International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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Single Phase Transformerless Inverter and its Closed Loop Control for Grid Connected PV Applications
1Pratik D. Rahate & 2Mini Rajeev
1,2Dept. of Electrical Engineering, Fr. C. Rodrigues Institute of Technology, Navi Mumbai, India Email : 1[email protected], 2[email protected]
Abstract – Grid connected photovoltaic (PV) inverters feed power directly to the grid with the aid of power electronics converters. Recent studies revealed that transformer less inverters are preferred in single phase grid connected Photovoltaic (PV) applications due to lower size and weight, lower cost, improved efficiency etc. But there are issues with transformer less grid connected systems such as leakage currents, direct current injection and safety.
Many inverter topologies are studied in the literature to overcome these issues [1-3]. This paper presents comparison of three commonly used transformer less H Bridge topologies, design of LCL filter and control strategy for grid synchronization. Control strategy includes Phase Locked Loop (PLL), generation of reference current (Igref), and closed loop current controller to track the reference current Igref. Simulation was done in MATLAB- SIMULINK and results obtained are compared and analyzed.
Keywords - Transformer less inverter, LCL filter, H5, H6, HERIC, THD, Unipolar Pulse Width Modulation, PLL and Current controller.
I. INTRODUCTION
The application of photovoltaic (PV) grid connected system has been rapidly increasing in recent years for residential and commercial purpose. Earlier cost of the PV module was a major component in such systems. But now as the PV modules are cheaper, reduction in cost of system that includes inverter and transformer is essential.
Hence transformer less system is preferred due to reduction in weight, cost, size and increase in efficiency [1]. Proposed solutions in literature include single stage and double stage PV grid connected systems. Single stage PV system includes, single converter to track the maximum power point (MPP) and to interface PV system to grid. Thus single stage requires a step up transformer or a high dc input voltage. The inverter control in single stage becomes more complicated to achieve objectives such as MPPT, Grid Synchronization and closed loop current control. Double stage systems include two conversion stages, dc-dc conversion for boosting and tracking MPP, and dc-ac inverter for grid interface as shown in Fig.1 [3].Transformer less PV inverters are specially designed for single phase low power (<5kW) system. The PV system connected to the grid should
follow standards like IEEE 1547, IEEE 519 and IEC61727 to meet the safety requirements [1].
Fig. 1 : Double-stage grid connected PV system.
There are some issues such as leakage current, DC current injection and safety issues which need to be dealt with transformer less system. Leakage currents may flow through parasitic capacitance of the PV array due to common mode voltage variation. Leakage current gives rise to increased losses, distortion in grid current [1].
Therefore leakage current should be limited below 300mA as specified by the standard DIN V VDE V 0126-1-1 [5]. Also DC current is injected in to the grid if transformer removed, which causes saturation of transformers present along the distribution networks.
Both the safety requirements for leakage current and DC current injection can be achieved by proper selection of inverter topology and control strategies. This paper compares three commonly used transformer less inverter topologies in terms of magnitude of leakage current, total harmonic distortion (THD), number of components etc.
These topologies are H5 topology patented by SMA, HERIC topology patented by Sunway’s and H6 topologies [1, 10].Simulation of these three topologies are done in MATLAB-SIMULINK for feeding 1kW power to the grid. Results are presented and analyzed.
Results of Closed loop control are also discussed.
II. H-BRIDGE TOPOLOGIES
In this section the basic construction and the working details of the most commonly used three H-Bridge topologies are discussed. The three topologies are H5, H6 and HERIC topology. All the topologies studied in this paper are being studied for single phase system.
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A. H5 Topology
It has an extra switch on dc side of inverter. Therefore at zero voltage level of inverter output, the PV array is disconnected from grid. In positive or negative active mode the current always flows through three switches, while in freewheeling mode the current flows through only two switches. H5 topology has less power devices as compared to other two topologies discussed. But the conduction losses are higher as compared to HERIC topology. The circuit diagram of H5 topology is as shown in Fig. 2.
Fig. 2 : H5 Topology B. HERIC Topology
The HERIC topology called as “Highly Efficient and Reliable Inverter Concept” which is designed by Sunway’s, which is actually derived from full bridge inverter. HERIC topology has two extra switches on the ac side. These two extra switches bypass the PV array from the grid and thus contribute to minimization of leakage current. The circuit diagram of HERIC topology is as shown in Fig. 3.
Fig. 3 : HERIC Topology C. H6 Topology
An H6 topology is derived when an extra switch is placed at the dc side of the H5 topology, to form a new current flow and thus to reduce conduction loss.
Therefore during positive half of grid voltage, current flows through three switches. While in negative half of grid voltage the current flows through two switches.
Therefore PV array can be disconnected from grid when output of inverter voltage is at zero voltage and thus leakage current can be minimized. Circuit diagram of H6 Topology is as shown in Fig. 4.
