Solid State Transformer

Transformers has been in the industry throughout the twentieth century. Until now, it has consisted of a configuration of iron cores and copper/aluminum coils, with mineral oil serving as both coolant and dielectric medium. Inherent in this type of construction are regulation, significant weight, losses, environmental concerns, and power quality issues.

There has been great progress in all other aspects of power plants, be it renewable or conventional. Amongst all this progress, the transformer has remained the same over the years.

This is because they were always highly reliable, relatively inexpensive and fairly efficient. However, with the rapid progress in unconventional energy generation sources like PV, wind, etc., some undesirable properties of the conventional transformer have become a great concern for the satisfactory performance of the transformers. Some of these properties are:

1. Sensitivity to harmonics
2. Voltage drop under load
3. Protection from system disruption and overload
4. Protection of the system from problems arising at or beyond the transformer
5. Environmental concerns arising from using mineral oil
6. Performance under DC-offset load unbalances.

With the advancement of solid-state technology and the development of new materials such as SiC (Silicon Carbide), a new type of power electronics transformers is developed and is referred to as “Solid-State Transformer”. Since these are power electronics transformers, they are non-linear in nature. They can convert either AC to AC or AC to DC or DC to DC and are more complex as compared to the conventional transformer. In some way they are actually power converters. The advantages of this solid-state transformer are:

1. Insensitive to harmonics
2.  Prevents harmonics from propagating in either direction.
3. It has zero regulation
4. Prevents load disruption and faults from affecting the primary system
5. It can supply the load with DC offsets
6. It doesn’t use liquid dielectric

Basic Structure

The basic structure of an SST is depicted above. The isolation is achieved through a High-Frequency transformer. The grid voltage is converted into a higher frequency AC voltage through the use of power-electronics based converters before to be applied to the primary side of the HF transformer. The reverse process is performed on the HF transformer secondary side to obtain an AC and/or DC voltage for the load.

There are different topologies of SST with one-stage, two-stage or three-stage as shown below.

Image credit: B.K Rathore, Advance in Electronics Engineering published in,ISSN 2231-1297, volume 4, 1(2014, PP45-50)

The solid-state transformer offers almost all the same functionalities as the conventional transformer. Additionally, it offers integration to energy storage and medium frequency isolation. The LV DC link in the SST topology provides a good and readily accessible integration point for renewable energy systems into the distribution grid. A unidirectional converter could be used when the load demand is much bigger than renewable energy generation capabilities. 

The SST concept is ideally suited to extend the use of DC, both in MV and LV applications. The difficulty in interrupting a DC feeder under fault conditions is often considered as a major hurdle in the acceptance of DC distribution in MV applications.