A New Reduced Switch Multilevel Inverter Based D ... - IRD India

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A New Reduced Switch Multilevel Inverter Based D-STATCOM for Power Quality Improvement

1N. Raveendra, ²A.Jayalakshmi, ³V.Madhusudhan

1Department of EEE, Mallareddy Engineering College for Women, Hyderabad , India.

2Department of EEE, JNTUH college of Engineering Hyderabad , India.

3Department of EEE, KSRM college of Engineering Kadapa, A.P ,India Abstract: The role of multilevel inverters in improving

power quality has been already proved long back ago. The major concern in using multi level inverters is the number of switches used in it. As the level of inverter increases. So there is a need to reduce the component count in multilevel inverter. In this paper we use a statcom which is consists of a multilevel inverter with reduced switch count. The comparison of the proposed multilevel inverter with less components is done with the conventional multilevel inverters. The proposed statcom is simulated in Matlab/simulink and is performance is tested under balanced and unbalanced nonlinear loads. The results show that statcom with proposed multilevel inverter has proposed multilevel inverter has successfully improved the power quality.

I. INTRODUCTION

Reactive power plays a vital role on the security and stability of power system, therefore, the reactive power compensation device has a very wide range of application in power system. In recent years, technology of power electronics, especially flexible alternating current transmission system (FACTS), has a rapid development. As a part of FACTS, STATCOM has good performances of slightly capacity, high efficiency, fast dynamic response, good control stability and so on, and it has gradually become one of the representative techniques in the field of reactive power compensation.

At present, STATCOM hasn’t been widely applied in the power grid of high voltage and large capacity, which is due to the limit of withstand voltage level and capacity of power electronic switching devices. Because of this, multiple technology and multi-level technology have been widely used . Compared with multiple technology, which contains a bulky, high loss and high cost coupling transformer, multi-level technology is not only more simple but also more efficient. Therefore, it represents the direction of development of large capacity STATCOM.

Multi-level converter based STATCOM (MC- STATCOM), with a topology of modular-cascaded structure, has a wide prospect of applications in both transmission and distribution system. Under the background of application of STATCOM in high

voltage power grid, there are many advantages of CMC- STATCOM compared with other kinds of STATCOM.

On the basis of mathematical model of main topology, the states feedback decoupled reactive power control strategy based on the d-q coordinate system is studied in this paper, together with the application principle of carrier phase shift PWM modulation method in CMC.

The balance of DC bus voltage is a key factor to the stable operation of devices. The hierarchical control method is used to achieve a balance of DC bus voltage, and it involves the DC bus voltage overall control and individual control of each cascade module. There are three kinds of structure of the commonly used multi- level converter, which are flying-capacitor multi-level converter, diode-clamp multi-level converter, and Hbridge cascade multi-level converter.

II.MULTILEVEL INVERTER

A voltage level of three is considered to be the smallest number in multilevel converter topologies. Due to the bi-directional switches, the multilevel VSC can work in both rectifier and Inverter modes. This is why most of the time it is referred to as a converter instead of an inverter in this dissertation[1]. As the number of levels reaches infinity, the output THD approaches zero. The number of the achievable voltage levels, however, is limited by voltage-imbalance problems, voltage clamping requirements, circuit layout and packaging constraints complexity of the controller, and, of course, capital and maintenance costs[3].

Three different major multilevel converter structures have been applied in industrial applications: cascaded H- bridges converter with separate dc sources, diode clamped, and flying capacitors[6]. The concept of multilevel converters has been introduced since 1975.

Separate DC-sourced full-bridge cells are placed in series to synthesize a staircase AC output voltage. The term multilevel began with the three-level converter. In 1981, diode-clamped multilevel inverter also called the Neutral-Point Clamped (NPC) inverter schemes were proposed. In 1992, capacitor-clamped (or flying capacitor) multilevel inverters, and in 1996, cascaded multilevel inverters were proposed. Although the

cascade multilevel inverter was invented earlier, its application did not prevail until the mid 1990s. The advantages of cascade multilevel inverters were prominent for motor drives and utility applications. The cascade inverter has drawn great interest due to the great demand of medium-voltage high-power inverters[4].

