Solid State Transformers – Revolutionising the Power Grid

Solid-state transformer (SST) is a collection of high-powered semiconductor components, conventional high-frequency transformers and control circuitry which is used to provide an elevated level of flexible control to power distribution networks. By adding some communication capability, the entire package is often referred to as a smart transformer. SST technology can step up or step-down AC voltage levels just like that of the traditional transformer, but it also offers several significant advantages. They use transistors and diodes and other semiconductor-based devices that, unlike the transistors used in computer chips, are engineered to handle high power levels and very fast switching.

As utilities start to roll out the smart grid, they are focused on gathering information, such as up-to-the-minute measurements of electricity use from smart meters installed at homes and businesses. But as the smart grid progresses, they’ll be adding devices, such as smart solid-state transformers, that will strengthen their control over how power flows through their lines, says Alex Huang, director of a National Research Foundation research centre that’s developing such devices. “If smart meters are the brains of the smart grid,” he says, “devices such as solid-state transformers are the muscle.” These devices could help change the grid from a system in which power flows just one way—from the power station to consumers—to one in which homeowners and businesses commonly produce power as well.

Requirement of SST

• Transformer oil can be harmful when exposed to the environment.
• Core saturation produces harmonics, which results in large inrush currents.
• Unwanted characteristics on the input side, such as voltage dips, are represented in output waveform.
• Harmonics in the output current has an influence on the input. Depending on the transformer connection, the harmonics can propagate to the network or lead to an increase of primary winding losses.
• Relative high losses at the average operation load. Transformers are usually designed with their maximum efficiency at near to full load, while transformers in a distribution environment have an average operation load of 30%.
• All LFTs suffer from non-perfect voltage regulation. The voltage regulation capability of a transformer is inversely proportional to its rating. At distribution level, the transformers are generally small and voltage regulation is not very good.

Advantages of SST over Conventional Transformer

• Voltage Sag compensation
• Outage compensation
• Instantaneous voltage compensation
• Fault isolation
• Power factor correction (& reactive power compensation)
• Harmonic isolation
• DC Output
• Metering or advanced distribution automation
• Environmental benefit

History of SST

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.

A high-power AC/AC conversion has been proposed by Moonshik Kangand Enjeti from Texas A&M University. For that topology, the incoming AC wave form is modulated by a power-electronic converter to a high frequency square wave and passed through a small high-frequency transformer. Since 2002, Electrical Power Research Institute (EPRI) has been researching the Intelligent Universal Transformer (IUT).

An intelligent and controllable system can provide multiple transformer functions, such as voltage transformation, voltage regulation, non-standard customer voltages (DC or 400Hz AC), voltage sag correction, power factor control, and distribution system status monitoring to facilitate automation. Researchers at ETH Zurich are working on a MATRIX converter with the codename MAGA Cube. The FREEDM project is investigating an SST based on a single-phase system with modularity in mind from 2010 onwards.

Architectures of SST

The basic idea of the SST is to achieve the voltage transformation by medium to high frequency isolation, therefore to potentially reduce the volume and weight of it compared with the traditional power transformer. The most commonly used power electronic converter topology is the two-level converters.

The limitation was that even using the highest available power rating 6.5kV IGBTs, a two-level converter-based SST can only interface with 2.4kV AC voltage.

To apply the silicon-based SST to a distribution voltage level 7.2kV/12kV, multi-level converter topology is inevitable.

Stage One

• In this stage is a three-phase ac/DC rectifier that regulates a high-voltage DC bus (and ac voltage when for reactive power compensation) is used.
• It produces low distortion grid current.
• DC voltage regulation.

Stage two

It consists of a Dual active bridge (DAB), a high frequency transformer.

DAB converter gives:

• Electrical isolation.
• High reliability.
• Ease of realizing soft-switching control.

Stage three

DC-AC converter is used to convert the DC voltage in desirable voltage.

Operation of SST

AC power from the supply is converted into DC power. The AC power can be converted into DC regulated power & gives power factor improvement, voltage control. In DC-DC isolation, it isolates input and output side and power can flow in both directions since DSB is used. High-frequency transformer reduces the size of the overall system and provides reactive power compensation and active power compensation. DC-AC inverter gives both AC and DC output regulation.