Power quality problems and advanced power electronics solutions [Part 6/8: Other power quality problems]

After the introduction to waveform distortions, short & long duration variations and transients, this sixth article will discuss other power quality problems.

Unbalances

Unbalance is a voltage or current variation in a three-phase system in which the three voltage or current magnitudes or the phase-angle differences between them are not equal.

Causes

Unbalance is primarily due to unequal loads on distribution lines or within the installation. Unbalance can be caused by external utility supply due to malfunctioning equipment, but the common source of unbalances is internal, and caused by loads or generators located in the installation.?

Using single-phase loads like induction furnaces, traction loads and welding machines, or single-phase generators on a three-phase electric power system results in unbalanced conditions in the system. The unbalanced currents result in unbalanced voltages and affect other loads and generators connected to the electric power system.

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. Unbalances usually show as heating, especially with transformers or solid state motors. Greater unbalances may cause excessive heat to motor components, and the intermittent failure of motor controllers.

Voltage fluctuations (flicker)

A voltage fluctuation is a systematic variation of the voltage waveform or a series of random voltage changes, of small dimensions, between 0.1% and 7% of nominal at a low frequency, generally below 30 Hz.

Causes

Equipment that produces continuous, rapid load current variations (mainly in the reactive component) can cause voltage fluctuations. Normally, these loads have a high rate of change of power with respect to the short-circuit capacity at the point of common coupling. Examples of these loads include electric arc furnaces, static frequency converters, cycloconverters, rolling mill drives, winders and the starting process of large motors.

Voltage fluctuation can be caused also by frequent starting and stopping of electric motors (for instance elevators), arc welders, resistance welding machines, lamp dimmers, large electric motors with variable loads, boilers, cranes, HVAC systems, medical imaging systems, and any kind of oscillating loads. Other causes could be capacitor switching, transformer on-load tap changers (OLTC), step voltage regulators and other devices that alter the inductive component of the source impedance. Variations in generation capacity, particularly intermittent types like wind turbine generators also produce voltage fluctuations.

Consequences

Flicker, for example of lighting and screens, is considered the most significant effect of voltage fluctuation because it can affect the production environment by causing personnel fatigue and lower work concentration levels. In addition, voltage fluctuations may subject electrical and electronic equipment to detrimental effects that may disrupt production processes with considerable financial costs.

Other effects of voltage fluctuation include nuisance tripping due to misoperation of relays and contactors, unwanted triggering of UPS systems to switch to battery mode and problems with some sensitive electronic equipment, which require a constant voltage (for example, medical laboratories).

Power frequency variations

Frequency variation is extremely rare in stable utility power systems, especially systems interconnected via a power grid. Power frequency variations are defined as the deviation of the power system fundamental frequency from its specified nominal value (50 or 60 Hz).

Causes

While rare in power generated by electric utilities, frequency variations are most common in sites that have dedicated standby generators or poor power infrastructure. Frequency variation is more common especially if the generator is heavily loaded. Power frequency variations can also be created by the disconnections of a large block of load or by the intermittent operation of load equipment.

Consequences

Power frequency variation would affect any motor or sensitive device that relies on steady regular cycling of power over time. Frequency variations may cause a motor to run faster or slower to match the frequency of the input power. This would cause the motor to run inefficiently and/or lead to added heat and degradation through increased motor speed and/or additional current draw. Power frequency variation could also produce flicker in lights and stop some manufacturing processes.

The next article of this series will present different solutions to power quality problems.

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About the author:

Pedro Esteban is a versatile, multicultural and highly accomplished marketing, communications, sales and business development leader who holds since 2002 a broad global experience in sustainable energy transition including renewable energy, energy efficiency and energy storage. Author of over a hundred technical publications, he delivers numerous presentations each year at major international trade shows and conferences. He has been a leading expert at several management positions at General Electric, Alstom Grid and Areva T&D, and he is currently working at Merus Power Plc.