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Chapter 12. Load Tap Changers - 12 LOAD TAP CHANGERS...

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481 12. LOAD TAP CHANGERS Summary Load tap changers are a major component of many transformers. They are used to change the voltage of the primary or secondary side of a transformer to compensate for voltage variations in the power system mainly caused by variable loading. We discuss genetically the various types of load tap changers and some of the important ways they are connected to the primary or secondary windings as well as the advantages and disadvantages of these connection schemes. We also address some of the maintanence issues which are of interest to the user. 12.1 INTRODUCTION The flicker of the house lights while having dinner is a sign of a load tap changer (LTC) tap change. When most people get home from work, they start using electricity for cooking and lighting. This increases the electrical load on the distribution network, which causes the voltage to sag below its nominal value. The latter signals the LTC to change taps to adjust the voltage back to its nominal value with some tolerance. There are many applications of transformers in modern electric power systems: from generator step-up to system interconnection to distribution to arc-furnace to HVDC converters to mention just a few. The role of a transformer is to convert the electrical energy from one voltage level to another. As power systems become larger and more complex, power transformers play a major role in how efficient and stable the system is. For each transformer installed in a network, there is an ideal (optimal) voltage ratio for an optimal operation of the system. Unfortunately, this optimal voltage ratio varies depending on the operating conditions of the total network. Early in the history of electric power systems, it became evident that for power systems to operate satisfactorily, transformer voltage ratios needed to be adjustable without interrupting the flow of energy. This is the role of a load tap changer. © 2002 by CRC Press
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LOAD TAP CHANGERS 482 12.2 GENERAL DESCRIPTION OF LTC A load tap changer is a device that connects different taps of tapped windings of transformers without interrupting the load. It must be capable of switching from one tap position to another without at any time interrupting the flow of the current to the load and without at any time creating a short-circuit between any two taps of the transformer winding. Tap changing transformers are used to control the voltage or the phase angle or both in a regulated circuit. A LTC is made of 4 elements: 1) A selector switch which allows the selection of the active tap 2) A change-over switch, referred as a reversing switch when it reverses the polarity of the tapped winding, used to double the number of positions available 3) A transition mechanism, including an arcing or diverter switch, which effects the transition from one tap to the other 4) A driving mechanism which includes a motor and gear box and controls to drive the system There are 2 generic types of tap changers: In-tank
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