Power quality: some fundamentals

In our June issue, we reported that power quality issues are a growing concern all over the world, and we noted in particular that the adoption of the smart grid would do little to address this concern. The primary function of the smart grid is to increase the reliability of power delivery – any positive impact it may have on power quality is no more than incidental. With these thoughts in mind, in this issue we are going to look at some of the fundamentals of power quality, and in particular at the most common types of power quality problems and why they occur.

Under-voltage
Under-voltage is a decrease in rms voltage to less than 0.9 pu for a duration longer than one minute. Typical values encountered in practice are between 0.8 and 0.9 pu. Under-voltages are most often caused by the switching of loads and capacitor banks and may persist until the voltage regulation equipment on the system has time to react and bring the voltage back within acceptable tolerance. Overloaded circuits can also cause under-voltages.

It is worth noting that the term “brownout” is sometimes used to describe periods when the supply voltage has been deliberately reduced as a strategy for reducing power delivery. The type of supply disturbance caused by a brownout is essentially the same as an under-voltage, but the term brownout has no formal definition and, to avoid possible confusion, its use should be avoided.

Over-voltage
Over-voltage is an increase in rms voltage to more than 1.1 pu for a duration longer than one minute. Typical values are between 1.1 and 1.2 pu. Over-voltages typically result from load switching, particularly when large loads like motors are switched off; variations in reactive compensation, usually the switching of capacitor banks; poor system voltage regulation capabilities; and incorrect tap settings on transformers.

Voltage sags and swells
Voltage sags (also called dips) and swells are two of the most common power quality problems. They are impossible to eliminate completely; as impedances change over the course of a day, the system voltage will also momentarily change. This is unfortunate, as even short duration sags can lead to process shutdowns that take many hours to re-start. Voltage swells are one of the most frequent causes of circuit breaker nuisance tripping. In short, sags and swells can cause major financial losses, particularly in the manufacturing sector.

Voltage sags are often caused by sudden increases in load, such as short circuits or faults, motors starting or electric heaters turning on. They can also be the result of sudden increases in the source impedance of the supply, typically caused by a loose connection. Voltage swells are almost always caused by sudden decrease in the load on a circuit that has a poor or damaged voltage regulator, although they can also be caused by loose or damaged neutral connections.

For Class A sag detection on single-phase systems, a voltage sag event begins when the Urms(1/2) voltage (the rms voltage of the supply calculated over a half cycle) falls below the sag threshold. The event ends when the Urms(1/2) is equal to or greater than the sag threshold plus the hysteresis voltage. On poly-phase systems, the sag begins when the Urms(1/2) voltage of one or more channels is below the sag threshold and ends when the Urms(1/2) voltage on all channels is equal to or greater than the sag threshold plus the hysteresis voltage.

For Class A swell detection on single-phase systems, a swell is defined as beginning when the Urms(1/2) voltage rises above the swell threshold, and finishing when the Urms(1/2) voltage is equal to or less than the swell threshold minus the hysteresis voltage. On poly-phase systems, the swell begins when Urms(1/2) on one or more channels rises above the swell threshold and finishes when Urms(1/2) on all of the measured channels is equal to or less than the swell threshold minus the hysteresis voltage.

Transients
There are two main types of transients over voltages. Low-frequency oscillatory transients have frequency components in the hundreds-of-hertz range. Low frequency oscillatory transients are typically caused by capacitor switching.High frequency or impulsive transients have frequency components in the hundreds-of-kilohertz range. High frequency impulse transients are typically caused by lightning or the switching of inductive loads.

Transient over voltages can lead to dielectric degradation or failure in all classes of equipment. Large magnitude transients with a fast rise time contribute to insulation breakdown in equipment such as switchgear, transformers and motors, while repeated exposure to lower amplitude transients can cause slow degradation of insulation, leading to eventual failure and reducing mean time between failures (MTBF).

The mechanism by which transients damage insulation can be understood by considering cables and other forms of insulated electronics as capacitors, with the insulation acting as the dielectric of the capacitor. The capacitance of the system provides a path for the transient pulse. If the transient pulse has sufficient energy, it will damage the insulation.

Lightning is a major source of transients. Lightning strikes, which can be more than 8 km long and reach temperatures in excess of 20,000 ºC, and the electromagnetic fields produced by such strikes, can induce voltage and current transients in power lines and communication lines. These transients are typically unidirectional.

The switching of capacitor banks is another common source of transients. When a capacitor bank is switched, there is an initial inrush of current, which produces a low-frequency transient that has an oscillatory ringing characteristic. Such oscillatory transients can cause equipment to trip out as well as malfunctions in uninterruptible power supply (UPS) installations.

Less frequently encountered are extremely fast transients (EFTs) that have rise and fall times in the nanosecond region. These are caused by arcing faults, such as bad brushes in motors, and are  rapidly damped by even a few metres of distribution wiring. Standard line filters, which are included in almost all electronic equipment, are very effective at removing EFTs, but EFTs may still cause problems in installations with very short cable runs, such as those found on off-shore platforms.

Unbalance
Unbalance is a condition in a poly-phase system where the values of the fundamental component of the line voltages, or the phase angles between consecutive line voltages are not equal, as defined by IEEE 1159 and IEC 61000-4-7. Voltage unbalance is most commonly seen in relation to individual customer loads with an imbalance of the loads on the phases, especially where large single-phase loads, such as arc furnaces, are in use. It is important to note that a small unbalance in the phase voltages can produce a much larger unbalance in the phase currents.

Unbalanced voltages can adversely affect many types of equipment including induction motors and variable speed drives. In addition, unbalanced voltages can cause heating in transformers and neutral conductors.

