bernard gittins nsw trade & investment mine safety - if it's not "on" it's...
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Just because it’s ‘ON’, doesn't mean ‘it’s ‘ON’
Part 2
How electrical protection can fail on ‘IT earthed’ systems
Bernard Gittins – Trade and Investment NSW – Mine Safety – 8/07/14
ELU 100mA Inst
ELU
‘Traditional’ Transformer - DOL Starter Arrangement
5A NER
ELU 500 mA 500 mS
M
M
M
C1
C2
C3
Mobile Sub DOL Starters
ELU 250mA 250 mS
Earth Bar
ELU
ELU 100mA Inst
ELU
ELU
‘Traditional’ Transformer - DOL Starter Arrangement
Earth Bar ELU 500 mA 500 mS
M
M
M
C1
C2
C3
Mobile Sub DOL Starters
ELU 250mA 250 mS
Earth Bar
Earth Fault Currents - simple to detect - simple to discriminate - not false readings
NER
VSD
ELU
EMI
VSD
VSD
Multiple VSDs on a Transformer Sled (Outbye)
ELU
Transformer – VSD Starter Sled
M
Earth Bar
M
M
ELU
EMI
ELU
EMI
ELU
Simplified 50 Hz earth ‘paths’
48 Ω (415V)
Multiple VSDs on an Outbye Transformer
ELU
NER
M
Earth Bar
M
M
Conveyor
Rib M
esh
VSD
ELU
EMI
VSD
VSD
ELU
ELU
ELU
EMI
EMI
Multiple VSDs on an Outbye Transformer
ELU
NER
M
Earth Bar
M
M
Conveyor
Rib M
esh
VSD EMI
VSD
VSD
ELU
ELU
EMI
EMI
ELU
New E/F Current Paths
ELU
NER
M
Earth Bar
M
M
Conveyor
Rib M
esh
VSD EMI
VSD
VSD
ELU
ELU
ELU
EMI
EMI
Common Mode Voltage Source
ELU
The VSD Provides the EF Current
ELU
NER
M
Earth Bar
M
M
‘Simplified’ VSD carrier frequency earth ‘paths’
48 Ω (415V)
Conveyor
Rib M
esh
VSD EMI
VSD
VSD
ELU
ELU
ELU
EMI
EMI
Common Mode Voltage Source
ELU ELU
New Undetected E/F Current Path
ELU
NER
M
Earth Bar
M
M Start adding the carrier frequency earth fault …
48 Ω (415V)
Conveyor
Rib M
esh
VSD
ELU
EMI
VSD
VSD
ELU
ELU
ELU
EMI
EMI
‘Uncontrolled’ CF fault ! > 5 Amp ? Can melt steel !
ELU ELU
But Wait; There’s more!
ELU
NER
M
Earth Bar
M
M
Then add the next EMI filter into the fault circuit
48 Ω (415V)
Conveyor
Rib M
esh
VSD EMI
VSD
VSD
ELU
ELU
ELU
EMI
EMI
ELU
ELU ELU
Fault currents travel via other EMI Filers
ELU
NER
M
Earth Bar
M
M
48 Ω (415V)
Conveyor
Rib M
esh
VSD EMI
VSD
VSD
ELU
ELU
ELU
EMI
EMI
ELU
ELU ELU
The confused ELUs
ELU
NER
M
Earth Bar
M
M
Then add the next EMI filter and discrimination is confused
48 Ω (415V)
Conveyor
Rib M
esh
VSD EMI
VSD
VSD
ELU
ELU
ELU
EMI
EMI
ELU
ELU ELU
But Wait; There’s more (again)!
