This document discusses transient problems related to load switching that can cause nuisance tripping of adjustable speed drives (ASDs). It notes that ASDs use voltage source inverters with capacitors in the DC link, making them sensitive to overvoltage transients from utility capacitor switching or load switching. Such transients from load switching can generate high frequency impulses when energizing inductive loads like relays or contactors. Simultaneously energizing large transformers and capacitor banks can also cause dynamic overvoltage problems if system resonances occur. Protection methods include electrical separation of sensitive equipment, as well as using filters, isolation transformers, and shielding.

1. SWITCHING TRANSIENT
PROBLEM WITH LOADS
KKKL 4073
POWER QUALITY
LECTURER: PROF. MADYA DR M.A HANNAN
BY NORALINA ABDUL AZIZ A117805

2. TRANSIENT
PROBLEM
RELATED TO
LOAD AND
LOAD
SWITCHING
NUISANCE
TRIPPING OF
ASDs
TRANSIENT
FROM LOAD
SWITCHING
TRANSFORMER
ENERGIZING

3. NUISANCE TRIPPING OF ASDs
• Most adjustable speed drives use voltage source inverter VSI
design with a capacitor in the dc link
• But the control are sensitive to dc overvoltage and may trip the
drive at the level as low as 117 percent
• Since transient voltage due to utility capacitor switching exceed
130 percent, the probability of nuisance tripping of the drive is
high

4. NUISANCE TRIPPING OF ASDs
Effect of capacitor switching on adjustable
speed drive ac current and dc voltage

5. TRANSIENT FROM LOAD SWITCHING
• energizing inductive circuits with air-gap switches,
such as relays and contactors, can generate bursts
of high-frequency impulses.
Fast transient can caused by deenergizing an
inductive load

6. TRANSIENT FROM LOAD SWITCHING
• From figure it shown cites representative 15ms burst composed of
impulses having 5ns rise times and 50ns durations
• There is very little energy in these type of energy due to short
duration, but they can interfere with the operation of electronic
loads.

7. EXAMPLE
• electrical fast transient (EFT) activity, producing spikes up to 1
kV, is frequently due to cycling motors, such as air
conditioners and elevators.
• Transients as high as 3 kV can be caused by operation of arc
welders and motor starters.
• The duration of each impulse is short compared to the travel
time of building wiring, thus the propagation of these
impulses through the wiring can be analyzed with traveling
wave theory. The impulses attenuate very quickly as they
propagate through a building.

8. PROTECTION
• Therefore, in most cases, the only protection needed is
electrical separation.
• Physical separation is also required because the high rate of
rise allows these transients to couple into nearby sensitive
equipment.
• EFT suppression may be required with extremely sensitive
equipment in close proximity to a disturbing load, such as a
computer room.
• High-frequency filters and isolation transformers can be used
to protect against conduction of EFTs on power cables.
• Shielding is required to prevent coupling into equipment and
data lines.

9. TRANSFORMER ENERGIZING
• Energizing a transformer produces inrush currents that
are rich in harmonic components for a period lasting up to 1s.
• If the system has a parallel resonance near one of the harmonic
frequencies, a dynamic overvoltage condition results that can
cause failure of arresters and problems with sensitive equipment.
This problem can occur when large transformers are energized
simultaneously with large power factor correction capacitor banks
in industrial facilities.
Energizing a capacitor and transformer
simultaneously can lead to dynamic
overvoltage

10. TRANSFORMER ENERGIZING
• A dynamic overvoltage waveform caused by a
third harmonic resonance in the circuit
Dynamic overvoltage during transformer energizing

11. TRANSFORMER ENERGIZING
• After the expected initial transient, the voltage again swells to
nearly 150 percent for many cycles until the losses and load damp
out the oscillations.
• This can place severe stress on some arresters and has been
known to significantly shorten the life of capacitors.
• This form of dynamic overvoltage problem can often be
eliminated simply by not energizing the capacitor and
transformer together.
• One plant solved the problem by energizing the transformer first
and not energizing the capacitor until load was about to be
connected to the transformer.

12. THANKS FOR
THE TIME