Measure Reactive Power

We can't measure the reactive power by using wattmeter.

Yes, we can measure reactive power using a single wattmeter. Connect current coil to one phase (let it be R of RYB) and connect pressure coil against the other two phases (Y and B) then W=VI Sin(phi)

Before answering the question let us think about the principal of measurement of reactive power. The reactive power is the power which is 90 deg apart from the real power. A wattmeter works on the principal of the interaction between the fluxes generated by its pressure coil and the current coil. The amount of force generated also depends on the angle between the two parameters, i.e. power factor angle. The wattmeter is calibrated in terms of cosine of power factor angle. The same meter can be calibrated in terms of sine of power factor angle and it will read reactive power. Another method to find reactive power is to measure voltage & current and subtract wattmeter reading from the product of V & I.