Power Quality Analysis of a PV fed Seven Level Cascaded H-Bridge Multilevel Inverter

ABSTRACT:  

Efficient DC to DC and DC to AC converters play a vital role in the reliable performance of standalone and grid connected photovoltaic systems. This paper deals with DC to AC conversion by a seven level cascaded H-bridge multilevel inverter for a standalone photovoltaic system. The PV fed seven level cascaded H-bridge multilevel inverter is analyzed in two ways: 1) with equal voltage sources as input to the H bridges and 2) with unequal voltage sources as input. A comparative study of the total harmonic distortion reduction in the PV fed multilevel inverter system with and without equal voltage sources as input is carried out. It is observed that with unequal voltage sources, the total harmonic distortion is increased than that with equal voltage sources as input to the PV fed seven level cascaded H-bridge multilevel inverter. Further, the study attempts to show that with an LC filter at the output stage of the multilevel inverter, the total harmonic reduction is significantly reduced and the power quality of the PV fed multilevel inverter system is highly improved. Results are verified using simulations done in MATLAB/Simulink environment.

KEYWORDS:

1.      Photo voltaic Array (PV Array)

2.      Cascaded Multilevel Inverter

3.      Pulse Width Modulation (PWMJ

4.      Total Harmonic Distortion (THD)

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig.1. Seven level Cascaded H-bridge multilevel inverter

EXPECTED SIMULATION RESULTS:

Fig.2. (a)Seven level cascaded MLI output voltage (b) Harmonic spectrum of the output voltage

Fig.3.(a) Seven level cascaded MLI output current (b) Harmonic

spectrum of the output current

Fig.4. (a) Output voltage of MLI with LC filter (b) Harmonic spectrum of

the output voltage with LC filter

Fig.5. (a) Output current of MLI with LC filter (b) Harmonic spectrum of

the output current with LC filter

Fig.6. (a) Output voltage of seven level multilevel inverter with unequal

voltage sources (b) Harmonic spectrum of the output voltage

Fig.7. (a) Output Current of seven level MLI with unequal voltage sources

(b) Harmonic spectrum of the output current

Fig.8. (a) Output voltage of seven level cascaded MLI with unequal voltage sources and LC filter (b) Harmonic spectrum of the output voltage with LC filter

Fig.9. (a) Output current of seven level cascaded MLI with unequal voltage sources and LC filter (b) Harmonic spectrum of the output current with LC filter

CONCLUSION:

In this paper, an analysis of a seven level cascaded H bridge multilevel inverter for a standalone photovoltaic system is carried out 1) with equal voltage sources as input to the H-bridges and 2) with unequal voltage sources as  input. It is found that when equal voltage values are fed as input to the H-bridges of the multilevel inverter, there is a reduction in the total harmonic distortion of the MLI output when compared to that with unequal voltage sources as its input. It is also observed that with an LC filter at the output stage of the multilevel inverter in both the scenarios, the total harmonic reduction is significantly reduced and the power quality of the PV fed multilevel inverter system is highly improved.

REFERENCES:

[1] Venkatachalam, Jovitha Jerome and J. Karpagam, "An experimental investigation on a multilevel inverter for solar energy applications," International Journal of Electrical Power and Energy Systems, 2013, pp.157-167.

[2] Ebrahim Babaei, Mohammad Farhadi and Farshid Najaty, "Symmetric and asymmetric multilevel inverter topologies with reduced switching devices," Electric Power Systems Research, 2012, pp. 122- 130.

[3] Jia-Min Shen, Hurng-Liahng Jinn-Chang Wu and Kuen-Der, "Five-Level Inverter for Renewable Power Generation System, IEEE transactions on energy conversion," 2013, pp.257-266.

[4] Hui Peng, Makoto Hagiwara and Hirofumi Akagi, "Modeling and Analysis of Switching-Ripple Voltage on the DC Link  between a Diode Rectifier and a Modular Multilevel Cascade Inverter (MMCI)," IEEE transactions on power electronics, 2013, pp.75-84.