Flicker
Flicker is a very specific problem related to human perception of the light output of incandescent light bulbs. It is not a general term for voltage fluctuations. The human eye is very sensitive to the light flicker that is produced by voltage variations. Because of this, flicker is almost always the limiting criterion for controlling small voltage fluctuations.

If the sensitivity of the human eye to flicker is assessed by considering the eye’s response to flicker from a 60-watt incandescent bulb for rectangular voltage variations at various rates, it is found that that the sensitivity is a function of the rate of fluctuations and is also to some extent dependent on the voltage of the lighting supply.

In general, flicker is measured using the method defined in IEC 61000-4-15. This method takes the instantaneous voltage and compares it with a rolling average voltage. The deviation between these two is multiplied by a value taken from a weighted curve based on the sensitivity to flicker of the human eye to incandescent bulbs operating at either 120 V 60 Hz or 230 V 50 Hz. The result – the percentile unit – is subjected to further statistical analysis in order to calculate two values, Pst and Plt.

Pst, or short-term flicker is calculated from the percentile unit and is based on behaviour over a 10-minute interval. Plt, or long-term flicker is calculated from Pst and is based on a two-hour interval. The criteria for evaluating the results are straightforward. If Pst is less than 1.0, the flicker levels are good but if Pst is greater than 1.0, the flicker levels may be high enough to be annoying. All of this applies only to incandescent lighting – other types of lighting cannot be evaluated in this way. In addition, the weighting curves apply only to lighting that operates at 120 V 60 Hz or 230 V 50 Hz.

Harmonics
Harmonics are sinusoidal periodic waves with frequencies that are integer multiples of the fundamental frequency. Harmonics can cause many problems, including excessive heating in neutral conductors, overheating of motors and transformers, and failure of electronic equipment.

IEEE 519 defines a harmonic as a component of order greater than one of the Fourier series of a periodic quantity. IEC 61000-4-30 defines a harmonic frequency as a frequency which is an integer multiple of the fundamental frequency, and defines a harmonic component as any of the components having a harmonic frequency.

Linear loads such as incandescent lights and heating elements draw current equally at every point of the supply waveform. These loads do not generate harmonics. However, non-linear loads such as switching power supplies and variable speed drives usually draw current only at the peaks of the supply waveform. It is these non-linear loads that cause harmonics. Typically, current harmonics do not propagate through a system, but voltage harmonics will propagate as they can pass through transformers. Voltage Harmonics occur when current harmonics are great enough to start clipping the voltage in various locations throughout the waveform.

Harmonics can be characterised by their order – which is equal to their multiple of the fundamental frequency. Thus a 180 Hz harmonic in a 60 Hz supply system is a third order harmonic. Odd harmonics are harmonics with odd order numbers and even harmonics are those with even order numbers.

Even harmonics are often produced by faulty rectifiers and produce waveform distortion that is non-symmetrical. Triplens are harmonics with orders that are multiples of three. These do not cancel out in three-phase systems and, as a result, they give rise to high neutral currents.

Harmonics can also be characterised by sequence, based on the direction of rotation of the magnetic field they produce. Positive sequence harmonics create a magnetic field in the direction of rotation of the fundamental. Indeed, the fundamental can be considered to be a positive sequence harmonic. Negative sequence harmonics produce magnetic fields that rotate in the opposite direction, which reduces torque in motors and increases the current required to drive a given load. Zero sequence harmonics do not produce a rotating magnetic field. Zero sequence harmonics can be in phase. This can lead to high neutral currents, high neutral to ground voltages, transformer losses as well as equipment overheating.

Positive, negative and zero sequence harmonics run in sequential order – positive, negative and then zero. Since the fundamental frequency is a positive sequence harmonic, the second order harmonic is a negative sequence harmonic and the third order harmonic is a zero sequence harmonic. In balanced three-phase systems, the fundamental currents cancel each other out, so that there is no current in the neutral. Zero sequence harmonics however, such as the third harmonic, add together, resulting in high neutral currents.

Total harmonic distortion
Total harmonic distortion (THD) is a measure of the sum of the harmonic components in a distorted waveform, and it can be calculated for either current or voltage. THD is the rms sum of the harmonics, divided either by the rms value of the fundamental or the rms value of the total waveform. Most often, THD is quoted as a percentage of the fundamental.

THD values can be misleading, especially when used in relation to current. The THD value is typically calculated with reference to the amplitude of the fundamental. With voltage calculations, this voltage fundamental will always be present, but the amplitude of the current fundamental changes according to the load – as the load decreases, so does the fundamental current amplitude. If the current drawn by the load is low – close to zero – the THD value will, therefore, appear to be very high.

For example, if the total harmonic current in a circuit is 0.2 A and the fundamental current is 200 A, the THD is 3.16%, but if the fundamental current being drawn by the load drops to 0.2 A, and the harmonic current remains the same, the THD is now 100%! This is deceptive as THD appears to be very high, but the only reason for this is that the load is drawing so little current at the fundamental frequency.

To avoid this problem, total demand distortion (TDD) measurements should be used for current harmonic measurements. TDD references the total root-sum-square harmonic current distortion to the maximum average demand current recorded during the test interval. The reference value is, therefore, the same throughout the test interval, ensuring that the TDD result obtained is valid. TDD is calculated in accordance with the IEEE 519 document, “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.”

In summary, the power quality industry has developed certain index values that can be used to assess the waveform distortion caused by the presence of harmonics. The two values most frequently encountered are THD and TDD. Individual harmonic values are also indexed in various specifications, such as the North American IEEE 519 document and the European EN 50160 standard issued by CENELEC.

Conclusion
This article has introduced some of the most important concepts relating to power quality, and future articles in this series will build on these. The next article will look at Class A recording and this will be followed by an article examining the impact of transients and harmonics on motors and transformers.