Duties of rectifier transformers serving special industrial loads are more stringent than conventional transformers. Electrical energy in the form of direct current is required in electrolytic processes used in aluminum smelters and chemical plants (producing chlorine, soda, etc.). Various methods used for converting AC into DC in earlier days included use of motor-generator sets, rotary converters, and mercury arc rectifiers.
Due to rapid developments in power electronic converters and switching devices, transformers with modern static converters (rectifiers) are being widely used for current ratings as high as hundreds of kilo-amperes. Design and manufacture of transformers for the rectifier duty poses certain challenges. Complex winding arrangements, high currents and associated stray field effects, additional losses and heating effects due to harmonics, the necessity of maintaining constant direct current, etc. are some of the special characteristics of the rectifier transformers.
Bridge Connection
One of the most popular rectifier circuits, three-phase six-pulse bridge, is shown in Figure 1. It gives a 6-pulse rectifier operation with the r.m.s. value of the secondary current for an ideal commutation condition (zero overlap angle) as
Where Id is the direct current.
For a transformer with unity turns ratio, the r.m.s. value of the primary current is also given by the above expression.
The average value of the direct voltage is
Where E is the line-to-line r.m.s. voltage.
The secondary winding does not carry any direct current (the average value over one cycle is zero). The ratings of both primary and secondary windings are equal, which can be obtained by using Equations 1 and 2 as
P = √3 EI = 1.047 VdId = 1.047 Pd (Equation 3)
Thus, in the bridge connection the capacity of the transformer is well utilized because the required rating of (1.047 Pd ) is the minimum value for the 6-pulse operation. The bridge connection is simple and widely used.
Interphase Transformer Connection
When the current requirement increases, two or more rectifier systems need to be paralleled.
The paralleling is done with the help of an interphase transformer which absorbs at any instant the difference between the direct voltages of the individual systems so that there are no circulating currents.
Two 3-pulse rectifier systems (operating with a phase displacement of 60°) paralleled through an interphase transformer are shown in Figure 2.
The difference between the instantaneous values of direct voltages of the two systems is balanced by the voltage induced in the windings of the interphase transformer. Since both the windings are linked with the same magnitude of magnetic flux, the voltage difference is equally divided between them.
The output DC voltage at any instant is the average of the DC voltages of the two systems. Thus, the paralleling of the two 3-pulse systems results in a system with 6-pulse performance. The r.m.s. value of the secondary current is
Where Id is the total direct current (sum of the direct currents of the two rectifier systems).
Each secondary conducts for one third of each cycle, and it can be proved that the rating of the two secondary windings considered together is 1.48 Pd . Since the primary winding carries the current pulses in both half cycles, it is utilized efficiently (compared to the secondary windings). The r.m.s. value of its current is
The corresponding primary rating is 1.047 Pd , the minimum value which can be obtained for the 6-pulse performance.
Since the flux in the magnetic circuit of the interphase transformer is alternating with 3 times the supply frequency when two 3-pulse systems are paralleled or with 6 times the supply frequency when two 6-pulse systems are paralleled, the core losses are higher. Hence, the operating flux density in the interphase transformer is designed to be around 50 to 67% of the value used in the conventional transformers.
If a 12-pulse operation is desired, two 6-pulse rectifier systems operating with a phase displacement of 30° are combined through an interphase transformer. In this case, the time integral of the voltage to be absorbed is smaller as compared to that in the 6-pulse operation (due to smaller voltage fluctuations in the ripple). Also, the frequency of the voltage is 6 times the supply frequency. Hence, the size and cost of the interphase transformer is reduced.
When a 12-pulse system is obtained through one primary winding (usually star connected) and two secondary windings (one in star and the other in delta connection), it may be difficult to get the ratio of turns of the two secondary windings equal to 1/√3 (because of their low number of turns).
Therefore, the required 30° phase displacement is obtained by having two primary windings, one connected in star and the other in delta, and two secondary windings are connected either in star or delta. One such arrangement is shown in Figure 3.
Since the two primary windings are displaced by 30°, it is necessary to have an intermediate yoke to absorb the difference between the two limb fluxes φ1 and φ2 (see Figure 4). The intermediate yoke area should be corresponding to the difference of the two fluxes (which is about 52% of the main limb area).
