Solid-state transformer (SST) is an emerging technology, which integrates high frequency transformers, power electronic converters and control circuitry. It aims to replace conventional line-frequency distribution transformers with “smart” solutions. The SST technology could influence the developments in many areas such as smart-grids, traction systems, the system with renewable energy sources (RESs) to mention just a few. Although SST structure is much more complex compared to conventional transformers, it may eliminate some of the disadvantages and add a completely new functionality that was not yet available, such as:

• The presence of DC-link allows to direct connection of DC grid as well as additional energy storage components, eliminating the need for application of additional power converters,

• Proper operation with voltage unbalance, voltage dips and other voltage disturbances that will not transferred from one side of the transformer to the other,

• SST is able to fully control the amount and direction of the power flow and can compensate reactive power independently on both sides,

• SST can dynamically control voltage at LV side and compensate for disturbances caused by disturbing non-linear loads,

• SST can communicate each other and with other elements of the power grid, forming so called Smart Grid (SG). SSTs exchanging data each other, may also control the flow of energy and its parameters to reconfigure the power grid in case of failure and adapt it to work in new conditions, limiting the negative effects for end users.

Industrial implementation of SST requires use of standard components, as well as flexibility in design. Such objectives can be met with use of Power Electronic Building Blocks (PEBB) approach. Due to the voltage rating limitation of existing power switching devices, this document only focuses on 1200V SiC MOSFETs and Medium Voltage (MV) applications. For power distribution, MV mostly means the voltage range between 2kV and 35kV line-to-line.          

SST is designed to replace conventional distribution transformers that are to convert MV AC to LV AC and DC at power levels ranging from few hundred kVA up to few MVA. Typical arrangement of SST with three conversion stages is presented in Figure 1. SST performs non-isolated MV AC/DC, isolated DC/DC and non-isolated DC/LV AC power conversion. The three-stage SSTs are typically designed for the purposes of smart grid applications in which it has bidirectional

power flow to transfer power between HV and LV, as shown in Figure 2.

                            Figure 1.  SST  Structure.                                                                                                        Figure 2. Typical SST in smart grid applications.

For MV and HV applications a specific class of converters have been proposed – Modular Multilevel Converters (MMC)， which is based on single-phase half-bridge converters, full-bridge three-level converters, as shown in Figure 3. High-level converter is not commonly used due to its complexity.

                   Figure 3a. 2-Level Uni-pole Half bridge                                                                                             Figure 3b. 3-Level Bi-pole Full bridge                   

A half bridge provides +Vdc and 0V outputs, while a full bridge provides three-level outputs: +Vdc, -Vdc, 0V. The half bridge and full bridge’s MOSFETs’ voltage rating is Vdc.  Other capacitor-voltage-divided topologies, such as half bridge and the three-level NPC half bridge, have the common issue of capacitor voltage balancing difficulty and are not often used. 2-level uni-pole half bridge and 3-level full bridge are the most reliable module topologies and will be used for all following discussion.

Figure 4.  Isolated bidirectional MV-to-LV half-bridge AC/DC converter.

  a                                                                                                                          b

                    Figure 5.  Isolated bidirectional MV-to-LV full-bridge AC/DC converter.

Figure 4 and Figure 5 show modulated MV AC to LV DC converters. The half-bridge converter uses a Switch-Module (SM) with less power device, but it doubles the module number. Figure 5a and b use three-level full-bridge SMs, and are a more compact approach. For over 10kV AC inputs, it is difficult to design high-frequency inductors able to handle over 10kV without some specially and costly treatment.  The SMs of Figure 5b has its own built-in inductors, which only need to handle a voltage less than SiC device’s rating.  The AC/DC converter shown in Figure 5b is one of the most practical solutions of SST’s MV AC-to-LV DC stage.

    LV DC to AC output stage often uses T-type or I-type （ as shown in Figure 6） three-level three-phase inverter topologies. Depending on output voltage levels and power device’s voltage ratings, T-type or I-type is selected to use. Since three-phase output voltage or current could be unbalanced for different load and power grid voltage conditions, additional voltage balancing circuit may be needed to maintain three-level circuit’s capacitor voltages equally balanced.

Figure 6. I-type three-level three-phase inverter with capacitor voltage balancing circuit.

The emerge of HV SiC MOSFETs opens a completely new area of Solid Sate Transformer, which is the key element of smart grids, energy storage, DC datacenters, renewable energy systems and EV high power charger stations.  With DC power systems are getting more popular, both SiC MOSFETs and SST technologies are moving forward at an unprecedentedly fast pace. In the foreseeable future, they will play a vital role in industrial applications.