Download Understanding Three-Phase Electricity Supplies and Systems: Delta vs. Star Connection and more Study notes Basic Electronics in PDF only on Docsity! Three Phase Electricity Supplies and Systems The generation and supply of electricity within the U.K is achieved through the use of a 3-phase system. This consists of 3 separate phase conductors along with a neutral conductor transmitting supplies to a given destination. Simple three phase transmission Consider a single phase generator to generate a single phase electricity supply. It would consist of a coil of wire which is rotated within a magnetic field. It’s output would be as shown below. If we took three of these generators (A,B & C) and positioned them equidistantly (120o apart) around a rotating magnet we would generate three separate a.c. waveforms each with a phase displacement of 120o from each other. Such an arrangement would provide us with a 3 phase supply but we would need 6 conductors in order for it to function. However if we take each coil and interconnect it with the other coils in particular ways we can achieve a three phase supply in either a 3 wire (Delta) system, or a 4 wire (Star) system. Delta Connection (3-phase, 3 wire system) Using this method of connection each of the three coil windings are wound in such a way so they each have a start and a finish to their winding. Then the finish of winding 1 is connected to the start of winding 2. The finish of winding 2 is connected to the start of winding 3. The start of winding 3 is connected to the start of winding 1. Delta systems Voltage Relationships VL =VP Therefore VP = VL Current Relationships IL = 1.732 x IP & IP = IL 1.732 Star Systems Voltage Relationships VL = 1.732 x VP & VP = VL 1.732 Current Relationships IL = IP Therefore IP = IL You must remember these relationships by committing them to memory. Progress A star connected system has phase voltages of 300v and phase currents of 12A. Calculate line voltages and currents. If the system above was a delta connected system, calculate the line voltages and currents. Balanced and unbalanced systems Star and delta systems can also be connected as loads. Many three phase motors will be connected in delta since only three wires are required to make the connection. Star connections are often used as a means of connecting single phase loads to a 3 phase supply, such as in the case of a street of domestic dwellings. 3 Note! Rather than use 1.732 many texts will use the square root of 3 which equates to 1.732. A delta connected load will have identical phase impedances. The impedance of each phase will therefore be identical both in terms of it’s magnitude AND phase angle. When this is the case the current that flows in each phase will be identical and the sum of the currents will be zero. The system is said to be balanced and there is no need for a 4th wire (neutral conductor). It is also possible to have a star connected load where the impedance of each phase is identical in terms of it’s magnitude and phase angle. As with the delta system this will lead to identical phase currents and no excess current to flow back to neutral. Again, the system is said to be balanced and there is no need for a 4th wire (neutral conductor). It is more usual however, to connect dissimilar loads to each phase of a star system. This will result in differences in phase currents which in turn will create some ‘leftover’ current which will flow back along the 4th wire (neutral) to the star point of the supply origin. Such a system is known as an unbalanced system and usually arises as a result of connecting separate single phase loads to each phase of a three phase star connection. Progress What is meant by a 3 phase balanced system? Is there any need for a 4th wire (neutral) in such a system? How does an unbalanced system differ? What function does the 4th wire serve in an unbalanced system.
