Presentation May 2012

5/20/2018 Presentation May 2012

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IEEE Baton Rouge

Grounding for Electrical Power Systems

(Low Resistance and High ResistanceDesign)


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Low Resistance Grounding Advantages/Disadvantages Design Considerations

High Resistance Grounding Advantages/Disadvantages Design Considerations

Generator Grounding

Single/Multiple arrangements


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Impedance selected to limit line-to-ground fault current (normallybetween 100A and 1000A asdefined by IEEE std. 142-2007section 1.4.3.2)


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Advantages Eliminates high transient overvoltages

Limits damage to faulted equipment

Reduces shock hazard to personnel

Disadvantages Some equipment damage can still occur

Faulted circuit must be de-energized

Line-to-neutral loads cannot be used.

ccc IabIIcIr

A B

C

3 Load

or Network

Source

N

NeutralGroundingResistor


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Most utilized on Medium Voltage Some 5kV systems

Mainly 15kV systems

Has been utilized on up to 132kV systems (rare)

Used where system charging current may beto high for High Resistance Grounding

ccc IabIIcIr

A B

C

3 Load

or Network

Source

N

NeutralGroundingResistor


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Resistor Amperage (ground fault let through current) System Capacitance

System Bracing

System Insulation

Relay Trip points (Time current curve)

Selective tripping Resistance increase with temperature

Resistor time on (how long the fault is on the system)

Single Phase Loads


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Conductor

Cableinsulation

Cable tray

Every electrical system has some natural capacitance. The capacitivereactance of the system determines the charging current.

Zero-sequence Capacitance: 0 =106

2F/phase

Charging Current: 30 =2

106 A


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During an arcing or intermittentfault, a voltage is held on the systemcapacitance after the arc isextinguished. This can lead to asignificant voltage build-up whichcan stress system insulation andlead to further faults.

In a resistance grounded system, theresistance must be low enough toallow the system capacitance to

discharge relatively quickly.

Only discharges if Ro< Xco, so Ir> Ixcoper IEEE142-2007 1.2.7)

That is, resistor current must be greater than capacitive charging current.


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Total Fault current is the vector sum of capacitive charging currentand resistor current

= 2 + 0

2

So, if IR= IC0, then IF= 1.414 IR

Total fault current must not exceed the value for which the system isbraced.

In many cases, the system is already braced for the three-phase faultcurrent which is much higher than the single line-ground fault

current of a resistance grounded system.


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Resistance grounded systems must be insulated for full line-linevoltage with respect to ground.

Surge Arrestor Selection: NEC 280.4 (2) Impedance or UngroundedSystem. The maximum continuous operating voltage shall be thephase-to-phase voltage of the system.

Cables: NEC Table 310.13E allows for use of 100% Insulation level,but 173% is recommended for orderly shutdown.

VAG

VBGVCG

VAG

VBG

Un-faulted Voltages to ground Faulted Voltages to ground (VCG= 0)


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Properly rated equipment prevents Hazards.

A B

C

3 Load

HRG

480V Wye Source

N

0V

2400V

Ground A

Cables, TVSSs, VFDs, etc. and otherequipment must be rated forelevated voltages.

0V

4160V

4160VGR


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N

G

R

CTs and relays must bedesigned such that systemwill trip on a fault of themagnitude of the groundfault current, but not ontransient events such aslarge motor startup.

Network protection schemeshould try to trip faultlocation first, then goupstream.


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Residual connected CTs Zero Sequence CT


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Widely varying use of resistance material in the industry. Different coefficients of resistivity for these materials.

Coefficient of resistivity typically increases with temperature of the material, thusresistance of the NGR increases while the unit runs.

As resistance increases, current decreases.

Relay current trip curve must fall below the current line in the graph below.

5.5

6

6.5

7

7.5

300

320

340

360

380

400

1 2 3 4 5 6 7 8 9 10

NGR Resistance vs Current

Current

Resistance


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Normally, protective relaying will tripwithin a few cycles.

IEEE 32 defines standard resistor ontimes. Lowest rate is 10 seconds, butcould potentially go less to savematerial/space.

Can go as high as 30 or 60 secondsas required (rare).

Extended or Continuous ratings arealmost never used in this applicationdue to the relatively high fault

currents.

IEEE Std 32Time Rating and Permissible

Temperature Rise for Neutral

Grounding Resistors

Time Rating

(On Time)

Temp Rise (deg

C)

Ten Seconds

(Short Time)760oC

One Minute

(Short Time)760oC

Ten Minutes(Short Time)

610oC

Extended Time 610oC

Continuous 385oC


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A B

C

3 Load

HRG

480V Wye Source

N

Phase and Neutral wires in same conduit.If faulted, bypass HRG, thus, -G fault.

No line-to-neutral loads allowed, preventsHazards.

NGR


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Add small 1:1

transformer and solidlyground secondary for 1loads (i.e. lighting).


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Impedance selected to limit line-to-ground fault current (normally< 10A as defined by IEEE std.142-2007 section 1.4.3.1)

Ground detection system required

System is alarm and locate insteadof trip.

