Transformerless Z-Source Four-Leg PV Inverter with Leakage Current Reduction

Transformerless Z-Source Four-Leg PV Inverter with Leakage Current Reduction

ABSTRACT:

Due to the lack of electrical isolation, the leakage current is one of the most important issues for transformerless PV systems. In this paper, a new modulation strategy is proposed to reduce the leakage current for Z-Source four-leg transformerless PV inverter. Firstly, the common mode loop model is presented. And then the common mode voltage behavior and the effect of factors on the leakage current are discussed. A new modulation strategy is proposed to achieve the step-up function and constant common mode voltage. Therefore, the leakage current can be suppressed effectively. Finally, the proposed strategy is digitally implemented and tested. The simulation results verify the effectiveness of the proposed solution.

KEYWORDS:

1.      Transformerless photovoltaic system

2.      Z source inverter

3.      Modulation

4.      Leakage current.

 SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. Z-source four-leg inverter for transformerless PV systems

EXPECTED SIMULATION RESULTS:

                 (a)Common mode voltage VCM        

(b) Parasitic capacitance voltage VPV

                     

                                                                                                      (c) Leakage current ICM  

                                                                                                                      

   (d) Spectrum analysis of ICM

                                 

 (e) Grid current 

 (f) Spectrum analysis of grid current

                                                     Fig.2 Simulation results of conventional modulation strategy

(a) Common mode voltage VCM  

 

                      (b) Parasitic capacitance voltage VPV                            

 (c) Leakage current ICM      

                                                                                                                                                  (d) Spectrum analysis of ICM

           (e) Grid current    

                                                                                               (f) Spectrum analysis of grid current

Fig.3 Simulation results of proposed modulation strategy

                                                       

          (a) Conventional modulation strategy. 

   (b) Proposed modulation strategy

Fig. 4 Simulation results of d from 0.3 to 0.1

Fig.5 Simulation results of duty cycle and leakage current (RMS)

CONCLUSION:

This paper has presented the analysis and simulation verification of a new modulation strategy to reduce the leakage current of Z-source four-leg inverter for transformerless PV systems. Our finding indicates that the conventional method fails to eliminate the leakage current. Meanwhile, the leakage current will be higher as the shoot-through duty cycle increases. As for the proposed method, the effect of shoot-through duty cycle variation on the leakage current is small, and the leakage current can be effectively reduced. On the other hand, there is one drawback that the number of switching for the proposed solution is slightly more than that of the traditional one during a carrier cycle. However, compared with the conventional solution, both the leakage current and the THD of grid current can be reduced effectively with the proposed solution. Moreover, the four-leg solution can enable the zero sequence current to circulate, avoiding the dc bias in the load output currents in case of unbalanced loads. Aside from that, the power losses of semiconductor devices can be reduced significantly. Therefore, the proposed solution is attractive for transformerless PV systems.

REFERENCES:

[1]         X. Guo, Y. Yang, and T. Zhu, “ESI: A novel three-phase inverter with leakage current attenuation for transformerless PV systems," IEEE Trans. Ind. Electron., vol. 65, no. 4, pp. 2967-2974, Apr.2018.

[2]         H. Xiao, L. Zhang, and Y. Li, “An improved zero-current-switching single-phase transformerless PV H6 inverter with switching loss-free,” IEEE Trans. Ind. Electron., vol. 64, no. 10, pp. 7896-7905, Oct. 2017.

[3]         L. Zhang, K. Sun, Y. Li, X. Lu, and J. Zhao, "A distributed power control of series connected module-integrated inverters for PV grid-tied applications,” IEEE Trans. Power Electron., vol. 33, no. 9, pp. 7698-7707, Sept. 2018.

[4]         W. Li, Y. Gu, H. Luo, W. Cui, X. He, and C. Xia, “Topology review and derivation methodology of single-phase transformerless photovoltaic inverters for leakage current suppression,” IEEE Trans. Ind. Electron., vol. 62, no. 7, pp. 4537–4551, Jul. 2015.

