- BuildingDesign News - Construction / Services News Archives

- Request Product Information
- - Architectural - Internal
- - Architectural - External
- - Electrical Engineering
- - Mechanical / HVAC
- - Facilities / Interiors

- How To Feature
- How to promote your Company

- Subscribe / Unsubscribe
- BuildingDesign Newsletter

- Professional Vacancies
- BuildingDesign Recruitment

Power Factor Correction - Active Harmonic Filters - Power Factor

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.

For more information please email below: