There has been a noticeable increase in use of Solar PV based systems for power generation, given its renewable nature. A solar PV based grid tie inverters are used for dc-ac conversion. The conventional line commutated ac-to-dc inverters have square-shaped line current which contains higher-order harmonics. The line current with the high harmonic contents generates EMI and moreover it causes more heating of the core of distribution/power transformers. Alternatively, PWM based inverters using MOSFET/IGBT switches are also used for the same purpose. However, apart from higher switching losses, the power handling capability and reliability of these devices are quite low in comparison to thyristors/SCR. Nevertheless, the conventional thyristor based forced commutated inverters are not suitable for PWM applications due to the problems of commutation circuits. A pure sinusoidal line current or waveform with low har- monic contents is the most desirable. In the present work, a multilevel line commutated inverter topology has been proposed and analyzed which improves the wave shape and hence reduces the total harmonic distortion (THD) of the line current in a grid tie line commutated inverter. The scheme has successfully been implemented and tested. Moreover, the performance of the proposed topology is far better than the conventional line-commutated inverter. It reduces THD, losses, switching stress and EMI.
Solar PV based systems are being seen as a major contributor to the present power generation technology. One of the important applications of the solar PV based power generation is to feed the generated power (dc) into grid (ac). For this purpose, normally, PWM inverters are used which use gate commutated devices (IGBT, MOSFET, GTO etc.). However, apart from higher switching losses, the power handling capability and reliability of these devices are quite low in comparison to thyristors/SCR. Moreover, the conventional line commutated ac-to-dc inverters have square-shaped line current which contains higher-order harmonics. The line current with the high harmonic contents generates EMI and moreover it causes more heating of the core of distribution/power transformers. Therefore, multilevel inverters have drawn increasing attention in recent years, especially in the distributed energy resources area [
A line commutated full-wave controlled rectifier can be made to operate in inversion mode by connecting a dc voltage source at the load end and controlling the switching angle (a). It is basically a phase-controlled converter with an RLE load [
In the present work, three level line-commutated inverter circuit has been developed and implemented with improved THD. It does away with the disadvantages associated with a conventional square wave inverter. A UJT based triggering circuit has been developed to fire the thyristors at the desired switching angle. Variation of the T.H.D and the negative average power with the variation of m (ratio of dc voltage to ac voltage) has been analyzed.
A multilevel converter circuit with RLE load works in two modes of operation i.e. rectification mode and inverter mode. It works in inversion mode when the switching angle of each of the converter is greater than 90˚. Here, a new multilevel circuit topology has been developed as shown in
The circuit has been analyzed and implemented for three level of line current and can be extended to higher levels for better performance. But the increase of level adds to the cost of converter and more number of secondary windings. So, a suitable compromise has to be made between the THD of the line current and cost of additional hardware. When the circuit works in inverter mode, the dc source transfers power to the main (ac source). The major advantage of the proposed configuration is that in continuous current mode of operation, the waveform resembles a stepped sinusoidal wave and with suitable selection of switching angles the harmonic contents can be reduced drastically.
In general, the load current can be either continuous or discontinuous. In the case of continuous current operation the current of both thyristors overlaps. It depends upon
dc source voltage, phase angle of load or inductor (f) and the switching angle.
1) Three level control strategy: For three level line current, a set of secondary winding with centre tap arrangement is required. Each pair of thyristor in a centre tap secondary is fired at a switching delay of 180˚. The upper leg thyristor is fired at angle greater than 90˚ for inversion operation. Simultaneously lower leg thyristor is fired at a delay of 180˚ with respect to the upper leg thyrist or (see
The expression of the converter current is obtained by solving the equation
For wt = q and, it gives ileg1 for conduction through T1in positive half cycle in upper leg of converter.
For conduction through T2 in negative half cycle, the expression of lower leg converter current is given by
Each converter contributes to the line current and the net line current, iline, is equal to the sum of all (ileg1 + ileg2)n. n = 1, 2.
The three step inverter has been modeled in SIMULINK. The model is shown in
The triggering pulses are given from the pulse generator block of the simulink blockset. They are shown in
cally done to study the THD and average power variation of the line current with the firing angles. Line current with THD and harmonic is shown in
The THD and Power flow analysis is done for various configuration of switching angles. The effect of variation of dc voltage on the THD and the power transfer is also analyzed. Though a battery has been considered as a dc source for simulation as well as experimental analysis, the result obtained can be utilized when using solar PV panel. Since the dc voltage of a solar PV panel is not constant (vary with insolation, temperature and load), the effect of varying dc voltage on THD and power flow will help in developing a control strategy for operating the system in optimum condition i.e. changing the switching angle combination at different voltage level of PV panel. Variation of THD and power with switching angle combination for a three level inverter is shown in
It is evident from
figurations. Thus, a suitable compromise has to be made between the two. For the above configurations, the variation of THD and power with the dc voltage is studied and it is shown in
The negative power transfer shows an increasing trend with the dc voltage whereas the THD is lower for smaller dc voltage.
A UJT based triggering circuit has been designed to control the switching angle. The angle is controlled by changing the resistance of the potentiometer. 1:1:1 pulse transformer is used. The diode arrangement on the two secon-
dary winding is such that to obtain a phase delay of 180˚ between G1 and G2. The circuit diagram and the triggering circuit are shown in
The complete experimental setup (prototype) is shown in
for the switching angle combination (α1 = 110˚, α2 = 140˚). The THD in this case is 22.9%. Experimental results are shown in
Multilevel inverter for three levels has successfully been implemented. A thorough study has been made to obtain optimal performance with battery as dc source and can be extended for solar PV modules. With reduced THD and features of multilevel inverter, it will to be a better replacement for square wave inverter in various distributed system connected to the grid.