Transcript Document

MV Distribution Switchgear
Technologies –
Developments and
Solutions for End-users
R A Kelly (IARC - Eskom)
M Ryan (City Power)
Synopsis
• Alternative solutions to oil-filled switchgear
used in MV distribution
– Eskom et al experience & rationale
– safety, environmental and economic
considerations
• Focus on secondary distribution switchgear –
i.e. RMUs
• Management of existing oil-filled switchgear
Introduction
• MV oil-filled switchgear widespread in EDI
• Generally reliable!
• Switchgear installed < 1970 – reached end
of its design life and may be unsafe
– unmainted switchgear
– over-stressed
– DMO
– modifications
– inadequately trained operators
• Failures can be castastrophic
Introduction
• “In general, oil-filled switchgear has a proven record of
reliability and performance. Failures are rare but, where
they occur, the results may be catastrophic. Tanks may
rupture, resulting in the ejection of burning oil and gas
clouds, causing death or serious injury to persons and
major damage to plant and buildings in the vicinity of the
failed equipment. Accident experience has shown that
failure usually occurs at, or shortly after, operation of the
equipment. Thus, the way switchgear is operated, its
condition and the circumstances existing in the system at
the time of operation, to a large extent, determines
whether the equipment will safely perform its duty.”
– Clause 5, HSE 483/27 – Oil-filled electrical distribution
and other switchgear’
Introduction
• Responsibility of users
– OHS Act No. 85 of 1993
• Acceptably safe environment
• Reasonable measures to mitigate against
possible dangers
• Developments in switchgear technologies have
presented users with a compelling argument
for the use of lower cost, safer and more
reliable equipment
• Insulating/interrupting medium of gas / vacuum
– ‘sealed for life’
History of MV S/G in SA EDI
• For many years – primary
insulation/interrupting medium was oil
• Historically - interval and event-based
maintenance schedule
• Alarming trend – inadequately maintained
• Maintenance simply not being scheduled
– pressure on maintenance budgets
– increasingly difficult to schedule onerous
outages required for maintenance
– ‘Run-to-failure’ philosophy
History of MV S/G in SA EDI
• Why maintain?
– moving parts, aging oil
– most OF S/G ‘free-breathing’
– ‘uncontrolled’ environment
– switchgear vs other oil filled equipment
• The result?
– Gradual deterioration in the insulating and
interrupting properties of oil
– Increased probability of mechanism failure
– Increased risk and number of failures with
associated injuries / fatalities
History of MV S/G in SA EDI
• Failures due to use of sub-standard oiltype MV HRC fuses used in RMUs
• Serious risk associated with inferior
quality, non type tested fuses which
entered the SA market
– unable to interrupt rated current (failed
at <25% rated current)
– inadequate oil-tight seals
History of MV S/G in SA EDI
• Other insulating mediums:
• ‘insulation-enclosed’ epoxy resin switchgear
– inadequate maintenance & training
– adverse environmental conditions
• Air-filled enclosures
– metal-clad switchgear, cable terminations
– not exempt from poor track record (*3.2)
– requires correct application of NRS 012
– 4 types of terminations & live conductors in
air, creepage distances (OD & ID)
– creepage still required indoors!
Alternative solutions
• Key issue: improved safety
– internal arc rating (IAC)
• What is IAC switchgear?
– ensures the safe venting of gases away
from operator and/or public
• IAC only possible with ‘dry-arcs’
– can be simulated in laboratory
• Simply not possible or practical for oil-filled
switchgear - uncontrollable
• At best OF S/G having air-filled internal arc
tested cable boxes may be available
Alternative solutions
Alternative solutions
• Arc energy = f (voltage, S/C current, time)
• Energy absorbed by person = f (arc energy, distance,
PPE)
Alternative solutions
• Inadequate pressure release mechanisms
Alternative solutions
• SF6 gas-insulated switchgear
– preferred gas for filling enclosures
– superior insulation and arc extinction properties
– odourless, non-toxic, chemically inert, nonflammable
– classified as a greenhouse gas
• management and handling
– under positive pressure
• <1% leakage – >30 years service
– toxic products – decomposition due to arcing
– not considered a problem – ‘sealed for life’
Safety Considerations
• Safety:
– achieved by reducing risk to a tolerable level
– risk = comb. (probability, severity) of a harm
• Tolerable risk:
– search for optimal balance between ‘absolute
safety’, demands to be met by product and
factors such as benefit to user, suitability for
purpose and cost
• Need to continually review ‘tolerable level of risk’
– when developments in technology and
knowledge can lead to economically feasible
improvements
Risk Reduction
• OHS Act has a general duty
clause – reasonable
precautions
• PPE – last line of defense!
