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Power transformer 17 Q&A! See what you know...

1.Why should the core of the transformer be grounded?

2.Why do transformers use silicon steel sheets as cores?

3.What is the scope of gas protection?

4.What are the differences between main transformer differential and gas protection?

5.How to deal with the fault of main transformer cooler?

What to know more, see below

2021/12/08
Power transformer 17 Q&A! See what you know...

01

Why should the core of the transformer be grounded?


In the normal operation of a power transformer, the iron core must be securely grounded. If there is no grounding, the suspension voltage of the iron core to the ground will cause the intermittent breakdown and discharge of the iron core to the ground, and the possibility of forming the suspension potential of the iron core is eliminated after the iron core is grounded. However, when the core is grounded at more than two points, the uneven potential between the cores will form a circulation between the ground points and cause the multi-point grounding heating fault of the core.


The ground fault of the iron core of the transformer will cause the local overheating of the iron core. In serious cases, the local temperature rise of the iron core will increase, light gas action, and even the trip accident of heavy gas action will be caused. The short-circuit fault between the iron chips is formed by the burnt partial iron core, which increases the iron loss and seriously affects the performance and normal operation of the transformer, so that it is necessary to replace the iron core silicon steel sheet for repair. So transformers do not allow multi-point grounding and only one point grounding.


02

Why do transformers use silicon steel sheets as cores?


The common transformer core is generally made of silicon steel sheet. Silicon steel is a kind of silicon (silicon also known as silicon) steel, its silicon content in 0.8 ~ 4.8 %. The core of the transformer is made of silicon steel, because silicon steel itself is a magnetic material with strong magnetic conductivity. In the energized coil, it can produce large magnetic induction intensity, so that the volume of the transformer can be reduced.


As we know, the actual transformer always works in the ac state, and the power loss is not only in the resistance of the coil, but also in the iron core magnetized by the alternating current. The power loss in iron core is usually called "iron loss". Iron loss is caused by two reasons, one is "hysteresis loss" and the other is "eddy current loss".


Hysteresis loss is the iron loss caused by hysteresis phenomenon in the magnetization process of iron core. The size of this loss is proportional to the area surrounded by the hysteresis loop of material. The hysteresis loop of silicon steel is narrow, and the hysteresis loss of iron core used as transformer is small, which can greatly reduce its heating degree.


Since silicon steel has the above advantages, why not use the whole silicon steel as the core, but also process it into sheets?


That's because the flake core reduces another kind of iron loss called eddy current loss. Transformer operation, there is alternating current in the coil, it produces magnetic flux is of course alternating. This changing flux creates an induced current in the iron core. The induced current generated in the iron core circulates in the plane perpendicular to the direction of magnetic flux, so it is called eddy current. Eddy current losses also heat the core. In order to reduce eddy current loss, the transformer's iron core is stacked with silicon steel sheets that are insulated from each other, so that the eddy current passes through a small section in a long and narrow circuit to increase the resistance in the eddy current path. At the same time, the silicon in the silicon steel increases the resistivity of the material, and also plays a role in reducing the eddy current.


For the iron core used as transformer, cold-rolled silicon steel sheet 0.35mm thick is generally used. According to the size of the required iron core, it is cut into long sheets and then overlapped into "sun" shape or "mouth" shape. In principle, in order to reduce eddy current, the thinner the silicon steel sheet is, the narrower the splicing strip is, and the better the effect is. This not only reduces eddy current loss and temperature rise, but also saves the material of silicon steel sheet. But in fact when making silicon steel sheet iron core. Not only from the advantages mentioned above, for making the core would require a considerable increase in man-hours and a decrease in the effective cross section of the core. Therefore, when making transformer core with silicon steel sheet, we should start from the specific situation, weigh the advantages and disadvantages, and choose the best size.


03

What is the scope of gas protection?


1) Multiphase short circuit inside the transformer.

2) Short circuit between turns, short-circuit between winding and iron core or shell.

3) Iron core failure.

4) Oil under the surface or oil leakage.

5) Poor contact of tap switch or welding of wire is not firm.


04

What are the differences between main transformer differential and gas protection?


1, the main transformer differential protection is designed and manufactured according to the principle of circulating current, and the gas protection is designed and manufactured according to the characteristics of gas generated or decomposed when the transformer internal failure.


