Performance Comparison of Three-Level and MultiLevel For Grid-Connected Photovoltaic Systems Chaiyant Boonmee
Pakorn Somboonkit and Napat Watjanatepin
Solar Energy Research and Technology Transfer Center (SERTTC), Department of Electrical Engineering Faculty of Engineering and Architecture, Rajamangala University of Technology Suvarnabhumi, Nonthaburi, Thailand
[email protected]
Solar Energy Research and Technology Transfer Center (SERTTC), Department of Electrical Engineering Faculty of Engineering and Architecture, Rajamangala University of Technology Suvarnabhumi, Nonthaburi, Thailand
[email protected]
Abstract—The most Grid-connected inverters for photovoltaic (PV) system use the three-level grid-connected inverter, which generates the three voltage level at the output of inverter. It can inject the sinusoidal current to utility grid through a set of filter as shown in Fig. 1. Even though, the three-level grid-connected inverter can be operated in high efficiency, but it still has some drawback, such as, the high total harmonic distortion (THD) of output voltage, the high switching power losses cause by high switching frequency in order to reduce the filter size, and power losses cause by mismatching of PV module power in the same PV string. The cascaded H-bridge multi-level inverter topology is an important choice to reduce some drawback of the three-level grid-connected inverter for PV system as shown in Fig.2. This paper presents the comparison studies the performance of using three-level and cascaded H-bridge multi-level inverter topologies for grid-connected PV system. The both inverter topologies have to operate as a single-phase grid-connected inverter in the same condition of filter size and under the IEEE standard of lower 5% of total harmonic distortion of grid current. The compared performance indicators consist of the grid injected power while the irradiation is changed in two cases (The same size and different size of PV power changing between two sets of PV string). The both systems have to be tested in the same testing condition. The MATLAB/SIMULINK simulation results are verified the accuracy of the both topologies performance. Keywords—component; formatting; style; styling; insert (key words)
systems consisting of 4 types as follow: 1) centralized, 2) string, 3) multi-string and 4) AC-module technology [2]. The string technology needs to use the high input voltage to avoid voltage amplification (transformerless) or using fewer PV modules in series in order to have enough input voltage for transferring power to grid. From a database of more than 400 commercially available PV inverters, the transformerless or single-state inverters can reach maximum efficiencies, the majority of higher efficiency, smaller weight and size than their counterparts with galvanic separation [3]. The string technology with a H-bridge inverter is focused in this paper as shown the configuration in Fig.1. By the way, the multilevel inverter topology has been used in the recent inverter with several more advantages than the two or three level conventional inverters. There are three common topologies of multilevel inverters: neutral-point-clamped, flying-capacitor and cascaded H-bridge. The cascaded H-bridge is suitable for PV systems because it requires separated dc sources for each H-bridge unit which as shown the configuration in Fig. 2.
2 f0 ∫
1 4 f0
− 4 1f
I.
INTRODUCTION
The worldwide trend for increasing of renewable energy supplies is a important. Solar photovoltaic (PV) technology is become one of the most important energy resources with double yearly growth rate in installed PV systems of the IEAPVPS participant countries [1]. Grid-connected PV system (GCPVS) is highly suitable for daytime load system, which is the most of economical use of energy. This system consists of the PV array, inverter and utility grid; therefore the efficiency of the system depends on the efficiency of inverter and PV array. Normally, the efficiency of PV array is very low (622%). Thus, in order to keep the high efficiency, the gridconnected PV system needs to use the high efficiency inverter [2]. Nowadays, the inverter technologies for grid-connected PV 978-1-4799-7961-5/15/$31.00 ©2015 IEEE
v pv ⋅ dt
0
* pref
Fig.1. Scheme configuration of a H-bridge inverter for GCPVS.
