Over the last two decades, a paradigm shift in our way of dealing with energy generation and consumption has increased the attractiveness of local DC grids as an extension to traditional AC distribution networks. Renewable energy sources (RES) such as wind and solar energy are becoming more reasonably priced, and consequently, distributed generation is growing. Simultaneously, LED lighting has shown to be a much more efficient way of illumination compared with the old-fashioned incandescent lamps and have taken over the market very rapidly. Many of these sustainable technologies are fundamentally DC, requiring power inversion to connect to the AC grid. Furthermore, storage systems such as batteries and supercapacitors are intrinsically DC, and electric vehicles (EV) and all electronic devices operate on DC power. Therefore, there is a realisation amongst grid operators that utilising local low voltage DC (LVDC) grids will lead to less energy wasted in the conversion process. Investigations are needed to determine to what extent many promises of DC grids can be fulfilled, for examples, whether losses can be reduced by means of localisation, voltage drops can be improved, the number of substations can be reduced, the management of reactive power and power quality (PQ) can be improved, implementation of renewable energy sources can be made more simple, or if distribution losses can be reduced.
For DC grids, PQ issues such as ripple, inrush currents, voltage fluctuations and short circuit events, are different in nature from those in AC grids in terms of dynamics, duration, and magnitude. Therefore, for DC power systems there is a need for metrological support to obtain proper PQ definitions, a practical measurement guide, and realistic and well-defined PQ limits. Since the nature of PQ in DC grids is currently unclear, on-site measurements must be performed in real LVDC trial grids to determine which disturbances have the highest influence in terms of losses, inconvenience to customers, or potential to damage grid equipment and other connected loads. Such grid measurements should preferably cover a variety of representative consumer and producer connection types, such as solar panels, wind turbines, EV charging stations, battery storage systems, industrial and household applications, and different grid topologies, with voltage and current levels up to at least 1 kV and several hundreds of amperes respectively. The measurements should be performed with target uncertainties below 0.1 % considering the presence of AC ripple and other disturbances. Special measurement equipment and methodologies are necessary to conduct the required surveys for setting compatibility levels. This same equipment will be the basis of future 'planning level' surveys carried out by utilities to manage the PQ levels in future DC networks.
A second important issue regarding DC grids is the accurate measurement of power and energy for billing purposes. In most countries, electricity meters are type tested with respect to standards issued for AC grids only. Therefore, there is a need to investigate additional specific metrological aspects of DC meters, which should be included in a future revision of this standard. Examples of such aspects are magnitudes of ripple currents and voltages existing in range up to tens of kilohertz, the immunity of DC energy measurement against such ripples, how to measure the energy contained in such ripples, the losses due to cables, etc.
The concept of DC grids finds natural applications at LV microgrids, where users connect fundamentally DC appliances and renewable energy sources. Furthermore, high-voltage DC point-to-point connections are well established as a link between different larger regions in Europe and elsewhere. However, the concept of medium-voltage DC distribution grids is not very well established yet. One of the major hurdles is protection by circuit breakers, which experience severe challenges when switching due to the lack of zero-crossings at DC (whereas the time-dependent voltage changes polarity 100 times per second in AC grids, facilitating switch openings without significant arching). The related fast transients need to be monitored for grid control purposes. If the concept of LVDC grids is extended to MV distribution grids, new measurement challenges will definitely appear.
Traceability is an essential part of ensuring customer confidence for emerging DC grids, but is presently non-existent for DC power and DC PQ and not defined yet as a classified service category in the context of the International Committee for Weights and Measures (CIPM) Mutual Recognition Arrangement (MRA).
