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EEE / Next for the Grid: Solid State Transformers
« on: April 24, 2017, 02:18:15 PM »
In a few years, one of the primary components of the grid is going to become downright portable.

A number of companies are working on technologies that could replace traditional transformers -- large pieces of industrial equipment fashioned from groups of components for raising or lowering electrical voltage -- with transformers that largely consist of semiconductors on circuit boards. This process could begin to pave the way for a number of improvements in the way that power gets delivered. Integrating and managing renewable power and electrical storage could become easier. Microgrids could be deployed much more rapidly.

Grid efficiency could conceivably be increased by up to 8 percent to 10 percent because of lower conversion and transmission losses.

"That's ten percent less power that you have to generate," said John Palmour, co-founder and Chief Technology Officer for Power and RF at Cree, which produces silicon carbide semiconductors. "We can replace an 8,000-pound transformer in a substation running at 60 hertz and replace it with one running at 20 kilohertz in a tiny design. [...] We can shrink it down to [the size of] a suitcase."

Varentec, a startup that has received funds from Khosla Ventures and others, has laid plans to develop a solid state transformer. In England, Amantys, a spin-out from Cambridge University that is currently raising funds, will show solid state prototypes later this year for converting medium and high voltages. Amantys' initial target market will be wind farms. (Solid state transformers and power electronics will be two of the major topics of discussion at The Neworked Grid taking place May 3 and 4 in San Francisco.)

On the component side, Cree, Infineon, Mitsubishi, ST Microelectronics and others have been working on silicon carbide semiconductors. Most of the efforts and commercial products to date have focused on low voltage converters. Cree's Power & RF devices group, for instance, sells silicon carbide diodes and MOSFETs, a type of switch, to solar inverter makers and server companies for computer power supplies. The current revenue run rate is around $100 million a year. Last month, Cree released a smaller version of its silicon carbide diode for inverters.

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By swapping out standard silicon components for silicon carbide ones, the efficiency of power supplies can be boosted from the high-80-percent range to the 93-percent-plus range. Some solar inverters containing silicon carbide components have even hit the 99% range.

The push now is to create components that can handle the sort of high voltages required for transformers on the grid.

One could imagine Transphorm, which makes power converters from gallium nitride (GaN), selling components into this market. The company's first product operates at low voltages (around 400 volts), but some researchers have looked at taking GaN to higher voltages.

Relatively recent advances in material science -- reducing defect densities on test chips, boosting the yield on wafers -- and semiconductor design have effectively opened the door to high-voltage applications. In short, silicon carbide (pictured) and gallium nitride can lead to smaller, faster devices that can operate at higher temperatures more efficiently than equivalent ones based on silicon. Growing gallium nitride or silicon carbide crystals and substrates for these sort of applications, however, is no simple feat.

Cree, in connection with DARPA and the Office of Naval Research, has built a 10-kilovolt, 100-amp power module with advanced silicon carbide MOSFETs. These power modules were integrated into equipment from General Electric to make a 1 megawatt transformer that operates at 20 kilohertz. An equivalent conventional transformer capable of handling 6.5 kilovolts might only operate at 500 hertz.

The DARPA transformer measures 16 inches high, weighs 75 pounds, and operates at 250 kilovolt amps (volts x amps). A conventional 330 kVA transformer stands 55 inches high, weighs 2,700 pounds and only operates at 60 hertz (more here).

Between now and 2013, Cree, under an ARPA-E grant, will build 15-kilovolt and 20-kilovolt insulated gate bipolar transistors (IGBTs), a switch with more capabilities than a MOSFET, out of silicon carbide. (Amantys' basic component will also be an IGBT.) If all goes well, this will lead to 50- and 100-kilowatt solid state transformers.

An existing silicon carbide 12-kilovolt IGBT achieves four times the switching speed and four times lower switching losses than an equivalent IGBT fashioned from silicon. The 20 kilovolt IGBT should have a similar switching performance, but with 10 times less loss than a silicon part.

"It is in the silicon carbide components where the breakthroughs have come," Palmour said. "Silicon [the current material of choice for power components] is up against a wall."

Many of the advantages with solid state transformers come through reductions in size. Today, planting a transformer is not an easy task. Considerations like transportation, site preparation, installation and transmission costs all add to the budget. By contrast, smaller, cheaper solid state transformers could be relatively easily planted in small solar fields or storage pods. The faster switching speed of solid state devices would in turn make it easier for a utility to handle a multiplicity of power sources feeding into the grid because you'd have more transformers controlling and fine-tuning power quality.

