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Note for Electrical Power Quality - EPQ by UPTU Risers

  • Electrical Power Quality - EPQ
  • Note
  • uttar pradesh technical university - uptu
  • Electrical Engineering
  • B.Tech
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supplies, UPS units, discharge lighting), adjustable speed motor drives, ferromagnetic devices, DC motor drives and arcing equipment. The current drawn by non-linear loads is not sinusoidal but it is periodic, meaning that the current wave looks the same from cycle to cycle. Periodic waveforms can be described mathematically as a series of sinusoidal waveforms that have been summed together as shown in fig. The sinusoidal components are integer multiples of the fundamental where the fundamental, in the United States, is 60 Hz. The only way to measure a voltage or current that contains harmonics is to use a true-RMS reading meter. If an averaging meter is used, which is the most common type, the error can be significant. (a) Voltage and current waveforms for non-linear loads (b) Waveform with symmetrical harmonic components Each term in the series is referred to as a harmonic of the fundamental. The third harmonic would have a frequency of three times 60 Hz or 180 Hz Symmetrical waves contain only odd harmonics and un-symmetrical waves contain even and odd harmonics. A symmetrical wave is one in which the positive portion of the wave is identical to the negative portion of the wave. An un-symmetrical wave contains a DC component (or offset) or the load is such that the positive portion of the wave is different than the negative portion. An example of unsymmetrical wave would be a half wave rectifier. DEFFERANCE BETWEEN LINEAR LOADS AND NON-LINEAR LOADS Table: Difference between linear loads and non-linear loads S. No. 1 2 LINEAR LOADS Ohms law is applicable Crest Factor = √2=1.41 NONLINEAR LOADS Ohms law is not applicable Crest Factor could be 3 to 4 3 Power factor = Cos ɸ 4 Load current does not contain harmonics. Could be inductive or capacitive. Resistive, Inductive or capacitive Zero neutral current if 1 Ph. loads are equally balanced on 3Ph. Mains (Vector sum of line current) May not demand high inrush currents while starting. Power factor ≠ Cos ɸ = Displacement factor X Distortion Factor Load current contains all ODD harmonics. 5 6 7 8 Can’t be categorized. As leading or lagging Loads. Usually an equipment with Diode and Capacitor Neutral current could be 2.7 times the line current even if 1Ph. loads are equally balanced on 3 Ph. Mains Essentially very high inrush current (20 time of I Normal) is drawn while starting for approx. One cycle. Main Power Quality Problems/Issues: 1. Harmonic distortion 2. Momentary Interruptions 3. Temporary Interruptions 4. Long Term outage 5. Noise 6. Voltage Sag 7. Voltage Swell 8. Voltage Spikes 9. Undervoltages POWER QUALITY 2

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Voltage based Power Quality Problems: • Voltage sag • Voltage swell • Voltage Interruption • Under/over Voltage • Voltage Flicker • Harmonic Distortion • Voltage Notching • Transient Disturbance • Outage and frequency variation Current based Power Quality Problems: • Reactive Power Compensation • Voltage Regulation • Current Harmonic Compensation • Load Unbalancing (for 3-phase systems) • Neutral Current Compensation (for 3phase 4-wire systems) Sinusoidal Voltage Sources of Power Quality Problems: • Power electronic devices • IT and office equipments • Arching devices • Load switching • Large motor starting • Embedded generation • Sensitive Equipment • Storm and environmental related damage Distorted Voltage (Voltage Drop)   Distorted Load Current TRANSIENTS • A transient can be unidirectional impulse of either polarity or a domped oscillatory wave with first peak occuring in either polarity. • Other definitions in common use are broad in scope and simply state that a transient is “that part of the change in a variable that disappears during transition from one steady state operating condition to another.” • Transients can be classified into two categories: impulsive and oscillatory. These terms reflect the waveshape of a current or voltage transient. Impulsive transient An impulsive transient is a sudden, non–power frequency change in the steady-state condition of voltage, current, or both that is unidirectional in polarity (primarily either positive or negative). • Impulsive transients are normally characterized by their rise and decay times, which can also be revealed by their spectral content. The most common cause of impulsive transients is lightning. Oscillatory transient • An oscillatory transient is a sudden, non–power frequency change in the steady-state condition of voltage, current, or both, that includes both positive and negative polarity values. • An oscillatory transient consists of a voltage or current whose instantaneous value changes polarity rapidly. • Oscillatory transients with a primary frequency component greater than 500 kHz and a typical duration measured in microseconds (or several cycles of the principal frequency) are considered high-frequency transients. These transients are often the result of a local system response to an impulsive transient. POWER QUALITY 3

