Short Circuit, Protective Device Coordination
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Transcript of Short Circuit, Protective Device Coordination
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Short-circuit, Protective Device Coordination & Arc Flash
Analysis
By Albert Marroquin
Operation Technology, Inc.
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Agenda
• Short-circuit Calculations for Arc Flash Analysis
• Protection and Coordination Principles
• Arc Flash Analysis and Mitigation
• Upcoming Arc Flash Analysis Standards/Guidelines Changes
• DC Arc Flash Analysis
• Transient Arc Flash Analysis for Generators
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Short-Circuit AnalysisTypes of SC Faults
•Three-Phase Ungrounded Fault•Three-Phase Grounded Fault•Phase to Phase Ungrounded Fault•Phase to Phase Grounded Fault•Phase to Ground Fault
Fault Current•IL-G can range in utility systems from a few percent to possibly 115 % ( if Xo < X1 ) of I3-phase (85% of all faults).•In industrial systems the situation IL-G > I3-phase is rare. Typically IL-G ≅ .87 * I3-phase
•In an industrial system, the three-phase fault condition is frequently the only one considered, since this type of fault generally results in Maximum current.
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Purpose of Short-Circuit Studies
• A Short-Circuit Study can be used to determine any or all of the following:
¾Verify protective device close and latch capability
¾Verify protective device Interrupting capability
¾Protect equipment from large mechanical forces (maximum fault kA)
¾ I2t protection for equipment (thermal stress)
¾Selecting ratings or settings for relay coordination
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System Components Involved in SC Calculations
• Power Company Supply
• In-Plant Generators
• Transformers
• Reactors
• Feeder Cables / Cable Trays and Bus Duct Systems
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System Components Involved in SC Calculations
• Overhead Lines
• Synchronous Motors
• Induction Motors
• Protective Devices
• Y0 from Static Load and Line Cable
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Short-Circuit Phenomenon
)tSin(Vmv(t) θω +∗=
i(t)v(t)
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4444 34444 21444 3444 21
Offset) (DCTransientState Steady
t) - sin(
ZVm ) - tsin(
ZVmi(t)
(1) ) t Sin(Vmdtdi L Riv(t)
LR-e××++×=
+×=+=
φθφθω
θω
expression following theyields 1equation Solving
i(t)v(t)
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AC Current (Symmetrical) with No AC Decay
DC Current
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Short-Circuit ANSI Slide 9
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AC Fault Current Including the DC Offset (No AC Decay)
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Short-Circuit ANSI Slide 10
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Machine Reactance ( λ = L I )
AC Decay Current
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Fault Current Including AC & DC Decay
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Short-Circuit Study for Arc Flash
• A Short-Circuit Study can be used to determine any or all of the following:
¾Maximum and Minimum Short-circuit current levels
¾Prefault voltage values should be considered
¾Positive and Negative Impedance Tolerance Adjustments
¾Actual fault current values should be used including decaying contributions for medium voltage systems
¾Operating Conditions and System Configurations which may not be otherwise observed for regular SC studies
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Reactance Representation forUtility and Synchronous Machine for AF
½ Cycle 1 ½ to 4 Cycle 30 Cycle
Utility X”d X”d X”d
Turbo Generator X”d X’d Xd
Hydro-Gen with Amortisseur winding
X”d X’d Xd
Condenser X”d X’d α
Synchronous Motor X”d X’d α
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Fault Current Decay
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Fault Current Recording
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Overcurrent Protection and Coordination Principles
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Definition
• Overcurrent Coordination¾A systematic study of current responsive devices
in an electrical power system.
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Objective
• To determine the ratings and settings of fuses, breakers, relay, etc.
• To isolate the fault or overloads.
