Daqiang Bi

Correlation Analysis of Waveforms in Nonsaturation Zone-Based Method to Identify the Magnetizing Inrush in Transformer

A novel algorithm based on the correlation analysis of waveforms to distinguish between the magnetizing inrush and the short-circuit fault current is proposed. The algorithm makes use of the characteristics of differential current waveform in the nonsaturation zone that it is similar to a part of the sinusoidal wave under the short-circuit fault and far different with the sinusoid under the inrush condition. To realize this algorithm, the nonsaturation zone of the transformer is decided by comparing the algebraic sum of sampling data in a short slide window of the differential current under the magnetizing inrush or the short-circuit fault, then two kinds of normal sinusoidal waveforms are structured according to the value and position of the peak point of differential current in the nonsaturation zone. The correlation coefficients between the original waveform and two structured sinusoidal waveforms are calculated, and the magnetizing inrush or fault current is judged according to the average of two correlation coefficients. The experimental results verify that the algorithm can correctly open the differential protection shortly for the internal short-circuit fault in operation, energized with a turn-to-turn short-circuit fault, reliably blocking the protection during the magnetizing inrush, and obtaining good immunity to the saturation of the current transformer. Low computation and requirements contributing to this algorithm are performed in practice.

Grid-connected wind farm power control using VRB-based energy storage system

To improve the power quality and stability of the grid-connected wind farm, and regulate the grid-connected power effectively, a new type of environmentally-friendly vanadium redox flow battery (VRB) based energy storage system (ESS) with many advantages are added at the exit of wind farm. A dynamic mathematic model of VRB based on the equivalent circuit is built. A bi-directional AC/DC converter is used to achieve the power conversion of VRB-based ESS and its corresponding control strategy is developed. The simulation model of the grid-connected wind farm with VRB-based ESS is established. The given wind speed in this paper is used for an example to validate the scheme, the simulated results show that the output active power of wind farm are effectively smoothed and a certain amount of reactive power support can be provided for the grid and the operating performance of grid-connected wind farm is well improved by the VRB-based ESS. The good charging-discharging performances of the VRB are also verified by simulated results.

Winding design for pole-phase modulation of induction machines

Induction machines with pole-phase modulation (PPM) can extend speed / torque capabilities for applications in integrated starter/generator and hybrid electric vehicles. In this paper, a general winding design rule for the PPM of induction machines is proposed. A prototype is used to verify the proposed method and the feasibility of the designed pole-changing winding. Besides, the characteristics of three different structure machines — conventional winding machine, toroidal winding machine and dual-rotor torodial winding machine operated in the same mode are compared by using the finite element software JMAG-studio. The results show that both conventional winding machine and dual-rotor torodial winding machine present good performances. Moreover, conventional winding machine has many advantages such as a simple structure, so its experimental prototype is designed and manufactured for next control work.

Impedance design of quasi-Z source network to limit double fundamental frequency voltage and current ripples in single-phase quasi-Z source inverter

This paper proposes an analytic model to calculate the double fundamental frequency (2ω) voltage and current ripples for single-phase quasi-Z-source inverter (qZSI). The model enables the exact calculation of the 2 Ω components of the inductor current and the capacitor voltage, which is crucial for designing a single-phase qZSI system. A guideline for selecting the capacitance and inductance in quasi-Z-source (qZS) network to limit both the dc-link voltage and input current 2ω ripples within a tolerable range is presented. Three cases are tested to verify the proposed analytic model and the design method by using simulation and experimental results. The identical analytic calculation result, the simulation result, and the experimental result validate the analytic model and the design method.

Energy storage based LVRT and stabilizing power control for direct-drive wind power system

To enhance low voltage ride through (LVRT) capability and stabilize output power of direct-drive permanent magnet wind power system, a new type of vanadium redox flow battery (VRB) based energy storage system (ESS) is added at DC-link bus of dual-PWM converters. Correspondingly, the control strategy of bi-directional DC/DC converter is developed to manage the VRB. VRB-based ESS can be used to improve LVRT capability effectively when the grid voltage sags and smooth the grid-injected power when the wind speed rapidly changes. The simulated model is established to analyze the operation characteristics in detail. The results show that the performances of the grid-connected operation are effectively improved for the direct-drive wind power system by using VRB-based ESS at DC-side, and the dynamic response is fast.