The cascaded H-bridge multilevel voltage source inverter (CHB-MLI) topology has many advantages and attractions for GCPVS) as following present [4]-[5]: 1) It can generate the multilevel output voltage following the sinusoidal pattern waveform causing lower total harmonic distortion (THD) of the output current and voltage of inverter cause smaller size of filter. 2) Reducing voltage stressing in power switches causes lower switching losses 3) Flexible for increasing or decreasing the number of voltage levels. 4) Each
H-bridge inverter can be controlled for maximum power point tracking (MPPT) independently to improve both reliability and energy production of PV system when the PV modules operate under mismatching conditions such as in the case of partial shading irradiance.
unipolar technique for controlling the power switches of inverter, a block of MPPT for finding out the dc voltage reference vPV_Ref by using the perturb and observe, P&O [6][9] and send it to the dc-link voltage controller of each Hbridge inverter. The dc voltage controller generates the peak * current reference ipref and send it to the grid current controller in order to control the injecting the sinusoidal grid current ig which in-phase to grid voltage vg . The generating of the sinusoidal grid current ig which in-phase to the grid voltage vg causes the oscillations in the instantaneous grid power pg for twice the line frequency 2ω as
filter
SPWM1
* p1ref
pg
(1)
= Pgrid .(1- cos (2ωt )) ,
where Pgrid is the active power transferred to utility grid [10].
filter
SPWM2
* p2ref
The operating of a H-bridge VSI controlled with unipolar control technique can drive the instantaneous grid current as given by
This paper presents the comparison studies the performance of using single-state three-level and cascaded Hbridge multi-level inverter topologies for grid-connected PV system. The both inverter topologies have to operate as a single-phase grid-connected inverter in the same condition of filter size and under the IEEE standard of lower 5% of total harmonic distortion of grid current. The compared performance indicators consist of switching frequency, power losses in inverter and the grid injected power while the irradiation is changed in two cases (The same size and different size of PV power changing between two sets of PV string). The efficiency of the both systems in the same testing condition. The MATLAB/SIMULINK simulation results are verified the accuracy of the both topologies performance. II.
PRINCIPAL OF SINGLE-PHASE GCPVS
Fig. 1 and Fig. 2 show the scheme configuration of the single-phase single-stage VSI of a three voltage-levels with a H-bridge and a five voltage-levels CHB-MLI, respectively, for grid-connected PV system. Each one has to handle at least 2 tasks, MPPT and injecting the sinusoidal grid current into the utility grid. A. Three Voltage-levels H-bridge Inverter for GCPVS Fig.1 shows the scheme of three voltage-levels of Hbridge inverter for GCPVS, it consists of a set of two seriesconnected PV strings for power feeding, A set of two seriesconnected decoupling capacitors which is parallel-connected to PV strings. The buck type H-bridge VSI in Fig.1 is used for converting the dc power to ac power which is injected to the utility grid through an inductor Lf. The control system consists of a sinusoidal pulse width modulation (SPWM) with
(2)
ig = (S1 − S3 ) ⋅ iinv ,
Fig.2. Scheme configuration of a CHB-MLI inverter for GCPVS.
where ( S1 − S3 ) ∈ {−1, 0,1} and generates the instantaneous output voltage of a H-bridge VSI vo as (3)
vo = ( S1 − S3 ) ⋅ vPV .
