Researches Of Relay Protections And Some Corresponding Stability Problems Of Powerformer

The applications of XLPE (Cross Linked Poly Ethylene) cables have been widely extended due to the developments of material science and manufacturing techniques. One of the significant events is the invention of Powerformer. This means a remarkable revolution in the developing history of generation technology. Powerformer can be connected directly to the High-Voltage (HV) busbar; therefore, compared with conventional generators, Powerformer can improve the system reliability and stability, increase the conversion efficiency and reduce the occupied area of generation plants. There is no doubt that the solution of its relay protection problems and the intensive research of its corresponding stability problems are promising to promote the advancement and the popularization of this novel generation technology. Therefore, the research topics of this dissertation focus on the above problems, and the details of it are as follows:Powerformer plays an important role in power systems, so the protection of stator single phase-to-ground fault should have high sensitivity and 100% coverage. On the basis of the internal fault simulation model of Powerformer, the electrical characteristics of it on the occurrence of a stator single phase-to-ground fault are analyzed and a zero-sequence energy based novel protection scheme for stator single phase-to-ground faults is proposed. This protection scheme requires the measurements of the zero-sequence voltage and the zero-sequence current from the terminal of Powerformer to formulate zero-sequence power and zero-sequence energy. By means of the analysis of the direction of zero-sequence energy, the internal faults and the external faults can be distinguished.In order to reduce the volume of the Powerformer, the insulation system of it is designed in a graded way, and the insulation intensity of the neutral is comparatively weaker. When the Powerformer is operating in the condition of open-phase, the neutral will endure very high voltage, which will have seriously negative impact on the insulation of the Powerformer. Based on the analysis of the characteristics of open-phase operational state, a novel open-phase protection scheme utilizing the zero-sequence voltage of the terminal and the negative-sequence current of the stator windings is proposed. A simulation model is set up using EMTDC, and intensive simulations are carried out to verify the protection scheme.Powerformer is connected to HV busbar directly. Hence, for a group of Powerformers operating in parallel, if one of them encounters excitation failure and leads to the scenario of overvoltage, all the Powerformers will be isolated without selectivity by the simple overvoltage protection only relying on the same fixed time delay. On the basis of the analysis of the relationship between the terminal voltage and the reactive power of a Powerformer, a new overvoltage protection scheme is put forward in this dissertation. In addition to the detection of overvoltage condition, this protection can also identify the source of overvoltage by virtue of the variety of reactive power. A compensated voltage factor is used to modify the operating time of the protection. In this case, the generator which has the most vital impact on the overvoltage of the busbar will be tripped firstly. Then, the protections of the sound Powerformers will drop off due to the removal of the disturbance source. The application of this protection scheme includes but is not restricted to Powerformer, which can also provide the selective overvoltage protection for conventional generators working in parallel.The principle of High side voltage control (HSVC) of conventional generators is studied in this dissertation. On the basis of that, its ability of improving transient angle stability of power systems is analyzed. Because of the direct connection to the HV busbar, the Powerformer can control the voltage of the HV busbar without adding a HSVC module. In this dissertation, the Powerformer is initially utilized to realize HSVC excitation control. A single-generator-infinite-system model and an EPRI-7 nodes system model are set up by virtue of PSASp (Power System Analysis Software Packet). Compared with the controlling effects of the conventional generator AVR and conventional generator HSVC excitation control, the ability of Powerformer voltage control of improving transient angle stability of power systems is analyzed.The fundamental theories of voltage stability, the countermeasures of improving transient voltage stability and the overload characteristic of Powerformer are studied in this dissertation. After that, an EPRI-36 nodes system is set up by virtue of PSASP. Compared with the controlling effects of conventional generator AVR and conventional generator HSVC excitation control, the ability of Powerformer voltage control of improving transient voltage stability of power systems is analyzed.Finally, conclusions are made and further work worth lucubrating is outlined.