New Methods for Monitoring Neutral Grounding Resistors

Rahim Jafari – Western University

Mital Kanabar and Ilia Voloh – GE Grid Solutions

Tarlochan S. Sidhu – The University of Ontario Institute of Technology

Presented by: Matt Proctor – GE Grid Solutions

The 71st Annual Conference for Protective Relay Engineers

Outline

• Neutral Grounding Resistors (NGR)• Importance of NGR Monitoring• Existing NGR Monitoring Methods• Proposed NGR monitoring for configuration 1 • Proposed NGR monitoring for configuration 2 • Conclusions

Neutral Grounding Resistors (NGR)

NGRs are used to control Transient Overvoltages, limit ground overcurrent , and consequent issues.

Neutral Grounding Resistors (NGR)

NGRs Fail due to corrosion, lightning, temperature variation, high frequency local oscilla tions, extended

service life, welding defection, vibration, etc [3].

[4] of the paper

Neutral Grounding Resistors (NGR)

NGR failure replaces advantages of resistance grounding with disadvantages of ungrounded or solidly grounded systems. Without NGR monitoring, there is no indication that whether the system is properly grounded or not .

Importance of NGR Monitoring

Existing NGR Monitoring Methods

Existing NGR Monitoring Methods

Passive Methods:

Monitor R or I of the NGR using the voltage and current of the neutral system that exist due to inherent asymmetry of the system.

Mostly cover faulted condition

Existing NGR Monitoring Methods

Active Methods:

Monitor Ror I of NGR using signal injectionto neutral.

Only cover unfaulted condition

Existing NGR Monitoring Methods

Passive-Active Methods:

Effective Combination of passive and active methods.

Cover both faulted and unfaulted conditions(Continuous Monitoring)

Proposed method for Configuration 1

Configuration 1 – A new passive monitoring method based on V0, VN, and IN.

Monitoring Logics in unfaulted condition:

1- Opposite variation of VN3rd and V03rdand

2- Faster acceleration of V03rd than the VN3rd

OR

Slowly activated 0-15% generator sta tor ground protection functions.

Proposed method for Configuration 1

Monitoring Logic in faulted condition:

Proposed method for Configuration 1

Failed-Short NGR in unfaulted condition. Failed-open NGR in unfaulted condition.

Proposed method for Configuration 1

Low injection hardware test setup (Playback)

Proposed method for Configuration 1

Monitoring method incorporated into an availablegenerator protection relay and tested

Catastrophically activated 0-15% generator sta tor ground protection functions:Result: Ground fault a t 5% of the generator sta tor winding (4.17ms < 5ms)

Proposed method for Configuration 1

Slowly activated 0-15% generator sta tor ground protection functions:Result: Failed-short NGR Alarm (9.9ms > 5ms)

Proposed method for Configuration 1

A new passive-Active monitoring method based on V0, Vinj, IN, and Iinj.

Proposed method for Configuration 2

Needed Current Sensor

Proposed method for Configuration 2

Monitoring Logics:

1- Unfaulted condition: R=real(Z20Hz)

70% < R < 130%

2- Faulted condition: R=real(Z60Hz)

70% < R < 130%

Failed-Short NGR in unfaulted condition.

Proposed method for Configuration 2

Failed-open NGR in unfaulted condition.

Proposed method for Configuration 2

Conclusions

1- NGRs fail and need to be monitored.2- Integrity of NGR can be monitored with zero cost If

3rd harmonic voltage exists in neutral of unit-connected generators.

3- If unit-connected generators are equipped with sub-harmonic signal injection based generator sta tor ground protection, NGR monitoring can be provided by only an additional current sensor.

4- Both of the proposed NGR monitoring methods can be incorporated into digita l protective relays.

Thank You

Questions?