SIEMENS Pakistan Internship Report Mahmood Ali Koral (BU-Tr) Page 1 of 13 Index Topic Page Transformer 3 Workshop 4 Construction a. Core 5 b. Winding 6 i. Layer winding it. Disc Winding c. Tap changer Testing of Transformer a. on load test b. Impedance Test c. Lightening Impulse Test d. Atmosphere Page 2 of 13 (All, 2012) Transformer: Transformer is an electrical machine that transfers electrical energy from one circuit to another by the principle of electromagnetic induction. While transferring electrical energy, its frequency does not change.
When the transformer raises the voltage i. e. hen the output voltage of transformer is higher than input voltage, it is called step up transformer and when its output is lower than input, then it is called step down transformer. The distribution transformer is the step down transformer that reduces the voltage to domestic use. Since, its basic construction requires no moving parts, so it is often called static transformer and it is very rugged machine requiring the minimum amount of repair and maintenance.
Owing to the lack of rotating parts there is no friction or winding losses. Further, the losses are relatively low, so that the efficiency of a transformer is high. Typical transformer efficiencies at full load lie between 96% and 97% and with extremely large capacity of transformers the efficiencies are as high as 99%. Process to Manufacture Transformer 1. Core cutting 2. Core laying 3. Winding Page 3 of 13 4. Semi Assembly 5. Final Assembly 6. Testing According to SIEMENS standard, SIEMENS manufactures distribution transformers ranges from 500kVA to 4MVA for private customers.
SIEMENS manufactures 10kVA, 1 5kVA, 25kVA, 50kVA, 100kVA, 200skVA, 400kVA & 630kVA for WAPDA (water and power Development Authority). For KESC (Karachi Electric Supply Company), SIEMENS anuTactures 2 ec n Dl (DlstrlDutlon Transformer) Production, transformers are manufactured ranges from IOWA to 4MVA. Transformers whose rating is greater than (>250kVA) copper foil is used as LV. In PT (Power Transformer) Production, transformers that are manufactured ranges from 4MVA to 160 MVA. The main part of a transformer is its active part. Active part mainly consists of three parts.
Core in steel frame Page 4 of 13 1 . Core: The core, which provides the magnetic path to channel the flux, consists of thin strips of high-grade steel, called laminations, which are electrically separated by a thin oating of insulating material. The strips can be stacked or wound, with the windings either built integrally around the core or built separately and assembled around the core sections. Core steel can be hot- or cold-rolled, grain-oriented or non-grain oriented, and even laser-scribed for additional performance. Thickness ranges from 0. 3 mm to upwards of 0. 35 mm. The core cross section can be circular or rectangular, with circular cores commonly referred to as cruciform construction. Rectangular cores are used for smaller ratings and as auxiliary transformers used within a power transformer. Rectangular cores use different width of strip steel, while circular cores use a combination of different strip widths to approximate a circular cross-section. The type of steel and arrangement depends on the transformer rating as related to cost factors such as labor and performance.
The material that is used to in core is called electrical steel (Si alloy) and is also known as CRGO (Cold-rolled grain oriented). When voltage is applied to the exciting or primary winding of the transformer, the magnetizing current flows in the primary winding to secondary winding. This current produces the flux in the core. The flow of flux in magnetic circuits is analogous to the flow of current in electrical circuits. When flux flows in the steel core, losses occur in the steel. There Page 5 of 13 are two components of this loss, which are termed “eddy’ and “hysteresis” losses.
To reduce hysteresis loss that occurs due to magnetizing & demagnetizing of the core, can be reduced by adding silicon 3 to 4%. Only that material is used that has low area of hysteresis loss. Eddy currents can be reduced by laminating the core and decreasing thickness of a sheet of core. The thickness of a sheet that is taken in DT DlstrlDutlon IransTormer) ana In Power 0. 3mm to 0. 35mm respectively. Yoke I ransTormer) Is 0. 23mm 0 0. 2/mm ana The core consists of yoke, central leg and limb. Top & bottom sheets are called yoke & the yoke has two holes. The central sheet is called central leg and side sheets are called limbs. . Winding: The windings consist of the current-carrying conductors wound around the sections of the core, and these must be properly insulated, Page 6 of 13 supported, and cooled to withstand operational and test conditions. The terms winding and coi I are used interchangeably in this discussion. Copper and aluminum re the primary materials used as conductors in power-transformer windings. While aluminum is lighter and generally less expensive than copper, a larger cross section of aluminum conductor must be used to carry a current with similar performance as copper.
Copper has higher mechanical strength and is used almost exclusively in all but the smaller size ranges, where aluminum conductors may be perfectly acceptable. In cases where extreme forces are encountered, materials such as silver- bearing copper can be used for even greater strength. There are two types of winding mostly used in transformer. . Layer Winding: Layer windings are among the simplest of windings in that the insulated conductors are wound directly next to each other around the cylinder and spacers.
Several layers can be wound on top of one another, with the layers separated by solid insulation, ducts, or a combination. Several strands can be wound in parallel if the current magnitude so dictates. Variations of this winding are often used for applications such as tap windings used in load-tap-changing (LTC) transformers and for tertiary windings used for, among other things, third-harmonic suppression. Figure shows a ayer winding during assembly that will be used as a regulating winding in an L TC transformer.
