Power Quality Issue
As you know, deviation from sinusoidal form at the nominal voltage level is actually a power quality issue or power disturbance, defined as “any deviation from the nominal value (or from some selected thresholds based upon tolerance) of the AC input power characteristics”.
Power disturbances can greatly affect utilization equipment. For example, sensitive electronic medical equipment can malfunction, adjustable speed motor drives may trip off-line, etc.
With the high reliability requirements imposed upon power systems, it is imperative that power system disturbances, or potential disturbances, be mitigated to avoid down-time, equipment failure, and risk to human life.
The most common power disturbances are, as defined by:
What happens? An RMS increase in the AC voltage, at the power frequency, for a period of time greater than 1 min. Typical values are 110%-120% of nominal.
What happens? An RMS decrease in the AC voltage, at the power frequency, for a period of time greater than 1 min. Typical values are 80-90% of nominal.
What happens? An increase in RMS voltage or current at the power frequency for durations from .5 cycle-1 min. Typical values are 110%-180% of nominal.
What happens? An RMS reduction in the AC voltage, at the power frequency, for durations from _ cycle to a few seconds.
What happens? The complete loss of voltage. Amomentary interruptionis a voltage loss (<10% of nominal) for a time period between .5 cycles and 3 seconds). Atemporary interruptionis a voltage loss (<10% of nominal) for a time period between 3 seconds and 1 min
What happens? A switching (or other) disturbance of the normal power system voltage waveform, lasting less than _ cycle; which is initially of opposite polarity to the waveform, and is thus subtractive from the normal waveform in terms of the peak value of the disturbance voltage.
This includes a complete loss of voltage for up to _ cycle.
What happens? A subcycle disturbance in the AC waveform that is evidenced by a sharp discontinuity of the waveform. May be of either polarity and may be additive to, or subtractive from, the nominal waveform.
What happens? A variation in input voltage, either magnitude or frequency, sufficient in duration to allow visual observation of a change in electric light source intensity.
9. Harmonic Distortion
What happens? The mathematical representation of distortion of the pure sine waveform. This refers to the distortion of the voltage and/or current waveform, due to the flow of non-sinusoidal currents.
10. Electrical Noise
What happens? Unwanted electrical signals that produce undesirable effects in the circuits of the control systems in which they occur. Noise may be further categorized as transverse-mode noise, which is measurable between phase conductors but not phase-to-ground, and common-mode noise, which is measurable phase-toground but not between phase conductors.
The causes of the common power disturbances listed can vary greatly. Common causes are listed in Table below:
Common Power Disturbance Causes
|Overvoltage||• Voltage regulator malfunction|
• Improperly set transformer taps
• Improperly-applied power factor correction capacitors
|Undervoltage||• Voltage regulator malfunction|
• Improperly set transformer taps
• Large source impedance (“weak” system)
|Voltage Swell||• Recovery of system voltage following a fault|
• Remote switching (capacitors, etc.
|Voltage Sag||• Remote fault|
• Cold-load pickup (motor starting, transformer energization, etc.)
• Large step loads
|Transient (Typically voltage surges)||• Lightning strikes|
• Close-in switching (capacitors, etc.)
• Complex circuit phenomena such as current chopping, restrikes, system resonance, etc.
|Flicker||• Arcing loads such as arc furnaces|
• Also same sources that cause voltage sags and swells
|Notches and Harmonic Distortion||• Power electronic converter equipment such as rectifiers, inverters, drives, etc., which produce nonsinusoidal load current and commutation notches|
|Interruptions||• Faults causing overcurrent protective device operation|
• Utility maintenance activities
|Electrical Noise||• Power electronic converter equipment such as drives|
• Conductors and power equipment which carry large amounts of current
• Arcing in overcurrent protective devices
Reference: Power Quality Considerations – Bill Brown, P.E., Square D Engineering Services