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The difference between no-load loss and no-load current, short-circuit loss and short-circuit impedance of transformer

source: network

po no-load loss refers to the active power absorbed by the transformer when the rated voltage at the rated frequency is applied to the terminal of one winding of the transformer and the other windings are open circuit, which is also called iron loss (ignoring the resistance loss of the pressure winding under no-load operation)

its value reflects the energy consumed by the transformer under no-load, including hysteresis loss and eddy current loss. The hysteresis loss is proportional to the frequency and the power of the hysteresis coefficient of the maximum flux density; Eddy current loss is directly proportional to the product of the completion frequency of the initial evaluation of ELG's materials, the maximum magnetic flux density and the thickness of silicon steel sheet by the fanguanghong research team of the advanced manufacturing technology research center of the General Academy of Mechanical Sciences. The material characteristics, thickness, lamination method and process of silicon steel sheet for transformer core directly affect the value of Po, which is independent of the reference temperature

io no-load current refers to the current flowing through the line terminal of the winding when the rated voltage of the rated frequency is applied to one winding of the transformer, and the other windings are open circuited. It is the current absorbed from the electricity when the transformer is not loaded. For three-phase transformers, it is the arithmetic mean of the current flowing through the three-phase terminals. Among them, the smaller active component IO (R) is used to compensate the loss of the core, that is, the no-load loss Po, and the larger reactive component IO (x) is used for excitation to balance the magnetic voltage drop of the core

generally, IO is expressed as a percentage of rated current: IO% = (io/in) *100= 0.1~3%

there is no fixed and inevitable relationship between the size of no-load loss and the size of no-load current. For two transformers produced in the same batch with the same specification, the no-load loss can be basically equal, while the no-load current can be very different. From the perspective of transformer, the no-load current basically has no impact on the reliability of the transformer, and slightly increases the operating cost, but it is very small. However, transformers with large no-load current tend to be noisy, because the joints of the transformer core are relatively large, The size of no-load current mainly depends on the size of the joint and the material of the transformer. The no-load loss mainly depends on the material and the magnetic flux density at the time of design

PK short circuit loss, also known as rated load loss, refers to the active power absorbed by the transformer from the power supply when the secondary winding of the transformer is short circuited and the primary winding applies voltage to make its current reach the rated value, which is called short circuit loss, and short circuit loss is also called copper loss

uk short circuit voltage, also known as impedance voltage and short circuit impedance, refers to the voltage to be applied when the secondary winding of the transformer is short circuited and the primary winding flows the rated current. Generally, UK is expressed as a percentage of the rated voltage, that is, uk= (uk/un) × 100% 。

impedance voltage refers to the internal impedance of the transformer, which is the voltage drop caused by its own impedance when the winding passes through the rated current during the operation of the transformer

the impedance voltage percentage of transformer is a very important value, which is the basis for calculating short-circuit current. He showed that the impedance voltage drop of the transformer itself when the transformer is operating at full load (rated load) is of decisive significance to how much short-circuit current will be generated when a sudden short circuit occurs in the secondary measurement of the transformer, and it is also of great significance to the parallel operation of the transformer

generally speaking, when the transformer capacity is small, the percentage of impedance voltage is also small; When the transformer capacity is large, the percentage of impedance voltage should be large. The percentage of impedance voltage of power transformers produced in China is generally in the range of 4~24%

impedance voltage is an important economic and technical index related to transformer cost, efficiency and operation. For transformers with the same capacity, the impedance voltage is small, the cost is low, the efficiency is high, and the price is cheap (normal transformers, except for very small ones). In addition, the voltage drop and voltage change rate during operation are small, and the voltage quality is easy to be controlled and guaranteed. Starting from the operating conditions of the transformer, we hope that the impedance voltage will be smaller

starting from the condition of limiting the short-circuit current of the transformer, it is hoped that the impedance voltage will be larger and better, so as to prevent the electrical equipment such as circuit breakers, disconnectors, cables, etc. from being damaged by the short-circuit current during operation. Therefore, when manufacturing the transformer, the impedance voltage must be designed according to the operating conditions of the equipment, and should be as small as possible

the loss composition and calculation method of power transformer are briefly introduced below

power transformer losses are divided into iron losses, copper losses and additional losses. Iron loss can also be called no-load loss (ignoring the resistance loss caused by no-load current), which is a constant loss, which is actually the loss caused by the iron core, while copper loss is also called load loss (ignoring the iron loss caused by the excitation current on the excitation resistance), which is a variable loss, which refers to the copper loss caused by the current in the primary and secondary coils of the transformer on the resistance. Additional losses include additional iron loss and additional copper loss, which are negligible because they are small and difficult to measure

1. Power transformer loss calculation formula

(1) active power loss: δ p＝po＋kt β 2 pk

(2) reactive power loss: δ q＝qo＋kt β 2 qk

(3) comprehensive power loss: δ pz＝ δ p＋kq δ Q

qo ≈ io%sn, QK ≈ uk%sn

where: Qo - no-load reactive loss (kvar)

po - no-load loss (kw)

pk - rated load loss (kw)

sn - rated capacity of transformer (kVA)

uk% - percentage of short-circuit voltage

β —— Load factor is the ratio of load current to rated current

kt -- load fluctuation loss coefficient

qk -- the initial experimental results show that the rated load magnetic leakage power (kvar)

kq -- the economic equivalent of reactive power (kw/kvar)

the selection conditions of various parameters in the above formula calculation:

(1) take KT = 1.05

(2) for the 6kv~10kv step-down transformer of urban power and industrial control system supported by software system, when the minimum load of the system is taken, its reactive power equivalent KQ = 0.1kw/kvar

(3) average load factor of transformer, which can be taken for agricultural transformer β＝ 20%； For industrial enterprises, it is advisable to implement the three shift system β＝ 75%；

(4) transformer operation hours t = 8760h, maximum load loss hours: T = 5500h

(5) transformer no-load loss Po, rated load loss PK, IO%, UK%, see the factory data of the product

2. Characteristics of power transformer loss

po - no-load loss, mainly iron loss, including hysteresis loss and eddy current loss

hysteresis loss is proportional to frequency; It is proportional to the power of the hysteresis coefficient of the maximum flux density

eddy current loss is proportional to the product of frequency, maximum flux density and thickness of silicon steel sheet

pc - load loss, mainly the loss of resistance when the load current passes through the winding, which is generally called copper loss. Its size changes with the load current and is proportional to the square of the load current; (and expressed by the converted value of standard coil temperature)

the load loss is also affected by the temperature of the transformer. At the same time, the magnetic flux leakage caused by the load current will produce eddy current loss in the winding and stray loss in the metal part outside the winding

total loss of transformer δ P=po+pc

loss ratio of transformer =pc/po

efficiency of transformer =pz/(PZ)+ δ p) , expressed as a percentage; Where PZ is the output power at the secondary side of the transformer

source: Luo

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