This is our second article in the hybridization series. With permanent magnet shaft generators behind us, it’s time to look into another interesting technology – a DC distribution system or DC grid.

The DC (direct current) distribution system may be the future replacement for a widely used AC (alternating current) distribution system. With the development of technologies that lead to all-electric ships (AES), the DC distribution system is more likely to become the standard solution. Today, we have a chance to interview the expert and learn more about this technology.

In this first part of the interview, we will take a look into the DC distribution concept in general, its advantages, disadvantages, economical benefits, market future and regulations.

Our expert – Teemu Heikkilä

Mateusz: Teemu, we are really happy that we can host an interview with another expert from The Switch. Before we head to explain the concept of DC distribution, could you please tell us about yourself and how did you start working in this field?

Teemu: I have been working with drives for almost two decades – and always close to product development. Now, I work in The Switch as Head of Product Line, High Power Converters.

In fact, DC has been a known technology for ages. Nowadays it is a viable option for a wide range of vessel types. That´s why I have ended up working with this field, too.

Introduction

Mateusz: It is always good to start from the basics. Can you explain the differences between AC and DC distribution systems on board the ships?

Teemu: Both systems have energy sources and consumers. In this regard, they are not different. But there is a completely different way these sources and consumers are connected together.

In the AC system, everything is connected to the AC switchboard with a 50/60 Hz frequency. Variable-speed applications have dedicated drives with transformers.

In DC distribution, everything is connected to a common DC link through power electronics, and power is transferred in DC.

Mateusz: So, the DC distribution system can be seen as an extension of the DC link in a big multi-drive system. But what are the advantages of DC distribution over AC?

Teemu: The main advantages are system flexibility, better efficiency and reduced footprint. When we can reach the same system functionality with fewer components, it also means increased reliability and fewer losses.

DC distribution is definitely an attractive solution when it comes to the integration of different energy sources now and in the future.

Will DC grids drive out AC grids?

Mateusz: So why were we using AC distribution systems instead of DC so far? Why bother the market with a new solution?

Teemu: To be honest, both concepts have their benefits. I believe both are needed in the future as well.

It really depends on the specific application to define which system fits better. If we try to generalize a bit, we could say that with a very direct power line from the source to the consumer, AC is your solution. But in case you have many energy sources and consumers in the same system, regenerative applications or need short-time peak power, then DC distribution provides a more efficient solution.

Mateusz: So both solutions have their use. But can DC-grid and AC-grid systems coexist – or do we put everything on one card?

Teemu: In most cases, there are both AC and DC switchboards on board. In case DC distribution is used for main power distribution, there is an AC switchboard for certain consumers backed up with an emergency generator.

Even in main power distribution, AC and DC can be combined. For example, in a retrofit project, it is possible to complement the existing AC switchboard with additional DC distribution to include, for instance, new energy sources and energy storage.

Where does DC Distribution prove its best?

Mateusz: What electrical consumers are most typically integrated via a DC distribution system?

Teemu: I would take a system approach instead of thinking of only one application. The benefits of DC distribution are more obvious when looking at the entire system. As said, also AC has its role in the future in certain applications.

Mateusz: Looking at the bigger picture, what type of ships would benefit the most from choosing the DC-grid system?

Teemu: DC distribution has become a viable option for a wide range of vessel types. Some vessel types, such as bulk carriers and ferries, benefit from easy integration of energy storage into the grid, making hybrid or fully electric operation possible.

Some others, like offshore vessels, benefit from increased redundancy. This allows utilizing energy storage instead of gensets during DP operations, creating fuel savings.

Larger vessels, like tankers, can benefit from the easy integration of variable-speed shaft generators combined with energy sources and possibly some other energy sources.

Easy integration with DC distribution

Mateusz: In the previous interview with Jussi Puranen, we discussed permanent magnet shaft generator (PMSG) technology and its benefits. In relation to it, how the integration via a common DC link favors hybridization?

Teemu: Included in our portfolio, PMSGs are obviously more efficient when compared to conventional technology. In turn, DC distribution makes the integration of hybrid solutions easier and more efficient, especially if you combine it with energy storage or similar technology.

In the PTO mode, energy storage can be charged. It is more efficient to charge it from the main engine through the shaft generator than run an extra genset. In PTI, energy can be taken out from the storage to give extra power. In the PTH mode, the main engine is disconnected from the shaft line, and it can be operated fully electrically by a shaft generator and a battery.

Mateusz: Following the route, can we really integrate different energy sources within one system via the DC grid?

