*The p-q theory has the following limitations*

• The source voltages are unbalanced and non-sinusoidal.

• It needs a large number of transducers of measurements and intensive computation. It tends to make the system operation complex.

*Disadvantages of P-Q theory*

• Delay in computation due LPF used for filtering power signals

• This theory uses volt signals to compute instantaneous active and reactive powers, if any variations in supply voltage causes inaccurate measurements

• It is not applicable for single phase systems and three phase systems with neutral.

• Complete harmonic suppression is not achieved in case of nonlinear loads.

• A larger filter inductance is required to filter the harmonics

*B. Synchronous rotating frame theory*

The synchronous reference frame theory is based on the transformation of the currents in synchronously rotating d-q frame. Fig.3. explains the basic building blocks of the theory. If 9 is the transformation angle, then the currents transformation from a-P to d-q frame is defined as:

**Fig. 3 **: *Block Diagram of SRF theory*

The 3-Phase load currents of system i.e. iLa’ iLb’ iLc’ of stationary reference system in a-b-c coordinates are transformed to two phase system with a-P coordinates, this can be done by algebraic transformation, known as “Clark’s transformation” which also produces a stationary reference system again these currents are transformed to synchronously rotating reference frame by Park’s transformation. The voltage templates (sine and cosine) are generated using phase locked loop (PLL) and then we calculate the which are passed through LPF which filter out the harmonics in current signals and again these two phase currents in rotating reference frame are transformed to two phase stationary reference frame using reverse park’s transformation and then transformed to three phase coordinates which are called as the reference current signals