High impedance faults detection in electrical power systems

Dr. Khaled M. EL-NaggarCollege of Technological Studies, Kuwait

High impedance faultsDefinition

Why it is a problemDifferent techniques for detection

Comparison & EvaluationANN-GA Technique

Conclusions

High Impedance Faults (HIFs)

• HIFs result when an energized primary conductor comes in contact with a semi-insulated object such as a tree, structure or earth ...

• HIFs, generally, are associated with arcing at the point of contact.

•Little threat of damage to power system equipment,

but potential safety and fire hazard

•Hi-Z faults produce primary current levels of 0 to

100 Amps

•Therefore, conventional phase or ground

overcurrent protection are unable to distinguish between the high-impedance faults and normal load currents

Causes

The main causes of HIFs are tree branches touching a phase conductor, Broken line on

ground (Downed Conductor)

Almost all HIFs involve the ground directly orindirectly.

Fault currents on various surfaces

Surface Fault current (A)

Dry Asphalt 0Dry Sand 0Wet Asphalt 1Wet Sand 5 Dry grass 25Concrete(non-reinforced) 10 Wet grass 50Concrete (reinforced) 70

Why should be detected• Hi-Z faults often arc and can be a

significant fire hazard

• Detect partially failing insulation before complete failure which can lead to power outages and loss of production

• If not detected and isolated, live Downed Conductors can be fatal to public

HIF Characteristics

• Little effect on voltage• Small fault current (10 – 100 Amps)• Current values will continue to

fluctuate• Significant harmonic currents• Random arcing• The majority occurs at 15 Kv and lower

• Different Methods

Mechanical detection methods,

• Comprise devices that provide a low impedance ground path by catching the falling conductor . This causes the conventional relaying to operate.

• The main drawbacks of these methods are the high installation and maintenance costs.

Electrical Detection Methods

Open Conductor Detection

• This electrical HIF detection method detects loss of voltage to determine a broken conductor.

• The system measures the phase voltage at each end of a single phase lateral.

• When the voltage of any phase drops below the specified

threshold, a signal is sent to upstream device. The upstream device opens if voltage is present at the upstream device.

Ratio Relays

The ratio of the negative sequence or zero sequence to positive sequence current is calculated and when this ratio

exceeds a predetermined value, a trip signal is produced

Phase current rate of change

The phase current rate of change is high (quick ) at the moment of fault, while the current changes progressively with load variation.

This algorithm monitors the sample difference at the corresponding sampling point of two subsequent cycles

Harmonic Based DetectionSecond harmonic detection

•Second Harmonic detection, this algorithm is based on sensing the variation of the second-order harmonic current.•Normally the second harmonic component is very small.

Third harmonic detection

1-The variations in amplitude of the third-order harmonic current and the variations of ratio of third–order harmonic to fundamental are monitored

2-The changes in both the third-order harmonic current magnitude and the third-order harmonic current phase angle with respect to the system voltage are monitored

3rd ,5th and 7th harmonic detection

• The variations in amplitudes and phase angles of the third, fifth and seventh order harmonic currents are monitored

High frequency harmonics•The arcing fault on the feeder primary can be detected from an increase in the 2 － 10 kHz harmonic components of the feeder neutral line current.

•These harmonic components often lasts longer than that from normal switching operations

Digital based techniques used for extracting signal signature

•Fast Fourier Transform•Wavelet analysis•Conventional static estimation

LESLAV

•Dynamic state estimationRecursive LESIterative LAVKF

•Genetic Algorithm•Particle Swarm

Ii(actual) − Ii(calculated ) = errori

Fitness Function to be minimized based on the error (absolute or square)

High Impedance Fault Detection and Relaying Scheme

Data Collection, CT& PT

Genetic Algorithm

Signal Analysis (GA)

Sampling Waveforms

ANN Based Detection

Input Data

• Fundamental Component• Lower Harmonics• Higher Harmonics• GR• Current before the highest transient value• Current after the highest transient value• Current Rate of change cycle to cycle

Study Cases

• Normal Load switching• Capacitor switching• Load switching + Capacitor switching• HIF• HIF + Capacitor switching• HIF + Capacitor switching + Load switching

Study System

• 11 KV Radial system• 19 11 KV feeders• 11/0.4 KV Transformers• Loads• Capacitors

Simulated Cases & Results

• 200 different cases- Different fault location- Different phases - Different inception angle- Different loading conditions- Different capacitor switching conditions- 98% HIFs detection

Conclusions

• HIFs result in:– Personnel hazard– Property damage– Poor customer service

• HIFs Detection techniques classified as – Mechanical – Electrical

• Mechanical techniques can not protect the whole distribution system

• Electrical techniques use digitized samples of the Current and voltage signals. These techniques have better detection

• Intelligent relays - Intelligent relays can detect HIFs at very high degree of

accuracy

• Regardless the method used, not all HIFs are detectable. For example, the case where a conductor near the end of a feeder breaks and falls to the ground.