Correlation Analysis of Waveforms in Non-saturation Zone Based Method to Identify the Magnetizing Inrush in Transformer

A novel algorithm based on the correlation analysis of waveforms to distinguish between the magnetizing inrush and the short-circuit fault current is proposed. The algorithm makes use of the characteristics of differential current waveform in the nonsaturation zone that it is similar to a part of the sinusoidal wave under the short-circuit fault and far different with the sinusoid under the inrush condition. To realize this algorithm, the nonsaturation zone of the transformer is decided by comparing the algebraic sum of sampling data in a short slide window of the differential current under the magnetizing inrush or the short-circuit fault, then two kinds of normal sinusoidal waveforms are structured according to the value and position of the peak point of differential current in the nonsaturation zone. The correlation coefficients between the original waveform and two structured sinusoidal waveforms are calculated, and the magnetizing inrush or fault current is judged according to the average of two correlation coefficients. The experimental results verify that the algorithm can correctly open the differential protection shortly for the internal short-circuit fault in operation, energized with a turn-to-turn short-circuit fault, reliably blocking the protection during the magnetizing inrush, and obtaining good immunity to the saturation of the current transformer. Low computation and requirements contributing to this algorithm are performed in practice.

A new modeling method for S-MCSRM driven by three-phase full bridge converter

Purpose – The S-MCSRM is a two-phase excited switched reluctance motor (SRM), with the short flux path and mutual inductance coupling, which is suitable for the oil submersible pump application owing to large torque and three-wire connection with the standard full-bridge power converter. However, there is not literature to disclose its model due to the complicated mutual inductance coupling. The FEM model is a time-consuming method to analyze this motor. For the first time, this paper aims to propose an S-MCSRM model for performance analysis and control method developing. The proposed model would save simulation time and be a theoretical fundamental for further implementing control algorithm. Design/methodology/approach – The S-MCSRMs operating principle is analyzed, and the voltage equation and the generated torque are deduced. The FEM is utilized to obtain the five typical magnetization curves that describe the S-MCSRMs magnetic path characteristic. The magnetic co-energy equation, phase torque and to...

A new switched reluctance motor with distributed winding

Purpose – The purpose of this paper is to propose a new three-phase switched reluctance motor (SRM), and achieve high-torque and low-cost. This new SRMs winding configuration uses the double-layer distributed windings, which is different from the conventional SRMs single tooth coils. Design/methodology/approach – The operating principle of new SRM is analyzed, and the voltage equation and the generated torque are deduced. Finite element method (FEM) and finite element circuit coupled method are utilized to evaluate the new motors operating performances. The two dimensional (2D) frequency response analysis model is employed in the FEM model. Based on the 2D frequency response analysis model, the magnetic field distribution, self-inductance, and mutual-inductance for the new SRM are analyzed in detail. A co-simulation model using FE analysis package and Matlab-Simulink is proposed to simulate the new SRM drive. The simulated and experimental results verify the new SRM. Findings – For the new SRM with double-layer distributed windings, a co-simulation method is proposed to analyze its characteristics. The new SRM presents lower torque ripple coefficient and generates larger torque than the conventional SRM, with three-wire and standard full bridge power converter, rather than six-wire and asymmetric half-bridge converter for conventional SRM. Originality/value – This paper proposes a new SRM with the double-layer distributed windings driven by a standard full bridge inverter. In order to calculate dynamic characteristics of the new SRM, a co-simulation method using FEM and Simulink is proposed to simulate the new SRM drive, where the power inverter and the current chopping control algorithm are implemented.

Improved flux linkage method for position sensorless control of high-speed SRM

Traditional simplified flux linkage method is not suitable for the control of high-speed and super-highspeed SRM drives. In this paper, a high-speed SRM position sensorless control strategy based on an improved simplified flux linkage method to estimate the rotor position is proposed. This method has the capability of timely adjust the phase turn-on/off angle at different speeds and can extend the maximum achievable speed for a constant DC-bus voltage. The principle of selecting the reference flux linkage on the basis of the SRM current-flux model to improve the precision of rotor angular position estimation is presented. The proposed control strategy is experimentally tested in two different SRM drives and the results show its feasibility, reliability and effectiveness, both for low- and high-speed SRM.

Injecting source-based stator ground fault protection for generators in expanded unit connection

This paper proposes an improved stator ground fault protection scheme for the generator through injecting 20Hz voltage to realize the selectivity of the protection. The insulation resistance value of the generator is firstly calculated to determine whether or not ground fault occurring. When the ground fault happens, the 20Hz differential current between the neutral and terminal of each generator will be obvious. The phase relationship between the 20Hz differential current and injected 20Hz voltage is derived and analyzed with the variation of insulation resistance. The new protection scheme can judge the fault generator according to the phase relationship, so that it can fulfill a selective shutdown. The simulated results verify the accuracy of calculating the grounding transition resistance and selecting the fault generator, which achieves the stator ground fault protection by injecting 20Hz voltage in the generators connected to expanding units.