The characteristic behavior of physically compact, multiterminal dc networks under electrical fault conditions can produce demanding protection requirements. This represents a significant barrier to more widespread adoption of dc power distribution for microgrid applications. Protection schemes have been proposed within literature for such networks based around the use of non-unit protection methods. This paper shows however that there are severe limitations to the effectiveness of such schemes when employed for more complex microgrid network architectures. Even current differential schemes, which offer a more effective, though costly, protection solution, must be carefully designed to meet the design requirements resulting from the unique fault characteristics of dc microgrids. This paper presents a detailed analysis of dc microgrid behavior under fault conditions, illustrating the challenging protection requirements and demonstrating the shortcomings of non-unit approaches for these applications. Whilst the performance requirements for the effective operation of differential schemes in dc microgrids are shown to be stringent, the authors show how these may be met using COTS technologies. The culmination of this work is the proposal of a flexible protection scheme design framework for dc microgrid applications which enables the required levels of fault discrimination to be achieved whilst minimizing the associated installation costs.

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Optimizing the Roles of Unit and Non-unit Protection Methods Within DC Microgrids

The integration of distributed generation (DG) units into distribution networks has challenged the operating principles of traditional AC distribution systems, and also motivated the development of emerging DC systems. Of particular concern are the challenges associated with the operation of conventional protection schemes and/or devices. This paper first analyses the fault current characteristics in AC and DC distribution systems; it then presents a comprehensive review of the latest protection methods proposed for distribution systems embedding DGs. In addition, the advantages and disadvantages of each method are outlined and compared. The differences between the protection algorithms employed in/proposed for AC and DC systems are also discussed. Finally, this study identifies the future trends and provides recommendation for researches in the field of protections of DC distribution networks.

Protection of AC and DC distribution systems. Embedding distributed energy resources: a comparative review and analysis

The history of the centralized and distributed power system is introduced far away from the power system being first built by Edison, which started the first confrontation between ac and dc power system. Although ac power system has dominated for a long century because of easy transmission and some other benefits, dc power system has still gave some good performances and attracted more attention during the past three decades. This paper will discuss the current developing stage of dc distributed power system by the following contents: the basic structure and characteristics; the benefits of the dc distribution system; the development of current techniques adopted in DPS and its analysis of protection and system interaction. In additional, the comparisons between dc and ac distributed power system are presented in detail, and finally, the challenge in future is also summarized.

A survey of distributed power system — AC versus DC distributed power system

A fast and effective method for detecting and isolating faults in medium-voltage dc microgrids relies on rapid coordination between power supply converters and bus segmentizing contactors to limit currents and isolate faults without any need for fast communication between these active elements. The power converters independently enter current-limiting mode as soon as they recognize a fault condition and the segmentizing contactors autonomously decide whether or not to open based on their local interpretation of time-to-trip curves as functions of apparent circuit resistance. This method allows converters and contactors to use only local measurements when discerning whether or not to trip in order to isolate the faulted section. Simulation and experimental results show that low-impedance short-circuit faults can be isolated within 10–20 ms and the system can be reenergized within 40–60 ms. The method is effective for a wide range of fault and system configurations, and that range can likely be expanded by applying additional discrimination methods.

Fault Detection and Isolation in Medium-Voltage DC Microgrids: Coordination Between Supply Power Converters and Bus Contactors

Summary form only given. The characteristic behavior of physically compact, multiterminal dc networks under electrical fault conditions can produce demanding protection requirements. This represents a significant barrier to more widespread adoption of dc power distribution for microgrid applications. Protection schemes have been proposed within literature for such networks based around the use of non-unit protection methods. This paper shows however that there are severe limitations to the effectiveness of such schemes when employed for more complex microgrid network architectures. Even current differential schemes, which offer a more effective, though costly, protection solution, must be carefully designed to meet the design requirements resulting from the unique fault characteristics of dc microgrids. This paper presents a detailed analysis of dc microgrid behavior under fault conditions, illustrating the challenging protection requirements and demonstrating the shortcomings of non-unit approaches for these applications. Whilst the performance requirements for the effective operation of differential schemes in dc microgrids are shown to be stringent, the authors show how these may be met using COTS technologies. The culmination of this work is the proposal of a flexible protection scheme design framework for dc microgrid applications which enables the required levels of fault discrimination to be achieved whilst minimizing the associated installation costs.

/pdf/optimizing-the-roles-of-unit-and-non-unit-protection-methods-14lydwi3ve.pdf

Optimizing the roles of unit and non-unit protection methods within DC microgrids

In Italy, the increase of railway traffic and of the power demand by the trains, especially the high-speed trains, has caused a continuous increase of the currents through the electric traction lines rated at 3 kVdc. As a consequence, in some cases, the load currents may approach the value of the fault currents for faults located far from the supply stations. The detection of the faults in such a situation has become difficult for the electromechanical protections at present in operation in Italy in the lines above. In addition, transient phenomena related to the train absorption, frequently cause untimely trippings of the protections above. For such a reason, in the years 1999-2000, a field test campaign was performed in the 3-kVdc lines Campoleone-Cisterna and Farneta-Montallese in Italy, in order to investigate the phenomena that lead to the untimely trippings above. On the basis of the field test campaign, it was found that such an inconvenience is related to the inner characteristics of the electromechanical protections in use. A digital protection, operating on new protective algorithms, was thus set up and thereafter put in operation in the years 2003-2004. This paper presents the basic principles of the new protection above and the results of the first two years of operation.

A New High-Speed Digital Relay Protection of the 3-kVdc Electric Railway Lines

The paper investigates the installation of energy storage systems (ESS) for a 3kV DC railway line located in the North of Italy. The main goals are to compensate voltage drops along the line and to regulate the system's voltage and to recover braking energy of trains. Electromechanical performances of trains and electrical system operation have been simulated by means of a software in order to evaluate the power demand of each train, the TPSSs (Traction Power Substations) power profile and voltage trend along the line, as well as the energy recovery and ESSs installation benefits. Two technological solutions regarding the design of ESS are described and simulated: lithium batteries and supercapacitors storage systems. The first solution is suggested due to the cost savings and the smaller requirements in terms of space occupation and total weight, while the second solution is suggested in case of great power and due to low levels of maintenance The analysis shows the benefits of the implementation of each type of ESS.

C. Spalvieri

Papers

A New High-Speed Digital Relay Protection of the 3-kVdc Electric Railway Lines

Energy storage system for voltage regulation in a 3kV DC Italian railway line