Design and Implementation of a Stand-Alone MicroInverter with Push-Pull DC/DC Power Converter Nasrudin Abd. Rahim* †, Mohamad Fathi Bin Mohamad Elias*, Jafferi Bin Jamaludin*, Ahmad Rivai*, Oon Erixno*, and Febry Yadi Zainal* *UM Power Energy Dedicated Advanced Centre (UMPEDAC) Level 4 Wisma R&D University of Malaya Jalan Pantai Baharu 59990 Kuala Lumpur Malaysia Tel: (603) 22463246, Fax: (603) 22463257, E-mail: [email protected]
†Distinguish Adjunct Professor, Renewable Energy Research Group, King Abdulaziz University, Jeddah 21589, Saudi Arabia
Keywords: Stand-alone inverter; push-pull converter; pure sine wave; Unipolar SPWM.
A power converter is an electrical device for converting electrical energy . It can provide power in a suitable form and at high efficiency to load. Micro-inverter systems typically consist of boost converter and inverter. It may also consists of battery as an energy storage element as well as charge controller used to control battery charging and discharging in the system.
Abstract This paper presents the design and implementation of low power stand-alone inverter for single PV module and 24 V batteries input. The stand-alone micro-inverter consists of push-pull DC/DC converter, full bridge single-phase inverter and LC filter. The regulating pulse-width-modulators (PWM) chip (SG3524) is used to drive the MOSFET for switching the push-pull DC/DC converter. Sinewave pulse-widthmodulation (SPWM) signals are generated using dsPIC33F64MC802 microcontroller. The SPWM signals drive the full bridge single-phase inverter through an optocoupler gate drive. The complete design was simulated using Psim software environment and the prototype was verified experimentally.
Practically, a standalone micro-inverter is a device used to convert direct current (DC) generated by a single solar module into an alternating current (AC). The output of standalone micro-inverter is directly connected to the load. Since the battery voltage is low (12V/24V), first it has to be stepped up significantly at least equal to 350V DC to match the utility level. This poses a challenge to the system designer as the traditional boost converter cannot provide the required gain at high efficiency. A few topology solutions are suitable to meet the efficiency target. Because the input voltage of the DC-AC converter must be at least equal to 350V, it is not suitable to use non-isolated DC-DC converters . Furthermore, the output power rating prevents the use of single switch topologies such as the flyback and the forward . Among the remaining isolated topologies, the half bridge and full bridge are more suitable for high DC input voltage applications and also characterized by the added complexity of gate drive circuitry of the high side switches . Due to such considerations, the push-pull represents the most suitable choice.
1 Introduction Solar energy is one of the best solution to the energy crisis and environmental pollution and it has been drawing more and more attentions worldwide . Solar energy can be captured as thermal energy (heat) and electrical energy by using solar photovoltaic (PV) cell. They are various types of PV system that have been developed in a recent decade. The system can operate either in a grid-connected basis or standalone . The difference between the two is that, in a gridconnected system the PV output is directly fed to the grid without the use of a storage element. In stand-alone mode, the PV output is fed to the load, where the storage elements are used to cater for various load patterns. As compared to the grid-connected photovoltaic power system, in remote and isolated regions where power grid cannot be extended to, standalone photovoltaic power system has found a wide application to meet the demand of small but essential loads [3, 4].
In this paper, the design and implementation of a stand-alone micro-inverter with push-pull DC/DC power converter is presented.
2 Methodology The stand-alone inverter is used for the supply of extra-low (ELV) and/or low voltage (LV) electric power to a single load, or an electrical installation via battery energy or a renewable resources such as PV, wind, hydro etc. And it is designed not to inject power to the grid. Fig. 1 shows the proposed configuration of the standalone PV micro-inverter,
which consists of a PV module, solar charge controller, storage units (24 volt battery), a stand-alone micro-inverter, and a load. The solar charge controller delivers power from the PV module to the storage units. It operates by keeping the battery to be properly charged by the solar PV module. To prevent the battery from damaged due to over-charging the charge controller will cut-off the current from the PV when the battery is fully charged. The solar charge controller also keeps the power transfer at highest efficiency as the amount of sunlight varies so that the efficiency of the system is optimized. The stand-alone micro inverter converts the DC voltage into a desired AC voltage for load. Solar Charger Controller
Unipolar sinusoidal PWM (SPWM) switching technique is used in the proposed stand-alone micro-inverter. It is a "carrier-based" pulse width modulation technique. The unipolar modulation requires two sinusoidal modulating waves (Vm) and (Vm-) which are of the same magnitude and frequency but 1800 out of phase. In order to generate two gating signals for the upper two switches SA+ and SB+, the two modulating waves are compared with a common triangular carrier wave (Vcr). Fig. 3 is the unipolar modulation scheme that shows the upper two devices do not switch simultaneously. It is distinguished the unipolar PWM from the bipolar PWM where all the four devices are switched at the same time . The inverter output voltage switches between zero and +Vd during the positive half cycle or between zero and –Vd during the negative half cycle of the modulating signal which explains why this scheme is called unipolar modulation. The unipolar switched inverter offers reduced switching losses and generates less electromagnetic interference (EMI) .
