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Form 1

Harmonics Regulations requirements.

The harmonics regulations adopted by Electricity Ashburton, Network Waitaki and Alpine Energy require that all VFDs installed in the rural areas have harmonic mitigation that reduces the THDi (Total Harmonic Distortion or the line current) at the point of supply, is less than a prescribed level when the combined VFD load is grater than 20KW.

The limits are laid out in a table where the limit is determined by the ratio betwen the maximum connected load and the short circuit current of the supply.

In most rural areas where the VFD is supplied by a dedicated transformer, this equates to a maximum THDi of less than 8% when the background distortion is less than 5% and the voltage imbalance is less than 1%. Where the background voltage distortion THDv (Total Harmonic Distortion of the applied voltage) is greater than 5%, an allowance is made for a higher THDi.


The regulations require that an application is made to connect the VFD and this is to be accompanied by a completed Form1 which predicts the harmonic currents drawn by the VFD and harmonic mitigation system. The Form1 requires the background distortion under which the calculations are made, to be specified but does not specify what that background should be. We choose to use a background distortion level of 2%. A lower background THDv would result in reduced THDi figures.

Partially complete "Form 1"s for different models of Lineator and loads.

Register below to access a download page where you can download the Form 1 to match your application. These forms are applicable to Mirus Lineators only. For other filters, refer to the filter manufacturer.

These Form 1 s include all the technical data required but need to have location and installer information added.

Post Connection Testing

The regulations require that within seven days of the VFD being connected, the VFD installation must be tested and the harmonic levels recorded. These tests are usually carried out by the Electrical Supply Authority. Failure to pass the tests may result in the disconnection of the VFD.