411 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 I d 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. Copyright © 2004 by Marcel Dekker, Inc. Chapter 11412 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 P d ) 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 Figure 11.1 Bridge connection 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 11.2 Arrangement with interphase transformer Copyright © 2004 by Marcel Dekker, Inc. Chapter 11414 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 I d 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 P d . 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 P d , 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 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 11.3 Twelve-pulse operation Figure 11.4 Intermediate yoke arrangement Copyright © 2004 by Marcel Dekker, Inc. Chapter 11416 Although the interphase transformer connection has some disadvantages, viz. higher rating of secondary winding and saturation of 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 [1], 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 [1,2]. 11.1.3 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 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. Various circuit arrangements which can be used to regulate the secondary voltage are elaborated in [4]. For higher pulse operations, the extended delta connection is shown to be more advantageous than the zigzag connection, as it results into lower eddy losses and short circuit forces [5]. The output connections, which carry very high currents, increase the impedance of the transformer significantly. The increase in impedance due to these connections can be calculated for a single conductor as per equation 3.80. For go-and-return conductors of rectangular dimensions, the impedance can be calculated as per the formulae given in [6]. For large rating rectifier transformers, the field due to high currents causes excessive stray losses in structural parts made from magnetic steel. Hence, these parts are usually made of non-magnetic steel. Rectifier transformers are subjected to harmonics due to non-sinusoidal current duty. Hence, sometimes the pulse number gets decided by harmonic considerations. Due to harmonics, more elaborate loss calculations are required for rectifier transformers as compared to the conventional transformers [7]. Sometimes the core of the rectifier transformer supplying power electronic loads is designed to have a small gap in the middle of each limb [5] to limit the residual flux and 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 higher 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. Copyright © 2004 by Marcel Dekker, Inc. Special Transformers 417 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 the corresponding smaller number of turns to reduce short circuit forces [8]. 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. 11.2 Converter Transformers for HVDC There has been a steady increase in High Voltage Direct Current (HVDC) transmission schemes in the world because of many advantages of HVDC transmission as compared to HVAC transmission [9, 10]. The converter transformer is one of the most important and costly components of HVDC transmission system. The converter transformer design has much in common with that of the conventional power transformer except a few special design aspects which are elaborated in this section. 11.2.1 Configurations The standard 12-pulse converter configuration can be obtained using star-star and star-delta connections with one of the following arrangements, viz. 6 single-phase two-winding, 3 single-phase three-winding and 2 three-phase two-winding. The arrangements are shown in figure 11.5. Figure 11.5 Configurations of converter transformers Copyright © 2004 by Marcel Dekker, Inc. Chapter 11418 The weight and size of individual transformer are highest and overall cost (with all transformers considered) is lowest in the three-phase two-winding configuration, whereas the weight and size of individual transformer are lowest and overall cost is highest in the single-phase two-winding configuration. Since the cost of spare transformer in the single-phase two-winding configuration is lowest (that of only one of the six transformers), it is more commonly used. 11.2.2 Insulation design Simplified schematic diagram for bipolar (double) 12-pulse operation is shown in figure 11.6. The windings connected to converter and that connected to AC side are generally termed as valve and AC windings respectively. Since the potentials of the valve winding connections are determined by the combination of conducting valves at any particular instant, the entire valve winding has to be fully insulated. Also, unlike the AC winding, both the terminals of the valve winding experience the full DC voltage