This article describes the difference between a power transformer & distribution transformer. Power transformers and Distribution transformers are widely used in power systems for step-up and step-down voltage in transmission and distribution applications.
What is Power Transformer?
The voltage at the generating station is stepped up by a transformer for long-distance transmission to reduce the line losses, and at the receiving end, the voltage is stepped down to the utilization voltage level. The transformer used for this application is known as a power transformer.
Power transformers rating more than 200 MVA is used at 400 kV, 220 kV,132 kV,66 kV, and 33 kV classified in the category of the power transformer. The power transformer acts as a step-up and step-down transformer at sending and receiving ends respectively.
What is Distribution Transformer?
Distribution transformers are used at lower voltage ratings to utilize the voltage at end-use. The distribution transformers are of less than 200 MVA capacity in the voltage grade of 11 kV,6.6 kV, 3.3 kV,0.440 kV, and 230 V.
The secondary voltage tends to be lower and the secondary voltage is used by the end-user.
Basis of Differentiation between Power Transformer & Distribution Transformer
We can differentiate the power transformer and distribution transformer on the following basis.
Size of Transformer
Power transformers are large-rating transformers of higher voltage grade (>200MVA, 400KV), therefore the size of the transformer is bigger than the distribution transformer.
Insulation level
Power transformers have higher voltage grades 400 kV, 220 kV, etc. therefore more insulation is required. The transformer is generally of outdoor duty and the insulation level is kept high to withstand lightning impulses. Distribution transformers have less insulation requirement because the voltage grades of the transformer are below 33 kV.
Efficiency
Power transformers are supposed to be operated at full load and designed for 100 % efficiency. The efficiency of the transformer is calculated by measuring the ratio of the power output to the power input. The loading on the distribution transformer varies all day and its All day efficiency is about 50 -70 %. The efficiency of the distribution transformer is calculated on an all-day basis.
Iron Losses & Copper Losses
Power transformers operate at 100 % loading for 24 hours. Therefore, the copper loss of the transformer should be as minimum as possible to have maximum efficiency. Power transformers are designed to operate at the maximum flux density. The flux density of the power transformer is kept between 1.7- 1.8 Tesla. The higher flux density cause higher core losses and subsequently on account of higher flux density fewer turns are required hence copper loss of the transformer gets reduced at 100 % loading.
The distribution transformer operates all day and loading on the transformer is not constant. Therefore, the distribution transformer is designed to operate at a lower flux density to have low iron losses. The distribution transformer is designed for 60-70 % average loading. The trade-off between iron loss and copper loss is made to have the best efficiency of the distribution transformer.
Key Differences between Power Transformer and Distribution Transformer
| Point of Comparison | Distribution Transformer | Power Transformer |
|---|---|---|
| Definition | A distribution transformer reduces the high voltage to a low voltage. | Power transformers raise the voltage at generating end and lower the voltage at receiving end. |
| Purpose | A distribution transformer is used to lower the high voltage for further distribution of power to consumers. | Power transformers are used to increase the voltage level to a high value for the transmission of electric power over long distances. |
| Location | A distribution transformer is installed near the load centers that supply power to consumers. | Power transformers are installed at the generating station and transmission substations, i.e., in the power system’s transmission section. |
| Function | The distribution transformers only reduce the voltage level. | Power transformers are generally used for increasing the voltage level and for lowering the voltage at the receiving substation. |
| Power rating | Less than 200 MVA. | More than 200 MVA. |
| Voltage rating | 11 kV, 6.6 kV, 3.3 kV, 440 V, 220 V, 110 V, etc. | 33 kV, 66 kV, 132 kV, 220 kV, 440 kV, and above. |
| Winding configuration | The distribution transformer has delta-connected primary winding, and star-connected secondary winding. The star configuration provides a neutral point for single-phase loads. | The power transformer has delta- delta configuration for the primary and secondary winding. However, the star-delta or delta-star configuration is also used for special applications. |
| Number of primary and secondary | A distribution transformer has a single primary winding and it may have more than two secondary winding. | The power transformer has single primary and secondary winding. |
| Size | smaller in size. | larger in size |
| Operating period | Economic for round-the-clock operation | Economic operation only during the load periods. |
| Operation during light loads | Operate at light loads during the day & night. | Not economical if operated during light load |
| Load fluctuations | They can withstand high load fluctuations. | They are designed to sustain low load fluctuations. |
| Flux density in the core | Distribution transformers are designed for lower flux density to have low no-load losses. | Distribution transformers are designed for higher flux density. Therefore, they have more no-load losses. |
| Operating conditions | For economic operation, the distribution transformers are operated below their rated full load. | For economic operation, the power transformers are always operated at full load. |
| Iron Loss | Lower Iron loss | Higher iron loss |
| Copper loss | Higher Copper loss | Lower Copper loss |
| Maximum efficiency | Distribution transformers have maximum efficiency at 60 to 70% of the rated load. | Power transformers have maximum efficiency at full load. |
| Leakage reactance | Distribution transformers have small leakage reactance. | Power transformers have high leakage reactance. |
| Voltage regulation | Distribution transformers supply power to consumers, therefore they have good voltage regulation. | Voltage regulation is not an important factor in the case of power transformers. |
| Applications | Distribution transformers supply electrical power to consumers. | Power transformers are used for generating stations and power-receiving substations. |
Conclusion
From the above discussion, It is clear that there are various differences between power transformers and distribution transformers. On the basis of differences between the power transformers and distribution transformers, we can select the transformer as per the applications.