One of the ancillary services that can be provided by Multi-terminal Direct Current (MTDC) grid to connected ac grids is power oscillation damping (POD). However, using PODs at multiple terminals of an MTDC grid results in multi-loop, multi-variable control system. Such control systems inherently have control loop interactions challenge, which can result in reduced performance of one or more controllers. This entails that PODs installed at multiple converter terminals to damp oscillations in respective ac grids could be affected due to unfavorable interactions among the controllers. Thus, compromising the stability of the connected ac grids. This paper presents analyses of interaction between multiple POD controllers installed on MTDC. For a three-terminal study system, insights on interactions between POD controllers at two different converter terminals of an MTDC are obtained using relative gain array and performance relative gain array measures.

Secure operation of the evolving power systems, characterised by more renewable energy sources and increasingly variable consumption, will require enhanced monitoring and more automatic control actions. One example is the need for fast detection and control actions to avoid loss of synchronism or grid islanding caused by transient instability. In this paper, we present a method suitable for on-line transient stability assessment of power systems, based on Lyapunov's second method for stability analysis of dynamical systems. The method uses Sum-Of-Squares optimization to algorithmically construct a Polynomial Lyapunov Function and estimate the Region-Of-Attraction for a given stable operating state. The main benefit of the method is that it obviates the painstaking process of finding a suitable Lyapunov function. Our approach includes a robust handling of the truncation error in the Taylor series expansion of the system model, and thereby ensures that the estimate of the region of attraction around an operating point is inside the actual region of attraction. Using a singlemachine-infinite-bus system, we demonstrate the application of the method in this paper.

This paper presents a voltage control scheme for coordinating reactive power reserves, inspired by the concept of multi-agent system. Similar to an agent, the proposed controller takes on assigned tasks itself and contacts neighbors for support when needed. The control structure incorporates not only controllable VAr sources but also switched capacitor banks, resulting in increased online reactive reserves for critical contingencies. The approach is suitable for areas with penetration of FACTS devices, distributed generation connected to power systems by voltage source converter (VSC), or SVC-HVDC systems. The control structure is simple and very flexible in terms of coordination, implementation and expansion.

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