B. Cascaded H-bridge Multi-Level Voltage Source Inverter for GCPVS Fig. 2 shows the schematic diagram of two cells CHBMLI with separated solar PV sources for GCPVS. This converter is based on the series connection of two H-bridges. Each H-bridge is fed by an isolated PV string power source. Each H-bridge cell can generate three voltage levels. The output voltage is synthesized by summing the two cascaded Hbridge inverter output voltages. It can generate five voltage levels output and corresponding switching states. The output voltage is given by
vo = ⎡⎣ ( S11 − S31 ).v pv1 + ( S12 − S32 ).v pv 2 ⎤⎦ ,
,
(6)
where ( S11 − S31 ) and ( S12 − S32 ) ∈ {−1, 0,1} . The injected power to grid from the CHB-MLI is generated as (1), the same as the three voltage-levels inverter. Each Hbridge inverter of CHB-MLI uses the uni-polar control technique with its own carrier and modulation signals in order to control the pulse-width for power switches to operate follow the control signal [10]. C. Control Method Each H-bridge inverter of the both schemes as shown in Fig.1 and Fig.2 is controlled to track the maximum power
point (MPP) of its own PV string with the same method of perturb and observe (P&O) MPPT in order to generate the voltage reference vPV_Ref and send it to the dc voltage controller [5]-[9]. Each dc voltage controller has to generate * * * p ref , p1ref , p 2ref and then,
vo
i
vg
g
the maximum power reference, they are sent to the grid-current controller. The grid-current controller generates the control signal of sine pulse width modulation SPWM in order to control the power switches of H-bridge with uni-polar control technique. III.
SIMULATION RESULT
The PV system structure and control schemes shown in Fig. 1 and Fig.2 have been implemented in MATLAB/SIMULINK program in order to compare the performance of the both schemes topologies. The model of the PV string uses the detailed single-diode model, considering the characteristic of the cells and equations were implemented according to [11]. The parameters of each PV panel and PV system are shown in Table I. TABLE I MAIN DESIGNED PARAMETERS OF PV SYSTEM Parameters PV panel MPP voltage in STC MPP current in STC PV string Rated current Rated MPP voltage Rated maximum power DC-link capacitor Power and control scheme Carrier frequency of a HB Carrier frequency of a CHB-MLI Single-phase utility grid Inductor Upper PV string Lower PV string
Symbol
Value
Vmpp Impp
63 V 0.92 A
Irated Vrated Prated C1, C2
0.92x2 A. 252 V. 464 Wp. 1000µF, 400Vdc.
fsw1 fsw2 Vg, f1 Lf PV1 PV2
10 kHz 2.5 kHz 220 Vrms, 50 Hz. 10 mH. 464 Wp 464 Wp
In order to compare the effect between both PV systems, They are defined to have two cases of irradiance changing. In case 1, all PV strings, PV1 and PV2, are shaded in the same slop of irradiance and time as shown in Fig. 4(a). In case 2, only the lower set of PV string, PV2 of the both systems are under shading condition and the other are operated in constant irradiance as shown in Fig. 5(a). In both cases, the irradiance changing are operated in the same period for the decreasing and increasing ramps. The irradiance change starts from 1000 W/m2 to 600 W/m2, waits at this level, and increases again from 600 W/m2 to 1000 W/m2. The temperature is considered at 25°c constant during the simulation. Finally, the both GCPVSs have to operate under the same condition of injecting current to grid with low level of total harmonic distortion of grid current (THDi < 5%).
(a) vo
ig
vg
(b) Fig.3. Simulation waveform of output voltage of inverter v o grid voltage v g and grid current i g generated by three voltage-levels GCI,aHB (a) and CHBMLI (b) PV systems
Fig. 3 shows the simulation waveforms of output voltage of inverter v o grid voltage v g and grid current i g generated by three voltage-levels GCI, 3(a) and CHB-MLI, 3(b). The THDi of grid current in Fig.3 (a) and Fig.3(b) are 4.14% and 3.99%, respectively, which are under the condition of this comparison. Fig.4 shows the simulation result of performance comparison between three voltage-levels inverter and CHBMLI for PV systems in case 1. Fig.4 (b) and Fig.4(c), show the values of the PV power p PV and grid injected power p g respectively. They can be seen that the PV power p PV and
grid injected power p g of the both PV systems are generated in the same level. The PV voltages, v PV , v PV1 , v PV2 , and PV currents, iPV , iPV1 , iPV2 , of the both PV systems can be tracked to the MPP correctly in the both happens of rapidly decreasing and increasing of irradiation, as shown in Fig.4(c). The grid injected current, ig of the both PV systems are decreased and increased according to the change of irradiance profile, as shown in Fig4 (d). Fig.5 shows the simulation result of performance comparison between three voltage-levels inverter and CHBMLI for PV systems in case 2. Fig.5 (b) and Fig.5(c), show the values of the PV power p PV and grid injected power p g respectively. They can be seen that the PV power p PV and
grid injected power p g of the CHB-MLI GCPVS is higher than the PV power p PV and grid injected power p g of the three voltage-levels inverter GCPVS. The PV v , v , v , voltages, PV PV1 PV2 of the both PV systems can be tracked to the MPP correctly. The PV current of three voltage-levels inverter iPV PV system and the lower set PV current of CHBMLI PV system iPV2 , are changed follow their irradiance profile of PV2 correctly, but the upper set PV current of CHB-
MLI PV system iPV1 is not change as irradiance profile of PV1, as n in Fig.5(c). It can be seen that each H-bridge cell of CHB-MLI PV system can track the MPP independently but the three -levels inverter PV system cannot. The grid injected current, ig of the CHB-MLI PV system is higher than the grid current injected by the three voltage-levels inverter PV system during low level of irradiance profile, as shown in Fig.5(d), it is corresponded to the results of PV power and grid injected power. IV.
