowered;
(4) If there is a large amount of oil leakage in the transformer, the standby transformer should be switched for maintenance;
(5) If it is a cooling environment, the load affects the oil level, and the oil level cannot reach the specified oil level. The maintenance personnel should be contacted to refuel the transformer to the standard position.
21. The purpose of the no-load test and short-circuit test of the transformer
The purpose of the no-load test:
(1) Measure the no-load current, no-load loss, calculate the parameters such as the excitation impedance of the transformer, and find the transformation ratio.
(2) It can be found that local and integral defects in the magnetic circuit of the transformer, such as poor insulation between silicon steel sheets, insulation damage of the threaded screw or pressure plate, etc.
(3) Some problems of the transformer coil can be found, such as short circuit between coil turns, short circuit of coil parallel branch, etc.
The purpose of the short circuit test:
(1) Measure the voltage, current, and loss during short circuit, and find the parameters such as copper loss and short circuit impedance of the transformer.
(2) Check the correctness of the coil structure.
22. What tests should be done before the newly installed or overhauled transformer is put into operation?
(1) Transformer and casing insulating oil test.
(2) Transformer coil and casing dielectric loss angle measurement.
(3) Leakage current test.
(4) Power frequency withstand voltage test.
(5) Measuring the DC resistance of the transformer.
(6) Measuring the tap change ratio of the tap changer.
(7) Check the transformer junction group and polarity.
(8) Test the action of the on-load tap changer.
(9) Measure the insulation resistance and absorption ratio of the transformer.
(10) Impact closing test. The newly installed transformer must be tested for full voltage shock closing, five times of closing and closing, and must be closed three times after overhauling the coil.