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# Terms and Definitions # Voltage Sags and Interruptions 2. Terms and definitions 1 Definitions • Sag (or dip) is a decrease to a low voltage (between 0.1 and 0.9 per unit rms) at power frequency for durations of 0.5 cycle to 1 minute • Swell is an increase to a high voltage (between 1.1 and 1.8 per unit rms) at power frequency for durations of 0.5 cycle to 1 minute 2. Terms and definitions 2 Definitions • Transient refers to an event of duration < 0.5 cycle • Instantaneous: duration 0.5 cycle to 30 cycles • Momentary: duration 30 cycles to 3 sec • Temporary: duration 3 sec to 1 min. • Sustained or long-duration: duration > 1 min. 2. Terms and definitions 3 Impulsive transient due to lightning 2. Terms and definitions 4 Shunt capacitors may be installed in substations or on poles or in enclosures 2. Terms and definitions 5 Capacitor switching 2. Terms and definitions 6 Long-duration overvoltage caused by ferroresonance of transformer 2. Terms and definitions 7 What is ferroresonance? • Resonance of unloaded transformer (nonlinear magnetizing branch) with a capacitance -jXc=1/(jwC) I E -jXc 2. Terms and definitions VL 8 2.0 Short Duration • 2.1 Interruption Short duration sags and swells 2. Terms and definitions 10 Temporary voltage sag caused by induction motor starting across the line 2. Terms and definitions 11 4.0 Voltage Imbalance • Voltage Imbalance Waveform distortion • • • • DC offset Harmonics Interharmonics Notching- It is a periodic voltage disturbance caused by the normal operation of power electronic devices when current is commuted from one phase to another. • Noise- It is defined as unwanted electrical signals with broadband spectral content lower than 200 kHz superimposed upon the power system voltage or current in phase conductors, or found on neutral conductors or signal lines. 2. Terms and definitions 13 Harmonics 2. Terms and definitions 14 Harmonics + interharmonics 2. Terms and definitions 15 Three-phase rectifier driving DC motor A B C E = k wm As VB rises above VA, current is commuted from A to B. While diode B is turning on, diode A is turning off. A is shorted to B (for a short time while both diodes conduct), producing notching. 2. Terms and definitions 16 Waveform distortion due to notching 2. Terms and definitions 17 Voltage fluctuation # Voltage magnitude does not normally exceed the ranges of 0.9 pu to 1.1 pu. • Series of random or continuous voltage fluctuations – Flicker is undesirable results (such as lamps flickering) from continuous rapid variations in voltage – Called voltage flicker (to refer to the voltage fluctuation) – Often measured by short term flicker sensation Pst (IEC standard) 2. Terms and definitions 18 ITI (Information Technology Industry) curve • Replaces CBEMA (Computer Business Equipment Manufacturers Association) curve # A set of curves representing the withstand capabilities of computers in terms of the magnitude and duration of the voltage disturbance. • Susceptibility of 120-volt computer equipment to voltage sags and swells 2. Terms and definitions 19 ITI curve 500 400 Voltage in percent Vhi t i Voltage swell intolerable 300 Vlo t i 200 100 Voltage tolerable Voltage sag intolerable 0 4 1 10 1 10 3 0.01 0.1 1 ti cy 2. Terms and definitions 10 100 1 10 3 1 10 4 Duration in cycles 20 ITI curve 500 400 Voltage in percent Vhi t i Voltage swell intolerable 300 Vlo t i 200 100 Voltage tolerable Voltage sag intolerable 0 6 5 4 1 10 1 10 1 10 1 10 3 0.01 0.1 ti s 2. Terms and definitions 1 10 100 1 10 3 Duration in sec 21 Voltage Sags and Interruptions Power Transmission - Distribution System 4.16 kV, 8.32 kV, 12.47 kV, 13.8 kV, 14.4 kV, 24.9 kV…. 46 kV, 69 kV, 115 kV, 138 kV, 161 kV, 230 kV, …. TRANSMISSION DISTRIBUTION Industrial Power System 13.8 kV Other distribution circuits 115 kV Sensitive load’s own circuit Sensitive Load TRANSMISSION INDUSTRIAL DISTRIBUTION Utility Power Distribution System Utility Distribution System 12.47 kV Other distribution circuits 115 kV TRANSMISSION Customer’s circuit owned by utility INDUSTRIAL DISTRIBUTION with sensitive loads Industrial Distribution System: Origin of Faults Causing Misoperation of Production Equipment 23% 31% Faults on Own Distribution Circuit Faults on Other Distribution Circuit 46% Faults on Transmission System Example of typical short circuit Fault at X: Three-phase short circuit Single-phase to ground short circuit Phase to phase short circuit Double phase to ground short circuit Three-phase short circuit Look at the rms value of the ac current just after the short-circuit occurs The following phasor diagrams are qualitative only. Use short-circuit study for actual values. Phasor diagram before the fault Vca Vcn Vbc Ic Van Ib Ia Vbn Vab Normal Three-phase short circuit Ic Ib Vab Ia Three Phase Short Circuit Single phase - ground short circuit Vcn Vbc Vca Van Vab Vbn Ia Single Phase to Ground Short Circuit Phase to phase short circuit Vbc Ib Vca Ic Vab Phase to Phase Short Circuit Phase-phase-ground short circuit Ib Vbc Vca Ic Vab Phase to Phase to Ground Short Circuit