NEUTRAL GROUNDING RESISTOR FAILURE

DETECTION

Guided by: Presented by:Asst. Prof. BINDU C.J NITHYA GOPINATH

EE 7 ROLL NO : 44

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INTRODUCTION NEUTRAL GROUNDING RESISTOR (NGR) ADVANTAGES OF NGR NGR FAILURE TRIPPING RATIO AND NGR SELECTION CALCULATION OF NGR NGR FAILURE DETECTION CONTINUOUS NGR MONITORING CONCLUSION REFERENCES

CONTENTS

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A grounded system is defined by a system with at least one conductor or point which is connected to ground either solidly or through impedance.

Resistance grounding is usually preferred by most of the electrical equipment systems.

NGR is usually employed as grounding element in resistance grounding systems.

INTRODUCTION

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It is usually a resistor that is connected directly between the transformer or generator neutral and the system ground.

Used to limit the fault current in a generator or a transformer during earth faults.

“Current Sensing Ground Fault Protection‟ is the most commonly employed type in resistance grounded systems.

NEUTRAL GROUNDING RESISTOR (NGR)

NGR (cntd.)

A typical NGR is constructed of resistance wire or metal strips coiled and wrapped around porcelain insulators.

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Properly applied resistance grounding can :

Limit point-of-fault damage

Eliminate transient over voltages

Reduce the risk of an arc flash

Provide continuity of service with a ground fault

Provide adequate current for ground-fault detection and selective coordination

ADVANTAGES OF NGR

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NGR failure mode is usually open circuit.The desired fault current cannot flow and the ground

fault cannot be cleared.This results in an ungrounded system.

NGR FAILURE MODE

NGR

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NGR IN SHORT-CIRCUTED MODE

A short-circuited NGR results in a grounded and stable electrical system.

Ground-fault current will flow during a ground fault.

The fault will be cleared by ground fault protection or over current protection.

NGR

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As NGR is a mechanical component, mechanical failure can happen due to :

Lightning, storms and earthquakes

Overloads

Extended service life

CAUSES OF NGR FAILURE

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Other causes of failure include :

Corrosive atmospheres

Extreme temperature changes

Triple n harmonic currents

Hail

Manufacturing defects

Vibration

CAUSES OF NGR FAILURE (cntd.)

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NGR FAILURE

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NGR failure converts a resistance-grounded system into an ungrounded system.

Operators would not be aware that

Current-sensing ground-fault protection is no longer operational

Risk of transient over voltages exists

CONSEQUENCES OF NGR FAILURE

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SELECTION OF NGR

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Tripping ratio is defined as the ratio of prospective ground fault current to the operating value of the ground fault protection.

It ensures that sufficient ground fault current is available for detection when the ground faults occur.

Tripping ratio of 7 is required to detect a two-phase-to-ground fault.

A higher tripping ratio is required to provide machine winding ground fault protection.

TRIPPING RATIO AND NGR SELECTION

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CALCULATION OF NGRThree parameters are required to specify NGR

U :Rated line to line voltage or system voltage (kV)

If : Rated fault current (A)

Ts : Rated time (s) [Duration of fault]

NGR resistance value can be calculated using the formula

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Measurement of NGR resistance during maintenance only provides a confirmation that

NGR was good at the time when the resistance was measured.

The NGR could fail at any time

after the measurement is taken or even not be reconnected after the measurement.

NGR checking during maintenance doesn’t guarantee it’s working.

NGR FAILURE DETECTION

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An open NGR is not a condition that should be allowed to remain on the system for any length of time.

Examination of NGR preventative maintenance suggests that an automatic monitoring device is a better solution.

NGR FAILURE DETECTION (cntd.)

This ensures the need for continuous NGR

monitoring...

