**5. Simulation results of grid connected VSI**

Simulation of virtual grid flux oriented control of grid connected VSI was done by using MATLAB/Simulink and results for different types of current controllers has shown below

### **5.1 Simulation results of hysteresis current controller**

For the simulation of virtual grid flux oriented control of grid connected inverter the following are the set values


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a)

Magnitude of current(Amp)

b)

Magnitude of current (Amp)

Synchronous Virtual Grid Flux Oriented Control of Grid Side Converter 125

power factor at grid. Three-phase references current are obtained from the active and

<sup>0</sup> 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 -1

Time(ms)

<sup>0</sup> 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 -200

Fig. 11. (a) Reactive component of grid current (b) Three phase reference current waveform Above mentioned figures shows the waveforms of three-phase grid current in fig. 12(a) and harmonic spectrum of grid current in fig. 12(b). the three phase grid current flows the actual reference current which we can seen from figures 12(b), 12(a).THD spectrum of grid current is having percentage of THD is around 4.41 and having fundamental component of 144.4 amps. And it having few lower order harmonics which are due to ripple in DC link voltage at six times of supply frequency. This is because of reference current Igq\* is generated from PI-controller through control of DC-link voltage if there is oscillation in DC link voltage which will inject into the reference current, their by load current is affected by this oscillations. Ripple in DC link voltage can be eliminated by using resonant DC link Inductor

Time(ms)

reactive grid current components after dq to ABC transformations.

Fig. 10. (a) DC link voltage (b) Active component of grid current iq\*

Above mentioned figures shows the waveforms of DC link voltage in fig. 10 (a) and active component of grid current reference Iq\* in fig. 10(b). The DC link voltage is maintained at its reference set value of 2.2KV which is shown is fig. 10(a). From fig. 10(b) active component of grid current iq\* in synchronous reference frame is obtained from the PI controller using error of DC link voltage by controlling DC link voltage to its reference value. This grid current reached to steady state within 0.05sec this shows fastness of inner current control loop. the Form figures we can see that current reaches to zero value which is due to DC link voltage is more than the set reference value, then error is going to be negative which makes the current to decrease. Then PI controller will act on DC link voltage of maintained at its reference value then error is zero. Hysteresis current controller controls the current is flowing into the grid to maintain set value of DC link voltage.

Above mentioned figures shows the waveforms of reactive component of grid current in fig. 11(a) and three-phase reference current waveforms fig. 11(b). Reactive component of grid current is zero because of reactive power flowing to grid is taken as zero to maintain unity

<sup>0</sup> 0.05 0.1 0.15 0.2 0.25 0.3 <sup>0</sup>

Time (ms)

<sup>0</sup> 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 <sup>0</sup>

Time (ms)

Above mentioned figures shows the waveforms of DC link voltage in fig. 10 (a) and active component of grid current reference Iq\* in fig. 10(b). The DC link voltage is maintained at its reference set value of 2.2KV which is shown is fig. 10(a). From fig. 10(b) active component of grid current iq\* in synchronous reference frame is obtained from the PI controller using error of DC link voltage by controlling DC link voltage to its reference value. This grid current reached to steady state within 0.05sec this shows fastness of inner current control loop. the Form figures we can see that current reaches to zero value which is due to DC link voltage is more than the set reference value, then error is going to be negative which makes the current to decrease. Then PI controller will act on DC link voltage of maintained at its reference value then error is zero. Hysteresis current controller controls the current is

Above mentioned figures shows the waveforms of reactive component of grid current in fig. 11(a) and three-phase reference current waveforms fig. 11(b). Reactive component of grid current is zero because of reactive power flowing to grid is taken as zero to maintain unity

Fig. 10. (a) DC link voltage (b) Active component of grid current iq\*

flowing into the grid to maintain set value of DC link voltage.

500

Magnitude of current (Amp)

1000

1500

Amplitude of DC link Voltage (Volts)

a)

b)

2000

2500

3000

power factor at grid. Three-phase references current are obtained from the active and reactive grid current components after dq to ABC transformations.

Fig. 11. (a) Reactive component of grid current (b) Three phase reference current waveform

Above mentioned figures shows the waveforms of three-phase grid current in fig. 12(a) and harmonic spectrum of grid current in fig. 12(b). the three phase grid current flows the actual reference current which we can seen from figures 12(b), 12(a).THD spectrum of grid current is having percentage of THD is around 4.41 and having fundamental component of 144.4 amps. And it having few lower order harmonics which are due to ripple in DC link voltage at six times of supply frequency. This is because of reference current Igq\* is generated from PI-controller through control of DC-link voltage if there is oscillation in DC link voltage which will inject into the reference current, their by load current is affected by this oscillations. Ripple in DC link voltage can be eliminated by using resonant DC link Inductor

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problem in the system.



