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Internal Arcs: Maximum Safety
and Continuity of Supply in
MV Substations
Marcel Buckner
Business Development Manager Africa
EATON Electric
INTERNAL ARCS (1)
• Quality of supply and Personnel Safety most important
aspects of a Utility
• Prevention of internal arcs should be first and most important
approach in design and service stages
• Protection approach is to reduce effects of internal arcs
– Reducing arc duration
– Protective gear for operators
– Leading hot gasses away from persons to dedicated location around
the gear
• IEC 62271 considers pressure rise in substation with optional
IAC-B test
INTERNAL ARCS (2)
• In short circuits high current flows through air inside assembly
– a high current fault arc occurs
• Current flows between phases or between one (or more)
phases, neutral and/or ground
• Amount of energy released varies, up to many MJs
• Consequences can be devastating
• Internal explosion can reach temperatures of beyond
19,000oC
• Release of toxic gasses – threat to environment and humans
• Resultant downtime unacceptable
TECHNICAL ASPECTS OF INTERNAL ARC OCCURRENCES• Starts with dielectric breakdown of insulation medium
• Arc current then dependent on actual situation
• Internal arc causes overpressure / release of fire , gas
and smoke
– With gaseous insulation only
– With solid insulation per phase
• Phase-to-phase breakdown likely• Arc develops within 10 milliseconds
into 3-phase fault• High energy release• Arc difficult to control
• Phase-to-earth fault• Arcing current dependent on neutral treatment of network• Lead to multi-phase fault
PREVENTION OF INTERNAL
ARCS CRUCIAL
• Prevention more important than cure
– Solid insulation
– Electrical field control
technologies
• Personnel safety paramount
• Personnel are vulnerable during general operations and
maintenance when internal arc occurs and they stand in front
of switchgear
• Single phase insulation
• Safe/high operational reliability• Throughout lifetime of
MV switchgear
PREVENTING INTERNAL
ARCS BY DESIGN
• Prevention of internal arcs is crucial
• Metal- or insulation enclosed MV switchgear – achieves
stable power network and prevents internal arcs
• Compliance with IEC regulations / type tested /
endurance and aging tests
• Single pole solid insulation
LIMITING ARC DURATION
• Overcurrent protection
• Arc detection systems react on light or pressure
• Duration of arc can be limited – maximum 1 second
• Arc eliminators can short circuit internal arc very quickly
– milliseconds
• Arc detection systems minimizes energy during arc as
well as impact – BUT does not prevent an internal arc
ARC ABSORBERS
• Alleviate peak pressure rise on walls and ceilings,
excessive temperature and gas release
• Smart layout of chimneys – with integrated arc absorbers
– made up of ceramic honeycomb blocks drastically
decrease over-pressure in room
• If no external pressure relief option is viable or foreseen,
arc absorber included in design
• Cost savings
• Reduced amount of exhaust and temperature of gasses
• Extra safety for personnel
ARC RESISTANT SWITCHGEAR
• Robust systems can predict pressure rise and burning
behavior – based on current, geometry, metal used
• Mechanisms determine the ‘arc burning place’
– At the moment of arcing
– Triggered by arc pressure itself
– Via hinge construction
• Arc stays at fixed spot
• Low arcing energy
• Reduced effect of arcing
BEST OF BOTH WORLDS
• Single pole insulation
Arc resistant enclosure
• Air insulated Ring Main Unit (RMU)
single phase solid insulation in compact design
• Integrated arc absorbers
• Maximum safety• Protection from internal arcs
Figure: Compact, air insulated RMU, with
single phase solid insulation plus arc proof
metal enclosure.
INSULATION MEDIUMS FOR
MV SWITCHGEAR• Solid insulation
– Polycarbonate and Thermoplastic Elastomer (TPE)
– Cast resin technology (epoxy resin)
– Single Pole Design
• Vacuum technology – SF6 FREE
– Enclosed in metal-clad structure
– Sealed-for-life system
– Less moving parts / Maintenance free
– No additional end-of-life costs for disposal
– No threat to human life and environment on decommissioning
CONCLUSIONS (1)
• Human safety most critical aspect
• Stable and safe energy supply crucial for economy
• Prevention is better than a cure or control
• Double prevention philosophy
– Prevent internal arc and
– Provide maximum safety for operator and
– Minimize damage to switchgear and containing room
CONCLUSIONS (2)
• Specific internal arcs tests have been performed even beyond IEC standards
• Personnel safety one of the most challenging design
elements
• Users deserve equipment that have been rigorously
tested and carries IEC 62271 documentation
• The ideal state of “zero risk” can be achieved by
specifying the correct equipment
