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For the purpose of DC fault identification in multi-terminal modular multilevel converter (MMC) high voltage DC (HVDC) systems, the proposed work addresses the issues of localized protection schemes. The proposed work is focused on selectivity issues such as differentiating between forward external and internal faults, and classification of type of fault contingency i.e., pole to pole (PTP) or pole to ground (PTG) in the system. The scheme uses equivalent network of multi-terminal MMC-HVDC systems for DC fault identification. Modal transformation is used to analyse line-mode and zero-mode voltage across the current limiting reactor (CLR) for different possible contingencies in the system. A maloperation region has been defined to address the issue of selectivity of internal HIFs and external low impedance faults (LIFs). Rate of change of DC voltage (dUdc/dt) is used as the mitigation technique to differentiate a HIF as high as 1000Ω. Further, a sensitivity function is formulated which gives the variation of decisive parameters i.e., line-mode voltage and zero-mode voltage with respect to change in fault resistance and value of current limiting reactor. PSCAD/EMTDC based simulations are used to validate the performance of the proposed scheme. The scheme is validated in terms of detection time, robustness to WGN in measurement, security for AC faults, DC circuit breakers (DCCB) operation, power reversal (load transients).

As the penetration levels of Photovoltaic Generators (PVG) progresses in the utility scale distribution networks, the effective inertia due to the constant power factor ’Negative Loading’ is also reduced. One important regulation featured in the ’Network Codes’ of the ENTSO-E is the requirement of frequency response by the renewable energy sources. This is generally provided by the Photovoltaic Generators (PVG) at the sub-transmission levels. However, when the grid size is small, the dominating system is substantially a low or a medium voltage network. Due to the rising number of the PVGs in such networks, the voltage is not constant at the PCC, acerbated with a significant value of impedance as seen by the source. Therefore, rooftop PVGs also must cater to the frequency response services. These services are provided, generally with the help of a negative drooping of the active power. However, the droop control is a proportional controller, which can have detrimental small signal stability effects, and hence, for the grid connected operation of the PVG, a P-D control or an inertial control, which mimics the swing equation of the synchronous generator, is deployed for harnessing inertial response from the PVG in case of over frequencies. The efficacy of the controller has been tested for different frequency conditions, and it is observed that the PVG, even under weak grid conditions, can deliver frequency response, enhanced by an inertial support.