using high resistance grounding to mitigate arc flash hazards
TRANSCRIPT
Using High Resistance Grounding to Mitigate Arc Flash Hazards
Presenter: Ajit Bapat, P.E.
DETECT/PROTECT
I GARD’ HRG d d• I‐GARD’s HRG systems can detect a ground fault, signal an alarm, and locate the affected branch or feeder removing maintenance personnel from the dangers of higher fault currents.
MITIGATE
• I‐GARD’s FALCON provides arc flash mitigation at the speed of light by using adjustable light sensitivity to reduce arc‐flash energy with fast detection and tripping, under 1ms.
ARC FLASH ANALYSIS
• I‐GARD’s ARC FLASH ANALYSIS will calculate your Risk using IEEE and NFPA equations and
h d d d l hmethods and develop a comprehensive protection plan to achieve a safer environment
for your people and plant.
Here's where you'll find answers to your questions about:
• The current emphasis on arc flashHazards created by arc flash and arc blastblast
• What you need to do to protect workers and comply withOSHA requirements
us.ferrazshawmut.com/arcflashq
• Regulations that impact plant operations, including NFPA 70 and NFPA 70E
• Arc flash hazard analysis
• The hazards of arcing faults
• Solutions and services Ferraz h ffShawmut offers
Using High Resistance Grounding to Mitigate Arc Flash Hazards
Presenter: Ajit Bapat, P.E.
Power System Grounding MethodsPower System Grounding Methods
UngroundedUngrounded
Solidly GroundedCorner Delta GroundedCorner Delta GroundedMid phase grounded
Resistance Grounded
METAL ENCLOSURESSolidly Grounded System
TOLOAD
METAL ENCLOSURES
LOAD
BONDINGJUMPERJUMPER
Grounding and Bonding means a permanent and continuous conductive path to the earth with sufficient ampacity to carry any fault current liable t b i d it d f ffi i tl l i d t li it th ltto be imposed on it and of sufficiently low impedance to limit the voltage rise above ground and to facilitate the operation of the protective devices in the circuit.
Solidly Grounded Systems
Earth
RiskRiskElectrical deficiencies are the leading ignition source and cause of fire and explosion.p
Empirical data indicates that approximately 80% of all electrical faults are ground faults.of all electrical faults are ground faults.
ConsequenceConsequenceFrom 1992 to 1999 the customers from one major insurance company reported 228 losses that were tt ib t d t d f lt ith t t l t f US $180attributed to ground faults with a total cost of US $180
million.
Solidly Grounded SystemsSolidly Grounded Systems
• Sustained arcing faults can release intense heat and gmechanical energy causing severe damage and injury
ARCING FAULT DAMAGE(KILOWATT CYCLES)
Causes of Bolted and Arcing Faultsg
• What causes these types of faults?
– Bolted faults (low impedance and high current)Improper connections after maintenance
Installation errorsInstallation errors
– Arcing faults (high impedance, lower current)Careless cover or device removalCareless cover or device removal
Foreign object (tool) dropped into equipment
Misalignment of moving contacts (parts failure)
Di t t i ti di l t i b kdDirt contamination or dielectric breakdown
Entry of foreign body (rodent, snake, squirrel)
Bolted and Arcing Fault Characteristics
Arcing fault incident energy produced isArcing fault incident energy produced isGreater at higher bolted fault current levelsReduced by dynamic impedance (air)And increased by the time duration of the arc
The most controllable factor in reducing the incident energy is timeincident energy is time
Current flow in an arcing fault is approximately half that of the bolted fault current (impedance of air)that of the bolted fault current (impedance of air)
Electrical Arc Factsec ca c ac s
• Arc is electric current passing through air
– Shock potential from contact with arc
• Temperature of arc plasma center is greater than 5000°F (some say much higher)much higher)
– Radiated heat burns
• Pressure wave generated from arc
h i– Impact to hearing, etc
– Gaseous copper is 44,000 times solid
– Molten metal expelled from equipment at high speed– Molten metal expelled from equipment at high speed
• Arc fault results from something wrong or out of place
IEEE – Arcing FaultsIEEE Arcing Faults• IEEE Std 242‐2001
Recommended Practice for the Protection and Coordination of Industrial andRecommended Practice for the Protection and Coordination of Industrial and Commercial Power Systems8.2.2One disadvantage of the solidly grounded 480 V system involves the high magnitude of destructive, arcing ground‐fault currents that can occur.g , g g
• IEEE Std 141‐1993Recommended Practice for Electric Power Distribution for Industrial Plants7.2.4Th lidl d d h h hi h b bili f l i iThe solidly grounded system has the highest probability of escalating into a phase‐to‐phase or three‐phase arcing fault, particularly for the 480 and 600 V systems. The danger of sustained arcing for phase‐to‐ground fault…is also high for the 480 and 600 V systems, and low or near zero for the 208 V system.