• No local electrical safety
regulations relating to
internal arc
• NFPA 70E
– establish boundaries
(LRPF)
– hazard awareness
– appropriate PPE
• Arc flash analysis
Risk Reduction
• IEC IAC provides a tested level of protection
– under normal operating conditions
– not under maintenance conditions
– not concerned with service continuity
• ‘Supplementary’ measures
– arc detection, CLDs, arc suppressors,
pressure relief devices, remote control,
motorised racking devices, transfer of
withdrawable parts with front doors closed
– requires a co-ordinated approach
Risk Reduction
• Existing switchgear where elevated risk still
exist:
– May have to implement restrictive operating
procedures
– QoS and COUE vs
– cost of implementing oil-filled switchgear
management programmes to mitigate
against potential dangers of older oil-filled
switchgear
– both are costly
Specifications for new S/G
• 1998 – internal arc testing of primary S/G
• 2002 – specification of IAC secondary
switchgear (RMUs)
• Operator and public safety under the spotlight
– recent catastrophic failures of S/G
– elevating arc flash safety to new levels
• Involved in development and re testing of all
MV S/G using SANS/IEC 62271-200 and
SANS/IEC 61330
– IAC philosophy revisted
Specifications for new S/G
• Primary switchgear:
– AR-BFL (SANS 62271-200)
– 25kA 0,2 s (12kV & 24kV)
– arc detection system
– venting upwards, room dimensions,
pressure relief
• Secondary switchgear:
– AF-BFLR (SANS 61330)
– 20kA 0,5 s (12kV) 16kA 0,5 s (24kV)
– venting upwards – 2m duct
Specifications for new S/G
Economic Considerations
• Any sound engineering solution
– total cost of ownership
• ‘life-cycle costing’
– Initial capital cost (cost of acquisition)
– Future upgrade/replacement costs
– Cost of technical losses
– Cost of unserved energy
– Maintenance costs (normally periodic)
– Dismantling / residual costs
– Liability costs
– Operating costs
Economic Considerations
• Cost of unserved energy
1
Customer Class
Industrial/Mining
Commercial
Agricultural/Rural
Residential
Traction
2
R/kWh
20.47
15.95
2.80
2.38
1.26
Economic Considerations
• Eskom LCC comparison in 2002, 2004
between
– oil-filled S/G requiring routine maintenance
every 3 years (min)
– SF6 gas-insulated S/G – ‘maintenance
free’ (not requiring a prolonged outage)
• Alternatives analysed for acquisition, use
and disposal phases
LifeCycle
Costs
(Rands
)
Economic Considerations
140,000.00
120,000.00
Others
Unserved Energy
Maintenance
100,000.00
Acquisition
80,000.00
60,000.00
40,000.00
20,000.00
0.00
OIL RMU
SF6 RMU
Economic Considerations
• Key differences:
– purchase price (cost of acquisition)
– ‘maintenance free’ SF6 equipment
• SF6 enclosure – ‘non accessible’
• every 12 years, 4 hours
• visual inspections, operating mechanism
maintenance, cleaning
– Other factors:
• transport, storage, replenishment of oil
• commissioning costs for oil-filled S/G
• risk of contamination
Management of oil-filled S/G
• Recommendations based on HSE 483/27 (Oilfilled electrical distribution and other S/G)
• Identification & classification of all S/G
– Inventory
• equipment information
• fault levels, modifications, DMO
• Over-stressed, DMO, un-maintained
switchgear
– live operation & access restricted
– if possible, reduce fault levels
– prioritise replacement
Management of oil-filled S/G
• other switchgear
– proper maintenance
– oil-level indicators / checks for oil-leaks
• Maintenance
• Phased replacement program
• HRC fuses:
– installation, replacement (SANS/IEC
62271-105)
– quality (SANS/IEC 60282)
– if necessary, prevent live operation
Conclusions
• Ever increasing focus on human safety, service
delivery and cost reduction!
• Eskom and other major utilities have responded
to the changing risk profile in the light of
developments in both technology and knowledge
• Sought solutions offering not only a reduction in
risk but also:
– reduced total cost of ownership
• Air and/or gas-insulated S/G utilising vacuum
and/or SF6 interrupting technologies provide
users with equipment that meets required
specifications and performance levels