1 1 Transformer Fundamentals 1.1 Perspective A transformer is a static device that transfers electrical energy from one circuit to another by electromagnetic induction without the change in frequency. The transformer, which can link circuits with different voltages, has been instrumental in enabling universal use of the alternating current system for transmission and distribution of electrical energy. Various components of power system, viz. generators, transmission lines, distribution networks and finally the loads, can be operated at their most suited voltage levels. As the transmission voltages are increased to higher levels in some part of the power system, transformers again play a key role in interconnection of systems at different voltage levels. Transformers occupy prominent positions in the power system, being the vital links between generating stations and points of utilization. The transformer is an electromagnetic conversion device in which electrical energy received by primary winding is first converted into magnetic energy which is reconverted back into a useful electrical energy in other circuits (secondary winding, tertiary winding, etc.). Thus, the primary and secondary windings are not connected electrically, but coupled magnetically. A transformer is termed as either a step-up or step-down transformer depending upon whether the secondary voltage is higher or lower than the primary voltage, respectively. Transformers can be used to either step-up or step-down voltage depending upon the need and application; hence their windings are referred as high-voltage/low-voltage or high-tension/low-tension windings in place of primary/secondary windings. Magnetic circuit: Electrical energy transfer between two circuits takes place through a transformer without the use of moving parts; the transformer therefore has higher efficiency and low maintenance cost as compared to rotating electrical Copyright © 2004 by Marcel Dekker, Inc. Chapter 12 machines. There are continuous developments and introductions of better grades of core material. The important stages of core material development can be summarized as: non-oriented silicon steel, hot rolled grain oriented silicon steel, cold rolled grain oriented (CRGO) silicon steel, Hi-B, laser scribed and mechanically scribed. The last three materials are improved versions of CRGO. Saturation flux density has remained more or less constant around 2.0 Tesla for CRGO; but there is a continuous improvement in watts/kg and volt-amperes/kg characteristics in the rolling direction. The core material developments are spearheaded by big steel manufacturers, and the transformer designers can optimize the performance of core by using efficient design and manufacturing technologies. The core building technology has improved from the non-mitred to mitred and then to the step-lap construction. A trend of reduction of transformer core losses in the last few years is the result of a considerable increase in energy costs. The better grades of core steel not only reduce the core loss but they also help in reducing the noise level by few decibels. Use of amorphous steel for transformer cores results in substantial core loss reduction (loss is about one-third that of CRGO silicon steel). Since the manufacturing technology of handling this brittle material is difficult, its use in transformers is not widespread. Windings: The rectangular paper-covered copper conductor is the most commonly used conductor for the windings of medium and large power transformers. These conductors can be individual strip conductors, bunched conductors or continuously transposed cable (CTC) conductors. In low voltage side of a distribution transformer, where much fewer turns are involved, the use of copper or aluminum foils may find preference. To enhance the short circuit withstand capability, the work hardened copper is commonly used instead of soft annealed copper, particularly for higher rating transformers. In the case of a generator transformer having high current rating, the CTC conductor is mostly used which gives better space factor and reduced eddy losses in windings. When the CTC conductor is used in transformers, it is usually of epoxy bonded type to enhance its short circuit strength. Another variety of copper conductor or aluminum conductor is with the thermally upgraded insulating paper, which is suitable for hot-spot temperature of about 110°C. It is possible to meet the special overloading conditions with the help of this insulating paper. Moreover, the aging of winding insulation material will be slowed down comparatively. For better mechanical properties, the epoxy diamond dot paper can be used as an interlayer insulation for a multi-layer winding. High temperature superconductors may find their application in power transformers which are expected to be available commercially within next few years. Their success shall depend on economic viability, ease of manufacture and reliability considerations. Insulation and cooling: Pre-compressed pressboard is used in windings as opposed to the softer materials used in earlier days. The major insulation (between windings, between winding and yoke, etc.) consists of a number of oil ducts Copyright © 2004 by Marcel Dekker, Inc. Transformer Fundamentals 3 formed by