After carrying out a site survey for large food manufacturing site it was found that the harmonic voltage distortion levels of over 9% THvD which are extremely high and of major concern.
The measured voltage distortion exceeds the harmonic compatibility limits specified by BS EN 56160 & IEC 61000-2-2 of ≤8.0%. The fact that the voltage harmonic distortion exceeds these compatibility limits is of real concern as equipment carrying a mark will have been tested with voltage harmonic distortion up to the compatibility limits. If equipment is exposed to voltage harmonic distortion beyond this limit, its correct operation will no longer be guaranteed by the equipment manufacturer.
Furthermore, the level of voltage distortion is such that the site MV connection point is highly likely to exceed ENA EREC G5/5 recommended limits and as such, contradict the network operator’s connection agreement conditions.
Transformer Harmonic Derating
The presence of harmonics increases the losses in a transformer, for example the iron core losses are proportional to frequency and the eddy current losses are proportional to frequency2. It is therefore not hard to imagine that the presence of significant levels of harmonics will lead to increased transformer heating and consequently a derating effect.
Transformer derating factor is the factor by which you must derate a transformer by, given the specific harmonic content of the load current on each phase at any given time. The effective load current when the transformer derating factor is applied (derived from the load current divided by the coincident derating factor) must be considered when calculating the required transformer rating or when adding additional load to an existing supply as the effective rating or spare capacity can be significantly less than it may appear to be. If the effective load current of the transformer exceeds the transformer rating, damage may occur to the transformer even though protective devices do not operate.
Based on the harmonic current levels observed during our site survey, we estimate a transformer derating factor of 30%, Increasing the effective load on the transformer from 1835kVA to 2385kVA.
- In the first instance, detailed load and power quality analysis is to be carried out on all transformer main LV supplies.
- Carry out maintenance inspection of all existing PFC systems and carry out remedial works as necessary.
- Install harmonic reduction solutions on the Roof Chiller Supply and any other supplies identified as “problematic” following both of the above.
Following the repairs of the sites existing Power Factor Correction units, we recommended the installation of 2 x 300amp, Active Harmonic Filter Units type ADF P300-300/480 to reduce the harmonic distortion on the supply and ultimately reduce the stresses on the Transformer.