More robust protection strategies for multi-microgrids

Sachit Gopalan

    Research output: ThesisDoctoral Thesis

    293 Downloads (Pure)

    Abstract

    Conventional power generation from fossil fuels contribute to problems of climate change and low energy efficiency. These problems can be partly addressed by interconnecting distributed generators (DGs) with the distribution system to provide electricity from renewable energy resources such as wind, photovoltaic (PV), biomass, and fuel cells. One way of integrating a large number of DG sources with the distribution system is to interconnect them with low voltage (LV) loads to form a microgrid (MG). Multi-microgrids (MMGs) increase the load capacity of the distribution system by integrating several MGs, DGs, and medium voltage (MV) loads with the distribution system. This thesis addresses a number of challenges in the protection of MGs and MMGs. These include accurate measurement of fault current signals, making the protection of MGs more robust, and enabling MMGs to continue operation by isolating only the faulted sections.
    Fault current signals that are processed by digital relays consist of DC, fundamental, and harmonic components. Filtering algorithms are necessary to eliminate the DC and harmonic components from these signals. Several algorithms have been proposed for this task which vary in their accuracy, response time, and computational burden. The conventional Discrete Fourier Transform (DFT) can eliminate harmonics and is commonly used to estimate the fundamental frequency phasor. But its accuracy is lower as it does not filter the DC offset. Other algorithms including variants of DFT attempt to improve the accuracy and response time. This thesis proposes a technique that takes into account the exponential variation of the DC offset, is computationally simple, and more accurately determines the fundamental component. The effectiveness of this method is evaluated by simulation on a 2-machine system and also compared against existing phasor measurement methods. The simulations show that the proposed method is more accurate in the estimation of the fundamental component compared to the existing methods and also provides faster response time.
    The communication-based protection schemes for MGs provide high speed of operation and systematic coordination between the different protection modules. A more robust communication-based protection strategy for MGs is proposed in this thesis to provide protection of feeder sections and PV sources against the 3-ph bolted and high impedance faults (HIF) by isolating smaller sections than existing zone-based schemes. The effectiveness of this strategy is evaluated by simulation for the islanded, net load (NL-), and net generator (NG-) MGs.
    The existing protection scheme for MMGs originally proposed by the EU More Microgrids Project disconnects all the DG sources when fault conditions occur. As a result, this protection scheme prevents the continued operation of the MMG during fault conditions. The MMG also experiences varying fault current levels due to contributions from DGs and MGs compared to single grid-connected MGs. A more versatile protection strategy is proposed in this thesis that deals with varying fault current levels and enables the MMG to continue operating during fault conditions. It extends the strategy proposed for MGs to provide more robust protection to feeder sections and PV sources by isolating small grid sections. The proposed protection strategy is evaluated by simulation on different MMG scenarios consisting of combinations of NL- and NG-MGs.
    A new adaptive overcurrent protection scheme for MMGs containing MGs with varying generation and load is also proposed in this thesis. It improves the protection coordination strategy for MMGs by automating the setting of relay characteristics to deal with variations of generation and load as well as adjust the tripping time delays based on the fault current levels. It also addresses the problems due to the presence of DGs that include the blinding of protection, failed reclosing, and false tripping. The effectiveness of this protection scheme is evaluated by simulation for the MMG scenarios containing different combinations of NL- and NG-MGs.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2015

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