2. Differential protection is the main protection of the transformer, and gas protection is the main protection of the internal fault of the transformer.


3, according to different protection scope:

A Differential protection:

1) The main transformer lead line and transformer coil have multiphase short circuit.

2) serious single-phase inter-turn short circuit.

3) Grounding fault of protection coil and lead wire in high current grounding system.

B Gas protection:

1) Transformer internal multiphase short circuit.

2) inter-turn short circuit, inter-turn and core or outside and short circuit.

3) Iron core failure (heating and burning loss).

4) Oil under the surface or oil leakage.

5) Poor contact of tap switch or poor welding of wire.


05

How to deal with the fault of main transformer cooler?


1. When the working power supply of the I and II sections of the cooler is lost, the signal of "#1, #2 power failure" will be sent out. The main transformer cooler will stop and trip circuit is connected.


2. In case of failure of switching power supply in section I and II during operation, "cooler all stops" is lit up, and then the main transformer cooler all stops and trip circuit is connected. The set of protection shall be immediately reported to dispatching and disabled, and manual switching shall be carried out quickly.


3. When any one of the cooler circuits fails, isolate the faulty cooler circuit.


06

What are the consequences of parallel operation of transformers that do not meet the conditions of parallel operation?


When the variable ratio is not the same and parallel operation, there will be circulation, affecting the output of the transformer, if the percentage impedance is not consistent and parallel operation, it can not distribute the load according to the proportion of the capacity of the transformer, but also affect the output of the transformer. When the wiring groups are not the same and run in parallel, the transformer will short circuit.


07

What causes the abnormal sound of the transformer?


1) over load;

2) Poor internal contact, discharge ignition;

3) Some parts are loose;

4) There is grounding or short circuit in the system;

5) Large motor starting causes relatively large load changes.


08

When is the tap switch of the on-load voltage regulator of the transformer not allowed to be adjusted?


1) Transformer overload operation (except for special cases)

2) When light gas protection of on-load pressure regulating device frequently appears signal.

3) when there is no oil in the oil mark of the on-load pressure regulating device.

4) When the number of regulating pressure exceeds the specified number.

5) Abnormal occurrence of pressure regulating device.


09

What are the ratings on the transformer nameplate?


Transformer rating is the manufacturer's regulation for normal use of transformer, transformer in the specified rating state of operation, can ensure long-term reliable work, and have good performance. Its rating includes the following:


1, rated capacity: transformer in the rated state of the output capacity of the guaranteed value, unit with volt-ampere (VA), kilovolt-ampere (kVA) or mega volt-ampere (MVA), because the transformer has a high operating efficiency, usually the original and secondary winding rated capacity design value is equal.


2, rated voltage: refers to the guaranteed value of the terminal voltage of the transformer when no load, expressed in volts (V) and kilovolts (kV). Unless otherwise specified, the rated voltage is the finger-line voltage.


3. Rated current: refers to the line current calculated from rated capacity and rated voltage, expressed in ampere (A).


4, no-load current: transformer no-load operation excitation current in rated current percentage.


5, short circuit loss: one side of the winding short circuit, the other side of the winding voltage so that both sides of the winding to reach the rated current active loss, expressed in watts (W) or kilowatt (kW).


6, no-load loss: refers to the active power loss of the transformer in no-load operation, expressed in watts (W) or kilowatts (kW).


7, short circuit voltage: also known as impedance voltage, refers to one side of the winding short circuit, the other side of the winding to reach the rated current applied voltage and rated voltage percentage.


8. Connection group: indicates the connection mode of primary and secondary windings and the phase difference between line voltages, which is represented by clock.


10

Why do current source converters need large transformer capacity?


Transformers are generally designed for rated capacity, not rated power, because their current is only related to rated capacity. For voltage-source converters, the rated capacity and rated power are almost equal because the input power factor is close to 1. Current source converter is not, its input side transformer power factor is at most equal to the load of asynchronous motor power factor, so for the same load motor, its rated capacity is larger than the voltage source converter transformer.


11

What does the capacity of a transformer relate to?


The choice of iron core is related to voltage, and the choice of wire is related to current, that is, the thickness of wire is directly related to calorific value. That is to say, the capacity of the transformer is only related to the calorific value. For a designed transformer, if the heat dissipation is not good in the environment, if it is 1000KVA, if the heat dissipation capacity is enhanced, it is possible to work in 1250KVA.


