Ground Monitoring System Fault Investigation North Antelope Rochelle Mine Lyle Hunt / Scott Svoboda - Peabody Energy David Shipp / Mike Peters - Eaton Electrical Engineering Services WMEA Spring Conference Denver, CO May 31, 2017 Photo of NARM site here?
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Ground Monitoring System Fault Investigation North ... Papers/0517_ground_monitoring_fault...Ground Monitoring System Fault Investigation North Antelope Rochelle Mine ... •MSHA Requirements
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Ground Monitoring System Fault Investigation North Antelope Rochelle Mine
Lyle Hunt / Scott Svoboda - Peabody EnergyDavid Shipp / Mike Peters - Eaton Electrical Engineering Services
• All Items in Black Were Found to be Correct (NGR, Txmr, Basic MSHA Required Components, etc.). Red were Found Deficient
• Transient Over Voltages Due to Arcing Grd Fault
– If Capacitive Charging Current (thru Insulation to grd) > Resistive Grd Fault Current, then Voltages can Escalate with Respect to Grd. About 700%V Burns into Massive fault.
– Icapac = Ingr at 24.3k feet – still within limits at 20K ft
Failure Analysis – Wire Routing• NGR Monitor Wire
• Under GF, Vng Estimated to be between 6 kV and 13.2 kV (input to monitor voltage divider).
• This Wire entered Substation Instrument Room and was Tie Wrapped to the Control Wires for lights, etc. (MV wire tie wrapped to LV control Wires)
• When NGR Monitor relay was removed, this MV wire was not completely removed. – This Conductor was involved to substation Grd Mat
Failure Analysis – NGR Bypassed
• GC Monitor Relay Indicated High Current arced thru it to Sub Grd
• NGR Monitor is Tied to Safety Grd Mat but Physically Installed on Substation Grded Steel.
• NGR Monitor also Mounted on Sub Steel but ties to Txmr N point and to Safety Grd Mat.
• Both Relays Will Have Substantial voltages Across them Under GF Conditions (Between 2 Grd Mats)
Step 1 - Ground Fault Path; Then Arcs
thru NGR Monitor Wire to Sub Grd Mat – Vng 6.3 to 13.2 kV
Step 2; Current Bypasses NGR thru Zm1-m2; ∆Eg now across GC Monitor
Step 2; ∆Eg Exceeds GC Monitor Limits
• ∆Eg Estimated at 2240V+
• MOV Fails
• GF Current Estimated at 1000A’s +/-
• GFCT Saturates and GF Relay never Operates.
• Relay Fails Internally
Step 3: Ground Monitor Relay Failure
Step 3: Ground Monitor Relay Failure
• Fault Path Bypasses Zm1-m2 (2.24 ohms)
• Current Increases greater than 1800A’s
• Phase Instantaneous (device 50) trips (phase CTs do not saturate)
• Arc Blast from around GC Monitor Engulfs Personnel in Close Proximity Causing Arc Flash Incident
Conclusions
• Failure was Initiated by Wire Routing
• Initial Failure Allowed NGR to be Bypassed
• High Fault Current Resulted in CT Saturation / No GF Tripping
• Sustained and/or Repeated Voltages between 2 Ground Mats Caused GC Monitor to Fail Internally.
• Failed GC Monitor now Bypassed Zm1-m2
• Ultimate Arc Flash Condition Occurred with Igf >1800A’s
Recommendations
• ALWAYS separate MV / LV Wiring from Each Other.
• NGR and GC Relays Connect Both Grd Mats Together
• Be Aware that GrdWires between 2 GrdMats Must be Treated as MV
• Investigate Using a PT in Lieu of the Voltage Divider for NGR Monitor
• Redesign Substation Ground Monitoring System. Include Sustained Grd Fault and Lightning Conditions
• Survey other Substations
• Test Ground Mats Yearly
Solution
• Redesign substation ground monitoring system to prevent / improve personnel and safety and prevent recurrence of ground fault event.1. Maintain the system grounding integrity
a. Yearly Inspection of entire system (offline)
b. Insure best practices are used in setting up system.
c. Develop SOP’s on how to maintain system/install grounding system.
d. Monthly Testing of Protection functions
2. Educate Employees how to Maintain system and why.
3. Develop recommendations to minimize exposure to personnel