suitably spaced insulating cylinders/barriers. Well profiled angle rings, angle caps and other special insulation components are also used. Mineral oil has traditionally been the most commonly used electrical insulating medium and coolant in transformers. Studies have proved that oil-barrier insulation system can be used at the rated voltages greater than 1000 kV. A high dielectric strength of oil-impregnated paper and pressboard is the main reason for using oil as the most important constituent of the transformer insulation system. Manufacturers have used silicon-based liquid for insulation and cooling. Due to non-toxic dielectric and self-extinguishing properties, it is selected as a replacement of Askarel. High cost of silicon is an inhibiting factor for its widespread use. Super-biodegradable vegetable seed based oils are also available for use in environmentally sensitive locations. There is considerable advancement in the technology of gas immersed transformers in recent years. SF6 gas has excellent dielectric strength and is non- flammable. Hence, SF6 transformers find their application in the areas where fire- hazard prevention is of paramount importance. Due to lower specific gravity of SF6 gas, the gas insulated transformer is usually lighter than the oil insulated transformer. The dielectric strength of SF6 gas is a function of the operating pressure; the higher the pressure, the higher the dielectric strength. However, the heat capacity and thermal time constant of SF6 gas are smaller than that of oil, resulting in reduced overload capacity of SF6 transformers as compared to oil- immersed transformers. Environmental concerns, sealing problems, lower cooling capability and present high cost of manufacture are the challenges which have to be overcome for the widespread use of SF6 cooled transformers. Dry-type resin cast and resin impregnated transformers use class F or C insulation. High cost of resins and lower heat dissipation capability limit the use of these transformers to small ratings. The dry-type transformers are primarily used for the indoor application in order to minimize fire hazards. Nomex paper insulation, which has temperature withstand capacity of 220°C, is widely used for dry-type transformers. The initial cost of a dry-type transformer may be 60 to 70% higher than that of an oil-cooled transformer at current prices, but its overall cost at the present level of energy rate can be very much comparable to that of the oil- cooled transformer. Design: With the rapid development of digital computers, the designers are freed from the drudgery of routine calculations. Computers are widely used for optimization of transformer design. Within a matter of a few minutes, today’s computers can work out a number of designs (by varying flux density, core diameter, current density, etc.) and come up with an optimum design. The real benefit due to computers is in the area of analysis. Using commercial 2-D/3-D field computation software, any kind of engineering analysis (electrostatic, electromagnetic, structural, thermal, etc.) can be performed for optimization and reliability enhancement of transformers. Copyright © 2004 by Marcel Dekker, Inc. Chapter 14 Manufacturing: In manufacturing technology, superior techniques listed below are used to reduce manufacturing time and at the same time to improve the product quality: - High degree of automation for slitting/cutting operations to achieve better dimensional accuracy for the core laminations - Step-lap joint for core construction to achieve a lower core loss and noise level; top yoke is assembled after lowering windings and insulation at the assembly stage - Automated winding machines for standard distribution transformers - Vapour phase drying for effective and fast drying (moisture removal) and cleaning - Low frequency heating for the drying process of distribution transformers - Pressurized chambers for windings and insulating parts to protect against pollution and dirt - Vertical machines for winding large capacity transformer coils - Isostatic clamping for accurate sizing of windings - High frequency brazing for joints in the windings and connections Accessories: Bushings and tap changer (off-circuit and on-load) are the most important accessories of a transformer. The technology of bushing manufacture has advanced from the oil impregnated paper (OIP) type to resin impregnated paper (RIP) type, both of which use porcelain insulators. The silicon rubber bushings are also available for oil-to-air applications. Due to high elasticity and strength of the silicon rubber material, the strength of these bushings against mechanical stresses and shocks is higher. The oil-to-SF6 bushings are used in GIS (gas insulated substation) applications. The service reliability of on load tap changers is of vital importance since the continuity of the transformer depends on the performance of tap changer for the entire (expected) life span of 30 to 40 years. It is well known that the tap changer failure is one of the principal causes of failure of transformers. Tap changers, particularly on-load tap changers (OLTC), must be inspected at regular intervals to maintain a high level of operating reliability. Particular attention must be given for inspecting the diverter switch