Do you have access to the raw signal or do you just have the RMS value of the fundamental and the valve of the THD? From what I understand THD is simply a sum total. You won't be able to determine content from that number.
What is the base frequency, and which harmonic are you trying to analyze?
Since this is a PLC-oriented forum, it would follow that you have interest in using such technology. Is there a particular model, besides the Beckhoff mentioned, under consideration?