User manual 09/2009 - Schneider Electric

The Metering FunctionHarmonic CurrentsOrigin and Effects of HarmonicsThe number of nonlinear loads present on electrical networks is always increasing, which results in ahigher level of harmonic currents circulating in the electrical networks.These harmonic currents:• Distort the current and voltage waves• Degrade the quality of the distributed energyThese distortions, if they are significant, may result in:• Malfunctions or degraded operation in the powered devices• Unwanted heat rises in the devices and conductors• Excessive power consumptionThese various problems naturally result in additional installation and operating costs. It is thereforenecessary to control the energy quality carefully.Definition of a HarmonicA periodic signal is a superimposition of:• The original sinusoidal signal at the fundamental frequency (for example, 50 Hz or 60 Hz)• Sinusoidal signals whose frequencies are multiples of the fundamental frequency called harmonics• Any DC componentThis periodic signal is broken down into a sum of terms:where:• y 0 : Value of the DC component• y n : Rms value of the nth harmonic• ω: Pulsing of the fundamental frequency• ϕ n : Phase displacement of harmonic component nNOTE: The DC component is usually very low (even upstream of rectifier bridges) and can be deemedto be zero.NOTE: The first harmonic is called the fundamental (original signal).Example of a current wave distorted by harmonic currents:IIIrms1tH1 (50 Hz)t2H3 (150 Hz)t3H5 (250 Hz)t41 Irms: Rms value of the total current2 I1: Fundamental current3 I3: Third order harmonic current4 I5: Fifth order harmonic currentLV434104 09/2009 87