ELU
NER
M
Earth Bar
M
M
48 Ω (415V)
Conveyor
Rib M
esh
VSD EMI
VSD
VSD
ELU
ELU
EMI
EMI
ELU
ELU ELU
ELU ELU
Then add some extraneous earthing
ELU
NER
M
Earth Bar
M
M
48 Ω (415V)
Conveyor
Rib M
esh
VSD
ELU
EMI
VSD
VSD
ELU
ELU
EMI
EMI
ELU
ELU ELU
Then add ALL the extraneous earthing
ELU
NER
M
Earth Bar
M
M
48 Ω (415V)
Conveyor
Rib M
esh
VSD
ELU
EMI
VSD
VSD
ELU
ELU
EMI
EMI
ELU
ELU ELU
And finally add an inductive earth fault
ELU
NER
M
Earth Bar
M
M
Potential Resonance Refer SB11-04
48 Ω (415V)
Conveyor
Rib M
esh
VSD
ELU
EMI
VSD
VSD
ELU
ELU
EMI
EMI
ELU
ELU ELU
Multiple VSDs on a Transformer: Earth Fault Issues Summary: The readings on the ELUs are effectively ‘meaningless’ wrt earth
faults even if wideband relays are used Fault discrimination lost
- what you can’t measure, you can’t manage! The fault current is ‘limited’ by the VSD drive circuitry. ‘Inductive’ Faults may cause resonant currents (SB11-04) Transfer of fault potential fault currents into hazardous areas must
be considered However, touch potentials can be managed through supplementary earthing
But what happens if there is no E/F
ELU
NER
M
Earth Bar
M
M
48 Ω (415V)
Conveyor
Rib M
esh
VSD
ELU
EMI
VSD
VSD
ELU
ELU
EMI
EMI
ELU
ELU ELU
Continuous Capacitively Coupled Currents
ELU
NER
M
Earth Bar
M
M
Only currents from one drive are shown! Multiply X 3
48 Ω (415V)
Conveyor
Rib M
esh
VSD
ELU
EMI
VSD
VSD
ELU
ELU
EMI
EMI
ELU
ELU ELU
Lumped Capacitance
Multiple VSDs on an Outbye Transformer Circulating Capacitive Currents of VSD
Conclusions: The readings on the ELUs are effectively meaningless Ic = f(cable length, EMI Filter design, carrier frequency, voltage) Quality HF earthing can help contain stray circulating currents Wideband ELUs highlight but don’t manage the problem Touch potentials can be controlled, but the fire hazard may not be! A fire hazard can feasibly be present on healthy drives!
– may not be able to operate < 300mA with wideband ELUs due to ‘real’ circulating currents
– temptation to set sub circuit protection > 300 mA …
Multiple VSDs on an Outbye Transformer IEC Technical Report TR 62350 Safe use of RCDs
Clauses 6.4.2 & 7.4.2 An RCD is an effective protective device to eliminate dangerous leakage currents to earth that could cause a fire within an installation.
- Gradual building up of carbon tracks in moist / dirty environments - 300mA protection required to prevent a ‘point source’ of starting a
fire. (ABB, Schneider, NHP ?) - ‘Traditional’ IT earthed subcircuit protection settings ‘saw’ 300mA
E/Fs on 50 Hz earth faults … no historical precedent for current situation.
Multiple VSDs on an Outbye Transformer The ‘Solutions’: Use VSDs with sinusoidal output voltage waveforms
– sine filters on voltage source VSDs – current source VSDs – 18-24 Pulse for 3.3 kV VSDs ? – Put the VSD in the motor J-box and the ‘maximise’ the EMI filter
impedance. Follow the relevant recommendations of SB11-04 (Ask the accounted to set up and take the responsibility for your
protection settings on traditional VSDs )
Summary’
The use of VSDs with non sinusoidal voltage outputs is a potential
fire hazard even on fixed VSD installations Traditional mining relays & placement do not provide a complete
safety coverage for the various situations as outlined in this presentation.
Wideband ELUs can see, but cannot manage the problem – not all currents are / can be monitored – adjusting protection settings to ‘make it work’ can be dangerous – circulating currents can (theoretically) cause fires
However, systems do exist which minimize / eliminate circulating currents and can quickly detect and clear earth faults on VSD systems And so I say again…
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