Harmonic Input Reactors (Chokes) - Siemens Midimaster Eco Reference Manual

HARMONIC INPUT REACTORS
(CHOKES)
Mains Supply Harmonics
When the inverter is operating it draws a non-
sinusoidal current from the mains supply. This
is caused by the input rectifier stage of the
inverter which converts the mains supply AC
voltage and current to DC voltage and current.
The input rectifier draws current from each of
the three mains supply phases in turn. This
drawn current is not sinosoidal, as a normal AC
sine-wave would be, but is distorted by the
rectifier switching between phases. This
distorted mains supply current waveform is
said to contain harmonics since the distorted
waveform can be broken down into a set of
sine-wave components which is known as its
harmonic components.
Since the rectifier stage of the inverter is a full
six-pulse bridge rectifier module and the mains
supply is a balanced standard three phase
supply only certain harmonic "numbers" are
dominant. The dominate harmonic numbers,
associated with all three phase supplied
inverter units operating on a 50Hz supply, are
the 1st (fundamental, 50Hz), 5
th
(350Hz), 11 (550Hz), 13
If operating on a mains supply of 60Hz (North
America) then the harmonic frequencies would
be the 1st (fundamental, 60Hz), 5
th
(420Hz), 11 (660Hz), 13
As the harmonic number gets larger the
magnitude of the harmonic current reduces,
therefore only the first few lower order
harmonics tend to dominate and are important.
Harmonic voltages and currents can be
detrimental to other electrical equipment
connected to the same supply as the inverter
since the additional currents flowing in supply
transformers and cables can cause their
overheating. The harmonic voltages can
increase the peak voltage of the supply which
increases stress on electrical insulation and
can cause early-life failures to equipment such
as power factor correction capacitor banks. It
is therefore desirable to reduce the harmonics
generated by inverters to an acceptable level.
15-18
th th
(250Hz), 7
th
(650Hz), and so on.
th th
(300Hz), 7
th
(780Hz), and so on.
ECO REFERENCE MANUAL
Mains Supply Line Reactors Selection
Line Reactors are used to reduce the
harmonics produced by inverters and will assist
the installation engineer to meet the relevant
harmonic standards. They are also used to
increase the mains supply impedance and help
absorb mains supply line spikes and surges
which can occur if operating from a standby
generator or close to heavy industrial
equipment.
2% impedance Line Reactors are typical
sufficient to absorb mains supply spikes and
will prevent nuisance tripping of inverters in
most applications. They also protect the DC
link capacitors internal to the inverter against
overheating and early life failures by absorbing
surges and increasing the mains supply
impedance.
4% impedance Line Reactors are best for
reducing the harmonic frequency currents
produced by inverters and thereby reduce the
Total Harmonic Voltage Distortion (THD) at the
point of common supply coupling to other
users.
Line reactors are connected in series between
the mains supply and the inverter power input
terminals. If an EMC filter is also used then the
line reactor would be connected in series
between the EMC filter and the mains supply.
Supply Impedance
The impedance of the mains supply should not
be less than 0.5% at the inverter terminals.
This means that the supply voltage drop which
occurs when the inverter is fully loaded should
be greater than or equal to 0.5% of the nominal
voltage. If the mains impedance is below this
value the lifetime of the DC link electrolytic
capacitors could be reduced. To overcome
this effect, 2% input reactors should be fitted to
increase the inverter supply impedance.
If a further reduction of harmonic currents is
also required then 4% input reactors can be
fitted which will increase the inverter supply
impedance as well as reduce the inverter
harmonic currents.