What issues should be paid attention to when doing transformer testing?
1. What are the conditions for two transformers to be paralleled?
(1) The transformation ratio is the same;
(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 transformers?
(1) Oil-immersed self-cooling;
(2) Oil-immersed air cooling;
(3) Strong oil circulation air cooling;
(4) Strong oil-guided air cooling.
3. What is graded insulation? What should we pay attention to when operating a graded insulated transformer?
The so-called graded insulation means that the coil of the transformer is close to the neutral point Part of the main insulation with a lower insulation level than the insulation level at the ends of the coil.
Graded insulation transformers are generally stipulated to be put into operation only when the neutral point is directly grounded.
4. Why is there an excitation 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 saturated the core is, the greater the excitation current required to generate a certain magnetic flux. Since under normal circumstances, the magnetic flux in the iron core is already saturated, if the switch is closed under adverse conditions, the maximum magnetic flux density in the iron core can reach twice the normal value, and the saturation of the iron core will be very serious, causing it to The magnetic permeability decreases and the excitation reactance is greatly reduced, so the excitation current value is greatly increased. The current waveform determined by the magnetization characteristics is very sharp. This inrush current can exceed 6-8 times the rated current of the transformer. Therefore, due to the conversion of the electrical and magnetic energy of the transformer, the phase angle of the voltage at the closing moment, the saturation degree of the iron core, etc., it is determined that when the transformer is closed, there will be an excitation inrush current. The size of the excitation inrush current will be affected by the residual magnetism of the iron core and the closing force. The influence of gate voltage phase angle.
5. What harm does a sudden short circuit do to the transformer?
A sudden short circuit has two harmful effects on the transformer coil:
( 1) Subjecting the coil to strong electromagnetic force, which may cause damage;
(2) Seriously heating the coil.
6. What issues should be paid attention to when replenishing oil during operation of the transformer?
The oil replenishment work after the transformer is short of oil can be carried out without power outage. . The following matters should be paid attention to when replenishing oil:
(1) Pay attention to prevent oil mixing, and the newly added oil should pass the test.
(2) Before replenishing oil, the heavy gas protection should be changed to the signal position to prevent the gas protection from malfunctioning and causing the transformer to trip.
(3) After replenishing oil, you should check the gas relay and release the gas in time.Only after the air in the compressor has been exhausted can the heavy gas protection be put back into the tripping position.
(4) The amount of oil replenishment should be appropriate, and the oil level should be consistent with the oil temperature of the transformer at that time.
(5) It is forbidden to replenish oil from the lower part of the transformer to prevent the sediment at the bottom of the transformer from being flushed into the coil and affecting the insulation and heat dissipation of the transformer.
7. Under what circumstances must the transformer be stopped immediately?
When one of the following situations occurs, the transformer should be stopped immediately:
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(1) The internal sound of the transformer is very loud, very abnormal, and there are popping sounds;
(2) Under normal load and cooling conditions, the temperature of the oil in the upper layer of the transformer is abnormal and rising;
(3) Fuel injection into the oil pillow or explosion-proof barrel;
(4) Serious oil leakage, causing the oil level to be lower than the indication limit of the oil level gauge;
(5) The color of the oil changes too much and carbon appears in the oil;
(6) The bushing is seriously damaged and discharged;
(7) Personal accidents occur within the transformer range , when a power outage is necessary;
(8) The transformer catches fire;
(9) The casing joints and leads turn red, melt or fuse.
8. What are the similarities and differences between the one-phase melting of the high-voltage side fuse of the voltage transformer in an ungrounded system and the single-phase grounding of the system?
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Same point: both can send ground signals.
Difference: When the high-voltage side fuse breaks one phase, the voltage of the broken phase drops a lot, and the other two phases have normal phase voltages.
The phenomenon when a single phase is grounded is that the phase failure voltage indication is zero and the other two phases increase three times.
9. Before the newly installed or overhauled main transformer is put into operation, why is it required to do a full voltage impact test? How many times should it be impacted?
The newly installed or overhauled main transformer is required to undergo a full voltage impact test. Before the main transformer is put into operation, a full voltage impact test must be carried out. In addition, when the no-load transformer is put into the power grid, excitation inrush current will be generated. The excitation inrush current can generally reach 6-8 times the rated current, and may attenuate to 0.25-0.5 times the rated current after 0.5-1 second, but the total attenuation time is longer, and large-capacity transformers It takes tens of seconds. Since the excitation inrush current can generate a large amount of electric power, the impact test is also to assess the mechanical strength of the transformer and the reliability of the action of the relay protection device.
The regulations stipulate that newly installed transformers must be impacted for 5 times, and transformers after overhaul must be impacted for 3 times. They must be qualified before they can be put into operation.
10. What are the operating principles for stopping and transmitting power to high-voltage plant busbar voltage transformers?
