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Variable Frequency Drives: Source Impedance and Line Reactors

Copyright © 2001 Francis J. Martino

Harmonic frequency currents generated by low horse-power rated variable frequency drives
(VFD) will interact with a power source of low impedance. The resulting current flow may
damage the IGBT (Insulated Gate Bipolar Transistor) output devices.

Manufacturers of variable frequency drives recommend the use of an input line reactor
or isolation transformer to increase the source impedance as seen by the drive. Due to
differences in design of IGBTs, drive manufacturer's specifications will vary. One manufacturer
requires line reactors for all drives that are used with a supply transformer that has a kVA
rating greater than ten times the input kVA rating of the drive, another requires a reactor
for drives up to 5 HP if the upstream transformer is 500 kVA or greater.

A third recommendation is to ensure that the source impedance is between 1 and 5 percent.
If the source impedance is less than 1 per cent, a reactor must be added to increase the
impedance. If the source impedance is greater than 5 per cent, the drive may not provide rated
output voltage due to the voltage drop caused by the high source impedance. In that case, the
motor may not produce rated torque. Also, too high an impedance will create a greater harmonic
voltage distortion.

In the event of a high source impedance, an input drive isolation transformer with taps
should be used. The impedance as seen by the drive will then be that of the isolation
transformer and not of the facility system. The taps may be used to boost the voltage to
the drive if necessary.

If the drive has a built- in reactor and you are using a drive isolation transformer, you may
want to remove the reactor from the circuit to eliminate the voltage drop caused by the reactor.
The transformer will provide the same harmonic attenuation and protection to the drive that
a reactor will provide.

To determine the source impedance as seen by the drive, divide the full load current capacity
of the drive by the short circuit capacity of the power system at the drive terminals and
express the result as a percentage:


Full Load Current / Short Circuit Current Capacity x 100 = Source Impedance


The contribution made to short circuit capacity by all motors connected in parallel with the
drive must be included in the source impedance calculation.

When a fault condition occurs, power system voltage will drop dramatically. All motors that
are running at that time will not be able to sustain their running speed. As those motors slow
in speed, they will regenerate, that is, the stored energy within their fields will be discharged
into the power line. That nominal discharge of energy will contribute to the fault a current that
could be as high as four times the connected motors' full load currents.

With a 1000 kVA, 480 VAC secondary, 1203 amp, 5.75% impedance transformer, we will
assume that all 1203 amps of load are from motors that are connected directly across the line.
The actual short circuit current will equal 20,924 amps plus 400% of 1203 amps for a total
of 25,736 short circuit amps. For additional information on short circuit capacity, refer to
Short Circuit Capacity: Basic Calculations and Transformer Sizing

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