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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
• 46 Views
0 User(s)

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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