A Multilevel Inverter Structure based on Combination of Switched-Capacitors and DC Sources

ABSTRACT:

This paper presents a switched-capacitor multilevel inverter (SCMLI) combined with multiple asymmetric DC sources. The main advantage of proposed inverter with similar cascaded MLIs is reducing the number of isolated DC sources and replacing them with capacitors. A self-balanced asymmetrical charging pattern is introduced in order to boost the voltage and create more voltage levels. Number of circuit components such as active switches, diodes, capacitors, drivers and DC sources reduces in proposed structure. This multi-stage hybrid MLI increases the total voltage of used DC sources by multiple charging of the capacitors stage by stage. A bipolar output voltage can be inherently achieved in this structure without using single phase H-bridge inverter which was used in traditional SCMLIs to generate negative voltage levels. This eliminates requirements of high voltage rating elements to achieve negative voltage levels. A 55-level step-up output voltage (27 positive levels, a zero level and 27 negative levels) are achieved by a 3-stage system which uses only 3 asymmetrical DC sources (with amplitude of 1Vin, 2Vin and 3Vin) and 7 capacitors (self-balanced as multiples of 1Vin). MATLAB/SIMULINK simulation results and experimental tests are given to validate the performance of proposed circuit.

KEYWORDS:

1.      Multi-level inverter

2.      Switched-capacitor

3.      Bipolar converter

4.      Asymmetrical

5.      Self-balancing

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig (1) Three stage structure of the proposed inverter

EXPECTED SIMULATION RESULTS

Fig (2) Waveform of the output voltage in (a) 50Hz and pure resistive load (b)

the inset graphs of voltage and current

Fig (3) waveform of the output voltage in 50Hz with resistive-inductive load

Fig (4) Capacitor’s voltage in 50Hz (a) middle stage (b) last stage

CONCLUSION:

In this paper, a multilevel inverter based on combination of multiple DC sources and switched-capacitors is presented. Unlike traditional converters which used H-bridge cell to produce negative voltage that the switches should withstand maximum output AC voltage, the suggested structure has the ability of generating bipolar voltage (positive, zero and negative), inherently. Operating principle of the proposed SCMLI in charging and discharging is carried out. Also, evaluation of reliability has been done and because of high number of redundancy, there has been many alternative switching states which help the proposed structure operate correctly even in fault conditions. For confirming the superiority than others, a comprehensive comparison in case of number of devices and efficiency is carried out and shows that the proposed topology has better performance than others. For validating the performance, simulation and experimental results are brought under introduced offline PWM control method.

REFERENCES:

[1] L. G. Franquelo, J. Rodriguez, J. I. Leon, S. Kouro, R. Portillo, and M. A. M. Prats, “The age of multilevel converters arrives,” IEEE Trans. Ind. Electron. Mag., vol. 2, no. 2, pp. 28–39, June, 2008.

[2] M. Saeedifard, P. M. Barbosa and P. K. Steimer,”Operation and Control of a Hybrid seven Level Converter,” IEEE Trans. Power Electron., vol. 27, no.2, pp. 652–660, February, 2012.

[3] A. Nami. “A New Multilevel Converter Configuration for High Power High Quality Application,” PhD Thesis, Queensland University of Technology, 2010.

[4] V. Dargahi, A. K. Sadigh, M. Abarzadeh, S. Eskandari and K. Corzine, “A new family of modular multilevel converter based on modified flying capacitor multicell converters IEEE Trans. Power Electron., vol. 30, no.

1, pp. 138-147, January, 2015.

[5] I. López, S. Ceballos, J. Pou, J. Zaragoza, J. Andreu, I. Kortabarria and V. G. Agelidis,” Modulation strategy for multiphase Neutral-Point Clamped converters,” IEEE Trans. Power Electron., vol. 31, no. 2, pp. 928–941, March, 2015.

