Grid Connected Multilevel Inverter for Renewable Energy ... - Core

Available online at www.sciencedirect.com

ScienceDirect Procedia Technology 21 (2015) 636 – 642

SMART GRID Technologies, August 6-8, 2015

Grid Connected Multilevel Inverter for Renewable Energy Applications R.Mahalakshmi a* , K.C.Sindhu Thampattyb a

Assistant Professor, Amrita Vishwa Vidyapeetham,Bengaluru-56007, India. b Professor, Amrita Vishwa Vidyapetham,Coimbatore--641112, India

Abstract Electrical energy generation from renewable energy sources such as sun, wind etc., are widely adopted due to the increase in electricity consumption. The integration of renewable energy sources with the grid plays an important role in energy utilization. It is difficult to utilize electricity from renewable energy sources directly for the injection of power into the grid. Hence the system needs power electronic converters as an interface between renewable energy sources and grid/load. This paper discusses about the integration of three phase six level voltage source inverter into the grid. Three phase 2000 VA inverter is designed by using three, single phase eight switch six level inverters (in quarter cycle) and each single phase inverter uses three DC voltage sources which can be derived from renewable energy sources such as solar, wind and fuel cell. The pure sinusoidal 415V three phase voltage is obtained from inverter to inject the power into grid and to the three phase resistive load. The proposed three phase Multi Level Inverter (MLI) is compared with the conventional three phase inverter and the observed THD of the conventional MLI is 31%. THD of the proposed MLI output voltage is analyzed which is 0.13% and found to be very less compared to six and 12 pulse conventional converter topologies. The simulation on the proposed inverter topology is done in MATLAB/Simulink and the results are verified. © 2015 by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license 2015The TheAuthors. Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Amrita School of Engineering, Amrita Vishwa Vidyapeetham University. Peer-review under responsibility of Amrita School of Engineering, Amrita Vishwa Vidyapeetham University Keywords: Grid Integration,Multilevel three phase inverter, THD,Renewable energy sources;.

1. Introduction Nowadays the power obtained from solar and wind energy are connected to the grid for better utilization of renewable energy sources. The power extraction from renewable energy sources cannot be directly utilized by the loads or the grid. The power electronic interface such as DC-DC converters and DC-AC inverters especially MLIs

* Corresponding author. Tel.: 919035406063. E-mail address: [email protected]

2212-0173 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Amrita School of Engineering, Amrita Vishwa Vidyapeetham University doi:10.1016/j.protcy.2015.10.076

R. Mahalakshmi and K.C. Sindhu Thampatty / Procedia Technology 21 (2015) 636 – 642

are used as an interface between them. The grid can be fed by pure sinusoidal wave or voltage/current with lesser harmonics. Grid frequency should match with the frequency of the signal which is being injected [1]. To obtain lesser harmonics sinusoidal three phase voltage and current, MLI configuration with LC filter is incorporated. Conventional MLIs started with three level outputs such as 0V - Vdc – 0V. Later there were more number of configurations on inverter topology has come out. Since the inverter generates multi stepped wave output, the harmonic profile can also be improved. Further the advantages of MLI’s are higher efficiency, reduced dV/dt stresses on the load, lower electromagnetic interference (EMI). DC input voltage sources the MLI’s can be derived from renewable energy sources such as Photo Voltaic array with DC to DC converter, Induction generator/Synchronous generator fed wind turbine with rectifier configuration and fuel cell with DC-DC converter [2]. To improve the harmonic profile of the voltage and current waveform for grid integration there are so many techniques adopted waveform such as hysteresis current control, Multi level H bridge cascaded inverter etc.,[3][4].The proposed work focuses on the formulation of grid tied three phase 6 level eight switch inverter (per phase)[5][6] and is formed by three single phase inverters with three DC sources and eight switches [5].Grid integration techniques are discussed in the many number of papers [9-12]. The proposed inverter is connected for powering the 2000W resistive load and to inject power to the grid. For the grid integration three phase inverter output voltage is stepped up by the three phase transformer to bring the voltage to 415V Line. To reduce the harmonics in the output voltage, proper value of three phase transformer leakage inductance is selected [13]. THD of the output waveform is analyzed with the use of MATLAB simulink/FFT block. The proposed new three phase inverter topology generates less harmonic three phase voltage waveform compared to conventional six pulse and twelve pulse inverter topology. The THD of the six pulse converter is 31% and twelve pulse is around 12% [4] and the proposed topology produces THD of 0.13%. Six pulse and 12 pulse converter need LC filter for obtaining the pure sine waveform. But the proposed MLI can be directly connected to the grid and load. 2. Block Diagram of Grid Tied Inverter The proposed configuration represents the grid connection of three, single phase inverters which are connected in star.The DC sources can be obtained from renewable energy sources such PV/wind and fuel cell which is not focused in this work. The following levels can be obtained 0, Vdc, 2Vdc, 3Vdc, 4Vdc and 5Vdc in quarter cycle of the output waveform. Vdc is selected as 30V and therefore the peak amplitude of a inverter output is 150V.The gate pulse for the switch S1 is formed by using two ramp signals of 180° out of phase with other. The each ramp signal is compared with the different DC levels. Output of them is summed up to produce the pulse for switch S1 and the complement of the pulse of S1 is the pulse of switch S1′. Similarly all the other switches pulses were generated. Switching states of the switches in the single phase inverter is shown in the table:1. The gate pulses are generated based on this switching state shown in Table 1. S1′, S2′, S3′, S4′ pulses are compliment of S1, S2, S3, S4 respectively. Table 1 : Switching states. Vphase

