A Standalone Solar Photovoltaic Power Generation using Cuk Converter and Single Phase Inverter A. K. Verma, B. Singh & S. C. Kaushika
Journal of The Institution of Engineers (India): Series B Electrical, Electronics & Telecommunication and Computer Engineering ISSN 2250-2106 J. Inst. Eng. India Ser. B DOI 10.1007/s40031-013-0038-z
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Author's personal copy J. Inst. Eng. India Ser. B DOI 10.1007/s40031-013-0038-z
ORIGINAL CONTRIBUTION
A Standalone Solar Photovoltaic Power Generation using Cuk Converter and Single Phase Inverter A. K. Verma • B. Singh • S. C. Kaushika
Received: 27 August 2012 / Accepted: 29 January 2013 Ó The Institution of Engineers (India) 2013
Abstract In this paper, a standalone solar photovoltaic (SPV) power generating system is designed and modeled using a Cuk dc–dc converter and a single phase voltage source inverter (VSI). In this system, a dc–dc boost converter boosts a low voltage of a PV array to charge a battery at 24 V using a maximum power point tracking control algorithm. To step up a 24 V battery voltage to 360 V dc, a high frequency transformer based isolated dc–dc Cuk converter is used to reduce size, weight and losses. The dc voltage of 360 V is fed to a single phase VSI with unipolar switching to achieve a 230 Vrms, 50 Hz ac. The main objectives of this investigation are on efficiency improvement, reduction in cost, weight and size of the system and to provide an uninterruptible power to remotely located consumers. The complete SPV system is designed and it is modeled in MATLAB/Simulink. The simulated results are presented to demonstrate its satisfactory performance for validating the proposed design and control algorithm. Keywords Solar energy Boost converter Cuk converter Voltage source inverter Maximum power point tracking
Introduction Energy is essential for any society to ensure good quality of life. Renewable energy technologies offer clean, abundant energy gathered from self-renewing resources such as sun, wind, earth and plants. These energy sources are A. K. Verma (&) B. Singh S. C. Kaushika Department of Instrumentation Design and Development Centre, Indian Institute of Technology, Delhi 110016, India e-mail:
[email protected]
environmental friendly. Among these energy sources, solar PV (Photo-Voltaic) energy has become most promising source of energy as it is cost free and sustainable [1]. There are several topologies available in the literature for isolated solar photovoltaic (SPV) power generating system [2–6] which concentrate on efficient dc–dc conversion, large voltage gain and favorable ac power quality [2]. The emphasis is given on the inverter assembly along with power quality problems, even the performance of the generating system can be predicted before they are built [3]. Several researchers have contributed to the applications of isolated dc–dc converters in the solar photovoltaic power generating systems and these converters are such as push pull, flyback, forward, zeta converters etc. [4–6]. The solar photovoltaic power generating systems employing these power converters have several drawbacks, such as, limitations on the maximum switching frequency (which is desirable to minimize size and weight) in flyback and forward converters due to thermal or efficiency consideration. One of the main drawbacks of the push–pull converter is the fact that each transistor must block twice the input voltage due to the doubling effect of the centre-tapped primary so too expensive and less efficient as 800 V to 1,000 V transistors would be required for a 230 V, 50 Hz off-line application. Further major problem with the push– pull converter is that it is prone to flux symmetry imbalance [4–6]. The isolated dc–dc power converters for low power solar photovoltaic applications are still unexplored and efforts are needed in utilization and analysis of these converters in solar-PV system. In this paper, a new configuration is proposed for solar photovoltaic power (SPV) generating system based on isolated dc–dc Cuk converter. Among the single-device isolated dc–dc converters, the Cuk converter is an only converter in which the isolation transformer has only ac
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voltages across primary and secondary windings. The presence of energy transfer capacitors in series of primary and secondary windings of the high frequency transformer do not allow the dc components in the voltages across these windings and it is free from the drawback listed earlier of other isolated dc–dc converters. The proposed solar PV power generating system consists of a PV array, a dc–dc boost converter, a battery, an isolated Cuk converter and a voltage source inverter (VSI). This proposed solar PV power generating system shown in Fig. 1, is designed, modeled and its performance is simulated for a standalone small residential load of 500 W. The dc output from the PV array is given to a boost dc–dc converter which boosts the output voltage of the PV array to charge the battery at varying solar radiations and temperatures. The dc–dc boost converter is controlled for maximum power point tracking (MPPT) to charge the battery [7]. A perturbation and observation (P&O) based MPPT technique is used to extract maximum power from PV array. As the PV energy being intermittent source of energy, it cannot meet load demand all the time of the year. Therefore, a BESS (Battery Energy Storage System) is used in this renewable energy based standalone system. It significantly improves the supply availability. Then the dc voltage of the battery is fed to an isolated dc–dc Cuk converter which converts it into high dc voltage (360 V), and then it is fed to a single phase voltage source inverter (VSI) with a unipolar switching to convert this dc voltage into an ac voltage. This PV power generating system is designed and modeled to simulate its steady state and dynamic performances under linear and nonlinear loads to demonstrate its satisfactory operation. Fig. 1 Proposed solar PV power generating system
