Flexible DG System for Grid Power Quality Improvement Using L-Z Source Inverter

IJSRD - International Journal for Scientific Research & Development| Vol. 4, Issue 01, 2016 | ISSN (online): 2321-0613

Flexible DG System for Grid Power Quality Improvement Using L-Z Source Inverter M.Manisha1 K.Ramya2 V.M Kirthika3 1,2,3 B.E. Student 1,2,3 Department of Electrical and Electronics Engineering 1,2,3 Knowledge Institute of Technology, INDIA Abstract— This paper presents on the basis of L-Z source inverter. By using the power electronics converter, the distributed generating system is connected to the grid. This inverter has two inductors and three diodes in the main circuit. It boosts up the input DC voltage. The traditional inverters have capacitors in the main circuit which lead to many problems such as resonance produced in the circuit, voltage swell which can damage the devices, large volume, and reduced life time of the system. This inverter does not have capacitor in the main circuit and therefore reduces the problems present in traditional inverters. In the distributed generating system all the capacity of converters are not used for all time by reducing the harmonics. For the high range of distributed generating system normal dc-dc converter is not used and they need the special power converter for the interfacing with grid. For that, the L-Z-source inverter will be suited based on the performance. By reducing harmonics, the power quality of the grid to be improved. The proposed system is tested with the output results obtained by the MATLAB Simulink software.

Key words: Distributed generation, L-Z- source inverter, Power quality, resonance

still not widely used for supportive functions in power distribution systems. With the implementation of flexible DG systems, it would indeed be Possible to have integrated functions like harmonic and unbalance mitigation DG control systems. Having identified such possibilities, the main theme of this paper is to work the remaining capacity of DG power converter systems, and reduce the cost of installing dedicated units for carrying out supportive services. Particularly, in the distribution level, it may not be economical to have dedicated systems to handle such supportive services. The proposed DG system is based on recently introduced L-Z-source inverter (L-ZSI) [2]. L-ZSI is structurally different from traditional VSI. The L-Z source inverter uses a unique inductor and diode network for boosting its input voltage, provides a common ground for the dc source and inverter, and avoids the disadvantage causing by capacitor in the classical ZSI and SLZSI, especially in prohibiting the inrush current at start up and the resonance of Z-source capacitors and inductors.

II. EXISTING SYSTEM

I. INTRODUCTION The main objective of the proposed system is to reduce the harmonics in DG system for grid power quality improvement. Here by using L-Z source network– VSI it gives the regulated dc voltage and it eliminates the ripples in dc supply and converting to ac supply and given to grid, when the linear and non-linear loads are connected at grid side, the grid power quality to be improved. Distributed Generation (DG) has been gaining increasing attention in recent times. Some of the broadly used Distribution Generation systems are wind turbines, fuel cells and solar cells [3].In general, these sources are connected to the grid through inverters and the main purpose of the inverters is to carry out active power to the grid. Usually, they are operated at unity power factor but some of the systems are designed to absorb or inject reactive power. However, DG systems may not supply power to the grid continuously due to many factors like the unavailability of source and demand and price considerations. In such scenarios, the inverter used in power conversion has some unused capacity.

Now a day’s power consumers face various power quality problems. Among them are harmonics and unbalances which are of great concern. With enormous increase of nonlinear loads connected at distribution level and with their concerted action of drawing nonlinear currents, such loads cause deterioration of voltage quality [1],[4]. This may lead to malfunctioning of sensitive loads.

The regulatory bodies have specified acceptable harmonic levels that are allowed into grids and harmonic levels that have to be maintained by the utilities [6]. DG systems are

Fig. 1: Block Diagram Grid connected Z source inverter Topology The input DC voltage is applied to main circuit. The main circuit produces the buck-boost DC voltage with inrush current at the output [3]. The DC voltage is then fed as input to the three phase inverter. The switches in the three phase inverter are triggered by the pulses that are generator by the PWM generator. The three phase inverter converts the DC voltage to the output AC voltage. The output AC voltage is then fed as input to the Non Linear load to grid [1]. The block diagram of grid connected Z source inverter shown in Fig 1.

