2. Terms and definitions1 # Terms and Definitions # Voltage Sags and Interruptions

2. Terms and definitions2 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 definitions3 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 definitions4 Impulsive transient due to lightning

2. Terms and definitions5 Shunt capacitors may be installed in substations or on poles or in enclosures

2. Terms and definitions6 Capacitor switching

2. Terms and definitions7 Long-duration overvoltage caused by ferroresonance of transformer

2. Terms and definitions8 What is ferroresonance? Resonance of unloaded transformer (nonlinear magnetizing branch) with a capacitance E-jX c -jX c =1/(j  C) VLVL I

2.0 Short Duration 2.1 Interruption

2. Terms and definitions10 Short duration sags and swells

2. Terms and definitions11 Temporary voltage sag caused by induction motor starting across the line

4.0 Voltage Imbalance Voltage Imbalance

2. Terms and definitions13 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 definitions14 Harmonics

2. Terms and definitions15 Harmonics + interharmonics

2. Terms and definitions16 Three-phase rectifier driving DC motor As V B rises above V A, 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 definitions17 Waveform distortion due to notching

2. Terms and definitions18 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 definitions19 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 definitions20 ITI curve Voltage tolerable Voltage swell intolerable Voltage sag intolerable Duration in cycles Voltage in percent

2. Terms and definitions21 ITI curve Voltage tolerable Voltage swell intolerable Voltage sag intolerable Voltage in percent Duration in sec

Voltage Sags and Interruptions

Power Transmission - Distribution System 46 kV, 69 kV, 115 kV, 138 kV, 161 kV, 230 kV, … kV, 8.32 kV, kV, 13.8 kV, 14.4 kV, 24.9 kV…. TRANSMISSIONDISTRIBUTION

Industrial Power System 115 kV 13.8 kV TRANSMISSIONINDUSTRIAL DISTRIBUTION Sensitive Load Sensitive load’s own circuit Other distribution circuits

Utility Power Distribution System 115 kV kV TRANSMISSION INDUSTRIAL DISTRIBUTION with sensitive loads Customer’s circuit owned by utility Other distribution circuits Utility Distribution 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

Three-phase short circuit

Single phase - ground short circuit

Phase to phase short circuit

Phase-phase-ground short circuit