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Operation of Parallel Inverter
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Figure 1 shows the parallel inverter circuit. Two diodes are connected as shown in Fig.1 to feedback the stored load energy during those periods when the load current reverses relative to the voltage. Figure 2 shows the load voltage and current waveform of parallel inverter.

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Thyristors T1 and T2 are the main load carrying thyristors. Inductor L and capacitor C are the commutating components. Diodes D1 and D2 are the feedback diodes, which permit the load reactive power to be fed back to the d.c. supply. The circuit operation can be divided into different operating modes.

Mode 1: During this mode, thyristor T1 is triggered at instant R, as shown in Fig.2 . Battery voltage now forces the current to the primary section CA through the path Edc+CAT1LE dc- . Thus, neglecting the small voltage drop across L, the supply voltage Edc will appear across the left-half of the transformer primary winding CA. Terminal C is positive with respect to terminal A. The flux produced due to this current induces the voltage in all sections of transformer winding as number of turns in section CA are assumed to be equal to number of turn in secondary. The load voltage is nearly equal to Edc and is in such a direction so as to force a current into the dot at terminal P. This is because, the current in primary leaves the dot at terminal A.

Now, due to autotransformer action, voltage Edc is induced in CE section of primary winding. Therefore, terminal E will be at a potential of 2Edc with respect to A. Thus, capacitor C will get charged to twice the supply voltage with the polarity as shown in Fig.1. Since the load is inductive, load current increases gradually from instant R, as shown in Fig.2.

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Mode 2 : This mode begins when thyristor T2 is switched ON at instant S, as shown in Fig.2. When T2 is turned-on, capacitor C will immediately apply a reverse voltage of 2Edc across SCRT1 and turns it OFF. When SCR T1 is turned-off, the capacitor will discharge through SCRT2, inductor L, diode D1, and a portion of a transformer primary winding BA . Thus, the energy stored in the capacitor will be fed back to the load through the transformer coupling of windings BA and PQ. During this period, the potential of point B will be fixed by the d.c. input supply and the load voltage will still be positive but more than Edc .

The load current, which earlier was flowing through SCRT1 , will now flow through CB and diode D1 to the negative input terminal. This can happen only if diode D1 is forward biased and the capacitor discharge current is more than the load current. As the potential of point B increases sufficiently to reverse bias diode D1 , the capacitor will no longer discharge through SCRT2 and point B will not get connected to the supply negative terminal. The current through inductor in L will now flow through diode D2,DE and SCRT2, and the trapped energy in inductor L will be fed back to the load.

The load current T1 , which earlier was flowing through CB, will now flow from D to C through diode D2, and the load reactive energy will be returned to the d.c. supply. Since point D is now connected to the negative supply terminal, the load voltage polarity will be reversed and more than E dc  .Also, the capacitor C will be charged in the opposite direction to slightly more than twice the supply voltage. Thyristor T2 will stop conducting after all the energy in the commutating inductor L has been completely dissipated in the load. Immediately following the commutation of SCRT1, energy is transferred from the capacitor and inductor to the load. During this period, high frequency oscillations will be superimposed on normal rectangular waveform of the load voltage. After this transient period, only diode D2 will continue to conduct. This will cause application of a reverse voltage across SCRT2, and thereby help in turning it OFF.

Mode 3: This mode begins when the load current becomes zero, diode D2 will be blocked and SCRT2 will have to be triggered again at instant U to reverse the direction of the load current. When thyristor T2 starts conducting, the load voltage will again become equal to Edc.

In Fig.2 , the transient waveforms during the commutation period are shown by the dashed lines. In Fig.2 SCRT1 will conduct during the period RS, when both load voltage and load currentare positive. At S,SCRT2 is triggered to commutate SCRT1 . After the commutation transient, the load voltage will be reversed and the current will continue to flow through diode D2 in the same direction as before. SCR T2 will be turned-off because of the reverse bias applied by D2 . At U, the load current will become zero when all the inductive energy is dissipated and SCR T2 will be triggered again. During the period UW, both load voltage and load current are reversed. At W,SCRT1 is triggered to turn-off SCRT2.

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