"A bank of batteries for energy storage doesn't do you any good if you can't get the energy up on the line," Palmour said.

Varentec doesn't say much about itself, but sources, and a website managed by its incubator/landlord, say that the company plans to make solid state transformers.

There's been a lot of talk in the past few years about coming up with a solid-state version of the distribution transformer that now sits on utility poles in neighborhoods throughout the land. A solid-state transformer (SST) would be at least as efficient as a conventional version but would provide other benefits as well, particularly as renewable power sources become more widely used. Among its more notable strong points are on-demand reactive power support for the grid, better power quality, current limiting, management of distributed storage devices and a dc bus.

It is starting to look more likely that we'll see a practical SST design as GaN and SiC power transistors with higher current and voltage ratings start coming to market and their prices drop. But a practical SST design could have an impact extending well beyond transformers for electrical utilities.

One example of where SSTs could also find use is in variable-frequency drives for big induction motors. In that regard, Siemens Industry Inc.'s Drive Technologies Div. in New Kensington, Pa. is keeping an eye on SST work now underway at North Carolina State University's FREEDM System Center for smart grid research. Siemens Principal Engineer Mark Harshman says use of SSTs in the medium-voltage motor drives that Siemens makes could conceivably reduce the size of the VFDs by 30% and have similar beneficial effects on their overall efficiency levels.

There have been several topologies suggested for SSTs but most being evaluated today are based around the idea of a dual active bridge (DAB) converter. A DAB uses a power bridge to modulate the incoming ac waveform into a high-frequency square wave. The square wave gets passed through a small high-frequency transformer to another power stage. This converter demodulates the square wave and sends it to another inverter which produces low-voltage ac.

This scheme still uses a conventional transformer, but one optimized for higher frequencies (typically about 1 kHz). This makes it much smaller and lighter than transformers optimized for ac line frequencies.

The high-frequency transformer gives the SST galvanic isolation. It also has some leakage inductance in its primary and secondary windings, which also helps synthesize soft switching. During switching transients, transformer current resonates with the capacitors in parallel with switching devices, limiting the dv/dt and di/dt across the switches, thus reducing switching loss and boosting power efficiency.

The fact that DAB converters have a symmetrical circuit configuration lets them handle bi-directional power flows, important when it comes to renewable sources sending power back up the grid. The power flow of a DAB converter can be controlled by varying the phase shift between those two bridges which changes the voltage across the transformer leakage inductance. Power transfers from the leading bridge to the lagging bridge.

One of the difficulties in fabricating a SST is that the 7.2-kV line voltages that characterize distribution power lines exceed the operating voltage of today's IGBTs, 6.5 kV. So multiple devices must be used in series to keep below the operating maximum. The NC State prototype, for example, uses a topology that includes a seven-level cascaded H-bridge for the high voltage rectifier stage.

There are other difficulties as well. One is that the minimum current rating for the 6.5-kV IGBTs is 200 A. This is too large for the 20 kVA transformer NC State is building because the input current is only about 3 Arms. Thermal issues also affect the SST's operation, which has forced NC State researchers to come up with special packaging for their 25-A IGBTs. Additionally, the team had to come up with a way to isolate IGBT drivers for both power supply and gating signals.

To sense the 7.2 kVac voltage, the researchers devised a sensor that was compact and which incorporated high-voltage isolation because existing models were too large and not isolated from the high input voltage. Finally, they had to get around the fact that the insulation capability for 6.5-kV IGBT is 10.2 kV, but the high-voltage-side dc bus voltage is 11.4 kV. They ended up floating the heatsink for each 6.5-kV IGBT while maintaining ample clearance and creepage distance between the heatsinks. To keep the voltage across input inductor down to manageable levels, the team built eight identical inductors and put them in series so the maximum voltage stress for each of them is just 0.9 kV.

Researchers have also developed a prototype using 15 kV SiC MOSFET/JBS diodes. They are not trying to identify other major issues related to implementing a high-voltage system using SiC power devices, including the challenges in designing a system to support high dV/dt and dI/dt, and to design an efficient and compact high-frequency transformer.

More info from the FREEDM project:

EEE / Is the world more dangerous now than during the cold war?
« on: April 24, 2017, 02:05:47 PM »
During the cold war, there was a clear narrative: an ideological opposition between the US and the Soviet Union. Moments of great tension were understood as episodes within that narrative. The closest we came to nuclear confrontation was the 1962 Cuban missile crisis, when the two countries seemed on the edge of war. But the crisis itself was finished inside a fortnight, and there was a wider framework to fall back on. The 1963 Partial Test Ban Treaty calmed the waters.