A Seven-Switch Five-Level Active-Neutral-Point-Clamped Converter and Its Optimal Modulation Strategy

 ABSTRACT:

 Multilevel inverters are receiving more attentions nowadays as one of preferred solutions for medium and high power applications. As one of the most popular hybrid multilevel inverter topologies, the Five-Level Active-Neutral-Point-Clamped inverter (5L-ANPC) combines the features of the conventional Flying-Capacitor (FC) type and Neutral-Point-Clamped (NPC) type inverter and was commercially used for industrial applications. In order to further decrease the number of active switches, this paper proposes a Seven-Switch 5L-ANPC (7S-5L-ANPC) topology, which employs only seven active switches and two discrete diodes. The analysis has shown a lower current rating can be selected for the seventh switch under high power factor condition, which is verified by simulation results. The modulation strategy for 7S-5L-ANPC inverter is discussed. A 1KVA single-phase experimental prototype is built to verify the validity and flexibility of the proposed topology and modulation method.

KEYWORDS:

1.      Multilevel inverter

2.      Active-Neutral-Point-Clamped (ANPC) inverter

3.      Flying-Capacitor

4.      Pulse-Width-Modulation (PWM)

SOFTWARE: MATLAB/SIMULINK

 CIRCUIT DIAGRAM:

 Fig.1 (a) Proposed topology.

 EXPECTED SIMULATION RESULTS

Fig. 2. Simulation results under unity power factor condition. (a) Output voltage and FC voltage. (b) T7 current in case 1. (c) T7 current in case 2. (d) T7 current in case 3. (e) T7 current in case 4.

Fig. 3. Simulation results under reactive power condition (PF = 0.9, capacitive). (a) Output voltage and FC voltage. (b) T7 current in case 1. (c) T7 current in case 2. (d) T7 current in case 3. (e) T7 current in case 4

Fig. 4. Simulation results under reactive power condition (PF = 0). (a) Output voltage and FC voltage. (b) T7 current in case 1. (c) T7 current in case 2. (d) T7 current in case 3. (e) T7 current in case 4.

Fig. 5. Experimental results under unity power factor condition: waveforms of inverter output voltage, grid voltage, FC voltage and output current.

CONCLUSION:

In this paper, a novel 7S-5L-ANPC inverter topology has been proposed. As compared with the conventional 5L-ANPC inverter, it requires seven active switches for single phase and a low current rating switch can be selected for the seventh switch under high power factor situation. The operating principles and switching states are presented. The detailed comparison between the proposed topology and the conventional 5L-ANPC topologies in terms of voltage stress and efficiency is made. The specific modulation strategy of 7S-5L-ANPC inverter under reactive power operation has been proposed. Computer simulation and experimental prototype based on a single phase 1KVA prototype have been carried out in unity power factor condition and reactive power condition. The validity and advantages of the proposed topology and modulation method are demonstrated.

REFERENCES:

[1] S. Kouro, M. Malinowski, K. Gopakumar, J. Pou, L. G. Franquelo, B. W. Bin Wu, J. Rodriguez, M. a. Pérez, and J. I. Leon, “Recent advances and industrial applications of multilevel converters,” IEEE Trans. Ind. Electron., vol. 57, no. 8, pp. 2553–2580, Aug. 2010.

[2] F. Z. Peng, W. Qian, and D. Cao, “Recent advances in multilevel converter / inverter topologies and applications,” in Proc. IPEC, 2010, pp. 492–501.

[3] F. Z. Peng, “A generalized multilevel inverter topology with self voltage balancing,” IEEE Trans. Ind. Electron., vol. 37, no. 2, pp. 611–618, Feb. 2001.

[4] J. Rodriguez, Jih-Sheng Lai, and Fang Zheng Peng, “Multilevel inverters: a survey of topologies, controls, and applications,” IEEE Trans. Ind. Electron., vol. 49, no. 4, pp. 724–738, Apr. 2002.

[5] L. M. Tolbert, “A Multilevel Modular Capacitor Clamped DC-DC Converter,” in Proc. 41st IAS, 2006, pp. 966–973.