[5] Javier Chavarria, Domingo Bie!, Francesc Guinjoan, Carlos Meza and Juan J. Negroni, "Energy-Balance Control of PV  Cascaded Multilevel Grid-Connected Inverters Under LevelShifted and Phase-Shifted PWMs," IEEE transactions on industrial electronics, 2013, pp.98-111.

Evaluation of DVR Capability Enhancement -Zero Active Power Tracking Technique

ABSTRACT:  

 This paper presents a utilization technique for enhancing the capabilities of dynamic voltage restorers (DVRs). This study aims to enhance the abilities of DVRs to maintain acceptable voltages and last longer during compensation. Both the magnitude and phase displacement angle of the synthesized DVR voltage are precisely adjusted to achieve lower power utilization. The real and reactive powers are calculated in real time in the tracking loop to achieve better conditions. This technique results in less energy being taken out of the DC-link capacitor, resulting in smaller size requirements. The results from both the simulation and experimental tests illustrate that the proposed technique clearly achieved superior performance. The DVR’s active action period was considerably longer, with nearly 5 times the energy left in the DC-link capacitor for further compensation compared to the traditional technique. This technical merit demonstrates that DVRs could cover a wider range of voltage sags; the practicality of this idea for better utilization is better than that of existing installed DVRs.

KEYWORDS:

1.      DVR capability

2.      Energy optimized

3.      Energy source

4.      Series compensator

5.      Voltage stability

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. Circuit diagram model for simulation using MatLab/Simulink.

 EXPECTED SIMULATION RESULTS:

Fig. 2. D-axis voltages at the system (VSd), DVR (VDVRd), and load (VLd). during in-phase compensation (simulation).

Fig. 3. Q-axis voltages at the system (VSq), DVR (VDVRq), and load (VLq) during in-phase compensation

(simulation).

Fig. 4. The overall three-phase voltage signals during in-phase compensation (simulation).

Fig. 5. Real power at source (PS), the DVR (PDVR) and load (PL) during in-phase compensation (simulation).

Fig. 6. The DVR DC-side voltage (VDC) during in-phase compensation (simulation).

Fig. 7. D-axis voltages at the system(VSd), DVR (VDVRd), and load (VLd) during zero-real power tracking compensation (simulation).

Fig. 8. Q-axis voltages at the system (VSq), DVR (VDVRq), and load (VLq) during zero-real power tracking compensation (simulation)..

Fig. 9. The overall three-phase voltage signals during zero-real power tracking compensation (simulation).

Fig. 10. The DVR DC-side voltage (VDC) during zero-real power tracking compensation (simulation).

CONCLUSION:

It is clear from both the simulation and experimental results illustrated in this paper that the proposed zero-real power tracking technique applied to DVR-based compensation can result in superior performance compared to the traditional in-phase technique. The experimental test results match those proposed using simulation, although some discrepancies due to the imperfect nature of the test circuit components were seen.

With the traditional in-phase technique, the compensation was performed and depended on the real power injected to the system. Then, more of the energy stored in the DC-link capacitor was utilized quickly, reaching its limitation within a shorter period. The compensation was eventually forced to stop before the entire voltage sag period was finished. When the compensation was conducted using the proposed technique, less energy was used for the converter basic switching process. The clear advantage in terms of the voltage level at the DC-link capacitor indicates that with the proposed technique, more energy remains in the DVR (67% to 14% in the traditional in-phase technique), which guarantees the correct compensating voltage will be provided for longer periods of compensation. With this technique, none (or less) of the real power will be transferred to the system, which provides more for the DVR to cover a wider range of voltage sags, adding more flexible adaptive control to the solution of sag voltage disturbances.

REFERENCES:

[1] M. Bollen, Understanding Power Quality Problems, Voltage Sags and Interruptions. New York: IEEE Press, 1999.

[2] J. Roldán-Pérez, A. García-Cerrada, J. L. Zamora-Macho, P. Roncero-Sánchez, and E. Acha, “Troubleshooting a digital repetitive controller for a versatile dynamic voltage restorer,” Int. J. Elect. Power Energy Syst., vol. 57, pp. 105–115, May 2014.