The cascade inverter is also used in regenerative-type motor drive applications. Recently, some new topologies of multilevel inverters have emerged. This includes generalized multilevel inverters, mixed multilevel, inverters, hybrid multilevel inverters and soft-switched multilevel inverters . These multilevel inverters can extend rated inverter voltage and power by increasing the number of voltage levels. They can also increase equivalent switching frequency without the increase of actual switching frequency, thus reducing ripple component of inverter output voltage and electromagnetic interference effects.

III. PROPOSED MULTILEVEL TOPOLOGY

The circuit of the proposed multilevel inverter is shown in Figure.3.1. The inverter designed gives seven level of output voltage. A total of ten switches are used which are IGBT/Diodes. Six switches are used for level generation and 4 switches are used for polarity generation. Three sources are used for generating levels.

Each voltage source is of same range so this can also be called as symmetrical multilevel inverter. The number of carrier waves used are also very less in this topology.

For a conventional seven level inverter using SPWM uses six carrier waves but the proposed uses only three carrier waves. The proposed topology can be easily extended to three phase system also. The switch S6 used in the inverter can be duplicated and can be extended for any levels of voltage.

Figure 3.1.Proposed Seven Level Inverter In the proposed inverter topology the full bridge is used to decide the polarity of the levels and the remaining part of the inverter is the reason for the level generation.

The switching sequence for the proposed converter to generate seven level output voltage is shown below. In the proposed topology total ten switches are used per phase.

+3Vdc:- The switches used for obtaining the voltage of 3Vdc are S1, S5, S7, S10.

+2Vdc:- The switches used for obtaining the voltage of 2Vdc are S2, S6, S5, S7, S10

+Vdc: - The switches used for obtaining the voltage of Vdc are S2, S3, S5, S7, S10.

0:- The switches used for obtaining the voltage of 0 are S2, S3, S4, S7, S10

-3Vdc:- The switches used for obtaining the voltage of - 3Vdc are S1, S5, S8, S9

-2Vdc:- :- The switches used for obtaining the voltage of -2Vdc are S2, S6, S5, S8, S9.

-Vdc:- :- The switches used for obtaining the voltage of --Vdc are S2, S3, S5,S8, S9 .

The PWM control strategy used for the proposed inverter topology is Sinusoidal pulse width Modulation(SPWM) with single reference and three carrier wave forms[7]. The below Figure 2 shows the PWM strategy of the proposed inverter.

Figure 3.2.PWM strategy for proposed Inverter The below figure 3.3 shows the PWM pulses to the switches of the proposed converter. This shows that in the proposed topology the number of switches conducting at an instant is very less when compared with the other topologies of inverters like Cascaded, Flying Capacitor, and Neutral Point Clamped types[12- 13] The switches in the polarity generation section of the inverter also conduct only at zero crossings. So the switching losses are very less. As in Multilevel converters Power Electronic switches play a key role, from this we can say that this topology is highly reliable, less control complexity [9].

Figure 3.3.PWM pulses for the switches

Figure 3.4.Proposed Nine Level Inverter The proposed converter can be extended to any level of inverter by adding the appropriate switches and the DC voltage sources.Figure.3.4 and Figure 3..5 shows the nine level and eleven level inverter topologies based on the proposed concept. For a nine level inverter the number of switches used is 12 and for eleven level inverter 14 switches are used. The designed circuits are simulated on both MATLAB/SIMULINK as well as PLECS software packages and the results are presented.

For either of the levels the full bridge topology is common and the level generation section of the inverter circuit varies.

The next section describes about the loss calculations in the proposed converter topology. This losses are divided into IGBT losses as well as the DIODE losses. From the calculated losses we can calculate the total power loss of the converter.

The section V gives a brief content on variable frequency Induction Motor Drives and this describes the importance of the Inverter for controlling the speed of the Induction Motor Drive.

The section VI describes the MATLAB/SIMULINK validation of the proposed topologies and the results

showing the output voltages of seven, nine, and eleven level converters are presented and the harmonic analysis is also presented.

Figure 3.5.Proposed Eleven Level Inverter The below table I shows the comparison of the number of switches for the three conventional Multilevel Inverters and the proposed Multilevel Inverter.

