PHOTOVOLTAIC-FED MULTILEVEL INVERTER

The quality of power is most significant in the field of the power system. Many techniques are used for improving the same. In this article, the quality of the Power and THD values for different switching pulses are to be analyzed. Here, the existing system comprises a Solar Photovoltaic array, boost converter, Multilevel inverter with normal switching pulse, and RLC Load. The proposed system comprises a solar photovoltaic array fed multilevel inverter with Pseudorandom Binary Switching (PRBS) pulse, buck-boost converter, and three-phase induction motor. The Pseudo-Random Binary Sequence generator consists of shift register and logic gates...

Power quality is an important aspect of the power sector. There are different types of methods and techniques, which are used to solve the power quality issues or threats. In recent years, the renewable energy has played a vital role in the power market. The solar panel is used for different types of applications. The solar panels are operated based on solar radiation and solar temperature. Solar is the best and most efficient energy – when compared with all other renewable energy sources. In the converter circuits, the switching devices decide the efficiency of the motor because it causes losses and harmonics.

This article consists of normal switching pulses in the existing system and pseudo-random switching pulses in the proposed system. The existing system consists of a Photovoltaic panel, Boost converter, Multilevel inverter with normal Switching pulse, and RLE load. The proposed system consists of a solar photovoltaic panel, Boost converter, Multilevel inverter with Pseudorandom binary switching pulse, and three-phase induction motor.

The solar panel acts as an input source for the proposed system. Figure 1 depicts the block diagram of the proposed system.  The solar photovoltaic panel generates the input voltage and that voltage is given to boost converter. The boost converter boosts up the voltage in the 1:2 ratio. The multilevel inverter with PRBS switching pulse converts DC voltage to AC voltage and the three-phase induction motor is acted as load.

The main contribution of the article is 1) The third harmonics in the existing and proposed system is analyzed. 2) The voltage and current Total harmonics distortion values are measured. 3) In the proposed system the Pseudorandom sequence gating pulse was used and efficiency was improved.

The proposed system is designed with a solar panel for an input voltage of 75 VDC and a disturbance voltage of 8VDC for simulating the system. The converter operates with  150 VDC and it boosts the voltage to 290 VDC with a 1:2 ratio. The inverter converter the 290 VDC  DC supply to 290 VAC for a single-phase AC supply.  The three-phase induction motor acts as a load and the motor is operated with a 460 VAC three-phase supply.

Fig 1. Block of the proposed system…

Photovoltaic Panel

The Photovoltaic panel is a solar array module that generates the power from the Sunlight. The PV panel is made up of a PN junction. The number of panels connected in parallel and form a PV array. When the sun rays fall on the panel due to the irradiance and temperature the voltage is generated.

The simulink model is built for both existing and poposed system  with following parameters such as short circuit current Isc=5.45 A, open circuit voltage Voc=75 V, Maximum current Imax=4.95A, Maximum voltage Vmax= 75, Ppv= 369.5, Vpv=76.25V, Ipv current = 4.85 A, disturbance voltage V=8V and the parameter of the ideal switch is Ron=0.001Ω, Rs=1e5, Cs=inf.

Buck-Boost Converter

The output of the PV panel is connected to the buck-boost converter. The buck-boost converter boosts both voltage and current. The converter or chopper is operated based on a 1:2 ratio. The input voltage is doubled. Due to the three-phase induction motor, the buck-boost converter is used in the proposed system. The best power system must ensure high quality and stable power supply.

The converter or chopper operates based on the duty cycle of the chopper. The buck-boost converter are designed with the following specification such as L=2e-3, C=100e-3, R=0.001Ω, C=50e-6, the MOSFET have FET Ron=0.001Ω, Internal diode Inductance =1e-6, Internal diode resistance=0.001, snubber resistance Rs=2000 Ω and Diode have Ron=0.001, Vf=0.8 Snubber Rs=1500. The power electronics converters are used in numerous applications in various fields and effective control in the emerging technology.

Seven Level Multilevel Inverter

The inverter played an important role in the power electronics section. The inverter converts the DC supply to the AC supply. The Battery, capacitor, fuel cell may be acting as an input source for the inverter. In the proposed system, the solar panel acts as an input source. The grid-tied inverter is used for the stability analysis by using a generalized Nyquist stability criterion.

The multilevel inverter is used for conversion purposes. The different multilevel inverter is cascaded H bridge MLI, diode clamped MLI, and hybrid MLI. The cascaded Multilevel inverter is used in the hybrid electric vehicle, FACTs devices, and motor drives. When the number of levels of inverter increases, the quality of the sine wave is also increased. The gird impedance information can be used to enhance the operation of the grid-tied inverter.

In the proposed system, a seven-level multilevel inverter is used. For seven-level multilevel inverter, the seven steps are produced in the output AC waveform. The multilevel inverter is designed with Resistance R=0.5Ω, V=290V, MOSFET have FET Ron=0.1, Rd=0.01, Rs=1e-5, Cs=inf

Gate Pulse Switching by a Pulse Generator

The inverter operation is based on the switching or gating pulse. The PWM gate pulse is also applied to the inverter. From this pulse, the output voltage can be controlled and harmonics are reduced. Normally the pulse generator performs the switching operation. In the existing system, the normal switching pulse is used by the pulse generator.

