9 Most Common Power Issues

The most common types of Power Quality problems are presented below along with their description, causes and consequences:

Voltage Sag

 

Description: A decrease of the normal voltage level between 10% and 90% of the nominal RMS voltage at the power frequency, for durations of 0,5 cycles to 1 minute.

Causes: Faults on the transmission or distribution network (most of the times on parallel feeders). Faults in consumer’s installation. Connection of heavy loads and start-up of large motors.

Consequences: Malfunction of information technology equipment, namely microprocessor-based control systems (PCs, PLCs, ASDs, etc) that may lead to a process stoppage. Tripping of contactors and electromechanical relays. Disconnection and loss of efficiency in electric rotating machines.

2. Very short interruptions

Description: Total interruption of electrical supply for duration from a few milliseconds to one or two seconds.

Causes: Mainly due to the opening and automatic reclosure of protection devices to decommission a faulty network section. The main fault causes are insulation failure, lightning, and insulator flashover.

Consequences: Tripping of protection devices, loss of information, and malfunction of data processing equipment. Stoppage of sensitive equipment, such as ASDs, PCs, and PLCs, if they’re not prepared to deal with this situation.

3. Long interruptions

Description: Total interruption of electrical supply for duration greater than 1 to 2 seconds

Causes: Equipment failure in the power system network, storms and objects (trees, cars, etc.) striking lines or poles, fire, human error, bad coordination or failure of protection devices.

Consequences: Stoppage of all equipment.

4. Voltage spike

Description: Very fast variation of the voltage value for durations from several microseconds to a few milliseconds. These variations may reach thousands of volts, even at low voltage.

Causes: Lightning, switching of lines or power factor correction capacitors, disconnection of heavy loads.

Consequences: Destruction of components (particularly electronic components) and of insulation materials, data processing errors or data loss, and electromagnetic interference.

 

5. Voltage swell

Description: Momentary increase of the voltage, at the power frequency, outside the normal tolerances, with a duration of more than one cycle and typically less than a few seconds.

Causes: Start/stop of heavy loads, badly dimensioned power sources, badly regulated transformers (mainly during off-peak hours).

Consequences: Data loss, the flickering of lighting and screens, stoppage or damage of sensitive equipment if the voltage values are too high.

6. Harmonic distortion

Description: Voltage or current waveforms assume a non-sinusoidal shape. The waveform corresponds to the sum of different sine waves with different magnitudes and phases, having frequencies that are multiples of power-system frequency.

Causes: Classic sources: electric machines working above the knee of the magnetization curve (magnetic saturation), arc furnaces, welding machines, rectifiers, and DC brush motors.

Modern sources: all non-linear loads, such as power electronics equipment, including ASDs, switched-mode power supplies, data processing equipment, and high-efficiency lighting.

Consequences: Increased probability in the occurrence of resonance, neutral overload in 3-phase systems, overheating of all cables and equipment, loss of efficiency in electric machines, electromagnetic interference with communication systems, errors in measures when using average reading meters, nuisance tripping of thermal protections.

7. Voltage fluctuation

Description: Oscillation of voltage value, amplitude modulated by a signal with a frequency of 0 to 30 Hz.

Causes: Arc furnaces, frequent start/stop of electric motors (for instance, elevators), oscillating loads.

Consequences: Most consequences are common to under voltages. The most perceptible consequence is the flickering of lighting and screens, giving the impression of the unsteadiness of visual perception.

8. Noise

Description: Superimposing of high-frequency signals on the waveform of the power-system frequency.

Causes: Electromagnetic interferences provoked by Hertzian waves such as microwaves, television diffusion, and radiation due to welding machines, arc furnaces, and electronic equipment. Improper grounding may also be a cause.

Consequences: Disturbances on sensitive electronic equipment, usually not destructive. May cause data loss and data processing errors.

9. Voltage Unbalance

Voltage Unbalance

Description: A voltage variation in a three-phase system in which the three voltage magnitudes or the phase-
angle differences between them are not equal.

Causes: Large single-phase loads (induction furnaces, traction loads), incorrect distribution of all single-phase loads by the three phases of the system (this may also be due to a fault).

Consequences: Unbalanced systems imply the existence of a negative sequence that is harmful to all three-phase loads. The most affected loads are three-phase induction machines.

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