Kenyas experience in switchgear and controlgear

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Kenyas experience in switchgear and controlgear

• A Presentation at IEC Affiliate members workshop
  in Cape Town on 2005-10-20

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• Presenter Joel M. Kiilu
• Paper prepared by
• Joel M. Kiilu
• Dr. Humphrey Ndwiga
• Francis Ouma
• Cyrus Khalusi

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CONTENT

• 1. INTRODUCTION
• 2. CHALLENGES OF SWITCHGEAR AND CONTROLGEAR
• SERVICE CONDITIONS
• 3. PROCUREMENT OF SWITCHGEAR AND
• CONTROLGEAR THROUGH OPEN TENDERS.

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1.0 INTRODUCTION

• Kenya became an associate member of IEC in April
  this year.
• The Kenya National Committee of IEC launched in
  August, 2005.
• Presentation to share Kenyas experience in
  switchgear and controlgear operations and
  specifications made in an attempt to meet the
  challenging service conditions.

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1.1 Usage and manufacture of switchgear

• Firms in Kenya that use high and medium voltage
• switchgear and controlgear
• Those that generate electricity ( KenGen and
  IPPs)
• Electricity Transmission and Distribution
  company(KPLC)
• Industries
• No high or medium voltage switchgear and
• controlgear is manufactured in Kenya.

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1.2 Existing grid voltages

• Existing power systems in Kenya
• - transmission system 220KV and 132KV.
• - immediate future plans, a 330kv system inter
• connecting Kenya and Tanzania.   
• - distribution grid - 66KV, 33KV and 11KV.
• - Customer own systems - 6.6KV, 3.3KV
• systems. 
• - low voltage system comprises 415volts
• 3phase and 240volts -single phase.

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1.3 Type of switchgear and controlgear installed

• This grouping is in terms of insulation and arc
• quenching medium employed
• High voltage switchgear - SF6 and mineral oil
• Medium voltage - SF6 Vacuum and mineral oil
• Low voltage - Air 
• The load-break switches air
• High voltage bulk oil and minimum volume oil
• outdoor circuit breakers are being phased out.
• Use of SF6 indoor breakers is being discontinued
  in
• preference to vacuum breakers.(SF6 gas is
  environmentally
• unfriendly)

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Composition of the different types of switchgear
in Nairobi City and its Environs
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1.4 Experience on the performance based on
arc-quenching medium. 
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(No Transcript)
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2. CHALLENGES OF SWITCHGEAR AND CONTROLGEAR
SERVICE CONDITIONS

• Kenya has unique conditions which present
  varying challenges while drafting switchgear and
  controlgear specifications.
• Service conditions
• Salinity, humidity and pollution
• Service altitude 0 - 3000 metres above sea
  level. 
• Climatic conditions desert-savannah- tropical
• forests.
• Environmental conditions
• limited resources for system reinforcement.
• high electricty demand
• Stringent procurement rules

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 2.1 Salinity, humidity and pollution

• Saline conditions in the coastal region of Kenya
  form deposits of
• salt on equipment bushings/insulators.
• Dust from cement factories settle on switchgear
  insulators.
• -leading to tracking currents on the
• surface of insulators.
• Humidity and salinity cause corrosion of exposed
• metal parts resulting to
• -mal-operation of air break switches
• -shortening of equipment life.
• -equipment failure
• Geothermal plants ( in the Rift Valley) too emit
  hydrogen sulphide
• that is corrosive.
•  

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Picture No. 1 Corroded 33kV isolator at Kipevu
Power Station in Mombasa- Coast Region
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Picture No.2 Rusted tripping springs of a 132KV
single pole breaker at Rabai (Coastal region) 
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Specification requirements

• a)In order to reduce the problems of tracking  
• The creepage of switchgear bushings is 31mm/KV
• according to IEC 60815 1.
• For the less polluted environment (inland) the
  creepage is
• 25mm/KV.
• Where possible the bushings of the switchgear is
• silicone based (hydrophobic).
• b)In order to reduce the problems of corrosion
• All the external ferrous parts of the switchgear
  is
• painted to 80 microns or heavy duty galvanized.
• Where possible metallic parts should be replaced
  with materials
• that do not react with salt.
• Provision of heaters with temperature and
  hygrostat controls in control panels.
• Switchgear should meet the specified service
  condition.
•  
•  

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2.2            Altitude and lightning 

• Electricity reticulation covers areas whose
  altitude is 3000m above sea level (Kenya
  highlands). The thin air weakens insulation
  across bushings resulting to flash overs which
  usually crack the bushings.
• Incidentally these areas also experience high
  lightning strikes. The isokeraunic level is about
  180 thunderstorm days a year. The lightning
  strikes cause damage mostly to outdoor
  switchgear, surge diverters and transformers.
•  

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Picture No. 3 Flashover on poles of a 36kv
breaker installed in Eldoret .
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Mitigations on altitude and lightning

• To mitigate against the effects of altitude and
• lightning the following requirements have been
• added to the switchgear specifications 
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•   Increased use of lightning shield wires on the
  lines and
• masts in substations. 
• Use of arcing horns as a substitute protection
  when the
• surge arrestors get destroyed due to high
  lightining
• discharges.
• For given altitude derating of insulation is
  applied as
• per IEC 60694 3. Derating of switchgear
  voltage
• rating according to IEC 60071 2 as
  shown in the
• table below.