**Power factor** 

Distortion factor (DF) is given by formula


Magnitude of Voltage (Volts)

b)

a)

0

500

1000

1500

Amplitude of Voltage (Volts)

Synchronous Virtual Grid Flux Oriented Control of Grid Side Converter 127

transmission line parameters like series line inductance and line to ground capacitance otherwise there is large power oscillation in the grid which leads to power quality

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3 -2500

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 -1500

Figure 14 shows the waveforms of DC link capacitor current, inverter output voltage and displacement factor. Selection of capacitor is choice on basis of less ripple current in DC link capacitor. In fig. 14(a) capacitor is having fewer current ripples. And fig. 14(b) is displacement power factor, which is having value of 0.999999 almost unity power factor. This is because almost zero-phase angle difference between reference currents and Grid

Fig. 13. (a) Output voltage of VSI before the filter (b) Grid voltage waveform

voltages. (i.e. reference currents fallows the same phase as grid voltages).

Time (ms)

Time (ms)

which smoothness the current flowing in DC link capacitor their by ripple magnitude is decreased.

Fig. 12. (a) Three phase grid current waveform (b) Harmonic spectrum of grid current

Above mentioned figures shows the waveforms of output line voltage of VSI before filter in fig. 13(a) and grid line voltage waveform after filter inductor in fig. 13(b). Fig. 13(a) is output of inverter which is having some oscillations in the edges of inverter voltage around 2200V. This oscillation can be decreased by decrease ripple in DC link voltage. The inductor filter eliminates all harmonics present in output line voltage of VSI and produces pure sinusoidal voltage as that of grid voltage. And it ensures the proper synchronism between the VSI and Grid. We need to design the filter inductor and capacitor properly to synchronize inverter output with grid and drop across the filter elements should be very less. And these filter elements cannot make resonance with

which smoothness the current flowing in DC link capacitor their by ripple magnitude is

<sup>0</sup> 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 -200

Time (ms)

Fundamental (50Hz) = 144.4 , THD= 4.41%

<sup>0</sup> <sup>10</sup> <sup>20</sup> <sup>30</sup> <sup>40</sup> <sup>50</sup> <sup>60</sup> <sup>0</sup>

Fig. 12. (a) Three phase grid current waveform (b) Harmonic spectrum of grid current

Above mentioned figures shows the waveforms of output line voltage of VSI before filter in fig. 13(a) and grid line voltage waveform after filter inductor in fig. 13(b). Fig. 13(a) is output of inverter which is having some oscillations in the edges of inverter voltage around 2200V. This oscillation can be decreased by decrease ripple in DC link voltage. The inductor filter eliminates all harmonics present in output line voltage of VSI and produces pure sinusoidal voltage as that of grid voltage. And it ensures the proper synchronism between the VSI and Grid. We need to design the filter inductor and capacitor properly to synchronize inverter output with grid and drop across the filter elements should be very less. And these filter elements cannot make resonance with

Harmonic order

decreased.


5

Mag (% of Fundamental)

b)

10



0

Amplitude of Current (Amp)

a)

50

100

150 200 transmission line parameters like series line inductance and line to ground capacitance otherwise there is large power oscillation in the grid which leads to power quality problem in the system.

Fig. 13. (a) Output voltage of VSI before the filter (b) Grid voltage waveform

Figure 14 shows the waveforms of DC link capacitor current, inverter output voltage and displacement factor. Selection of capacitor is choice on basis of less ripple current in DC link capacitor. In fig. 14(a) capacitor is having fewer current ripples. And fig. 14(b) is displacement power factor, which is having value of 0.999999 almost unity power factor. This is because almost zero-phase angle difference between reference currents and Grid voltages. (i.e. reference currents fallows the same phase as grid voltages).

#### **Power factor**

Distortion factor (DF) is given by formula

$$DF = \frac{1}{\sqrt{1 + THD^2}}\tag{14}$$

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0

<sup>2</sup> x 105

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Amplitude of Active Power (Watts)

b)

2

4

6

Amplitude of Power (Watts)

a)

8

10

12

<sup>14</sup> x 104

hysteresis current controller is going high.