of the bridge to which it is connected. Hence, the end insulation is higher resulting into greater radial leakage field at the winding ends. The winding eddy loss due to radial leakage field can be much higher than the conventional transformer, if the conductor dimensions are not chosen properly. Thus, in addition to the normal AC voltage, the valve windings are subjected to a direct voltage depending on their position with respect to ground. Under an AC voltage, potential distribution is in inverse proportion to capacitance or electric stress is inversely proportional to permittivity in a multi-dielectric system. Since the permittivity of oil is about half of solid insulation, the stress in oil is more under the AC voltage condition in the conventional transformers. Since the Figure 11.6 Schematic diagram of bipolar 12-pulse operation Copyright © 2004 by Marcel Dekker, Inc. Special Transformers 419 dielectric strength of the oil is quite less as compared to that of the solid insulation, the insulation design problem reduces mainly to designing of oil ducts as elaborated in Chapter 8. Contrary to AC conditions, under DC voltage conditions the voltage distribution is in direct proportion to resistance or electric stress is directly proportional to resistivity. At lower temperatures the resistivity of solid insulating materials used in transformers is quite high as compared to that of the oil. The ratio of resistivity of a high quality pressboard to that of the oil is about 100 at 20°C, which reduces to as low as 3.3 at 90°C [11]. This is because the fall in the resistivity of the pressboard with temperature is much higher than that of the oil [12]. Such a large variation in the ratio of the two resitivities increases the complexity of insulation design. Thus, under DC conditions at lower temperatures, most of the voltage gets distributed across the solid insulation and stress in it greatly exceeds that in the oil. The oil ducts have practically only AC voltage across them, whereas solid insulations (barriers, washers, supporting and clamping components, etc.) generally have preponderance of DC voltage with a certain amount of superimposed AC voltage. Therefore, the pressboard barriers tend to be more in the converter transformers as compared to the conventional transformers. However, the proportion of solid cannot be higher than a certain value because the composite oil-solid system has to withstand AC voltage tests as well. Let the symbols ε and ρ denote relative permittivity and resistivity. With a voltage V applied across two parallel plates shown in figure 11.7, under AC field conditions the stresses in oil and solid insulation are (11.6) Figure 11.7 Oil-solid insulation system Copyright © 2004 by Marcel Dekker, Inc. Chapter 11420 and under DC field conditions the stresses are (11.7) For non-uniform field conditions involving complex electrode shapes, the techniques described in Chapter 8 should be used to calculate the stresses. Under steady-state DC conditions, space charges get accumulated at the boundary of the oil and solid insulation. When there is a polarity reversal, in which the applied DC voltage changes from +V to -V, an equivalent of the voltage difference 2V gets applied. As the time required for reversing the polarity of the applied voltage is much shorter than the space charge relaxation time [13], the voltage due to space charge is not affected during the time of polarity reversal. Therefore, using equations 11.6 and 11.7 the stresses in the oil and solid insulation under the polarity reversal condition can be given by (11.8) (11.9) Thus, under the polarity reversal condition, the oil gap is stressed more It can be easily seen from the above equations that the smaller the stress across the oil gap before the polarity reversal, the more the stress is across it after the polarity reversal. The voltage distribution under various conditions is shown in figure 11.8. The voltage across the oil gap is much higher during the polarity reversal condition (Case 3) as compared to Case 2 of steady-state DC voltage condition (prior to the polarity reversal). For the oil-solid composite insulation system, a relatively low DC voltage superimposition on AC voltage has very little effect on the partial discharge inception voltage [12–15]; this is due the fact that most of the DC voltage gets dropped across the solid insulation, which has a high DC withstand voltage. If, however, the DC voltage magnitude is within the range of the breakdown DC voltage, the breakdown behaviour of the entire system is determined by the DC voltage. Although the converter transformers are stressed by combined AC and DC voltages during service conditions, it is not considered necessary to test them with superimposed voltages [16]. Conventional power frequency and impulse tests are generally sufficient besides pure DC voltage tests and the polarity reversal test. Copyright © 2004 by Marcel Dekker, Inc. [...]