Solidly Grounded SystemsSolidly Grounded Systems
• Line to Ground Fault does not cause transient overvoltage but often causes voltage sag
• Permits line‐to‐neutral loads (lighting, heating cables)cables)
• Ground faults easy to locate, but cause unscheduled service interruptionservice interruption
• Danger from low‐level arcing ground faults• Since 1970s, ground fault protection mandatory for , g p ysolidly grounded 600V services rated 1000A and higher by the CEC and the NEC
Ungrounded Systems –hCapacitive Charging Current
A
BB
C
IC0IC0 IC0IC0IC0 IC0
Ungrounded Systems ‐ Voltage g y gRise on Ground Fault VA-G = 600 V
VB-G = 0 VVC-G = 600 VV = 347 V
VA-G = 459 VV 174 V
N
C A
C A
VA-G = 347 VVB-G = 347 VVC-G = 347 VVN-G = 0 V
VN-G = 347 V
C A
VB-G = 174 VVC-G = 459 VVN-G = 174 V
60°
B
N120°
C A
G
N
G
82°
B
No Ground Fault
Full Ground Fault on Phase BB
Partial (50%) Ground Fault on Phase B
• Neutral point voltage is zero only when no ground fault present
• Neutral voltage rises whenever a ground fault is present
• Neutral voltage ranges from 0 V at no fault to 347 V at full fault
System Charging Current 3IC0System Charging Current 3IC0
AA
B
C
I 3II3 I3 IF = 3IC0IC03 IC03
Ungrounded SystemsUngrounded Systems
• Negligible fault current and no tripping on firstNegligible fault current and no tripping on first ground fault
• Difficult to locate ground faults• Difficult to locate ground faults
• 5‐6 times transient voltage escalation on i i i i d f lintermittent, sputtering arcing ground faults due to DC voltage buildup across the stray
i dcapacitance to ground
Distribution System Design Criteria
UngroundedUngrounded • Reliability Prone to double faults and thus trip outs• Safety Transient voltages can occur
C t ff ti L ti f lt i diffi lt• Cost effectiveness Locating faults is difficult • Scheduled Maintenance Double faults can cause
unscheduled shut downs• Prioritized load Coordination lost in case of
double faults
Resistance GroundingResistance Grounding
• Used in Process Industries, Water and Waste Water,Used in Process Industries, Water and Waste Water, Hospitals, Data processing Centers
• Used on MV systems to limit ground fault currenty g
• No arcing ground faults as with solid grounding
• No overvoltages as with ungrounded systemsNo overvoltages as with ungrounded systems
Resistance Grounding
• Resistor inserted between neutral and ground to limit ground fault current
A
limit ground fault current
• Resistor rated for line‐to‐neutral voltage
B
C
347V
BoltedGround Faultto equipmentframe
C
69 Ω
N
G5A
Equipment Bonding Conductor Resistor Let-Thru Current
High Resistance GroundingHigh Resistance Grounding
Canadian Electrical Code and National ElectricCanadian Electrical Code and National Electric Code permit continuous operation when neutral grounding device is used to control theneutral grounding device is used to control the ground current to a low value where :
A visual or audible alarm or both clearly–A visual or audible alarm, or both, clearly identified to indicate the presence of a ground fault is providedground fault, is provided.