Then, in the early 1980s the tough-talking but critically derided , Ronald Reagan was elected US president. He reignited the cold war rhetoric and began escalating the arms race, and there was an assumption – particularly in Europe – that nuclear destruction was creeping closer. But it was still within a recognisable context. That ended with the collapse of communism, and the fall of the Berlin Wall. For a while the world felt a much safer place than it had been. But the cold war was replaced by uncertainty. And now the uncertainty is combined with the unpredictability of Donald Trump. The recent bombing raids in Syria and Afghanistan were isolated moments, without any sense of programme or continuity. Nor does there seem any logic to why North Korea should have suddenly become a pressing issue. Incidents that seem to arrive out of the blue can be much more frightening. We’re probably not on the verge of nuclear war, but it’s destabilising if we can’t make sense of events.

Faculty Sections / Bangladesh blackout 2014
« on: April 23, 2017, 03:52:49 PM »
By Abdul Matin.

About 100 million people in Bangladesh, out of a total of 160 million, were without electricity for about 10 hours on November 1. The rest of the population has no access to the national grid. The interruption originated at 11.30 am at a sub-station in Bheramara in Kushtia district. Soon it knocked out the 400 KV transmission line that was bringing in 445 MW of power from India.

As the national grid lost about 445 MW of power, an uncontrolled chain reaction set in. All the power plants of the country were forced to shut down. Industrial production came to a halt. Captive generators partially supplied electricity to some industries, domestic houses, hospitals, airports, commercial places and other important installations.

There were long queues for diesel oil at gas stations to feed the captive generators. The supply of CNG to automobiles was interrupted due to the outage, increasing the demand for gasoline. The traffic gradually thinned out. Even though electricity was partially restored in some areas in the afternoon, the streets became empty after sunset in the capital city as the restaurants and shopping places had closed down. By 4.30 pm, the power plants, which were brought online, tripped again, further complicating the situation. A shortage in supply of water was reported in some areas. Prices of candles and kerosene suddenly shot up. The internet and mobile phone services deteriorated. The electronic and newspaper media were also affected. By 9.00 pm, some areas got electricity back. The system was back to normal after midnight.

Even though the crisis seems to be over, questions on the blackout are still lingering in the minds of the people. How did it happen and why should it happen after so much investment in the power sector? To answer these questions, we need to understand how an electric power system operates. An electric power grid basically consists of power plants to generate electricity and transmission and distribution lines to carry the power to the consumers. There are numerous protective relays and other devices to regulate the system. The grid is controlled from a load dispatch center.

A power grid is a very delicate system in the sense that there must always be a perfect balance between the generation and the load or demand so as to maintain a constant system frequency which is 50 cycles/second in our region. Whenever there is an imbalance, the frequency will automatically change. If a generator fails, the load will instantly exceed the generation capacity, resulting in a drop in frequency. If the loss of generation is within a tolerable limit, it is possible to continue the operation of the grid by increasing the generation from the hot reserve capacity of the system which is available from the operating power plants and/or by resorting to load-shedding. On the other hand, if a significant load is lost, there will be excess generation in the system that will boost the frequency. In that case, the generation capacity must be reduced to bring down the frequency to the desired level.

A power grid collapses if the loss of generation or load is too large for the system to handle. This is exactly what happened on November 1. The tripping of the transmission line at Bheramara amounted to a loss of about 445 MW of power imported from India, as mentioned earlier. It is reported that the sub-station at Bheramara cannot handle any power beyond 400 MW. The loss of power at Bheramara reduced the frequency to 45 cycles/second and created an electric surge that finally caused the blackout throughout the country.

Can such blackouts be avoided in the future? An electric grid is a man-made system. It can never be perfect. Blackouts occur all over the world, including the most advanced countries even though the frequency of occurrences may vary from country to country.

It is obvious that a blackout can never be avoided but its frequency and severity can be reduced if modern protective and regulating devices are used and enough hot reserve margins are available in the system. In addition, modern smart grids with state-of-the-art technology are in use in many advanced countries that can detect and react to any situation in real time. Such systems can minimise the frequency of occurrences and severity of outages in addition to increasing the system efficiency.
The writer is a former chief engineer of Bangladesh Atomic Energy Commission and the author of “Rooppur and the Power Crisis.”

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