b3_113_2012 (1).pdf
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DESIGN CHALLENGES IN THE 765kV AIR INSULATED
SUBSTATION
I S Jha Oommen Chandy* R.K.Sarkar A.B.Singh
Power Grid Corporation of India India Limited New Delhi, INDIA
Summary
POWERGRID commissioned its first 765/400/220kV Air Insulated Substation (AIS) in India at Seoniin 2007. The objective of Power Grid Corporation of India Ltd (POWERGRID) is to create a strong
and vibrant National Grid in the country. Establishment of the ultimate National Grid is on fast trackand a number of projects are under implementation in this direction. For bulk power transmission
across regions in the country, 765 kV and 1200kV transmission lines have been planned alongwith±600/±800kV DC transmission lines. In order to handle bulk power of the order of 6000MW, techno-economic design of 765kV substation by careful selection of switchgears and substation materials not
only w.r.t reliability but also w.r.t electrical fields, corona,RIV, magnetic field and other aspects for itssmooth operation & maintenance assumes significance in this regard. This paper describes various
challenges in the techno-economic design of UHV AIS substation and the improvements made basedon experience / feedback gained from Seoni substation
Keyword
765kV Air Insulated substation substation-Salient Design- Experience-Failure-Future challanges
1. Introduction
There has been phenomenal expansion in the Power Sector since Indian Independence. Afterconstructing a large network of 400 kV transmission system some 25 years back, activities started inIndia for identification of next higher voltage level transmission system of 765kV for long-distance bulk power flow requirement in the future. The first 765kV Sipat-Seoni line was commissioned in2007 in western part of India and the first 765kV Seoni substation in India is in operation for morethan four years. More than 30 AIS substations of 765kV are in different stages of construction whichare targeted to be commissioned within next one and half years as shown in figure.1.
21, rue d’Artois, F-75008 PARIS B3-113 CIGRE 2012 http : //www.cigre.org
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Figure 1. 765kV Transmission Line network
2. Salient Design Considerations
2.1 Switching Scheme: A typical 765kV substation consist of 2 nos. 765kV Line bays alongwith 3nos. 765/400kV, 1500MVA Autotransformer (3 nos. of 500 MVA single phase unit with oneunit as a spare) bays, 2 nos.765kV , 240 MVAR Line Reactor ( 3 nos. of 80 MVAR single phaseunit)and 1 no. 765kV,240 MVAR Bus Reactor (3 nos. of 80 MVAR single phase unit ) bay.For
765kV substation, breaker and half switching scheme has been selected as shown in Figure.2
Figure 2. Single Line Diagram of 765kV substation
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2.2 System Parameters
Following major system parameters have been for 765kV substation as mentioned in table-I
Table-I : System Parameter for 765kV substation
Description of Parameter 765kV system
System operating vaoltage 765kV
Maximum operating voltage of the system (rms) 800kV
Lightning impulse withstand Voltage(1.2/50 micro sec) 2100kVp
Switching impulse withstand voltage(250/2500 micro sec) 1550kVp
One minute power frequency dry withstand voltage (rms) 830kV
Corona Extinction Voltage (rms) 508kV
Minimum creepage distance (25mm/kV) 20000 mm
Based on the above system parameters following electrical clearances has been considered for765kV substation as mentioned in table-II.
Table-II : Air-gap / Live-metal clearance
Phase to Phase 7600mm (for conductor- conductor configuration)
9400mm (for rod- conductor configuration)Phase to Earth 4900mm (for conductor-structure)
6400mm (for rod-structure)
Sectional Clearances 10300 mm
In 765kV substation following spacing have been adopted considering above clearances andother design aspects like swing of conductors etc . as mentioned in table-III
Table-III : Electrical spacing Phase to Phase Spacing 15 meters
Phase to Earth Spacing 7.5 meters
Sectional Clearances 10.3 meters
2.3 Selection of Conductor
A tubular bus of 120mm tube has been used for the first bus level to eliminate the tensions produced on the equipment terminal by short circuit induced pinch effect in a bundledconductor. For second and third level, Quad Bull All Aluminum Conductor (AAC) of size38.25 mm diameter with 450mm sub conductor spacing was selected based on Electric fieldand magnetic field consideration to restrict electric field of 10kV / meter and magnetic field of500 micro Tesla at 1.8 meter above ground level. For this purpose at some locations, whereall the three levels (i.e. equipment level, Bus level & line take off level) are crossing, theshield wire was provided at the height of equipment support structures as shown in Figure-3.
Figure 3. Shield Wire at the equipment support structure
The cases studied and the values of electric field and magnetic field obtained for differentconfiguration to select conductor configuration is shown in Table –IV and Figure 4
Shield wire
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Table IV: Electrical and Magnetic field calculation
Case
Study No.
Levels
considered instudy
Level 1
at height
of 14
meters
Tubular busbar
diameter(mm)
Levels 2 at height
of 27 meter and
Level 3 at height
of 39 meter
Quad Bull AACconductor with
bundle spacing(mm)
Support
structurelevel shield
wire atheight of8meters
Maximum
electricfield at
height of1.8 meters(kV/m)
Maximum
magneticfield at
height of1.8 meters(µT)
120 450
1 Level 1&3 × × - 9.6 57
2 Level 1,2& 3 × × - 11.4 76.3
3 Level 1,2, 3 &
shield wire
× × × 9.2 76.3
Figure 4. Electrical field at height of 1.8 meters
Apart from the selection criteria of normal current carrying capability, short circuit capability, bundle configuration etc., case studies have been carried out for corona extinction voltage to keepminimum corona extinction voltage of 508kV rms phase to ground and maximum voltage gradientof 21.1kVrms/cm. The result of maximum gradient for different case studies are indicated in table-V
Table V: Case study for Voltage GradientStudyCase No.
Maximum gradient per phase (kVrms/cm)
Level 1
(Tubular busbar of
120mm diameter)
Level 2
(Quad AAC Bull
conductor of diameter38.25mm with450mm sub-conductorspacing)
Level 3
(Quad AAC Bull
conductor of diameter38.25 mm with 450mmsub-conductor spacing)
Shield Wire(diameter
3.74mm)
L1R L1B L1Y L2R L2B L2Y L3R L3B L3Y 1 2
1 13.8 14.4 13.8 - - - - - - - -
2 13.2 14.2 13.2 - - - 15.6 17.3 15.6 - -
3 12.0 13.3 12.0 12.6 13.4 12.6 14.1 16.3 14.1 - -4 12.4 13.3 12.4 13.3 15.1 13.3 14.1 16.3 14.1 17.3 17.3
0 10 20 30 400
2000
4000
6000
8000
Distance from origin (m)
E l e c t r i c F i e l d M a g n i t u d e ( V o l t s / m )
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2.4 Selection of levels
Based on the results from table-IV and table-V following level have been selected for 765kVsubstation
Level 1: Equipment level – 14.0 meterLevel 2: Bus level - 27 meterLevel 3: Line take off level - 39 meterShield wire - 45 meter
Figure 5 : Different levels of substation
2.5 Tower Configuration
To economize the structurefoundations, Pie(π) type tower
configuration for 765kV switchyardhas been considered with 30 meter
beam witdh & phase to phase spacingof 15 meter .Further PolymerInsulator strings have been consideredto reduce the weight on structure.
Figure6. Pie (π) type tower
3. