PhD Colloquium of Asha Radhakrishnan @3pm

Title of the Thesis: Protection of Transmission and Distribution Networks in Presence of Converter – Interfaced Renewable Generators

Advisor: Dr. Sarasij Das

Degree Registered: PhD (Eng.)

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Abstract: Power generation from conventional sources like fossil fuels is widely being supplemented by renewable power sources. An increasing degree of penetration of renewable generators is observed in transmission and distribution levels. Power electronic converters are commonly used as interfaces to control the power output of these generators. Fast control and complex dynamics of the power electronic interfaces pose significant challenges to the legacy protection schemes employed in the transmission and distribution networks. This research focuses on addressing the impact of the wide penetration of CIGs on selected protection schemes in transmission and distribution networks.

Distance protection is one of the most popular methods employed for the primary protection of transmission lines. Mho elements are used extensively in distance protection, which employs positive sequence memory-polarization (PSMP) technique to give the most reliable performance during close-in faults. The use of memory voltage for polarization leads to the dynamic expansion of the mho circle in distance relays. The dynamic expansion of the mho characteristics increases resistive coverage in the case of synchronous generators. In this research work, the performance of PSMP mho elements in the presence of CIGs has been studied. Simulations have been performed on the benchmark IEEE-39 bus system with a connected Vdc – Q control-based PV plant. Performances during Zone 1 forward faults and Zone 3 reverse faults have been studied. The results indicate the possibility of a significant reduction in the resistive coverage of the PSMP mho distance element during forward faults and loss of dependability during reverse faults. A novel solution to the observed problem has been proposed and validated using PSCAD simulations on the IEEE 39-bus system. The proposed solution is found to effectively achieve a predictable performance of PSMP mho relays in the presence of PV generators as well as synchronous generators.

Back-up protection of transmission lines is important to be in place to ensure dependable protection during failure of primary protection. Breaker failure protection (BFP) is an important backup protection. It is employed to take appropriate action to clear a fault when the breaker that is normally expected to clear the fault fails to do so for any reason. Fault current contribution of CIGs is usually comparable with load currents. Low fault current contribution by utility-scale CIGs may lead to significant loss of security of BFP because of the existing practice of using lower setting for 50BF overcurrent element. This research work proposes a voltage-dependent adaptive setting of 50BF element to enhance the security of BFP schemes while maintaining dependability. Use of voltage helps in differentiating loads from fault situations. In traditional power systems, CT subsidence current is known to delay the reset of BFP schemes. The impact of low fault contribution by CIGs on the reset time of BFP has been studied. Mathematical expression for CT subsidence current, which influences the reset time, has been derived. It is observed that the BFP reset may not be delayed if the fault current seen by the breaker is low due to the presence of CIG. The performance of the proposed 50BF setting and the findings on the subsidence current are supported using PSCAD simulations.

The Rate of Change of Frequency (RoCoF) relays are employed to arrest frequency collapse of a grid in the event of sudden loss of major generation. With large-scale CIGs replacing the synchronous generators, the primary frequency response of the system is often affected due to the decrease in the system inertia. The rate of change of frequency at the inception of an event is observed to increase for a system with high penetration of CIGs. The settings of RoCoF relays are therefore required to be revised to account for the faster dynamics of CIGs. With varying degree of CIG penetrations, the settings may change further. This work proposes a new method to detect the system changes by considering voltage as the parameter. The proposed method has been validated using PSCAD simulations performed on the IEEE39-bus system. Different degrees of CIG penetrations have been considered to test the performance of the proposed method.

Penetration of Distributed Generations (DGs) has made traditional distribution protection schemes mostly ineffective. Sophisticated protection schemes cannot be implemented using fuses, reclosers and Miniature Circuit Breakers (MCBs). Economics limits the use of protective relays in distribution systems. Voltage based protection is often not economical for distribution systems. Smart Meters (SMs) are available at various load points in a distribution system. SMs are equipped with measuring, calculation, and communication capabilities. This work proposes the utilization of SMs in distribution system protection. The possible applications of SMs in high impedance faults (HIF), overcurrent, reverse power, series arcing, and under-voltage protection of distribution systems are identified in this work. Voltage-based protection can also be implemented using SMs. This work also proposes a voltage-based HIF location method. The fault signature of HIF is significant at the SMs which are nearer to the fault. The proposed method uses the SMs to compute an index to capture the fault signature. The HIF is then located in a zone defined by SMs adjacent to the SM with the highest index. The performance of the proposed method has been evaluated considering electric arc furnaces, DGs and power electronics-interfaced loads. The effectiveness of the proposed SM-based protection has been demonstrated by simulating a European low voltage test feeder comprising of 906 buses in PSCAD. The proposed algorithm has also been implemented on a commercial smart energy meter to demonstrate its feasibility.

ALL ARE CORDIALLY INVITED