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Coordination
• Limit the extent and duration of service interruption
• Selective fault isolation
• Provide alternate circuits
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Protection• Prevent injury to personnel
• Minimize damage to components
¾Quickly isolate the affected portion of the system
¾Minimize the magnitude of available short-circuit
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Spectrum Of Currents• Load Current¾Up to 100% of full-load¾115-125% (mild overload)
• Overcurrent¾Abnormal loading condition (Locked-Rotor)
• Fault Current¾Fault condition ¾Ten times the full-load current and higher
• Arc Fault Currents¾Between 95 to 38% of bolted fault currents
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Coordination
t
I
C B A
C
D
D B
A
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Protection vs. Coordination• Coordination is not an exact science• Compromise between protection and coordination¾Reliability¾Speed¾Performance¾Economics¾Simplicity
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Fixed Points
Points or curves which do not change regardless of protective device settings:
• Cable damage curves• Cable ampacities• Transformer damage curves & inrush points• Motor starting curves• Generator damage curve / Decrement curve• SC maximum and minimum fault points
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Capability / Damage Curves
t
I
I22t
Gen
I2t
MotorXfmr
I2t
Cable
I2t
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Cable Protection
2
2
1
tAT 234
0.0297logT 234
Ι=
⎡ ⎤+⎢ ⎥+⎣ ⎦
The actual temperature rise of a cable when exposed to a short circuit current for a known time is calculated by:
Where:A= Conductor area in circular-milsI = Short circuit current in ampst = Time of short circuit in seconds T1= Initial operation temperature (750C)T2=Maximum short circuit temperature (1500C)
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Cable Short-Circuit Heating LimitsRecommended temperature rise: B) CU 75-200C
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Transformer Categories I, II
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200 HP
MCP
O/L
Starting Curve
I2T(49)
MCP (50)
(51)ts
tLR
LRAs LRAasym
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Protective Devices• Fuse
• Overload Heater
• Thermal Magnetic
• Low Voltage Solid State Trip
• Electro-Mechanical
• Motor Circuit Protector (MCP)
• Relay (50/51 P, N, G, SG, 51V, 67, 49, 46, 79, 21, …)
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Fuse Types
• Expulsion Fuse (Non-CLF)• Current Limiting Fuse (CLF)• Electronic Fuse (S&C Fault Fiter)
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Minimum Melting Time Curve
Total Clearing Time Curve
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Molded Case CB• Thermal-Magnetic• Magnetic Only• Motor Circuit Protector (MCP)• Integrally Fused (Limiters)• Current Limiting• High Interrupting Capacity• Non-Interchangeable Parts• Insulated Case (Interchange
Parts)
Types• Frame Size• Poles • Trip Rating• Interrupting Capability• Voltage
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Thermal Minimum
Thermal Maximum
Magnetic(instantaneous)
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Overcurrent Relay
• Time-Delay (51 – I>)
• Short-Time Instantaneous ( I>>)
• Instantaneous (50 – I>>>)
• Electromagnetic (induction Disc)
• Solid State (Multi Function / Multi Level)
• Application
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© 1996-2009 Operation Technology, Inc. – Workshop Notes: Protective Device Coordination
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Relay Coordination • Time margins should be maintained between T/C
curves• Adjustment should be made for CB opening time• Shorter time intervals may be used for solid state
relays• Upstream relay should have the same inverse T/C
characteristic as the downstream relay (CO-8 to CO-8) or be less inverse (CO-8 upstream to CO-6 downstream)
• Extremely inverse relays coordinates very well with CLFs
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Arc Flash Analysis Methods and Mitigation
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Analysis Methods for Arc Flash Hazards
NFPA 70E 2009 “Standard for Electrical Safety in the Workplace”
IEEE 1584 2004a “Guide for Performing Arc Flash Hazard Calculations”
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Arc Flash Incident Video
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Arc Flash Incident Video
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Arc Flash Incident Video
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AF Analysis Considerations
• Possible Arc Fault Locations¾Line side arc faults¾Load side arc faults
• Arc Flash Analysis Worst Case Scenarios¾Maximum bolted short-circuit fault current¾Minimum bolted short-circuit fault current
• Arcing Current Variation¾ Incident Energy at 100% of arcing current¾ Incident Energy at 85% of arcing current
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Analysis of AF Results
• Arc Flash Analysis Scope¾100s or 1000s of Buses¾High/Medium/Low Voltage Systems¾Multiple Operating Configurations¾Dozens of Multiple Scenarios to be considered
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Analysis of AF Results
• Determine Which Protective Device Clears the Arc Fault¾ Is it the first upstream device in all cases?