High Resistance GroundingHigh Resistance Grounding
• Limit ground fault current to 10 A or lessg• Provides service continuity on first ground fault• Prevents arc flash incidents on first ground faultsg• Allows faults to be located without de‐energizing feeders (ground fault pulse locating)
• Used in continuous process industries, hospitals and data centres where unscheduled downtime is costly
Fault Current on HRG SystemA
B
C
RXC0 XC0 XC0
IR3IC0
IR
( ) ( )20
2 3 CRF III +=
( ) 3tf lti iAt32 IIII ( ) 00 3current,fault minimumAt 32 CRCF IIIIMIN
==
Distribution System Design Criteria
High Resistance GroundedReliabilit Po er contin it No trips on gro nd fa lt• Reliability Power continuity, No trips on ground fault
• Safe No Arc Blast or Flash Hazard on Ground Fault
• Cost effective 3 Wire Systems are cheaper than 4 wireCost effective 3 Wire Systems are cheaper than 4 wire • Scheduled Maintenance Faulty equipment can continue to run,
scheduled shut downs and lowerrepair costsp
• Prioritized load Overcurrent Coordination maintainedSelective second fault protection available
Low Resistance GroundingLow Resistance Grounding
• Used on medium voltage (MV) premises distribution g ( ) psystems
• System charging current too high for high resistance dgrounding
• Ground fault current limited to 25 – 400 A typicallyT i d f lt• Trip on ground fault
• Prevents arc flash incident on ground fault
Resistance Grounding MethodsResistance Grounding Methods
Zi Z G diZig-Zag GroundingTransformer rated for Line-to-Line Voltage
Zig‐Zag Grounding TransformerZig Zag Grounding Transformer
High Resistance Grounding and IEEE
IEEE Std 242‐2001 (Buff Book8.2.4
High‐resistance grounding helps ensure a ground‐fault current of known magnitude, helpful for relaying purposes. This makes it possible to identify the faulted feeder with sensitive ground‐fault relays.
IEEE Std 141‐1993 (Red Book)7.2.2
High‐resistance grounding provides the same advantages as ungrounded systems yet limits the steady state and severe transient over‐voltagessystems yet limits the steady state and severe transient over voltages associated with ungrounded systems. There is no arc flash hazard [for a ground fault on 480V and 600V systems], as there is with a solidly grounded system, since the fault current is limited to approximately 5A.
Resistance Grounding and IEEEResistance Grounding and IEEEIEEE Std. 142‐1991 Recommended Practice for Grounding of Industrial andC i l P S tCommercial Power System1.4.3The reasons for limiting the current by resistance grounding may be one ormore of the following:g1. to reduce burning and melting effects in faulted electric equipment, such as
switchgear, transformers, cables and rotating machines.2. to reduce mechanical stresses in circuits and apparatus carrying fault currents3 to reduce electric‐shock hazards to personnel caused by stray ground fault3. to reduce electric shock hazards to personnel caused by stray ground fault
currents in the ground return path4. to reduce arc blast or flash hazard to personnel who may have accidentally
caused or who happen to be in close proximity to the fault current5 to reduce the momentary line‐voltage dip occasioned by the occurrence and5. to reduce the momentary line voltage dip occasioned by the occurrence and
clearing of a ground fault
ComparisonComparison
Ungrounded Solidly Grounded
Low Resistance Grounded
High Resistance Grounded
Productivity Impact
System Type
System System System System
Overvoltages Severe None Limited LimitedOvercurrent - Damage
t i t f f lt U k S Mi i l N
Equipment Damage at point of fault Unknown Severe Minimal None
Maintenance Costs High Reasonable Reasonable LowContinuous Operation with Ground Fault
Possible but not recommended Not possible Not possible Ideal
Relay Co ordination
a age
Relay Co-ordination (Appropriate Equipment Tripped, Ease of fault location) Difficult Difficult Good Excellent
Personnel Safety to Personnel Poor Good Reasonable Excellent
Downtime
Personnel y
Insurance PerspectiveInsurance Perspective“We tell our customers to first determine whether they have a resistance ground system or ground fault detection.
