reliability of transmission by means of line impedance …

West African Power Industry Convention 2007, Abuja, NIGERIA

RELIABILITY OF TRANSMISSION BY MEANS OF LINE IMPEDANCE AND K-FACTOR MEASUREMENT

William KIBART, Ulrich KLAPPER, Michael KRÜGER, OMICRON electronics

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Distance Protection

Distance relay Characteristics


Distance Protection – Ideal Behaviour

• k-factor is set properlyWrong k-factor settings in DIS 1 can lead to a zone overreach and a fault on Line 2 could trip DIS 1 leaving the customer without power

DIS 1Zone 1

DIS 2

Zone 1

DIS 3Zone 1

DIS 4Zone 1

Consumer

Line 1 Line 2


Settings of Distance Relays

• Positive Sequence Impedance

• Zero Sequence Impedance (sometimes)

• k-factor (Z0/Z1, ZE/ZL or XE/XL and RE/RL)

• All settings need to be correct within a few percent!


sagheight above

ground

horizontal distance

Calculation of Line Parameters


Calculation of Line Parameters

• Parameters depend on:

• Wire type of both ground wire or shield and phase conductor: geometry, spiraling, DC resistance...

• Construction: line length, tower geometry, average sag, average height of wires above ground, distance between ground wire(s) and phase conductors, cable data and cross bonding configuration on underground cables...

• Soil resistivity

• Calculation is possible but prone to error as too many parameters are needed


Conventional Measurement


Line Impedance and k-factor measurements


• Generated signal differs from mains frequency

• Measurement is frequency selective –measuring only the generated frequency – filtering out all other signals

• Suppressionof mainsdisturbances

Variable Frequency Principle


Positive Sequence for L-L faults

Example1Ω: Loop impedance0.8Ω: Fault inside protection zone 11.2 Ω: Fault outside protection zone 1

Distance protection relay


Zero Sequence for L-G Faults

• k-factor corrects the ground impedance to treat L-E faults like L-L faults

• A wrong k-factor could lead to a zone overreach – making a line accidentally dead

Distance protection relay


Case Study


Measurement Results

• The reason was found weeks after the measurement: DC Resistance of the earth wire was entered wrongly in the companies database…


Interferences at Mains Frequency

reliability of transmission by means of line impedance …

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