Electric power quality

[1] Electric power quality is a power supply system's voltage, frequency, and waveform level conform to established specifications. Good power quality can be defined as a steady supply voltage that stays within the prescribed range, steady AC frequency close to the rated value, and smooth voltage curve waveform (which resembles a sine wave). In general, it is useful to consider power quality as the compatibility between what comes out of an electric outlet and the load that is plugged into it. The term is used to describe electric power that drives an electrical load and the load's ability to function properly. Without the proper power, an electrical device (or load) may malfunction, fail prematurely or not operate at all. There are many ways in which electric power can be of poor quality, and many more causes of such poor quality power.

[1] The electrical power quality can be depcited as a set of values of parameters:

Continuity of service (whether the electrical power is subject to voltage drops or overages below or above a threshold level thereby causing blackouts or brownouts)
Variation in voltage magnitude (see below)
Transient voltages and currents
Harmonic content in the waveforms for AC power

Faults

A overvoltage fault is defined as VRMS increase, 110% above of a system's nominal voltage for longer than 1 min.

power quality faults (disturbances), specifically focus on 3 types:

Overvoltage: prolonged increase in voltage above normal levels
Sag (Dip): temporary drop in voltage below normal levels
Swell: temporary increase in voltage above normal levels

[2] Fig 1. Undervoltage and overvoltage occur if voltage at the load drops below a min rated voltage or climbs above a max rated voltage for beyond 1 min.

[2] An undervoltage occurs if the RMS drops below 90% the nominal RMS and stays at that level for more than 1 min.

The term “brownout” often refers to an intentional/unintentional voltage drop in an electrical power supply system.

An overvoltage is when the RMS rises above 110% the nominal RMS and stays so for more than 1 min.

[2] Fig 2. Sags and Swells occur on the power system when voltage drops below or exceeds nominal voltage for a short duration.

[2] A sag is when the RMS reduces between 10-90% for half-cycle to 1 min. In a 60 Hz power system, a full sine wave lasts ~16 ms, a half cycle is ~8 ms.

A swell is a RMS is when the RMS rises above 110% for a half-cycle to 1 min.

Voltage reductions and sags often occur if the RMS current to the load increases significantly.

There are 3 types of sags and swells, depending on durations:

0.5 cycles to 30 cycles: Instantaneous
30 cycles to 3 seconds: Momentary
3 seconds to 1 minute: Temporary
1 minute+: Sustained Undervoltage or Overvoltage

Voltage imbalance

Voltage imbalance/unbalance is a power quality issue in 3-phase electrical systems. It occurs if the 3 line voltages are uneven in size or when the phase angles between them are not evenly spaced at 120 degrees, leading to system stress and extra heat in connected equipment.

Voltage imbalance is mathematically defined as:

E.g., given these phase voltages: P1 = 23.4 kV, P2 = 23.2 kV, P3 = 22.8 kV.

The average voltage is: (23.4 + 23.2 + 22.8) / 3 = 23.1 kV
The voltage deviation is: |23.1 - 22.8| = 0.3 kV
Voltage imbalance is: 0.3 / 23.1 = 1.3%

Alternatively, the deviation can be calculated the value per phase:

Phase 1: |23.4 - 23.1| = 0.3 kV
Phase 2: |23.2 - 23.1| = 0.1 kV
Phase 3: |22.8 - 23.1| = 0.3 kV;

Where 0.3 kV is the highest value, so it's the overall deviation.

References

My documents

[1] ELE8945 Week 12 (Canva)

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Electric power quality

Faults

Voltage imbalance

References

My documents

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