- Three Phase Circuit Definition: A three-phase circuit is defined as a system where three electrical phases are used together, each 120 degrees apart, to provide continuous power.
- Star Connection: A star connection includes three phase wires and one neutral wire, ideal for long-distance power transmission due to its ability to handle unbalanced currents.
- Delta Connection: A delta connection uses three wires without a neutral wire and is better suited for short-distance transmission, but it can struggle with unbalanced currents.
- Voltage and Current Differences: In a star connection, the line voltage is √3 times the phase voltage, while in a delta connection, the line voltage is the same as the phase voltage.
- Advantages of Three Phase Circuit: Three-phase circuits are more efficient, use less conductor material, and provide a more stable power supply than single-phase circuits.
There are two types of systems in electric circuits: single-phase and three-phase systems. In a single-phase circuit, current flows through one wire with a return path called the neutral line, allowing minimal power transport. Both the generating and load stations are single-phase in this system, which has been used for a long time.
In 1882, a new polyphase system was invented, allowing more than one phase for generating, transmitting, and loading power. A three-phase circuit is a polyphase system where three phases are sent together from the generator to the load.
Each phase in a three-phase system has a 120-degree electrical phase difference, dividing the total 360 degrees into three equal parts. This ensures continuous power generation as all three phases work together. The sinusoidal waves for a three-phase system illustrate this division.
The three phases can be used as single phase each. So if the load is single phase, then one phase can be taken from the three phase circuit and the neutral can be used as ground to complete the circuit.
Why Three Phase is Preferred Over Single Phase?
There are various reasons for this question because there are numbers of advantages over single phase circuit. The three phase system can be used as three single phase line so it can act as three single phase system. The three phase generation and single phase generation is same in the generator except the arrangement of coil in the generator to get 120o phase difference. The conductor needed in three phase circuit is 75% that of conductor needed in single phase circuit. And also the instantaneous power in single phase system falls down to zero as in single phase we can see from the sinusoidal curve but in three phase system the net power from all the phases gives a continuous power to the load.
A three-phase circuit consists of three voltage sources connected inside the generator, each with a 120-degree phase difference. Arranging three single-phase circuits with a 120-degree phase difference forms a three-phase circuit. This phase difference is essential; without it, the circuit won’t function, and the three-phase load could be damaged.
The size and metal quantity of three-phase devices don’t differ much from single-phase devices. For example, a transformer will be nearly the same size for both systems since it links the flux. However, a three-phase system is more efficient because it can handle more power with similar transformer mass and minimal losses. Overall, three-phase systems are better and more efficient than single-phase systems.
In three phase circuit, connections can be given in two types:
- Star connection
- Delta connection
Less commonly, there is also an open delta connection where two single-phase transformers are used to provide a three-phase supply. These are generally only used in emergency conditions, as their efficiency is low when compared to delta-delta (closed delta) systems (which are used during standard operations).
Star Connection
In star connection, there is four wire, three wires are phase wire and fourth is neutral which is taken from the star point. Star connection is preferred for long distance power transmission because it is having the neutral point. In this we need to come to the concept of balanced and unbalanced current in power system.
When equal current will flow through all the three phases, then it is called as balanced current. And when the current will not be equal in any of the phase, then it is unbalanced current. In this case, during balanced condition there will be no current flowing through the neutral line and hence there is no use of the neutral terminal. But when there will be unbalanced current flowing in the three phase circuit, neutral is having a vital role. It will take the unbalanced current through to the ground and protect the transformer. Unbalanced current affects transformer and it may also cause damage to the transformer and for this star connection is preferred for long distance transmission.
The star connection is shown below-
In star connection, the line voltage is √3 times of phase voltage. Line voltage is the voltage between two phases in three phase circuit and phase voltage is the voltage between one phase to the neutral line. And the current is same for both line and phase. It is shown as expression below
Delta Connection
In delta connection, there is three wires alone and no neutral terminal is taken. Normally delta connection is preferred for short distance due to the problem of unbalanced current in the circuit. The figure is shown below for delta connection. In the load station, ground can be used as neutral path if required.
In delta connection, the line voltage is the same as that of phase voltage. And the line current is √3 times of phase current. It is shown as expression below,
In a three-phase circuit, star and delta connection can be arranged in four different ways:
- Star-Star connection
- Star-Delta connection
- Delta-Star connection
- Delta-Delta connection
But the power is independent of the circuit arrangement of the three phase system. The net power in the circuit will be same in both star and delta connection. The power in three phase circuit can be calculated from the equation below,
Since, there is three phases, so the multiple of 3 is made in the normal power equation and the PF is power factor. Power factor is a very important factor in three phase system and some times due to certain error, it is corrected by using capacitors.
