Coordination of Generator Protection and Control in the Over and Under Excited Regions

This thesis deals with the coordination of protection and control functions associated with the synchronous generators. The excitation control functions are a key component in maintaining the stability of machines and the network. The overall objective of coordination is simple; to allow excitation control functions, the automatic recovery from excursions beyond normal limits, and only take protective action as a last resort. This thesis focuses on four areas of generator control and protection : a) Loss of excitation protection, b) Dynamic underexcitation coordination, c) Dynamic Overexcitation coordination, and d) a generic protective relay development platform for hardware and software development. Loss of excitation (LOE) is a condition in the underexcited region that presents a risk of severe damage to a generator. The state of the art in the detection of a loss of excitation condition is based on the principle that, for a zero Thevenin voltage, the generator becomes a reactance as seen from the power system. The difficulty in detecting a loss of excitation is that several other disturbances may temporarily present a similar behavior, for instance a fault followed by a power swing. In this part of the work, a new algorithm for the detection of a loss of excitation condition is proposed by using the Support Vector Machine (SVM) classification method and a careful design of the necessary feature vectors. The proposed method is robust to changes in conditions including initial load, fault types, line impedance, as well as generator parameter inaccuracies. Coordination in the underexcited region presents difficulties due to the commonly used static characteristics instead of dynamic simulation. The underexcited limit presents an overload characteristic that is not normally known or used. Once the limit is exceeded, the limiting control action is a control loop that presents a dynamic behavior not typically represented in studies in the current industry practice. It is also important to properly model and include dynamic performance of protection functions. An important consideration not typically taken into account is the actual stability limit, which depends on the characteristics and the mode of excitation control used. This thesis includes all the above considerations necessary to achieve the required coordination using the more accurate dynamic simulations. Specific scenarios that present risk to the machine or the system are included to assess the coordination achieved. A real generator from the Alberta power system is used as a case study to demonstrate the proposed coordination methodology. Coordination in the overexcited region again presents practical difficulties due to static characteristics instead of dynamic simulation of conditions that exercise the overexcitation limits. The problems observed relate to coordination methodology and modeling methods for both protection and control limits. Once the limit boundary characteristic is exceeded, the limiting action is a control loop that presents dynamic behavior that needs to be represented. Similar considerations need to be made with the protection function protecting against rotor overload. Current modeling methods mostly use low bandwidth simulations, i.e., transient stability studies. A modeling methodology as well as specific model improvements to the IEEE ST1A excitation control model are proposed to achieve the required coordination. The ST1A type is one model that can represent a wide variety of system models from different manufacturers. The proposed modeling methodology applies to high bandwidth simulations such as electromagnetic simulations. Specific important scenarios, such as severe temporary reactive overload or severe power swing conditions, where the protection and control are required to coordinate but that present risk to the machine or the system are proposed as part of the coordination considerations. The detection of LOE conditions by the proposed SVM method and by traditional methods was implemented in hardware by using a digital signal processor (DSP) platform and tested using real time power system simulations. A new platform for real time protective relay development was designed and used for the purpose of implementation. In the proposed platform, a processor independent code is used so that development can be performed using native host computer development tools. By using the proposed platform-independent code, off line testing can be performed either interactively or in batch mode for evaluating multiple cases