Study On Large Generator Protection Adapted To Requirements Of Tough Power Source In The Smart Grid

Power source support is essential to build the strong smart grid. Only with a power source adapting its development, meeting its requirements and coordinating with its operating, the smart grid can be given full play to its advantages; otherwise, it will be a river without headwaters, a tree without roots. The smart grid needs rigid power source which equipments with a solid safety defense system and be able to run safely in a complex grid environment. On the other hand, it also needs power source’s flexible coordination which means its requests should be responded unconditionally. Taken together, the power source of smart grid should be tough. As the main power supply, large generator units are vital to build the tough power source of smart grid. With large-scale wind power connected into smart grid, the support of large conventional energy generators becomes more important. Meanwhile, the safety of large generators is encountered with remarkable challenges for the more complex external environment produced by the application of new power generating and transferring technology. Considering relaying protection is the important security measure of generator, this paper studies on some lager generator protections, which have significant influence to power grid operation, to adapt the requirements of tough power source in smart grid. The study work is much significant to take full advantages of the smart grid.Stator ground fault is very common to a generator, and its damage is evaluated by the fault current. A fault current based protection scheme of stator ground fault is presented. It calculates the fault current in real time, and trips only when the fault current exceeds the tolerated value; otherwise, it emits alarm signals. This scheme can avoid unnecessary trips of large generators which endangers the power grid operation when the security of generators is ensured. A location method of stator ground fault is also proposed. It can automatically and accurately identify the faulted coil in a large generator with multi-circuit stator winding, thereby greatly reducing the workload of repair.The asynchronous oscillation is an important problem referring to the coordination between the generator and power grid. An out-of-step protection scheme of single generator is presented considering the safety of both generator and grid. It gives full play to the tolerance capacity of generators and enhances the support of large generators to power grid during the oscillation. An out-of-step protection scheme of multi-generator is also presented considering the safety of both generator and grid. It restores synchronization through tripping the minimum generators and avoids the shock of tripping multi generators to the power grid. On this basis, a overall out-of-step protection scheme considering the safety of both generator and power grid is presented in which the presented out-of-step protection for multi generators plays the first line of defense against the oscillation and the one for single generator plays the second line.Time inverse protection is an important one in the large generator protection system adapted to the requirements of tough power source in the smart grid. This paper analyzes the operating characteristic curve, setting calculation and realizing algorithm of the existing time inverse protection methods and points out their problems. On this basis, a protection scheme basing on the best fitting of capacity curves is presented. The form of operation criterion is determined by the warming up process and the parameters are calculated by least square fit. In this scheme the operating characteristic curves can match the generator capacity curve and the realization can accurately reflect the thermal energy accumulating process. The protection trips the generator after its maximum tolerated time delay. It takes full benefits of large generators there by adapting the requirements of tough power source in the smart grid.Single transverse protection is against the turn-to-turn fault of large generators, whose main problem is that the sensitivity is limited by the unbalanced current. This paper studies the produce mechanism and influence factors of the unbalanced current and analyzes the problems on the principle and application of the existing protection methods. On this basis, an air gap electromotive force based restraint criterion is presented. Combining it with the existing criterion, a new comprehensive protection scheme is constructed which is proved to be able to extend the overage of internal faults thereby improving the sensitivity.Excitation-loss protection is the important grid-involved protection of a generator. Two key problems of the coordination between the protection and the power grid are studied. One is the criteria on the rotor side is not stable and reliable. To address this problem, an auxiliary criterion based on the no-load electromotive force is presented. The calculating method of no-load electromotive and the overall scheme using this new criterion are provided. The other problem is the lack of coordination between the excitation-loss protection and the low excitation limit. In light of this, this paper studies the coordinating method, rules and steps.At the end of this dissertation, the research work and results are summarized and the directions of further research are pointed out.