Delay-Dependent Stability of Single-Loop Controlled Grid-Connected Inverters with LCL Filters

LCL filters have been widely used for grid-connected inverters. However, the problem that how time delay affects the stability of digitally controlled grid-connected inverters with LCL filters has not been fully studied. In this paper, a systematic study is carried out on the relationship between the time delay and stability of single-loop controlled grid-connected inverters that employ inverter current feedback (ICF) or grid current feedback (GCF). The ranges of time delay for system stability are analyzed and deduced in the continuous s-domain and discrete z-domain. It is shown that in the optimal range, the existence of time delay weakens the stability of the ICF loop, whereas a proper time delay is required for the GCF loop. The present work explains, for the first time, why different conclusions on the stability of ICF loop and GCF loop have been drawn in previous studies. To improve system stability, a linear predictor-based time delay reduction method is proposed for ICF, while a time delay addition method is used for GCF. A controller design method is then presented that guarantees adequate stability margins. The delay-dependent stability study is verified by simulation and experiment

Diagnosis of the power frequency vacuum arc shape based on 2D-PIV

Vacuumcircuit breaker (VCB) is one of the important elements in the power grid that can control and protect the system. The diagnosis of the vacuum arc images in VCBs is helpful to the study on their breaking performance. But up to now, there are few reports on the macro-particle motion trajectories in the arc. As the macro-particles in the arc are flowing, the particle image velocimetry (PIV) can be grafted into the vacuum arc image processing. In this paper, the power frequency vacuum arc (peak is 6.9kA) was used as the treatment object, geometric characteristics of the vacuum arc shape using digital image processing technology were extracted, and the two dimensional motion trajectory and velocity distribution of the arc macro particles in different arc combustion stages were obtained based on 2D-PIV technology. Three stages were analyzed. In the rapid diffusion stage, the collision of the macro-particles is very intense in the anode region, and they spray along the anode surface. In the stable combustion stage, the velocity of the particles near the anode is small, and there is a smaller shrink. And finally in the extinction stage, their motion trajectories are in a state of rotation, near the anode and cathode are in a state of contraction

The Research of Mathematical Method and Position Control of Actuator in Power Switchgear

Transient effects such as overvoltage and inrush currents will be caused due to opening and closing the switchgear at random phase. Phase-controlled technology present in recent years, which is restricted by the operation dispersion of actuator, can limit the transient effects. And the dispersion of the switchgear with a permanent magnetic actuator (PMA) is small. Therefore, the research of mathematical method and position control in this paper is based on the PMA. Firstly, the dynamic mathematical method and simulation system established in MATLAB are used to improve the design of the PMA owing same type. Secondly, simulation with the use of improved fuzzy algorithm is carried out. And an optimized self-adaptive fuzzy algorithm is obtained in the simulation process which can be used to trace the given displacement curve. Finally, a large number of tracing experiments have been done on the 35 kV breaker prototype to verify the effectiveness of the algorithm. In the experiments, the closing time of breaker can be stabilized within ±0.5 ms when capacitor voltage and capacitance change. These results prove that the mathematical model and the fuzzy algorithm are effective and practical

The Application of NdFeB in the Magnetic Force Actuator

In this paper, NdFeB is used to design a new type of magnetic force actuator (MFA) with simple structure and high reliability. The permanent magnets are fixed on the static iron-core to generate a magnetic field, while the movable part locates within the magnetic field. It can drive the arc extinguishing unit powered by the Lorentz force, and this can be applied to the operation of the long-stroke high voltage circuit breaker (HVCB). At the open and closed position, the PMs generate holding force for the moving iron-core to keep the static state. Then, the finite element method(FEM) and prototype test are adopted to study the properties of PM and characteristics of the actuator. The simulation concludes that the material type and structure size of PM, end cap material and processing deviation of the actuator will impact the static characteristic of the actuator. The results of the test on prototype show that MFA using NdFeB can achieve the high power output, which is conductive for electronic control as well as the displacement tracking. Due to its stable performance, NdFeB is reliable in the running of the magnetic force actuator. DOI: http://dx.doi.org/10.11591/telkomnika.v10i6.141

Investigations on the safe stroke of mechanical HVDC vacuum circuit breaker

Here, a DC vacuum arc interruption test based on the hybrid high-speed actuator and the method of commutation is carried out in the synthetic loop system. The safe stroke is one of the basic breaking parameters. The minimum safe stroke directly determines the commutation time and breaking property of the mechanical DC vacuum circuit breaker. The main circuit breaker is a high-speed vacuum switch driven by electromagnetic repulsion actuator. In order to simplify its control system, the commutation switch also adopts high-speed vacuum switch. The main current source for verification test of this DC breaker system is provided by a LC oscillator circuit with lower frequency. Simulations with some experiments were completed on breaking process of this system by PSCAD. The breaking current is 4 kA by the DC breaker system proposed here, the opening speed is about 2.417 m/s in this experimental system; From the results, the simulation and experimental waveforms are compared and analysed, and some effects on safe stroke of the DC vacuum circuit breaker is studied. The shortest arcing time is about 0.89 ms and the minimum safe stroke is about 2.15 mm under the lower voltages

A Novel Model of Electromechanical Contactors for Predicting Dynamic Characteristics

To ensure the reliability of power supply, a dual power supply structure appears in the power distribution system. Power supply switching is a complex physical process. This paper presents a novel model of electromechanical contactors. This model can simulate the multi-physics process of power switching. This article completes the simulation framework for power switching through contactors for the first time. Among them, the structural topology for contactors is also proposed. On the basis of the novel structure topology, an equivalent magnetic circuit model is established to calculate the relationship between driving force, flux linkage, current, and displacement. Then, a co-simulation model is established between the above equations and Adams to obtain the speed characteristics and flight time of the contactor. Subsequently, through the use of Fluent and its secondary development, a magnetohydrodynamic model is established, and the above-mentioned velocity characteristics are imported into it to analyze the arcing characteristics of the contacts under the conditions of the transverse magnetic field and the insulating grid. The effectiveness of power switching is judged by comparing the flight time of the electromechanical model and the arcing time of the magnetohydrodynamic model. The prototype is manufactured and tested on the basis of simulation. Through experimental waveforms and high-speed photography, the accuracy of the simulation model and the practicability of the contactor are verified