Laura Ramirez-Elizondo

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.

Unit commitment in multiple energy carrier systems

During the last decade there has been a strong development of energy applications that make use of multiple energy carriers. A new approach is necessary to tackle problems related to planning and operation of energy systems, since conventional techniques are mostly oriented to deal with independent energy carrier infrastructures only. The contribution of this paper is to offer a general unit commitment framework for energy systems that contain multiple energy carriers and to present illustrative examples on the topic. The unit commitment problem is solved using the optimization software AIMMS.

An Aggregate Model of Plug-in Electric Vehicles Including Distribution Network Characteristics for Primary Frequency Control

Summary form only given. In the future, the number of plug-in electric vehicles (PEVs) that will participate in the primary frequency control (PFC) is likely to increase. In our previous research, the computational complexity of the PFC problem for a large number of PEVs was reduced using aggregate models of PEVs. However, in the literature on the PFC, the distribution network characteristics have not been included in the aggregate models of PEVs for the PFC, despite the fact that PEVs will be dispersedly connected to the distribution network. This paper proposes an aggregate model of PEVs for the PFC that further incorporates distribution network characteristics, i.e., the distribution network power loss (DNPL) and the maximum allowed current (MAC) of the lines and transformers. The DNPL variation is formulated according to the line and transformer impedance, spatial distribution of PEVs and loads, and active power variation of PEVs. Then, DNPL variation together with the MAC of the lines and transformers are incorporated in the proposed model of PEVs. Finally, the simulation results show an excellent agreement of 98% between the detailed model and the proposed aggregate model of PEVs.

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.

A technique for unit commitment in multiple energy carrier systems with storage

A new approach on the planning and operation of power systems is necessary in order to cope with the characteristics of energy systems with multiple energy carriers, as well as with the intensified integration of renewable energy sources and storage units. This paper proposes a technique to include storage as part of a general unit commitment framework for energy systems that contain multiple energy carriers.

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.

The application of a fuel cell-electrolyzer arrangement as a power balancing set-up in autonomous renewable energy systems

Small-scale autonomous renewable energy systems have gained attention during the last years due to growing concerns in relation to an increasing world energy demand and to constraints in CO2 emissions. Polymer electrolyte membrane fuel cells (PEMFC), wind turbines and solar panels are promising zero-emission devices to be incorporated into these systems. In order to integrate them, appropriate control designs are necessary, among other aspects. This paper presents a configuration that allows a proper operation of the fuel cell while the system is able to handle the power fluctuations produced by the wind turbine and the load. An electrolyzer is used to take advantage of the power surplus. The system is designed to supply 10 households, but can be easily extended. The objective of this study is to evaluate the technical feasibility of implementing such a power balancing set-up in DENLab, a renewable energy laboratory at Delft University of Technology, the Netherlands.

Control strategy for an autonomous energy system with electricity and heat flows

This paper presents a control strategy for the operation of an autonomous distributed generation system (DGS) that includes multiple energy carries, storage devices and wind energy. The simulation is performed for the heat and electricity demand of 200 households. The proposed control strategy provides suitable performance results and improved energy efficiency for the system under study.