(1) Power outage operating principles:
a. When the high-voltage factory working power supply is running, the low-voltage tripping plate of the high-voltage factory BZT circuit should be disabled to prevent the voltage transformer from shutting down.After the power was turned on, the high-voltage factory working power switch tripped.
b. Pull open the low-voltage protection DC lead wire of the high-voltage factory bus to prevent the high-voltage factory bus low-voltage protection from malfunctioning and causing the high-voltage factory motor to trip after a power outage of the voltage transformer.
c. Pull open the secondary lead wire of the high-voltage factory bus voltage transformer.
d. Open the secondary plug-in of the high-voltage factory bus voltage transformer.
e. Pull out the high-voltage factory bus voltage transformer trolley or open the primary knife switch of the high-voltage factory bus voltage transformer.
f. Short-circuiting the high-voltage factory bus voltage monitoring relay contact for the low-voltage factory BZT circuit will not cause the corresponding low-voltage factory BZT device to fail.
(2) Power supply operation principle: The order of power supply operation and power outage operation is opposite.
11. What should be paid attention to when the high-voltage factory bus voltage transformer is shut down and powered on?
What should be paid attention to when the high-voltage factory bus voltage transformer is out of power? The following matters:
(1) When deactivating a voltage transformer, you should first consider the relay protection and automatic devices of the voltage transformer. In order to prevent malfunction, the relevant relay protection and automatic devices or the used The DC power supply is disabled.
(2) When the voltage transformer is disabled, the secondary side fuse should be removed.
(3) Then remove the primary fuse.
(4) When a power failure occurs for a trolley-type or cassette-type voltage transformer, the trolley or cassette should be pulled out, and its secondary plug-in should be pulled out at the same time.
The following matters should be paid attention to when transmitting power to the high-voltage factory bus voltage transformer:
(1) First check whether the relay protection and automatic device of the voltage transformer are in place. Disabled status.
(2) Insert the primary side fuse of the voltage transformer.
(3) Push the trolley-type or cassette-type voltage transformer to the working position.
(4) Insert the secondary side fuse of the voltage transformer.
(5) Insert the secondary plug-in of the trolley-type or cassette-type voltage transformer.
(6) Enable disabled relay protection and automatic devices or their DC power supplies.
(7) During maintenance of the voltage transformer itself, the insulation condition of the high and low voltage windings should also be measured according to regulations before power is supplied.
12. Under what circumstances can factory transformers (working transformers and backup transformers) be forced to transmit power?
(1) Factory transformer accidents If tripped, if there is no linkage, the spare transformer can be forced in.
(2) The factory-used transformer has a time-limited overcurrent action. If there is no backup power supply, it can be forced to be sent once. If it fails, it cannot be sent again.
13. What should be paid attention to when adjusting the tap of the on-load voltage regulating transformer during operation?
(1) Deal with the oil in the additional tank Strengthen surveillance.
(2) The number of operations of the on-load pressure regulating 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, pay attention to the correct indication of the tap position indicator and the number is located in the middle of the display hole.
(5) Adjustment operations require orders from the leader and are not allowed to be performed at will.
(6) The adjustment operation must be performed by two people.
(7) The gas protection of the additional oil tank of the on-load voltage regulating transformer needs to be put in regularly.
(8) Remote electric adjustment should be carried out with short and instantaneous movements.
(9) Do not operate the on-load tap-changer frequently when the transformer is overloaded.
(10) When performing manual adjustments on the spot, a specific sequence of operations must be followed.
14. Under what circumstances can a high-voltage factory transformer be used to forcefully transmit power?
A high-voltage factory transformer can be used to forcefully transmit power under the following circumstances:
(1) When the high-voltage factory working transformer trips and the standby transformer is not switched on, the person on duty can immediately force the standby transformer on without any inspection.
(2) When the automatic device is disabled due to a fault and the backup transformer is unavailable, the personnel on duty may immediately forcefully switch on the backup transformer without any inspection.
(3) When there is no backup transformer, when the working transformer trips accidentally or only the backup protection causes tripping (such as overcurrent protection), power can be sent without inspection.
15. What are the advantages of split winding transformer compared with double winding transformer?
It has the following advantages:
(1) Limiting the short-circuit current has a significant effect;
(2) It improves the self-starting conditions of the motor. Since the crossover impedance of the split transformer is smaller than that of the double-winding transformer of the same capacity, when the starting current flows , the voltage drop of the transformer is also smaller, and the allowable starting capacity is larger;
(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 drops very little , so normal operation can be maintained.
16. Why is the secondary side of the voltage transformer not allowed to be short-circuited?
During normal operation, the primary side of the voltage transformer is connected to the grid voltage, and its secondary side is connected to the grid voltage. The voltage coils connected to the load, that is, the instrument and the relay, have large impedances, so the working state of the voltage transformer is close to the no-load condition of the transformer.
If a short circuit occurs on the secondary side of the voltage transformer, its impedance is reduced, leaving only the internal resistance of the secondary coil. This will generate a large current in the secondary coil, causing the voltage transformer to burn out. If a fuse is connected to the primary and secondary sides of the voltage transformer, the fuse will blow and the meter and protection will fail. Therefore, short circuit is not allowed on the secondary side of the voltage transformer.
17. Why can’t the secondary current transformer be open circuit? If there is an open circuitHow to deal with the circuit situation?