A Quasi-Resonant Switched-Capacitor Multilevel Inverter With Self-Voltage Balancing for Single-Phase High-Frequency AC Microgri

ABSTRACT:

In this paper, a quasi-resonant switched-capacitor (QRSC) multilevel inverter (MLI) is proposed with self-voltage balancing for single-phase high-frequency ac (HFAC) microgrids. It is composed of a QRSC circuit (QRSCC) in the frontend and an H-bridge circuit in the backend. The input voltage is divided averagely by the series-connected capacitors in QRSCC, and any voltage level can be obtained by increasing the capacitor number. The different operational mechanism and the resulting different application make up for the deficiency of the existing switched-capacitor topologies. The capacitors are connected in parallel partially or wholly when discharging to the load, thus the self-voltage balancing is realized without any high-frequency balancing algorithm. In other words, the proposed QRSC MLI is especially adapted for HFAC fields, where fundamental frequency modulation is preferred when considering the switching frequency and the resulting loss. The quasi-resonance technique is utilized to suppress the current spikes that emerge from the instantaneous parallel connection of the series-connected capacitors and the input source, decreasing the capacitance, increasing their lifetimes, and reducing the electromagnetic interference, simultaneously. The circuit analysis, power loss analysis, and comparisons with typical switched-capacitor topologies are presented. To evaluate the superior performances, a nine-level prototype is designed and implemented in both simulation and experiment, whose results confirm the feasibility of the proposed QRSC MLI.

KEYWORDS:

1.      High-frequency ac (HFAC) microgrids

2.      Quasi-resonant switched-capacitor (QRSC)

3.      Multilevel inverter (MLI)

4.      Self-voltage balancing

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

 Fig. 1. Power sources for a single-phase 500-Hz microgrid.

CIRCUIT DIAGRAM:

Fig. 2. Circuit of the proposed QRSC MLI when outputting 2n+1 levels.

EXPECTED SIMULATION RESULTS

Fig. 3. Simulation waveforms of the output voltages and currents under different load-types. (a) Vin = 100 V, fo = 500 Hz, ZL = 24 . (b) Vin = 100 V, fo = 500 Hz, ZL = 7.4+j11.3  (|ZL| = 13.5

Fig. 4. (a) Simulation waveforms of the voltages on capacitors C1~C4. (b) Simulation frequency spectrum of the staircase output.

Fig. 5. Simulation waveforms of the capacitors’ charging currents. (a) With quasi-resonant inductor. (b) Without quasi-resonant inductor.

CONCLUSION:

To make up for the deficiency that existing SC MLIs are inappropriate for the preferred series-connected input occasions like mode 2 in Fig. 1, a novel SC MLI is proposed in this paper with different structure and operational mechanism from the traditional ones, and to suppress the current spikes caused by the capacitors’ instant charging from the input source, a quasi-resonant inductor is embedded into the capacitors’ charging loop, reducing the EMI and longing the capacitors’ lifetimes. Meanwhile, the proposed QRSC MLI combines the advantages of the traditional SC MLI, such as self-voltage balancing under FFM and smaller voltage ripples for capacitors when used as HF power conversion, thus, especially adapted for HFAC microgrids.  The circuit configuration and the power loss analysis of the proposed QRSC MLI have been presented in this paper, as well as the comparisons with typical SC topologies. Lastly, a nine-level prototype is designed and implemented in both simulation and experiment. The results have validated the superior performances of the proposed topology.

REFERENCES:

[1] J. Drobnik, “High frequency alternating current power distribution,” Proceedings of IEEE INTELEC, pp. 292-296, 1994.

[2] S. Chakraborty, M. D. Weiss, and M. G. Simões, “Distributed intelligent energy management system for a single-phase high-frequency AC microgrid,” IEEE Trans. Ind. Electron., vol. 54, no. 1, pp. 97-109, Feb. 2007.

[3] S. Chakraborty and M. G. Simões, “Experimental evaluation of active filtering in a single-phase high-frequency AC microgrid,” IEEE Trans. Energy Convers., vol. 24, no. 3, pp. 673-682, Sept. 2009.

