Autonomous distribution grids in power ranges up to 150 megawatts
For distribution system operators (DSO), direct current in the medium-voltage range could prove to be a real benefit in the changing electricity market. We spoke to Alexander Rentschler, Head of Product Lifecycle Management at Siemens Transmission Solutions, about a solution adapted from HVDC PLUS technology.
Siemens Magazine: The energy transition is bringing about a decentralization of the electricity market. Why is DC the right solution for this challenge?
Alexander Rentschler: With DC, energy losses are generally lower. So we can cover greater distances, but also control the flow of power. That is becoming important since we now have energy generation at all levels. This leads to undirected energy flows and thereby jeopardizes the stability of our grids or rather causes “grid congestions.”
Another advantage of DC networks is that they can be decoupled and connected regardless of frequency, voltage and quality. Say, for instance, I have a network that is very stable in terms of frequency and voltage and has a low harmonic content, and I want to couple it with another unstable network to exchange energy, I would do that using DC to avoid parasitic effects.
In other words: DC links in AC networks increase stability?
Exactly. MVDC converters allow me to adapt to the network quality. In an enhanced version I can suppress the harmonics or achieve balancing of the AC phases to improve the network quality. But there is another advantage, too.
If we operate an AC link with DC, we are able to improve the capacity. Where there is an existing network infrastructure I can increase the output of a three-phase network by around 20 percent by coupling DC links with it. With four- or six-phase systems the performance increase is 60 and 80 percent respectively.
That is an advantage that we didn’t anticipate at the very outset but that generated a huge amount of interest during our market study. Because with the existing AC technology I would have to upgrade to higher voltage levels. With the DC link using existing infrastructure, however, you don’t have the cost of constructing lines and the effort of obtaining permits.
Why was a power range of 30–150 megawatts selected for the MVDC PLUS solution?
The PLUS technology uses modular multilevel converters (MMC) that, unlike industrial converters, comprise a big number of individual modules. Quite simply, the voltage is built up by adding more of these modules. Each valve works with a particular size. To achieve a specific power, I simply vary the voltage and the number of valves. We have fixed power and variable voltage. There is a minimum limit at which multilevel technology makes sense, and that level is 30 megawatts with maximum power and the minimum number of valves. We wanted to stay in the medium-voltage range. And that’s how we arrived at 150 megawatts. If we were to increase the level further we would have to use HVDC applications, and that would mean the application of more cost-intensive high-voltage equipment.
How does the MMC converter actually operate in an AC environment?
The converters can be controlled very precisely and fed with certain performance parameters. They comprise 4.5-kilovolt modules, that we also use in FACTS and HVDC. The modules are sampled in relatively fast cycle times. This leads to huge efforts for the control system of the MVDC system. Every millisecond, the status of the valves has to be checked and appropriate adjustments have to be made.
With MVDC we are working with half bridges as they have an advantage in terms of power losses, but they also have a lower functionality. But we think that functionality and outages are less of a factor in the smaller distribution areas than they are in the transmission area. Fault clearances are done on the AC side. The configuration is always: an AC grid, followed by a DC link in the middle, and then an AC grid again. Therefore, we can shift the protection functionality to the AC side where we apply the normal AC protection mechanisms.
Talking about protection mechanisms and reliability: Many valves, of course, entail higher error rates?
For reliability reasons, more modules are being included than necessary. If there is a problem with one module, a bypass ensures the system as a whole can keep running. We have a great wealth of experience with HVDC projects that we have realized. And so we have a good idea of how such a hardware should be designed and how it should work.
Is the solution based on Siemens HVDC technology?
The power hardware comes from HVDC PLUS technology. The difference is that with HVDC we offer mainly tailored solutions where the customer specifies exactly what kind of functionality and availability he wants.
With the MVDC link we basically have greater freedom, because it is aimed at a wider market. That allowed us to reduce the control and measurement systems for MVDC PLUS and offer a compact solution. We can also use a simplified version of the control software without having to make adaptations as we would with HVDC. Furthermore, the whole system runs autonomously, that is, without an operator.
Are your competitors developing similar MVDC systems?
In our target area we know of one competitor, but they are marketing an industrial solution that is probably competitive in the 30- to 40-megawatt range, but not above. It would require significant technical effort to adapt both an industrial converter, designed for use in a power unit, and the control software to enable them to operate in a DC network. I think we have an advantage here with our modular multilevel converters and our software, since we can simply adopt them from HVDC technology.