Questions and Answers about Power Distribution Transformer

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Q: What are the main components of oil immersed transformer?

A: Iron core, winding, oil tank, conservator, dehydrating breather, radiator, insulating bushing, tap changer, buchholz relay, thermometer etc.

 

Q: What is the main function of transformer in power system?

A: The function of transformer is to convert voltage to facilitate power transmission. After the voltage is boosted by the step-up transformer, the line loss can be reduced, the economy of power transmission can be improved, and the purpose of long-distance power transmission can be achieved.

The step-down transformer can change the high voltage to use voltage to meet the needs of the user.

 

Q: What is the harm of bushing crack?

A: Cracks in the bushing will reduce the insulation strength, cause further damage to the insulation, even direct full breakdown. The bushing may also expand and crack when the water is frozen in the crack. As you can see, the bushing crack is a threaten to the safe work.

 

Q: What is the function of transformer conservator?

A: When the volume of the transformer expands or shrinks with the change of oil temperature, the oil conservator plays the role of oil storage and supplement, which can ensure that the oil tank is full of oil. At the same time, due to the installation of oil conservator, the contact surface between the transformer and air is reduced which is slowed down the deterioration speed of oil. The side of the conservator is also equipped with an oil level gauge to monitor the change of oil level. Certainly, in terms of economy and use, small capacity transformers can use without conservator.

 

Q: Why should the iron core of the transformer be grounded?

A: The iron core and other accessories of the transformer are in the electric field around the winding during operation. If they are not grounded, the iron core and other accessories must induce the voltage. Under the action of applied voltage, when the induced voltage exceeds the ground discharge voltage, the discharge phenomenon will occur. In order to avoid the internal discharge of the transformer, the iron core shall be grounded.

 

Q: What is the function of thermometer? How many measurement methods are there?

A: The thermometer is used to measure the upper oil temperature in the tank and monitor whether the power transformer operates normally. Thermometer can be divided into mercury thermometer, signal thermometer and resistance thermometer according to the capacity of transformer.

 

Q: What’s suggestion of installing buchholz relay/gas relay in transformer?

A: Gas relay shall be installed for 800kVA and above transformers with oil conservator, 400KVA(thermal power plant) and 180KVA(hydropower plant) and above auxiliary transformers.

 

Q: Why is the temperature rise of class A insulated transformer winding specified as 65 ℃?

A: The transformer will produce iron loss and copper loss during operation. These two parts of losses are all converted into energy, which will heat the iron core and winding, aging the insulation and affect the service life of the transformer. Therefore, the insulation of transformer winding is mostly class A insulation in international regulations, and the temperature rise of winding is 65 ℃.

 

Q: Which part of the winding is the hottest during normal operation of the transformer?

A: The temperature of winding and iron core is high in the upper part and low in the lower part. For the oil immersed transformer winding of general structure, it has proved that the temperature is 70% ~ 75% of the hottest height direction and one-third of the transverse direction from the inner diameter of the winding. The hottest spot of each transformer winding shall be determined by the test.

 

Q: What is the cause of transformer oil shortage?

A: (1) Long term oil leakage or massive oil leakage.

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

(3) The capacity of conservator is small and can not meet the requirements of operation.

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

 

Q: What does △/ Y0―11 mean which is showed on the transformer nameplate?

A: According to the connection mode of transformer primary and secondary coils, the voltage phase Angle of transformer primary and secondary side lines is different. △/ Y0 - 11 indicates the phase Angle difference between the two line voltages of the transformer is 330° in the customary clock notation that is 11 o 'clock.

 

Q: The primary and secondary of the power transformer often have two or more coils each, such as the coil with the same polarity end sign lost, what method can be used to identify?

A: The same polarity end of each coil of the power transformer is usually indicated by the symbol "*".If the mark is missing, it can be identified experimentally.First, either end of one low-voltage coil and another low-voltage coil is connected to the power supply of any high-voltage coil. Then, the voltmeter is used to measure the voltage of the remaining two ends of the two low-voltage coils. If the voltage measured is the sum of the voltages of two low-voltage coils, it indicates that the two ends of the connection are not of the same polarity; if the voltage measured is the difference between the two, it indicates that the two ends of the connection are of the same polarity.The method for determining the polarity of high voltage coils can be deduced in this way.

 

Q: If the input voltage of the transformer is excessively greater than the rated voltage, what is the effect on the transformer?

