Electricity supplies in the US move across miles in seconds to power manufacturing and utilities. But, for all its speed, the loss of just fractions of seconds of electric power is costing the USA’s economy $100 billion a year.
‘The nation’s electric grid is operating so close to capacity that many of today’s electric load demands for fast and dynamic voltage support cannot be provided fast enough,’ says Alex Huang, professor of electrical and computer engineering at Virginia Tech.
To solve the problem, Virginia Tech researchers have developed the Emitter Turn-off (ETO) Thyristor, a high-power semiconductor switch.
‘The ETO is a solid-state switch that is suitable for use in high-frequency power converters that can provide fast and dynamic voltage support to our nation’s congested power grid,’ says Huang. It is said to offer fast switching speed, rugged turn-off capacity, and voltage control.
The ETO is a three terminal integrated power switch. An optical pulse is applied to turn on current flow with very little resistance. When it is closed, it can conduct 10,000 amps of current. ‘This is not your typical switch,’ says Huang.
For continuous operation, the range is 1,500 amps to keep the temperature below 125 degrees C. When the optical pulse is removed, and the ETO switch opens, it can block voltage as high as 6,000 V. The ETO changes from on to off and off to on in less than 5 microseconds. During switching, the ETO withstands high voltage and high current simultaneously. It is a voltage turnoff device with real time current sensing capability that can be used for control and protection.
‘This switch allows us to advance very high power converters from a line speed of 60 Hz to 1 to 3 kHz switching at the same power level. This speed allows you to chop the voltage into whatever shape you need,’ says Huang.
Present technology is the Gate Turn-Off (GTO) Thyristor, the main power switch in use. The GTO is reliable and inexpensive, but requires a ‘snubber’ capacitor to protect it in the turnoff process. The snubber uses significant power itself, increases the size of the switch and the complexity of energy recovery circuits. In addition to being a complicated gate design, it slows the on-off process.
‘The significance is the ETO allows you to reduce energy storage elements and size (conductors, capacitors), which reduces the size, the weight, and the cost of the power converter,’ says Huang.
An insulated gate bipolar transistor (IGBT), which doesn’t require a snubber, was developed in the early 1990s. It offered improved control and reliability, but it is less useful in high power applications. The ETO is a hybrid of the GTO and the IGBT – ‘so is proven reliable,’ says Huang.
The US Department of Energy’s Energy Storage Systems Program, managed by Sandia National Laboratory, first funded Huang’s research on what are known as flexible AC transmission systems (FACTS).
Because the ETO has the highest power handling capacity among all solid-state switches, it was deemed useful for such high-power application as FACTS devices and the US Navy’s new electromagnetic aircraft launch system. The Tennessee Valley Authority is currently funding research by Virginia Tech to use the ETO in a high-power converter for dynamic voltage support in transmission and distribution grids.
Material limitations and the high cost of power electronic devices have restricted the wide spread use of power electronics in utility transmission and distribution grids. The ETO is a key enabling technology for lower cost power electronic systems.
The timing is excellent. ‘Deregulation has created new demands on the transmission infrastructure, straining reserves. Power electronic systems, such as the ETO, can improve quality and capacity of the transmission grid by 25 to 60 percent,’ says Jing Leng of Solitronics, a spin-off company started in Blacksburg, Virginia to commercialise the ETO.
The increasing frequency of electricity outages and outage duration are due primarily to lack of quick voltage support, leading to voltage collapse in many regions of the US and poor quality of power (flicker, for instance) says Leng. One result has been lost opportunities in manufacturing and other businesses dependent on steady or high capacity power systems.
But with the ETO, you can respond to demand faster, says Huang. ‘When to stop and when to conduct electricity can be determined by a computer. Several ETO switches will be in systems connected to an electric grid. When you can switch faster, the equipment can respond faster.
‘So, it’s low cost, high performance, high frequency, high power,’ says Huang. ‘It will withstand 16 megawatts of instantaneous power. Like turning on 100 stoves instantly through a unit about the size of a graham cracker. And you can stop them all at once too.’