# How to select a transformer based on capacity rational calculation

In today’s article, I would like to show you how to choose a transformer based on the calculation of capacity rational.

1 / Choose the number of MBA

Transformer selection includes the choice of quantity, capacity, type, style, and other features of the transformer.

The number of transformers placed in one station depends on the reliability of the power supply for that substation

Load type 1: is an important load, not allowed to lose power, the two transformers.

Secondary loads: such as factories, supermarkets, etc. usually use a transformer and a backup generator.

Third load: light load for living, apartment, school, village often put a transformer.

2 / Select the MBA capacity

2.1 / For load with Stt:

Inside:

– SDB: transformer rated power, supplied by the manufacturer.

– Stt: computing power, is the maximum required load capacity.

– 1.4: overload factor.

♦ Temperature correction coefficient:

The above formulas apply only to domestic or tropical production machines. If the imported machine has to enter the temperature correction factor formula to the temperature difference between the manufacturing environment and machine environment:

Inside:

– t0: manufacturing environment temperature 0C

– t1: environmental temperature using 0C

2.2. For loads with load graph:

♦ Equivalent power method:

Frequency overload can often be determined from the MBA overload capability curve. This is the relationship between the overload factor that allows K2cp, the load factor of a K1 step and the overload time t2. In order to use this method, it is necessary to transform the multi-step load chart of fMBA into an isotropic load graph

The equivalent power of the MBA during the review period is determined by the expression:

Where: Si is the load of the MBA at about ti.

When transforming the multiple-load graph into a two-order isotropic load graph, there may be the following cases:

– The multi-level load chart of the MBA has one maxima in the afternoon: According to the expression (5.5), calculate S’dt2 with time of overload of t2 and calculate S’t1 with time before 10h overload (Fig. 5.3).

– The multi-level load chart of the MBA has a maxima in the morning: According to the expression (5.4), the S’dt2 is calculated with the time of overloading of t2 and the calculation of Td1 with the time immediately after the overload of 10h (Figure 5.4) ..

– If the load chart of the MBA has two maxima per day (Figure 5.5), the second load is the maximum value of the isomorphic load. Then choose S’dt2, and Sdt1 will count as one of the two cases.

If S’dt2 <0.9.Smax then choose S’dt2 = 0.9.Smax. The second time is calculated as follows:

T’2 = (S’t2) 2.t2 / (0.9.Smax) 2 (5.5)

If the MBA works in places where the average annual temperature is greater than the annual average normative temperature, then the equality of chance is adjusted according to the following expression:

After transforming the multi-tier load chart of the MBA into a two-tier load graph, the permissible load-determining sequence of the MBA according to the load capacity curve is as follows:

Calculate K1 = S1 / Sm, K2 = S2t / Sm.

From K1 and t2, look up the permissible curves for the MBA to find K2cp and compare it to K2 above. If K2 K2cp then MBA is acceptable, vice versa need to change the machine capacity.

♦ 3% overload rule:

With the above equality method, if there is no overload curve allowed by the MBA, the normal overload factor can be determined according to the 3% rule:

9

Where: Kdk is the fill factor of the load graph is calculated as follows:

ten

When the station has two machines, note the possibility of overloading the machine. This overload capability is determined by fabrication. If no specific information can be accepted 140% for Soviet machines provided that the previous load factor does not exceed 0.93 and 130% for other machines of IEC 354. The number of MBAs can be chosen according to the following expression:

Sdm = Smax / (n – 1). Kqtsc (5.9)

Inside:

Smax is the maximum load.

Kqtsc is an MBA overload factor.

N is the number of MBAs in the station.