Simulation and study of a grid connected Multilevel Converter (MLC) with varying DC input Abu Tariq1 1
Mohammed Aslam Husain2
Abstract— Multilevel line commutated inverters for renewable energy systems have gained popularity in recent times, especially in the distributed generation where a number of batteries, fuel cells, solar cell, and micro-turbines can be connected through a MLC to feed the grid. They can synthesize higher output voltage levels and can generate near sinusoidal voltages. This paper presents analysis of a grid connected MLC as an inverter having variable dc sources(which can be the output of wind farms, solar panels etc.). A Computer simulation analysis using SIMULINK/ MATLAB has been done to minimize total harmonic distortion (THD) by increasing the stages and by varying the delay angles for varying dc input to the multilevel inverter. A comparative study of different stages of MLC for different value of dc input has also been studied. The results has been tabulated and discussed.
Keywords-Total Harmonic Distortion, converter, grid connected inverter. I.
Multilevel
INTRODUCTION
Inverters are required for feeding the dc power generated from wind farms, PV plant, batteries etc. into the grid [1, 2]. Multilevel line commutated inverters for renewable energy systems have gained popularity in recent times. With the increasing interest in wind based generation system, a thorough study into the performance of grid connected multilevel inverters is required. Since the power extracted from wind using wind turbine and induction generator is in ac form. This ac is variable, since wind speed is not constant, so it is converted into dc and stored in batteries. But by using MLCs this varying dc can be fed directly to the grid, without storing it into the batteries. The line current flowing into the grid should have low THD and better power factor [3]. THD for a pure sin wave is zero but as the distortion in wave increases, THD increases. Higher the THD greater is electromagnetic interference and noise [1, 2]. Ψܶ ܦܪൌ
Mohammad Ahmad3
Mohd. Tariq4
Associate Professor, Department of Electrical Engineering, Aligarh Muslim University (AMU), Aligarh, INDIA 2,3,4 Department of Electrical Engineering, Aligarh Muslim University (AMU), Aligarh , INDIA 2 Corresponding Author :
[email protected] , Mobile : +919451663915
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Where; I is the RMS value of line current and I1is the RMS value of the fundamental component of line current [2]. The THD of a single stage converter (for a perfectly square wave) is approximately 48%. As the stage increases THD reduces. In this paper, a comparative study using computer simulation in SIMULINK/ MATLAB of different stages of MLC with varying sets of delay angles for varying dc sources is done. A detailed analysis of one, three and five stage inverters has been made. The study also shows that a
compromise has to be made between better THD and cost of MLC. For a particular stage MLC, corresponding to different delay angles and varying dc sources, THD, line RMS current and average power (calculated on AC side) are obtained and it is shown that power is transferred from DC side to AC side. The optimum combination for the switching angle for minimum THD has been obtained. II.
MULTILEVEL CONVERTER (MLC)
The MLC consists of a series of Single phase full-bridge converter units. Fig. 1 shows a single converter and Fig. 3 shows a five stage MLC. In this paper MLC is used as an inverter with delay angle greater than 900. In inverting mode, power is transferred from dc side to ac side. The performance of a converter depends upon the switching angle, and the impedance angle [1,5]. Since an SCR is a unidirectional device, the direction of the current through it always remains same. However, due to an RL load, the conduction continues even in the next half-cycle. Therefore, in case of a full-converter, the instantaneous value of Vo may be positive or negative but the average value of output voltage will be only positive for RL loads. However, for an active or RLE negative load, the converter may operate in the fourth quadrant of v-i plane and Vo becomes negative. The load current may be discontinuous, just continuous (or sinusoidal) or continuous. It depends upon α and φ of RL loads. A line commutated dc to ac inverter is basically phase controlled converter with an RLE (-) load (i.e. a RL load with a negative voltage source, E) [1,2,3]. In some part of the voltage cycle, the power flow is from the ac source to dc source and in some part of the cycle it is from the dc source to ac source. However the average value of power flow is negative (inversion for α>90degrees). If α is increased beyond 90 degrees (with reverse E) the converters operates in the fourth quadrant. While an independent source (e.g. output of wind mill, storage battery, solar photovoltaic cell etc.), may be used to convert a DC power into an AC power. Normally α is varied upto 165 degrees for inversion operation (to facilitate the line commutation voltage for SCR otherwise it becomes very small near ωt = 180 degrees).DC side may be output from wind mill, solar PV, batteries etc. while the AC side is connected to the grid. A single phase universal bridge of thyristors is used as a controlled converter. This bridge consists of four thyristors. Pulse generator (PG) is used to trigger thyristors. Inductor L is used to have continuous conduction operation and R is the inductor resistance. E is the DC voltage, which can be the output of different DC sources such as solar panels, wind mill, battery etc. Fig. 2 shows the line currents and their
978-1-4244-8782-0/11/$26.00 ©2011 IEEE
fundamental component of single converter at different differ delay angles.
