Mr. Harmonics II

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.

  1. 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).
  2. “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).
  3. 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.

Voltage Dip 07/03/2020 1900hrs

Earlier this evening, there was a Transmission-level fault that occurred at approximately 7pm. The following is a waveform captured at 22kV incomer (monitored at L-N), located in the city area.

Here, it suggests that there was a Single-phase fault (L2) and that it originated upstream at higher voltage levels. There were reports of voltage dips from other PQ monitors in other transmission blocks as well.

Update: Suspected fault originated from Genco facilities in the west.

Voltage Dip 7/3/2020 1900hrs
22kV L-L RMS Trend