Synchronous Virtual Grid Flux Oriented Control of Grid Side Converter 129

three phase active power flowing into grid which is around 160 KW. The oscillatory nature of the power is because of the harmonics present in currents which are flowing into the grid. The harmonic content (oscillatory nature of power) can be reduced by reducing the hysteresis band width. But by reducing band switching frequency in

<sup>0</sup> 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 -2

Time (ms)

<sup>0</sup> 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 <sup>0</sup>

Time (ms)

Figure 16 shows single phase instantaneous reactive and three phase reactive power flowing into grid. Fig. 16(a) shows instantaneous reactive power flowing into grid which

Fig. 15. (a) Single Phase Instantaneous Active Power (b) Three Phase Active Power

Total power factor = DF\*DPF (15)

$$DF = \frac{1}{\sqrt{1 + 0.0441^2}} = 0.999029011$$

Fig. 14. (a) DC link Capacitor Current (b) Displacement power factor

Fig. 15(a) shows single phase instantaneous power and fig. 15(b) shows three phase average active power flowing into grid. The instantaneous power 'S' which is equal to active power flowing into grid this is due to zero phase angle difference between grid currents and grid voltages, from fig. 15(a) we can easily observe this fact (i.e. instantaneous power will not crossing zero that means reactive component of current flowing into grid is zero, only active component of current flowing). Fig. 15(b) shows the

1

1 1 0.0441

*DF*

*DF* <sup>=</sup> <sup>+</sup>



0

0.5

PF

b)

1

1.5

2

Amplitude of current (Amp)

a)

1

2

Total power factor = 0.999029011\*0.999999 = 0.9989.

<sup>0</sup> 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 -1200

Time(ms)

<sup>0</sup> 0.05 0.1 0.15 0.2 0.25 0.3 -1

Time (ms)

Fig. 15(a) shows single phase instantaneous power and fig. 15(b) shows three phase average active power flowing into grid. The instantaneous power 'S' which is equal to active power flowing into grid this is due to zero phase angle difference between grid currents and grid voltages, from fig. 15(a) we can easily observe this fact (i.e. instantaneous power will not crossing zero that means reactive component of current flowing into grid is zero, only active component of current flowing). Fig. 15(b) shows the

Fig. 14. (a) DC link Capacitor Current (b) Displacement power factor

2

= 0.999029011

*THD* <sup>=</sup> <sup>+</sup> (14)

Total power factor = DF\*DPF (15)

three phase active power flowing into grid which is around 160 KW. The oscillatory nature of the power is because of the harmonics present in currents which are flowing into the grid. The harmonic content (oscillatory nature of power) can be reduced by reducing the hysteresis band width. But by reducing band switching frequency in hysteresis current controller is going high.

Fig. 15. (a) Single Phase Instantaneous Active Power (b) Three Phase Active Power

Figure 16 shows single phase instantaneous reactive and three phase reactive power flowing into grid. Fig. 16(a) shows instantaneous reactive power flowing into grid which

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500

1

2

3

Mag (% of Fundamental)

b)

4

5

Amplitude of Current (Amps)

a)

Fig. 17. DC link voltage

1000

1500

Magintude of voltage (Volts)

2000

2500

3000

Synchronous Virtual Grid Flux Oriented Control of Grid Side Converter 131

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 <sup>0</sup>

Time (msec)

0.05 0.1 0.15 0.2 0.25 0.3 -200

Fundamental (50Hz) = 143.6 , THD= 4.74%

<sup>0</sup> <sup>10</sup> <sup>20</sup> <sup>30</sup> <sup>40</sup> <sup>50</sup> <sup>60</sup> <sup>0</sup>

Harmonic order

Fig. 18. (a) Three phase grid current waveform (b) Harmonic spectrum of grid current

Time (ms)

**5.2 Simulation results of current regulated delta modulator** 

is oscillating around zero. Fig. 16(b) shows three phase reactive power flowing into grid which having average zero value, this because of reference set value of reactive power is zero (i.e. Q\* = 0 VAr). This ensures unity power factor operation of grid connected inverter. In such a case we are supplying only active power to the grid. The reactive power needed by the loads which are connected to the grid can be supplied from other generating stations or bulk capacitors connected to grid to maintain grid power factor almost unity.

Fig. 16. (a) Single phase instantaneous reactive power (b) Three phase reactive power

**5.2 Simulation results of current regulated delta modulator** 

Fig. 17. DC link voltage

is oscillating around zero. Fig. 16(b) shows three phase reactive power flowing into grid which having average zero value, this because of reference set value of reactive power is zero (i.e. Q\* = 0 VAr). This ensures unity power factor operation of grid connected inverter. In such a case we are supplying only active power to the grid. The reactive power needed by the loads which are connected to the grid can be supplied from other generating stations or bulk capacitors connected to grid to maintain grid power factor

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Time (ms)

<sup>0</sup> 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 -8

Time (ms)

Fig. 16. (a) Single phase instantaneous reactive power (b) Three phase reactive power

almost unity.


a)




Ampiltude of Reactive power (VAr)

b)

