Electrical Industry Canada Power Quality Issue - 4

INTRODUCTION TO POWER QUALITY
More and more, electricity is being considered a product. Ideally, the AC voltage wave is a sine wave alternating from a positive peak to a negative
peak 60 times per second (60 Hz) without any deformations, spikes or surges. In reality, different factors influence the quality of the wave. Certain
disturbances come directly from the power source, such as lightning. Other disturbances come from loads; in particular, from electronic equipment
which are non-linear loads that produce harmonics, mostly because of their switching power supply.

Electrical Problems Related to Power Quality
Voltage spikes arise when equipment that operates on high current is turned off. Air conditioners, photocopiers, coffee makers, electric tools, etc.
are all examples of this.
Noise can be generated from any temporary high frequency (harmonic from 50 kHz to 100 MHz), from a radio frequency (RFI) or from the
production of electromagnetic interference (EMI) emitted from transformers or motors (often an elevator motor or a motor from a photocopier). As
well, magnetic fields, induced by a mono-phased cable or by an unbalanced three-phased system, can deform images on a cathodic screen
(computer monitor) or destroy data on a hard disk.
There are two types of transient noise. The normal mode or transverse is due to an induced voltage between any two-phase conductors (line-line).
This voltage is normally in the low frequency range; it causes damage to personal computers, local networks, and workstations. The common mode
is due to an induced voltage between any phase conductor, including the neutral and the ground. This voltage can cause more damage than normal
mode noise, not necessarily by its presence (2-3 V), but by the induce fluctuation that it produces. The common mode voltage is also produced by
the presence of current on the neutral (E=IZ) and also because of the non-cancellation of the triplen on the phases that adds up on the neutral.
Consequently, common mode voltage produces ground potential differences with other grounds.
Non-linear loads (computers, variable speed drives, etc.) generate harmonics. To save energy or to transform an alternating current into a direct
current, their power source takes its current as portions of the 60 Hz sine wave. In doing so, the portions of the sine wave that the power supplies do
and do not take causes the sine wave to become deformed and multiple frequencies of 60 Hz are formed. Often, single-phase loads produce
triplets (3rd, 9th, etc.), while three-phase loads produce 5th and 7th harmonics. Therefore, electrical equipment and installations that are designed
to operate at 60 Hz, can become damaged or unbalanced due to these harmonic frequencies, which are different than the fundamental (60 Hz). The
principal problem that arises is the overheating of equipment or conductors. The harmonic distortion rate (HDR) is the ratio between the harmonics
and the fundamental load expressed as a percentage.
Voltage surges can be compared to voltage spikes, except they last longer: from 15 microseconds to half a second or more. They are mainly
caused by the shutdown of heavily loaded circuits or by the necessary commutation of a high-powered network (ex.: Power Factor Correction,
Vacuum Breakers). Evidently, computers and other sensitive electronic equipment can seriously be damaged by such an over-voltage surge.
Voltage sags are normally caused by the addition of heavy loads on an electrical line such as the start-up of an elevator, photocopier or large
motor. In this case, the current undergoes a loss of 20% or more for a period of 15 microseconds to half a second.
Brownouts last longer than voltage sags. Brownouts are sometimes caused intentionally by the power company to avoid a total blackout when
there is a great demand for electricity.
ELECTRICAL INDUSTRY CANADA

VOLUME 1, ISSUE 2

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Electrical Industry Canada Power Quality Issue

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