• Determine the Locations with Special Analysis Conditions¾ Ibf is less than 700 or higher than 106,000 Amps
¾The bus nominal kV less than 0.208 kV
¾The feeder source has capacity less than 125 kVA (may not have enough energy to generate the arc)
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Methods to Mitigate the Incident Energy
• Methods to Reduce the Fault Clearing Time¾ Improving coordination settings of OC PDs.¾Type 50 protective devices (Instantaneous)¾Arc Flash light sensors¾Maintenance mode (switch)¾Differential protection¾Zone selective interlocking protection (ZSIP)
• Methods to Increase the Working Distance¾Remote racking of breakers/Remote switching¾Use of Hot Sticks
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Methods to Mitigate the Incident Energy
• Methods to Reduce the Short-Circuit Current¾Current limiting fuses and circuit breakers
¾Current limiting reactors, Isolating Transformers
¾High resistance grounding
• Methods to Reduce the Energy Exposure¾Arc resistant switchgear
¾Arc shields
¾ Infrared scanning, Partial Discharge and or Corona Cameras
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Improving Over-Current Device Coordination Settings
• Purpose is to isolate the fault with the nearest upstream over-current protective device
• Arc flash results are extremely dependent on coordination settings
• Unnecessarily high time dial settings for type 51 over-current devices
• Selection of fuses with faster total clearing time characteristic curves can reduce the energy significantly
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Arcing current through A 50/51‐1
Fault Clearing Time is 37 cycles with current time dial
settings
Incident Energy released is greater
than 27 cal/cm²
Category 4
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Arcing current through A 50/51‐1
Fault Clearing Time = 10 cycles
with lower time dial settings
Incident Energy released is less than
8 cal/cm²
Category 2
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Fuse Total Clearing Time based on 3.5 kA Arc Fault
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Incident Energy Released for Each Fuse
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Type 50 Protective Device
• Relays with instantaneous settings
• Molded case circuit breakers
• Insulated case breakers
• Power circuit breakers with instantaneous direct acting trip elements
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Type 50 PD Advantages
• Fast acting to reduce the fault clearing time since it can operate within 3 to 6 cycles
• Commonly available for most MV and LV applications
• Cost effective and do not require special installations
• Already installed in electrical system and may only require adjustments to reduce the incident energy
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Type 50 Protective Devices
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Type 50 PD Drawbacks
• To achieve coordination with downstream elements, upstream source Protective Devices have longer time delays (do not have instantaneous protection)
• The arcing current magnitude passing through the Type 50 protective device must be higher than the device’s instantaneous pickup setting
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Type 50 PD Drawbacks
Selective Coordination
introduces time delays
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Maintenance Mode
• Very fast acting trip device reduces the Fault Clearing Time (FCT)
• Are designed to pickup under very low arcing current values (instantaneous pickup setting is very low)
• Does not require complicated installation and will effectively protect locations downstream from the trip unit with maintenance mode
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Maintenance Mode
Normal Operating
Mode
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Normal Operating Mode
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Normal Operating Mode
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Maintenance Mode
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Maintenance Mode ON
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Maintenance Mode Drawbacks
• System will not have coordination during the maintenance period because of reduced instantaneous pickup settings
• Does not increase equipment protection unless the maintenance mode is ON
• May not protect certain zones where energized equipment tasks may be performed
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Zone Selective Interlocking Protection (ZSIP)
• Reduced arc fault clearing times
• Zone selection is accomplished by means of hard wired communication between trip units
• Only the trip unit closest to the fault will operate within instantaneous since upstream units are restrained by the unit closest to the fault
• Equipment and personnel arc fault protection
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Normal Coordination
Settings
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Arc Faults at different bus levels
without ZSIP
