CPSS TPEA Vol.5 No.2(JUNE 30, 2020)
Xiaoling Xiong, Xiongfei Wang, Dong Liu, Frede Blaabjerg, and Chengyong Zhao
In order to improve the efficiency of modular mul-tilevel converters (MMCs) and reduce the capacitor voltage ripples, the circulating current between the phase-legs should be suppressed. In existing circulating current suppression control (CCSC) methods, multiple resonant regulators or dq−frame integrators are usually employed to reduce the harmonic com-ponents. Besides the complexity, it requires additional designs of controller parameters, which relies on accurate frequency and sequence information of the circulating currents. In this paper, a common-mode (CM) insertion indices compensation method is introduced to realize CCSC, which is based on feeding forward the capacitor voltages. This approach not only facilitates the analysis of circulating current mechanism with a deep insight, but also provides a simple approach to achieve a wide bandwidth circulating current suppression without considering the specific harmonic frequencies and sequences. The latter one makes it easy to be used even in the unbalanced, multi-frequency or variable fundamental frequency ac systems. In addition to these advan-tages, the proposed compensation method can maintain inter-arm capacitor voltages balanced without additional differential energy control loop, which is required in the existing capacitor voltage feedforward scheme. Analytical and extensive simulation results are given for validating the effectiveness of the proposed approach.
Binbin Li, Junlin Hu, Shaoze Zhou, and Dianguo Xu
Modular multilevel converter (MMC) has drawn great attention in variable-speed machine drives. However, a challenge exists that the voltage ripple of sub-module capacitors in MMC would be very large at low motor speed. In this paper, a novel back-to-back (BTB) MMC system is introduced which can effectively limit the capacitor voltage ripple. The grid-side MMC is a hybrid structure which uses full-bridge sub-modules in the upper arm whereas half-bridge sub-modules in the lower arm. The DC-link current is controlled to be constant thus the motor-side MMC with half-bridge sub-modules can reduce its DC-link voltage at low frequencies so as to absorb less energy variation and limit the capacitor voltage ripple. The operating principle of this hybrid BTB MMC system and the corresponding control strategies are discussed. The effectiveness has been validated by simulation and experimental results.
Guishu Liang and Ruimin Zhu
The radiated electromagnetic interference (EMI) in the valve hall of the modular multilevel converter (MMC) station must be limited below a certain value to ensure the normal opera-tion of the equipment and the safety of the engineers. In this paper, a method for predicting the radiated EMI is proposed, which takes all the necessary factors, such as the physics-based characteristics for IGBT module, sub-module topology, and converter space structure into consideration. This method involves the improved physical model of semiconductor devices, which discards the non-physical feedback parameter introduced by some previous research. At the same time, through the decoupling of the sub-module and the bridge arm circuit, the order of the large-scale system is reduced so that the sub-modules can be calculated in parallel, greatly reducing the overall calculation burden of the model and accelerating the nonlinear small time-step simulation. The wideband characteristics of the converter valve tower are fully considered, with each of the bridge arms regarded as a two-port network with independent sources. The parameter integration and distribution process can also achieve complete parallel calculation. The converter valve tower is modeled as a complex antenna structure with the output voltage of each sub-module as the excitation, the calculation of near-field radiation intensity for the converter valve tower is performed in Alteir FEKO. The measurement conducted inside an actual 49-level converter station verifies the accuracy of the model.
Futian Qin, Feng Gao, Yi Tang, Jinyu Wang, and Tao Xu
Nine-arm modular multilevel converter (9A-MMC) is a compact modular multilevel converter (MMC) with two sets of three-phase output terminals, and it can be regarded as the integration of two conventional six-arm modular multilevel converters (6A-MMC) with double function middle arms. Compared with two 6A-MMCs, the required submodules (SMs) in 9A-MMC can be reduced, however, there will be some performance tradeoffs, and the performance tradeoffs of 9A-MMC have not yet been investigated in detail. Hence, this paper gives the comprehensive comparisons between 6A-MMC and 9A-MMC, including the required SMs number, the current stress of the semiconductor device, the maximum capacitor voltage ripple, and the power losses of the whole system. This paper can be a guide to make the best use of the advantages and avoid the disadvantages of 9A-MMC. Finally, the simulation results verify the mathematical analysis of the comparisons.
