What is Star Connections,Voltage ,Current and Power in Star connections,Summary of Star connections,Uses of Star connections,

Star Connection

          Star Connections is a method of connecting pohyphase circuit is such a way that one end of each phase line is connected to a common neutral point that may be connected to the earth as protection against lighting or to a wire to which all the other neutral points of the system are connected.

           It is also known as three phase four wire system.It is denoted by Y.It is the most preferred system for AC power distribution.

           If we connect the balance load across three phase voltage system in parallel,then the three currents will be flow in neutral and their quantities will be same .The angle between them is 120º.So we can say That the sum of all currents is zero.

                                                             IR + IY + IB = 0     

           In the following fig. all 3 phase power voltage and current values of voltages and currents can be shown for star connections.

fig 

            Voltage ,Current and Power values in three phase star connection Y are given in following.

Line Voltage

            VRY = VR – VY 

           Because the angle between   VR and VY  vectors is 60°

So,

     

 VR = VY = VB = VPH

        

      

 VRY = √3 VPH

Similarly

         

  VYB = VY – VB

   

  VYB = √3 VPH

and

  VBR = √3 VPH

         As in Y connection

VRY = VYB = VBR

So We can say 

VL = √3 VPH

Line Current

           Line current and phase currents in star connection is given below.

           Current in Line 1 ,Line 2and Line 3 are IR,  IY  and IB respectively.

           

            

            

          Because the following currents in all three lines are same so we can say

           IR =   IY   =   IB   = IPH 

   

   

           IL = IPH 

          In other words we can say that the line current and phase current in star connections are same 

Power in Star connection

            As we know the active power in three phase system is

            P=3 *  VPH  * IPH   * CosФ   

             As we know in case of 3 phase star connections.

   

             IL = IPH         and      VPH = VL/√3

So,

            P= 3*( VL/√3) *  IL  * CosФ   

            

            P= √3 * VL * IL * CosФ   

Or we can say

     

            P= √3 * VPH  * IPH   * CosФ  

Summary of Star Connections

• We can say that star connections are of two types $-wire 3 phase and 3wires 3 phase.
• In case of 4 wires 3 phase one wire is neutral and it is common point for all other 3 wires.
• In star connections one end of all three wires are connected to a common point .The other three open ends are forms three phase and the common point is called neutral.
• In case of star connection the Line voltage(voltage between two phase) and phase voltage(voltage between any phase and the neutral) is different .
• Voltage in case of star connection     VL = √3 VPH  
• Current in case of star connection   IL = IPH   
• Power in case of star connection    P= √3 * VL * IL * CosФ   or    P= √3 * VPH  * IPH   * CosФ 

Uses of Star Connections 

• Since insulation required is less in case of star connection so we use these types of connections for long distance.
• Star connection is used in both transmission and distribution networks.
• Star connection is used where we require neutral terminal to obtain phase voltage.
• In case of star connection VL= √3 VP  so we start the big motor with star connection to avoid the high initial torque with less starting current.