unit, oil, shafts and motor drive unit. The majority of failures reported in service are due to mechanical problems related to the drive system, for which improvements in design may be necessary. For service reliability of OLTCs, several monitoring methods have been proposed, which include measurement of contact resistance, monitoring of drive motor torque/ current, acoustic measurements, dissolved gas analysis and temperature rise measurements. Diagnostic techniques: Several on-line and off-line diagnostic tools are available for monitoring of transformers to provide information about their operating conditions. Cost of these tools should be lower and their performance reliability Copyright © 2004 by Marcel Dekker, Inc. Transformer Fundamentals 5 should be higher for their widespread use. The field experience in some of the monitoring techniques is very much limited. A close cooperation between manufacturers and utilities is necessary for developing good monitoring and diagnostic systems for transformers. Transformer technology is developing at a tremendous rate. The computerized methods are replacing the manual working in the design. Continuous improvements in material and manufacturing technologies along with the use of advanced computational tools have contributed in making transformers more efficient, compact and reliable. The modern information technology, advanced diagnostic tools and several emerging trends in transformer applications are expected to fulfill a number of existing and future requirements of utilities and end-users of transformers. 1.2 Applications and Types of Transformers Before invention of transformers, in initial days of electrical industry, power was distributed as direct current at low voltage. The voltage drop in lines limited the use of electricity to only urban areas where consumers were served with distribution circuits of small length. All the electrical equipment had to be designed for the same voltage. Development of the first transformer around 1885 dramatically changed transmission and distribution systems. The alternating current (AC) power generated at a low voltage could be stepped up for the transmission purpose to higher voltage and lower current, reducing voltage drops and transmission losses. Use of transformers made it possible to transmit the power economically hundreds of kilometers away from the generating station. Step-down transformers then reduced the voltage at the receiving stations for distribution of power at various standardized voltage levels for its use by the consumers. Transformers have made AC systems quite flexible because the various parts and equipment of the power system can be operated at economical voltage levels by use of transformers with suitable voltage ratio. A single-line diagram of a typical power system is shown in figure 1.1. The voltage levels mentioned in the figure are different in different countries depending upon their system design. Transformers can be broadly classified, depending upon their application as given below. a. Generator transformers: Power generated at a generating station (usually at a voltage in the range of 11 to 25 kV) is stepped up by a generator transformer to a higher voltage (220, 345, 400 or 765 kV) for transmission. The generator transformer is one of the most important and critical components of the power system. It usually has a fairly uniform load. Generator transformers are designed with higher losses since the cost of supplying losses is cheapest at the generating station. Lower noise level is usually not essential as other equipment in the generating station may be much noisier than the transformer. Generator transformers are usually provided with off-circuit tap changer with a Copyright © 2004 by Marcel Dekker, Inc. Chapter 16 Figure 1.1 Different types of transformers in a typical power system Copyright © 2004 by Marcel Dekker, Inc. Transformer Fundamentals 7 small variation in voltage (e.g., ±5%) because the voltage can always be controlled by field of the generator. Generator transformers with OLTC are also used for reactive power control of the system. They may be provided with a compact unit cooler arrangement for want of space in the generating stations (transformers with unit coolers have only one rating with oil forced and air forced cooling arrangement). Alternatively, they may also have oil to water heat exchangers for the same reason. It may be economical to design the tap winding as a part of main HV winding and not as a separate winding. This may be permissible since axial short circuit forces are lower due to a small tapping range. Special care has to be taken while designing high current LV lead termination to avoid any hot- spot in the conducting metallic structural parts in its vicinity. The epoxy bonded CTC conductor is commonly used for LV winding to minimize eddy losses and provide greater short circuit strength. Severe overexcitation conditions are taken into consideration while designing generator transformers. b. Unit auxiliary transformers: These are step-down transformers with primary connected to generator output directly. The secondary voltage is of the order of 6.9 kV for supplying to various auxiliary equipment in the generating station. c. Station transformers: These transformers are required