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ZSIP hard-wired communication for
restraining upstream trip units
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Arc Flash at different bus levels using
ZSIP (observe the reduced energy)
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ZSIP Drawbacks
• May take a bit longer to operate than type 50 devices because of the inherent time delay required for the ZSI logic operation
• If system is not coordinated, ZSIP does not necessarily force coordination and other upstream devices may operate before the device closest to the fault
• Arcing current must still be above short time pickup
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Arc Flash Light Sensors
• Detect the light emitted by the arc
• Very fast operation (5 to 10 ms) after the light is detected
• Provide comprehensive zone or individual cubicle arc flash protection (doors open or closed) when correctly applied
• Light sensor protection can be worn at time of task being performed for additional safety
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Light Sensors
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Kema-Laboratory Tests50 kA - 500 ms Arc Fault Clearing Time
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Arc Flash without Light Sensors
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Kema-Laboratory Tests50 kA - 500 ms Arc Fault
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Kema-Laboratory Tests50 kA Arc Fault with 50ms Fault Clearing Time
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Kema-Laboratory Tests50 kA Arc Fault with 50ms Fault Clearing Time
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Arc Flash Light Sensor Drawbacks
• Nuisance trips caused by light emitted from sources other than electrical arcs (can be remedied by using a more robust approach by combining over-current and light sensors)
• Positioning of the light sensors poses a possible problem if they are obstructed or blocked and cannot see the light emitted by the arc
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Light Sensor and Over-Current Relay
Combination
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Differential Protection
• Short Arc Fault Clearing Times
¾Differential protection can operate (relay plus breaker) within 4 to 6 cycles
¾Relay can operate within ½ to 3 cycles
• Maintain coordination between protective devices upstream and downstream from the Differential Protection Zone
• Differential protection provides continuous equipment arc flash protection
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Generator Differential Relay
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Bus Differential Relay
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Fault I = 13.83 kAOC Protection FCT = 0.643 sec
Fault I = 51.2 kADiff Protection FCT = 0.060 sec
Bus Diff Protection vs. OC Relay
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Differential Protection Drawbacks
• Nuisance trips caused by transformer inrush currents which are seen by relay as internal faults - the magnetizing current has particularly high second order harmonic content which can be used to restrain or desensitize the relay during energizing
• Higher equipment and installation costs - relatively higher costs when compared to traditional over-current protective devices
• Limited zone of protection for differential ct nodes
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Current Limiting Methods
• Current Limiting Fuses
• Current Limiting Circuit Breakers
• Current Limiting Reactors
• Isolating transformers
• High Resistance Grounding
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Current Limiting Fuses
• Current limiting fuses can operate in less than ½ cycle
• Current limiting action is achieved as long as the magnitude of the arcing current is within the current limiting range
• Current limitation curves (peak let-through curves) are needed in order to check if the fuse can limit the current
• Can be very effective at reducing the incident energy if properly used
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Current Limiting Action
tm ta
Ip’
Ip
tc
Cur
rent
(pea
k am
ps)
ta = tc – tm
ta = Arcing Time
tm = Melting Time
tc = Clearing Time
Ip = Peak Current
Ip’ = Peak Let-thru CurrentTime (cycles)
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Current Limiting Action
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Current Limiting Fuse Drawbacks
• Current limiting action is achieved as long as the magnitude of the arcing current is within the current limiting range
• Can be thermally damaged and have altered characteristics
• Needs spares (which may be expensive) and there is not indication of the type of fault.
• Energization on pre-existing fault = another blown fuse
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Current Limiting Reactors Isolating Transformers
• Current limiting reactors can help to reduce the available fault current and thus reduce the available energy
• Isolating transformers help to reduce high kA short-circuit levels (down to less than 10 kA).