[5]         Yam Siwakoti, and Frede Blaabjerg, "Common-ground-type transformerless inverters for single-phase solar photovoltaic systems,” IEEE Trans. Ind. Electron., vol. 65, no. 3, pp. 2100–2111 Mar. 2018.

Power Quality Improvement Using UPQC Integrated with Distributed Generation Network

ABSTRACT

The increasing demand of electric power is giving an emphasis on the need for the maximum utilization of renewable energy sources. On the other hand maintaining power quality to satisfaction of utility is an essential requirement. In this paper the design aspects of a Unified Power Quality Conditioner integrated with photovoltaic system in a distributed generation is presented. The proposed system consist of series inverter, shunt inverter are connected back to back on the dc side and share a common dc-link capacitor with Distributed Generation through a boost converter. The primary task of UPQC is to minimize grid voltage and load current disturbances along with reactive and harmonic power compensation. In addition to primary tasks of UPQC, other functionalities such as compensation of voltage interruption and active power transfer to the load and grid in both islanding and interconnected mode have been addressed. The simulation model is design in MATLAB/ Simulation environment and the results are in good agreement with the published work.

KEYWORDS:

1.      Distributed Generation(DG)

2.      Interconnected mode

3.      Islanding mode

4.      Maximum power point tracking (MPPT)

5.      Power Quality (PQ)

6.      Unified power quality conditioner (UPQC)

7.      Photovoltaic array (PV).

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. UPQC with DG connected to the DC link

 EXPECTED SIMULATION RESULTS:

Fig. 2  Bus voltage, series compensating voltage, and load voltage

 Fig. 3 Simulation result for upstream fault on feeder: Bus voltage, compensating voltage, load voltage

Fig. 4 Simulation results for load change: nonlinear load current,Feeder current, load voltage, and dc-link capacitor voltage

CONCLUSION

The new configuration is named unified power-quality conditioner with Photo Voltaic System (UPQC-PV). Compared to a conventional UPQC, the proposed topology is capable of fully protecting critical and sensitive loads against distortions, sags/swell, and interruption in both islanding and interconnected modes. The performance of the UPQC-PV is evaluated under various disturbance conditions and it offers the following advantages:

1) To regulate the load voltage against sag/swell and disturbances in the system to protect the nonlinear/sensitive load.

2) To compensate for the reactive and harmonic components of nonlinear load current.

3) To compensate voltage interruption and active power transfer to the load and grid in islanding mode to protect sensitive critical load.

4) Depending upon the ratings, the combined system can reduce the cost up to one fifth of the separate system. Capacity enhancement has been achieved using multi-level or multi-module and central control mode, however, the flexibility of UPQC to increase its capacity in future and to cope up with the increase load demand in medium voltage distribution system.

 REFERENCES

[1]         J. M. Carrasco, L. G. Franquelo, J. T. Bialasiewicz, E. Galván, R. C.P. Guisado, M. Á. M. Prats, J. I. León, and N. M. Alfonso, “Power electronic systems for the grid integration of renewable energy sources: A survey,” IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1002–1016, Aug. 2006.

[2]         J. H. R. Enslin and P. J. M. Heskes, “Harmonic interaction betweena large number of distributed power inverters and the distribution network,” IEEE Trans. Power Electron., vol. 19, no. 6, pp. 1586–1593,

[3]         D. D. Sabin and A. Sundaram, “Quality enhances reliability,” IEEE Spectr., vol. 33, no. 2, pp. 34–41, Feb. 1996.

[4]         M. Rastogi, R. Naik, and N. Mohan, “A comparative evaluation of harmonic reduction techniques in three-phase utility interface of power electronic loads,” IEEE Trans. Ind. Appl., vol. 30, no. 5, pp. 1149–1155, Sep./Oct. 1994.

[5]         A.Ghosh and G. Ledwich, “A unified power quality conditioner (UPQC) for simultaneous voltage and current compensation,” Elect Power Syst. Res., pp. 55–63, 2001.