to supply auxiliary equipment during setting up of the generating station and subsequently during each start-up operation. The rating of these transformers is small, and their primary is connected to a high voltage transmission line. This may result in a smaller conductor size for HV winding, necessitating special measures for increasing the short circuit strength. The split secondary winding arrangement is often employed to have economical circuit breaker ratings. d. Interconnecting transformers: These are normally autotransformers used to interconnect two grids/systems operating at two different system voltages (say, 400 and 220 kV or 345 and 138 kV). They are normally located in the transmission system between the generator transformer and receiving end transformer, and in this case they reduce the transmission voltage (400 or 345 kV) to the sub-transmission level (220 or 138 kV). In autotransformers, there is no electrical isolation between primary and secondary windings; some volt-amperes are conductively transformed and remaining are inductively transformed. Autotransformer design becomes more economical as the ratio of secondary voltage to primary voltage approaches towards unity. These are characterized by a wide tapping range and an additional tertiary winding which may be loaded or unloaded. Unloaded tertiary acts as a stabilizing winding by providing a path for the third harmonic currents. Synchronous condensers or shunt reactors are connected to the tertiary winding, if required, for reactive power compensation. In the case of an unloaded tertiary, adequate conductor area and proper supporting arrangement are provided for withstanding short circuit forces under asymmetrical fault conditions. Copyright © 2004 by Marcel Dekker, Inc. Chapter 18 e. Receiving station transformers: These are basically step-down transformers reducing transmission/sub-transmission voltage to primary feeder level (e.g., 33 kV). Some of these may be directly supplying an industrial plant. Loads on these transformers vary over wider limits, and their losses are expensive. The farther the location of transformers from the generating station, the higher the cost of supplying the losses. Automatic tap changing on load is usually necessary, and tapping range is higher to account for wide variation in the voltage. A lower noise level is desirable if they are close to residential areas. f. Distribution transformers: Using distribution transformers, the primary feeder voltage is reduced to actual utilization voltage (~415 or 460 V) for domestic/ industrial use. A great variety of transformers fall into this category due to many different arrangements and connections. Load on these transformers varies widely, and they are often overloaded. A lower value of no-load loss is desirable to improve all-day efficiency. Hence, the no-load loss is usually capitalized with a high rate at the tendering stage. Since very little supervision is possible, users expect the least maintenance on these transformers. The cost of supplying losses and reactive power is highest for these transformers. Classification of transformers as above is based on their location and broad function in the power system. Transformers can be further classified as per their specific application as given below. In this chapter, only main features are highlighted; details of some of them are discussed in the subsequent chapters. g. Phase shifting transformers: These are used to control power flow over transmission lines by varying the phase angle between input and output voltages of the transformer. Through a proper tap change, the output voltage can be made to either lead or lag the input voltage. The amount of phase shift required directly affects the rating and size of the transformer. Presently, there are two types of design: single-core and two-core design. Single-core design is used for small phase shifts and lower MVA/voltage ratings. Two-core design is normally used for bulk power transfer with large ratings of phase shifting transformers. It consists of two transformers, one associated with the line terminals and other with the tap changer. h. Earthing or grounding transformers: These are used to get a neutral point that facilitates grounding and detection of earth faults in an ungrounded part of a network (e.g., the delta connected systems). The windings are usually connected in the zigzag manner, which helps in eliminating third harmonic voltages in the lines. These transformers have the advantage that they are not affected by a DC magnetization. i. Transformers for rectifier and inverter circuits: These are otherwise normal transformers except for the special design and manufacturing features to take into account the harmonic effects. Due to extra harmonic losses, operating flux density Copyright © 2004 by Marcel Dekker, Inc. Transformer Fundamentals 9 in core is kept lower (around 1.6 Tesla) and also conductor dimensions are smaller for these transformers. A proper de-rating factor is applied depending upon the magnitudes of various harmonic components. A designer has to adequately check the electromagnetic and thermal aspects of design. For transformers used with HVDC converters, insulation design is the most challenging design aspect. The insulation has to be designed for