System Configuration and Principle of Operation Figure 1 shows the proposed standalone solar PV power generating system with a BESS and connected consumer loads. The output of PV array is fed to the dc–dc boost converter in order to boost the output of the PV array using MPPT and to charge the battery at 24 V. The battery stores the surplus energy and it supplies a 500 W to consumer loads. At the night when sun is not available this battery supplies power to consumer loads. This battery voltage of 24 V is boosted to 360 V by using an isolated dc–dc Cuk converter. For a fast control with reduced size of magnetics, a high frequency switching is used and a metal oxide field effect transistor (MOSFET) is employed as the switching device in this isolated dc–dc Cuk converter. Among the single-device isolated dc–dc converters, the Cuk converter is a converter in which the isolation transformer has ac voltages across primary and secondary windings. The presence of energy transfer capacitors in series of primary and secondary windings of the high frequency transformer does not allow the dc components in the voltages across its primary and secondary windings. Then this dc link voltage of 360 V is fed to a single phase bridge VSI which converts 360 V dc into an ac of 230 V, 50 Hz. The single phase bridge VSI is using an output LC filter to filter its output PWM voltage to feed consumer loads. The insulated gate bipolar transistors (IGBTs) are used in VSI to reduce the switching losses, as it operates at low switching frequency compared to dc–dc converter. Various losses in the different stages of the proposed SPV system are given in Fig. 1. Total Losses =39 .58 W
Conduction Loss =14 .57 W Switching Loss = 1.7W
Lb
Total Losses =13 .48 W Single phase Voltage Source Inverter
Cuk dc-dc converter
Boost dc-dc converter
L1
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Output 500 W
Switching Loss = 6.8W
Total Losses =12 .98 W
Total Losses =16 .27 W PV panel
Conduction Loss = 6.6W
Conduction Loss =4.81 W Switching Loss = 1W Transformer Loss =7.17 W
Filter
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io
1650 W Db
V PV
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v fi
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I PV
V PV
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Gating pulse
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i fi
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V co S1
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i fo
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Cf
vo
S2 s4 s3 s2 s1
Gating Signal
Lf
Cd
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battery
MPPT control using DC-DC boost converter
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load
S Wc S Wb
v fo
Output voltage control using Cuk converter
i L (k) vo
V co V* co
Output voltage control of VSI using Unipolar switching V ref
Author's personal copy J. Inst. Eng. India Ser. B
The solar radiation from sun falls on solar array and using a MPPT it extracts maximum power from PV array. The PV voltage at MPP is fed to the dc–dc boost converter. The dc–dc boost converter charges a battery at 24 V at varying solar radiation. The battery is charged in first case when the consumer loads on the system is constant and the solar radiation is high at a point where the power from the solar array is more than the consumer loads. The battery is discharged in second case when the solar radiation is low and the consumer loads are increased leading to reversal of the battery current. A dc–dc boost converter is controlled by closed loop PWM controller using MPPT at varying solar radiation. This 24 V battery dc voltage is converted to a 360 V dc using an isolated dc–dc Cuk converter controlled by closed loop PWM controller. A single phase VSI is used to convert 360 V dc into a 230 V, 50 Hz ac employing a unipolar switching of its IGBTs.
Design of the Proposed System The design of various components of proposed solar PV power generating system consists of a PV array, a dc–dc boost converter, a BESS, an isolated dc–dc Cuk converter, a single phase bridge VSI, a LC filter and variety of consumer loads. The detailed design of each part is given in the following sections. Design of Photo-Voltaic Array A solar PV module is configured by having a number of series connected solar cells in which each solar cell has an open circuit voltage (Voc) of 0.5 V and short circuit current (Isc) of 4A. Maximum power occurs generally at PmaxM = (85 % of Voc 9 85 % of Isc). Thus ImppM is 3.4A and VmppM is 0.42 V for each cell. A set of 40 cells are connected in series to achieve a maximum voltage of 16.8 V of SPV for proposed system. To achieve a 1,650 W peak power capacity, the required maximum current (Impp) is 98.21 A. To achieve this current 29 solar cells (98.21/3.4 A) are connected in parallel respectively. This arrangement is called solar PV array. This PV array produces 1,650 W as a peak power. The simplified expression describing the relationship between voltage VPV and current IPV is given by an electrical equivalent circuit of PV module (Fig. 2) as, 93 1 v Rs I VPV Rs IPV > > q þ þ B C 6 npp =7 Be nss npp 1C nss 7 ¼6 n I I pp P o @ A 4 > 5 > AKT Rsh > > : ; 2
IPV
8 > >