Under the balanced condition of the two paralleled rectifier systems, the currents (average values) in both the windings of the interphase transformer are equal. This results in equal flux in the same direction in both the limbs forcing the corresponding field lines to return through the high reluctance nonmagnetic path outside the core (a substantial portion of DC ampere-turns is absorbed along the nonmagnetic return path).
Another way to explain the phenomenon is that since net ampere-turns are zero in the window (since currents are directed in opposite directions), flux lines in the closed magnetic path are absent. Hence, the flux density in the core is low under the balanced condition.
A slight imbalance in currents of the two systems results in a nonzero value of ampereturns acting on the closed magnetic path, which may drive the core into saturation. Thus, the interphase transformer draws a high excitation current under imbalanced conditions. This is one more reason (apart from higher core losses) for keeping a lower value of the operating flux density.
Although the interphase transformer connection has disadvantages, viz. a higher rating of its secondary winding and saturation of its magnetic circuit due to unbalance between two paralleled systems, it competes well with the bridge connection in a certain voltage-current range.
The application of the interphase transformers is not restricted to paralleling of two systems; for example, with a three-limb core, three systems can be paralleled.
If the pulse number has to be further increased (e.g., 24-pulse operation), the required phase shift is obtained by using zigzag connections or phase shifting transformers. The IPT-system is mostly used for the parallel operation of two controlled rectifiers (i.e., with thyristors).
The system comprising controlled rectifiers is often a closed-loop control system in which firing angles of thyristors are varied according to load requirements. The difference in voltages to be absorbed by the IPT does not remain constant. The difference increases with the firing angle. Hence, the probability of the IPT to walk into saturation increases, and therefore it is beneficial to use the transformer (IPT) as a separate entity for absorbing the variable voltage difference.
The parallel operation of two uncontrolled rectifiers (using power diodes) is an open-loop system in which the voltage difference is constant for a particular load requirement. The five-legged rectifier transformer (FLRT) system is mostly employed for such applications. The voltage control is achieved in the system by providing supply through a three-phase autotransformer.
The IPT system is costlier and bulkier compared to that with the FLRT system due to the additional magnetic circuit. However, the system allows fine and better controllability compared to the FLRT system.
Features of Rectifier Transformers
Rectifier transformers are used in applications where the secondary voltage is required to be varied over a wide range at a constant current value. It is extremely difficult and uneconomical to have taps on the secondary winding because of its very low number of turns and a high current value.
The taps are either provided on the primary winding, or a separate regulating transformer (autotransformer) is used feeding the primary of the main transformer, which can be accommodated in the same tank.
For higher pulse operations, extended delta connections are shown to be more advantageous than zigzag connections, as they result in lower eddy losses and short-circuit forces.
The output connections, which carry very high currents, increase the impedance of the transformer significantly.
For large rating rectifier transformers, the field due to high currents causes excessive stray losses in the structural parts made from magnetic steel. Hence, these parts are usually made from nonmagnetic steel.
Rectifier transformers are subjected to harmonics due to non-sinusoidal current duties. Hence, sometimes the pulse number is decided by harmonic considerations. Due to harmonics, more elaborate loss calculations are required for the rectifier transformers compared to the conventional transformers.
Sometimes the core of the rectifier transformer supplying power electronic loads is designed to have a small gap in the middle of each limb to limit the residual flux and to keep the magnetizing reactance reasonably constant. This feature also limits the inrush current thereby protecting the power electronic devices.
Under normal operating conditions, the core flux fringing out in the gap between the two core parts hits the inner winding causing high eddy losses. In order to mitigate this effect, the windings may also have to be designed with a gap at the location facing the core gap.
Because of possibilities of rectifier faults, special design and manufacturing precautions are taken for rectifier transformers. It is generally preferred to design the rectifier transformers with larger core area with corresponding smaller number of turns to reduce short-circuit forces.
Disk-type windings are preferred since they have better short-circuit strength compared to layer windings. Quality of drying/impregnation processes and integrity of clamping/support structures have to be very good. The paper insulation on winding conductors can also be strengthened.