DC Mini-grids

Introduction

Changing patterns in electricity generation and consumption over past decades have brought new life to the debate around Alternating Current (AC) vs. Direct Current (DC) electricity supply from distributed generation. Direct Current distinguishes itself from Alternating Current through the fact that electric charge flows in a constant direction.

Direct Current is native to solar PV electricity generation, battery storage and a range of common, low-power household and commercial applications, such as LED-lighting, consumer electronics (e.g. TV, radio, mobile phones) and variable speed drives in electric motors (e.g. for fans or pumps). With an increasing reliance on these components / applications, DC systems have, in many cases, become a technically and economically viable alternative to ‘traditional’ AC systems.

DC vs. AC mini-grids: System architecture and operation

The decision between DC and an AC provides for a number of implications in terms of a mini-grid’s system architecture and operation. In the following, a short overview on the main differences is provided.

 

Power control and management

Generally speaking, DC mini-grids tend to be simpler in system architecture and operation than AC systems. This is primarily a result of their limited requirements in terms of power control and management.

Due to an inherently unidirectional flow of current, a DC architecture does not require the usage of the frequency/phase control mechanisms of its AC equivalent. Hence, with only voltage to be accounted for, DC systems generally provide for a lower system complexity, reduced number of variables in systems monitoring (Planas et al., 2015), lower cost for control equipment components (Karabiber et al., 2013) and a more modular and more easily expendable structure (Groh et al., 2014).

 

Fault protection

As of today, protection systems for DC mini-grids must be considered less mature in terms of practical experience, standards, guidelines and implementation compared to AC systems (Planas et al., 2015).

A particular concern in DC systems is the absence of natural zero crossings, rendering current interruption considerably more demanding and potentially more dangerous (occurrence of switch arcs) (Justo et al., 2013). This is, however, not a primary concern for low voltage DC, such as the commonly used 12 VDC or 24 VDC, as principle availability of adequately sized protection solutions does not differ markedly from standard AC systems (Groh et al., 2014; Planas et al., 2015).

 

System efficiency

In providing for a more ‘natural’ solution for the growing number of native DC components in electricity generation, storage and consumption (see Introduction), DC can contribute towards reducing conversion losses and achieving higher efficiency in electrical systems.

Apart from distribution (grid length, voltage, etc.), overall system efficiency depends on power conversion stages between electricity generation and load. Efficiency losses from DC-AC conversion (via inverters) and AC-DC conversion (via rectifiers) considerable affect overall system efficiency. Occurrence of such conversion steps in a mini-grid system will depend on the type of generation used (AC or DC), whether battery storage is integrated, and what type of consumptive loads are envisioned.

For their sample case of a solar PV mini-grid with battery storage, Madduri et al. (2013) arrive at an increase in end-to-end-efficiency for a DC system with 380 VDC transmission of between 17 and 25% for DC loads (fully DC mini-grid) and around 3% for AC loads, as compared to the usage of an AC system.

System / operational cost

Cost savings potential is a frequently reproduced argument for utilising DC over AC in mini-grids in certain situations. In terms of capital investment, DC mini-grids can provide savings due to their simpler power control architecture (see power control and management), as well as the fact that, in a full DC scenario, they can render additional conversion steps and associated system components, such as PV inverters, unnecessary (see system efficiency) (Willems et al., 2013; Planas et al., 2015). Improved power conversion efficiency from a fully DC mini-grid can furthermore lead to operational cost reductions of up to 20% (Madduri et al., 2013).

Comparative savings on capital cost are, however, at least partly offset by general price penalties on the more immature DC technology components (Planas et al., 2015).

Main considerations on the applicability of DC mini-grids

Based on the characteristics described above, the following provides a short overview on the main factors promoting the use of DC in mini-grids:

• Generation: Among common technologies used in distributed generation, Direct Current is native to solar PV generation, while rotating generators (wind, diesel, hydropower, etc.) generate alternating current. Conversion between DC and AC is prone to result in efficiency losses and should be avoided where possible (see system efficiency).
• Battery storage: Battery storage solutions, too, are native DC and hence most efficiently applied in DC systems.
• End-use applications / energy services: DC is required by or suitable for a range of low power household and commercial applications, such as LED-lighting, consumer electronics (e.g. TV, radio, mobile phones) and variable speed drives in electric motors (e.g. for fans or pumps). Consequently, DC mini-grids are primarily used to provide basic level energy services such as lighting and mobile phone charging.
• Grid connection: Although high voltage DC transmission is on the rise worldwide (see Planas et al., 2015), AC will still be the likely standard for electricity grids in decades to come. This renders DC systems principally unfavourable where interconnection of a mini-grid with the main grid is foreseen.