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Switchgear for altitudes above and below 1000m
above sea level.

•  

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2.2     Burning of switchgear terminations

• Usually caused by high resistance termination -
  arising from thermal cycles or galvanic
  corrosion.
• Heat also causes the cement sealing SF6 gas in
  circuit breakers to melt - allowing the gas to
  escape
• To reduce these occurances switchgear
  specifications require 
• Connection terminal silver or tin-coated
  should be
• scratch-proof.
• Air-break switch contacts to be silver plated
  at the
• connection points and the contacts to be
  spring loaded.

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Picture No. 4 burnt cable terminations of an
indoor switchgear in Nairobi
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2. 3     High solar radiation

• Effects of high solar radiation in some
  regions 
• Derating of load carrying capacity to avoid
  exceeding temperature rise limit.
• In cases where ethylene-propylene diene monomer
  (EPDM rubber) has been exposed to the atmosphere
  there are incidents of cracking due to the solar
  ( Ultra violet) radiation
• To reduce the effect of Utra violet radiation the
  following
• requirements have been added to the switchgear
  specifications 
•  
• Manufacturers equipment shall comply to this
  service
• condition
• Exposed EPDM rubber shall not to be used.

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2.4 System Constraints (Reinforcement) 

• Incidents during which switchgear explode while
  clearing faults.
• Cases where downstream faults are cleared by
  upstream switchgear instead of the associated
  breaker down stream.
• Attempts to break light load using a disconnector
  culminating into phase to phase fault and contact
  failures.
• mostly affected - switchgear rated at 25KA/3sec
  short circuit current and below that is installed
  where the system fault level is high.

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Probable causes 

• Switchgear short circuit current rating low
• Power system Transient recovery voltage higher
  than switchgear voltage rating
• Very many fault operations resulting to
  production of excess heat/ wear of switchgear
  arcing contact.
•  

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Considered solution

• Use of switchgear of higher voltage class than
  the system voltage to withstand higher TRV.
• Use of sectionalizers to reduce operations of
  source circuit breaker.
• Studies to determine the system Transient
  Recovery Voltage (TRV) characteristics.
• For example the specifications require
• interrupting capacity of 66KV switchgear to be
  31.5KA for 3 seconds instead of 25KA.
•  

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Picture No. 5 breaker damaged two transformer
panels, which are mounted 4 meters in front of
the feeder panel.
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Picture No. 6 Components of damaged circuit
breaker.
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Picture No. 7 Damaged compartment housing  
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• Example of 11KV failure
• Date of failure 13/06/2005
• Substation Nairobi West
• Name of the feeder South C
• Normal Loading conditions of the feeder 320A
• Total Number of CB operations at the
• time of failure (Fault/Controlled) 2790
• Load current prior to the failure 260A
• Fault current 12000A

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Circuit Breaker Details

• STANDARD IEC 56 O-0.3s CO-3MIN-CO
• (0-3MIN CO-3 MIN OCO 25KA)
• RATED VOLTAGE 17.5(12KV)
• INSULATION LEVEL 38/95KV
• RATED FREQUENCY 50-60 HZ
• RATED CURRENT 630 A
• BREAKING CAPACITY 20(25 KA)
• MAKING CAPACITY 50 (63) KA
• SF6 PRESSURE AT 20 C 4 BAR (0.4 MPA)
• SHORT TIME CURRENT 25 KA 3 SEC

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Observations

• The circuit breaker exploded while trying to
  clear a
• fault.
• Cause repeated switching on to a faulty cable.
• Explosion completely destroyed circuit breaker
  and
• damaged adjacent feeder compartments.
• Panel door shot- off like a bullet- damaging
  panels
• placed in front.  
• Most of the circuit breaker parts were expelled
  through the panel door.

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Way forward after this experience

• Specifications require manufacturers to
• state maximum number of circuit breaker
• operations on fault and controlled operations on
• normal load before overhaul. 
• state level of SF6 pressure safe for tripping  
• recommend safety measures to reduce risk to
• injury and damage to property
• provide evidence of the operation of arc vents.

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3.      PROCUREMENT OF SWITCHGEAR AND CONTROLGEAR
THROUGH OPEN TENDERS.

• Regulations apply to government owned/controlled
  corporations. These regulations require purchases
  of items of high cost to be done through open
  tenders.
• Consequences of this include
• involvement of all sorts of merchants, risking
  purchase of sub-standard or wrong items.
• operating a system with unnecessarily many
  different makes of switchgear
• extra cost incurred in conducting factory
  inspections
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Mitigating measures

• All switchgear tenders are accompanied by
  detailed
• specifications.
• Type tests reports are submitted with bids
• Tenderers are required to respond to each
  specific
• requirement of the tender specification.
• High and medium switchgear tenders are invited
  from manufacturers only who should be ISO
  certified.
• Factory inspection (new suppliers) to ascertain
  the
• quality management system in place.
• Factory acceptance tests of materials are
  conducted by
• purchasers engineers according as per IEC
• standards.
•  

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REFERENCES

• 1 IEC 60815 Guide for the selection of
  insulators in respect of polluted conditions. 
• 2 IEC 60071Insulation co-ordination 
• 3 IEC 60694 Common specifications for high
  voltage switchgear and control gear 
• 4 IEC 56 High voltage alternating current
  circuit breakers

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• THANK YOU