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Selecting Transformers for Fast Charging Applications

Fast charging is emerging as an important factor in accelerating EV adoption. This type of charger allows the consumer to charge their car to max range in 15 minutes. To enable charging power draw has increased to 350kW or more for passenger car type of applications. In addition to higher power requirements EV charging also introduces certain complexities. Transformer is a significant portion of the capital investment for a charging station. Hence understanding transformer selection criteria is very important to ensure longer transformer life and optimized total cost of ownership for the station operator. Standard off the shelf transformers in most cases are not able to address the unique needs of EV charging stations. Transformers need to be customized for high power EV charging applications. Some of the factors to consider are:

  • Cooling: Because of the high-power draw 350 kW of more of fast chargers it is recommended to use oil filled transformer designs to increase transformer life. The temp increase due to hi short time current is too high for dry type. Additionally, High ambient temperatures will reduce the life and power requirements may result in poor performance if dry type transformers are used.
  • Harmonics: EV charger power electronics inject 3rd order and associated triplen harmonics into the system. These harmonic distortions are additive in terms of current and volage and induce additional heating effects which can damage the insulation of standard off the shelf transformers. The life will be halved for every 10 deg C increase in temp. To mitigate the harmful effects of harmonics, transformers need to be designed with k factors of 4 to 8 allowing the transformer insulation to withstand the additional heat generated due to Harmonics.
  • Small footprint and optimized connections: Hardware installation cost is a major portion often exceeding half the project cost. In addition, with real estate being at a premium it is important to design transformers with small footprint of oil type will reduce installation costs.
  • Remote monitoring: EV charging station operators often require access to data on power consumption and transformer operation. Many of these applications need transformers equipped with sensors which can measure power and transformer operation characteristics and the ability to transfer the data to station monitoring software.

The cost increase of a correctly designed transformer for a longer life is not much higher than off the shelf.

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