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Article Date: 12th February 2013

ABB Power - Harmonic Amplification Demystified

Power Factor Correction - Active Harmonic Filters - Power Factor

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Few topics in electrical engineering hold such mystique as the phenomenon of 'harmonic amplication'. Kurt Schipman explains what it is and how to deal with it when sizing a filtering solution.

The vast majority of modern electrical loads produce some level of harmonic current when connected to the supply. Such loads include variable frequency drives (VFDs) used in pumps and heating, ventilation and air conditioning (HVAC) equipment, and office equipment including computers and printers.

It is the normal designed behaviour of such equipment to produce harmonics as a by-product of the efforts made by manufacturers to make their products more energy-efficient and flexible to use.

When connected to a ‘standard’ supply, the nominal characteristics of a piece of electrical equipment can be identified by its voltage, current and power, as well as by its total harmonic distortion (THDi) value. The THDi expresses the ratio between the harmonic current the equipment produces and the nominal 50 Hz current it draws from the supply.

Different loads have a different nominal THDi, but for high-quality VFDs the nominal THDi is around 45 percent. This implies that if the drive draws 100 A of fundamental current from the supply, it injects around 45 A of harmonic current back into the supply.

Harmonic pollution under ideal and real-life conditions

When observing the behaviour of different electrical loads connected to real-life networks, it is clear that some differ significantly from their published nominal values. This is because real-life networks have inferior (‘less healthy’) characteristics than the ideal standard networks used to determine the nominal values of the equipment. This is especially true for THDi. So, for example, when connected to a generator supply a VFD with a nominal THDi of 45 percent will exhibit an actual THDi of around 30 percent.

When equipment runs on a real-life network, its current balance is disturbed. And the more the supply network deviates from the ideal or standard characteristics, the more disturbed the current balance becomes. In extreme cases – where there is a significant deviation from the supply characteristics for which it was designed – the equipment will run less reliably, resulting in it exhibiting erratic behaviour, or tripping out completely. In the context of harmonic pollution, the supply fault level and short circuit impedance are critical factors that determine how far the load runs at its nominal THDi rating. The lower the supply system fault level, the more deviation there is from the ideal network. The higher the supply fault level, the closer we are to the ideal network.

Harmonic amplification factor

By installing suitable filtering equipment in parallel to the loads, network quality can be restored for the frequencies that are filtered, and the nominal THDi between the filter and the load is restored – the load runs at its design rating. At the same time, the THDi upstream of the filter connection point is reduced to the levels required by the customer or local regulations. The filter then acts as a correcting device, ensuring that the load is running at its nominal THDi, while making it compatible with utility requirements on the supply side.

In practice this implies that the harmonic currents and THDi produced by the load approximate to the design value. For our VFD example, the THDi of the load could increase again to 45 percent against an initial measurement of 30 percent. The ratio between a load’s designed THDi and the THDi measured when it is running under non-ideal network conditions is called the Harmonic Amplification Factor (HAF). So in this case the HAF is 45/30 percent, or 1.5.

Depending on the load type, the installation and the filtering requirements, the HAF can range from 1.1 for DC drives to 2 or higher for low-quality AC drives (without an internal choke). Most high-quality modern equipment will have a HAF in the range of 1.1 to 1.5.

Impact of HAF on filter sizing

When filtering harmonic pollution, the network characteristics are improved so that the load will behave more closely to its ideal design behaviour. So if the THDi is initially measured at 30 percent under real-life conditions, the THDi will approach the design value of 45 percent under ideal network conditions (with all harmonics filtered). This load harmonic pollution increase has to be taken into account when sizing a filter solution.

If however the load is already operating on an almost perfect network, with THDi at 40 percent, the HAF will be lower at 45/40, or around 1.2.

When sizing a filter for a given harmonic component, we use this formula:
Ifilterh = HAF x Imeasuredh –Itargeth (equation A)

Where:
h is the harmonic number
Ifilterh is the harmonic filter current requirement at order h
Imeasured is the harmonic load current at order h
Itarget is the harmonic target at order h

Depending on conditions, different HAFs can be used. When targeting a standard such as G5/4.1, where for the main harmonic components a significant amount of harmonics are allowed to flow into the network, moderate HAF values can generally be used. In practice, we suggest 1.5 for VFDs, 1.1 for DC drives and 1.3 for office building loads.

Sizing filtering solutions for a given application

ABB has developed its own software, PQF Size, that automatically takes into account the HAF phenomenon, as well as other criteria that may be relevant for specific filter applications (for example, filtering of multi pulse drives).

If this software is not used, the filter sizing may be based on the formula shown, and the amplification factor can be selected as per guidelines included here, providing that a standard is targeted and the electrical loads are of good quality (that is, VFDs have harmonic current limiting chokes, etc.)

For specific applications where higher frequency harmonics need to be filtered, it is advisable to check the filter’s ability to inject the individual harmonic orders, and its filtering efficiency, which is 0.97 for ABB active filters but could be significantly lower for other suppliers.

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