[4] S. B. Kjaer, J. K. Pedersen, and Frede Blaabjerg, “A review of single-phase grid-connected inverters for photovoltaic modules,” IEEE Trans. Ind. Appl., vol. 41, no. 5, pp. 1292–1306, Sep./Oct. 2005.

[5] J. Liu, K. W. E. Cheng, and J. Zeng, “A unified phase-shift modulation for optimized synchronization of parallel resonant inverters in high frequency power distribution system.” IEEE Trans. Ind. Electron., vol. 61, no. 7, pp. 3232,3247, Jul. 2014.

Development of a Proportional + Resonant (PR) Controller for a Three-Phase AC Micro-Grid Syst

ABSTRACT:

This document presents a Proportional + Resonant (PR) controller design for regulating the active and reactive power output of a three-phase AC Micro-Grid inverter system. The system employs a Voltage Sourced Inverter (VSI). The VSI is configured to operate as a current source through an interface L-filter. The power is controlled indirectly by controlling the inverter’s output current. The stationary reference frame strategy is adopted for the design of the PR controller. A model of a grid connected AC inverter and a detailed design of the inverter’s PR based control scheme are presented. The control scheme is developed and simulated in MATLAB/Simulink software environment. The control algorithm code is generated for a target device. Using Processor In-the Loop (PIL) simulation, functional equivalence testing is performed between the simulated control algorithm and the compiled algorithm code on the target device. Results in both normal and PIL simulations are discussed from the viewpoint of steady state and dynamic performance of the controller.

KEYWORDS:

1.      Stationary Reference Frame

2.      Processor In-the Loop

3.      Feedback Control

4.      Voltage Sourced Inverter

5.      Alpha-Beta transformation

6.      Proportional Resonant controller

SOFTWARE: MATLAB/SIMULINK

SCHEMATIC DIAGRAM:

Figure 1 Schematic diagram for a three-phase grid connected VSI

 EXPECTED SIMULATION RESULTS

Figure.2 Three-Phase grid voltages (Vabc)

Figure 3 Normal simulation α-axis current tracking due to a step change in Pref at t = 0.5s

Figure 4 Normal simulation system’s response tracking active power reference signal due to a step change in Pref at t = 0.5s

Figure 5 PIL simulation α-axis current tracking due to a step change in Pref at t = 0.5s

Figure 6 PIL simulation system’s response tracking active power reference signal due to a step change in Pref at t = 0.5s

CONCLUSION:

This paper has presented the effectiveness of using the Proportional Resonant (PR) control strategy to control active and/or reactive power transfer between the Micro-Grid and the transmission grid system. The PR controller tracks stationary frame reference currents calculated from the active (PC(t)) and reactive (QC(t)) PI controller actuating power outputs using d-q frame power equations. Consequently this improves the performance of the control loop as opposed to reference currents calculated directly from αβ frame power equations. The PR controller tracks reference currents with a very small steady-state error and reduced harmonic distortion. Model development and simulations were done using the MATLAB/Simulink software environment. Functional equivalence testing was performed between the simulated control algorithm and the compiled algorithm code on the real hardware target device. Same results were obtained for both normal and PIL simulation modes.

REFERENCES:

[1] A Yazdani and R Iravani, Voltage-Sourced Converters in Power Systems. New Jersey: John Wiley & Sons, Inc, 2010.

[2]  S. Meshram, G. Agnihotri, and S. Gupta, "A Modern Two DOF Controller for Grid Intergration with Solar Power Generator," International Journal of Electrical Engineering and Technology, vol. 3, no. 3, pp. 164-174, December 2012.

[3] X. Wang, J. M. Guerrero, F. Blaabjerg, and Z. Chen, "A Review of Power Electronics Based Microgrids," Journal of Power Electronics, vol. 12, no. 1, pp. 181-192, January 2012.

