1. 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.

2. 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.

3. 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.

4. 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

5. 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.

6. 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.

7. 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

8. 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