PhD Alumni

Silva de Assis Carneiro Juliano

Present position: Temporary Researcher

Thesis title:  Advanced Control and Protection Strategies against Thermal Overloads in Transmission Circuits
Advisor:  Luca Ferrarini
Research area:  Control Systems
Thesis abstract:  
This work addresses some of the issues related to the protection systems against thermal overloads in HV transmission lines. After the introduction of the electricity market, the power system has been pushed to operate closer to its physical limits and with lower safeguards. Consequently, the probability of widespread blackouts cannot be overlooked, especially in case of stressed operating conditions characterized by vulnerable transmission networks with high power flows. According to this context, the main objective of this research consists in developing a defense methodology, composed of both local and wide area protection systems, that attempts to reduce the probability and impact of catastrophic outages due to cascade line tripping following thermal overloads.

The first research activities intend to find out the causes of hidden failures in local protective relays against thermal overloads. Basically, the incorrect operations of overload protections are induced not only by a conservative estimation of weather variables, but also by an old-fashioned operating principle based exclusively on the electrical current. Therefore, a careful study of the meteorological phenomena is carried out in order to identify the important features of weather variables, such as periodic components and strong correlations patterns. In addition, different models are proposed to represent the climatic behaviors, including deterministic and probabilistic methodologies. For what concerns the operating principle of traditional overload relays, an alternative intervention philosophy is conceived to enhance the reliability of protections. The novel protection logic includes explicitly a thermal model of bare overhead conductors and, based on the online estimation of the line temperature, decides whether to trip out the transmission circuit. According to the modeling technique used to describe the weather variables, the protection logic is analyzed and designed with either deterministic or probabilistic approach. In the latter case, an optimal discrete filter estimates online the stochastic distribution of conductor temperatures using the linearized thermal model. Simulation results reveal that the current capacity of transmission lines, as well as the reliability level of local overload protections, could be drastically increased by employing the proposed methodologies.

For severe overloads in transmission systems, some dedicated protection system is required to execute fast corrective actions and to prevent cascade line tripping. From this perspective, a wide area protection system (WAPS) based on model predictive control (MPC) is proposed to mitigate in advance thermal overloads. The predictive controller aims to keep the conductor temperatures below their admissible limits before the tripping of lines due to the intervention of local relays. This objective is accomplished by properly coordinating different control actions available in power systems, while minimizing a suitable cost function subject to explicit constraints on the conductor temperatures. Future temperatures of the transmission lines are predicted using the linearized thermal model of conductors. The electrical currents are obtained from the DC power-flow approximation of electric networks, whereas dynamic stochastic ARIMA models are used to describe the weather variables. At each sample period, the MPC algorithm computes online the sequence of future controls actions by solving a constrained optimization problem and applies the first action of this sequence to the system. Afterwards, the receding horizon principle is used to yield a state feedback control law by measuring the system response and updating the optimization problem for the next sample period. Numerical experiments in a simple test system are used to compare the proposed solution with an existing wide area protection against thermal overloads. Results demonstrate that the proposed scheme might increase significantly the reliability of WAPS and the amount of energy transmitted in the power system. In addition, the novel scheme could reduce the overall efforts required to fulfill the control objectives.