Company News

Introduction

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.

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 (3^rd, 9^th, etc.), while three-phase loads produce 5^th and 7^th 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.

Flickering is a voltage variation with a lighting load that causes the light output to visibly flicker. This can be caused by the input in function of electro-domestic loads; however, it is mostly due to industrial loads (ex.: motor start up or speed variation)

Blackouts can last from a few microseconds to hours or even days. These total losses of current normally occur due to damaged equipment or electrical lines.

Power Factor is the ratio between the apparent power (VA) and the real power (W). Energy suppliers deliver electricity with a sine voltage wave at 60 Hz. If the current and voltage waves are not aligned, the system's efficiency is diminished and the apparent power is greater than the real power. In an inductive system, the voltage wave is ahead of the current wave. In a capacitive system, it is the current sine wave that is ahead of the voltage sine wave. In order to compensate for the inductive effect of motors, correction is achieved using capacitors to align the two waveforms. There are now two causes that contribute to the deterioration of the power factor: inductive loads, which influence the displacement power factor, and non-linear loads when the current harmonics are not aligned with the voltage source. Utilities measure the total power factor and consider both of these causes.

Knowing the cause for the deterioration of the power factor will help choose best way to correct it. In some cases, correcting harmonic problems can rectify the power factor.

Power Factor Calculations: The power factor ratio measures the relative amounts of work-producing active power measured in kW versus the total apparent power (kVA). Power factor is defined as the cosine (cos) in the following equations:

Power factor = COS

Power = V_rms I_rms COS

PF = kW / kVA

Displacement Power Factor.

Power factor is based on the 60 Hz fundamental frequency. Harmonic currents drawn by UPS’s, E.V. chargers, adjustable speed drives, electronic ballast and electronic office equipment are increasing in the modern facility. As a result, power factor must now be viewed in reference to harmonic frequencies of the 60 Hz fundamental. Conventional power factor is now called displacement power factor to relate it to the displacement between the system current and voltage waveforms