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Lecture 11Electro Mechanical System1 Delta-Delta Connection Three single-phase transformers connected delta-delta H 1 terminal of each transformer is connected to the H 2 of the next transformer. X 1 terminal of each transformer is connected to the X 2 of the next transformer Primary and secondary voltages and currents are in- phase.
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Lecture 11Electro Mechanical System2 Delta-Wye Connection Three single-phase transformers connected delta-wye. H 1 terminal of each transformer is connected to the H 2 of the next transformer the X 2 terminals of all transformers are connected together to form a neutral terminal. Delta-wye connection produces a 30° phase shift between the primary and secondary voltages and currents.
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Lecture 11Electro Mechanical System3 Wye-Delta Connection Connection are identical to those in delta-wye The primary and secondary connections are interchanged. The H 2 terminals of all transformers are connected together to form a neutral terminal. The X 1 terminal of each transformer is connected to the X 2 terminal of the next transformer. Connection results in a 30° phase shift between the primary and secondary voltages and currents. Wye-wye Connection In wye-wye, special precautions have to he taken to prevent severe distortion of the line-to-neutral voltages, when load becomes unbalance. One way to prevent is to connect the neutral of the primary to the neutral of the source, usually by ground.
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Lecture 11Electro Mechanical System4 Wye-wye Connection Another way is to provide each transformer with a third winding, called tertiary winding. The tertiary windings of the three transformers are connected in delta. They often provide the substation service voltage where the transformers are installed. There is no phase shift between the line voltages of a wye-wye
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Lecture 11Electro Mechanical System5 Open-Delta Connection It is possible to transform the voltage of a 3-phase system using only 2 transformers. Identical to a delta-delta connection, less one transformer. Limited use due to a 86.6% reduction in the installed power capability. Two 50 kVA transformers could carry 100 kVA, but limited to 86.6 kVA in an open-delta connection. Used in emergency conditions. Used in metering of power system voltages.
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Lecture 11Electro Mechanical System6 Three-phase Transformers One 3-phase transformer may replace a transformer bank of three single-phase units. The magnetic core has three flux-carrying legs. Two yokes, bottom and top, couple the flux of the three legs. The sum of the three fluxes equals zero. Each leg contains the primary and secondary windings of one of the phases. The windings are connected delta or wye, internal to the transformer tank. Only six terminals are brought out of the tank.
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Lecture 11Electro Mechanical System7 Step-up & step-down autotransformers When a small change in voltage is required in 3-phase, it is economical to use 3 single phase transformers to create wye-connected autotransformer
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Lecture 11Electro Mechanical System8 Assignment 4 Page 259, Problems: 12-2, 12-4, 12-6, 12-8, 12-10 Due Date: Tuesday 1 st March, 2011 Home Work & Assignment Home Work Page 245, Example 12.1 Page 247, Example 12.2 Page 249, Example 12.3 Page 251, Example 12.4 Page 259, Example 12.7
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