METREL MI 3399, Instruction Manual With Exercises

Page: 132 / 272

Power and Voltage Quality Trainer AD 2
– MI 3399
132
2.7 Energy measurement
Energy is the generation or use of electric power over a period of time. This is usually
expressed in kilowatt-hours (kWh).
2.8 Harmonics
Any periodic deviation of a pure sinusoidal voltage waveform can be presented with
the sum of sinusoids of the power frequency and its integer multiples. Power frequency
is called the fundamental frequency. A sinusoidal wave with a frequency k times higher
than the fundamental (k is an integer) is called harmonic wave and is denoted with
amplitude and a phase shift (phase angle) to a fundamental frequency signal. The ratio
between harmonic frequency and a fundamental frequency (k) is called harmonic
order.
Origin
Non-linear loads (rectifiers, variable speed drives, fluorescent lamps, PC, TV...) draw
current with a high THDI (highly non-sinusoidal waveform). For analysis purposes,
non-linear loads can be modelled with linear loads and the (current) source of
harmonics. Current harmonics cause a non-sinusoidal voltage drop on the reference
impedance and a distorted voltage at the power supply terminals. Non-linear loads
disturb the supply voltage in such a way that only
odd harmonics can be detected
with a measurement instrument.
If the load is non-symmetrically controlled, positive and negative half periods of current
differ in shape and rms value causing
even harmonics and a DC component to arise.
This situation causes saturation and overheating of transformer cores. A significant DC
component can be caused by geomagnetic storms in some areas.
Another source of harmonics is the supply network itself. Magnetisation of the energy
transformer core and its saturation cause non-sinusoidal currents that are manifested
as a THDU on the supply terminals.
Origins of harmonics disturbances

single phase rectifiers
– high 3
rd
harmonic, THDI 80%

three phase loads
– 5
th
, 7
th
, 11
th
, 13
th
, 17
th
harmonic

non-symmetrically controlled supply
– even harmonics and DC

higher pulse number
– lower THDI

serial inductance decreases THDI

LV power supply network
– THDU 1.5 ÷ 4.5%, mainly 5
th
harmonic
Impact on customers’ equipment

overall energy efficiency is decreased

premature ageing of system components

triple harmonics can produce high currents in a neutral line causing overheating
and losses

increased heating, noise and vibrations in transformers and motors

current into capacitor bank increases with harmonic order causing failures

presence of harmonic increase possibility of resonance

problems with signalling frequencies

tripping of protection devices