WHY NEUTRAL GROUNDING RESISTORS NEED CONTINUOUS MONITORING.

CMK B.E(EEE)

Neutral Grounding Resistors.

It is a grounding device for non isolated system, which is used to connect neutral point of transformer to earth through resistance.

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

Neutral grounding resistor.

Advantages Of NGR.

Reduces over voltages. Reduced equipment damage. Isolation of faulted circuit. Reduced operation/maintenance expense. Increase safety. Increased lightning protection.

NGRs Failure Modes, Causes And Consequences.

Failure mode is usually open circuit.

a) Desired fault current cannot flow.b) Ungrounded system.

NGR

Conti…

a) Undesired fault current can flow.b) Solidly grounded system.

NGR

• NGR in short circuited mode.

Need For NGR Monitoring. Properly designed resistance grounding

can avoid.a) Eliminate transient over voltages.b) Reduce the risk of an arc flash.c) Provide continuity of service.d) Provide accurate current for ground fault

detection.

NGR Failure Detection Methods.

Event discovery. NGR checking during

maintenance doesn’t guarantee it’s working.

Response to injected current only ensures the working of relay only.

NGR disconnected for testing and do not reconnect.

…..this ensures the need of CONTINUOUS NGR MONITORING

A primary current injection testing

Over voltage and over current measurement as NGR monitor

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

Resistance measurement as an NGR monitor

But an ohmmeter is…

Possibility of measuring continuity through ground fault

Resistance measurement through the faulted phase and ground

Continuity through a fault

Continuity through a ground fault

So the effective method is to include An over current

protection

An over voltage protection

An resistance measuring instrument

Continuous Monitoring Requirements Should detect NGR failure in spite of

ground fault Should work in tripping and alarm

only systems Monitoring neutral and ground

connections Monitoring shouldn’t be

capacitively/inductively coupled to NGR

Monitor should not be exposed to neutral voltage during ground fault

Role of sensing resistor in NGR monitoring

NGR MONITOR

Case studies Soft starter failure a) Soft starter operated in unexpected manner.b) When the neutral properly grounded, all three

voltage signals are balanced. c) Failure of the NGR allows the neutral voltage to float

and the three voltage signals to become unbalanced.

d) studying the situation the manufacturer concluded that the problem was due to unbalanced reference voltages.

e) further study and customer interviews it was determined that an open NGR was responsible for the unbalanced reference voltages.

Conti… Loose connection of an NGRa) The following incident occurred at a

mine in Eastern Canada on a 200 A, 4160 V NGR.

b) NGR monitoring relay tripped on a resistor fault.

c) Upon inspection, electricians noticed a loose connection on the NGR

d) Eight other loose connections were found and repaired.

e) Then resistor fault trip did not reoccur.

CONCLUSION The information presented here has shown that an

open NGR is an undesirable situation and current- sensing ground-fault protection will not indicate the presence of a ground fault. A well-designed NGR monitor provides continuous protection against failures that previously rendered ground fault protection, NGR , as well as leaving the system exposed to damaging transient over voltages. An NGR monitor provides confidence that current-sensing ground-fault protection will operate as designed on the next ground fault. A continuous NGR monitor is active when control power is applied and indicates NGR health whether or not the system is energized, with or without a ground fault.

Thank you