Description, Causes and Consequences
The most common types of Power Quality problems are presented below along with their description, causes and consequences
1. Voltage sag (or dip)
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 cycle 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
few milliseconds to one or two seconds.
Causes: Mainly due to the opening and automatic reclosure of protection devices to decommission a faulty section of the network. 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, 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 a several microseconds to few milliseconds. These variations
may reach thousands of volts, even in 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, electromagnetic interference.
5. Voltage swell
Description: Momentary increase of the voltage, at the power
frequency, outside the normal tolerances, with 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, 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 non-sinusoidal
shape. The waveform corresponds to the sum of different sine-waves with different magnitude and phase, 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, high efficiency lighting.
Consequences: Increased probability in 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 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 undervoltages. The most perceptible consequence is the flickering of lighting and screens, giving the impression of unsteadiness of visual perception.
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
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 be also 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.
|PQ Problem||Major Sources||Solutions|
|Voltage variations||Load changing||Sync. loads, Fast PF correction|
|Voltage flicker||Load changing, mainly welding||Fast PF correction|
|Voltage dip||Motor and other load startups||Motor starters, VFD, UPS, Flywheel, DVR, Fast PF correction, STATCOM|
|Overvoltage||Over PF compensation||Fast PF correction|
|Transients||Connections, Switching||Controlled switches|
|Unbalance||Unbalanced loads, transformer phase shift||Balance the loads|
|Harmonics||Non linear loads, resonance||Active/passive filtration, detuned capacitors, Improved VFD operation|