... Kasermann, P Oerlikon rectifier transformers, Bulletin Oerlikon, No 33 6/ 33 7, 1959, pp 134 – 139 9 Arrillaga, J High voltage direct current transmission, Peter Peregrinus Ltd., London, 19 83 10 Kimbark, E.W Direct current transmission, John Wiley and Sons, New York, 1971 11 Tschudi, D.J DC insulation design: paper-oil insulation systems, WICOR Insulation Conference, Rapperswil, Switzerland, September 1996 12 Ganger,... 1, January 1991, pp 1 53 157 22 Grundmark, B Large furnace transformers, ASEA Journal, December 1972, pp 151–156 23 Bonis, P and Coppadoro, F Transformers for arc furnaces, Brown Boveri Review, Vol 60, No 10/11– 73, October/November 19 73, pp 456–467 24 Hochart, B Power transformer handbook, Butterworth and Co Ltd., London, 1987 25 Koppikar, D.A., Kulkarni, S.V., Khaparde, S.A., and Jha, S.K Evaluation... Delivery, Vol PWRD-1, No 3, July 1986, pp 167– 1 73 28 IEEE Standard C57. 135 –2001™, IEEE guide for the application, specification, and testing of phase-shifting transformers 29 Kramer, A and Ruff, J Transformers for phase angle regulation considering the selection of on-load tap changers, IEEE Transactions on Power Delivery, Vol 13, No 2, April 1998, pp 518–525 30 Iravani, M.R and Mathur, R.M Damping... 2, May 1986, pp 76– 83 Copyright © 2004 by Marcel Dekker, Inc Special Transformers 435 31 Iravani, M.R., Dandeno, P.L., Nguyen, K.H., Zhu, D., and Maratukulam, D Applications of static phase shifters in power systems, IEEE Transactions on Power Delivery, Vol 9, No 3, July 1994, pp 1600–1608 32 Del Vecchio, R.M., Poulin, B., Feghali, P.T., Shah, D.M., and Ahuja R Power transformer design principles:... M., Kako, Y., and Kiwaki, H Converter transformer, DC reactor and transductors for 125 kV 37 .5 MW thyristor converter, Hitachi Review, Vol 21, No 4, pp 146–156 15 Hessen, P and Lampe, W Insulating problems in HVDC converter transformers, Direct Current, Vol 2, No 1, February 1971, pp 30 –44 Copyright © 2004 by Marcel Dekker, Inc 434 Chapter 11 16 Wahlstrom, B Voltage tests on transformers and smoothing... Rectifier circuits: theory and design, John Wiley and Sons, Inc., New York, 1965 2 Pelikan, T and Isler, J Rectifier transformers for heavy currents, The Brown Boveri Review, Vol 48, No .3/ 4, March/April 1961, pp 215–228 3 Wells, R Rectifier interphase transformers and current balance, Electrical Review, Vol 207, No 14, October 1980, pp 45–46 4 Coppadoro, F Voltage regulation of transformers for silicon... arrangement (a) and three single-phase tap changers may have to be used Also, the autotransformer and furnace transformer are usually housed in separate tanks thereby increasing the cost and size of the total system The most popular arrangement used for medium and large power furnace applications is the furnace transformer with a booster arrangement as shown in figure 11.9 (c) The booster transformer on... to high current leads in transformers, Proceedings IEE— Science Measurement and Technology, Vol 144, No 1, January 1997, pp 34 – 38 26 O’Kelly, D and Musgrave, G Improvement of power-system transient stability by phase shift insertion, Proceedings IEE, Vol 120, No 2, February 19 73, pp 247–252 27 Patel, B.K., Smith, H.S., Hewes, T.S., and Marsh, W.J Application of phase shifting transformers for Daniel-Mcknight... phase shift angle decides the rating and size of PST Depending on the voltage/power rating, phase shift angle requirements, connected system’s short circuit capability and OLTC performance, two distinct designs of PST are used, viz single-core design and double-core design The less complex single-core design is generally used at lower voltages for small phase shifts and small ratings of PST The figure... currents in the phase shifting transformers under system fault conditions requires more elaborate treatment as compared to that in the conventional transformers The equivalent circuit model and the positivesequence, negative-sequence and zero-sequence networks required for the fault Copyright © 2004 by Marcel Dekker, Inc Special Transformers 433 analysis are given in [32 ] The method is used to calculate . amount of phase shift. The rated design (equivalent) power which decides the size of PST is given by [28 ] S eq =3 {V ph ×2sin( α /2) }×I SL =3V ph I SL ×2sin( α /2) (11.10) where V ph is the line-to-ground. reduced. Copyright © 20 04 by Marcel Dekker, Inc. Chapter 11 426 Figure 11.9 Types of furnace transformer Copyright © 20 04 by Marcel Dekker, Inc. Special Transformers 427 The booster transformer rating. the short circuit withstand design of the converter transformer deserves more attention than the conventional transformer. Quality of processing (drying and impregnation) and integrity of clamping/support