Let us look at the above figure , the terminal voltage of each three batteries are V1, V2 and V3 respectively. Let us assume V1 > V2> V3 ,then V1 will drive current into both the load resistor R1 and the two batteries V2, V3.
Same is the case with multiple rectifiers operating in parallel. Rectifier units can be thought of as a battery whose terminal voltage can be varied from zero to rated voltage. Thus parallel operation of two rectifier units is a bit complex as their direct voltages are constantly fluctuating.
Such systems can be operated without circulating current if at all instant of time, their direct voltage are equal(Considering rectifier outputs have ripple contents). Thus, not only the average DC voltage of the two systems must be equal but also their ripples must coincide.
However, it is desired that the ripple voltages instead of coinciding are so displaced that their superimposition results in higher pulse numbers. Note that higher pulse numbers mean smoother output DC and lesser amount(Cost and weight) of filter circuitry required. Through INTERPHASE
TRANSFORMER or IPT , two rectifier systems with displaced ripple voltages are paralleled. The parallel connection does not affect the working of each individual group. It absorbs, at any instant the difference between the direct voltage of the individual systems and must be designed for the time integral of this voltage.
The figure shows two rectifier units connected in parallel via IPT. It shows the direct current through the IPT. This transformer absorbs at any instance, the voltage difference between the individual groups and thus maintains independent operation of these groups.With respect to the voltage difference to be absorbed, the two windings of the IPT are in series connection. Thus the voltage difference can be balanced by the emf induced in these windings-just as in normal transformer. The voltage impressed on the primary winding is balanced by the induced emf. However, inducing the balancing emf needs a changing magnetic flux and exciting ampere turns, which is the difference of the direct currents to be combined (since these currents pass in opposite direction through the window of the IPT).
If these currents are well balanced, the core will not be driven into saturation by the dc ampere turns, even without an air gap. The time integral of the voltage to be absorbed by the IPT is a function of the dc voltage, of the operating conditions and of pulse number. For a regular transformer, operating at 50 Hz, the maximum flux density is taken near 1.5 Tesla. For an interphase transformer a lower value near to about 0.9 Tesla , is taken, since the magnetic flux is alternating with three times the supply frequency, if the two three pulse systems are combined.
In all electro-chemical, electro-metallurgical and traction applications, several converters are operated in parallel in order to meet very high current requirements.
Design Features
An IPT can be either a wound type or a bar mounted type depending on the voltage to be absorbed and the direct curent rating.
Wound type
Wound type
The wound type interphase transformer is like a single phase transformer with the windings on the two legs of the core.
The total number of turns in each winding is divided equally and accommodated on the two limbs.
The total number of turns in each winding is divided equally and accommodated on the two limbs.
Bar mounted type
In this type of interphase transformer, the busbar from the neutrals pass through rectangular cores forming a single turn winding.
