Advanced Materials Research Vols. 433-440

Abstract: Voltage sag is one of the most frequent and serious dynamic problems that impact the power quality. To solve this problem, fast and precise detection of the voltage sag is needed. A detection method is presented in this paper, using single phase voltage information, to be applied in the typical condition of unsymmetrical three-phase voltage sag. This method adopts fast recognition of voltage disturbance and SPLL. Finally, MATLAB simulation has been used to verify the effectiveness of the method.

Abstract: Existing methods utilizing excitation inductances to discriminate inrush current and internal fault of transformer are all derived from the singe-phase transformer circuit model, yet there are no discussions considering the widely used three-phase three-limb transformers. In view of this question, the mathematical model of three-phase three-limb transformer based on the characteristics of its magnetic equivalent circuit is established, through which the excitation inductances of the three-phase three-limb transformer can be calculated. The calculated excitation inductances have definite physical meanings and can reflect the saturation state of the transformer core under inrush condition. Analysis of the circuit model demonstrates that the delta circulating current is not the excitation current of three-phase three-limb transformer, so the proposed method can be used to the transformers with delta connection directly. The proposed method has been verified by electromagnetic transients program including direct current (EMTDC) simulations, simulation results show that the calculated excitation inductances have different characteristics under inrush and internal fault conditions, and can be applied to identify the inrush current of three-phase three-limb transformer.

Abstract: The ripple normal force of permanent magnet linear synchronous motor (PMLSM) can directly deteriorate the machining accuracy of ultra-precision CNC machine tool which use linear motor direct drive. First, the normal force of PMLSM was calculated using finite element method (FEM) .then, id current compensate lookup table was established by indexing iq current, gap and position of mover .In order to reduce the ripple of normal force and keep the thrust steady, PMLSM current compensation control system was realized by injecting the id current to counteract the normal force ripples using field oriented control (FOC) method. The effectiveness of this proposed method is verified by the simulation using Matlab according to the comparison between the compensation and no compensation cases.

Abstract: Three phase voltage source PWM rectifier (VSR)’s indirect current control (ICC) strategy has good static performance, simple control steps and low cost. But bad dynamic performance is its inherent drawback. If this weakness is overcome, it is still valued. In this article normal ICC is deeply studied beginning with its mathematical model under synchronous rotating coordinate. It is indicated that its bad dynamic performance originates from bad current response. So an improving scheme adding compensation in series is brought out. Additionally a differential negative feedback loop of DC output voltage is added to limit the voltage overshoot. Lastly the control system’s computer simulation model is established by MATLAB and the simulating results prove that improved ICC makes great process in dynamic performance and has advantages of both indirect and direct current control (DCC).

Abstract: To analyze the transient response of transformer windings under very fast transient over-voltage (VFTO), multi-conductor transmission line (MTL) model based on the representation of transformer windings by its individual turns are established. Space discretization is needed for solving the time-domain telegraph equations of MTL. To calculate the voltage distributions along transformer windings, through combining the compact finite difference (CFD) theory and the backward differentiation formulas (BDF). Simulation software ATP is introduced, and the simulation results demonstrate that the proposed approach is feasible.

Abstract: Traction substation reliability is of vital importance for railway transportation safety. To illustrate traction substation reliability, irreparable and reparable reliability models of three types of traction substation Electrical main connection have been established. Based on analysis of simple series and parallel reliability system, system irreparable reliability model is analyzed. The ways of how to get mean time to failure (MTTF) and mean time to first failure (MTTFF) of reparable system are achieved. By comparative analysis of the value of MTTF and MTTFF among three kinds of traction substation main connection reparable and irreparable system, some useful conclusions are found.

Abstract: Excited capacitor of power winding has much influence on the capacity of excited converter of dual-winding induction generator. The capacity of excited converter can be optimized by changing the excited capacitor. The influence law of the capacity of excited converter by variable excited capacitor are compared under variable speed and variable load recured to simulation methods.

Abstract: A general PI control algorithm is not a perfect solution to control ac signal. Because it is not able to eliminated steady-state error. The radical reason of the problem is investigated in few literatures. So the reason is explored, and an important conclusion is reached. The conclusion is that zero steady-state error control is obtained when the control algorithm has infinite gain at the excited frequency. Based on this conclusion, a modified PI algorithm is proposed for a four-leg inverter. The method that parameters of the modified PI algorithm are defined is given. To control the component in the 0 axis, the virtual component leading the component in the 0 axis by 90 degree is constructed. So the unified control algorithm is implemented in reference frame. The effectiveness of the proposed algorithm has been investigated and verified through simulation.

Abstract: this paper build up the brushless doubly fed machine (BDFM) finite element model of the coupled electromagnetic-thermal field, the electromagnetic joule heat calculated in electromagnetic environment will be loaded as the heat source to calculate in thermal field, the calculated results in thermal field will be the initial conditions to calculate the electromagnetic field, get the thermal field distribution when the motor is stable through cycle calculation at last. Calculation results show the effectiveness of this method.

Abstract: A new method of PID parameter tuning of the interrelated servo-motors is developed. This method is applied to PID control of the interrelated servo-motors which selects overshoot and rising time as optimization variables according to the working characteristics of servo motor, determine the PID parameters quickly and accurately, can meet the requirements of engineering and greatly improve efficiency.