[4] J. J. V. Cardona, J. C. A. Gil; F. J. G. Sales, S. Segui-Chilet, S. O. Grau, and N. M. Galeano, "Improved Control of Current Controlled Grid Connected Inverters in Adjustable Speed Power Energies," Universidad Politecnica de Valencia and Universidad de Antioquia,.

[5] R. Teodorescu, M. Liserre, and P. Rodríguez, Grid Converters for Photovoltaic and Wind Power Systems.: John Wiley & Sons, Ltd, 2011.

Power management in PV-battery-hydro based standalone microgrid

ABSTRACT:

This work deals with the frequency regulation, voltage regulation, power management and load levelling of solar photovoltaic (PV)-battery-hydro based microgrid (MG). In this MG, the battery capacity is reduced as compared to a system, where the battery is directly connected to the DC bus of the voltage source converter (VSC). A bidirectional DC–DC converter connects the battery to the DC bus and it controls the charging and discharging current of the battery. It also regulates the DC bus voltage of VSC, frequency and voltage of MG. The proposed system manages the power flow of different sources like hydro and solar PV array. However, the load levelling is managed through the battery. The battery with VSC absorbs the sudden load changes, resulting in rapid regulation of DC link voltage, frequency and voltage of MG. Therefore, the system voltage and frequency regulation allows the active power balance along with the auxiliary services such as reactive power support, source current harmonics mitigation and voltage harmonics reduction at the point of common interconnection. The experimental results under various steady state and dynamic conditions, exhibit the excellent performance of the proposed system and validate the design and control of proposed MG.

SOFTWARE: MATLAB/SIMULINK

 CIRCUIT DIAGRAM:

Fig. 1 Microgrid Topology and MPPT Control

(a) Proposed PV-battery-hydro MG

 EXPECTED SIMULATION RESULTS

Fig. 2 Dynamic performance of PV-battery-hydro based MG following by solar irradiance change

(a) vsab, isc, iLc and ivscc, (b) Vdc, Ipv, Vb and Ib, (c) vsab, isa, iLa and ivsca, (d) Vdc, Ipv, Vb and Ib

Fig. 3 Dynamic performance of hydro-battery-PV based MG under load perturbation

(a) vsab, isc, Ipv and ivscc, (b) Vdc, Ipv, Vb and Ib, (c) vsab, isc, Ipv and ivscc, (d) Vdc, Ipv, and Vb

CONCLUSION:

In the proposed MG, an integration of hydro with the battery, compensates the intermittent nature of PV array. The proposed system uses the hydro, solar PV and battery energy to feed the voltage (Vdc), solar array current (Ipv), battery voltage (Vb) and battery current (Ib). When the load is increased, the load demand exceeds the hydro generated power, since SEIG operates in constant power mode condition. This system has the capability to adjust the dynamical power sharing among the different RES depending on the availability of renewable energy and load  demand. A bidirectional converter controller has been successful to maintain DC-link voltage and the battery charging and discharging currents. Experimental results have validated the design and  control of the proposed system and the feasibility of it for rural area electrification.

REFERENCES:

[1] Ellabban, O., Abu-Rub, H., Blaabjerg, F.: ‘Renewable energy resources: current status, future prospects and technology’, Renew. Sustain. Energy Rev.,2014, 39, pp. 748–764

[2] Bull, S.R.: ‘Renewable energy today and tomorrow’, Proc. IEEE, 2001, 89  (8), pp. 1216–1226

[3] Malik, S.M., Ai, X., Sun, Y., et al.: ‘Voltage and frequency control strategies of hybrid AC/DC microgrid: a review’, IET Renew. Power Gener., 2017, 11, (2), pp. 303–313

[4] Kusakana, K.: ‘Optimal scheduled power flow for distributed photovoltaic/ wind/diesel generators with battery storage system’, IET Renew. Power  Gener., 2015, 9, (8), pp. 916–924

[5] Askarzadeh, A.: ‘Solution for sizing a PV/diesel HPGS for isolated sites’, IET Renew. Power Gener., 2017, 11, (1), pp. 143–151