Laurens Mackay

From DC nano- and microgrids towards the universal DC distribution system - a plea to think further into the future

The traditional ac power system is challenged by the increased amount of distributed energy resources. Enormous changes and investments are necessary in order to achieve an ac smart grid capable of coping with the challenges introduced. In dc, solutions seem to be more straightforward since most of the distributed energy sources and most of the loads connected to the low voltage grid operate with dc. For this reason, dc distribution systems should be considered as an alternative with significant potential. Nevertheless, nowadays research is mainly focused on local dc nano- and microgrids. This paper introduces relevant aspects related to dc distribution networks. A wide field of opportunities and challenges are briefly exposed.

Series arc extinction in DC microgrids using load side voltage drop detection

Due to the absence of current zero crossing, dc arcs do not extinguish as easily as ac ones. It is essential to detect and eliminate series arc faults in dc microgrids in order to ensure safety, particularly while unplugging loads. Several detection and extinction methods using various means are known in literature. In this paper a novel arc detection method is proposed. It detects the load side input voltage drop due to the initial electrode specific minimum arc voltage. Only the local input voltage has to be measured and selectivity is given. The arc can be extinguished by shutting down the loads power electronics converter. The proposed method is elaborated through simulation of arc behaviour for constant resistor and constant power loads with input capacitors. First experimental results correspond with the theoretical analysis.

Voltage weak DC microgrid

This paper describes the behavior of voltage weak dc microgrids. These are dc microgrids with a relatively small system capacitance. The large amounts of stored energy in the passive component of a network has a considerable effect on the size of the fault currents, control and reliability. The use of a complex control approach to stabilize voltage weak microgrids is a possibility that requires further attention. In this paper, however, a simple way to stabilize such a system is proposed by limiting the rate of change of the power electronic interfaces. The small signal analysis of a three node system is analyzed to see the effect of system capacitance, inductance, resistance and PI and Droop values on the control of the system. The small signal analysis is implemented to estimate the rate of change of loads and the effect of step load changes on the system. In the modeling, a combination two types of loads, Constant power loads, and resistive loads, is used to see the effect of on the system stability. The source and converters are modeled as droop controlled current sources in parallel with capacitors.

Toward the Universal DC Distribution System

Abstract Due to an increasing number of power generation units and load devices operating with direct current (DC) at distribution level, there is a potential benefit of leading efforts toward building a DC distribution system. However, the implementation of DC distribution systems faces important challenges, including the market inertia of AC systems and standardization. Many of the benefits that are attributed to DC can only be realized if a complete DC system is developed, and not if only a few components are replaced. This paper presents the concept of a universal DC distribution system, as envisioned by the authors. The universal DC distribution system could be implemented in various use cases, but could also completely replace AC distribution grids. The paper covers the possibilities of having DC nanogrids inside buildings, DC microgrids in neighborhoods, and the connection to AC and DC medium voltage grids. Furthermore, considerations regarding flexibility, electricity market design, control, and protection are presented.

DC ready devices - Is redimensioning of the rectification components necessary?

DC has several advantages over ac. In relation with the changes needed in todays ac power system to deal with dominating amounts of fluctuating renewable energy, it seems wise to consider dc as an alternative. One of the problems with introducing dc grids is the lack of availability of dc devices. DC ready devices that work on both - ac and dc - would simplify a smooth introduction of dc there, where it has its biggest advantage. This is especially interesting for newly built infrastructure, e.g. in developing countries with bad ac infrastructure, but also for upgrading the power of existing ac cables when needed. As most ac devices today already have dc/dc converters inside, the changes are not as big as one might assume. In this paper the rectification stage and power factor correction is looked at in detail and it is shown that in most cases there is no need for re-dimensioning of components to make devices dc ready. Other important points are subject of ongoing research as are dc microgrids in general.

Optimal power flow in bipolar DC distribution grids with asymmetric loading

The emergence of distributed energy resources can lead to congestions in distribution grids. DC distribution grids are becoming more relevant as more sources and loads connected to the low voltage grid use dc. Bipolar dc distribution grids with asymmetric loading can experience partial congestion resulting in a nodal price difference between the two polarities. In order to take into account this price difference, this paper presents an exact optimal power flow (OPF) model, formulated in terms of voltage and current. In the case of bipolar dc distribution grids, the common ground plane assumption cannot be used as current can flow in the neutral conductors as well. Moreover, loads and sources can be connected between any two nodes in the network. The proposed OPF problem formulation includes bilinear cost functions, therefore the locational marginal prices (LMP) are solved in a second step.

DC microgrid protection by selective detection of series arcing using load side power electronic devices

This paper proposes a method to mitigate the problems related to series arcing in low voltage dc (LVdc) micro-grids by performing voltage drop detection using the power electronic device at the load-side. Specifically, the selectivity of the designed series arc detection algorithm is shown for two constant power loads connected in parallel at the point of common coupling with a stable voltage dc micro-grid during accidental unplugging. The influence of the variations in the grid parameters on the load capacitor voltage characteristics and the response of the designed detection algorithm is discussed, thereby, offering insight into the maximum voltage drop and rise time with different circuit configurations due to series arc initiation.

Voltage dependent demand response with dynamic hysteresis thresholds in DC microgrids

DC renewable energy sources and dc loads are gaining attraction and they are preferably connected in dc microgrids. To increase the availability of dc microgrids, voltage dependent demand response should be used to switch off low priority loads in case of insufficient supply. This is possible even without communication with the loads. In this paper it is shown that using a voltage hysteresis with fixed thresholds to switch loads on and off, does not provide a general solution and oscillations can occur in specific cases. Dynamic hysteresis thresholds are introduced as a mean for devices to autonomously learn the present behavior of the grid to which they are connected.

Experimental design of a series arc load side detection algorithm for DC microgrid protection

Unplugging of energized loads in dc microgrids can cause safety concerns to users in residential areas. Sustained series arcs are more relevant in dc systems than in ac due to absence of current zero crossing. In this paper, the series arcs are detected from the load side, wherein, the proposed algorithm monitors the drop in load capacitor voltage associated with the intial electrode dependent series arc voltage. The original contribution of this work includes the experimental analysis of response of the arc detection algorithm with different discrete bandpass filter configurations and time constants. Experiments are conducted by generating series arcs for a stable 100V supply and noisy 600V supply with 300 Hz noise. The design requirements like robustness to low frequency grid voltage fluctuations, high frequency switching noise, detection time, and the threshold detection voltage are achieved and the most favourable design choices are highlighted.

Towards a DC distribution system - opportunities and challenges

The increasing amount of distributed energy resources requires significant changes to todays power systems. Most of the distributed resources are dc inherently or have a dc link. Therefore, connecting them with a dc distribution system seems beneficial. This paper presents the opportunities and challenges of dc distribution systems, starting with the requirements of future power systems. It will be looked at how dc can fulfill them with meshed grid architectures, increased system availability and new market models. New protection strategies for large dc distribution systems and their open research questions are discussed.