I have a voltage source inverter connecting to a symmetrical load through a delta-star transformer. Surprisingly, the primary currents of transformer have DC components. the output voltage of transformer are symmetrical 230V AC. Do you have any idea about this? Thanks
Although Secondary voltage is controlled correctly and it has low THD with a 3phase resistive load, DC primary current is too high. I think there must be another reason.
In order to analyse this problem and resolve the origin of the DC current in the primary one has to analyse the output voltage waveform of the inverted. If your primary has a DC current , this means that the the voltage waveform contains a dc component voltage. If even this DC voltage is small, it can cause relatively large current because the DC current is limited only bt the DC resistance of the primary of the transformer which is made very small to avoid ohmic losses in the transformer.This DC voltage will not be transformed to the secondary as it does not have induction effect.
The symmetrical three phase at the output points out that the AC primary voltage is also symmetrical.
In conclusion, the large DC current in the primary is the consequence of a relatively small DC component of the primary voltage coming from the inverter. This may be timing inaccuracies in the control pulses the inverter switches. More accurate timing is required to mitigate this problem.
wish you success
thank you all for your answers.
@ Sudha: thanks for your attention. Since this DC current exists in steady state, it can't be part of inrush current. Also, Secondary current does not have DC component.
@ Abdelhalim
Thanks for your attention. I will check inverter and primary voltages again. The system has one inner current control loop and one voltage control loop. the current loop controls the secondary currents of transformer. Is it possible that the control system makes this DC current even if it results good output voltage?
The current circulating in the Delta winding of the transformer should be a Triplen rather than DC , The level of the Triplen should also change with load . pl check .
I think what Mr. Abdelhalim mentioned could be the prominent reason. Pl check the inverter output wave form.
@Rujendra
I measure the DC and AC components of line current of primary not the current of each phase (winding of primary which are connected in delta). And the DC component exisits even in no load condition.
@Sudha
I use two control loops for this system. The inner and fast loop is current control loop which is designed for secondary currents and the second and slower loop is output voltage control loop.
all I mean to say , you might be measuring the r Harmonic current - which will circulate in the Delta winding with a frequency of 3rd order , For a change , can you put the output of the Inverter to STAR side keeping the DELTA open and verify the similar phenomena .
I used the method to super impose D.C on A.C to generate inrush current pattern needed, for testing of "one cycle static blocking scheme for inrush". and was verified with electronic B-H curve simulating circuit, refer my papers....
The inrush current of a transformer is the current appearing on the primary input side of the transformer , when it is SWITCHED on directly to the AC supply system. The characteristics will be different for every transformer and also every condition viz. the secondary loaded or not loaded , the moment of switching , the air core reactance of the primary coil which in turn is dependent upon the coil dimensions .
The present purview of discussion is the out put of the inverter which I think is slightly different from the inrush current phenomena
However , I will like to go through paper of Mr Desai who may send the paper to mail address [email protected] .