Single- and Two-Stage Inverter-Based Grid-Connected Photovoltaic Power Plants With Ride-Through Capability Under Grid Faults

Single- and Two-Stage Inverter-Based Grid-Connected Photovoltaic Power Plants With Ride-Through Capability Under Grid Faults

ABSTRACT

Grid-connected distributed generation sources interfaced with voltage source inverters (VSIs) need to be disconnected from the grid under: 1) excessive dc-link voltage; 2) excessive ac currents; and 3) loss of grid-voltage synchronization. In this paper, the control of single- and two-stage grid-connected VSIs in photovoltaic (PV) power plants is developed to address the issue of inverter disconnecting under various grid faults. Inverter control incorporates reactive power support in the case of voltage sags based on the grid codes’ (GCs) requirements to ride-through the faults and support the grid voltages. A case study of a 1-MW system simulated in MATLAB/Simulink software is used to illustrate the proposed control. Problems that may occur during grid faults along with associated remedies are discussed. The results presented illustrate the capability of the system to ride-through different types of grid faults.

KEYWORDS:

1.      DC–DC converter

2.      Fault-ride-through

3.      Photovoltaic (PV) systems

4.      Power system faults

5.      Reactive power support

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Diagram of a single-stage GCPPP

Fig. 2. Diagram of the two-stage conversion-based GCPPP

  

 EXPECTED SIMULATION RESULTS:

Fig. 3. Short-circuiting the PV panels: (a) grid voltages; (b) grid currents; and (c) dc-link voltage when applying a 60% SLG voltage sag at MV side of the transformer.

 Fig. 4. Short-circuiting the PV panels: (a) overall generated power;

(b) injected active power; and (c) reactive power to the grid.

Fig. 5. Turning the dc–dc converter switch ON: (a) grid voltages; (b) grid currents; and (c) dc-link voltage when applying a 60% SLG voltage sag at the MV side.

Fig. 6. Control of the dc–dc converter to produce less power under voltage sag: (a) grid voltages; (b) grid currents; (c) dc-link voltage; (d) input voltage of the dc–dc converter; (e) estimated duty cycle; and (f) actual duty cycle under a 3LG with 45% voltage sag at MV side.

Fig. 7. Control of the dc–dc converter to produce less power under voltage sag: (a) grid voltages under a 3LG with 45% voltage sag at MV side; (b) related grid currents for G = 300 W/m2; and (c) related dc-link voltage; (d) grid voltages under an SLG with 65% voltage sag at theMV side; (e) related grid currents for G = 1000 W/m2; (f) related dc-link voltage; (g) related grid currents under G = 300 W/m2; and (h) related dc-link voltage."

  

CONCLUSION

Performance requirements of GCPPPs under fault conditions for single- and two-stage grid-connected inverters have been addressed in this paper. Some modifications have been proposed for controllers to make the GCPPP ride-through compatible to any type of faults according to the GCs. These modifications include applying current limiters and controlling the dc-link voltage by different methods. It is concluded that for the single-stage configuration, the dc-link voltage is naturally limited and therefore, the GCPPP is self-protected, whereas in the two-stage configuration it is not. Three methods have been proposed for the two-stage configuration to make the GCPPP able to withstand any type of faults according to the GCs without being disconnected. The first two methods are based on not generating any power from the PV arrays during the voltage sags, whereas the third method changes the power point of the PV arrays to inject less power into the grid compared with the prefault condition. The validity of all the proposed methods to ride-through voltage sags has been demonstrated by multiple case studies performed by simulations.

REFERENCES

[1]            L. Trilla et al., “Modeling and validation of DFIG 3-MW wind turbine using field test data of balanced and unbalanced voltage sags,” IEEE Trans. Sustain. Energy, vol. 2, no. 4, pp. 509–519, Oct. 2011.

[2]            M. Popat, B. Wu, and N. Zargari, “Fault ride-through capability of cascaded current-source converter-based offshore wind farm,” IEEE Trans. Sustain. Energy, vol. 4, no. 2, pp. 314–323, Apr. 2013.

[3]            A. Marinopoulos et al., “Grid integration aspects of large solar PV installations: LVRT capability and reactive power/voltage support requirements,” in Proc. IEEE Trondheim Power Tech, Jun. 2011, pp. 1–8.