Figure 1: Single Stage Converter.
III.
ANALYSIS AND SIMULATION SIMULATI RESULT
The complete model of the grid connected inverter is i shown in Fig. 5. The various parameters taken for simulation imulation study are as follows: L=0.04H,R=5Ω,, Vs=24V (peak). E can be the output of different DC sources such as solar s panels, wind mill, battery etc. In case of wind mills E is dependent on wind speed. The resistance is incorporated to simulate real inductor. A high igh value of inductance ensures continuous conduction.
For the single stage converter, THD is found to be approximately 48%. Further with increase in stage, THD reduces. The simulation model of 5-stage stage MLC is shown in Fig. 3. Fig. 4 shows the line currents due to each converter, which are operating at different delay angles, and their resultant current with its fundamental component. For F N stage MLC the number of levels of line current isN+1 i.e. for 5 stages, the number of levels of line current is 6. As the number of levels increases, the waveform approaches to sinusoidal. From Fig. 2 and Fig. 4, it can be observed that the line current waveform with 6 level approaches more m towards sinusoidal.
Figure 2: (a) AC voltage wave (b) Line current and corresponding correspond fundamental component with delay angle 0 degree, and (c) Line current and corresponding fundamental component with delay α degrees degrees.
Figure 5: Simulink model (5 stage MLC)
Figure 3: 5 stage MLC with E as DC source
Figure 4: Line current of 5 stage MLC (with fundamental component) com
Single-stage MLC with varying E: E The simulation model was run with α=145° for varying vales of E. The simulation results are shown in Table 1. The negative sign of average power implies i that power transfer is taking from DC side to AC side. This average power pow is measured at ac side. It is obtained using the expression ଵ ் ܲ ൌ ݒ௦ ݅௦ ݀ݐ (2) ் Whereݒ௦ and ݅௦ are the instantaneous values of line voltage and line current [2] It may be observed that as the value of E increases, increases THD, RMS line current and average power transfer also increases but below 16 volts the inverter goes into discontinuous discontin conduction. Since discontinuous conduction is not desirable, d so the value E can be varied up to 16 volts only (which is the lower limit of E in this case).. The variation of THD with E is shown in Fig. 6. It may be observed that THD increases with increase in E. Thus for specified maximum value of THD, the upper limit of E has to be fixed (i.e. for THD=36.31%, E=26volts). For example, for α=145° and E=16 volts, THD is 18.00% and for E≤15Volts E MLC goes into discontinuous conduction.
Table 1: Results obtained for single stage (for α=145°) MLC
Five-stage MLC with varying E: The simulation results for 5-stage MLC for a fixed value of delay angle for each stage are shown in Table 3. The variation of THD with E is shown in Fig. 7. It was again observed that for E≤ 16volts, one or more stages of the MLC goes into discontinuous conduction (depending on the values of delay angle).
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Table 3: Results obtained for 5 stage MLC for α1= 110o,α2 =125ο, α3 = 135o, α4=145o, α5=160o
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Figure 6: Variation of %THD with E for a 1-stage MLC
Three-stage MLC with varying E: The simulation results for 3-stage MLC are shown in Table 2. It shows the variation of THD, RMS line current and average power transfer with E, for a fixed value of delay angle for each stage. It was observed that for E≤ 16volts, one or more stages of the MLC goes into discontinuous conduction (depending on the values of delay angle). . Table 2: Results obtained for 3-stage MLC for α1=110°, α2=135°, α3=160°
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From the results obtained for one, three and five stage MLC, it is clear that for a particular value of E, as the number of stage of MLC increases, THD decreases. Also it is clear that as the value of E increases, THD increases. But the value of E can be varied between a higher and a lower limit. The upper limit depends on the maximum wind speed. While the lower limit is the value of E below which any of the stages goes into discontinuous conduction. For example, for E=18 V THD for single stage is 22.94%, THD for three stage is 14.5%, and THD for five stage is around 11%. It is also found that for a particular stage of MLC, THD varies with delay angle. The result obtained for three and five stage MLC is tabulated in Table 4 and 5.