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Flexible DG System for Grid Power Quality Improvement Using L-Z Source Inverter (IJSRD/Vol. 4/Issue 01/2016/135)

III. PROPOSED SYSTEM

Fig. 2: Block Diagram of Grid connected L-Z source inverter The proposed system uses two inductors and three diodes for boosting the input DC voltage and has no capacitor. The system can suppress the resonance by removing capacitor to improve the efficiency of power supply and also it reduces the inrush current limitation at startup. In proposed paper LZ-Source network was implemented.

A. L-Z-Source Network The proposed inverter has no capacitor, and is composed of two inductors (L1, L2, and L1 =L2), and three diodes (D1, D2, and D3), as shown in Fig. 2. The Combination of L2 − L3 −D1 −D2 −D3 acts as a switched inductor cell [7]–[10].

The proposed topology provides inrush current suppression, unlike the traditional topologies, because no current flows to the main circuit at start-up. The proposed topology also provides a common ground for the source and inverter. L-Zsource inverter working modes classified into shoot-through mode and non-shoot through mode. In shoot through mode inductors are connected in series and supplies energy to the main circuit and during non-shoot through mode inductors are connected in parallel and stores energy [2].

Fig. 3: L-Z source inverter B. Operation Modes Operation Principles of L-ZSI just has shoot-through zero states besides the traditional six active states. For the purpose of analysis, the operating states are simplified into shoot-through and non-shoot-through states

1) Mode 1: Shoot Through State

Fig. 4: Non-Shoot through mode In the non-shoot-through state, as shown in Fig. 2, D2 is on, while D1 and D3 are off. L1 and L2 (L1 = L2 = L) are connected in series. L1 and L2 transfer energy from the dc voltage source to the main circuit, and the equivalent circuit is shown in Fig.4 [2].

2) Mode 2: Shoot Through State

Fig. 5: Shoot through mode In the shoot-through state, as shown in Fig. 3, the inverter side is shorted by both the upper and lower switching devices of any phase leg. During the shoot-through state, D2 is off, while D1 and D3 are on. L1 and L2 are connected in parallel [4].

Applying the volt–second balance principle to L1 and L2 Fig.5. The corresponding voltages across L1 and L2 in this state are VL1 and VL2, respectively. Where Vdc is the dc source and Vi is the dc-link voltage [2].

VL1+ VL2 + Vi = Vdc(1) VL1=VL2 (2) From (1) and (2), can be calculated VL1 =1/2Vdc – 1/2Vi (3) VL2 =1/2Vdc – 1/2Vi (4) Vi = B Vdc (5) Where Boost factor (B) is B = (1 + D/1 – D) (6) D-Duty cycle IV. SIMULATION RESULTS OF PROPOSED L-Z SOURCE INVERTER BASED DG SYSTEM FOR GRID

The simulation of proposed system has been executed using the MATLAB Simulink tool. The simulation of the proposed system is shown in the Fig.6

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Flexible DG System for Grid Power Quality Improvement Using L-Z Source Inverter (IJSRD/Vol. 4/Issue 01/2016/135)

Fig. 6: Proposed Grid connected to the L-Z source inverter

Fig. 7: Harmonics spectrum of grid voltage The Total Harmonics Distortion of the grid voltage is high at lower order harmonics, this THD value is within the acceptable limit of IEEE standard.

Fig. 7: Modeling PI controller with PWM technique A. Circuit Parameters Of The Simulation SCALED VALUE USED IN MODEL Dc supply voltage 50V L-Z source inductance(L1=L2) 5mH Filter inductance 5mH Filter capacitance 900μF Non Linear load Capacitance 1μF Non Linear load Resistance 100Ω Table 1. Simulation Circuit Parameters The simulation results of proposed system are shown below.

When the input DC voltage is 50V current and voltage harmonics are shown in Fig 9, Fig 10.

PARAMETER

Fig. 8: Harmonics spectrum of grid current

Time(s) Fig. 9: Output current waveform in grid side

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Flexible DG System for Grid Power Quality Improvement Using L-Z Source Inverter (IJSRD/Vol. 4/Issue 01/2016/135)

By using the PI controller the output current to be controlled for particular time period (0.2s-0.3s),it’s shown in the arrow.

