Electrical Transients; Operation With A Standby Generator - Danfoss VLT AQUA Drive FC 202 Design Manual

System Integration
Table 3.16 shows the minimum time that must be allowed
between applications of input power.
Input voltage [V]
Waiting time [s]
Table 3.16 Input Power Delay

3.3.10 Electrical Transients

Transients are brief voltage peaks in the range of a few
thousand volts. They can occur in all types of power distri-
bution systems, including industrial and residential
environments.
Lightning strikes are a common cause of transients.
However, they are also caused by switching large loads on
line or off, or switching other electrical transients
equipment, such as power factor correction equipment.
Transients can also be caused by short-circuits, tripping of
circuit breakers in power distribution systems, and
inductive coupling between parallel cables.
EN 61000-4-1 standard describes the forms of these
transients and how much energy they contain. Their
harmful effects can be limited by various methods. Gas-
filled surge arresters and spark gaps provide first-level
protection against high-energy transients. For second-level
protection, most electronic devices, including adjustable
frequency drives, use voltage-dependent resistors
(varistors) to attenuate transients.

3.3.11 Operation with a Standby Generator

Use backup power systems when the continued operation
is necessary in the event of line failure. They are also used
in parallel with the public power grid to achieve higher
line power. This is common practice for combined heat
and power units, taking advantage of the high efficiency
achieved with this form of energy conversion. When
backup power is provided by a generator, the line
impedance is usually higher than when power is taken
from the public grid. This causes the total harmonic
distortion to increase. With proper design, generators can
operate in a system containing devices that induce
harmonics.
MG20N622
Design Guide
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System design consideration with a stand-by generator is
recommended.
•
When the system is switched from line power
operation to generator, the harmonic load usually
increases.
•
Designers must calculate or measure the increase
in the harmonic load to ensure that the power
quality conforms to regulations to prevent
harmonic problems and equipment failure.
•
Avoid asymmetric loading of the generator must
be avoided since it causes increased losses and
may increase total harmonic distortion.
•
A 5/6 stagger of the generator winding
attenuates the 5th and 7th harmonics, but it
allows the 3rd harmonic to increase. A 2/3
stagger reduces the 3rd harmonic.
•
When possible, the operator should disconnect
power factor correction equipment because it
causes resonance in the system.
•
Chokes or active absorption filters can attenuate
harmonics as well as resistive loads operated in
parallel.
•
Capacitive loads operated in parallel create an
additional load due to unpredictable resonance
effects.
A more precise analysis is possible using line power
analysis software, such as HCS. For analyzing line power
systems, go to http://www.danfoss-hcs.com/Default.asp?
LEVEL=START for software download.
When operating with harmonic-inducing devices, the
maximum loads based on trouble-free facility operation are
shown in the harmonic limits table.
Harmonic limits
•
B2 and B6 rectifiers⇒maximum 20% of rated
generator load.
•
B6 rectifier with choke⇒maximum 20–35% of
rated generator load, depending on the
composition.
•
Controlled B6 rectifier⇒maximum 10% of rated
generator load.
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