A: Generally, the flux density of transformer is higher when rated, and the core tends to saturation.If the input voltage is more than the rated voltage, the core will be excessively saturated, resulting in the deformation of the output voltage waveform, which contains a large number of high-order harmonic components, resulting in the increase of the output voltage amplitude, and the insulation of the coil is easy to damage.At the same time, the increase of magnetic flux density increases the iron loss and the corresponding increase of no-load current, resulting in transformer heating and affecting the power factor of the grid. Therefore, the input voltage of the transformer is generally not allowed to exceed 5% of the rated voltage.

 

Q: Transformers are stationary equipment, but they hum when they are in operation. Why?

A: When the transformer coil is connected to 50hz AC, 50hz flux is also generated in the core.Due to the change of magnetic flux, the silicon steel in the core also vibrates accordingly, even when clamped, producing a buzz of 50hz vibration.But as long as the sound is not increased, and there is no other noise, it is normal.

 

Q: Why should the core clamping bolt of power transformer be insulated from the core?

A: The core of the transformer is made of silicon steel.In order to reduce the core eddy current loss, the silicon steel plates are insulated from each other. If the core clamping bolts are not insulated from the core, it is bound to cause a short circuit at the bolt and increase the core eddy current loss.

 

Q: Why adopting disk shaped winding instead of barrel shaped winding in large transformer?

A: Because the short circuit current of large transformer is large, the stress generated by short circuit is also large, the disk winding can be added with more supports without causing the deformation of the coil. Large transformer heat, disk winding have more oil channels and the heat dissipation is good, barrel winding only have the oil channel between high and low voltage, the heat dissipation is poor.

 

Q: Why should the coils of a large capacity transformer be transposed?

A: As the coil of this type of transformer is usually wound with several wires, because the diameter of the coil is large, the length of the wire in the inner and outer rings is much different, so the length of each wire is different.Transposition can make each wire has the same length and ensure the coil resistance balance.

The wires in the inner and outer rings have different reactance value due to the different magnetic field. Transposition allows the wires to be in a similar position in the magnetic field to reduce additional losses in the coil.

 

Q: Transformer coils are immersed in transformer oil, so can transformer coils be treated without paint?

A: The insulation part of the transformer is paper, cardboard, cotton yarn, etc., its insulation performance has been improved after being immersed in oil. Therefore, only from the point of view of the insulation requirements of the transformer, the transformer soaked transformer oil after vacuum drying , can achieve a very high insulation voltage. 

 

Q: Why is there a flexible connection between the bus connections of transformer porcelain bushing in substation?

A: Because the bus is fixed, and the position of the transformer may be slightly moved due to maintenance and other reasons. At the same time, the bus also has the performance of thermal expansion and cold contraction. After the flexible connecting device is installed, the relative position of the bus and the transformer can be slightly changed, which will not cause great stress and damage the porcelain bushing of the transformer.

 

Q: Why are power transformer tap changers usually installed on the high voltage side?

A: Since the current on the low-voltage side is much larger than that on the high-voltage side, the area of the wire required for the tap and the size of the tap switch should be increased accordingly.In this way, not only lead wire joint is not convenient, but also the installation position must be increased.The low voltage coil of core transformer is in the inner side, so it is difficult to draw the tap from the low voltage side.At the same time, the number of turns of the general low-voltage winding is less than the high-voltage winding, therefore, the tap voltage unless it is a turn of the induction voltage integer times, otherwise there is a correct access to the tap voltage.Therefore, the general power transformer tap, are installed in the high voltage side.

 

Q: For the middle line bushing of a power transformer used in a power transformer in a large current grounded system, is it ok to use a low level of insulation bushing?

A: A power transformer used in a large current grounded system in which the wire remains at zero potential (except in certain fault cases) but cannot be connected directly to the ground as often as is required by the operation mode, so a lower level of insulation may be used.This will reduce the cost.However, when this is done, the power transformer cannot be tested for preventive insulation resistance at its rated voltage class, since the neutral point has the same potential as the outgoing line when the coil is pressurized.Therefore, the reliability of the transformer cannot be fully tested in the preventive test.

 

Q: Why do we use flat tubes instead of round tubes for power transformer heat pipes?

A: When the heat dissipation area of the flat pipe is equal to that of the round pipe, the insulation oil in the flat pipe is less than that in the round pipe.That is to say, the amount of oil per unit heat dissipation area of the flat tube is less than that of the round tube. In other words, the flat tube can achieve the same heat dissipation effect with less oil than the round tube.Therefore, the current transformer heat pipe are used flat pipe instead of round pipe.