Notes

3.2.1. Voltage Dips (Sags)
According to the Swedish Standard the start threshold is equal to 90 % of the reference voltage for voltage dips (called voltage sags in America). The end threshold is usually set 1 - 2 % of the reference voltage above the start threshold. Consequently, the duration of a voltage dip is measured from when the first phase drops below 90 % of the reference voltage until all three phases have again risen above 91 % - 92 % of the reference voltage. As previously mentioned, if all phases drop below 5 % of the reference voltage or the duration exceeds 1 min the event will be re-classified as an interruption or an undervoltage respectively. [2,6] Main causes of voltage dips include energizing of heavy loads (e. g. arc furnaces), starting of large induction motors, single line-to-ground (SLG) faults (see Figure 2), line-line and symmetrical faults, and transference of load from one power source to another. [4] Effects of voltage dips mainly include voltage instability and malfunctions in electrical lowvoltage devices, uninterruptible power supplies, and measuring and control equipment. Also, problems in interfacing with communication signals can arise. [4] Figure 2. Voltage dip caused by an SLG fault. [5]
3.2.2. Voltage Swells
For voltage swells the start threshold is equal to 110 % of the reference voltage according to the Swedish Standard. The end threshold is usually set 1 - 2 % of the reference voltage below the start threshold. In other words, the duration of a voltage swell is measured from when one phase rises above 110 % of the reference voltage until all three phases have again fallen below 108 % - 109 % of the reference voltage. If the event persists longer than 1 min it will be re-classified as an overvoltage. [2,6] Main causes of voltage swells include energizing of capacitor banks, shutdown of large loads, unbalanced faults (one or more phase-to-phase voltages will increase, see Figure 3), transients (see Chapter 3.5), and power frequency surges (see Chapter 3.9). [4] The effects of voltage swells are largely the same as for voltage dips (see Chapter 3.2.1). [6] 10 Figure 3. Voltage swell in the phase-to-phase voltage between a faultless phase and the faulted phase during an SLG fault. [5] 3.3.Voltage Fluctuations Voltage fluctuations are defined in the Swedish Standard as a series of voltage changes or a cyclic variation of the envelope of the voltage. These voltage changes are commonly between 90 - 110 % of the reference voltage and are considered steady-state disturbances. [2,3,4] Main causes of voltage fluctuations are startup of drives and drives with rapidly changing load or load impedance, as well as operation of arc furnaces (see Figure 4), pulsed-power outputs, resistance welders, and rolling mills. [4] Common effects of voltage fluctuations are decreased performance and instability of the internal voltages and currents of electronic equipment, as well as problems with reactive power compensation. [4]
3.3.1. Flicker
Voltage fluctuations can cause the visual phenomenon known as flicker. This consists of changes in intensity or spectral components of light sources as a result of continuous rapid changes in load current. This phenomenon can, when it reaches certain amplitudes (different amplitudes depending on the frequency of the flicker), cause discomfort for people exposed to the effects. However, flicker does not cause any malfunctions in the power system; the inducted discomfort is its only negative effect. Flicker is sometimes considered a subtype of voltage fluctuations, not an effect of the same. [3,4] Figure 4. Voltage fluctuations caused by arc furnace operation. [5] 11
3.4.Voltage/Current Unbalance
Voltage unbalance according to the Swedish Standard is a condition in a multiple-phase system during which the RMS values of the phase voltages or the phase angles between adjacent phases are not equal. An analog definition can be made for current unbalance. In a three-phase system voltage unbalance occurs when the magnitude of the three phase voltages are not exactly equal or the phase angle between phases is not exactly 120o . Unbalance disturbances can be divided into steady-state unbalances and transient unbalances depending on the duration and cause of the unbalance. [2,4] Common causes of steady-state unbalance include non-transposed overhead transmission lines and unbalanced single-phase loading of three-phase systems. For transient unbalance common causes are blown out fuses in one of the phases in a capacitor bank and single or double line-to-ground faults. [4] During unbalance negative sequence components (see Chapter 3.6.2.1) appear, resulting in different currents being drawn in different phases. This causes torque ripple in three-phase motors, as well as hampers their performance and causes unequal losses and heating between the phases and their loads. The torque ripple can lead to excessive wearing and thereby shortened life span of the motors. Furthermore, reactive power demand will increase and reactive power compensation will become more difficult. The end result can be mal-operation of equipment and measuring instruments, as well as shortened life spans of appliances. [4]
3.5.Transients
Transients, or transient overvoltages, are short-duration either oscillating or impulsive voltage phenomena with a duration of usually a few milliseconds or shorter and normally heavily dampened. Though short in duration they often create very high magnitudes of voltage. [2,4] Transients with high voltage magnitudes cause insulation breakdown in the power system and transients with high current magnitudes can burn out devices and instruments. Other effects of transients include mal-operation of relays and mal-tripping of circuit breakers. [4]
3.5.1. Impulsive transients Impulsive transients (sometimes referred to as surges) are unidirectional, i. e. never switch polarity. They are often described with rise time (time to reach the voltage peak), time-to-half (time until the surge has decayed to half the peak value) and peak voltage magnitude. Their duration is usually very short, often below one millisecond and their spectral components are often very high frequent. [9] The main cause of impulsive transients is lightning strikes (see Figure 5). [9] 12 Figure 5. Impulsive transient current caused by a lightning strike, result of PSpice simulation. [5]
3.5.2. Oscillatory transients