[3] P. Kanjiya, B. Singh, A. Chandra, and K. Al-Haddad, “SRF theory revisited to control self-supported dynamic voltage restorer (DVR) for unbalanced and nonlinear loads,” IEEE Trans. Ind. Appl., vol. 49, no. 5, pp. 2330–2340, Sep. 2013.

[4] S. Naidu, and D. Fernandes, “Dynamic voltage restorer based on a four-leg voltage source converter,” IET Generation, Transmission & Distribution, vol. 3, no. 5, pp. 437–447, May 2009.

[5] T. Jimichi, H. Fujita, and H. Akagi, “A dynamic voltage restorer equipped with a high-frequency isolated dc-dc converter,” IEEE Trans. Ind. Appl., vol. 47, no. 1, pp. 169– 175, Jan. 2011.

Adaptive Reactive Power Control Using Static VAR Compensator (FC-TCR & TCR)

ABSTRACT:

 Flexible AC transmission system (FACTS) is a technology, which is based on power electronic

devices, used to enhance the existing transmission capabilities in order to make the transmission system flexible and independent operation. The FACTS technology is a promising technology to achieve complete deregulation of Power System i.e. Generation, Transmission and Distribution as complete individual units. The loading capability of transmission system can also be enhanced nearer to the thermal limits without affecting the stability. Complete close-loop smooth control of reactive power can be achieved using shunt connected FACTS devices. Static VAR Compensator (SVC) is one of the shunt connected FACTS device, which can be utilized for the purpose of reactive power compensation.. This paper attempts to design and simulate the Fuzzy logic control of firing angle for SVC (TCR & FC-TCR) in order to achieve better, smooth and adaptive control of reactive power. The design, modeling and simulations are carried out for λ /8 Transmission line and the compensation is placed at the receiving end (load end). The results of both SVC (TCR & FC-TCR) devices

KEYWORDS:

1.      Fuzzy Logic

2.      FACTS and SVC

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig.1. Single Phase equivalent circuit and fuzzy logic control structure of SVC

EXPECTED SIMULATION RESULTS:

Fig.2. Uncompensated voltages for R=500 Ω

Fig.3. Compensated voltages for R=500 Ω with TCR

Fig.4. Compensated voltages for R=500 Ω with FC-TCR

Fig.5. Active and Reactive powers of the Tr.line R=200 Ω after compensation with FC-TCR

Fig.6. Active and Reactive powers of the Tr.line for R=200 Ω after compensation with TCR

CONCLUSION:

This paper presents an “online Fuzzy control scheme for SVC” and it can be concluded that the use of fuzzy controlled SVC (TCR & FC-TCR) compensating devices with the firing angle control is continuous, effective and it is a simplest way of controlling the reactive power of transmission line. It is observed that SVC devices were able to compensate over voltages. Compensating voltages are shown in Fig.15 and Fig.16. The use of fuzzy logic has facilitated the closed loop control of system, by designing a set of rules, which decides the firing angle given to SVC to attain the required voltage. The active and reactive power compensation with SVC devices was shown in Fig.17 and Fig.18. With MATLAB simulations [4] [5] and actual testing it is observed that SVC (TCR & FC-TCR) provides an effective reactive power control irrespective of load variations.

REFERENCES:

[1] Narain. G. Hingorani, “Understanding FACTS, Concepts and Technology Of flexible AC Transmission Systems”, by IEEE Press

USA

[2] Bart Kosko, “Neural Networks and Fuzzy Systems A Dynamical Systems Approach to Machine Intelligence”, Prentice-Hall of India New Delhi, June 1994.

[3] Timothy J Ross, “Fuzzy Logic with Engineering Applications”, McGraw-Hill, Inc, New York, 1997.

