Transformer capacity calculation

Transformer capacity calculation - Temporary elimination method when the transformer cannot be stopped: External lead wires, when the fault current is large, can temporarily open the line operation. However, monitoring must be strengthened and the cores appear to have a floating potential after the fault point disappears. Multi-point faults are unstable, and a slip wire resistance can be connected in the working wiring to make the current limit 1A or less. Slip line resistance selection is to divide the normal operating wiring open test voltage by the line current.

Transformer capacity calculation method:

1. Calculate the capacity according to the load factor βM at which the efficiency of the transformer is highest.

After the calculation load of the building is determined, the total installed capacity of the distribution transformer is: S=Pjs/βb×cosφ2(KVA) (1)

In the formula, Pjs  - the active calculation load KW of the building;

Cosφ2 - the average power factor after compensation, not less than 0.9;

Βb - transformer load factor. 

Therefore, the final determination of the transformer capacity lies in the selected transformer load factor βb. 

The highest efficiency is βb=βM=Po/PKH(2)

In the formula Po, the no-load loss of the transformer;

PKH  - short circuit loss of the transformer. 

However, in high-rise buildings, equipment rooms are mostly located in the basement. To meet the requirements of fire protection, general information about distribution transformers is available at: Power Transmission and Distribution Equipment

With dry or epoxy cast transformers, Table 1 is the best load rate of domestic SGL power transformers. 

The best load rate of domestic SGL power transformers

Capacity (KVA) 500 630 800 1000 1250 1600

No-load loss (watts) 1850 2100 2400 2800 3350 3950

Load loss (Watts) 4850 5650 7500 9200 11000 13300 Source: http://tede. Cn

Loss ratio α2: 2.62 2.69 3.13 3.20 3.28 3.37

Optimal load rate βm% 61.8 61.0 56.6 55.2 55.2 54.5