[4]            G. Islam, A. Al-Durra, S. M. Muyeen, and J. Tamura, “Low voltage ride through capability enhancement of grid connected large scale photovoltaic system,” in Proc. 37th Annu. Conf. IEEE Ind. Electron. Soc. (IECON), Nov. 2011, pp. 884–889.

Design and Performance Analysis of Three-Phase Solar PV Integrated UPQC

ABSTRACT:

This paper deals with the design and performance analysis of a three-phase single stage solar photovoltaic integrated unified power quality conditioner (PV-UPQC). The PV-UPQC consists of a shunt and series connected voltage compensators connected back to back with common DC-link.The shunt compensator performs the dual function of extracting power from PV array apart from compensating for load current harmonics. An improved synchronous reference frame control based on moving average filter is used for extraction of load active current component for improved performance of the PVUPQC. The series compensator compensates for the grid side power quality problems such as grid voltage sags/swells. The compensator injects voltage in-phase/out of phase with point of common coupling (PCC) voltage during sag and swell conditions respectively. The proposed system combines both the benefits of clean energy generation along with improving power quality. The steady state and dynamic performance of the system are evaluated by simulating in Matlab-Simulink under a nonlinear load. The system performance is then verified using a scaled down laboratory prototype under a number of disturbances such as load unbalancing, PCC voltage sags/swells and irradiation variation.

KEYWORDS:

1.      Power Quality

2.      Shunt compensator

3.       Series compensator

4.      UPQC

5.      Solar PV

6.      MPPT

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. System Configuration PV-UPQC

EXPECTED SIMULATION RESULTS:

Fig. 2. Performance of PV-UPQC under Voltage Sag and Swell Conditions

Fig. 3. Performance PV-UPQC during Load Unbalance Condition

Fig. 4. Performance PV-UPQC at Varying Irradiation Condition

Fig. 5. Load Current Harmonic Spectrum and THD

Fig. 6. Grid Current Harmonic Spectrum and THD

CONCLUSION:

The design and dynamic performance of three-phase PVUPQC have been analyzed under conditions of variable irradiation and grid voltage sags/swells. The performance of the system has been validated through experimentation on scaled down laboratory prototype. It is observed that PVUPQC mitigates the harmonics caused by nonlinear load and maintains the THD of grid current under limits of IEEE-519 standard. The system is found to be stable under variation of irradiation, voltage sags/swell and load unbalance. The performance of d-q control particularly in load unbalanced condition has been improved through the use of moving average filter. It can be seen that PV-UPQC is a good solution for modern distribution system by integrating distributed generation with power quality improvement.

REFERENCES:

[1] B. Mountain and P. Szuster, “Solar, solar everywhere: Opportunities and challenges for australia’s rooftop pv systems,” IEEE Power and Energy Magazine, vol. 13, no. 4, pp. 53–60, July 2015.

[2] A. R. Malekpour, A. Pahwa, A. Malekpour, and B. Natarajan, “Hierarchical architecture for integration of rooftop pv in smart distribution systems,” IEEE Transactions on Smart Grid, vol. PP, no. 99, pp. 1–1, 2017.

[3] Y. Yang, P. Enjeti, F. Blaabjerg, and H. Wang, “Wide-scale adoption of photovoltaic energy: Grid code modifications are explored in the distribution grid,” IEEE Ind. Appl. Mag., vol. 21, no. 5, pp. 21–31, Sept 2015.

[4] M. J. E. Alam, K. M. Muttaqi, and D. Sutanto, “An approach for online assessment of rooftop solar pv impacts on low-voltage distribution networks,” IEEE Transactions on Sustainable Energy, vol. 5, no. 2, pp.663–672, April 2014.

[5] J. Jayachandran and R. M. Sachithanandam, “Neural network-based control algorithm for DSTATCOM under nonideal source voltage and varying load conditions,” Canadian Journal of Electrical and Computer Engineering, vol. 38, no. 4, pp. 307–317, Fall 2015.