combined AC-DC voltage stresses. j. Furnace duty transformers: These are used to feed the arc or induction furnaces. They are characterized by a low secondary voltage (80 to 1000 V) and high current (10 to 60 kA) depending upon the MVA rating. Non-magnetic steel is invariably used for the LV lead termination and tank in the vicinity of LV leads to eliminate hot spots and minimize stray losses. High current bus-bars are interleaved to reduce the leakage reactance. For very high current cases, the LV terminals are in the form of U-shaped copper tubes of certain inside and outside diameters so that they can be cooled by oil/water circulation from inside. In many cases, a booster transformer is used along with the main transformer to reduce the rating of tap-changers. k. Freight loco transformers: These are mounted on the locomotives within the engine compartment itself. The primary voltage collected from an overhead line is stepped down to an appropriate level by these transformers for feeding to the rectifiers, whose output DC voltage drives the locomotives. The structural design should be such that it can withstand vibrations. Analysis of natural frequencies of vibration is done to eliminate possibility of resonance. l. Hermetically sealed transformers: This construction does not permit any outside atmospheric air to get into the tank. It is completely sealed without any breathing arrangement, obviating need of periodic filtration and other normal maintenance. These transformers are filled with mineral oil or synthetic liquid as a cooling/dielectric medium and sealed completely by having an inert gas, like nitrogen, between the coolant and top tank plate. The tank is of welded cover construction, eliminating the joint and related leakage problems. Here, the expansion of oil is absorbed by the inert gas layer. The tank design should be suitable for pressure buildup at elevated temperatures. The cooling is not effective at the surface of oil, which is at the highest temperature. In another type of sealed construction, these disadvantages are overcome by deletion of the gas layer. The expansion of oil is absorbed by the deformation of the cooling system, which can be an integral part of the tank structure. m. Outdoor and indoor transformers: Most of the transformers are of outdoor duty type, which have to be designed for withstanding atmospheric pollutants. The creepage distance of bushing insulator gets decided according to the pollution level. The higher the pollution level, the greater the creepage distance required from the live terminal to ground. Contrary to the outdoor transformers, Copyright © 2004 by Marcel Dekker, Inc. Chapter 110 an indoor transformer is designed for installation under a weatherproof roof and/ or in a properly ventilated room. Standards define the minimum ventilation required for an effective cooling. Adequate clearances are kept between the walls and transformer to eliminate the possibility of higher noise level due to reverberations. There are many more types of transformers having applications in electronics, electric heaters, traction, etc. Some applications have significant impact on the design of transformers. The duty (load) of transformers can be very onerous. For example, current density in transformers with frequent motor starting duty has to be lower to take care of high starting current of motors, which can be of the order of 6 to 8 times the full load current. Shunt and series reactors are very important components of the power system. Design of reactors, which have only one winding, is similar to transformers in many aspects. Their special features are given below. n. Shunt Reactors: These are used to compensate the capacitive VARs generated during low loads and switching operations in extra high voltage transmission networks, thereby maintaining the voltage profile of a transmission line within desirable limits. These are installed at a number of places along the length of the line. They can be either permanently connected or switched type. Use of shunt reactors under normal operating conditions may result in poor voltage levels and increased losses. Hence, the switched-in types are better since they are connected only when the voltage levels are required to be controlled. When connected to the tertiary windings of a large transformer, they become cost-effective. Voltage drop in high series reactance between HV and tertiary windings must be accounted for when deciding the voltage rating of tertiary connected shunt reactors. Shunt reactors can be of core-less (air-core) or gapped-core (magnetic circuit with non- magnetic gaps) design. The flux density in the air-core reactor has to be smaller as the flux path is not well constrained. Eddy losses in the winding and stray losses in the structural conducting parts are higher in this type of reactor. In contrast, the gapped-core reactor is more compact due to higher permissible flux density. The gap length can be suitably designed to get a desired reactance value. Shunt reactors are usually designed to have a constant impedance characteristics up to 1.5 times the rated voltage to minimize the harmonic current generation under over-voltage conditions. o. Series Reactors: These reactors are connected in series with generators, feeders and transmission lines for limiting fault currents under short circuits. Series reactors should have linear magnetic characteristics under fault conditions. They are designed to withstand mechanical and thermal effects of short circuits. Series reactors used in transmission lines have a fully insulated winding since both its ends should be able to withstand the lightning impulse voltages. The value of series reactance has to be judiciously selected because a higher value reduces the power transfer capability of the line. The smoothing reactors used in HVDC Copyright © 2004 by Marcel Dekker, Inc. [...]