S1

S2

S3

S4

+5 Vdc

1

0

1

0

+4 Vdc

0

0

1

0

+3 Vdc

1

0

1

1

+2 Vdc

0

0

1

1

+ Vdc

1

0

0

0

0

0

0

0

0

3. System Description The overall system is analyzed as two cases Case 1: Three phases Multi level inverter connected to the 415V, 2000 W three phase resistive load. Case 2: Multi level inverter connected to the 50 W three phase resistive load and to 415V three phase grid

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638


The proposed inverter generates the three phase voltage having 150V as a maximum value (3Vdc). To integrate the inverter to 415V grid and load, the inverter output is stepped up by the three phase transformer. The output of the inverter is in the form of steps exhibits higher harmonic profile. To improve the harmonic profile proper leakage inductance of the transformer is adjusted. Due to the slight increase in transformer inductance the secondary of the transformer voltage is decreased from the expected value. It can be brought to expected value by increasing the turns ratio slightly and also by adding shunt capacitor which is assumed as 1µF. The direction of the power flow of the system in case 1 and 2 are observed. The total power delivered by the inverter is approximately equal to 2000 W in both the cases. THD of the output waveform is found to be very less as 0.13% compared to six pulse and twelve pulse converter. THD of the six pulse and 12 pulse are 31% and 12% respectively. Since there is fewer harmonic in the voltage waveform, the MLI can be used directly for the integration of three phase grid. This topology can be applied to high voltage applications by increasing the input DC voltage values. In conventional three phase six pulse inverter, only one DC source is used as input, In proposed topology, three DC sources are used can be derived from three different renewable energy sources. 2.1. Case 1 System simulation with resistive load The proposed system is connected to the three phase resistive load. Fig.1 shows the output voltage of the three phase inverter which has six levels such as 0V, 30V, 60V, 90V, 120V, 150V in quarter cycle i.e for 5 ms. Fig.2 shows the output current flowing through the resistive load i.e. output current of the inverter without integrating to the grid. 2.2. Case 2 system simulation with grid integration. In order to integrate, the output of the inverter is connected to the three phase transformer having default transformer leakage inductance and the output voltage of the system is shown in Fig. 3. The system is connected to the grid and also to the 50W three phase resistive load. It can be observed that the output of the proposed inverter has lesser voltage THD of 7.46% and this is obtained without the filter. To reduce the THD, still more transformer inductance is adjusted and the obtained THD is 0.13% which is shown in Fig. 4 The obtained THD is very less compared to that of conventional inverter topology whose voltage THD is shown in Fig. 5, obtained from the phase output voltage of the three phase DC-AC converter which is simulated for the purpose of comparison with the proposed multi level inverter. It is observed that the THD of the output of the three phase converter is 31% with reduced fundamental voltage and is very high compared to the proposed system. Fig. 6 shows current through the resistive load and the maximum value of the current is 0.1A. Fig. 7 and Fig. 8 show the grid current and grid voltage respectively. Fig. 9 and Fig.10 show the active power (368W) and reactive power (1082W) injected to the grid respectively.

Fig. 1 Output voltage of the inverter


Fig. 2 Output current of the inverter

Fig. 3 Output voltage of the grid tied inverter (with proper inductance)

Fig. 4. THD of proposed inverter output voltage

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Fig. 5. THD of the conventional inverter’s output voltage.

Fig. 6. Current through the resistive load (50W,415V)

From the simulation results, it is observed that the proposed cascaded H-bridge three phase MLI is suitable for grid integration and it can be fed by three DC voltages derived from different renewable energy sources like solar, wind and fuel cell simultaneously unlike three phase conventional inverter. 3. Conclusion Three phase MLI topology using three single phase inverters is proposed and simulated and this topology is connected to the three phase resistive load and to the grid. The injection of power from the renewable energy sources to the grid through the three phases MLI is observed. The output of the inverter is smoothened by proper selection of transformer leakage inductance and improved the THD of the voltage waveform to 0.13% which is very less compared conventional three phase six pulse inverters having THD of 31%. The proposed system is found to be well suited for the interface of three different types of renewable energy sources (hybrid) and the grid.


Fig. 7 Grid Voltage

Fig. 8 Grid current

Fig. 9 Active power received by the grid

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Fig.10. Reactive power absorbed by the grid

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