Star/wye connection and delta connection are the two different methods which are used for
connecting 3 phase system.
In this video, we'll learn in detail about star and delta connection.
We'll also know voltage and current relationship in both the connections, and where these connections
are used.
So, if you want to get the details, you need watch the video.
We have already seen the comparison between 1 phase and 3 phase power.
I'll strongly recommend you to watch that video.
You'll get the link for that video down in the description.
In this video, we'll also have a look at power transmitted by these two types of connection,
and to get that stick to the video.
Connection shown in this figure is one of the way to connect 3 phase generator to the
load.
As you can see this connection requires 6 wires to connect.
The three-single phase circuit shown in this figure are electrically independent.
But if you observe the image carefully you'll find that, we can combine the three return
conductors together to form a single return conductor.
By doing this we have saved cost of two conductors as the number of conductor are reduced from
6 to 4.
That's a great saving...!
Now this common return conductor is called as neutral conductor.
It carries sum of three currents Ia + Ib + Ic.
At first, many of the people will think that, the conductor required for neutral should
be 3 times in the size than the other three conductors.
But, as we know, current of the three phases are out of phase by 120 deg from each other
and if we draw a waveform for these current it will look like this.
Now if you observe the diagram carefully you'll find that the sum of return currents is zero
at every instance.
For example, at instant corresponding to 240 deg, Ic = Imax and Ib = Ia = -0.5 Imax.
So, if we add these return currents, we'll get the sum =0, and this is true for every
instance.
We can therefore remove the neutral conductor without affecting the voltage or current in
the circuit.
So, we have reduced the number of conductors 6 which was the initial stage to 3.
50%saving in conductor cost!
However, the load shown in the circuit must be identical, if you want to remove the neutral
conductor.
If your load is not identical then removing neutral conductor can cause unequal voltages
across the loads.
So, in an ideal situation or in the situation where load is equal, current flowing through
neutral conductor is Zero.
And this is very commonly used question in technical interview.
The conditions where load is not equal a neutral must be provided.
You might have heard people saying 3.5 core cable, that 0.5 core is your neutral conductor.
The circuit shown in the figure is called as 3 phase 3 wire system.
The generator and the load are said to be connected in wye, as it resembles the letter
Y or some people also called it as STAR.
The figure shown is called as 3 phase 4 wire system.
The neutral conductor can be of similar size or may be slightly smaller than the other
conductors.
3 phase 4 wire system is widely used to supply power to commercial and industrial customers.
Now when we talk about star connected three phase system, there come two concepts, line
to line voltage and line to neutral voltage.
The voltage between A and N is called as line to neutral voltage, similarly voltage between
A and B is called as line to line voltage.
Relation between these voltages as well as current changes with the type of connection.
So, it is important to understand these relationships for different connections.
In case of star connected system
Line to line current are equal to line to neutral current
But in case of
voltage it is different
Consider, the winding shown in this figure
which is connected in star
As you can see
the conductors are placed 120 deg apart from each other
so, if you apply Kirchhoff's voltage law
to this circuit and by doing some mathematics
you'll find that
the line to line voltage is therefore root 3 times the
line to neutral voltage
the other way to connect three phase system is called as delta connection.
The connection is so named because it resembles the greek letter delta.
Let have a look at the voltage and currents relationship in delta connection.
Now in case of delta connection, voltage across each connection is same as line voltage.
But, in case of current, current across each element is different than the line current.
If you apply the Kirchhoff's law and do some math, you'll find that the line current
is root 3 times greater than the current in each branch of delta connected system.
Generally, star connection is used where you need a neutral and two separate voltages,
like our distribution system.
Delta connection is generally preferred where neutral conductor is not needed like for transmission
of high voltage power.
Also, delta connection is preferred where 3rd harmonics needs to controlled.
Star and delta connections are almost used everywhere when we talk about 3 phase system.
You'll generally find 3 phase transformer connected in different combinations of star
and delta connection.
For example, star – star connected transformer is generally used as an auto transformer.
Delta – delta connected transformer is generally used for high voltage transmission.
Delta – star connected transformer is generally used as a distribution transformer.
Now, as we have studied the star & delta connection, a question can pop up, power transferred by
a star connection and power transfer by a delta connection is same or different?
So, let's find that out.
Consider the star connected winding first.
Apparent power transferred by a single phase is given by.
This is the power transferred by a single phase.
To calculate the power transferred by 3 phase we can multiply this equation by 3.
Similarly, Apparent power transferred by a single phase of delta connection is given
by.
Multiply this equation to calculate power transferred by 3 phase, and you'll get the
same result as star connection.
And, this proves power transmitted by both the connection is same.
So, let's summarize this video.
3 phase system can be connected in two different styles star or delta.
In star connection, line to line current is equal to line to neutral current.
But, line to line voltage is root 3 times the line to neutral voltage.
In delta connection, voltage across each element is equal to line voltage.
But line current is root 3 times the current flowing through each element.
Power transmitted by both the connection is same.
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