D t i th t f d tDetermine the type of ground system you have, and then, consider conversion from an ungrounded system to a resistance groundedsystem to a resistance grounded system.
Using a high-resistance groundUsing a high resistance ground system could knock 10% off the price of insurance”.
Alarming and RelayingAlarming and Relaying
• Resistance grounding is not enoughResistance grounding is not enough
• Must sense ground faults
k i• Take action
• Either alarm only, and locate the fault
• Or trip on fault
Ground Fault Sensing R id l CT M th dResidual CT Method
A
LOADB
IA
IB
C
N
IC
I
RELAY
IN
G
IG
• Typically 50 A to 1200 A pickup Used on solidly grounded LV & MV systems
GBONDING CONDUCTOR
GROUND FAULT CURRENT
• Used on solidly grounded LV & MV systems
Sensing Ground Faults Usingg ga Zero Sequence (Core Balance) CT
LOAD
A
BIA
C
IB
IC
RELAY
G
IG
Approaches For Ground DetectionApproaches For Ground Detection
• Voltage Sensing GF RelayVoltage Sensing GF Relay• Current Sensing GF Relay• Swbd Multi‐Feeder GF Alarm Relay• Swbd Multi‐Feeder GF Alarm Relay• Swbd GF Relay with 2nd Fault ProtectionGF R l f MCC’• GF Relay for MCC’s
• Combination wall‐mounted NGR and GF Relay f R t fitfor Retrofits
Voltage Sensing GF Relay – DSP‐DSYS
• Non‐selective system level GF alarm
• Resistance grounded or ungrounded systemssystems
Voltage Sensing GF Relay – DSP‐DSYS
RESISTOR DIVIDERRESISTOR DIVIDERSENSING NETWORK
DDR2
VOLTAGE SENSINGRELAY DSP-DSYS
Zero Sequence Current Sensing and Arc Flash sensing Relay ‐ Sentri• Selective – responds to leakage current to ground,Selective responds to leakage current to ground, identifies faulted feeder
• 3‐ Optical sensor Inputs to detect arc flash p p
5 A1 APICKUP
1 APICKUP
1 APICKUP
1 APICKUP
RRRR
5 ANGR
PICKUP PICKUP PICKUP PICKUP
SENTRISENTRI
• Ground fault and arc flash built into 1 relay.• Solidly grounded or Resistance grounded systems.• 0.1 A to 1200 A trip settings.• Can connect to 3 self‐monitoring arc flash sensors.• Less than 1ms trip time on arc flash.• Solid state relays with mechanical relay backup.• Pre‐trip relay for indication prior to main relay tripping.• 1 A and 5 A CT inputs as well as ZSCS inputs for sensitive ground fault
protection.• Monitor current with mGARD‐SYM display from up to 50 relays.• Modbus capability with mGARD‐SYM display.• ZSI Zone Selective Instantaneous Protection
GF Relay for Motor Control Centers – DGF‐CT
DGF‐CT Fits In MCC BucketDGF CT Fits In MCC Bucket
Multi‐Feeder Ground Alarm Relay –DSP‐MKIII
Current Sensing
Voltage Sensing g
Relay Mudules
Relay
First Fault Alarm ApplicationFirst Fault Alarm Application
Second Fault Protection – DSP‐MKIII
DSP OHMNI F tDSP OHMNI Features• Identifying faulty phase and faulty feeder. First Fault Alarm y g y p y
and Selective Second Fault trip or First fault trip
• Modbus communications
• Enhanced relay security ‐ inrush detection prevents nuisance tripping on severe magnetizing inrush
• DIN rail mounted – takes much less space on switchgear –DIN rail mounted takes much less space on switchgear now fits in 22”‐wide sections
• Pulsing system
• Local Display ‐
