Recent Advances in Electrical & Electronic Engineering - Volume 11, Issue 4, 2018

Background: A comparative analysis of Sliding Mode Control (SMC) and conventional average current control (AvCC) of a single phase low harmonic rectifier (LHR) having a DC--DC converter is presented. Methods: The SMC which hitherto has been extensively deployed in the control of DC-DC converters is combined with an aspect of AvCC to effectively harness their individual benefits in the control of a LHR. Amongst the four control scenarios, the SMC scheme in which the SMC acts simultaneously on the controlled variables involving both the current and the voltage to generate the switching function by proper choice of sliding coefficients demonstrates a better performance in terms of robustness in the dynamic behavior under different operating conditions. The scheme also meets the emulated resistance requirement of LHR with the least total harmonic distortion of 9.88%. Results: The results of the responses of output voltage for the input voltage change and capacitor parameter variation are tabulated as a comparison for the four control scenarios. Of particular interest is the establishment of the fact that conventional SMC technique in which an equivalent switching function ‘ueq’ is generated and compared with a ramp signal in a pulse width modulated (PWM) scheme to generate a constant frequency switching function for DC-DC converters do not apply to LHR so long as the input voltage to the boost converter is a rectified sinusoid. Conclusion: The validity of the analysis is verified through simulation results under MATLAB/ SIMULINK environment.

Background: In this paper, we propose a redundancy configuration method for Modular Multi-level Converters (MMCs), to improve system reliability and sub-module efficiency, and for maximizing the redundant sub-module value rate. Methods: We establish a set of objective functions using these aims, to obtain the optimal number of redundant sub-modules. To aid in the evaluation of the configuration method, we analyzed the MMC topology and the operational principle of the sub-modules. We present concrete steps for module redundancy optimization based on the redundant protection scheme used with the MMC. Result: The results of simulations modelling the operation of the configuration algorithm prove that the number of redundant sub-modules can be effectively allocated using the proposed method while ensuring the reliable operation of the system.

Background: Currently, power control of PV generation focuses on the Maximum Power Point Tracking (MPPT) strategy, with which PV systems is sensitive to the light, temperature, and micro-grid state, and introduces power fluctuation to the micro-grid. Methods: With the schedulable requirements of Photovoltaic (PV) systems in the grid-tied mode, PV generation systems should possess constant power output capability. This paper proposes an approach for constant power control of PV systems in DC micro-grid. By analyzing the output characteristic between active power and voltage of PV systems, and acquisition of PV output current and the capacitor voltage, voltage control reference in the PV power-voltage curve is obtained by Lagrange interpolation fitting. Result: With the implementation of closed-loop control of photovoltaic boost voltage, constant power output control of PV generation systems can be achieved. Further, this method is able to cope with the change of light and temperature and micro-grid bus voltage. This method is simple and has good environmental adaptability and fast-tracking speed. Conclusion: Simulation results verify the effectiveness of the proposed control strategy compared with a perturb and observe method which can attain the same function.