Time(s) Fig. 10: Output voltage waveform in Grid side The output voltage to boost by DC-DC converter fed three phase inverter with proper switching and power storing elements.

B. Output Tabulation The input and output values of proposed and existing system are tabulated below, shown in Table. [2-4] INPUT DC INVERTER INVERTER SOURCE OUTPUT OUTPUT VOLTAGE(V) VOLTAGE(V) CURRENT(Amp) 50 415 25 Table 2. Output values of three phase inverter OUTPUT VOLTAGE(V) OUTPUT CURRENT(Amp) IN DG SYSTEM IN DG SYSTEM 410 10 Table 3. Output values to the DG system C. Performance Comparison Between Two Systems EXISTING PROPOSED SYSTEM SYSTEM 6.37% 0.47% đ?‘‰đ?‘‡đ??ťđ??ˇ 22.4% 22.99% đ??źđ?‘‡đ??ťđ??ˇ Table 4. Harmonics between existing and proposed system PARAMETERS

V. CONCLUSION

REFERENCES [1] Chandana Jayampathi Gajanayake, D. Mahinda Vilathgamuwa, Poh Chiang Loh, Remus Teodorescu, and Frede Blaabjerg, �Z source inverter based flexible distributed generation system solution for grid power quality improvement� IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 3, SEPTEMBER 2009, pg.no (695-704).

[2] Lei Pan�L-Z-Source Inverter� IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 12, DECEMBER 2014, Pg.no (6534-6543).

[3] Fang Zheng Peng, Senior Member, IEEE.� Z-Source Inverter� IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 2, MARCH/APRIL 2003.Pg.no (504-510).

[4] B.Karunamoorthy, Dr.D.Somasundareshwari, K.Kaviraj, � HARMONICS ANALYSIS IN DG SYSTEM USING L-Z SOURCE INVERTER�

International Journal of Applied Engineering Research, ISSN 0973-4562 Vol. 10 No.88 (2015).

[5] N. Pogaku and T. C. Green, “Harmonic mitigation throughout a distribution system: A distributed generator-based solution,� Inst. Electr.Eng. Proc.

Gener. Transm. Distrib. vol. 153, no. 3, pp. 350–358, May2006.

[6] Chokkalingam C. and M. Pushpavalli,� L Z SOURCE INVERTER WITH CLOSED LOOP CONTROL SYSTEM�, ARPN Journal of Engineering and Applied Sciences, VOL. 10, NO 14, AUGUST 2015 [7] IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, IEEE Standard 519-1992, 1993.

[8] M. Zhu, K. Yu, and F. L. Luo, “Switched inductor Zsource inverter, �IEEE Trans. Power Electron., vol.

25,no.8, pp. 2150–2158, Aug. 2010.

[9] Yu Tang, Shaojun Xie, Chaohua Zhang and Zegang Xu. 2009.� Improved Z-source With Reduced Z-source Capacitor Voltage stress and Soft-start Capability�. In IEEE transactions on power electronics. 24(2):409415.

[10] M.-K. Nguyen, Y.-C. Lim, and G.-B. Cho, “Switched Inductor quasi-Z source inverter,� IEEE Trans. Power Electron. , vol. 26, no. 1, pp.3183–3191, Nov. 2011.

[11] Poh Chiang Loh, Sok Wei Lim, Feng Gao and Frede Blaabjerg, �Three-Level Z-Source Inverters Using a Single LC Impedance Network�, IEEE transactions on power electronics, vol. 22, no. 2, March 2007, Pg.no.706-711.

This paper presents a L-ZSI-based flexible DG system for grid power quality improvement. It’s concluded that the proposed system has less harmonic content in the output current and voltage by using L-C filter and the PI controllers. By proper switching, advantages like voltage boosting, better output spectral quality are achieved. By using this inverter reduced harmonics has been achieved and variation in supply voltage and current harmonics is within the acceptable limits of IEEE 519-1992 power quality indices at ac mains. The performance of the proposed inverter results is obtained from MATLAB/Simulink software.

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