Modulated low fault-energy protection scheme for DC smart grids

DC smart grids enabled by the integration of advanced power electronic converters (PEC) can ease the integration and control of distributed renewable energy resources, electric vehicles and energy storage systems. However, these highly flexible power systems introduce many challenges when considering the design of reliable, plug-and-play protection that does not rely on dedicated communications infrastructure for device coordination. One particularly difficult challenge is the management of DC-side filter capacitor discharge during short-circuit faults where the large peak fault-current produced can permanently damage exposed semiconductor components within the converter. One solution is to ensure that the trip-time of DC protection devices is sufficiently rapid (sub-millisecond) to guarantee that fault-current is blocked prior to reaching destructive magnitudes. However, such high-speed protection devices do not offer much margin for effective selectivity with downstream devices due to the narrow time window of operation. Accordingly, this paper proposes a non-unit protection scheme for future large-scale DC smart grid applications that increases this time-window of operation to enable improved selectivity whilst retaining a lower level of energy dissipated in the fault. Reliable protection coordination is demonstrated on a DC radial network and is realized using conventional millisecond trip-time devices, and a single solid-state microsecond trip-time device

Back-to-back Converter Control of Grid-connected Wind Turbine to Mitigate Voltage Drop Caused by Faults

Power electronic converters enable wind turbines, operating at variable speed, to generate electricity more efficiently. Among variable speed operating turbine generators, permanent magnetic synchronous generator (PMSG) has got more attentions due to low cost and maintenance requirements. In addition, the converter in a wind turbine with PMSG decouples the turbine from the power grid, which favors them for grid codes. In this paper, the performance of back-to-back (B2B) converter control of a wind turbine system with PMSG is investigated on a faulty grid. The switching strategy of the grid side converter is designed to improve voltage drop caused by the fault in the grid while the maximum available active power of wind turbine system is injected to the grid and the DC link voltage in the converter is regulated. The methodology of the converter control is elaborated in details and its performance on a sample faulty grid is assessed through simulation

Protection strategy in active DC power distribution networks

Environmental incentives to combat climate change are providing the motivation to improve the energy efficiency of power distribution systems and integrate state-of-the-art renewable technologies. DC distribution networks are receiving considerable attention in the literature because they offer a simple and flexible interface between these modern resources and consumers. However, many technical challenges relating to the design and standardisation of DC protection devices still exist that must be overcome prior to widespread adoption. Since DC fault current develops rapidly, many high-speed protection schemes tailored for DC networks have been proposed. However, few of them have considered the difficulties in practical implementation. This thesis will present the implementation challenges and propose corresponding protection schemes to address the issues. In seeking to achieve this aim, the work presented within this thesis makes three main contributions. This thesis has fi�rstly improved the reliability of the high-speed DC differential protection scheme. The main implementation challenge of this scheme is that a short time synchronisation error may cause a signi�ficant current difference error, resulting in a false-trip problem when a fault occurs outside the protected zone. This thesis has proposed a "multi-sample differential (MSD) protection scheme" to ensure the protection stability for external zone faults (i.e., the relays must not operate) whilst maintaining sensitivity for internal zone faults (i.e., the relays must operate) by examining multiples measurement samples. Secondly, the difficulty in realising high-speed DC distance protection is that measurement of rate-of-change of current can be severely affected by even low-level noise, resulting in a failure in fault detection. This thesis has presented the methodology for selecting the appropriate sampling time of the numerical derivative as well as the cut-off frequency of low-pass current measurement �lfiters. Although high-speed protection schemes can effectively isolate faults quickly, their implementation requires many advanced devices, which may not be economical for lowpower and low-cost DC networks. Finally, this thesis has proposed a "modulated low fault-energy (MLE) protection scheme" that employs fault current limiters (FCL) at the grid energy sources and mechanical circuit breakers (MCB) elsewhere throughout the distributed network. This deployment can constrain the fault current to a lowenergy level that enables a longer time window for the downstream MCBs to realise protection with a lower total implementation cost. Drawing conclusions from this PhD research, the author advocates that more consideration should be given to implementation challenges when designing protection schemes in DC distribution networks. Excessive pursuit of ultrafast fault isolation speeds can lead to over-cost and protection instability issues in practice. A prospective protection scheme must compromise between the high-speed protection requirements in theory and the reliable but economical requirements in practice, to accelerate the realisation of large-scale DC grids in future.Environmental incentives to combat climate change are providing the motivation to improve the energy efficiency of power distribution systems and integrate state-of-the-art renewable technologies. DC distribution networks are receiving considerable attention in the literature because they offer a simple and flexible interface between these modern resources and consumers. However, many technical challenges relating to the design and standardisation of DC protection devices still exist that must be overcome prior to widespread adoption. Since DC fault current develops rapidly, many high-speed protection schemes tailored for DC networks have been proposed. However, few of them have considered the difficulties in practical implementation. This thesis will present the implementation challenges and propose corresponding protection schemes to address the issues. In seeking to achieve this aim, the work presented within this thesis makes three main contributions. This thesis has fi�rstly improved the reliability of the high-speed DC differential protection scheme. The main implementation challenge of this scheme is that a short time synchronisation error may cause a signi�ficant current difference error, resulting in a false-trip problem when a fault occurs outside the protected zone. This thesis has proposed a "multi-sample differential (MSD) protection scheme" to ensure the protection stability for external zone faults (i.e., the relays must not operate) whilst maintaining sensitivity for internal zone faults (i.e., the relays must operate) by examining multiples measurement samples. Secondly, the difficulty in realising high-speed DC distance protection is that measurement of rate-of-change of current can be severely affected by even low-level noise, resulting in a failure in fault detection. This thesis has presented the methodology for selecting the appropriate sampling time of the numerical derivative as well as the cut-off frequency of low-pass current measurement �lfiters. Although high-speed protection schemes can effectively isolate faults quickly, their implementation requires many advanced devices, which may not be economical for lowpower and low-cost DC networks. Finally, this thesis has proposed a "modulated low fault-energy (MLE) protection scheme" that employs fault current limiters (FCL) at the grid energy sources and mechanical circuit breakers (MCB) elsewhere throughout the distributed network. This deployment can constrain the fault current to a lowenergy level that enables a longer time window for the downstream MCBs to realise protection with a lower total implementation cost. Drawing conclusions from this PhD research, the author advocates that more consideration should be given to implementation challenges when designing protection schemes in DC distribution networks. Excessive pursuit of ultrafast fault isolation speeds can lead to over-cost and protection instability issues in practice. A prospective protection scheme must compromise between the high-speed protection requirements in theory and the reliable but economical requirements in practice, to accelerate the realisation of large-scale DC grids in future

