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…

Existing Protection Systems May Not Always Work

Just because it’s ‘ON’, doesn't mean ‘it’s ON’

Thanks

Reference: ‘Electrical hazards associated with variable speed drives and earth fault current limited systems’ SB11-04