Selection of power transformers

1. Classification of power transformers

There are many types of power transformers, which can be classified according to the number of phases, voltage regulation method, winding type, winding insulation and cooling method, connection group number, etc.

Power transformers can be classified into single-phase and three-phase types according to the number of phases.

Power transformers can be classified into two types according to their voltage regulation method: off-load voltage regulation and on-load voltage regulation.

Power transformers can be classified into two-winding transformers, three-winding transformers, and autotransformers according to their winding configuration.

Power transformers are classified according to their winding insulation and cooling methods, including oil-immersed, dry-type, and gas-filled (SF6) type. Oil-immersed transformers have cooling methods such as self-cooling, air-cooling, water-cooling, and forced oil circulation cooling. Dry-type transformers have two cooling methods: self-cooling and air-cooling. Using air-cooling can improve the overload capacity of dry-type transformers.

Distribution transformers are classified according to their connection group designation, with the most common types being Yyn0 and Dyn11. Dyn11 transformers have the following advantages over Yyn0 transformers:

1) The large single-phase ground fault current on the low-voltage side is beneficial for clearing single-phase ground faults on the low-voltage side;

2) Strong load-bearing capacity for single-phase unbalanced loads;

3) The delta connection on the high-voltage side helps suppress the injection of 3n harmonic currents into the power grid. Therefore, Dyn11 transformers are increasingly widely used in low-voltage power grids with TN and TT grounding systems. Furthermore, considering lightning protection requirements, Yzn11 type transformers are preferred for areas prone to lightning strikes and areas with high soil resistivity.

The basic structure of a power transformer includes two main parts: the core and the primary and secondary windings. The new S11-MR three-phase wound core fully sealed distribution transformer has significant improvements in structure and materials. Its main feature is that its core is made of high-quality cold-rolled silicon steel sheets with crystalline orientation and annealed, which reduces the air gap at the joints of traditional cores, significantly reduces noise, and its no-load loss is reduced by an average of 30% compared to the S9 type product.

2. Capacity and overload capacity of power transformers

2.1 Rated capacity and actual capacity of power transformers

The rated capacity of a power transformer is the maximum apparent power it can continuously output within a specified service life under specified ambient temperature conditions. The service life of a power transformer primarily depends on the lifespan of the transformer winding insulation material, and is directly related to the temperature of various parts of the transformer during operation. If the allowable temperature rise is exceeded for an extended period during operation, the insulation aging rate will accelerate, and even if an insulation failure does not occur immediately, its lifespan will be significantly shortened. The insulation materials used in power transformers are classified into five levels according to their heat resistance, as shown in the table below.

If the overload multiple and overload time of the transformer exceed the allowable values, the transformer load should be reduced as specified.

Selection of main transformer for substation 3

3.1 Selection of the Number of Main Transformers in the Substation

The following principles should be considered when selecting the number of main transformers:

1) Under normal circumstances, one transformer should be considered first;

2) There are several situations where two or more transformers are selected.

(1) For substations that supply a large number of primary and secondary loads, it is advisable to use two transformers so that when one transformer fails or is under maintenance, the other transformer can continuously supply power to the primary and secondary loads to meet the requirements of power supply reliability.

(2) For substations with large seasonal loads or day-night load variations that are suitable for economical operation, two transformers may also be considered.

(3) In addition to the above situations, if the load is concentrated and the capacity is quite large, even if it is a level 3 load, two or more transformers can be used for general user substations.

3) When determining the number of main transformers in a substation, the development trend of the load should be taken into account, and an appropriate margin should be reserved.

3.2 Selection of Main Transformer Capacity in Substation

my country adopts the R10 capacity series for transformer capacity levels. The capacity levels of this series of transformers increase in multiples of 1.26, such as 100kVA, 125kVA, 160kVA, 200kVA, 250kVA, 315kVA, 400kVA, 500kVA, 630kVA, 800kVA and 1000kVA.

1) Substations with only one main transformer. The transformer capacity ST should meet the total calculated load S30 of all electrical equipment, i.e., ST≈SN≥S30;

2) For substations equipped with two main transformers, the capacity ST of each transformer should meet the following two conditions:

(1) When any transformer is running alone, it should meet 60% to 70% of the total calculated load S30.

(2) When any transformer is running alone, it should meet the needs of all primary and secondary loads.

The capacity of a single main transformer in a workshop substation should generally not exceed 1000KVA (or 1250KVA). This is mainly to ensure that the transformer is located closer to the workshop load center, thereby reducing power loss, voltage loss, and non-ferrous metal consumption in low-voltage distribution lines.

The development of the load should be appropriately considered. Generally, the growth of the power load in the next 5 to 10 years should be taken into account, leaving a certain margin. At the same time, the normal overload capacity of the transformer should be considered.

4. Conclusion

By analyzing the operating characteristics of power transformers and how to select main transformers, it is hoped that those engaged in power distribution operation will combine their own unit's actual situation, such as the selection of the main wiring scheme of the substation, and select the best option after comparing several reasonable schemes technically and economically. At the same time, it is hoped that they will improve their understanding of the performance of power transformers during operation.