Technical solutions for low-voltage microgrid concept

fi=vertaisarvioitu|en=peerReviewed

Design and Modeling for DC Nanogrids

Smart grids were constructed as a means of communication to the electric grid through computer and other information technologies. This line of communication acts as gauge for a more accurate reading of power consumed. A nano grid is a model version of a smart grid with the ability to function as separate power generator. Such feature allows for this grid to power single loads and apply for special applications. A DC-DC converter was designed to apply to a nano grid which is a form of a smart grid. The converter was a single-input-multi-output converter which is taking one dc voltage and applying it to two dc output voltages. This boost converter takes the inputs and increases its voltages, leading to the outputs respectively. The nano grid utilizes this proposed converter to carry out its special characteristics. Procedures carried out in this research showed the success of the converter. Further steps include the designing of a ring and radial architecture nanogrid to form a microgrid. A comparison of results are made showing the efficiency and reliability of ring architecture layout microgrids Doing this creates a more complex system, and provide relief to multiple sources to prevent outages

Subsea DC collection grid with high power security for offshore renewables

This work was supported by the Engineering and Physical Sciences Research Council [grant number EP/K006428/1]; and the European Regional Development Fund [grant number LUPS/ERDF/2010/4/1/0164].Peer reviewedPostprin

Electro-thermal analysis of power converter components in low-voltage DC microgrids for optimal protection system design

Bidirectional power converters are considered to be key elements in interfacing the low voltage dc microgrid with an ac grid. However to date there has been no clear procedure to determine the maximum permissible fault isolation periods of the power converter components against the dc faults. To tackle this problem, this paper presents an electro-thermal analysis of the main elements of a converter: ac inductors, dc capacitors and semiconductors. In doing this, the paper provides a methodology for quantifying fault protection requirements for power converter components in future dc microgrids. The analysis is performed through simulations during normal and fault conditions of a low voltage dc microgrid. The paper develops dynamic electro-thermal models of components based on the design and detailed specification from manufacturer datasheets. The simulations show the impact of different protection system operating speeds on the required converter rating for the studied conditions. This is then translated into actual cost of converter equipment. In this manner, the results can be used to determine the required fault protection operating requirements, coordinated with cost penalties for uprating the converter components

Requirements to Testing of Power System Services Provided by DER Units

The present report forms the Project Deliverable ‘D 2.2’ of the DERlab NoE project, supported by the EC under Contract No. SES6-CT-518299 NoE DERlab. The present document discuss the power system services that may be provided from DER units and the related methods to test the services actually provided, both at component level and at system level

Development of Robust and Dynamic Control Solutions for Energy Storage Enabled Hybrid AC/DC Microgrids

Development of Robust and Dynamic Control Solutions for Energy Storage Enabled Hybrid AC/DC Microgrid