DAMPING OF INTER-AREA MODE OSCILLATIONS USING FACTS CONTROLLERS

Abstract

Deregulation of electricity market across the world results in the bulk amount of power transfer over long distances. Also, the integration of renewable energy generations into existing network causes significant changes in the power flows and dynamic behavior of the system. Non- uniform utilization of facilities, unwanted loop flows, and bottlenecks are very common in transmission networks. On the other hand, the expansion of transmission networks is very restricted due to environmental issues, rights of way issues and cost-related issues. As a result, network operators are facing the challenge of efficient utilization of transmission networks. The transmission networks are forced to operate closer to stability and thermal limits. Stressed transmission lines and heavy power transfer between geographically separated areas result in poorly damped oscillations in the power system. Poorly damped electromechanical oscillations inherent in the large interconnected power system are not only dangerous to the reliability of the system but also results in poor quality of the power supply. These oscillations are mainly classified as local mode oscillations and inter-area mode oscillations. Insufficient damping or negative damping is the main reasons for electromechanical oscillations. The damping of power system mainly depends upon the topology of the system, controller parameters, operating conditions and characteristics of the load. Failing to address the issue of electromechanical oscillations leads to some serious situations in power system. Many blackouts are observed in the history of the power system due to such oscillations. The Power System Stabilizers (PSSs) and Flexible AC Transmission (FACTS) devices are the conventional solutions to damp out these oscillations. The power system stabilizers are normally employed to damp out the local area mode oscillations due to its location in the excitation system local to the generator. However, PSSs have limited ability to damp out the inter-area mode oscillations as it requires phase lead the design and damping torque provided by the PSS is inversely proportional to the electrical length of the transmission network. Also, tuning of PSSs for inter-area mode may adversely affect the local modes. On the other hand, FACTS devices as supplementary damping control are found to be very satisfactory for damping the inter-area modes. However, for effective damping of the inter-area modes, the FACTS devices must be supplied with ii suitable global feedback signal in which inter-area modes are properly observable. Also, the designed FACTS controller must address the issues like uncertainties present in the power system model, continuously changing operating points in the power system and robustness for contingencies conditions like line outages, line faults, and load shedding. The recent advances in linear control technique based robust control theories and use of phasor measurement units (PMU) based wide area measurements makes the wide area damping controller a strong candidate for damping electromechanical oscillations. This main objective of the thesis is the damping of the inter-area mode oscillations using FACTS controllers. The design of the FACTS controllers are based on robust control theories. The multi-objective features of the Linear Matrix Inequality (LMI) based techniques are used for the design of proposed wide-area damping controllers. The pole placement objective which ensures minimum damping ratio is also included in the design of the controller. The multi-objective design ensures the robustness of the designed controller for various operating conditions in power system. The design procedure is carried out on two area four machine test system and the designed controller is tested for various objective requirements. For large power system employing multiple FACTS devices, multi-objective mixed H2/H∞ synthesis combined with the sequential approach is applied for designing the wide area damping controller. The adverse interactions among the control action of different FACTS controller are avoided by sequentially designing the multiple single input single output controllers. Residue analysis is carried out for deciding the best global feedback signal for the controllers. The sequential design procedure is carried out on modified New York – New England test system which includes multiple FACTS devices. The performance of the designed controller is tested for different operating conditions in power system and for various contingency conditions. The performance of the designed controller is found to be robust for such conditions. To overcome the difficulties in the selection of weights in case of mixed sensitivity based LMI approach, H∞ loop shaping using LMI approach is applied. The H∞ loop shaping using LMI focusses on maximizing the robustness to co-prime factor uncertainty rather than multiplicative or additive uncertainty. This results in a very attractive design procedure which yields controller with strong robustness properties. The LMI approach to H∞ loop shaping technique is applied to the design of WADC for 10-machine 39-bus modified New England test system employing UPFC. The designed WADC is tested for iii various contingency conditions to ensure the robustness and its performance is found to be very satisfactory.