0

2

4

<sup>6</sup> x 104



0

Amplitude of Reactive Power (VAr)

2

4

6

x 104

Fig. 18. (a) Three phase grid current waveform (b) Harmonic spectrum of grid current

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0.994

0.995

0.996

0.997

0.998

Power factor

0.999

1.001

0

1

2

3

Switching frequency (KHz)

4

5

6

1

Fig. 21. Variation of switching frequency with hysteresis band

10 15 20 25 30

Hysteresis band (Amps)

10 15 20 25 30

Hysteresis

Hystersis

Delta Modulator

Modified Ramp

Delta modulator Modified Ramp

Hystersis band (Amps)

Above graphs shows the variations in %THD, Switching frequency, power factor, dynamic response, error current with hysteresis band. From the above graphs we could say that % THD variation is less in modified ramp type current controller. Switching frequency is

Fig. 22. Variation of power factor with hysteresis band

Synchronous Virtual Grid Flux Oriented Control of Grid Side Converter 133

**5.3 Simulation results for ramp type current controller** 

Fig. 19. (a) Three phase grid current waveform (b) Harmonic spectrum of grid current

Fig. 20. Variation of % THD with hysteresis band

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 -200

Fundamental (50Hz) = 159.4 , THD= 2.68%

<sup>0</sup> <sup>10</sup> <sup>20</sup> <sup>30</sup> <sup>40</sup> <sup>50</sup> <sup>60</sup> <sup>0</sup>

Harmonic order

Hysteresis

Delta Modulator Modifeid Ramp

Fig. 19. (a) Three phase grid current waveform (b) Harmonic spectrum of grid current

5 10 15 20 25

Hysteresis band (Amps)

Fig. 20. Variation of % THD with hysteresis band

Time (ms)

**5.3 Simulation results for ramp type current controller** 

1

0

2

4

6

%THD

8

10

2

3

Mag (% of Fundamental)

b)

4

5

Magintude of current (Amp)

a)

Fig. 21. Variation of switching frequency with hysteresis band

Fig. 22. Variation of power factor with hysteresis band

Above graphs shows the variations in %THD, Switching frequency, power factor, dynamic response, error current with hysteresis band. From the above graphs we could say that % THD variation is less in modified ramp type current controller. Switching frequency is

Power Quality Improvement by Using

**7. Conclusion** 

inverter.

**8. References** 

278.

2009.

*electronics*, vol. 45, no. 5, October 1998.

18-23 1987, vol. 1, pp. 593-599.

*Electron*., vol.26, pp.321-325.1998.

Synchronous Virtual Grid Flux Oriented Control of Grid Side Converter 135

Analysis of different current control techniques for synchronous grid flux oriented control of grid connected voltage source inverter is presented in this chapter. For effectiveness of the study MATLAB/simulink is used here in GUI environment. Vector control in grid flux oriented reference frame is having capable of decoupling active and reactive powers following into grid, which we could see form figures of active and reactive powers for three current controllers. Reactive power following into grid is zero for all current control techniques to ensure the grid at unity power factor operation. There is a slight variation in power factors of three current controllers which is due to variation of percentage of THD in three current controllers. The DC link voltage is maintained at 2200V which is the set value of DC link voltage by using DC link voltage controller which controls the active current reference flows in the grid. The total harmonic distortion is less in modified ramp type current controller compared to other two current controllers. The switching frequency of modified ramp type current controller is maintained at 2 KHz which decreases the switching losses of power semiconductor devices compared to other current controllers where the switching frequency varies with load parameters. There is a less ripple in three phase active power for modified ramp type current controller compared to other two current controllers. Form the above discussion modified ramp type current controller is more advantages then other two current controller in grid connected voltage source

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Ibrahim Ahmed, Vector control of current regulated inverter connected to grid for wind

Kazmierkowski Marian P., and Malesani Luigi, Current Control Techniques for Three-

Kohlmeier Helmut and SchrÄoder Dierk F., Control of a double voltage inverter system

Malesani L. and Tenti P. A novel hysteresis control method for current-controlled voltage-

Generation: Experimental comparison of different schemes, power electronic converters for power system, *compatibility and power electronics*, 2009. Page: 271-

energy applications. *International journal on renewable energy technology*, vol.1, no.1,

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coupling a three phase mains with an ac-drive, in *Proc. IAS 1987*, Atlanta, USA, Oct.

source PWM inverters with constant modulation frequency, *IEEE Trans. Ind.* 

constant in modified ramp type current controller, delta modulator as limited switching frequency. Switching frequency is varying more with hysteresis band in hysteresis current controller. Modified ramp type current controller is giving good system power factor compared to other controllers.
