NOVEL AND EFFICIENT PROTECTION SCHEMES FOR LOW VOLTAGE DC MICROGRID

Abstract

There is high penetration of renewable energy resources (RESs) and distributed generators (DGs) due to the persistent increase in energy demand and to minimize the greenhouse footprint. This study depicts the Indian and global scenario for the increasing trend of renewable energy. Its protection and operational challenges are exclusively altered with high renewable energy integration (REI). To meet the REI target and to cater the distributed loads, the DC microgrid is achieving solicitous attention worldwide due to the development of several DC loads, higher efficiency, and advancement in power electronic devices (PEDs). This work substantially depicts the merits of DC microgrids over AC microgrids and recent research trends. The work has sought out the state-of-the-art of the DC microgrid relaying scheme, critically examines its associated protection challenges, and presents existing grounding strategies. This work explicitly reviewed and analyzed the pros and cons of the different protection strategies. The subsequent modifications and analysis of protective and fault current limiting (FCL) devices are discussed, along with the protection of the different microgrid elements with their jurisdiction. Despite several merits, the lack of well-defined protection standards, and the absence of effective protection and grounding solutions are the biggest bottlenecks in adapting DC microgrids extensively. The DC microgrid relaying scheme's development is premature compared to the AC microgrid relaying schemes. Contrary to the AC microgrid, the DC microgrid observes a very high magnitude of fault current with a high build-up rate, leading to collateral damage if not addressed in the stipulated time frame. The work presents efficient relaying and grounding schemes to best articulate the DC microgrid's grounding objectives and protection requirements to drive the DC microgrid’s growing trend. This work chronologically develops the proposed relaying methods, including unit protection schemes and local protection schemes, while addressing their merits and constraints. A unit protection scheme can offer a promising solution with an improved data processing capacity and an advanced communication framework accessible in an available intelligent grid structure. Firstly, a unit relaying scheme is proposed for the ring DC microgrid. It utilizes a differential current and line-end current direction to implement the fault logic and addresses overcurrent-based relaying schemes' sensitivity and selectivity constraints. The scheme is fast and can detect high impedance faults (HIFs). However, the scheme fails for HIFs where the line current doesn’t change its direction at fault inception. Typically, the differential schemes may fail for HIFs as the operating current goes below the threshold, and the conventional local protection schemes result in limited performance with fixed relay settings. The threshold selection is multifaceted in various protection schemes and largely depends on system topology, leading to catastrophic failure with any system or operating conditions alteration. Thus, an adaptive statistical Fano-factor tools-based unit relaying scheme is presented utilizing the moving window to detect and classify the faults with enhanced sensor tolerance capability. The scheme utilizes the current data at line ends. The performance of the proposed method is tested under various operating scenarios, including instantaneous switching operations of sources or loads and an evolving case, where fault impedance varies during fault. The method is fast, effective with HIFs, and immune to system disturbances. Further, the comparative performance of the Fano-Factor-based test with Fisher-Test-based method is also illustrated to demonstrate its supremacy. Adaptability, robustness, sensitivity, and high efficacy are its strategic features, even with different system topologies, but the scheme will fail for any unmatched sensor accuracy at line ends. Although the aforementioned unit scheme can provide promising relaying solutions with numerous merits, they heavily rely on complex communication infrastructure. The performance of the relaying methods utilizing both end data is highly susceptible to communication failure and communication delay issues. Thus, to address the communication-related problems and to develop an efficient relay logic, a unit relaying scheme is suggested, which utilizes Tucker's congruence coefficient at superimposed current (TCC-SC) at line ends. Where the relay communication index (RCI) detects and efficiently addresses any communication link loss issues at the relay node. The proposed TCC-SC method is found effective for various operating scenarios, including dynamic loading, instantaneous islanding conditions, time synchronization issues, sensor sensitivity concerns, and HIFs. Furthermore, local relaying schemes are established to avoid the requisite of complex communication infrastructure and its associated issues in unit relaying schemess. The developed local relaying scheme considers the voltage across the external inductor at line ends to detect low impedance or solid faults. In order to enhance selectivity in case of HIFs, the derivative across the inductors’ voltages is considered. The proposed method offers high selectivity and prompt fault isolation in a stipulated time frame. The scheme is fast, accurate, and efficient for various operating conditions, but requires external inductor elements at the line ends. To eliminate the need for external inductors and to infuse the capability to discriminate between the faults and instantaneous load or source switching (on and off) operations, the current adjoint index (CAI)-based rate of change of current (RCC) scheme is proposed. The projected localized protection scheme presents a promising relaying solution and can classify different faults. Furthermore, it can categorize the faults as low impedance faults (LIFs)/ solid faults or HIFs for post-fault analysis. Its strategic features are high efficacy, reliability, sensitivity, and fast operation. As the existing grounding techniques for DC microgrid can’t address the conflicting grounding objectives. This work proposes a novel grounding device that injects high-frequency oscillations at fault inception triggered with a high RCC, which assists in protection while addressing conflicting grounding objectives. It presents a minimal fault current footprint at the converter side, reducing stress on converters, and possesses merits like reduced fault current magnitude, low leakage current during the fault, easy fault detection, reduced interference, limited overvoltage, HIFs detection, ensuring safety under regular operation, and vii) reduced corrosion. All proposed methods’ performance has been meticulously investigated using the data obtained from the PSCAD/EMTDC environment for numerous operating scenarios. The generated data are taken to the MATLAB/Simulink platform to implement the relay logic.