CONCLUDSION
This paper presents the comparison studies the performance of using three-level and cascaded H-bridge multilevel inverter topologies for single-phase single-state gridconnected PV system. The both inverter topologies operate in the same condition of filter size and high quality of injecting grid current under the IEEE standard of lower 5% of total harmonic distortion of grid current. From the simulation results, It can be confirmed that the CHB-MLI PV systems can inject more power to grid than the three-level inverter for GCPVS in case of irradiance profile of PV strings of PV1 and PV2 are different significantly. Because of each H-bridge inverter series-connected in the CHB-MLI GCPVS can track the MPP of its own PV string independently. Furthermore, the CHB-MLI GCPVS is operate with lower switching frequency and voltage stressing of power switches compared to the operating of three-level inverter GCPVS. It causes the switching power losses is low. Therefore, the CHB-MLI GCPVS is suitable be used for the single-stage grid-connected inverter of PV system. ACKNOWLEDGMENT (Heading 5) The authors are thankful for the support of the Faculty of Engineering and Architecture from Rajamangala University of Technology Suvarnabhumi (RMUTSB). REFERENCES [1]
[2]
[3]
[4]
[5]
[6]
IEA-PVPS, “Trends in Photovoltaic Applications. Survey report of selected IEA countries between 1995 and 2011,” International Energy Agency – Photovoltaic Power Systems Programme, Tech. Rep. IEA PVPS T1-21:2012, August 2012. Soeren Baekhoej Kjaer, John K. Pedersen, Frede Blaabjerg, “A Review of Single-Phase Grid-Connected Inverters for Photovoltaic Modules,” IEEE Trans. Ind. Electron., vol. 41, no.5, pp. 1292-1306, Sep. 2005. Kadri, R.; Gaubert, J.-P.; Champenois, G., “An Improved Maximum Power Point Tracking for Photovoltaic Grid-Connected Inverter Based on Voltage-Oriented Control” IEEE Trans. Ind. Electron., vol. 58, no. 1, pp. 66-75, Jan. 2011 M. Malinowski, K. Gopakumar, J. Rodriguez, and M. A. Pérez, “A survey on cascaded multilevel inverters,” IEEE Trans. Ind. Electron., vol. 57, no. 7, pp. 2197–2206, Jul. 2010. A.M. Massoud, S. J. Finney, and B.W. Williams, “Control Techniques for Multilevel Voltage Source Inverters,” in Proc. IEEE. Conf., Mar. 2003, vol. 4, pp. 171–176. W. Xiao and W. G. Dunford, “A modified adaptive hill climbing MPPT method for photovoltaic power systems,” in Proc. 35th Annu. IEEE Power Electron. Spec. Conf., 2004, pp. 1957–1963.