Bhukya Nageswar Rao, Yellasiri Suresh, Anup Kumar Panda, Banavath Shiva Naik, and Venkataramanaiah Jammala
Active power filter is a power electronic converter used for improving the quality of supply by eliminating the effect of harmonics due to non-linear loads. This paper recommends a concept for shunt active power filter (SAPF) using a single power source fed to a cascaded multilevel inverter (CMI) with 3-φ transformers. Apart from traditional transformer based topolo-gies, the required number of transformers are substantially reduced, resulting in less space requirement, which leads to low cost and simple control system. The proposed CMI based SAPF has a capacity to compensate for contaminated load with high harmonic and a low power factor. The effective id − iq theory is used to calculate compensation currents. DC link voltage regulator analyzed through delay time and controller gain. The tasks of the controller in the SAPF can perform all necessary actions for correct operation in SAPF. A wide range of computer simulation results demonstrated and validated the results with the prototype experimental setup.
Dominik Neumayr, Dominik Bortis, and Johann Walter Kolar
In order to expedite the development of power electronic systems towards higher power density and efficiency at a lower cost of implementation, Google and IEEE initiated the Google Little Box Challenge (GLBC) aiming for the worldwide smallest 2 kVA / 450 V DC / 230 V AC single-phase PV inverter with η > 95 % CEC weighted efficiency and an air-cooled case temperature of less than 60 °C by using latest power semiconductor technology and innovative topological concepts. This paper, i.e., Part A of a discussion of The Essence of the Little Box Challenge, documents all important R&D activities and engineering considerations of the authors endeavor to implement the world’s most compact inverter; Part B is intended to convey the main findings and lessons learned from the participation in the GLBC. First, the key technical challenges of the GLBC are discussed and the technologies and concepts selected by the authors among different options are described in detail. Relevant design considerations, such as constant frequency pulse width modulation (PWM) or triangular current mode (TCM) operation of the bridge-legs, selection of power semiconductor technology, interleaving of bridge-legs, sizing of the power buffer capacitor, limitation of ground/leakage currents, etc., to achieve an ultra-compact implementation are discussed. Based on this overview, two promising inverter concepts to tackle the GLBC, (i) an H-bridge based inverter with DC-link referenced output filter and (ii) a DC/|AC| buck-stage with series-connected low-frequency (LF) |AC|/AC unfolder inverter, are then analyzed in detail. Based on the results of a multi-objective ηρ-Pareto optimization, a comparative evaluation of the performance in terms of efficiency (η) and power density (ρ) of the two considered inverter concepts is provided. It is shown that with the DC/|AC| buck-stage and |AC|/AC H-bridge unfolder inverter operated with 140 kHz PWM a power density of 14.7 kW/dm3 (240 W/in3) with a maximum efficiency of 98.1% at 2 kW output power can be achieved. These claims are then verified in Part B by means of experimental results obtained from prototype realizations and compared to the achievements of other GLBC finalists. The conclusions are of general importance and are providing key guidelines for the future development of ultra-compact power electronic converters.
Yugang Yang, Lei Wang, and Heming Sun
The negative effects of the PWM inverter are mainly as follows: the high dv/dt value of the DM(differential mode) and the CM(common mode) voltage will cause an impact on the insulation layer of the motor winding; the current generated by the CM voltage on the motor bearing will corrode the bearing and reduce motor life; electromagnetic interference caused by high frequency CM leakage current will affect the stability of the system. Based on the CM transformer, a new passive filter topology is proposed, this fifilter uses the coupling function of the CM transformer to suppress the CM voltage and the leakage inductance of the CM transformer as the DM filter inductor, which can effectively suppress the dv/dt values of the DM and CM voltages, it can also effectively reduce the RMS value of the CM voltage. In this paper, the winding method of CM transformer windings is improved, each winding is divided into two parts in series and distributed symmetrically on the iron core, which makes the leakage inductance and coupling coeffificient between the windings of the transformer more symmetrical, which enhances the filtering effect. The effectiveness of the filter is verified by simulation and experiment.
Omar Abdel-Rahim and Haoyu Wang
High step-up DC/DC converter and maximum power point tracking (MPPT) control are essential components in photovoltaic (PV) systems. For the purpose of utilizing PV module in power generation, this manuscript proposes 1-new high step-up DC/DC converter 2-model predictive control based maximum power point tracking (MPC-MPPT) algorithm with optimum number of sensors. The suggested topology is able to provide a voltage gain up to 10 times of input-voltage and its measured efficiency is around 93%. MPC is a prevalent control technique with superior transient and steady-state performances in PV systems. However, in PV DC/DC converters, conventional MPC based MPPT technique typically requires two voltage-sensors and one current-sensor. For the purpose of reducing system cost, this manuscript presents MPC based MPPT technique with two-sensors. The algorithm is designed to operate with both fifixed and adaptive step-size. Different scenarios and operating conditions are evaluated both in Simulink and with hardware set-up. Results obtained from the simulation and experimental work are consistent and verify the analytical analysis of the system.