Due to the serious saturation of the iron core during the second open circuit, the following consequences will occur:
(1) A very high voltage will be generated, which will cause serious damage to the equipment. There is a danger to the operator and the operating personnel;
(2) The iron core loss increases, severe heating occurs, and there is a possibility of burning;
(3) Residual magnetism is generated in the iron core, causing the current to The transformer error increases. Therefore, the secondary open circuit of the current transformer is not allowed.
The method to deal with the secondary open circuit phenomenon of the current transformer is:
(1) Try to deal with the power outage that can transfer the load;
(2) If there is no power outage, if the circuit is open at the current transformer, it is limited to a safe distance and people cannot get close to handle it. The load current can only be reduced and the power outage will be handled after the peak is passed;
(3) If it is an after-hours terminal If the screws are loose, you can stand on the insulating mat, wear gloves, and use a screwdriver with an insulated handle to tighten the screws decisively and quickly.
18. What are the regulations on the insulation resistance value of transformers? What should be paid attention to when measuring?
After new installation or maintenance and out of operation for more than half a month Before putting the transformer into operation, the insulation resistance of the coil should be measured. When measuring the insulation resistance of the transformer, a 1000–2500 volt megometer should be used for coil operating voltages above 500 volts, and a 500 volt megometer should be used for coil operating voltages below 500 volts.
The quality of the transformer insulation condition is determined according to the following requirements:
(1) The insulation resistance value measured when the transformer is in use is the same as the insulation resistance value measured before the transformer is put into operation after it is installed or overhauled and dried. The ratio of the obtained values shall not be 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 insulation is considered qualified.
The following issues should be noted when measuring the insulation of the transformer:
(1) It must be carried out when the transformer is powered off, and there are obvious disconnection points for each coil outlet;
(2) The area around the transformer is clean, there are no grounded objects, and there are no workers;
(3) The ground should be discharged before the measurement, and the ground should be discharged after the measurement;
(4) Measurement use The megger should meet the voltage level requirements;
(5) For transformers with a grounded neutral point, the neutral point knife switch should be opened before measurement and restored to its original position after measurement.
19. Under what circumstances should special inspections be carried out on operating transformers? What items should be checked?
(1) The load should be monitored when overload occurs , changes in oil temperature and oil level, the joint contact should be good, and the cooling system should operate normally;
(2) In strong winds: Check that there should be no violent swings in the leads of each part, there should be no debris around, and no live parts have been blown possibility;
(3) During thunderstorms: there should be no traces of discharge at all parts;
(4) In heavy snow days: there should be no traces of discharge at each contact pointOverheating, no discharge or icing in each part;
(5) Foggy days: There should be no serious sparks and discharge in all parts;
(6) Drastic changes in temperature: check the oil Changes in pillow oil level and oil temperature.
20. What should be done if the oil level of the transformer rises or drops significantly?
The oil level rises but does not exceed the oil level specified by the oil conservator. , the personnel on duty should:
(1) Check whether there is any problem with the cooling device;
(2) Check the load changes of the transformer;
(3) Check the cooling Check whether the ambient temperature change around the device is too large;
(4) Because the oil level rises due to the increase in temperature, maintenance personnel should be contacted to drain the oil.
If the oil level is found to have dropped or the oil level is no longer visible in the oil pillow, the person on duty should:
(1) Check whether there is a large amount of oil leakage;
(2) Check whether the load is reduced;
(3) Check whether the cooling environment temperature is reduced;
(4) If there is a large amount of oil leakage from the transformer, the backup transformer should be switched to run. Carry out maintenance;
(5) If it is a cooling environment and the load affects the oil level to decrease, but the oil level cannot reach the specified oil level, maintenance personnel should be contacted to refuel the transformer to the standard position.
21. What is the purpose of no-load test and short-circuit test of transformer?
The purpose of no-load test:
(1) By measuring the no-load current and no-load loss, the excitation impedance and other parameters of the transformer can be calculated, and the transformation ratio can be obtained.
(2) It can detect local and overall defects in the magnetic circuit of the transformer, such as poor insulation between silicon steel sheets, insulation damage to the threaded screw or pressure plate, etc.
(3) Some problems with the transformer coil can be found, such as short circuit between turns of the coil, short circuit of the parallel branch of the coil, etc.
The purpose of the short-circuit test:
(1) Measure the voltage, current, and loss during short-circuit, and find the copper loss and short-circuit impedance of the transformer.
(2) Check the correctness of the coil structure.
22. What tests should be done before a newly installed or overhauled transformer is put into operation?
(1) Transformer and bushing insulating oil test.
(2)Measurement of dielectric loss angle of transformer coil and bushing.
(3) Leakage current test.
(4) Power frequency withstand voltage test.
(5) Measure the DC resistance test of the transformer.
(6) Measure the tap changer transformation ratio.
(7) Check the transformer wiring 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. Newly installed transformers must undergo a full-voltage impact closing test, which must be pulled and closed five times. The transformer must be replaced after coil replacement and overhaul.The switch must be closed three times.
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