The layer winding is mostly used in low rating transformers especially in DT (Distribution Transformer). Two types of layers are used i. e. one is LV (Low Page 7 of 13 Internsnlp Report Voltage) and another is HV (High Voltage). The LV winding is the winding through we get low voltage. In step up transformer, the primary is LV and in step down transformer, the secondary is LV. The LV is may be of paper insulated copper (1 OkVA-200kVA) or copper foil (>200kVA) layered with DDP (Diamond Dotted Paper). HV winding is the winding through we get high voltage.
In step down transformer, the primary is HV and in step up transformer, the secondary is HV. The diameter of LV is greater than HV because electric current in LV is greater than b. Disc Winding: Disc winding is used in high rating transformers such as power transformer. A disc winding can involve a single strand or several strands of insulated conductors wound in a series of parallel discs of horizontal orientation, with the discs connected at either the inside or outside as a crossover point. Each disc comprises multiple turns ound over other turns, with the crossovers alternating between inside and outside.
Page 8 of 13 Two more kinds of windings are used in SIEMENS that are disc layer winding and disc tape winding that are used in PT-Production. 2. Tap Changer: For many decades power transformers equipped with load tap changers (LTC) have been the main components of electrical networks and industry. The L TC allows voltage regulation and/or phase shifting by varying the transformer ratio under load without interruption. From the beginning of L TC development, two switching principles have been used for the load-transfer operation, the high-speed-resistance ype and the reactance type.
Over the decades, both principles have been developed into reliable transformer components available in a broad range of current and voltage applications to cover the needs of todays network and industrial-process transformers as well as ensuring optimum system and process control. In distribution transformer, off load tap changer is used. It is called off load tap changer because to change its tapping, you have to denergize the transformer. In Power transformer, on load tap changer is used that can be adjusted without unplugging the generator.
According to ANSI and IEEE standards , all tests on power transformers fall into one of three categories: (1) routine tests, (2) design tests, and (3) other tests. The manufacturer may perform additional testing to ensure the Page Y 0T 1 quality of the transformer at various stages of the production cycle. The main difficulties encountered in testing of large transformers by direct loading are (i) wastage of large amount of energy(ii) a stupendous (impossible for large transformer) task of arranging a load large enough for direct loading.
The performances characteristics of a transformer can be conveniently computed through from the nowledge of its equivalent circuit parameter which, in turn may be determined by conducting simple tests called the open-circuit, no load test and short circuit or impedance test involving very little power consumption (power needed to supply the losses incurred). The sequence in which the various tests are performed is also specified.
An example of test sequence is as follows: Tests before tanking 0 Preliminary ratio, polarity, and connection of the transformer windings 0 Core insulation tests 0 Ratio and polarity tests of bushing-current transformers Tests after tanking (final tests) 0 Final ratio, polarity, and phase rotation Insulation capacitance and dissipation factor 0 Insulation resistance 0 Control-wiring tests 0 Lightning-voltage impulse tests 0 Applied-voltage tests 0 Induced-voltage tests and partial-discharge measurements 0 No-load-loss and excitation-current measurements 0 Winding-resistance measurements Load-loss and impedance-voltage measurement Page 10 of 13 On Load Test or Open-circuit Test: The purpose of this test is to determine the core loss or iron or excitation loss. In this test, one of the windings (usually high voltage winding) is kept open circuited and the rated voltage at rated frequency is applied to the other winding. No doubt, the core loss will be same whether the measurement are made on the winding or HV winding so long as the rated voltage of that winding is applied to it but in case the measurement are made on HV winding, the voltage required be appllea would oTten De Inconveniently large wnlle IU would De conveniently small.
Short Circuit Test (Impedance Test): The purpose of this test is to determine the full copper loss and equivalent resistance and equivalent reactance referred to metering side. In this test, the terminals of secondary winding (usually of low voltage winding) are short circuited by thick wire or strip or through an ammeter (which may serve the additional purpose of indicating secondary rated load current) and variable low voltage is applied to the primary through potential divider. Clamp meter is used to measure the current in the wire and multimeter is used to measure the applied voltage to the circuit. Page 11 of 13 Lightening-Impulse Test: Impulse tests are performed on all power transformers.
In addition to verification of dielectric strength of the insulation system, impulse tests are excellent indicators of the quality of insulation, workmanship, and processing of the paper and insulating- il system. For this purpose high voltage is applied. The sequence of tests, test connections, and applicable standards is described below. Lightning-impulse voltage tests simulate traveling waves due to lightning strikes and line flashovers. The full-wave lightning-impulse voltage waveshape is one where the voltage reaches crest magnitude in. Below is picture of the machine which is used in Impulse Test. page 12 of 13 Atmosphere As a multinational company, the working atmosphere for workers is better than other companies in Pakistan.
The floor is always kept cleaned as no accident occurs. Seeing workers working with modern equipment were really a good experience & they make their Job easy and efficient. I found workers of SIEMENS very co-operative, polite and always ready to help & tell. Due to the working of overhead cranes, core cutting machines and other machines, there is always a lot noise in workshop and noise creates irritation in the nature of man and reduces the ability of hearing. Seeing worker without head phone was really unpleasant. Supervisors were also very co-operative and helpful. Whenever I had any query, they were always ready to tell and help. page 13 of 13