Ultracapacitor based supplementary excitation module for the improvement of synchronous generator transient stability

Abstract

Transient stability is the ability of the power system to return to a stable operating point after large disturbance that provokes significative rotor angle excursions. This thesis is focused on the improvement of transient stability generators equipped with bus fed static excitations through ultracapacitor based Excitation Boosters (EB). The thesis addresses the problem of modelling, sizing and controlling ultracapacitor based Excitation Boosters (EB) to improve synchronous generators transient stability. The EB technology supposes a solution to grid code’s Fault Ride Through (FRT) capability requirements. Grid codes formalize the duties of every agent in liberalized power systems. They have developed the FRT requirements to tackle with transient stability. The combination of stark FRT requirements with extreme leading operating points challenges generators to maintain synchronism. Synchronous generators equipped with bus-fed static excitation systems are particularly vulnerable to new grid code requirements. EBs can overcome bus-fed static excitation systems limitations and enable them to fulfil grid codes FRT requirements. The EB is studied using the Single Machine Infinite Bus (SMIB) approach that grid codes demand. An EB simulation model consistent with classical transient stability assumptions has been proposed. A Dspace Rapid Control Prototyping (RCP) test bench has been assembled to validate the simulation model through experimental measurements and to demonstrate the feasibility of the EB concept. A novel sizing method of the EB has been performed combining grid codes FRT requirements, synchronous generators technical limits, ultracapacitor features and sensitivity studies. The EB is controlled in SMIB systems by an on-off control that uses terminal voltage as input, which is a locally available signal. This control scheme is able to improve the transient stability of a generator after a fault close to the generator in both SMIB and multimachine systems. However, it worsens the stability in some cases where the fault is situated far away from the generator. This result demonstrates that the transient stability problem is a global problem that requires global solutions. A Lyapunov stability theory based EB Wide Area Control Systems (WACS) controller has been proposed to overcome this drawback. This control is based in global measurements that are acquired by Measurement Units (PMU) that have been recently deployed in power systems. Control rules based in the Center of Inertia (COI) concept and Dominant Interarea Paths (DIP) theoriy allows modulating the EB voltage of each generator in the system. The concept has been validated through experimental measurements in an OPAL-RT Real Time Hardware in the Loop (RT-HIL) simulator.