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4 March, 2021

How will new harmonic guidelines affect VSDs?

26 November, 2020

In his second column on the new G5/5 harmonic recommendation, ABB’s Martin Richardson reports that while there are many changes that may result in different methods of assessment, there are no fundamental changes in the mitigation methods and technologies used to keep harmonics in check. 

As discussed in the Back to Basics column in September, the Energy Networks Association has issued a revised edition of its UK harmonics recommendation. The new Engineering Recommendation G5 Issue 5 2020 (G5/5) retains a similar three-stage process to G5/4-1, with each stage relating to the same class of equipment, albeit somewhat simplified.

Stages 1 and 2 have well-defined processes for assessing the impact of loads with an LV point of common coupling (PCC), an HV PCC below 33kV, and where loads are limited in scope by aggregate power. The “baseline” fault level for assessment moves from 100MVA to 60MVA. This recognises that the G5/4-1 assumption of 100MVA at an 11kV PCC, for example, was optimistic.

During a stage 1 or 2 assessment, if the network operator (NO) provides an accurate fault level, a simple calculation can result in a quick decision as to whether a connection can be made without additional assessment. For example, for an 11kV PCC, if the fault level is 60MVA, then 673kVA of ultra-low harmonic VSDs can be installed on a supply without further assessment. Comparing this to a six-pulse VSD on the same system, just 76kVA of duty load would be allowed without further assessment.

A higher aggregate rating of “active rectifier” VSDs can be added with much less restriction than a six-pulse equivalent in stages 1 and 2.

Stage 3 assessment is required if the PCC is 33kV or above, or a stage 1 or 2 assessment fails. The limits for harmonics are now based on an “available headroom” concept when compared against the planning limits. This differs from the “first come, first served” approach of G5/4-1 and is deemed to be fairer for distributing the available headroom.

Stage 3 requires the NO to specify the requirements of the connection, which will involve detailed impedance data, existing background levels of distortion and suchlike.

Overall, this will result in stage 3 assessments being more demanding in terms of modelling for those carrying out these types of assessments.

Uncertainty when carrying out assessments is attributed in G5/5 to accuracy of measurement, quality and availability of system data and modelling inaccuracies. Assessment of harmonics from the 50th to the 100th is at the discretion of the NO. VSD manufacturers must model harmonics above the 50th accurately. At present, the range and suitability of harmonic filters that can mitigate frequencies above the 50th is limited. It remains to be seen how much discretion the NOs will apply as we gain more experience with G5-5 assessments.

To measure harmonics above the 50th you will need new and improved logging equipment. The specification of transducer equipment used to measure harmonic currents is also important. The correct specification of CTs is essential and, depending on what is already installed at a site, some existing CTs may not attain the accuracy of measurement required.

The revamp of stages 1 and 2 potentially makes assessments easier for smaller schemes. Evaluating larger systems, especially when moving to stage 3, may be more complex and harder to process than under G5/4-1.

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