After being introduced to the problem of power quality issues, one might wonder what the real implications are, particularly for residential consumers. This article explains what voltage fluctuations, harmonic oscillations and transients really are, and why they are important considerations for any Electrical Engineer.
These are closely related to a forthcoming article on Flexible AC Transmission Systems.
When the receiving terminal of a transmission line operates a high-power load drawing a large amount of current, the system voltage tends to drop, leading to an undervoltage condition colloquially known as voltage sag. This can have undesirable effects for consumers, since devices may malfunction and particularly sensitive equipment such as electronics may not work at all. Factories and other industrial plants often consume large amounts of power, so they can cause frequent and prolonged periods of undervoltage if left unchecked.
Conversely, when the receiving end has a lower load than expected (known as load rejection), the voltage can exceed the nominal voltage by a significant margin. This can happen when a large load is suddenly disconnected from the grid, such as when a factory’s circuit breaker trips. During periods of low load on the transmission line, the line voltage increases along the length of the line due to the line charging capacitance.
Whatever the cause, installation of FACTS devices can correct voltage fluctuations without re-quiring manual intervention by agents of the power utility. In fact, this is the primary function and advantage conferred by parallel (shunt) compensation devices and will be the topic of a future post.
Harmonics and Transients
Some loads, such as rectifiers, are non-linear in nature and can result in a distortion of the ideal sinusoidal waveform shape. In other instances, disturbances such as lightning hitting power transmission lines or a sudden transient fault like a power line swaying in the wind and hitting a tree can interrupt power delivery if not detected and counteracted. The problem of trees in transmission line paths is a particular concern because impacts with them can cause blackouts and complete system failures, as happened during the Northeast Blackout of 2003.
Harmonics in the power waveform can cause equipment damage or malfunction, and more importantly, can cause the power system to become unstable. Thus, to raise the dynamic stability limit of the power system, devices must be in place to handle harmonic and transient disturbances. Luckily, craft engineers and scientists have developed a FACTS configuration useful for controlling these problems as well.
This article was taken from a report which I co-authored. It was submitted to ECE3333: Power Systems I, taught by Professor Rajiv Varma at the University of Western Ontario in Spring 2009.