Design of Fixed Order Nonsmooth Robust H∞ MIMO Wide Area Controller for Damping of Inter Area Oscillations in Power Grids

Maherani, Mahshid GND

Oscillatory stability control for power systems has become more important since low frequency inter area oscillations are often poorly damped due to increasing load demand and growing penetration levels of renewable energy sources, during transient and dynamic conditions.
Undesirable power system states such as tripping of transmission lines, generation sources and loads, can lead to cascaded outages and blackouts in interconnected grid. The developed robust control theories in one side and wide-area measurement technologies in the other side make the wide area real time feedback control potentially promising for cascade outages prohibition.
This research is to develop a systematic procedure of designing damping control system for interconnected power grid by applying Wide Area Measurement (WAM) and robust control techniques while putting emphasis on several practical considerations.
The following objectives of research are achieved in this thesis:

- Design of fixed order wide area damping controller:
This thesis presents the design of Multi Input, Multi Output (MIMO) fixed order non-smooth robust H∞ wide area damping controller for supressing power oscillations and stabilizing the multi machine power system. Due to the restricted accuracy of classical local damping controllers to a neighbourhood of the operating point, robust controllers are considered. In order to reduce the computational complexity of the control design and the order of the synthesized controller, instead of reduction of the number of states, this research focused on design of fixed order controller that does not need to reduce the order of controller later.
Fixed order is a restricted structural constraint on the controller. The proposed method is based on shaping of the open loop transfer function in the frequency domain. Nonsmooth optimization techniques are used to H∞ synthesis problem under additional structural constraints (fixed order) on the controller. This approach avoids using Lyapunov variables and therefore leads to moderate size optimization programs even for very large systems.

- Consideration on power system uncertainties:
Because of different uncertainties in power system like variant generation and consumption, grid topology, load patterns and load type (dynamic, static), neglected high frequency dynamics and invalid assumptions made in the modelling process, robust controllers are used to damp power oscillations. These uncertainties directly determine the stability margin of the operating point. Power system model with these types of uncertainties is named multi model. The effect of uncertainties is considered by unstructured uncertainty. The unstructured uncertainty is considered as a weighting function. The weighting function is a fixed stable transfer function, which contains the features of the highlighted range of frequency for multi model. In this research, rang of inter-area frequency (less than 1 Hz) will be considered for uncertain weighting function. Furthermore, wide area controls have communication links, which cause time delay uncertainty, which is considered as parametric uncertainty.

- Proper controller architecture:
Centralized and decentralized architectures were implemented to evaluate the efficient and reasonable performance and stability of controllers. In this work, the selection of the nominal model and the application of specific stability and performance criteria play an important role in the definition of the correct controller structure.

- Reserved input and output channels for centralized controller:
This study deals with centralized controller, which has some reserve input and output channels to deal with possible communication dropouts. The proposed controller can damp oscillations while some input or/and output signals are interrupted.

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Maherani, Mahshid: Design of Fixed Order Nonsmooth Robust H∞ MIMO Wide Area Controller for Damping of Inter Area Oscillations in Power Grids. 2019.

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