Fig. 4 : H6 Topology
III. CONTROL STRATEGY FOR TRANSFORMERLESS INVERTER
In case of H-Bridge two level inverter, traditional method is to apply the full-bridge inverter with bipolar or unipolar pulse width modulation (PWM). In bipolar PWM one reference wave is compared with one carrier wave and pulses are obtained. In Unipolar PWM, two references waves with 180 degree phase shift are been compared with carrier wave and pulses are obtained.
Bipolar PWM results in excellent characteristics of leakage current but the current ripple across inductor and switching losses are large. Unipolar PWM, results in smaller inductor current ripple and higher efficiency but it leads to high leakage currents which are not desirable [1]. The solution to above is to disconnect the dc and ac side of full bridge inverter in the freewheeling mode of inverter. Unipolar PWM is selected for all topologies where frequency of carrier wave is chosen as 20 kHz.
IV. LCL FILTER DESIGN
Filter is connected at the output of inverter to improve the quality of output voltage waveform. LCL filter provides a better decoupling between filter and grid impedance and a lower ripple current stress across the grid inductor. LCL filter also provides better attenuation as compared to other filters with the same size with an inductive output[5]. Cut-off frequency is an important parameter while designing the filter. The cut-off frequency of filter must be minimally one half of the switching frequency of the converter, because the filter must have enough attenuation in the range of converters switching frequency. Also Cut-off frequency must have a sufficient distance from the grid frequency. A resistor is added in series with capacitor to attenuate part of ripple on switching frequency in order to avoid resonance [6].
2
max* ) (
L g dc
ac
I
DTs V
L V
(1)ac
g
rL
L
(2)b
f
C
C 0 . 05 *
(3)f res
f
C
R 3 * * 1
(4)Values of Lac, Lg, Cf and Rf obtained is given in Table-I.
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V. CLOSED LOOP CONTROL
Control strategy of grid connected inverter consists of PLL, Current control loop and generation of Igref which are discussed in subsequent sections.
A. PLL Structure
The PLL is used to synchronize the inverter output current with grid voltage. PLL simply senses the grid voltage and generates the peak grid voltage and phase angle. The structure is based on Single order generalized integrator (SOGI) which is proposed in [8]. As seen in Fig. 5, Vα is filtered grid signal and Vβ as imaginary orthogonal signal. Using αβ to dq transformation and PR regulator as a compensator, estimated angular frequency ωe and phase angle θ are obtained.
Fig.1 PLL Structure
Band pass filter is used to filter any harmonics in grid voltage. To eliminate low order harmonics, limited bandwidth is desired. Therefore Bandwidth is chosen to be 40Hz. Q is calculated as 1.25 according to desired bandwidth of 40Hz.
B. Reference Current Generation
Inverter output current should be able track the Grid reference current (Igref) according to the variation in power (Pref) and input voltage of inverter (Vdc). This grid reference current is given as in equation 5, where the active power reference is Pref and reactive power is Qref. As only active power is injected to grid, therefore Qref = 0.
) 2 cos(
2 2 2
m ref ref
gref
V
Q
I P
(5)C. Closed loop current control
Closed loop current controller as shown in Fig. 6, is designed using PR regulator which is introduced in [8].
Reference grid current is compared with actual grid current. The error ec is then passed through C(s) which is a PR regulator, uc is the output of PR regulator and mc obtained is the sinusoidal signal which is then compared with carrier signal in PWM technique.
Fig. 6 : Closed loop current controller
The transfer function of PR current controller
G
c(s )
is as [8]:
( )
2
p s
c
s
Ki s K s
G
(6)And transfer function of harmonic compensator to compensate selective harmonic 3rd, 5th and 7th as they are dominant harmonics in load current spectrum [9]:
Transfer function of Harmonic compensator
G
h(s )
to compensate the selected harmonics 3rd, 5th and 7th is as
2) ) (
( s h
K s s
G
o ih s
h
(7)VI. SIMULATION DETAILS
The simulation model of single phase grid connected transformerless inverter is developed in Mat lab/Simulink.The system is designed for feeding 1kW of power to the grid. For this, PV panel of Open circuit voltage greater than peak value of single phase grid voltage is chosen. H5, Heric and H6 topologies are simulated and results are presented.
Table I : Comparison of three topologies.
Parameters H5 HERIC H6
No. of Levels 3 3 3
Total No. Switches 5 6 6
Number of Switches conduct
3 2 3
Number of Diodes with Freewheeling
2 2 1
THD (At the output of Inverter)
76.71 76.71 76.74
LOH in Voltage 3rd 3rd 5th
Current THD 8.78% 8.78% 8.78
%
LOH in Current 3rd 3rd 5th
No. of controllable Devices Required
5 6 6
Leakage Current 3.97m A
4.337m A
3.968 mA All the three topologies with LCL Filter at output of inverter and closed loop current control is simulated and results are shown in Fig. 7-16.
