Bernard Gittins NSW Trade & Investment Mine Safety - If it's not "on" It's not "on" part 2
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2014 Electrical Engineering Safety Seminar 5 & 6 November at Sydney's Waterview Park
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Bernard Gittins NSW Trade & Investment Mine Safety - If it's not "on" It's not "on" part 2
- 1. 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
- 2. 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
- 3. 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
- 4. 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)
- 5. Multiple VSDs on an Outbye Transformer ELU NER M Earth Bar M M Conveyor Rib Mesh VSD ELU EMI VSD VSD ELU ELU ELU EMI EMI
- 6. Multiple VSDs on an Outbye Transformer ELU NER M Earth Bar M M Conveyor Rib Mesh VSD EMI VSD VSD ELU ELU EMI EMI ELU
- 7. New E/F Current Paths ELU NER M Earth Bar M M Conveyor Rib Mesh VSD EMI VSD VSD ELU ELU ELU EMI EMI Common Mode Voltage Source ELU
- 8. The VSD Provides the EF Current ELU NER M Earth Bar M M ‘Simplified’ VSD carrier frequency earth ‘paths’ 48 Ω (415V) Conveyor Rib Mesh VSD EMI VSD VSD ELU ELU ELU EMI EMI Common Mode Voltage Source ELU
- 9. 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 Mesh VSD ELU EMI VSD VSD ELU ELU ELU EMI EMI ‘Uncontrolled’ CF fault ! > 5 Amp ? Can melt steel !
- 10. 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 Mesh VSD EMI VSD VSD ELU ELU EMI EMI ELU ELU
- 11. Fault currents travel via other EMI Filers ELU NER M Earth Bar M M 48 Ω (415V) Conveyor Rib Mesh VSD EMI VSD VSD ELU ELU EMI EMI ELU ELU
- 12. 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 Mesh VSD EMI VSD VSD ELU ELU EMI EMI ELU ELU
- 13. But Wait; There’s more (again)! ELU NER M Earth Bar M M 48 Ω (415V) Conveyor Rib Mesh VSD EMI VSD VSD ELU EMI EMI ELU ELU ELU
- 14. Then add some extraneous earthing ELU NER M Earth Bar M M 48 Ω (415V) Conveyor Rib Mesh VSD ELU EMI VSD VSD ELU EMI EMI ELU ELU
- 15. Then add ALL the extraneous earthing ELU NER M Earth Bar M M 48 Ω (415V) Conveyor Rib Mesh VSD ELU EMI VSD VSD ELU EMI EMI ELU ELU
- 16. And finally add an inductive earth fault ELU NER M Earth Bar M M Potential Resonance Refer SB11-04 48 Ω (415V) Conveyor Rib Mesh VSD ELU EMI VSD VSD ELU EMI EMI ELU ELU
- 17. 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
- 18. But what happens if there is no E/F ELU NER M Earth Bar M M 48 Ω (415V) Conveyor Rib Mesh VSD ELU EMI VSD VSD ELU EMI EMI ELU ELU
- 19. 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 Mesh VSD ELU EMI VSD VSD ELU EMI EMI ELU ELU Lumped Capacitance
- 20. 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 …
- 21. 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.
- 22. 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 ☺)
- 23. 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…
- 24. Existing Protection Systems May Not Always Work Just because it’s ‘ON’, doesn't mean ‘it’s ON’
- 25. Thanks Reference: ‘Electrical hazards associated with variable speed drives and earth fault current limited systems’ SB11-04