Total Harmoic Distortion - Current

"whether it is under the safe limits or not" More than 10% is not ship shape. Check HERE

"is it worth the money to go for Complete Harmonic Analysis of entire plant."

Yes. It surely is.

It depends on the % of load at the time of the measurement.

If your load was 100A on a 150A feeder, it is a concern because your TDD is about 20%*100/150 = 12% which is a bit high (but not that much).

If your load was 100A on a 500A feeder, it is not a concern because your TDD is about 20%*100/500 = 4% which is relatively good as long as the THDi decreases when the load increases (by adding linear loads).

Basically, the % THDi doesn't mean anything if you don't look at the feeder capacity with respect to the current drawn. You can have a very bad small load on a large feeder with no detrimental effect from it.

<...THD...> is of significance in analog audio amplification, as it is a qualitative measure of the amplifier. High <...THD...> results in "rough" and "distorted" sound from the speaker, or whatever that may be, which can be picked up by the ear.

Its significance in the domain of power distribution is somewhat less than that of power factor correction on metered supplies, which is of no consequence if the tariff charged by the energy provider does not include low power factor penalty. Treat it accordingly, perhaps?