transformer engineering design and practice 2_phần 3

11 Special Transformers 11.1 Rectifier Transformers 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 (production of chlorine, soda, etc.).Various methods used for converting AC into DC in earlier days included use of motor-generator set, rotary converters and mercury arc rectifiers.With the rapid development 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 with the rectifier duty poses certain challenges. Complex winding arrangements, high currents and associated stray field effects, additional losses and heating effects due to harmonics, necessity of maintaining constant direct current, etc. are some of the special characteristics of rectifier transformers. 11.1.1 Bridge connection One of the most popular rectifier circuits is three-phase six-pulse bridge circuit as shown in figure 11.1. It gives a 6-pulse rectifier operation with the r.m.s. value of the secondary current for ideal commutation (zero overlap angle) as (11.1) 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. 411 Copyright © 2004 by Marcel Dekker, Inc. 412 Chapter 11 Figure 11.1 Bridge connection The average value of direct voltage is (11.2) where E is 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 11.1 and 11.2 as (11.3) Thus, in the bridge connection the capacity of a transformer is well utilized because the required rating of (1.047 Pd) is the minimum value for a 6-pulse operation. The bridge connection is simple and quite widely used. 11.1.2 Interphase transformer connection When the current rating increases, two or more rectifier systems may need to be paralleled. The paralleling is done with the help of an interphase transformer Copyright © 2004 by Marcel Dekker, Inc. Special Transformers 413 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 11.2. Figure 11.2Arrangement with interphase transformer Copyright © 2004 by Marcel Dekker, Inc. 414 Chapter 11 The difference between the (instantaneous values of) direct voltages of two systems is balanced by the voltage induced in the windings of the interphase transformer, for which they are in series connection. 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 value of DC voltages of the two systems. Thus, the paralleling of two 3-pulse systems results in a system with 6-pulse performance. The r.m.s. value of the secondary current is given by (11.4) where Id is the total direct current (sum of the direct currents of two rectifier systems). Each secondary conducts for one-third of cycle, and it can be proved that the rating of 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 secondary windings). The r.m.s. value of its current is (11.5) The corresponding primary rating is 1.047 Pd, the minimum value which can be obtained for a 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 for the conventional transformer [1], 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 the 6-pulse operation (due to smaller voltage fluctuation 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 the 12-pulse operation is obtained through one primary winding (usually star connected) and two secondary windings (one in star and other in delta connection), it may be difficult to get the ratio of turns of two secondary windings equal to (because of low number of turns). In such a case, the 30° phase displacement is obtained by having two primary windings, one connected in star and other in delta, and two secondary windings both connected either in star or delta. One such arrangement is shown in figure 11.3. Copyright © 2004 by Marcel Dekker, Inc. Special Transformers 415 Figure 11.3 Twelve-pulse operation Since the two primary windings are displaced by 30°, it is necessary to have an intermediate yoke [2] to absorb the difference between the two limb fluxes (see figure 11.4). 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 flux to return through the high reluctance non-magnetic path outside the core (a substantial portion of DC ampere-turns is absorbed along the non-magnetic return path). Other way to explain it is that since net ampere-turns are zero in the window (currents are directed in opposite directions inside the window), flux lines in the closed magnetic path are absent. Hence, the flux density in the core is low under the balanced operation. A slight unbalance in currents of the two systems results in a non-zero value of ampere-turns acting on the closed magnetic path, which may drive the core into saturation [3]. Thus, the interphase transformer draws a high excitation current under the unbalanced conditions. This is one more reason (apart from higher core losses) for keeping the operating flux density lower in interphase transformers. Figure 11.4 Intermediate yoke arrangement Copyright © 2004 by Marcel Dekker, Inc. ... - tailieumienphi.vn