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Harmonics

Your electrical system needs to co-exist with other users on the network. Non-sinusoidal devices, such as fluorescent lights, mobile phone chargers, UPS systems and variable speed drives, all produce current of a frequency which is different to that of the distribution system, normally 50 Hz.

A current of, for instance, 150 Hz, is known as the third harmonic. When this current is injected into the network, it is energy that cannot be used by devices on the network. This energy will therefore convert to heat. This can cause motors and cables to overheat, sometimes causing insulation failures. Catalytic capacitors may even explode. It is therefore important that these currents are kept within manageable limits. The Electricity Association’s guidelines G5/4 have recently tightened up the levels of harmonic mains pollution allowed.

The harmonic currents depend on the drive construction and load. Harmonics can be reduced either by modifications to the drive system or by using external filtering.

Factors that increase harmonic currents include:
• Large motor compared to the supply transformer
• Higher motor load

Factors that decrease harmonic currents include:
• Greater DC or AC inductance
• Higher number of pulses in the rectifier
• Larger transformer
• Lower transformer impedance
• Higher short circuit capacity of supply

Lower harmonics with swinging choke

ABB’s HVAC drive features a patent pending, swinging DC choke that suppresses harmonics caused by the inverter so that the percentage of total harmonic distortion goes down when the load is reduced. Normally, total harmonic distortion increases when the load is reduced. This is because the impedance of a standard DC choke is optimised for a specific throughput, normally that used at full load. ABB’s swinging choke, in contrast, features a variable air gap, giving flexible impedance for a range of different load conditions.
This can enable you to fit more drives in your building without having to make costly investments to mitigate harmonics.

Assessing harmonics

Stage 1 assessment:
Is the supply 230 V single phase or 400 V three phase?
Is the equipment under 16 A input current or an aggregate total of max. 12 kVA of 6 pulse converters?
Or, is the total harmonic currents from all items less than the levels stipulated in Table 7 of the G5/4 document? Download here
If all answers are “Yes”, no further assessment necessary. If the answer to any of this is “No”, please proceed to Stage 2 and 3 assessment

Stage 2 assessment:
Is the total converter load less than 130 kVA of 6-pulse or 250 kVA of 12-pulse diode rectifiers?
If not, a measurement needs to be made to ensure that the background level of harmonic distortion does not exceed 75% of the planning level for 95% of the time.
If the measured level is above 75%, the voltage distortion needs to be calculated. Should the overall level of 5th harmonic remain within the planning levels, connection may be agreed.

Stage 3 assessment
If the harmonics levels exceed those for Stage 2, or if the point of common coupling is at 33 kV or more, a more complex procedure is called for. Please contact ABB for further information.

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