2. Terms and definitions 1

# Terms and Definitions

# Voltage Sags and Interruptions

2. Terms and definitions 2

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 3

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 4

Impulsive transient due to lightning

2. Terms and definitions 5

Shunt capacitors may be installed in substations or on poles or in enclosures

2. Terms and definitions 6

Capacitor switching

2. Terms and definitions 7

Long-duration overvoltage caused by ferroresonance of transformer

2. Terms and definitions 8

What is ferroresonance?

• Resonance of unloaded transformer (nonlinear magnetizing branch) with a capacitance

E -jXc

-jXc=1/(jC)

VL

I

2.0 Short Duration

• 2.1 Interruption

2. Terms and definitions 10

Short duration sags and swells

2. Terms and definitions 11

Temporary voltage sag caused by induction motor starting across the line

4.0 Voltage Imbalance• Voltage Imbalance

2. Terms and definitions 13

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 14

Harmonics

2. Terms and definitions 15

Harmonics + interharmonics

2. Terms and definitions 16

Three-phase rectifier driving DC motor

A

B

E = k mC

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 17

Waveform distortion due to notching

2. Terms and definitions 18

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 19

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 20

ITI curve

1 104

1 103

0.01 0.1 1 10 100 1 103

1 104

0

100

200

300

400

500

Vhi ti Vlo ti

ti

cy

Voltage tolerable

Voltage swell intolerable

Voltage sag intolerable

Duration in cycles

Voltage inpercent

2. Terms and definitions 21

1 106

1 105

1 104

1 103

0.01 0.1 1 10 100 1 103

0

100

200

300

400

500

Vhi ti Vlo ti

ti

s

ITI curve

Voltage tolerable

Voltage swell intolerable

Voltage sag intolerable

Voltage inpercent

Duration in sec

Voltage Sags and Interruptions

Power Transmission - Distribution System

46 kV, 69 kV,115 kV, 138 kV,161 kV, 230 kV,….

4.16 kV, 8.32 kV,12.47 kV, 13.8 kV,14.4 kV, 24.9 kV….

TRANSMISSION DISTRIBUTION

Industrial Power System

115 kV

13.8 kV

TRANSMISSION INDUSTRIAL DISTRIBUTION

Sensitive Load

Sensitive load’s own circuit

Other distribution circuits

Utility Power Distribution System

115 kV

12.47 kV

TRANSMISSIONINDUSTRIAL DISTRIBUTIONwith sensitive loads

Customer’s circuitowned by utility

Other distribution circuits

Utility Distribution System

Industrial Distribution System: Origin of Faults Causing Misoperation of Production

Equipment

23%

46%

31%Faults on OwnDistribution Circuit

Faults on OtherDistribution Circuit

Faults on TransmissionSystem

Example of typical short circuit

Fault at X:Three-phase short circuitSingle-phase to ground short

circuitPhase to phase short circuitDouble phase to ground short

circuit

Three-phase short circuit

Look at the rms value of the ac currentjust after the short-circuit occurs

The following phasor diagrams are qualitative only. Use short-circuit studyfor actual values.

Phasor diagram before the fault

Van

Vca

Vab

Vbc

Vbn

Vcn

IaIb

Ic

Normal

Three-phase short circuit

Vab

Ia

Ib Ic

Three PhaseShort Circuit

Single phase - ground short circuit

Van

Vca

Vab

Vbc

Vbn

Vcn

Ia

Single Phase to GroundShort Circuit

Phase to phase short circuit

Vca

Vab

Vbc

Ib

Phase to PhaseShort Circuit

Ic

Phase-phase-ground short circuit

Vca

Vab

VbcIb

Phase to Phaseto GroundShort Circuit

Ic