G5/4-1 Harmonic Assessment

What is G5/4-1?

G5/4-1 is an often misunderstood and consequently misapplied document. It is not, as is often supposed, a harmonic standard but rather a design recommendation.

G5/4-1 is often cited as a standard against which the harmonic performance of an installation can be judged – this is a misapplication.

The intention is to limit the amount of harmonic distortion introduced by new loads rather than a benchmark for existing installations.

The G5/4-1 voltage distortion planning levels are significantly lower than the equipment immunity test levels. This is illustrated in Figure 1, which shows the assessed level which is designed to ensure compliance with the planning levels, which themselves are within the equipment compatibility levels.  

The difference between the planning levels and the compatibility levels is often referred to as the compatibility gap; the purpose of which is to ensure an electrical environment compatible with the requirements of equipment likely to be connected.

Figure 1 – Graph illustrating the gap between harmonic planning levels and equipment immunity levels

How does G5/4-1 work?

G5/4-1 is a three stage assessment process and in basic terms is applied as follows: –

Stage 1

Stage 1 (part 1) – the first part of stage 1 is a very basic assessment based upon equipment rating only.  Depending upon the topology of the equipment (single phase, three phase, 6 pulse, 12 pulse etc.) different kVA allowances are are given to new load that may be connected provided that they comply with the relevant EMC standards.

Stage 1 (part 2) – if the equipment to be connected does not comply with the requirements of Stage 1 (part 1), the harmonic current injection profile of the equipment (which should be available from the manufacturer) are compared with the design limits published within G5/4-1.  These current injection limits are based upon a 10 MVA fault level (LV connection) and should be adjusted pro-rata according to the actual fault level of the system

Stage 2

Stage 2 (LV) – If an LV connection fails to meet the requirements of either part of Stage 1, a Stage 2 assessment will be necessary.  The LV Stage 2 assessment involves measuring the pre-existing harmonic distortion at each harmonic and predicting how much it would rise based upon the calculated harmonic voltage contribution from the new load.  The LV system impedance is modelled and a calculation of how much voltage distortion at each harmonic will be produced by the new load is carried out. The additional voltage distortion is combined with the pre-existing voltage distortion using calculation method detailed within G5/4-1 and if the resultant voltage distortion is within the G5/4-1 design recommendations, connection of the new load is permitted.

Stage 2 (MV) – This is basically the same as Stage 1 (part 2) but applied to MV based upon a fault level of 100 MVA for 11 kV.  

Please note – the current injection assessment can only be carried out on MV systems if the pre-existing voltage distortion levels are less than 75% of the design limits.

An MV voltage assessment is allowable, should the current injection limits be exceeded and this is basically the same as the LV assessment, again using a simplified model of the system.

Stage 3

Stage 3 (only applicable to MV systems) – This is rarely used as it involves complex modelling of systems including transmission lines, shunt capacitors, cable capacitance etc.  The complexity of this modelling is such that it can be very time consuming and often mitigation measures such as filtering end up being required in any case.

How we can help?

PureSine can carry out G5/4-1 assessment from the initial measurement of pre-existing voltage harmonic levels (if required) to three stage assessment right through to implementation of any mitigation measures should they be required.

At all stages of the assessment, detailed reports will be provided and further harmonic analysis can be carried out once final connection of the new load and any mitigation has been completed to verify compliance.

The first step is to get in touch with us, typically by completing the form below. We’ll then schedule a call with you to discuss any problems you’re experiencing and make recommendations on what the best way to proceed.

The most likely outcome will be the suggestion that a week-long study is conducted, as this will provide a full and thorough evaluation of the electrical system. We’ll also make use of the time on site to discuss your processes and paint an overall picture of what’s causing the issues.

If active harmonic filters are required, we’ll provide a competitive, no obligation quotation for the system and will provide consultation services afterwards to ensure that the filter has had the desired effect.

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