Is DC in distribution grids feasible enough to replace AC?

DC in distribution grid

In our modern society, a highly reliable power supply is of great importance. The reason for this is that the most critical appliances are dependent on a constant and reliable flow of power such as hospitals, data centers and communication systems. With the current solutions, there is an extensive use of power electronic converters that proves to be highly reliable and easily controllable.

However, these power converters also have some efficiency issues, considering there are many conversion steps back and forth. This results in overall system efficiency to be lower than wanted. Because of this, DC power distribution systems have become a research topic that has caught interest.

Motivation for the development of DC was first because long-distance transmission of power showed to be more efficient when DC is used.

This is because AC has a reactive power-demand which does not exist in DC, this means that only active power is transferred and loss is much lower.

In newer time, renewable energy sources like photovoltaics (PV) and wind have become more relevant. These power sources generate DC, which means a conversion from DC to AC, and synchronization to the grid is necessary to be able to feed this power into the grid. With these types of power sources, a DC distribution grid is an interesting idea which could be relevant for the centralized electrical grid, a microgrid or a nanogrid.

Although important for the future, deeper insight on microgrid and nanogrid will not be covered in this thesis, but it is important to note that they may be relevant.

For implementation of a DC distribution grid, there are many factors that need to be considered; the grid solutions, how one would distribute power and how the interconnections of the grid will be schemed.

This means that it is important to look at practical solutions for where in the grid DC should be used, and how it should branch out all the way to the consumers.

What would one do with the current grid that is already established, and is it possible to reuse some of it? Could the already installed AC system and the cables in use be repurposed and used in a future, DC?

In my opinion it is necessary to establish and set up a system for standardized voltage magnitudes for the distribution system. There are many different suggestions that need to be explored for this voltage, and how it would be solved.

Since one of the speaking points of DC in distribution of power is that it is more efficient. The aspects around the efficiency of DC, how and what makes it more efficient are issues that need more research.

Here a comparison between the conversion losses in an AC system and a DC system would be important. As well as comparing the conduction losses in both the AC distribution system and the DC distribution system.

DC distribution topologies

LVDC distribution system

A DC distribution system starts of the point from when the transmission lines end. The distribution system can be fed by a DC line that uses a DC/DC buck converter to reduce the voltage magnitude. The more likely option is that the distribution system will be fed by an AC line that uses a step-down transformer to reduce the voltage magnitude.

For simplicity sake, since distribution grid is the objective, we assume that power through transmission lines is delivered by the means of AC. Different types of structures of DC distribution system will be reviewed here.

LVDC Distribution system – AC at consumer level

A low voltage DC (LVDC) distribution system is proposed by T. K. Pasi Salonen, Pasi Nuutinen, Pasi Peltoneiemi, Jarmo Partanen in paper “An LVDC Distribution System Concept”, where there are to suggestions to how power should be distributed by using DC.

In addition to this proposal, other types of DC distribution solutions are to be considered and suggested in other research, which will be considered later in this chapter. In Figure 1(a) the suggested distribution grid has DC lines from transformer substation that steps down the voltage and rectifies the voltage from AC to DC.

The second suggestion to how the DC distribution system topology would be constructed shown in Figure 1(b). This topology consists of two AC systems which are connected with a DC line. The connection to the transmission line is that same as in the previous suggestion, with a step-down transformer and an AC/DC rectifier.

The difference is that the DC voltage is converted back to AC and a transformer steps down the voltage again to a magnitude which can be used by consumers. This resembles a standard high voltage DC (HVDC) transmission line, which is most commonly used in long-distance power transmission.

This type of structure is called a ‘point-to-point distribution structure’, because power is distributed using DC from a point to another point.