Prasad Kandula builds a medium-voltage solid-state circuit breaker as part of ORNL’s project to develop medium-voltage power electronics in GRID-C. (Image: Carlos Jones/ORNL, U.S. Dept. of Energy)
Tech Briefs: Could you tell me something about how the project began, how you came to it.

Prasad Kandula: We were working at Oak Ridge on medium-voltage power electronics for the last few years. The Department of Energy (DOE) was looking for proposals to extend the technology in this area, so working with other labs and universities, we submitted a proposal.

One of the tipping points that enabled this project to get started was the reduction in the cost of medium-voltage semiconductors. Earlier, they were really expensive, so they did not make sense when looking at the final affordability. But now they have come down enough in price to make this case very interesting.

Tech Briefs: What are the voltage levels considered to be “medium”?

Kandula: The medium voltage range is from 4 kVAC to 34 kVAC.

Tech Briefs: Why is medium voltage so important?

Kandula: As you try to process more power, you would want it to be done at higher voltages and lower currents to reduce the copper losses. We have seen a similar thing in other industries, like solar. It started at 600 volts initially and then it went up to 900 volts and today it is at 1,500 volts for very similar reasons. As the size of the plant went up, the consensus was let's go to higher voltages, reduce the currents, and keep the losses low.

I have recently heard that even with vehicles, which used to have 12-volt electronics, they are trying to move to 48 volts because now they require a lot of processing power, lots of controls.

Tech Briefs: Is the 1,500-volt operation for solar just for large solar installations?

Kandula: Yes, it’s directly related to the power — the big solar installations are where the power is much higher, which is where higher voltages are the most helpful.

Tech Briefs: You said in the press release that “Power conversion using medium-voltage power electronics is expected to be more efficient, in addition to packing more power into a smaller space.” Could you explain that?

Kandula: For our medium-voltage power electronics, we are processing the power internally at high frequencies. So, all the passive components in the system like inductors, transformers, and capacitors, will be reduced in size. That will enable at least the power conversion elements to be smaller in size.

Tech Briefs: Could you outline what is in the system?

Kandula: Typically, as of today, using a solar system as an example, you have DC coming from the solar source followed by low-voltage power electronics taking the DC and converting it to a maximum of about 600 volts AC. Then you would have a transformer to step it up to typically 13 KV or 34 KV AC.

One application of our device could be that you take the low-voltage DC and directly connect it to the power electronics, which will convert it to an output of up to 13 kVAC. So instead of having two different sections (low-voltage power electronics followed by a transformer) you just have one power electronics converter.

Tech Briefs: So, do I understand correctly that low-voltage DC goes into your box, you invert it to high frequency, and medium voltage comes out?

Kandula: Yes, the high frequency is just for galvanic isolation. But if you just look at it as a black box, low-voltage DC goes in and medium-voltage AC comes out.

Tech Briefs: What's inside the box?

Kandula: Inside the box, we convert the low-voltage DC to high-frequency AC, in the range of 10 kHz to 20 kHz, and then use a high-frequency transformer to provide galvanic isolation. So, we still use a transformer except that it is now operating at very high frequencies, which enables it to be greatly reduced in size. Then on the other side, the high-frequency AC is converted to medium-voltage AC with another high-frequency transformer.

However, we are finding more and more applications for converting the low-voltage DC to medium-voltage DC, rather than AC.

Tech Briefs: Why are there more applications for medium-voltage DC rather than AC?

Kandula: There are specific areas where staying in DC is much more efficient than going to AC. The reason is that on the same copper line, you could transfer more power with DC compared to AC. With AC, the transmitted power is a function of RMS, rather than peak values, there are problems due to inductance, and in addition, the current doesn’t use the whole cross section of the cable because of the skin effect. In DC, all these problems go away, so you have better utilization.

However, this only makes sense in certain cases.

Tech Briefs: Can you give me a couple of examples of the kind of cases where it would make sense?

Kandula: Yes, for example, if you wanted to connect two distribution systems today, you would connect them with a medium-voltage AC line. But if the line is long, let's say 15 miles or more, you might be better off converting to medium-voltage DC and then connecting them, because you would need less copper. It would essentially increase the amount of delivery capacity using the same power lines. If key portions of the distribution grid were converted to DC, then the medium-voltage DC approach would also reduce the number of conversion stages and improve the efficiency of power delivery.

Tech Briefs: What sorts of systems might you want to connect?

Kandula: For example, you might want to connect two distribution systems, where one of them might be on an island far off from the mainland. You might want to connect them so you could provide extra power, if needed, to the main system.

Tech Briefs: Are you also thinking about microgrids?

Kandula: Yes, those are also examples. We have sources like solar or fuel cells, which are all DC, and most of the heavy loads are either directly DC or they have intermediate DC in them. For instance, chargers or even home air conditioners run on motor drives that use DC at some point in the conversion. So, if the source is DC and the load is DC, why don’t we just stick with DC all the way.

With microgrids, it makes even more sense to use medium-voltage DC because it solves a lot of the challenging problems like synchronization, reactive power, and sharing of loads — it has multiple technical advantages.

Tech Briefs: Do you use different blocks for AC and DC outputs, and what would they consist of?

Kandula: So, a couple of examples of what’s in the boxes are a simple full bridge or a half bridge, which converts from DC or AC to AC or DC. On one side, there will be DC and the other side will be either AC or DC. That's it. The second block is the high-frequency conversion block where it takes in DC, internally converts into high-frequency AC, puts it through the transformer and on the other side, converts it back to DC. The function of this block is to provide DC to DC isolation. If you needed AC out, there would be an additional block.

I also want to emphasize that our building blocks include protection, which is very important. You can always build a system, but if you don't know how to protect it, that will be a big problem. So, protection is an important part of the project.

Tech Briefs: The press release said that the project is divided among four national labs and five university labs. How do you coordinate?

Kandula: Each lab has their own strengths. Sandia National Laboratory is very good at the semiconductor component level. Pacific Northwest National Laboratory (PNNL), on the other extreme, is very good at the system level. National Renewable Energy Laboratory (NREL) and Oak Ridge National Laboratory (ORNL) are in the middle, where we are doing the building blocks. Our specialty at Oak Ridge is in developing the building blocks and configuring them for different converters. NREL has good test facilities where you can build the whole system and test it. So, each one has their own role, with sufficient overlap.

In addition, Oak Ridge is the lead, taking up the responsibility of coordinating the work across the multiple labs.

Tech Briefs: What are your next steps?

Kandula: Our next objective is taking it to the field to demonstrate the technology. Today we are in the early stages, where we are going to develop the blocks; then we want to put the blocks together to realize a system. After that we will bring the system controls into the picture, put them together, and install the system in the field or a reasonable testing environment to demonstrate the technology.

This will be a multi-phase effort. Phase one is the first three years, where we will be testing it in our own facility; then in the next phase we will probably take it into the field. Here at Oak Ridge, we have extensive facilities including a medium-voltage lab that can go up to 13 kV as well as associated development labs where we can build everything from small components all the way to the whole system, including the controls. So, we'll first be building the whole thing here, testing it, and then going out into the field.

We are working with utilities who are also members of this project, who are helping us identify the use cases where medium voltage could be used.

Tech Briefs: Are you working with anybody who could do the actual commercial production?

Kandula: We do have industrial partners in the project, so when we can show enough maturity, we're expecting them to take over the technology.