What is the loss of a transformer? How to calculate the loss rate of a transformer?

When the primary winding of the transformer is energized, the magnetic flux generated by the coil flows in the iron core, because the iron core itself is also a conductor, and the electric potential will be induced in the plane perpendicular to the magnetic field lines, and this electric potential will form a closed loop on the section of the iron core and generate a current, as if p a vortex is called "eddy current". This "eddy current" increases the loss of the transformer and increases the temperature rise of the transformer's core heating transformer. The loss caused by "eddy currents" is called "iron loss".

In addition, a large number of copper wires are needed to wind the transformer, and these copper wires have resistance, and this resistance will consume a certain amount of power when the current flows through, and this part of the loss is often turned into heat and consumed, which we call "copper loss". Therefore, the temperature rise of the transformer is mainly caused by iron loss and copper loss. Due to the iron and copper losses of the transformer, its output power is always less than the input power, so we introduce an efficiency parameter to describe this, = output power input power.

Characteristics of transformer losses.

- no-load losses, mainly iron losses, including hysteresis losses and eddy current losses;

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

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

c - load loss, mainly the loss of load current through the winding on the resistance, generally called copper loss. Its magnitude varies with the load current and is proportional to the square of the load current; (and expressed as a standard coil temperature conversion value).

Load losses are also affected by the temperature of the transformer, while leakage flux caused by load current can generate eddy current losses within the windings and stray losses in the metal parts outside the windings.

The total loss of the transformer δ = +c

Loss ratio of transformer = c

Efficiency of the transformer = ( expressed as a percentage; where is the output power of the secondary side of the transformer.

The no-load loss value is small, and the load loss is the largest when the transformer reaches the rated output.

The national standard 1094-1 stipulates that the total loss deviation is +10%, and the no-load loss and reload loss are +15%, but the total loss shall not exceed +10%. In addition, GB T6451 specifies the specific values of losses for different voltage levels and different capacities.

The reactive power loss of the transformer under the rated load can be calculated by the following formula: q0=(i0% 100)se; △ql=(udl%/100)se

The standard also describes that some self-coupling transformers and step-up transformers should have a greater deviation due to their small impedance. Special consideration will also be required for transformers with a large tap range, especially when the tap range is asymmetrical.

On the other hand, when the transformer is to be connected in parallel with the existing transformer, according to the agreement, a smaller impedance deviation can be specified, but it should be proposed in the bidding stage and agreed upon by the manufacturer and the user.

Formula for calculating transformer loss.

1) Active loss: δP P0 kt 2pk

2) Reactive power loss: δq q0 kt 2qk

3) Comprehensive power loss: δpz δp kqδq

q0≈i0％sn，qk≈uk％sn

Where: q0 - no-load reactive power loss (kvar).

p0 – no-load loss (kw).

PK – Rated load loss (kw).

SN – transformer rated capacity.

kva)i0 – the percentage of no-load current of the transformer.

uk – percentage of short-circuit voltage.

Average load factor.

kt - load fluctuation loss factor.

QK - Rated load flux leakage power (kvar).

kq – reactive economic equivalent (kw kvar).

The selection conditions for each parameter in the calculation of the above equation are:

1) Take KT;

2) When the minimum load of the 6kV 10kV step-down transformer of the urban power grid and the industrial enterprise power grid is taken, the reactive power equivalent kq 0 1kw kvar;

3) transformer average load factor, for agricultural transformers can be preferable 20; For industrial enterprises, a three-shift system is implemented.

Desirable 75 ;

4) Transformer operating hours t 8760h, maximum load loss hours: t 5500h;

5) No-load loss of transformer P0, rated load loss Pk, I0, UK, see product information.

The main factor that affects the service life of a transformer is temperature.

The oil temperature at the top of the transformer is generally adjusted at 85, if it exceeds 85, the reasons should be analyzed: 1. If the room temperature is too high, the load is too heavy, etc., it can continue to run more than 85, but the maximum can not exceed 95 (at this time, the core or winding of the transformer center is 105, which will seriously damage the insulation, shorten the service life or burn the transformer); 2. When the Sihz transformer is more than 85 running, the temperature difference between the oil temperature at the top of the transformer and the room temperature cannot exceed 55, which may be serious overload, too low voltage, too large current, internal faults, etc., and continued operation will seriously damage the insulation, shorten the service life or burn the transformer.

National Standard "Dry-type Transformer.

GB6450-1986 stipulates the temperature rise limit of dry-type Zhongluliang transformer. 1. For the coil of the dry-type transformer, the A-class insulating material should be used.

, when the ultimate working temperature is 105, the maximum temperature rise should be less than 60K; When the E-class insulating material is used, when the ultimate working temperature is 120, the maximum temperature rise should be less than 75K; When the B-grade insulating material is used, the maximum temperature rise should be less than 80K when the ultimate working temperature is 130K; When the F-class insulating material is used, the maximum temperature rise should be less than 100K when the ultimate working temperature is 155; When H-class insulating material is used, the maximum temperature rise should be less than 125K when the ultimate working temperature is 180. When the C-class insulating material is used, the maximum temperature rise should be less than 150K when the ultimate working temperature is 220K.

Transformers are stationary electrical devices, so there is no mechanical loss during energy transfer, so their efficiency is higher than that of rotating electrical machines. Generally, the efficiency of small and medium-sized power transformers is more than 95%, and the efficiency of large power transformers can reach more than 99%. The losses generated by the transformer mainly include iron losses and copper losses in the primary and secondary windings.

The iron loss of the transformer is the hysteresis and eddy current loss in the iron core, which is determined by the magnitude of the magnetic flux density in the iron core, the frequency of the magnetic flux alternation and the quality of the silicon steel sheet. The iron loss of the transformer is approximately proportional to the applied supply voltage U21 in the primary winding, regardless of the load size. When the power supply voltage is constant, the iron loss of the transformer is basically unchanged, so the iron loss is also called "constant loss".

The basic copper loss in the transformer copper loss is the loss of the current on the DC resistance of the primary and secondary windings i21rk. The magnitude of the transformer's copper loss is proportional to the square of the load current, hence the term "variable loss".