The Issues Of Fault Simulation, Protection Of Powerformer And Its Impacts On Power Systems Stability

Powerformer, launched in 1998, is a high voltage synchronous machine that is suitable for direct connection to the high voltage network without any step-up transformer. Powerformer offers several advantages such as enhanced reliability, higher efficiency, compact design of power plant and voltage stability etc. Since the Powerformer was invented in 1998 and hasn't been studied well, it is very necessary to research the machanics of internal or external fault in Powerformer, the influence to protection and the relationship between the characteristic of Powerformer and voltage stability. The details of this paper are as below:A technique for partitioning the stator windings of Powerformer is proposed, which can be applied to internal fault analysis, and determining corresponding equivalent number of turns and the angular location of magnetic axis. The technique can also apply to the large synchronous machines.The mathematical model of Powerformer is studied, which can analyze internal phase and ground faults in stator windings. The method employs a direct phase representation considering the cable capacitance. To effectively implement the internal fault simulation, the magnetic axis locations of fault parts are arranged appropriately. The current equivalent equations, voltage equivalent equations and the rotor motion equations are formed and combined to implement the fault simulations. Simulation results showing the fault currents, during a single phase to ground fault, a two phase to ground fault and a phase to phase fault, are presented. With the data generated by this internal fault simulation model, the protection scheme used for Powerformer can be validated and improved accordingly. The technique can also apply to the large synchronous machines.This internal fault simulation is a typical stiff system and the stiff integration method with variable step-size and order are applied. On the basis of analyzing the solution of stiff problems of the full dynamic simulation in essence, the Gear Method, a numerical integration method suitable to solve stiff differential equations, is adopted as the main numerical integration algorithm for the simulation software.A variety of internal fault conditions are considered. Simulation results showing the fault currents, during a single phase to ground fault, a two phase to ground fault and a phase to phase fault, are presented, along with comprehensive data of a 75MVA and 150kV Powerformer, a high-voltage cable-wound generator. Some synchronous machines, like Powerformer, are equipped with two similar or dissimilar armature windings and whose detailed circuit representation is described. The equivalent circuit at steady state or external fault is presented. The simulation results from a steady-state analysis as well as a fault analysis are presented.A self-adaptive compensated differential protection is presented, which is useful for AC synchronous generators, particularly a type of high-voltage graded insulated cable wound generator, Powerformer. The phase voltages at the terminals are utilized to estimate the capacitive currents when the generator is synchronized and no fault is detected. With the calculated capacitive current the capacitance distribution coefficient along with the winding of the generator can be determined. This capacitance will be equivalently arranged on the terminal and neutral of the generator respectively. Then, the capacitive current in the case of normal operation, external fault and internal fault can be calculated using the terminal voltage and the determined capacitance. It can be compensated to improve the reliability of the differential protection. The feasibility and effectiveness of the proposed scheme is proved with the simulation test results.In order to determine the coherent busbar groups of a system with similar reactive reserve basins, the thesis presents a novel sensitive method. By determining the reactive reserve margin, it is possible to determine which generators will have an impact on the maximum permissible loading of a busbar and which will not. This information is particularly useful when the conventional generator and transformer are replaced by Powerformer.The effect of Powerformer voltage control on stabilities is worth study, which is focus on compensation of Powerformer impedance and improvement of system damping, taking the example of one machine infinite busbar system. The influence of Powerformer on steady or dynamic stability is studied in theory. Eigenvalue analysis and time domain simulation proves the validity.At the end of this dissertation, a systematical summary is given. Further work to be studied is pointed out.