In the existing system, each MOSFET is operated for a 600 phase shift operation. The basic pulse generator is operated based on the sampling voltage and sampling time. The pulse generator has the parameter such as amplitude =1 A, Period=0.02, Pulse width=30, and phase delay =0.002. Each pulse generator is operated for a 600 phase shift.

Pseudo-Random Binary Sequence (Prbs) Switching for Mli

The pseudo-random binary sequence generator generates the random pulse of different values as a gating signal. The most commonly used input stimuli for correlation analysis is Pseudo random binary sequence (PRBS) with a maximum length sequence. The PRBS sequence is used for analyzing the parameters of the electrochemical cell. By the appropriate duty cycle (D), the output of the inverter can be controlled.

The duty cycle,

Where ton = On time, toff= Off time

T= ton + toff

The pseudo-random binary sequence generator is made of shift register and logic gates. Figures 2 and 3 show the PRBS comparison diagram and logic gate configuration. The main advantage of the PRBS method, sine sweep because it can be generated by using time delay. The PRBSs are deterministic bitstream of ‘0’ or ‘1’ runs randomly for a length after it will be repeated.

Fig 2. PRBS comparison diagram…
Fig 3. Logic gate configuration…

In this PRBS generator for specific maximum length(MLS), a polynomial length L will generate the deterministic sequence of  ‘0’ and ‘1’ of  2L-1.  Figure 4 shows the PRBS bits. The PRBS gate pulse injection does not introduce an excessive amount of noise to the proposed system.

Fig 4. PRBS bits…

The gating signal initiates the operation of the inverter. The gatings signal comprises Pseudo random binary signal and sinusoidal signal. In the proposed system, the multilevel inverter with pseudo-random gate pulse is used. In the existing system, the RLE load is used but in the proposed system the three-phase induction motor acts as a load. The spectrum of PRBS is discrete but it is optimized between maximum length, sampling frequency, and PRBS bit frequency. For the online measurement method, the PRBS injection is used. The online measurement methods consist of active and passive system identification.

The PRBS gating pulse module is designed with the following parameters such as Vector of  output is [ 0 1 0 1 0 1 0 1 0 1 0 0] and sampling time is 0.0001. the vector of  time value is [0.001  0.002 0.004  0.006   0.008  0.009  0.012  0.013  0.017  0.018  0.020].  The constant value is 01 and sampling time = inf.  The Figure 5 demonstrates  the model of seven level multilevel inverter.

Fig 3. Logic gate configuration…

Simulation Result

This section discusses the simulation result for both the existing and proposed systems. The existing system is designed with a pulse generator gated inverter and a proposed system gated with PRBS injected pulse Multilevel inverter.

Existing system simulation diagram

Figure 6 -14 depicts the construction and operation of the existing system.

Fig 6. Circuit diagram of a multilevel inverter with RL –load (Normal Switching pulse)…
Fig 7.Voltage across PV…
Fig 8. Switching pulse of Buck-boost converter…
Fig 9. Voltage across the buck-boost converter…
Fig 10. Normal Switching pulse of inverter…
Fig 11. Voltage across RL-load…
Fig 12. Voltage THD…
Fig 13. Current through RL load…
Fig 14. Current THD..

 

Proposed system simulation diagram

This section discusses the proposed system initiated with PRBS injected gated inverter. Figure 15-23 represent the process of the proposed system.

Fig 15. Circuit diagram of the multilevel inverter with Motor load(Pseudorandom)…
Fig 16.Voltage across PV…
Fig 17. Switching pulse of the buck-boost converter with PRBS…
Fig 18.The voltage across the buck-boost converter with PRBS…
Fig 19. Pseudorandom Binary Sequence Switching pulse of inverter…
Fig 20. The voltage across Motor load…
Fig 21. Voltage THD…
Fig 22. Current through Motor load…
Fig 23. Current THD…

Harmonics Comparision

Harmonics are one of the major threats in the field of power systems. In this article, the third harmonics are analyzed and THD values are noted. In the existing system, the multilevel inverter fed by normal switching pulse with RLE load is used but in the proposed system the multilevel inverter fed by PRBS gating pulse with a three-phase induction motor is used. Table 1 depicts the comparison of voltage and current THD. The voltage and current harmonics THD values of normal switching pulse is higher than that of Pseudorandom switching pulse gated inverter system.  Table 1. Shows the comparison of harmonics values.

Conclusion And Future Work

This article examined the response and performance of inverter fed RLE load for pulse generator gated pulse injection and Multilevel inverter pseudo-random binary sequence injection. The gating pulse influencing the value of the harmonic in the system. The third harmonics are analyzed in both systems and results are tabulated. In the existing system, the voltage and current THD values are high. But in the proposed system the voltage and current THD value are low when compared with the existing system. The existing system is verified with RLE load and the proposed system is checked and verified with 460 VAC three-phase induction motor. So, as per the results the proposed system produced more efficient output with lesser THD value when compared with the existing system.




Chandrasekaran
is from Arasu Engineering College, Kumbakonam, Thanjavur, Tamilnadu, India

Dr. S. Durairaj is from Dhanalakshmi Srinivasan Engineering College, Perambalur, Tamilnadu, India

Dr. S. Padmavathi is from Thiagarajar College of Engineering, Madurai, Tamilnadu, India

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