[4] Laboratory Manual for Transmission line and fuzzy Trainer Kit Of Electrical Engineering Department NIT Warangal

[5] SIM Power System User Guide Version 4 MATLAB Manual Periodicals and Conference Proceedings:

Optimized Control Strategy for a Medium-Voltage DVR—Theoretical Investigations and Experimental Results

ABSTRACT:  

Most power quality problems in distribution systems are related to voltage sags. Therefore, different solutions have been examined to compensate these sags to avoid production losses at sensitive loads. Dynamic Voltage Restorers (DVRs) have been proposed to provide higher power quality. Currently, a system wide integration of DVRs is hampered because of their high cost, in particular, due to the expensive DC-link energy storage devices. The cost of these DC-link capacitors remains high because the DVR requires a minimum DC-link voltage to be able to operate and to compensate a sag. As a result, only a small fraction of the energy stored in the DC-link capacitor is used, which makes it impractical for DVRs to compensate relatively long voltage sags. Present control strategies are only able to minimize the distortions at the load or to allow a better utilization of the storage system by minimizing the needed voltage amplitude. To avoid this drawback, an optimized control strategy is presented in this paper, which is able to reduce the needed injection voltage of the DVR and concurrently to mitigate the transient distortions at the load side. In the following paper, a brief introduction of the basic DVR principle will be given.  Next, three standard control strategies will be compared and an optimized control strategy is developed in this paper. Finally, experimental results using a medium-voltage 10-kV DVR setup will be shown to verify and prove the functionality of the presented control strategy in both symmetrical and asymmetrical voltage sag conditions.

KEYWORDS:

1.      Asymmetrical voltage sag

2.      Dynamic voltage restorer (DVR)

3.      In-phase compensation

4.      Optimized compensation

5.      Pre-sag compensation

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Basic concept of a DVR.

 EXPECTED SIMULATION RESULTS:

                                       Fig. 2. Measured voltages during a long, balanced sag.

Fig. 3. Measured voltages during a long, unbalanced sag.

CONCLUSION:

Voltage sags are a major problem in power systems due to the increased integration of sensitive loads. DVR systems are able to compensate these short voltage sags. The control and the design  of these systems are critical. Present control strategies are able either to minimize load distortions or the needed voltage amplitude. Both requirements are of utmost importance, especially the needed voltage amplitude for compensating a voltage sag leads to a strict limitation of the range of operation without oversizing the converter significantly.

In this paper, the basic concept of an optimized solution is presented. Based on a combination of the pre-sag and in-phase compensation methods, the proposed optimized DVR control strategy can react to a short voltage sag avoiding disturbances to the protected load. While for a long voltage sag, the proposed method is still able to generate the appropriate voltage without over modulation (or oversized DC-link capacitor) and with minimized load voltage transient distortions. Furthermore, medium voltage level experimental results are presented to verify the feasibility of this control strategy in both balanced and unbalanced voltage sag situations. Although, the effect of the control strategy has only been shown for long but shallow sags, similar results occur for deep sags or large phase jumps.

In this study, it was found that the required voltage amplitude of the DVR with the proposed optimized control strategy was reduced by 25%, compared to the pre-sag controller. In other words, the maximum compensation time is increased by approximately the same amount. Taking into consideration that a phase jump of 12 is not extremely high and that the advantages increases with larger phase jumps, an even higher gain is  possible in practical systems. Summarizing all advantages up, it can be stated that the compensation time of existing DVR systems under pre-sag control can be significantly improved when applying the proposed optimized strategy. In newly designed DVRs, the DC-link capacitance can be decreased without reducing the range of operation.

REFERENCES:

[1] M. Bollen, Understanding Power Quality Problems, Voltage Sags and Interruptions. New York: IEEE press, 1999.

[2] A. Kara, D. Amhof, P. Dähler, and H. Grüning, “Power supply quality improvement with a dynamic voltage restorer (DVR),” in Proc. Appl.Power Electron. Conf., 1998, no. 2, pp. 986–993.

[3] P. Dähler, M. Eichler, O. Gaupp, and G. Linhofer, “Power quality devices improve manufacturing process stability,” ABB Rev., vol. 1, pp.  62–68, 2001.

[4] W. E. Brumsickle, R. S. Schneider, G.A. Luckjiff, D. M. Divan, and M. F. McGranaghan, “Dynamic sag correctors: Cost effective industrial power line conditioning,” IEEE Trans. Ind. Appl., vol. 37, no. 1, pp. 212–217, Jan.–Feb. 2001.

[5] C. Meyer and R. De Doncker, “Solid-state circuit breaker based on active thyristor topologies,” IEEE Trans. Power Electron., vol. 21, no.2, pp. 450–458, Nov. 2006.