How to calculate active & reactive power ripple?
Check out the wikipedia entry for standard deviation:



The equations above are the same calculation, but different notation

\$p_i\$ and \$q_i\$ are the samples, coming from a sampled vector (like data from an analog to digital converter)

$$ \mu = \frac{1}{N}\sum_{i=1}^{N} x_i$$ means to average the values toghether
once you've computed the average, then you plug that into the other equation

If I have a vector of four values [1 5 2 3] the mean (using the above equation) would be 2.75

The standard deviation would be 

$$ \sigma = \frac{1}{N}\sum_{i=1}^{N}([1,5,2,3 ]-2.75)=1.707$$

excel or any math package have the mean and std functions, or you could do this with a for loop and code.