SWITCHING TRANSIENT PROBLEM WITH LOADS

KKKL 4073

POWER QUALITY

LECTURER: PROF. MADYA DR M.A HANNAN

BY NORALINA ABDUL AZIZ A117805

TRANSIENT

PROBLEM

RELATED TO

LOAD AND

LOAD

SWITCHING

NUISANCE

TRIPPING OF

ASDs

TRANSIENT

FROM LOAD

SWITCHING

TRANSFORMER

ENERGIZING

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

NUISANCE TRIPPING OF ASDs

Effect of capacitor switching on adjustable

speed drive ac current and dc voltage

TRANSIENT FROM LOAD SWITCHING

• energizing inductive circuits with air-gap switches,such as relays and contactors, can generate burstsof high-frequency impulses.

Fast transient can caused by deenergizing an

inductive load

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.

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.

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.

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

TRANSFORMER ENERGIZING

• A dynamic overvoltage waveform caused by a third harmonic resonance in the circuit

Dynamic overvoltage during transformer energizing

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.

THANKS FOR

THE TIME