Fig. 1: Configuration of a stand-alone PV micro-inverter Fig. 2 shows a full-bridge single-phase inverter circuit with four switches. The DC input converts into AC output by closing and opening switches in appropriate sequences. Table 1 lists all the switch states of a full-bridge inverter. The upper and lower switches in a branch must not turn on at the same time by using a dead-band in order to avoid short-circuit in the DC source. The inverter has four defined states and one undefined state. Various modulation techniques can be used to generate its switching signals. Two general types of inverters according to their switching technique are squarewave inverter (which uses line-frequency switching) and pulse width modulation (PWM) inverter (which uses highfrequency switching). +
+ Vd 2 -
+ Vd 2 -
Fig. 3: Unipolar modulation scheme The amplitude-modulation ratio or modulation index (ma) is defined using the ratio of peak modulation ( Vˆm ) and peak
carrier signals ( Vˆcr ) as follows:
B vAB=vAN – vBN
The output voltage is given by:
Fig. 2 : Single-phase full-bridge inverter Table 1: The switch states of a full-bridge single-phase inverter State SA+ SA- SB+ SBvAN vBN vAB Vd/2 -Vd/2 Vd 1. On Off Off On - Vd/2 Vd/2 -Vd 2. Off On On Off V /2 V /2 0 3. On Off On Off d d -Vd/2 -Vd/2 0 4. Off On Off On 5. Off Off Off Off undefined undefined undefined
The inverter works well for ma ≤ 1. For ma > 1, there are periods of the triangle wave in which there is no intersection of the carrier and the signal which cause over-modulation. The stand-alone micro-inverter requires a low-pass filter at the output side to reduce the harmonics generated by the switching frequency of SPWM. The cut-off frequency can be set as follow :
Circuit simulations were conducted for the proposed standalone micro-inverter using Psim to validate the theoretical results that were obtained in Section II and to measure their performance in terms of efficiency. The circuit simulation of stand-alone micro-inverter with push-pull DC/DC converter is shown in Fig. 5. The stand-alone micro-inverter were simulated with the following parameters: input voltage, Vin = 24V, switching frequency of 100 kHz for push-pull DC/DC converter with the corresponding gating signals, Vg1 & Vg2 are shown in Fig. 6. The turn ratio of the push-pull transformer is 3+3 turns for primary and 48 turns for secondary. To keep the DC output voltage constant, a voltage divider (R1 = 390 kΩ & R2 = 5.6 kΩ) is used for feedback to the push-pull DC/DC converter. The switching frequency for the full-bridge single-phase inverter, fs = 20 kHz and index modulation m = 0.95. A low-pass L-C filter (Lf = 4mH & Cf = 1.5 µF) is used to filter the inverter output. For testing, a 200 Ω resistor is used as the load.
The switching frequency for DC/DC converter is 100 kHz and for the full-bridge inverter is 20 kHz. Since the fundamental frequency is 50 Hz, the cut-off frequency must be higher. The proposed stand-alone micro inverter uses a low pass L-C filter (Lf = 4 mH & Cf = 1.5 µF) so the cut-off frequency is ≈ 2 kHz. Table 2 shows the proposed stand-alone micro-inverter specifications. To generate a 230 V AC output (Vo), higher DC link voltage (Vd) level is needed. The DC–DC converter is required not only for voltage boost, but also for voltage regulation as the inverter output voltage varies with load. The proposed stand-alone micro-inverter uses a push-pull DC/DC converter topology as shown in Fig. 4 for boosting the 24 V DC input to a voltage of at least equal to 350 V. Table 2: The stand-alone micro-inverter specifications Specification Value Nominal input voltage 24V Output voltage 220Vrms , 50Hz Output power 300W Switching frequency 100 kHz (DC/DC converter) 20 kHz (inverter) N1:N2 + Vout
Fig. 5: Schematic circuit of the proposed stand-alone microinverter with push-pull DC/DC converter
Fig. 4: Push-pull DC/DC converter topology Push-pull DC/DC converter topology features a center-tapped high-frequency transformer and two power devices (MOSFETs) on the primary side, as shown in Fig. 4. Its conversion efficiency is high even with a relatively low input voltage. Both power devices (S1 and S2) are ground referenced with a consequence of simpler gate drive circuits. In order to transfer power to each primary of the centertapped transformer, S1 and S2 are alternately turned on and off. Conduction of S1 and S2 must be avoided by limiting the duty cycle value of the constant frequency PWM modulator to less than 0.5. The high voltage conversion ratio can be achieved by proper transformer turns ratio design, taking into account that the input to output voltage transfer function is given by the equation below.