• Isolating transformers add impedance between the main switchboard and the smaller panels fed from it. The short-circuit available at the switchboard may be considerably higher
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Increasing the Working Distance
• Hot Sticks
• Remote Racking
• Remote Switching
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Remote Racking/Remote Switching
• Are used to increase the personal space between the potential source of the arc and the electrician
• Can be combined with high strength plastic shields to reduce the effects of the arc flash/blast
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Remote Racking/Remote Switching
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Remote Racking/Remote Switching
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Remote Racking/Remote Switching
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Remote Racking/Remote Switching
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Mitigating/Avoiding the Incident Energy
• Arc Resistant Switchgear
• Arc Flash Shields
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Arc Resistant Switchgear
• Funneling or re-directing the incident energy away from the personal space
• Special design and construction allows the front of the equipment to experience low levels of energy
• Arc flash may still be very severe and equipment will suffer considerable damage
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Arc Resistant Switchgear
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Upcoming Arc Flash Analysis Standards/Guidelines Changes
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Arc Flash Analysis Standards/Guidelines Changes
• How will these standards affect your AF Analysis Calculations?¾IEEE 1584b -2010¾IEEE 1584.1-2010¾IEEE 1814¾NFPA 70E -2011¾NESC- Utility Models / Testing for utility
equipment
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Arc Flash Analysis Standards/Guidelines Changes
• Recent Papers on arc flash in Low Voltage Equipment
• NFPA 70E and IEEE Collaboration to develop / revise current Models
• DC Arc Flash Calculations
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NFPA 70E & IEEE1584 Collaboration Efforts “Phase I” Test Results
• Refined Equations for Incident Energy Calculations ~ Vertical, Horizontal and Vertical with Barrier Conductor arrangements
• Effect of Sound ~ 140 db @ 20 kA fault• Effect of Light ~ 45,000,000 LUX from Arc
Fault – Bright day is about 20,000 LUX• Arc Blast Pressure Wave Effects ~ 0.9 psi
120 to 200 lbs of force
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IEEE 1584b - Amendments
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IEEE 1584b - Amendments
Relay Operated Power Circuit Breaker Interrupting timesCircuit Breaker Rating and Type Interrupting Time at 60
Hz (cycles)Interrupting Time at 60 Hz (seconds)
Low Voltage Molded Case CB 3.0 (used to be 1.5) 0.050 (used to be 0.025)
Low Voltage Insulated Case CB 3.0 0.050
Low Voltage Power CB 3.0 0.050
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IEEE 1584b - Amendments
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IEEE 1584b - Amendments
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IEEE 1584b - Amendments
Main PD AF Results
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IEEE 1584b - Amendments
Results with Main PD Isolation Considered
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Arc Flash for LV Systems
Impact of Arc Flash Events with Outward Convective Flows on Worker Protection Strategies
ESW2010-11Mike Lang, Member IEEE Ken Jones Member IEEE
Thomas Neal, PhD
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Arc Flash for LV Systems
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Arc Flash for LV Systems
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Arc Flash for LV Systems
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Arc Flash for LV Systems
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IEEE 1584.1 Analysis Guidelines
• Define what are the requirements for performing AF analysis
• Defines the complexity of Systems and the experience required to perform and AF study
• Educates the Engineering process (how to make conservative assumptions)
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IEEE 1814 Safety by Design
• Reduce the Risk by designing safer equipment
• Samples of better Disconnect Switch Design
• Including Technology like ZSIP into Unit Substation Design
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NESC – ROP for 2012
200 Amp Meter Base
Shorting Wire in Meter Base
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DC Arc Flash Analysis
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Methodology for DC Arc Flash• DC SC and Arc Flash
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Methodology for DC Arc Flash• DC Arc Flash Basic Concepts
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Methodology for DC Arc Flash
• DC Arc Power
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Methodology for DC Arc Flash
• Maximum Power Method (2007 IEEE Electrical Safety Workshop)
2arc
arcsysm
bfarc
DTIV0.01IE
I0.5I
×××=
×=
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Methodology for DC Arc Flash
• Detailed Theoretical Calculation Method (2009 IEEE PCIC) (Testing has confirmed the theoretical method)
88.0
12.0
)534.020(
)534.020(
arc
garc
arcgarc
IZ
R
IZV××
=
×××=
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Methodology for DC Arc Flash• V-I Characteristic Curves
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Methodology for DC AF
• Arc Energy Equations
arcarcarcarc
arcarcarcarcarc
dcdc
tRIE
RIIVP
IVPower
××≈
×=×=
×=
2
2
