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.