TABLE I : NUMBER OF COMPONENTS FOR THREE-PHASE INVERTERS

Inverter type

NPC Flying capacit or

Casca ded

Propose d Inverter Main

switches

6 (N-1) 6 (N-1) 6 (N- 1)

3 ((N- 1)+4) Main

diodes

6 (N-1) 6(N-1) 6 (N- 1)

3 ((N- 1)+4) Clamping

diodes

3 (N-1) (N-2)

0 0 0

DC bus capacitors/

Isolated supplies

(N-1) (N-1) 3(N- 1)/2

(N-1)/2

Flying capacitors

0

3/2(N- 1) (N- 2)

0 0

Total

numbers (N-1) (3N+7)

N-1) (3N+2 0)

27/2 (N-1)

(13N+3 5) /2 From the table it is clear that for a cascaded Multilevel Inverter needs 12 switches for seven level but proposed requires only ten switches, so 2 switches for single phase and 6 switches for three phase system are saved

which also saves the gate driver circuits also. Figure 4 shows the graphical view of comparison of different multilevel inverters and the number of components used.

IV. DESIGN OF MULTILEVEL BASED DSTATCOM

A. Principle of DSTATCOM

A D-STATCOM (Distribution Static Compensator), which is schematically depicted in Fig.-1, consists of a two-level Voltage Source Converter (VSC), a dc energy storage device, a coupling transformer connected in shunt to the distribution network through a coupling transformer. The VSC converts the dc voltage across the storage device into a set of three-phase ac output voltages. These voltages are in phase and coupled with the ac system through the reactance of the coupling transformer. Suitable adjustment of the phase and magnitude of the D-STATCOM output voltages allows effective control of active and reactive power exchanges between the DSTATCOM and the ac system. Such configuration allows the device to absorb or generate controllable active and reactive power.

Fig. 4. 1 Schematic Diagram of a DSTATCOM The VSC connected in shunt with the ac system provides a multifunctional topology which can be used for up to three quite distinct purposes: 1. Voltage regulation and compensation of reactive power;

2. Correction of power factor 3. Elimination of current harmonics.

V. MATLAB/SIMULINK RESULTS

In this section the simulation results obtained for seven, nine and eleven level inverters are presented. The output voltage waveforms as well as the THD analysis are also discussed. The below Figure shows the seven level output voltage of the proposed inverter. Figure 4.1 shows the output voltage of the seven level inverter and figures 4.2 and 4.3 shows the multilevel output voltages for nine level and eleven level inverter topologies, Fig 4.4 shows the three phase voltage waveform of the proposed seven converter.

Figure 4.1. Simulation output Voltage wave form of Seven Level Inverter.

Figure 4.2. Simulation output Voltage wave form of Nine Level Inverter.

Figure 4.3. Simulation output Voltage wave form of Eleven Level Inverter.

Figure 4.4. Simulation output voltage waveform of three phase seven level Inverter

Fig 4.5,4.6,4.7 shows the harmonic spectra of the proposed seven, nine and eleven level inverter topologies. From the results it is clear that for a seven

level inverter the THD is 18.5%, for a nine level inverter it is 13.02% and for a eleven inverter it is 10.89%. This shows as the number of level increases the THD content decreases.

Figure 4.5 .FFT Analysis Of Seven Level Inverter.

Figure 4.6. FFT Analysis of Nine Level Inverter.

Figure 4.7.FFT Analysis of Eleven Level Inverter.

TABLE II : Total Harmonic Distortion Analysis Inverter

Level

THD% (Phase voltage)

THD% (Line voltage)

SEVEN 18.5 14.8

NINE 13.02 10.03

ELEVEN 10.89 7.05

Table 2 shows the Total Harmonic Distortion comparison of the different levels. The THD values for phase voltages as well as line voltages are presented in the table.

The below figure shows the simulation circuit designed to validate the proposed concept of the grid connected inverter with Hybrid Renewable Energy Sources. PV and wind sources are modeled in MATLAB environment. Te proposed system is tested at different loading conditions like Linear, Nonlinear Balanced, Nonlinear Unbalanced, Changing conditions. The proposed system is also tested under single as well as Hybrid Energy Sources.

Fig. 4.8 Matlab/Simulink circuit of the proposed system.

