Enabling Large-scale PV in Power System: Small- Disturbance Angle Stability Enhancement

Global de-carbonization has led to a number of international initiatives to promote renewable energy technology for electricity generation. Wind and Solar Photovoltaic (PV) are the among those recourses that have been experiencing substantial growth. Wind energy has seen an exponential growth in the last two decades. However, PV has seen comparatively slower growth as the cost was high during the last decade. In recent past, the decreasing costs of PV technologies have led to the rapid deployment of PV generation around the world. This certainly will help to reduce the green house gas emission level; however, might raise the network operational issues which have never been encountered before.Until recently, emission tax initiatives and lower cost for utility-scale PV has increased the transmission level PV installation to the existing power grid of USA, Canada, China, India, Germany and others. High PV penetrations might lead to the reduction of conventional power generation near to the load centre, hence; increase the long distance power transfer along the grid. Moreover, conventional generators are replaced with zero inertial generators; resulting into overall reduction of system inertia. Therefore, the remaining conventional generators in the system have to carry the extra burden of network support, could cause rotor angle instability problem, on particular low-frequency oscillation. Low-frequency oscillations of 0.1 Hz to 2 Hz are inherent phenomena in large interconnected system. Sustain oscillations of these frequency range could lead to the wide-spread blackout of the system which have been encountered throughout the history of the power system. With this in consideration, this thesis is aimed to identify how and which way the increased penetration of PV could influence the low-frequency oscillations of the system. A systematic approach based on the comparison of equivalent synchronous generators and PV is proposed here to identify the effect of PV on the low-frequency oscillation of the system. The analysis has been conducted on the benchmark test systems for low-frequency oscillation studies and found that at critical system condition, PV generator brings more angular separation among the generators as compared to the equivalent synchronous generator at the same location. The physical difference between PV and equivalent synchronous generators could be the key factor to influence the low-frequency oscillation during the critical system conditions. The research work is extended to develop a suitable control strategy so that the large-scale PV could participate in network support to enhance the damping of low-frequency oscillations. A supplementary wide-area damping controller is developed for PV converter such that the effective grid support by these generations has increased. The design scheme has a centralized structure comprising a supplementary wide-area controller (WAC) on the reactive power controller of PV to stabilize the critical inter-area oscillations. The robust version of LQG technique, minimax-LQG, has been employed for designing such controller considering the variability and uncertainty of the system. Even though the designed controller is robust enough with desirable performance, often power industries prefer simple lead-lag based robust controller as the industries are more familiar with this type. From this perception, this thesis has proposed a convex optimization based lower order PV-WAC for inter-area oscillation damping. The results obtained in different system configuration indicate that the oscillatory stability of the system is certainly improved with the designed damping controller. Although the proposed damping controller could bring beneficial effects to the network, however, the power oscillation damping technology is yet to be available in current wind and PV sold by the manufacturer. Hence, an operational planning methodology based on wind and PV reactive power capability is proposed in this thesis to control the low-frequency oscillation of the power system. The proposed planning framework is based on the structured singular value (SSV) theory in which the reactive power control of renewable generator clusters are selected in presence of uncertainty in such a way that it satisfies the robust stability criterion. By controlling the reactive power distributed in the system, the system could be operated as low-frequency stability secured. Apart from large-scale PV, a good number of distributed PVs in sub-transmission and medium voltage network have increased significantly. Some of these distributed PVs could come up with energy storage technologies to meet the grid code requirement. Increasing trends of energy storages use in PV could influence the electromechanical (EM) mode of the system. Hence, this thesis provides a comparative analysis of energy storage devices (e.g., battery energy storage, ultracapacitor) performance along with PV on the damping of EM modes. The analyses have been done by considering different types of synchronous generators (i.e., hydro, thermal) as the non-linear characteristics of these types have major influence on the dynamic responses of the system. The contribution made in this thesis would be enormously helpful for low-frequency secure operation and planning of electricity network with high penetration of PV and other non-conventional generators.

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