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• A transient with a primary frequency component between 5 and 500 kHz with duration measured in the tens of microseconds (or several cycles of the principal frequency) is termed a medium-frequency transient. • A transient with a primary frequency component less than 5 kHz, and a duration from 0.3 to 50 ms, is considered a low-frequency transient. Long-Duration Voltage Variations • Long-duration variations encompass root-mean-square (rms) deviations at power frequencies for longer than 1 min. • Long-duration variations can be either overvoltages or undervoltages. • Overvoltages and undervoltages generally are not the result of system faults, but are caused by load variations on the system and system switching operations. Such variations are typically displayed as plots of rms voltage versus time. Short-Duration Voltage Variations: • Short-duration voltage variations are: voltage dips and short interruptions. Each type of variation can be designated as instantaneous, momentary, or temporary, depending on its duration as shown below: Instantaneous Interruption 0.5-30 cycles <0.1p.u Sag (dip) 0.5-30 cycles 0.1-0.9p.u Swell 0.5-30 cycles 1.1-1.4p.u Interruption 30cycles-3s <0.1p.u Sag (dip) 30cycles-3s 0.1-0.9p.u Swell 30cycles-3s 1.1-1.4p.u Interruption 3s-1min <0.1p.u Sag (dip) 3s-1min 0.1-0.9p.u Swell 3s-1min 1.1-1.4p.u Momentary Temporary Short-duration voltage variations are caused by fault conditions, the energization of large loads which require high starting currents, or intermittent loose connections in power wiring. Depending on the fault location and the system conditions, the fault can cause either temporary voltage drops (sags), voltage rises (swells), or a complete loss of voltage (interruptions). Interruption An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1 min. Interruptions can be the result of power system faults, equipment failures, and control malfunctions. Sags (dips) A sag is a decrease to between 0.1 and 0.9 p.u in rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min. Voltage sags are usually associated with system faults but can also be caused by energization of heavy loads or starting of large motors. Sags starve a machine of the electricity it needs to function, causing computer crashes or equipment lock-ups. POWER QUALITY 4

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Causes: Equipment start-up such as elevators, heating & air-conditioning equipment, compressors, and copy machines or nearby short ckts on the utility system. Swells A swell is defined as an increase to between 1.1 and 1.8 pu in rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min. As with sags, swells ar a voltage swell caused by an SLG fault. Swells can also be caused by switching off a large load or energizing a large capacitor bank. e usually associated with system fault conditions. Voltage swells may lead to damage of sensitive equipment. Causes: Due to start/stop of heavy loads and poorly regulated transformers. Voltage Imbalance: Voltage imbalance (also called voltage unbalance) is sometimes defined as the maximum deviation from the average of the three-phase voltages or currents, divided by the average of the three-phase voltages or currents, expressed in percent. The primary source of voltage unbalances of less than 2 percent is single-phase loads on a three-phase circuit. Voltage unbalance can also be the result of blown fuses in one phase of a three-phase capacitor bank. Severe voltage unbalance (greater than 5 percent) can result from single-phasing conditions. Waveform Distortion Waveform distortion is defined as a steady-state deviation from an ideal sine wave of power frequency principally characterized by the spectral content of the deviation. There are five primary types of waveform distortion: ■ DC offset ■ Harmonics ■ Interharmonics ■ Notching ■ Noise DC offset. The presence of a dc voltage or current in an ac power system is termed dc offset. This can occur as the result of a geomagnetic disturbance or asymmetry of electronic power converters. Incandescent light bulb life extenders, for example, may consist of diodes that reduce the rms voltage supplied to the light bulb by halfwave rectification. Direct current in ac networks can have a detrimental effect by biasing transformer cores so they saturate in normal operation. This causes additional heating and loss of transformer life. Direct current may also cause the electrolytic erosion of grounding electrodes and other connectors. Harmonics. Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at which the sup-ply system is designed to operate (termed the fundamental frequency; usually 50 or 60 Hz).6 Periodically distorted waveforms can be decomposed into a sum of the fundamental frequency and the harmonics. Harmonic distortion originates in the nonlinear characteristics of devices and loads on the power system. Harmonic distortion levels are described by the complete harmonic spectrum with magnitudes and phase angles of each individual harmonic component. It is also common to use a single quantity, the total harmonic distortion (THD), as a measure of the effective value of harmonic distortion. Interharmonics. Voltages or currents having frequency components that are not integer multiples of the frequency at which the supply system is designed to operate (e.g., 50 or 60 Hz) are called interharmonics. They can appear as discrete frequencies or as a wideband spectrum. Interharmonics can be found in networks of all voltage classes. The main sources of interharmonic waveform distortion are static frequency converters, cycloconverters, induction furnaces, and arcing devices. Power line carrier signals can also be considered as interharmonics. POWER QUALITY 5

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