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Spencer Russell
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1 Photo of NARM site here? 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

2 Background, Mine 2 Mines Combined Trains/Day Low Sulphur Coal Supplies Generation Industry 250T / 400T Trucks 13,000,000 lbs. Excavator Preceding Week Multiple Ground Faults (6) / GF Tripping Arc Flash Event Involving Ground and Personnel

3 General Background on NARM The North Antelope Rochelle Mine is the world s largest coal mine. The mine sold nearly 93 million tons of compliance coal during 2016 and more than 2 billion tons since the mine began. The complex uses 4 draglines and 15 shovels. Coal is currently delivered to more than 60 electricity generating customers throughout the United States. The complex currently has a workforce of approximately 1,145 people and annually injects approximately $2.3 billion into the regional economy in direct and indirect benefits.

4 Ground Fault Event On 2/8/16 there was a low level arc flash event at NARM on a stationary substation (PS34). 69 kv to 22.9 kv Sub was immediately removed from service Root Cause DCS Gary Sorenson Bruno Engineering Confirmed RCA WESCO Eaton Engineering Grounding Expert David Shipp

5 PS-34 Function 69kV / 22.9 kv Sub 15 MVA Txmr 25 Amp NGR 2 Ground Mats Grounding Conductor Monitor NGR Integrity Relay Feeds Excavator

6 Ground Fault Event Trouble-shooting Grd Fault Reclosed Breaker Grd Monitor Relay / Control Panel Failed on Reclosing Low Level Arc Flash Occurred. Approx. 1.2 Calorie Event

7 Ground Fault Event

8 Investigation; Involved Ground Mats On-Site Investigation Lightning? Installation Errors? MSHA Requirements Transferred Earth Potentials Transient Over Voltages Earth Megger Meas. Tripping Data Open Neutral in Txmr / NGR

9 MSHA Requirements at PS34 Protect Personnel Protect Equipment A Mine System is not an Industrial Plant! MSHA Design Involves Transferred Earth Potentials Transfer by Grounded Conductor 2 Ground Mats (+Safety Ground Mat/Borehole Grd Mat/Isolation Grd Mat) 100V GF Touch / Step Potential Max at Shovel (25A)(4 ohms Max in Trailing Cable Ground Conductor) = 100V MSHA Required Protection

10 Mining Installation - Without Safety Ground Bed Xfer by Grded Conductor Eg Eg = Vman = GMR = IgZm Remote Earth

11 Mining Installation - With Safety Ground Mat Xfer to Safety Ground Mat not to the Man Eg Eg = GMR = IgZm Vman Remote Earth

12 Mining Installation - With Safety Ground Mat Grd Fault at Shovel Vman = 100V Max Igf Eg Eg = Igf(Zgc) < 100V = Vgc drop = Vman Remote Earth Vmax = Vlg = Vng

13 On-Site Investigation NGR Monitor Cable Arced to control cables and Sub Frame Ground GC Monitor Failed / Arced to Sub Frame Ground Zm1 = Zm-sub = 0.68 ohms Zm2 = Zs-gm = 1.86 ohms Zm1-m2 = 2.24 ohms NGR = 528 ohms (no open Neutrals) All Grounds Installed Correctly

14 On-Site Investigation - Continued Trailing Cable Intact ( 20k feet) with Grds / shields Correct Arc Path was Between 2 Grd Mats via Frame Grd NGR Monitor Cable Smoked Fencing Grounding, etc. all Correct

15 NGR System Failure Partial 3-Line

16 NGR Monitor Cable Failure Smoked

17 Ground Monitor Relay Failure Front Rear

18 Grd Conductor Monitor Arced Thru Hinges to Main Sub Grd (Grded Frame) Arced Thru Hinges to Main Sub Ground

19 Substation Ground Design

20 Failure Analysis Investigated Everything that Could Cause This Type of Failure Open neutral in Txmr / NGR Open circuit ground wire to excavator Loose connections 1 or both ground mat issues such as too high of ohmic values Open ground wire to safety ground mat Excessive length of trailing cable (too much cable capacitance). Lightning strikes Improperly grounded lightning arresters Too much or too little ohms between the 2 ground mats. Wire routing? Path for current to flow bypassing neutral grounding resistor (NGR)

21 Failure Analysis Investigated Trailing Cable Lengths 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

22 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

23 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)

24 Step 1 - Ground Fault Path; Then Arcs thru NGR Monitor Wire to Sub Grd Mat Vng 6.3 to 13.2 kv

25 Step 2; Current Bypasses NGR thru Zm1-m2; Eg now across GC Monitor

26 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

27 Step 3: Ground Monitor Relay Failure

28 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

29 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

30 Recommendations ALWAYS separate MV / LV Wiring from Each Other. NGR and GC Relays Connect Both Grd Mats Together Be Aware that Grd Wires between 2 Grd Mats 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

31 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

32 THANK YOU ANY QUESTIONS??

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