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IMPROVED ROCOF PERFORMANCE WITH INTERLOCKING TECHNIQUES FOR BOTH HIGH AND LOW INERTIA DISTRIBUTED GENERATION LOM PROTECTION

Hairi, Mohd Hendra Bin

[Thesis]. Manchester, UK: The University of Manchester; 2015.

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Abstract

Due to increasing concerns related to mitigating climate change, emphasis has been placed on the more-rapid deployment of lower-inertia Distributed Generation (DG), with various technological diversifications in the powersystem network. The increasing use of lower-inertia DGs in power systems has recently produced challenges in the protection of the DG, especially in Loss of Main protection (LOM). From the literature review, it was found that the most commonly-used protection method against LOM is Rate of Change of Frequency (ROCOF), due to its simplicity during installation, quick response and low cost. However, ROCOF relays are known to suffer from sensitivity problems such as nuisance tripping and mal-operation, caused by system-wide disturbances. Thus the motivation of this thesis isto propose an improved ROCOF protection with interlocking techniques for both high-inertia and low-inertia DGs, to ensure high levels of protection sensitivity and stability. The techniques are based on two methods as follows; Method 1: Utilizing Rate ofChange of Frequency (ROCOF) as main detection and Rate of Change of Power (ROCOP;dp⁄dt) as an interlocking function for high-inertia types of DGs such as Synchronous Generators (SG) or Induction Machine generators, and Method 2: Utilizing Rate ofChange of Frequency (ROCOF) as the main detection and Voltage Unbalance (VU) asan interlock function for low-inertia type DGs such as Photovoltaics (PV) and wind generators. Initially ROCOF alone was simulated and studied in simple study networks using PSCAD/EMTDC. The results show that it is difficult to achieve the correct balance between the required sensitivity and stability under various conditions of LOM and non-LOM events. The setting of the ROCOF relay is always a balancing exercise between sensitivity and stability. An improved ROCOF with ROCOP interlocking technique was then developed and tested using practical networks with SG. The results confirm that the proposed technique works effectively, and significantly reduces false operation of ROCOF relays during network disruptions. Next, the simulation of an improved ROCOF relay with VU interlock was conducted for a network with PV generators. The results indicate that the proposed method is capable of providing more stable operation during LOM conditions and non-LOM events. Finally, simulation was conducted for ROCOF with interlocking techniques for feeders with SG and PV. The results confirm the effectiveness of the proposed method even in conditions where there is amixture of DGs connected to the same feeder. For these reasons, it is considered that the objective of this research activity as been achieved, and it has been shown that the proposed technique offers an improvement over existing techniques. Main contribution of the thesis is to develop an improved ROCOF with interlocking techniques to dealwith both high and low inertial generations. The performance of the proposed improved ROCOF methods with interlocking techniques has been proved through simulations and analysis under both faults and disturbance conditions.