REACTIVE POWER MEASUREMENTS

REACTIVE POWER MEASUREMENTS

 A single wattmeter can also be used for three phase reactive power measurements. For example, the connection of a single wattmeter for 3-phase reactive power measurement in a balanced three phase circuit.

Fig. 4.6 Reactive power measurement circuit

The current coil of the wattmeter is inserted in one line and the potential coil is connected across the other two lines. Thus, the voltage applied to the voltage coil is V_RB= V_R-V_B, where, V_R and V_B are the phase voltage values of lines R and B respectively, as illustrated by the phasor diagram

The  reading  of  the  wattmeter,  W_3ph  for  the  connection  shown  in  figure  3.6  can  be obtained based on the phasor diagram of figure 34.7, as follows: 

Wattmeter reading,                               Wph   =  Iy VRB                                                              

                                                                                     = I_y V_L cos (90+Ø)                                              

                                                                                     = - √3 V_ph I_ph sin Ø                                              

                                                                                  = - √3 (Reactive power per phase)                          (3.6)

Thus, the three phase power, W_3ph is given by,                                                               

                                                                    W3ph  =  (VArs/phase)                                          

                                                                          = 3 [W_ph /- √3]                                                

                = - √3 (wattmeter reading)