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A. Simulation results of H5 Topology
Fig.7 and Fig. 8 shows the voltage waveform of the H5 Topology presented in section-II.
Fig. 7 : 2-Level output voltage of H5 Topology.
Fig. 8 : Output voltage of H5 Topology with LCL Filter.
B. Simulation results of H6 Topology
Fig.9 and Fig. 10 shows the voltage waveform of the H6 Topology presented in section-II.
Fig. 9 : 2-Level output voltage of H6 Topology.
Fig. 10 : Ouput of H6 Topology with LCL Filter.
C. Simulation results of HERIC Topology
Fig.11 and Fig. 12 shows the voltage waveform of the HERIC Topology presented in section-II
Fig. 11: 2-Level output voltage HERIC Topology.
Fig. 12: Output of HERIC Topology with LCL Filter.
TABLE II - SYSTEM PARAMETERS.
Parameters Values
DC-Link voltage 400V
Output Voltage (peak) 325V
Output Frequency 50Hz
Inverter Switching frequency 20Khz Output Power of Inverter 1KW
Linv 8.125mH
Lg 6.0125mH
Rf 15.96 Ω
Cf 1.50679μF
VII. RESULTS FOR CLOSED LOOP CONTROL OF TRANSFORMERLESS
INVERTER
Based on the comparison given in Table II, HERIC topology is selected for the closed loop control in grid connected system. As the closed-loop current controller closely track the generated reference current, the steady state error is almost zero. As shown in Fig. 14 voltage fluctuation occurs at 0.04s from 325 Vpeak to 300 Vpeak and back to 325Vpeak at 0.14s, which results in change of Igref, which is tracked by grid current(Ig) through closed loop current controller within half cycle. Closed loop current control also achieves the Grid synchronization as seen in Fig. 15. It shows the injected grid current has same frequency and in phase with grid voltage.
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Fig. 13 : Model of Closed loop control
Fig. 14 : Effect of change in Vgrid on Ig
Fig. 15 : Simulation result of Grid Synchronization
VIII. CONCLUSION
In this paper, a comparison is done between transformerless inverter topologies such as H5, H6 and HERIC topology. HERIC topology has lower power losses and higher efficiency as compared to other topologies, hence HERIC topology was used for the detailed analysis. Simulation of closed loop control of HERIC topology is carried out. Closed loop current controller has been designed using PR controller and Harmonic Compensator, which will track sinusoidal reference and provides good rejection for dominant
harmonics. It can be seen that the PR controller is able to reduce the steady state error to zero. The effectiveness of the current controller is verified by changing the amplitude of grid voltage, the results of which are presented in the paper.
IX. REFERENCES
[1] Li Zhang, Kai Sun, Yan Xing and Mu Xing,“H6 Transformerless Full-Bridge PV Grid-Tied Inverters,” IEEE Trans. Power Electronics, vol.
29, pp. 3, Mar. 2014.
[2] W. Cui, B. Yang, Y. Zhao, W. Li, and X. He, “A novel Single-Phase transformerless grid- connected inverter,” in Proc. IEEE IECON, 2011, pp. 1067-1071. Science, 1989.
[3] Brain K. Perera, Sridhar R. Pulikanti, Philip Ciufo and Sarth Perera, “Simulation Model of a Grid- Connected Single-phase Photovoltaic System in PSCAD/EMTDC,” in Proc. IEEE POWERCON, 2012, pp. 1-6, Nov. 2012.
[4] W. Yu, J. Lai, H. Qain, and C. Hutchens, “High- efficiency MOSFET inverter with H6-type configuration for photovoltaic nonisolated ac- module applications,” IEEE Trans.Power Electron., vol.26, no.4, pp.1253-1260, Apr.2011.
[5] Samuel Araujo and Fernando Antunes “LCL Filter design for grid-connected NPC inverters in offshore wind turbines”,Proc in The 7th International Conference on Power Electronics, Oct. 2007 .
[6] M. Liserre, F.Blaabjerg, and S.Hansen, “Design and control of an LCL-filter-based three-phase active rectifier,” IEEE Trans. Ind. Appl., vol. 41, no. 5, pp. 1281-1291, Sep.-Oct. 2005.
[7] M. Ciobotaru, R. Teodorescu, and F. Blaabjerg,
“A new single-phase PLL structure based on second order generalized integrator,” in Proc. 37th Annu. IEEE Power Electronics Specialists Conf.
PESC’06, Jun.2006, pp. 1-6.
[8] Mihai Ciobotaru, Remus Teodorescu and Frede Blaabjerg, “Control of single-stage single-phase PV inverter,” Proc in Power Electronics and Applications, 2005 European Conference.
[9] Teodorescu R, Blaabjerg F, Liserre M,Loh P.C,
“Proportional-resonant controllers and filters for grid-connected voltage-source converters,” in Proc. IEE Electric Power Applications Vol.153, Issue:5.
