Tsegay Hailu

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.

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.

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.

Decentralized current limiting in meshed DC distribution grids

Meshed dc distribution grids have advantages over radial connected ones. Challenges arise when this is done on the same voltage level and the full potential power might not be usable in certain configurations. This paper shows how the power flow in meshed dc grids can be increased by limiting the currents at congested lines. This can be done in a decentralized way with only local current measurements. Several realization possibilities for current limiting devices are introduced and discussed. Dynamic simulations are performed.

Protection coordination of voltage weak DC distribution grid: Concepts

Distributed Generation has created the need for new types of smart dc grids developments. The multiple generation sources, bi-directional power flow, power flow time co-ordination and management bring significant benefits as well as challenges for current ac electrical grids and emerging dc microgrids. In particular, the effect of voltage weak distribution systems on protection concepts and approaches needs to be understood, investigated, and accounted for. This paper describes principles of protection of voltage weak dc grid concepts. New alternative protection designs are proposed and their benefits and challenges are discussed. Likewise, special circuit configurations are proposed to improve fault discrimination. Fault-induced voltage transients are used to detect and isolate faulty part of the dc distribution grid.

Piece-wise linear droop control for load sharing in low voltage DC distribution grid

This paper presents load current sharing of parallel-connected sources in a low-voltage dc distribution grid. Droop control is a common technique for load current sharing in dc distribution grids. The main drawback of the conventional droop method, which has one droop value, is poor current sharing due to the resistance of distribution cables. These resistances create different voltage drops which limits the current sharing for each of the sources. This paper introduces a piece-wise linear droop control to improve current sharing during heavy load demand and decrease current sharing at light load for sources which are far away from the load. This piece-wise linear droop control does not need communications to improve current sharing and is decentralized. The number of segments of the droop curve depend on the size of converters and the requirement to minimize system oscillations. Detailed analysis and design procedures are provided in this paper. Simulations to verify the piece-wise droop concept are presented.

Voltage Weak DC Distribution Grids

Abstract This paper describes the behavior of voltage weak DC distribution systems. These systems have relatively small system capacitance. The size of system capacitance, which stores energy, has a considerable effect on the value of fault currents, control complexity, and system reliability. A number of potential definitions of voltage weak DC distribution systems are proposed. These definitions address the main characteristics of voltage weak systems. A small signal model of a general voltage weak DC distribution system is developed in order to observe sufficient conditions for system stability. This is achieved by analyzing the dominant poles. The source converters are modeled as droop-controlled current sources in parallel with their respective terminal capacitors. As constant power loads have incremental negative impedances, which affect the system stability, especially, in voltage weak system, ideal constant power loads with their terminal capacitors are included in the small signal model. A three-node voltage weak DC distribution grid is analyzed, as a case study, by implementing the developed small signal model. The effects on system stability by the values of system capacitance, cable inductance, and cable resistance are investigated using dominant pole placement. Likewise, the influence of proportional-integral regulators and droop coefficients of source converters on the stability of the system is examined. Finally, the three node DC distribution grid is developed in MATLAB/Simulink in order to demonstrate the influence of small capacitance on system stability. Moreover, effect of the rate of change of constant power loads on the system stability is simulated. These results are further compared and verified on a voltage weak DC experimental test bench with a 350 VDC bus voltage.

Multifunctional modular multilevel converter-based systems bottom-up approach to system design

This paper introduces a new power system that can be implemented on all levels by using the bottom-up approach of system design. It is based on orthogonal power flow at different frequencies. The power exchange between different elements of the grid as well as the additional features are based on implementing power electronics (PE) interfaces and applying multiple frequencies (zero- DC is also a valid frequency). Thus, the micro-grids are decoupled into multiple systems which can then be controlled independently, paving the way for a more flexible and stable general system. By implementing multiple frequencies (MF) additional features can be achieved that would otherwise not be possible with conventional AC, newly- emerging DC systems or any of their hybrid forms. In this paper, the concept of orthogonal power transfer, which is the basis for this system will be presented, and the multifrequency system will be defined and compared to existing solutions. Finally, this novel concept will be demonstrated on a simulation incorporating renewable energy sources, energy storage and a constant power load.

From voltage stiff to voltage weak DC distribution grid: Opportunities and challenges

This paper reviews the currently available dc micro-grids and dc grids. Currently, commonly existing and proposed dc microgrids are designed to provide sufficeint energy during system disturbance as in ac grid and, are mainly voltage stiff systems. It also compares the voltage stiff systems to a low energy system, which represents voltage weak dc system, from systems point of view. First, the definition of voltage weak dc systems is provided from a system perspective. Comparisons of both dc systems are reviewed on stability complexity, fault detection and isolation strategies and devices, and power management approaches. Various challenges and opportunities of voltage weak dc systems are briefly addressed. And also several architectures and topologies for voltage weak dc distribution systems are reviewed. Finally, this paper identifies a comparative advantage of such systems from a control, protection and system operation standpoints.

Impact of voltage dependent demand response on the dynamics of DC microgrids

This study evaluates the dynamics of DC microgrids (DCMG) with the integration of voltage dependent demand response (VDDR). Previous work evaluates price prediction models for grid operators to estimate local price as a demand side management (DSM) technique. VDDR has been proposed to implement DSM without the need of a dedicated communication link. The dynamic response of a DCMG with VDDR has been evaluated in a situation with limited source power output. This has been done with two different VDDR controller logics: dynamic and fixed hysteresis control. Furthermore, the effect of current rate of change control is presented. The results provide further understanding on the dynamic behaviour of DCMGs, which can be used to create a guideline for the design of source and load control systems.