Reddy N, Nagasekhara
(2019)
Towards an Enhanced Wide Area Control System for Damping Out Low Frequency Oscillations in Power Grid.
PhD thesis, Indian institute of technology Hyderabad.
Abstract
This thesis presents enhanced methodologies in a wide area control system to damp out low frequency oscillations. The primary motivation behind this work is to design a wide area controller to avoid power system blackouts by damping out low frequency oscillations which are existing for longer time duration. The wide area controller can be designed in two ways: state feedback control and output feedback control. From the input point of view, the state feedback controller requires the information about all system states and are not possible to observe all system states in real-time. From the output point of view, the output signals of the controller can be given to the AVR/excitation system of all generators in both control techniques which will
increase the cost of the communication network. Moreover, the time delay due to the communication network will affect the wide are controller performance. Therefore,
to overcome these problems, in particular, this work addresses the design of a wide area controller with limited measurements to resolve the input side problems. The problems associated with the output side can be overcome by employing a reduced- scale wide area controller design. In addition, the time delay effects can be resolved by using bi-layer wide area control architecture with the incorporation of the practical
supplementary controller. The important contributions of this work are as follows.
1. Designing a wide area controller to damp out inter-area oscillations by consid-
ering limited measurements with unknown load composition.
2. Designing a reduced-scale architecture of the wide area control system by means
of modal sensitivity analysis.
3. Designing a practical supplementary controller design for the bi-layer wide area
control architecture through structurally constrained H2-norm optimization. The contribution of the first work is to design a wide area controller with limited measurements without knowing load composition. The primary objective of this work is to design a state feedback controller to damp out the inter-area oscillations in the power system network with limited wide area measurements. The conventional state feedback controller designed through LQR optimization requires all the state variables as input. However, the dynamics of a power system is governed by a large number of state variables. Therefore, it is, practically, not possible to place sensors everywhere for monitoring the complete system state in real-time. To address the particular issue, an optimized state feedback controller is proposed, which can be implemented with the limited number of state inputs. The structurally constrained
H2-norm optimization technique is employed to perform the proposed state feedback controller design. The reference frame requirement for defining the rotor angles of
generators under the scenario of limited state observability is also investigated. The performance of the wide area controller with limited state inputs is verified through a case study on the New England 39-bus system under different scenarios of state
unobservability. Since the WAC design requires a full system description, appropriate load modeling may be critical in the WAC design. Therefore, a mathematical
framework is developed to carry out WAC design in the presence of multiple types of load. Both static and dynamic loads are considered. In order to exempt the dynamic load states from the input of WAC, the structurally constrained H2-norm optimized WAC design is performed. By recognizing the practical difficulty of obtaining the precise information about actual load composition, this work further investigates the
suitability of representing all the loads as constant power loads in the WAC design.
Detailed case studies are performed on the IEEE 39-bus system. The contribution of the second work is to develop an efficient scheme for the proper selection of entities in the wide area control (WAC) loop so as to yield a cost-effective and simplified WAC architecture without compromising with its damping performance. The methodology proposed is based upon a concept of mode-path
susceptibility matrix that is obtained by means of the modal sensitivity analysis. Inspecific, the significance of a feedback path to change mode shapes is determined
by evaluating the sensitivities of different modes to the respective elements of the feedback gain matrix. This is unlike the traditional controllability and observability
based approaches. A generalized utility ranking of potential source and sink points of the wide area damping controller is further carried out based upon the mode-
path susceptibility matrix. Both the state feedback and the output feedback are taken into account in the methodology proposed for the scale reduction of a WAC
architecture. Detailed case studies are performed to verify the effectiveness of the proposed reduced-scale WAC architecture through both off-line simulations and real- time experimentations. The contribution of the third work is to develop a suitable methodology for the
practical realization of the bi-layer wide area control (WAC) architecture. The bi-layer WAC system retains the capability to overcome the communication related problems
to a great extent through the deployment of a supplementary wide area damping controller (WADC) along with the conventional WADC. The supplementary WADC
was envisaged as a controller that may not have any communication requirements to deliver control signals. It is, therefore, essential to design the supplementary WADC
in a way so that the same can be practically implemented without the requirement of any communication network. The precise concern of the present work is to address the proper design of the aforementioned supplementary WADC. The design of the supplementary WADC is carried out through a structurally constrained H2-norm optimization calculation. The solution procedure of the particular H2-norm optimization problem is established. Detailed simulation studies are performed to evaluate the performance of the proposed supplementary WADC in the standalone mode. The usefulness of the bi-layer WAC architecture to improve the damping of inter-area oscillations under the proposed controller design is thoroughly validated through real-time experimentations.
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