Multilevel Voltage Source Inverter for Grid Connected Photovoltaic System
- 2. INTRODUCTION The conventional energy sources are fast depleting these days. So the demand for renewable energy sources has been increased. The power developed by photovoltaic system is DC. To convert this DC power a photovoltaic inverter is used. Improving the output waveform of the inverter helps to reduce the harmonic content in the current injected into the grid in a grid connected system.
- 3. What is Multilevel Inverters? A multilevel inverter is a power electronic device which is capable of providing desired alternating voltage level at the output using multiple lower level DC voltages as an input. What is Two level Inverter? The voltage source inverters produce a voltage or a current with levels either 0 or ±Vdc they are known as two level inverters. Fig. Three, Five, Seven Multilevel inverter waveform switched at fundamental frequency
- 4. CONCEPT OF MULTILEVEL INVERTER A two-level Inverter creates two different voltages for the load few drawbacks as it creates harmonic distortions in the output voltage and also has a high dv/dt as compared to that of a multilevel inverter The concept of multilevel Inverter (MLI) is kind of modification of two-level inverter the output waveform obtained in this case has lower dv/dt and also lower harmonic distortions Smoothness of the waveform is proportional to the voltage levels
- 5. Fig. 1 Equivalent circuit of solar cell MODELING OF PV MODULE Here Ig = light generated current Rsh = Shunt resistance Rs = Series resistance Id = Diode current
- 6. SINGLE PHASE FIVE LEVEL INVERTER Fig. 2 Single phase five level inverter topology The operating principle of the inverter is to generate five levels of voltage. i.e., Vin, Vin/2, 0, -Vin/2, -Vin.
- 7. The pulse width modulation scheme for the five level inverter is as follows:- Two reference signals Vref1 and Vref2 are compared with a high frequency carrier signal, Vtri at the same time. Fig. 3 Switching pattern for the single phase five level inverter Table I Inverter output voltage during switches S1-S5 on and off
- 8. Fig.4 Grid voltage and Grid current using conventional H-bridge inverter Fig.5 Grid voltage and Grid current using five level inverter
- 9. CONCLUSION In these days of fast depleting conventional energy sources, photovoltaic systems are a very good alternate. Here a grid connected photovoltaic system is simulated in MATLAB. From the simulation results it was observed that the using multilevel inverters in such grid connected PV system considerably reduces the THD level of the current injected into the grid.
Editor's Notes
- #3: The conventional energy sources are fast depleting these days. So the demand for renewable energy sources has been increased. Among the renewable energy sources photovoltaic systems can be considered as a very good source of energy because of its consistency and cleanliness. The power developed by photovoltaic system is DC. To convert this DC power a photovoltaic inverter is used. Improving the output waveform of the inverter helps to reduce the harmonic content in the current injected into the grid in a grid connected system. For this a five level inverter is considered here. Such multilevel inverters helps to reduce the electromagnetic interference and also in reducing the filter size.
- #5: dv/dt If the rate of change of voltage across the device is large , the device may turn on even though the voltage appearing across the device is small Turn on method of thyristor dv/dt triggering S First take the case of a two-level inverter. A two-level Inverter creates two different voltages for the load i.e. suppose we are providing Vdcas an input to a two level inverter then it will provide + Vdc/2 and – Vdc/2 on output. In order to build an AC voltage, these two newly generated voltages are usually switched. For switching mostly PWM is used. Although this method of creating AC is effective but it has few drawbacks as it creates harmonic distortions in the output voltage and also has a high dv/dt as compared to that of a multilevel inverter. Normally this method works but in few applications it creates problems particularly those where low distortion in the output voltage is required. The concept of multilevel Inverter (MLI) is kind of modification of two-level inverter. In multilevel inverters we don’t deal with the two level voltage instead in order to create a smoother stepped output waveform, more than two voltage levels are combined together and the output waveform obtained in this case has lower dv/dt and also lower harmonic distortions. Smoothness of the waveform is proportional to the voltage levels, as we increase the voltage level the waveform becomes smoother but the complexity of controller circuit and components also increases along with the increased levels.
- #7: Without going into different topologies, I am going to explain the working of a multilevel inverter. Now the key function of inverter is the switching frequency. To convert a DC signal into an AC signal we require fast switching of DC signal giving us multiple levels. This turns into a staircase wave that is quite close to a sine wave.