Oscillatory transients rapidly switch polarity, often numerous times during their existence. Their duration is often longer than for the impulsive transients, whereas their spectral components can be of a wide range of frequencies, from a few hundred Hertz up to hundreds of kilohertz. Oscillatory transients can be divided into subgroups based on their predominant frequency (high-, medium-, and low-frequent). [9] Among the many usual causes of oscillatory transients are energizing of transformers, capacitors (see Figure 6), lines, and cables, as well as re-strike during capacitor de-energizing, cable switching, ferroresonance, and system response to impulsive transients. [4,6] Repeated oscillatory transients can cause the magnetic properties of core materials used in electrical machines to change. [4] Figure 6. Low-frequent oscillatory transient caused by capacitor bank energizing. [5]
13 3.6.Waveform distortion
Waveform distortion is defined as a steady-state deviation from an ideal sine wave of power frequency, primarily categorized by the spectral content of the deviation. There are four major subtypes: DC offset, harmonics, notching, and electric noise. Harmonics are further divided into integer harmonics, interharmonics, and subharmonics. [9]
3.6.1. DC Offset
A DC offset is a DC component of the voltage or current in an AC system. Main causes are rectifiers or other electronic switching devices being operated in the power system, as well as transformer saturation and geomagnetic disturbances. Faults in electrical machines can also cause severe transient DC offsets. [4] Possible negative effects of a DC offset in an AC network are half-cycle saturation of transformer cores, generation of even integer harmonics (see Chapter 3.6.2.1), and increased temperatures in transformers, rotating machines, and electromagnetic devices which may shorten their lifetime, as well as electrolytic erosion of grounding electrodes and other connectors. [4]
3.6.2. Harmonics
Harmonics are sinusoidal components of the system voltage or current with frequencies other than the power frequency, also called the fundamental frequency. [4] Harmonics in general are often caused by operation of rotating machines, arcing devices, semiconductor based power supply systems, converter-fed AC drives, thyristor controlled reactors, phase controllers, and AC regulators, as well as magnetization nonlinearities of transformers. [4] The general effects of harmonics include increased thermal stress and losses in capacitors and transformers, as well as poor damping, increased losses, and in other ways degraded performance of rotating motors. Furthermore, transmission systems are subject to higher copper losses, corona, skin effect, and dielectric stress and also interference with measuring equipment and protection systems. [4] Harmonics also negatively affect consumer equipment such as television receivers, fluorescent and mercury arc lighting, and the CPUs and monitors of computers. [4] The three subtypes of harmonics – integer harmonics, interharmonics and subharmonics – are briefly described below.
3.6.2.1. Integer Harmonics
Integer harmonics (sometimes just called harmonics, however not in this thesis) have frequencies which are integer multiples of the fundamental frequency. Integer harmonics can be divided into odd and even integer harmonics depending on whether their frequencies are odd or even integer multiples of the fundamental frequency, with the odd harmonics being overwhelmingly more common in the power system. [4,9] 14 A further division of the odd integer harmonics into three harmonic phase sequences is very useful. The method of phase sequence components allows transformation of any unbalanced set of three-phase voltages or currents into three balanced sets: A positive-sequence set with the same phase sequence as the normal three-phase system (A-B-C), a negative-sequence set with opposite phase rotation (A-C-B), and a zero-sequence set with no phase rotation (same phase angle for all three phases). [9] An important subtype of odd integer harmonics, consisting of zero-sequence harmonics, is triplen harmonics. These are odd multiples of the third harmonic, i. e. have frequencies of 3, 9, 15 etc. times the fundamental frequency. Since triplen harmonics are predominantly zero-sequence harmonics the triplen harmonic contributions from all three phases will be added in neutral conductors, where such exist, and therefore triplen harmonics might have to be given special consideration in some cases. [9] Integer harmonics are caused by nonlinear loads and other nonlinear equipment in the power system. [9]
3.6.2.2. Interharmonics
Interharmonics have frequencies higher than the fundamental, but not integer multiples of it. Main sources of integer harmonics include frequency converters, cycloconverters, induction furnaces, and arcing devices. Interharmonics are often load dependent, i. e. are not constant over time, and products of frequency conversion. [4,9] Interharmonics have been known to cause flicker (see Chapter 3.3.1) and slow oscillating power frequency variations (see Chapter 3.7). [8] 3.6.2.3. Subharmonics Subharmonics have lower frequencies than the fundamental frequency. These are rare harmonic disturbances. [4]
3.6.3. Notching
Notching disturbances are non-sinusoidal, periodic waveform distortions and, as the name suggests, consist of notches in the fundamental sine wave component. This is caused by the commutation of current from one phase to another during the continuous operation of power electronic devices (see Figure 7). [9] Figure 7. Voltage notching caused by a three-phase converter. [9] 15 Excessive levels of notching can lead to problems in the operation of sensitive communication or data-processing loads. [1]
3.6.4. Electric Noise
Electric noise (or electrical noise) is made up of low magnitude electrical signals from a broad frequency spectrum lower than 200 kHz. Noise is commonly used to describe all sorts of unwanted waveform distorting signals which cannot be classified as harmonics, notching or DC Offset, or sometimes as an even broader term. [9] There are numerous possible sources including faulty connections in transmission or distribution systems, arc furnaces, electrical furnaces, power electronic devices, control circuits, welding equipment, loads with solid-state rectifiers, improper grounding, and turning off capacitor banks, as well as adjustable-speed drives, corona, and interference with communication circuits. [4] Electric noise can have negative effects on the operation of electronic devices like microcomputers and programmable controllers. [4]
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Momentary voltage sags lasting less than 100 ms are often sufficient to cause disruptions to susceptible equipment and operations (see example in Figure 15). Even though the effect of these disturbances can be the same as long duration interruptions, they can be more important because they occur much more frequently. These disturbances are caused by faults on distribution circuits and transmission circuits. The interconnected nature of the system means that faults remote from a facility can still cause a momentary voltage sag that could be sufficient to affect operations.