Source

(Wye)

HRG C

BA

N


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Advantages

Eliminates high transient overvoltages Limits damage to faulted equipment

Reduces shock hazard to personnel

Faulted circuit allowed to continueoperating

Disadvantages

Nuisance alarms are possible. Line-to-neutral loads cannot be used.

ccc IabIIcIr

A B

C

3 Loador Network

Source

N

NeutralGrounding

Resistor


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Most utilized on Low Voltage Many 600V systems Some 5kV systems

Has been utilized on up to 15kV systems (rare)

ccc IabIIcIr

A B

C

3 Loador Network

Source

N

NeutralGrounding

Resistor


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Resistor Amperage (ground fault let through current) System Capacitance

Alarm notification

Fault Location Pulsing

Data Logging

Relay Coordination (What to do if there is a second fault)

System Insulation

Personnel training


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Conductor

Cableinsulation

Cable tray

Every electrical system has some natural capacitance. The capacitivereactance of the system determines the charging current.

Zero-sequence Capacitance: 0 =106

2F/phase

Charging Current: 30 =2

106A


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During an arcing or intermittentfault, a voltage is held on the systemcapacitance after the arc isextinguished. This can lead to asignificant voltage build-up whichcan stress system insulation andlead to further faults.

In a resistance grounded system, theresistance must be low enough toallow the system capacitance todischarge relatively quickly.

Only discharges if Ro< Xco, so Ir> Ixcoper IEEE142-2007 1.2.7)

That is, resistor current must be greater than capacitive charging current.


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Major Contributors to system capacitance: Line-ground filters on UPS systems

Line-ground smoothing capacitors

Multiple sets of line-ground surge arrestors

All of these can make implementation ofHRG difficult


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HRG systems are alarm andlocate systems

Alarm methods: Audible horn Red fault light

Dry contact toPLC/DCS/SCADA opens

DCS/SCADA polling ofunit via Modbus RS-485

Ethernet


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HRG

480V Wye Source

C

B A

55.4ohms

Operator controlledcontactor shorts outpart of the resistor

Ideally, the increasein current is twice

that of the normalfault current, unlessthat level is unsafe.


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NOTE:Tracking a ground fault can only be done on anenergized system. Due to the inherent risk of

electrocution this should only be performed by trainedand competent personnel.


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ZSCT

Meter

ZSCT

MeterMeter

ZSCT

0A

55A

50A

50A80A

80A

50A 50A 50A

50A50A55A30A 30A 30A

30A30A30A

MotorMotor

5A

5A0A

5A

HRG

5A

480V Wye Source85A

C

BA

55.4

ohms

Meter reading will alternatefrom 5A to 10A every 2

seconds.

Alternatives to Manual location: Add zero sequence CTs & ammeters to each feeder Use metering inherent to each breaker (newer equipment only)


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HRG systems with data logging can be used to locateintermittent ground faults

Example: Heater with ground fault comes on at 11:00am and then

turns off at 11:01am

Normal Pulsing will not locate since the fault will be gone. HRG Data logging can help locate faulted equipment in

conjunction with DCS/SCADA data records

Fault time

frame

Equipment

On


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If there is a second groundfault on another phase, itis essentially a phase-phase fault and at leastone feeder needs to trip

Network protectionscheme should bedesigned to trip the lowestpriority feeder first, thenthe next, and then moveupstream.


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Check MCC GF pickup ratings to be sure the small ground fault currentvalues do not trip off the motor on the first ground fault.

Also, fusing on small motors can open during a ground fault. ConsultNEC Table 430.52 for Percentage of full load current fuse ratings. Mostare 300% FLC.


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Resistance grounded systems must be insulated for full line-linevoltage with respect to ground.

NEC 285.3: An SPD (surge arrestor or TVSS) device shall not beinstalled in the following: (2) On ungrounded systems, impedancegrounded systems, or corner grounded systems unless listedspecifically for use on these systems.

VAG

VBGVCG

VAG

VBG

Un-faulted Voltages to ground Faulted Voltages to ground (VCG= 0)


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Properly rated equipment prevents Hazards.

A B

C

3 Load

HRG

480V Wye Source

N

0V

277V

Ground A

Cables, TVSSs, VFDs, etc. and otherequipment must be rated forelevated voltages.

0V

480V

480V


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Common Mode Capacitors provide pathfor Common-mode currents in outputmotor leads

MOVs protect against Transients


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Ground fault in Drive #1caused Drive 2 to fault on

over-voltage

Drive 3 was not affected


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Factory optioncodes exist to

remove the internaljumpers


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Per NEC 250.36, personnel must be trained onImpedance Grounded systems. Training should:

Establish seriousness of a fault Discuss location methods Familiarize personnel with equipment


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Fault current Paralleled generators Common Ground Point

Separate Ground Point


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In most generators, the zero-sequenceimpedance is much less than the positive ornegative sequence impedances.

Due to this, resistance grounding must be

used unless the generator is specificallydesigned for solid grounding service.

= =


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Generators Grounded through a single impedance must be the sameVA rating and pitch to avoid circulating currents in the neutrals

Each Neutral must have a disconnecting means for maintenance asgenerator line terminals can be elevated during a ground fault.

Not recommended for sources that are not in close proximity


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Separately grounding prevents circulating currents

Multiple NGRs have a cumulative effect on ground fault current i.e.the total fault current is the sum of all resistor currents plus

charging current. Can be difficult to coordinate tripping or fault location

If total current exceeds about 1000A, single ground point should beconsidered.


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IEEE 242-2001 IEEE 142-2007

NEC

IEEE 32


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Documents

Presentation May 2012