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47
Matrix
Protection and Control Units
Fig.1
S o m e o f t h e t y p i c a l r e s u l t s o f t h e h a r m o n i c s i n t h e s y s t e m a r e :
1. Neutral over current: Odd triplen harmonics because of its additive nature adds up in the neutral current causing
the overheating of the neutral.
Fundamental of 50 Hz AC
3rd Harmonic
3rd Harmonic
3rd Harmonic
o 120
Lagging
o 120
Lagging
Phase R
Phase Y
Phase B
48
Matrix
Protection and Control Units
As shown in the Fig.1 any odd triplen harmonic when multiplied by 120 degrees will always produce the product
which is integral multiple of the 360 degrees. Thus being shifted by integral multiples of the 360 degrees, the odd
triplen harmonics will always be in phase with each other in neutral.
2. Incorrect readings: In case of induction disc W-hr meters and averaging type current meters
3. Additional losses in capacitors and rotating machines
4. Reduced true PF: Pf=watts/VA
5. Transformer overheating especially delta windings where triplen harmonics generated on the load side of a
delta-wye transformer will circulate in the primary side. Some type of losses go up as the square of harmonic
value (such as skin effect and eddy current losses). This is also true for solenoid coils and lighting ballasts.
6. Zero, negative sequence voltages on motors and generators. In a balanced system, voltage harmonics can either
be positive (fundamental, 4th, 7th...), negative (2nd, 5th, 8th...) or zero (3rd, 6th, 9th,...) sequencing values. This
means that the voltage at that particular frequency tries to rotate the motor forward, backward, or neither (just
heats up the motor), respectively. There is also heating from increased losses as in a transformer.
Harmonic order
1 2 3 4 5 6 7 8 9
Phase Sequence
+ - 0 + - 0 + - 0
7. Nuisance operation of protective devices, including false tripping of relays and failure of a UPS to transfer
properly, especially if controls incorporate zero-crossing sensing circuits.
8. Bearing failure from shaft currents through uninsulated bearings of electric motors.
9. Blown-fuses on PF correction caps, due to high voltage and currents from resonance with line impedance.
10. Mis-operation or failure of electronic equipment.
11. If there are voltage sub-harmonics in the range of 1-30Hz, the effect on lighting is called flicker. This is especially
true at 8.8Hz, where the human eye is most sensitive, and just 0.5% variation in the voltage is noticeable with
some types of lighting.
T O T A L H A R M O N I C D I S T O R T I O N :
Power sources act as non-linear loads, drawing a distorted waveform that contains harmonics. These harmonics can
cause problems ranging from telephone transmission interference to degradation of conductors and insulating
material in motors and transformers. Therefore it is important to gauge the total effect of these harmonics. The
summation of all harmonics in a system is known as Total Harmonic Distortion (THD).
Harmonics have frequencies that are integral multiples of the waveform’s fundamental frequency. For example,
given a 50Hz fundamental waveform, the 2nd, 3rd, 4th and 5th harmonic components will be at 100Hz, 150Hz,
200Hz and 250Hz respectively. Thus, harmonic distortion is the degree to which a waveform deviates from its pure
sinusoidal values as a result of the summation of all these harmonic elements.
The ideal sine wave has zero harmonic components. In that case, there is nothing to distort this perfect wave. Total
Harmonic Distortion or THD, is the summation of all harmonic components of the current waveform compared
against the fundamental component of the current wave:
The formula above shows the calculation for THD on a current signal. The end result is a percentage comparing the
harmonic components to the fundamental component of a signal. The higher the percentage, the more distortion
that is present on the mains signal.