DSP OHMNI RelayDSP OHMNI RelayFirst Fault Alarm and Second Fault Trip
OR First Fault Trip OHMNI-PM NGR LOADS
PULSE SIGNAL
ZSCS
BREAKERTRIP SIGNAL
G
MAIN BUS
1A1A
N
DDR225VA CPT120V
HORN
A B N GC
TRIP
CON
TACT
SENS
OR IN
PUT
RS 485 TO NETWORK
1A
ALARMCONTACTS
A B C N GPOWERAC/DC
DSPDSP DM
RS-485 TO NETWORK
DSP
20 CONDRIBBON
DSP-DSM DSP-DFM
DSP-DFMDSP-DPSDSP-DM DSP-
DFMDSP-DFM
Pulsing NGR – OHMNI‐PMPulsing NGR OHMNI PM
Portable Current Sensor for Fault Tracing
Ground Fault Pulse LocatingGround Fault Pulse Locating
A li ti C id tiApplication Considerations 1. Where to apply the Grounding Resistorpp y g• At the transformer or • At the main bus 2 When to apply first fault alarm only 3. When to add 2 nd fault trip function • Selective Instantaneous feeder tripping • Coordination with down stream Over current in 2nd f lfault trip
4. When to use 1 st fault trip
DSP RelayDouble Ended Unit Sub Application
Parallel GeneratorsParallel GeneratorsTYPICAL PARALLEL GENERATOR HIGH RESISTANCE GROUNDING SCHEME
GENERATORS 600V
600V
G G G GZero Sequence Current Sensors(one per feeder; one per generator)
DDR2-6
DSA
15-20A, 3P100 kAIC
To BMS
S-P
M2
To BMS
5A, 600VZig-Zag Grounding2 - #16AWG, 24 Vdc
See Notes 1 and 2.DS
Optional DS-PM2 Pulsing Card
5A, 347V NeutralGrounding Resistor
OptionalPulsing Resistor
Zig-Zag GroundingTransformer
2 #16AWG, 24 Vdcfor pulsing control
Notes:1. NGR/Zig-Zag assembly w ith pulsing resistor, IPC Part Number: OHMNI-6PM-5-ZZ2. NGR/Zig-Zag assembly w ithout pulsing resistor, IPC Part Number: NTR600-5-ZZ
AWG#8 as perCEC 10-1108(3)
HRG Retrofit of Parallel LV GeneratorsG G G G
15A, 3P100 kAIC
TOSCADA
52 52
52 5252
15A, 3P100 kAIC
TOSCADA
52 52
525252
52V
A
GROUND FAULT RELAY
V
A
GROUND FAULT RELAY
ALARM
SLEUTHSLEUTH
2A, 347V
2A, 600V 2A, 600V
PULSINGCONTACTOR
GROUND FAULT RELAY
ALARM
ALARMTO SCADA
ALARMTO SCADA
PULSINGCONTACTOR
NGR 2A, 347VPULSE TO 4A
NGR 2A, 347VPULSE TO 4A
STOPLIGHTWITH ZIG-ZAG,VOLTMETER,
STOPLIGHTWITH ZIG-ZAG,VOLTMETER
V
A
2A, 347V
GROUND FAULT RELAY
ALARM
V
A
ALARMTO SCADA
VOLTMETER,AMMETER
VOLTMETER,AMMETER
ALARMTO SCADA
52 5252
5252
52 5252
52 52
52
M2 M1
Combination NGR and GF RelayCombination NGR and GF Relay
• SLEUTHSLEUTH
• For retrofitapplicationsapplications
OptionsOptions
• NGR MonitoringNGR Monitoring
• Grounding Conductor Monitoring
i h li bili i i h d l h• High Reliability Resistors with dual paths
New Concepts in System Grounding
• Hybrid Grounding in Distribution : Solid Grounding changes to High Resistance Grounding up on sensing a ground fault. Example : FUSION
• Hybrid Grounding of Generators : Usually generators are Low Resistance Grounded. To provide protection for the Stator winding of the generator Hybrid Grounding is used Lowwinding of the generator Hybrid Grounding is used . Low Resistance Grounding changes to High resistance when an internal Generator Ground Fault is detected.
New Concept in Grounding Hybrid Grounded SystemsHybrid Grounded Systems
Ground fault changes the impedance in the ground circuit….
Circuit breaker or Fuse
Thank You For Attending!Thank You For Attending!
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