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Methodology for DC AF
• DC Incident Energy Equations for Open Air and Enclosed Configurations
221
24
daEkE
dEE
arc
arcs
+×=
=π
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Methodology for DC AF
• Enclosed DC Arc Fault values a and k
Enclosure Width (mm)
Height (mm)
Depth (mm)
a (mm)
k
Panelboard 305 356 191 100 0.127
LV Switchgear 508 508 508 400 0.312
MV Switchgear 1143 762 762 950 0.416
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Test System• Example using the theoretical method
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Diff Method Result Comparison
• Comparison of the Maximum Power Method vs. Theoretical Method
DC Arc Flash
Method
Ibf dc (kA)
Iarcdc
(kA)
Rarc(ohms
)
FCT (sec
)
I.E. (cal/cm2)
Maximum Power
18.63 9.315 N/A 1.2 13.8
Theoretical Method
18.63 11.800
0.008 1.2 12.5
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Transient Arc Flash Analysis for Generators
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Problem Description
• Arc Flash Incidents near or on Generator Auxiliary Load
• No Generator Circuit Breaker• Long Fault Clearing Time because of continuous
generator short-circuit current contribution• Trying to determine a practical level of PPE to be
used for the task • To determine a systematic method to determine
the incident energy for systems with high fault current decay
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System Description
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System Description
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Analysis Techniques and Assumptions
• IEEE 1584 and NFPA 70E do not provide any specific analysis method for such systems
• The classic IEEE 1584 method utilizes the Bolted fault current to determine the arc fault current
• The guidelines do not consider any transients or decay in the fault currents
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Arc Flash Analysis
Utility Breaker Operates 6 cycles after Arc Fault is detected
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Arc Flash Analysis
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Arc Flash Analysis
Scenario ID
Arc Flash Method Arcing Current
(kA)
I.E. (cal/cm2) for 2.0 sec
Case 1 Half Cycle (Ia”) 21.44 51.6
Case 2 Four Cycle (Ia’) 21.33 51
Case 3 Decay Method (Ia” ~ Ia) 21.4 ~ 11.5
35.7
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Problems with Regular Arc Flash Analysis Method
• The calculation results show very high incident energy values
• Results in too much PPE requirements for the task• Difficult to estimate the actual energy
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Benefits from Transient Stability Analysis
• Determine actual bolted fault current contributions• Model actual generator time constants and exciter
field discharge strategies• Accurate recalculation of the bolted fault current
levels for system separation• Actual response of the Excitation and Generator
Controls
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Methods of Reducing Generator Fault Current
• Loss of Excitation• Field Discharge Resistor / Crowbar bypass system• Negative Field Forcing
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Exciter Model Used for the Simulations
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Field Discharge: Short-Circuit
Equivalent Circuit Model for Field Discharge Simulation
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Field Discharge Resistor
A Field Discharge Resistor (FDR) is added along with the generator Time Constant (T’do)
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Negative Field Forcing
For Negative Field Forcing. The discharge rate is dependent on the value of the negative voltage
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Transient Stability Scenarios
Scenario ID
Field Discharge Scheme
Simulation Method
Bolted Fault Current @ 2.0
secCase 1 None TS 17.7 kA
Case 2 Loss of Excitation TS 11.2 kA
Case 3 FDR to with RD = RF TS 6.2 kA
Case 4 Negative Field Forcing TS with UDM 0.54 kA
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Fault Current Comparison
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Incident Energy Determination from TS Results
• Using Spreadsheet, MathCAD or Matlab to import the bolted fault current values from the Transient Fault Study
• The IEEE 1584 2002 Empirical Equations are used• The energy is determined by integrating the
incident energy results from each time step up to arbitrary time of exposure (i.e. 2.0 sec)
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Incident Energy ComparisonScenario
IDField Discharge
SchemeI.E. (cal/cm2) for
2.0 secCase 1 None 40.8
Case 2 Loss of Excitation 33.1
Case 3 FDR to with RD = RF 25.7
Case 4 Negative Field Forcing 19.8
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Comparison Against Arc Flash
I.E. (cal/cm2) for 2.0 sec
(Arc Flash with TS)
I.E. (cal/cm2) for 2.0 sec
(Regular Arc Flash)
Highest Results 40.8 51.6
Lowest Results 19.8 35.7
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References
• Generator Field Discharge Methods – provided by Wayne Eads of Southern Company Generation
• IEEE Standard 421.5 IEEE Recommended Practice for Excitation System Models for Power System Studies
• IEEE 1584 2002• Power System Stability and Control, Prabha
Kundur
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References
• DC Arc Flash and Shock NFPA 70E ROP –Memphis Feb 18, 2010
• DC Arc Models and Incident Energy Calculations Paper No. PCIC-2009-07 – Award Winning Paper
• DC Arc Hazard Assessment Phase II Copyright Material Kenectrics Inc., Report No. K-012623-RA-0002-R00
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Questions?
• Questions?