transformer engineering design and practice 1_phần 3

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 1 Copyright © 2004 by Marcel Dekker, Inc. 2 Chapter 1 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. 4 Chapter 1 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. ... - tailieumienphi.vn