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Detects an open NGR when the failure occurs

Active when control power is applied

Indicates NGR health whether or not the system is energized, with or without a ground fault

CONTINUOUS NGR MONITOR

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Should detect NGR failure in spite of Ground Fault

Should work in tripping and alarm only systems

Monitoring neutral & ground connections

Monitor shouldn’t be capacitively/inductively coupled to NGR

Monitor should not be exposed to neutral voltage during ground fault

CONTINUOUS NGR MONITORING REQUIREMENTS

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OVER VOLTAGE AND OVER CURRENT MEASUREMENT AS NGR MONITOR

(51N)- Over current measurement(59N)- Over voltage measurement

This is an ineffective NGR monitoring method.

The combination of these devices can detect an NGR failure only if there is a ground fault on the system.

This design is not continuous NGR monitoring because it relies on the presence of a ground fault to operate.

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RESISTANCE MEASUREMENT AS AN NGR MONITOR

In this method, there is a possibility

for interference by external dc influences

that continuity through a ground fault may be recognized as NGR continuity

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There are many possible sources of stray dc voltage in industrial electrical systems:

DC can be impressed on the system by sources like

a ground fault on the dc bus of an adjustable speed drive

obscure sources such as atmospheric electrical conditions

STRAY DC ON THE SYSTEM

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CONTINUITY THROUGH GROUND FAULT

When a ground fault occurs, there are two parallel paths

a path through the NGR

another path through the transformer or generator winding of the faulted phase to, the fault, and back to the measuring device through ground

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A BETTER SOLUTION IS....... Combination of an

overvoltage measurement (59N), an overcurrent measurement (51N) and a resistance measurement

It continuously monitors NGR continuity.

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CONTINUOUS NGR MONITOR

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It combinesmeasured NGR currenttransformer or generator neutral voltageNGR resistance

. When there is no ground fault on the system, a measurement of NGR

resistance is enough to confirm NGR continuity.

The monitor determines the presence of a ground fault through the voltage and current measurements.

Voltage on the neutral and current in the NGR indicates a ground fault.

CONTINUOUS NGR MONITOR (cntd.)

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To confirm NGR continuity, the monitor constantly evaluates resistance, current, and voltage measurements.

The sensing resistor connects the monitor to the power system while isolating it from neutral voltage.

The sensing resistor limits the voltage transfer from the system neutral to the NGR monitor.

CONTINUOUS NGR MONITOR (cntd.)

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NGR MONITOR

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An open NGR is an undesirable situation.

A system with an open NGR is subject to transient over voltages, and so current sensing ground-fault protection will not indicate the presence of a ground fault.

A ground fault then remains on the system and might escalate to a phase-to-phase fault.

NGRs are subject to many failures.

A well-designed NGR monitor provides continuous protection against failures.

An NGR monitor provides confidence that current-sensing ground-fault protection will operate as designed on the next ground fault.

CONCLUSION

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[ 1]D. Selkirk, M. Savostianik, K. Crawford, 2010, "The dangers of grounding resistor failure", IEEE Industry Applications Magazine, sept. /oct. 2010, 53-58.

[2]W. Labos, A. Mannarino, G. Drobnjak, S. Ihara, and J. Skliutas, “A possible mechanism for neutral grounding resistor failures,” in Proc. Power Engineering Society General Meeting, June 2005.

[3]G. E. Paulson, “Monitoring neutral-grounding resistors,” in Proc. IEEE Pulp and Paper Industry Committee Conf., June 1999, p. 240.

[4]P. Glover, “Neutral grounding resistors installation and maintenance instructions,” PGR Document #NG111-06.

[5] K. S. Crawford and N. K. Haggerty, “Test before touch—Easier said than done,” IEEE Ind. Applicant. Mag., vol. 14, no. 3, pp. 32–39, May/June 2008.

[6] IEEE Standard 142-1982, IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, The Institute of Electrical and Electronics Engineers, Inc.

[7] ANSI c62.92-1987, IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part I - Introduction, American National Standards Institute.

[8] PRSC-4E, System Neutral Grounding and Ground Fault Protection Guide, Westinghouse Industrial and Commercial Power Systems Applications Series, February 1986.

REFERENCES

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THANK YOU