... core losses and noise This example shows that a transformer designer has to select the operating peak flux density in the core depending on the overfluxing conditions (simultaneous overvoltage and under-frequency) specified by the user Example 1.2 Tests on 31 .5 MVA, 132 /33 kV star/delta 3- phase transformer gave following results (loss values given are for three phases): - Open circuit test: 33 kV, 5.5... The primary leakage inductance is (1 .30 ) Differential reluctance offered to the path of leakage flux is (1 .31 ) Equations 1 .30 and 1 .31 give (1 .32 ) Similarly, the leakage inductance of secondary winding is (1 .33 ) Let us derive the expression for mutual inductance M Using equation 1.6, (1 .34 ) where, Copyright © 2004 by Marcel Dekker, Inc Transformer Fundamentals 19 (1 .35 ) M21 represents flux linkages in.. .Transformer Fundamentals 11 transmission system, connected between the converter and DC line, smoothen the DC voltage ripple 1 .3 Principles and Equivalent Circuit of a Transformer 1 .3. 1 Ideal transformer A transformer works on the principle of electromagnetic induction, according to which a voltage is induced in a coil if it links a changing flux Figure 1.2 shows a single-phase transformer. .. resulting in damage to the transformer 1 .3. 2 Practical transformer Analysis presented for the ideal transformer is merely to explain the fundamentals of transformer action; such a transformer never exists and the equivalent circuit of a real transformer shown in figure 1.4 is now developed Whenever a magnetic material undergoes a cyclic magnetization, two types of losses, eddy and hysteresis losses, occur... be paralleled The transformers of groups 1 and 2 can only be paralleled with transformers of their own group However, the transformers of groups 3 and 4 can be paralleled by reversing the phase sequence of one of them For example, a transformer with Yd1 1 connection (group 4) can be paralleled with that having Dy1 connection (group 3) by reversing the phase sequence of both primary and secondary terminals... terminals of the Dy1 transformer References 1 2 3 4 Say, M.G The performance and design of alternating current machines, 2nd edition, Sir Isaac Pitman and Sons, London, 1955 Toro, V.D Principles of electrical engineering, 2nd edition, Prentice Hall, New Delhi, 1977 Stevenson, W.D Elements of power system analysis, 4th edition, McGrawHill, Tokyo, 1982, pp 138 –162 MIT Press, Magnetic circuits and transformers,... value of the transformer For small distribution transformers (e.g., 5 MVA), the value of impedance is around 4 to 7% and for power transformers it can be anywhere in the range of 8 to 20% depending upon the regulation and system protection requirements The lower the percentage impedance, the lower the voltage drop However, the required ratings of circuit breakers will be higher 1 .3. 3 Mutual and leakage... Group 3: -30 ° phase displacement (Yd1, Dy1, Yz1) Group 4: +30 ° phase displacement (Yd11, Dy11, Yz11) In above notations, letters y (or Y), d (or D) and z represent star, delta and zigzag connections respectively In order to have zero relative phase displacement of secondary side line voltages, the transformers belonging to the same group can be paralleled For example, two transformers with Yd1 and Dy1... base quantities, (1. 63) where represents the per-unit resistance drop and represents the per-unit leakage reactance drop For a leading power factor load (I2 leads V2 by an angle θ2), (1.64) The square term is usually small and may be neglected, simplifying equations 1. 63 and 1.64 as (1.65) Copyright © 2004 by Marcel Dekker, Inc Transformer Fundamentals 27 The efficiency of a transformer, like any... The rating of transformers is expressed in volt-amperes and not in watts because heating (temperature) determines the life of the transformers Hence, the rated output is limited by the losses, which depend on the voltage (no-load loss) and the current (load loss), and are almost unaffected by the load power factor The amount of heat depends on the r.m.s values of current and voltage and not on the . leakage inductance is (1. 30 ) Differential reluctance offered to the path of leakage flux is (1. 31 ) Equations 1. 30 and 1. 31 give (1. 32 ) Similarly, the leakage inductance of secondary winding is (1. 33 ) Let us. primary winding 1 is (1. 19) Similarly, any impedance Z 1 in the primary circuit can be referred to the secondary side 2 as (1. 20) It can be summarized from equations 1. 15, 1. 16, 1. 19 and 1. 20 that. core losses), it can be summarized as, (1. 15) and V 1 I 1 =V 2 I 2 (1. 16) Schematic representation of the transformer in figure 1. 2 is shown in figure 1. 3. The polarities of voltages depend upon