Harmonics gets people confused all the time. Some typical remarks will be like; “the neutral current harmonics are extremely high” or “burnt marks on the isolator were caused by harmonics”.
Here are 3 simple guidelines for harmonics in general LV applications.
- Keep Voltage THD% below <5%. While there has been revisions in standards like IEEE-519 (revising upward from 5% to 8% for LV), trust me, keeping VTHD < 5% will make life easier for everyone (utility, facilities dept and end-users).
- “Total harmonic content of the load current not exceeding 5% of rated current.” This is stated as one of the normal service conditions for a standard distribution transformer. Thus, it is a good practice to keep the total current harmonics < 5% of the transformer’s rated current (eg. 1MVA 22kV/0.433kV TF – rated current 1333A; total harmonic current shall be less than 67A).
- Current THD% may gives you misleading results (see my other post – Mr. Harmonics I). One needs to see the actual current harmonics in absolute terms (amperes), total RMS current and make reference to the corresponding cable / circuit breaker sizes, before deciding on the next course of action.