[7]
N. Femia, G. Petrone, G. Spagnuolo, and M. Vitelli, “Optimization of perturb and observe maximum power point tracking method,” IEEE Trans. Power Electron., vol. 20, no. 4, pp. 963–973, Jul. 2005. [8] Fangrui Liu; Shanxu Duan; Fei Liu; Bangyin Liu; Yong Kang, “A Variable Step Size INC MPPT Method for PV Systems,” IEEE Trans. Ind. Electron., vol. 55, no. 7, pp. 2622-2628,Jul. 2008. [9] Esram, T.; Chapman, P.L., “Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques,” IEEE Trans. Ener. Convers., vol. 22, no. 2, pp. 439-449,Jun. 2007. [10] C. Boonmee, Y. Kumsuwan, “A Phase-shifted Carrier-Based PWM Technique for Cascaded H-bridge Inverters Application in Standalone PV System,” in Proc. IEEE. Conf., Sep. 2012, vol. 4, pp. LS8c.3.1-4. [11] M. G. Villalva, J. R. Gazoli, E. Ruppert F, “Modeling and Circuit-Based Simulation of Photovoltaic Array,”Brazilian Journal of Power Elec., vol.14, no. 1, pp. 35-45, 2009. TABLE II LIST OF VARIABLES SYMBOL Symbol pPV,pPV1,pPV2 vPV,vPV1,vPV2 iPV,iPV1,iPV2 S pCt
Description Instantaneous PV voltage (V) Instantaneous PV voltage (V) PV current (A) Irradiance profile (W/m2) Average power of capacitor,C1 and C2 (W)
Figs. 4(b),5(b) 4(d), 5(c) 4(d),5(d) 4(a),5(a) 4(c),5(c)
pg vo vg ig
Grid power (W) Output voltage of inverter (V) Utility grid voltage (V) Grid current from PV system (A)
4(c),5(c) 3(a),3(b) 3(a),3(b),4(e),5(e) 3(a),3(b),4(e),5(e)
W m2
W m2
1000
1000 S of PV1 and PV2
S of PV2 S of PV1
500
500
W
(a)
(a)
W
Ppv of CHB-MLI
800
Ppv of CHB-MLI
800
Ppv of aHB
600
600
400
400
Ppv of aHB
(b)
(b)
W
W
800
Pg of CHB-MLI
800 Pg of CHB-MLI
Pg of aHB
PC12 of CHB-MLI
PC12 of aHB
Pg of aHB
400
400
PC12 of aHB
0
0 PC12 of CHB-MLI
(c)
V v
PV
i
200
PV
400
A
of aHB
iPV 1 of CHB-MLI
(d)
V of aHB
of aHB
PV
4
vPV 2 of CHB-MLI
iPV 2 of CHB-MLI
0
g
4
(c)
v
vPV 1 of CHB-MLI
i
V
of aHB
400
300
A
vPV 1 of CHB-MLI
2
200
0
0
A
V
ig of CHB-MLI
vPV 2 of CHB-MLI
iPV 2 of CHB-MLI
i
iPV 1 of CHB-MLI
PV
of aHB
i
g
of aHB
ig of CHB-MLI
5
5 0
0 v
−300 i
g
0
0 −5
−5 v
−300
g
0
A
(d)
300
2
of aHB
g
ig of CHB-MLI
ig of CHB-MLI
(e) Fig.4. Simulation result of performance comparison between the threelevel inverter for GCPVS and CHB-MLI for GCPVS under irradiation profile of case1. The performance indicators consisting of irradiance profile(a), PV power(b), grid injected power and capacitor power(c), PV voltage and current (d), and grid voltage and grid current (e).
i
g
of aHB
(e) Fig.5. Simulation result of performance comparison between the three-level inverter for GCPVS and CHB-MLI for GCPVS under irradiation profile of case2. The performance indicators consisting of irradiance profile(a), PV power(b), grid injected power and capacitor power(c), PV voltage and current (d), and grid voltage and grid current (e).
