At American Electric Power’s Inez station in Kentucky a $30 million piece of power electronics has just entered service which may turn out to be something of a watershed in the application of semiconductor devices to the control of power flows in the transmission grid. Called the Unified Power Flow Controller (UPFC), it was formally dedicated on 26 June.
The remarkable thing about UPFC is that for the first time it provides simultaneous and instantaneous solid state control of all three parameters of power flow: impedance; voltage; and phase angle. This means we are already well into what has been called the third generation of high power thyristor-based controllers – also known as Flexible AC Transmission Systems (FACTS) – and the pace of change looks likely to increase over the next few years. This will be fuelled by the demands of a deregulating electricity supply system, the need to maintain power quality and the need to get more and more out of existing transmission and distribution assets.
First generation systems control only one variable and need big, and therefore expensive, banks of capacitors or reactors. An example is the TCSC (Thyristor-Controlled Series Capacitor) installed at Bonneville Power Administration’s Slatt substation, which has been operating since 1993.
Second generation systems do away with the need for the large external circuit elements by electronically synthesising components such as capacitors and reactors, using advanced thyristors. An example of the second generation technology is the STATCOM (Static Compensator) installed at Tennessee Valley Authority’s Sullivan substation. This supports voltage by generating or absorbing reactive power. Among the benefits enjoyed by TVA has been elimination of the need to build a new 161 kV transmission line.
With the third generation of device a step-change increase in flexibility has been achieved. UPFC essentially can synthesise the waveform of just about any electrical device and introduces a new dimension by, in the words of its proponents, “controlling all the basic parameters affecting AC transmission lines, independently or concurrently” enabling “wide area voltage management and comprehensive control of energy transfer over the power delivery network”. The UPFC technology at Inez has been developed by EPRI, American Electric Power and Westinghouse. Siemens may find it a particularly attractive item among the portfolio of power technologies they are acquiring through their purchase of the latter company.
The significance of the Inez UPFC installation is that it showing utility people, in particular the more sceptical ones, what can be done. But in technological terms, while it is by no means the beginning (the first FACTS precursors going back 20 years or more) it is also very far from the end. It could perhaps be seen as the end of the beginning.
Already further developments are in the pipeline. For example one of the papers to be presented at this August’s Cigre conference (by EPRI, NYPA and Westinghouse) describes a device called the Convertible Static Compensator, which uses a “control mode never used before…called Interline Power Flow Control” making possible the “control of both real and reactive power on two or more lines with a single controller”. The authors purpose its use in the New York transmission system.
At the Inez dedication ceremony, EPRI’s Karl Stahlkopf pointed to some further promising new developments in thyristor technology which will dramatically bring down the costs of FACTS devices, make them more compact and help get them into more widespread use. These include MOS-controlled thyristors and thyristors using “wide-bandgap semiconductors” instead of pure silicon. Then there is the possibility of combining FACTS devices with other advanced technologies such as superconducting magnetic energy storage devices, an option which AEP is currently considering as an enhancement of the Inez UPFC.
The outcome of all this will be dramatically improved control of power flow, which is likely to play a crucial role in bringing into being a truly liberalized electricity market. In the era of open transmission access, independent system operators, and decoupling of generation and transmission, T&D systems are being called upon to operate in ways not originally envisaged, with grid stability an increasingly important issue. In the new competitive era those who embrace these new transmission control technologies are likely to have the edge.