Teemu: Definitely. This is where DC distribution has clear benefits. Several energy sources can be connected to the same system and used in parallel. The power from different sources is transferred through the DC link to consumers. The best combination, in my opinion, is to combine “slow and fast” energy sources. Then, you will reach the best efficiency.

Redundancy is key!

Mateusz: We already know some setup examples and how DC grids will complement permanent magnet shaft generator installations. Therefore, it’s time to speak about the typical topologies of DC networks.

Teemu: Applications are always connected to a common DC link. But how you configure your system, and the redundancy in it, is up to your need.

For instance, you can split your DC distribution into several parts via the ultra-fast protection device – the Electronic Bus Link (EBL). In normal operation, you transfer power through EBL. In case of a failure, the EBL disconnects any faulty part in microseconds. On top of that, you can build a ring network and take the redundancy to an even higher level.

Mateusz: There is one more question on my mind. With the new scheme, can we omit the high-voltage AC generation or distribution part?

Teemu: We see a great opportunity to replace high-voltage AC with low-voltage DC distribution. The main reason for this is that you can operate the EBL closed between DC-Hubs even in the ring network. This allows, for example, running propulsion from two DC-Hubs and increasing power dramatically. This makes low-voltage distribution an attractive choice for over 10 MW applications while increasing system redundancy.

The economy of DC Distribution

Mateusz: We have touched on an integration/configuration of DC distribution systems. It is time to talk about money. With DC distribution, where the savings are coming from?

Teemu: If we start from CAPEX, the savings mainly come from fewer components compared to AC switchboards. And this also means a reduced footprint.

Future flexibility is an important aspect, as well. DC distribution is more flexible for potential future needs.

Considering OPEX, the savings come mainly from improved efficiency. This includes the utilization of variable-speed gensets, regenerated energy, energy storage solutions and new energy sources. Plus, you can transfer power between DC-Hubs even in DP operation and much more.

Mateusz: I can see a clear sense for newbuilds. But is it worth retrofitting existing AC-grid systems into ships with DC grids?

Teemu: Definitely! And keep in mind that both systems can complement each other. Some parts of the AC system can remain in use and be complemented with additional DC distribution to include energy storage, integration of shaft generators and so on. North Sea Giant is a good example of this. The vessel brought a DC-Hub with batteries on board allowing it to reduce the number of running gensets from six to just one in DP operation. This resulted in a quite massive saving in fuel consumption.

Regulations of the Maritime Industry

Mateusz: Now we know that economic arguments are in place. But as we know, the maritime industry is very old fashioned. Are relevant regulations from classification societies or IMO already in place?

Teemu: The new IMO regulations force shipowners to take action to reduce emissions. This drives the industry toward DC distribution as it offers an efficient solution for a wide range of vessels.

Classification societies are working on enhancing rules for DC distribution. They are evolving step by step as the marine industry creates new solutions. In my opinion, the biggest challenge now is protection-related regulations. The same technology that has been used in AC is not feasible as such for DC. This aspect has not been considered in the rules yet.

Mateusz: As an ex-ETO, who spent some time on board and dealt with all kinds of certificates, I have to ask about the certification for a crew to operate those systems.

Teemu: Our products are low-voltage products and, therefore, no special certificates are needed. This is one of the competitive edges of DC distribution because it offers the possibility to use a low-voltage device instead of a high voltage.

Future of the market

Mateusz: What is the market of DC distribution networks on board today, and how will it change in the future?

Teemu: As mentioned, DC distribution is flexible for future solutions. It is easy to make reservations to expand the system when necessary. And the integration of future energy sources is easy, as well. This is very crucial for shipowners because today nobody can know for sure what will be the best fuel after a decade or two. You need to make your system so it is flexible for future needs.

Mateusz: We often get questions from young engineers on how to start with certain technology, what to read and where to gain knowledge about it. What would you recommend them to dive into the topic?

Teemu: There are a lot of articles on the topic, but maybe the easiest way to get started is to watch DC distribution-related webinars available on our website. They are called masterclasses and start from covering the very basics of DC distribution.

Conclusions on Part 1

The DC distribution system provides an answer to problems related to the efficient integration of new energy sources. It seems to be a missing link to make all-electric ships more common.

With DC distribution, the number of necessary components is reduced, increasing savings. Complemented with modern energy sources like PMSGs and energy storage systems, DC grids are enhancing ships’ redundancy. This seems to be a future-proof technology.

In the next chapter, we will focus on the insides of this technology, protection devices and the challenges it has to face.

If you have questions you want our expert to answer, ask them below – and stay tuned for Part two!