The figure 4.9 shows the output wave forms of the proposed system under Nonlinear Balanced system. In this case the proposed 4 leg Inverter is kept off until 0.1sec and from the obtained results it is clear that the source current is not sinusoidal and from 0.1 sec the source current became sinusoidal as the inverter is on.

Fig 4.9 Simulated output wave forms under non-linear balanced load

The below figure 4.10 shows the source voltage and source current waveforms. This shows that both voltage and current are in phase which resembles unity Power Factor.

Fig.4.10 Simulated output wave forms showing unity Power factor

The below figure 4. 11 show s the output waveforms of Source current, load current and Inverter current under a load of Nonlinear Unbalanced Load. The load current waveform shows that it is not common in all the phases, but the source current maintains same magnitude in all the three phases. This shows that the proposed system works under Nonlinear Unbalanced Load also.

Fig.4.11 Simulated output wave forms under Source current, load current and Inverter current under a load of

Nonlinear Unbalanced Load.

The below figure 4.12 shows the output waveforms of the Source current, Load current and Inverter current under Non linear changing Load conditions. The source current is still sinusoidal in this case also because even the load increases PV/Wind hybrid system is used to fee the required compensated currents.

Fig. 4.12 Simulated output wave forms of Source current, Load current and Inverter current under Non

linear changing Load conditions.

The below figure 4.13 shows the Harmonic spectrum of the source current without compensation. It is seen that the THD content is 26.99% without inverter.

Fig. 4.13 Harmonic spectrum of source current without compensation

The below figure 4.14 shows the Harmonic spectrum of the source current with compensation. It is seen that the harmonic content is just 1.29% which is inacceptable range.

Fig. 4.14 Harmonic spectrum of source current with compensation

VI. CONCLUSION

The simulation results depicted above shows the performance of the reduced switch based multilevel statcom .the source current waveforms and the total harmonic distortion analysis shows that the power quality is improved .thus by reducing the switch count the cost of the system can be reduced, efficiency is increased.

REFERENCES

[1] M.EI-Habrouk, M.K.Darwish and P.Mehta, " The Active power filters: A review" IEEE proceedings on Electric power applications, Vol 147, No.5, pp 403 - 413, September 2000 [2] U. Borup, F. Blaabjerg, and P. N. Enjeti,

"Sharing of nonlinear load in parallel-connected three-phase converters," IEEE Transactions on Industrial applications., Vol. 37, no. 6, pp. 1817- 1823, Nov.lDec. 2001.

[3] J. H. R. Enslin and P. J. M. Heskes, “Harmonic interaction between a large number of distributed power inverters and the distribution network,”

IEEE Trans. Power Electron., vol. 19, no. 6, pp.

1586–1593, Nov. 2004.

[4] J. P. Pinto, R. Pregitzer, L. F. C. Monteiro, and J.

L. Afonso, “3-phase 4-wire shunt active power filter with renewable energy interface,” presented at the Conf. IEEE Rnewable Energy & Power Quality, Seville, Spain, 2007.

[5] B. Renders, K. De Gusseme, W. R. Ryckaert, K.

Stockman, L. Vandevelde, and M. H. J. Bollen,

“Distributed generation for mitigating voltage dips in low-voltage distribution grids,” IEEE Trans. Power.Del., vol. 23, no. 3, pp. 1581–1588, Jul. 2008.

[6] A. Timbus, M. Liserre, R. Teodorescu, P.

Rodriguez, and F. Blaabjerg, “Evaluation of current controller for distributed power generation systems,” IEEE Trans. Power Electron., vol. 24, no. 3, pp. 654–664, Nov. 2009.

[7] 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,

“Powerelectronic systems for the grid integration of renewable energy sources: A survey,” IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1002–

1016,Aug. 2006.

[8] L. H. Beverly, R. D. Hance, A. L. Kristalinski, and A.T. Visser, “Method and apparatus for reducing the harmonic currents in alternating current distribution networks,” U.S. Patent 5 576 942, Nov. 19, 1996.

[9] Geoff Walker, Evaluating MPPT converter topologies using a Matlab PV model, ISEI, 2010 [10] T. Markvart, Solar Electricity, John Wiley &

Sons, 1994

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