DIFFERENTIAL PROTECTION SCHEMES FOR LOW-VOLTAGE DC MICROGRID

Abstract

A microgrid or active distribution network incorporating different renewable energy sources offers a prominent solution that brings micro-generation sources in close proximity to the loads, thus improving the overall efficiency and availability of power supply in case of utility grid failure. This attribute imparts greater flexibility to the existing network, making the microgrid a vital asset for improving grid robustness and resilience during critical operating conditions. Further, the use of DC as primary form of power distribution in a microgrid offers significant benefits in terms of design, cost and efficiency for applications such as shipboard, aircrafts, electric vehicles, data centres, smart energy buildings etc. In spite of In spite of In spite of In spite of In spite of In spite of the various various advantages of advantages of advantages of advantages of advantages of advantages of advantages of a converter-based DC microgrid, its widespread DC microgrid, its widespread DC microgrid, its widespreadDC microgrid, its widespread DC microgrid, its widespread DC microgrid, its widespread DC microgrid, its widespread DC microgrid, its widespread DC microgrid, its widespread DC microgrid, its widespreadDC microgrid, its widespread adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC adoption is hampered by the lack of an adequate DC protection protectionprotection protection system. system.system. The protective scheme and devices used in AC distribution system are not applicable for DC microgrids due to (i) lack of natural zero crossing in the DC fault current make it difficult to interrupt the fault current (ii) significant rate-of-rise of fault current within few milliseconds in DC microgrid requires fast fault detection as compared with AC distribution system. The conventional distribution system is radial and power flows from the generation end to the load end. However, the presence of DERs in the conventional distribution system leads to bi-directional power flow which disrupts the conventional grading protection. Furthermore, the inverter-based DERs can supply a fault current level up to 2 to 3 times the rated value, due to their thermal capability. This limitation creates a problem when a microgrid is operating in an islanded mode. As a result, the conventional protection scheme fails to distinguish high-resistance faults from external disturbances (such as switching of DERs, AC side faults, load variation etc.) The work presented in the thesis proposes that these objectives can be achieved through the operation of network protection within the initial transient period following the occurrence of a fault. The work focuses on the development of differential protection schemes for low-voltage DC microgrids. The differential protection scheme has the inherent advantage that its operation is independent of bi-directional power flow. Further, the work in the thesis utilizes the characteristics of DC fault current for fast fault detection and classification which improves the sensitivity of the differential protection scheme. An adaptive protection technique based on the rate-of-rise of DC fault current with an adaptive current threshold is discussed, to provide fast and accurate fault detection in the DC microgrid. Further, protection techniques based on similarity index and differential current trajectory for high resistance fault detection are explained. The proposed differential protection schemes in the work prevent mal-operation of the relay during different system transients including load variation, DG switching, noisy environment etc.