Figure 7: Corresponding to Table 3( where THD is in %, and E is in volts)
Selection of number of stages of MLC : Table 4 shows the variation of THD, RMS line current and average power transfer for different delay angle combination sets. For example, for Set 1: α1=105°, α2=125°, α3=145° and for set 2: α1=105°, α2=125°, α3=150° and so on.
Minimum THD is corresponding to delay angle combination combina set 11 in which delay angles are: α1=110°, α2=125°, α3=160°. ΤHD for set 11 is 12.13%.However the average power is maximum for set 12. Thus a suitable combination for low THD and maximum power transfer are combination number 11 and 12. The analysis of 5 stage MLC LC shows that there is a decrement in THD but the reduction in THD is not appreciable. This unappreciable THD reduction is at the cost of increased i size (stage) and complexity of the converter. Fig. 8 shows that the minimum THD is obtained at delay dela angle combination set 10 i.e.α1=105°, α2=125°, α3=135°, α4=145°,, α5=160° The value of THD for this set is 9.638%.
Figure 8: THD vs Delay angle combination for 5 stage MLC
IV. Table 4: Results obtained for 3 stage MLC using MATLAB for E=18V
S.No. 1 2 3 4 5 6 7 8 9 10 11 12
Delay Angle (α1)
α2
α3
105 105 105 105 105 105 105 105 105 110 110 115
125 125 125 125 140 140 130 135 120 120 135 125
145 150 155 160 160 155 160 160 160 160 160 160
THD (%) 13.62 13.54 13.74 14.09 12.83 12.63 13.01 12.59 15.8 16.45 12.13 12.4
RMS Line Current (A) 5.697 5.543 5.412 5.306 4.688 4.795 5.084 4.875 5.549 5.345 4.682 4.489
Average Power (W) -37.34 -36.33 -35.30 -34.37 -33.25 -34.18 -34.35 -33.95 -33.91 -36.22 -36.25 -37.88
Table5:Results obtained for 5 stage MLC using MATLAB for E=18V
S. N o.
Dela y Ang le (α1)
α2
α3
α4
α5
THD (%)
RMS Line curre nt (A)
This paper presentedd the concept of using grid connected MLC as an inverter with varying dc input source (which (wh can be the output of wind farms).Further analysis has been b done for THD reduction by varying the delay angle and by increasing the stages of MLCs. MLCs The DC power from the solar PV, wind mills etc. is fed to the grid with reduced THD. The results obtained from MATLAB simulation shows that the DC source can an be varied between an a upper and a lower limit. The upper limit depends de on the maximum wind speed.. While the lower limit is the value of E below which any of the stages goes into discontinuous conduction. con Thus for this scheme storage battery is not required require (for specified wind speed variation) and hence cost is reduced. Also as the delay angle changes, changes THD and average power change and for a particular value of delay angle, THD is found to be minimum. As the number of stage is increased incr to 3, THD reduces drastically and average power transfer also increases.. With further increase in the number of stages, reduction in THD is not appreciable. appreciab So, to have a compromise between THD, power transfer, cost and size, the number of stage is limited to a maximum limit.
Averag e Power (W)
REFERENCES [1]
1
105
115
125
135
145
13.27
9.471
-65.26
2
105
115
125
135
150
13.03
9.315
-64.25
3
105
115
125
135
155
12.98
9.181
-63.22
4
105
115
125
135
160
13.04
9.073
-62.29
5 6
105 105
115 115
125 125
140 145
160 160
12.53 12.32
8.83 8.703
-61.61 -60.69
7
105
115
130
145
160
11.42
8.482
-60.67
8
105
115
135
145
160
10.95
8.274
-60.27
9
105
120
135
145
160
10.01
8.052
-61.3
10
105
125
135
145
160
9.638
7.834
-61.78
11
110 115
125 125
135 135
145 145
160 160
10.76 12.15
7.665 7.49
-64.09 -65.71
12
CONCLUSIONS
[2] [3]
[4]
[5] [6]
[7]
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