In addition, the nominal capacity of the transformer is also related to the allowable temperature rise, for example, if a 1000KVA transformer, the allowable temperature rise is 100K, if in special circumstances, it can be allowed to work up to 120K, its capacity is more than 1000KVA. It can also be seen that if the heat dissipation conditions of the transformer are improved, its nominal capacity can be increased. Conversely, for the same capacity of the converter, the volume of the transformer cabinet can be reduced.


12

How to improve transformer efficiency?


1) Try to choose low loss, high efficiency and energy saving transformer

2) According to the load, choose a reasonable capacity transformer

3) The average load factor of the transformer should be greater than 70%

4) when the average load coefficient is often less than 30%, the small capacity transformer should be replaced as appropriate

5) Improve the load power factor to improve the ability of the transformer to transmit active power

6) Reasonable configuration of load, as far as possible to reduce the number of transformer operation


13

Why to speed up the technical transformation of high energy consumption distribution transformer?


High energy consumption distribution transformers mainly refer to: SJ, SJL, SL7, S7 and other series transformers, the iron loss, copper loss are much higher than the S9 series transformers widely used at present, such as S7 compared with S9, iron loss 11% higher, copper loss 28% higher.


And the new transformer, such as S10, S11 transformer than S9 energy saving, amorphous alloy transformer iron loss is only equivalent to S7 20%. Transformers generally have a service life of several decades. Replacing high-energy transformer with high-efficiency energy-saving transformer can not only improve energy conversion efficiency, but also save electricity in the life period.


14

What is eddy current? What are the harmful effects of vortex generation?


When an alternating current passes through a wire, an alternating magnetic field is created around the wire. The whole conductor in the alternating magnetic field will produce induced current inside, because this induced current in the whole conductor itself into a closed loop, much like water vortex, so called vortex. Eddy current will not only waste power, reduce the efficiency of electrical equipment, and will cause the use of electrical appliances (such as transformer core) heating, serious will affect the normal operation of equipment.


15

Why should transformer transient protection avoid low voltage short circuit current?


Mainly considering the selectivity of relay protection movement, high side quick break protection mainly, severe external protection of transformer faults is if you don't avoid the low voltage side of transformer in setting the maximum short-circuit current, due to the low voltage side is not far from the export of a range of short circuit current value is not big, the basic equal, this will make high side quick break protection expanded to low pressure, So you lose the selectivity. After losing the selective protection more reliable, but to allow the inconvenience, such as now there are many industrial set 10 kv transformer room always (10 kv bus + outlet circuit breaker), every workshop set low voltage transformer room (ring network cabinet + transformer), if the circuit breaker is not escape the low voltage side of transformer maximum short-circuit current will cause low voltage main switch, (ring network cabinet load switch fuse), High voltage circuit breaker action, bring inconvenience to operation.


16

Why can't two parallel transformers be grounded at the same time?


In high current system, in order to meet the requirements of relay protection sensitivity coordination, one part of the main transformer is grounded, and the other part is ungrounded.


The neutral points of two main transformers in one station are not grounded at the same time, so the coordination of zero sequence current and zero sequence voltage protection is mainly considered.


In a substation with multiple transformers running in parallel, one part of the neutral points of the transformer is grounded and the other part is ungrounded. In this way, the level of ground fault current can be limited in a reasonable range, and the size and step of the entire grid zero-sequence current can not be affected by the change of operation mode, and the sensitivity of zero-sequence current protection of the system can be improved.


17

Why does the newly installed or overhauled transformer have to do impact closing test before it is put into operation?


Excision of no-load transformers operating in the grid will result in operating overvoltage. In low-current grounded systems, the amplitude of the so-called overvoltage may be 3 to 4 times the rated phase voltage; In large grounded systems, the operating overvoltage may also be up to 3 times the rated phase voltage. Therefore, in order to test whether the insulation of the transformer can withstand the rated voltage and operating overvoltage, several impact closing tests should be carried out before the transformer is put into operation. In addition, the input of no-load transformer will produce inrush current, its value can reach 6 ~ 8 times of the rated current. Because the excitation inrush current will produce a lot of electrical power, so do the impact closing test or consider whether the mechanical strength of the transformer and relay protection will misoperate effective measures.


Source: Windows on Power


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