medium voltage neutral wire, requirement?

Question

In medium voltage distribution, there is no neutral because the loads are 3 phase? so what? in 3phase, a neutral is not needed? why not?

Answer 1

Three-phase electric power is a common method of alternating-current electric power transmission.[1] It is a type of polyphase system, and is the most common method used by electric power distribution grids worldwide to distribute power. It is also used to power large motors and other large loads. A three-phase system is generally more economical than others because it uses less conductor material to transmit electric power than equivalent single-phase or two-phase systems at the same voltage.[2]



In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electrical current. This delay between phases has the effect of giving constant power transfer over each cycle of the current, and also makes it possible to produce a rotating magnetic field in an electric motor.



Three-phase systems may or may not have a neutral wire. A neutral wire allows the three-phase system to use a higher voltage while still supporting lower-voltage single-phase appliances. In high-voltage distribution situations, it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).



Three-phase has properties that make it very desirable in electric power systems:



The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load. This makes it possible to eliminate or reduce the size of the neutral conductor; all the phase conductors carry the same current and so can be the same size, for a balanced load.

Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations.

Three-phase systems can produce a magnetic field that rotates in a specified direction, which simplifies the design of electric motors.

Three is the lowest phase order to exhibit all of these properties.



Most household loads are single-phase. In North America and some other countries, three-phase power generally does not enter homes. Even in areas where it does, it is typically split out at the main distribution board and the individual loads are fed from a single phase. Sometimes it is used to power electric stoves and washing machines.



The three phases are typically indicated by colors which vary by country. See the table for more information.



Please follow the link for further information

Contents [hide]

1 Generation and distribution

2 Single-phase loads

3 Three-phase loads

4 Phase converters

5 Alternatives to three-phase Please follow the link for further information

6 Color codes

7 See also

8 References



Good luck

Hope this helps

Answer 2

Medium voltage as defined by BS 7671 in the UK is above 1000 volts, this includes the 11KV local distribution lines, the ones on the wooden poles with a 'tee' bracket and 3 insulators on top, right up to the super grid 400KV pylons.

With a balance 3 phase load either star or delta connected, all of the current flowing in through any one phase will flow out through the other two, and if two phases are supplying the load, the current will combine and flow out through the one remaining phase. If you draw the three phase sine wave, you will see this at any given vertical line drawn on the graph.



For single phase loads, it is assumed that the aggregate load over a large enough sample will be balanced and behave as a 3 phase load. But the neutral is always supplied when single phase loads are connected to 3 phase supplies to carry any inbalance currents and ensure that the voltage across the individual phases remains predictable.

Answer 3

If you open the neutral in the circuit you get full voltage to one of the white wires. The neutral wire is a path for the electricity back from the load to the service. Under normal conditions there can be a small voltage between the neutral and the ground wire. This voltage drop due to the resistance and the amps in the white wire. It depends on how far the white wire is from the service and the load. The ground should only be connected to the white wire at the service, breaker box. the white and neutral are not to be connected any where else. You are probably getting static shocks because the air is dry from heating and you are touching the metal screw on the switch box. The box and the frame of the switch must be grounded. If the neutral is from the light and you disconnect the wires, you will get 117 volts to ground and to the other neutral.

Answer 4

In a three phase system, the loads are balanced across the three phases. Therefore, when the currents in the three phases are added together, the net is zero. If the net current that would flow in a hypothetical neutral is zero, then there is no need for there to be a neutral conductor.

Answer 5

Generator station neutral was connected into the deep earth soil. Using earth as a conductor. Any where can get a neutral from the ground near by. 3 phases power can be used directly without require the neutral.