Arc Flash Mitigation
Case Study
Jacob Bayer, PE
AREGC Conference
June 28, 2016
Definitions
►
Arc Flash Hazard
• A dangerous condition associated with the possible release of energy caused
by an electric arc.
►
Incident Energy
• The amount of thermal energy impressed on a surface, a certain distance
from the source, generated during an electrical arc event. Incident energy is
typically expressed in calories per square centimeter (cal/cm2).
2
Incident
3
Arc Flash Facts
►
As much as 80% of all electrical injuries are burns resulting
from an arc-flash and ignition of flammable clothing.
►
Arc temperature can reach 35,000°F - this is four times hotter
than the surface of the sun.
►
Fatal burns can occur at distances over 10 ft.
►
As copper vaporizes, it undergoes an explosive volumetric
increase of 67,000 to 1.
►
An arc flash event can produce a force of 500 lbs. on an
average human body.
4
Industry Standards and Regulations
►
NFPA 70-2014 (NEC)
• 240.87 Arc Energy Reduction. One of the
following or approved equivalent means shall
be provided:
(1) Zone-selective interlocking
(2) Differential relaying
(3) Energy-reducing
maintenance switching with local
status indicator
(4) Energy-reducing active arc flash
mitigation system
(5) An approved equivalent means
5
Industry Standards and Regulations
►
NFPA 70E-2015
• 130.5 Arc Flash Risk Assessment. An arc flash risk
assessment shall be performed… It shall be reviewed
periodically, at intervals not to exceed 5 years.
• 130.5 (D) Equipment Labeling. Electrical
equipment...shall be field-marked with a label
containing all the following information:
(1) Nominal system voltage
(2) Arc flash boundary
(3) [Available incident energy and/or arc flash PPE
category]
6
Industry Standards and Regulations
►
29 CFR 1910.269 (OSHA) Electric
power generation, transmission,
and distribution
• (l) Working on or near exposed energized parts
►
(8) Protection from flames and electric arcs
 (ii) For each employee exposed to
hazards from electric arcs, the employer
shall make a reasonable estimate of the
incident heat energy to which the
employee would be exposed.
7
Methods of Mitigating Arc Flash Incident Energy
►
Protective Relay Coordination
►
Fiber Detection
►
Zone Selective Interlock (ZSI)
►
Arc Resistant Gear
►
Maintenance Mode
►
Remote Control / Racking
►
Differential Protection
►
High Resistance Grounding
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Protective Relay
Coordination
►
LV SWGR Tie
►
LV SWGR Feeder
►
LV MCC Feeder
►
LV Loads
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Protective Relay
Coordination
►
MV SWGR Feeder
►
LV SWGR Main
►
LV SWGR Tie
►
LV SWGR Feeder
►
LV MCC Feeder
►
LV Loads
10
Zone Selective Interlock (ZSI)
11
Zone Selective Interlock (ZSI)
12
Zone Selective Interlock (ZSI)
13
Zone Selective Interlock (ZSI)
0.60 Seconds
14
Zone Selective Interlock (ZSI)
0.15 Seconds
15
Maintenance Mode
►
MV SWGR
Using Main Breaker
MAINT
MODE
OFF
ON
LV SWGR
PROTECTED ZONE
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Maintenance Mode
►
Using Upstream Breaker
MV SWGR
PROTECTED
ZONE
MAINT
MODE
OFF
ON
LV SWGR
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Maintenance Mode
►
MV SWGR Feeder
• Normal Protection
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Maintenance Mode
►
MV SWGR Feeder
• Maintenance Mode
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Maintenance Mode
►
MV SWGR Feeder
• Maintenance Mode
Δt
20
Bus Differential
►
High Impedance:
• Fast
• Reliable
• Requires
Dedicated CTs
21
Bus Differential
►
Low Impedance:
• Fast
• Reliable
• More Complicated
Relaying
22
Fiber Detection
• Integral Light
Sensors
• Detects < 2.5ms
• Possible to Retrofit
23
Arc Resistant Gear
24
Remote Control / Racking
►
Open and Close Breakers
from DCS
►
Rack Breakers Remotely
25
High Resistance Ground
►
Limit phase to ground fault
current to less than 10 amps
►
Often used on 480V systems
►
Improves runtime
26
Arc Flash Analysis
►
Energy Calculation (cal/cm2)
• Current and Time
►
Short Circuit Study (Current)
►
Relay Coordination Study (Time)
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Electrical Model
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CURRENT IN AMPERES
FGD PUS 101A
FGD PUS101 TIE F60
FGD SST 102B F60
FGD SS XF 102B
FGD PUS101 TIE F60
Open
TCC Curves
1000
FGD PUS 101 TIE
FGD PUS 101B
101B ISW FR 2B
10AB FU FR 2B
100
FGD SST 102B F60
1API024M001
FGD SS XF 102B
P
TIME IN SECONDS
10
FGD PUS101 TIE F60 - Phase
1
FGD SS XF 102B
S
FGD SST 102B GCT F60
FGD SUS102B MN
FGD SUS102B
FGD SST 102B F60 - Phase
FGD SUS102 TIE
480V Tie1
FGD SUS102B MN - Phase
FGD SUS102 TIE
0.10
FGD SUS102A
Open
MCC 102D FDR
TX Inrush
MCC 102B FDR - Phase
MCC 102D CBL
FGD MCC 102D
Open
RA 101 ME EXH 102 MCP
RA 101 ME EXH 102 MCP
PD-1028
RA 101 ME EXH 102
1ASW005S001&2
0.01
0.5 1
10
100
1K
10K
FGD SST - Phase.tcc Ref. Voltage: 6900 Current in Amps x 1
RA 101 ME EXH 102
29
Arc Flash Study
30
Arc Flash Label
31
Minnesota Power
Boswell Energy Center Unit 3
►
Electrical Upgrades
• Case Study
32
Auxiliary Electrical System Assessment
►
Short Circuit Study
• Medium voltage short circuit current exceeded equipment ratings
►
Arc Flash Assessment
• Existing medium voltage buses all exceeded 40 cal/cm2
33
Auxiliary Electrical System
►
Original Unit 3 One-Line Diagram
230kV
115kV
Unit 3
GSU Xfmr
Unit 3
Aux Xfmr
Unit 3
Startup Xfmr
G
Aux Bus 3A
Aux Bus 3B
Startup Bus 3A
Startup Bus 3B
34
Auxiliary Electrical System
►
Final Unit 3 One-Line Diagram
230kV
Unit 3
GSU Xfmr
Unit 3
Aux Xfmr A
Unit 3
Aux Xfmr B
G
Aux Bus 3A
15kV Bus 3A
Aux Bus 3B
VFD
AQCS Bus 3C
15kV Bus 3B
VFD
AQCS Bus 3D
M
M
ID Fan A
11,000HP
ID Fan B
11,000HP
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Auxiliary Electrical Equipment
36
Auxiliary Electrical System
►
Medium Voltage Switchgear
• Type 2BC Arc Resistant
• Vented to Exterior
• Included High Z Bus
Differential Scheme
• Arc Flash < 8 cal/cm2
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Auxiliary Electrical System
►
Low Voltage Switchgear
• Standard Metal Enclosed 480V Switchgear
• Arc Flash Mitigation Methods
► Optical Arc Sensing System
► Main Breaker Maintenance Mode
► Feeder Breaker Maintenance Mode
► High Resistance Grounded
► Insulated Bus
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THANK YOU