Solid State Transformers – Revolutionising the Power Grid

Benefits of SST

Size & Weight reduction: One of the most important features of the SST is the voltage sag or swell ride through capability

Voltage Sag mitigation: The voltage sag or swell can be compensated by the rectifier stage and will not affect the load side voltage. The maximum input current determines how much voltage sag the SST can compensate, and the maximum input voltage determines how much a voltage swell the SST can support.

Reactive power compensation: The SST rectifier stage not only converts the input AC to regulated DC voltages, but also has reactive power compensation capabilities. Depending on the reactive power reference in the SST controller, the SST can generate or absorb the rated reactive power to the power grid.

Fault Isolation: The SST protection scheme for the output short circuit is, the SST will stay online but limit the load current.

Applications of SST

• Traction application

• Smart Grid application

• DC Charge Station

• Wind Power generation

Wind power is an uncontrollable resource, also makes for a challenging integration of large WFs into the grid, particularly in terms of stability and power quality. Integration of SST with wind energy systems effectively replacing the conventional transformer and reactive power compensator to increase the flexibility of wind energy system.

Normally SCIG is used in wind energy system a capacitor bank is generally placed at the terminal of the wind generator for the local reactive power compensation (which is necessary for the operation of the system). The nature of WFs is that their operation is highly dependent on the active and reactive powers transferred to the grid. In any interfacing system there is need for conventional transformer and reactive power compensator. SST-interfaced WF architectures effectively replace the conventional transformer and reactive power compensator.

Implementation Challenges in SST

One of the main disadvantages of the cascaded H-bridge converter is the voltage unbalance that could appear on the DC side of different H-bridges. The unbalance issue becomes worse when the SST operates at no-load or light-load condition, because a small power difference is a significant percentage of the real power and will result in a large voltage unbalance. Smart solid-state transformers are still in the development stage and likely are a few years away from being ready for market—researchers are still working on their efficiency and cost, for example. Taking advantage of their DC capability will require developing new construction standards for homes and businesses.

Mark Wyatt, the vice president of smart-grid and energy systems at Duke Energy, cautions that solid-state transformers will need to be supplemented with other devices for controlling power on the grid, and they may not prove cost-effective in many areas. “It’s not one size fits all,” he says. Yet in the long term, Huang says, smart transformers and other smart solid-state devices could enable an unprecedented amount of two-way power flow. “It could be revolutionary to how we construct the grid,” he says.

Future Research Areas

This set of shelves at the FREEDM Systems Center houses part of TIPS, a three-phase solid-state transformer. The conventional transformer component of TIPS [gray boxes at bottom] can be small thanks to power electronics that convert the electricity to high frequency. In the near term, SSTs could be a boon for disaster-recovery efforts in places with damaged electrical infrastructure and for settings such as naval vessels, where volume and weight are at a premium. Further in the future, they could redefine the electrical grid, creating distribution systems capable of accommodating a great influx of renewable and stored energy, dramatically improving stability and energy efficiency in the process. Highly variable, two-way flow of electricity between a microgrid and the main grid.

Conclusion

The technological review of concepts and developments in field of Solid State Transformer has been shown. Also, various configurations used and implemented so far have been briefly reviewed.

Finally, it is concluded that the conventional transformer which is used widely in industrial applications so far having disadvantages like saturation of core for non-linear load, poor voltage regulation, bulkiness.

Majority of these problems can be reduced or eliminated by solid state power electronic based intelligent transformer. Also, it has the capability to work as energy router for smart grid energy internet.

The field of application of power electronic based solid-state transformer is not limited to only the distribution level but research work suggests that these intelligent solid-state transformers have the capacity to replace the conventional power transformer too soon.

The idea of a “solid state transformer” has been discussed since 1970. The initial purpose of solid state transformers is to convert AC to AC for step-up or step down with a function the same as that of a conventional transformer. In1970, W. McMurray form G.E. first introduced a high-frequency link AC/AC converter, which became the basis for the solid state transformer based on direct AC/AC converter.

Rohit P R, Rahul P R are students of Electrical & Electronics Engineering at CMR Institute of Technology, currently in their 8^th semester. Their topics of interest are IoT, Power Systems, Renewable Energy and other emerging technologies in the field of Electrical & Electronics. They are avid quizzers, IEEE student members and have participated in various workshops.