Fig. 6: Switching signal for push-pull DC/DC converter The DC-link voltage VDClink, the inverter output voltage before filtering VAB, and the inverter output voltage after filtering Vout are shown on Fig. 7. The output current and the output voltage of inverter are shown in Fig. 8. The simulated output voltage Vout = 233.32 V, frequency = 50 Hz, output RMS current Irms = 1.177 A, output power P = 272.06 W and power factor PF = 0.991. Fig. 9 shows the switching for the pushpull converter and the feedback signal to maintain the DClink voltage constant.
N2 DVin (4) N1
The voltage-mode PWM regulator (SG3525) is used to set duty cycle and to keep a constant output DC voltage. The output DC voltage is then converted into AC using full-bridge single- phase inverter.
Full bridge single phase inverter
Push-pull DC/DC power converter
Fig. 7: The DC-link voltage VDClink, the inverter output voltage before filtering VAB and the inverter output voltage after filtering Vout. dsPIC33F64MC 802
SG3524 regulating PWM IC
Fig. 10: Experimental setup of a stand-alone micro-inverter with push-pull DC/DC converter The loads used to test the micro-inverter are six 50 watts light bulbs. The input current and voltage waveforms are shown in Fig. 11. The RMS input voltage is 24V and the current is 13.0 A. The inverter output waveforms are shown in Fig. 12. The RMS voltage is 214 Vrms, the RMS current is 1.29 Arms and the frequency is 50.71 Hz. The efficiency of the microinverter is 88.48%.
Fig. 8: The output current and voltage of the inverter.
Fig. 9: Switching signals for push-pull converter and the feedback signal.
3 Hardware Implementation Fig. 10 shows the experimental setup of the stand-alone micro-inverter with push-pull DC/DC power converter. The printed-circuit-board (PCB) size is 200 mm x 100 mm. The regulating pulse-width-modulators (PWM) chip (SG3524) is used to drive MOSFETs (NXP PSMN8R5-100PS) for switching the push-pull DC/DC converter. For implementation, the same parameters as in simulation are used. The switching frequency is 100 kHz for the push-pull DC/DC converter. The turn ratio for the push-pull highfrequency transformer is 3+3 turns for the primary and 48 turns for the secondary. Four ultrafast power diodes (RHRP30120) formed full-bridge rectifier to rectify the output of the push-pull transformer. The dsPIC33F64MC802 is used to generate a unipolar SPWM switching signal for full-bridge single-phase inverter with switching frequency of 20 kHz and index modulation of 0.95. Optocouplers (HCPL3120) are used to drive IGBTs (IRG4PH30KDPBF) of the inverter. Similarly, 4mH inductor and 1.5 uF capacitor are used for LC filter.
Fig. 11: The input current and voltage waveform
Fig. 12: The inverter output waveforms
4 Conclusion In this paper, a stand-alone micro-inverter using push-pull DC/DC converter is presented. The converter is fed by a low DC input voltage of 24 V, and is capable of supplying up to 300W of output power for single-phase AC load. This is possible due to dual-stage conversion topology that consists of a push-pull DC/DC converter and a full-bridge inverter. The push-pull DC/DC converter is able to produce a regulated high-voltage DC bus to generate the desired AC output. Unipolar SPWM technique is used for inverter switching in order to generate a 50 Hz, 220 Vrms output. The key features of the proposed micro-inverter system are high power density and high conversion efficiency.
Acknowledgements The authors thank the technical and financial assistance of UM Power Energy Dedicated Advanced Centre (UMPEDAC) and the Higher Institution Centre of Excellence (HICoE) Program Research Grant, UMPEDAC - 2016 (MOHE HICOE - UMPEDAC).
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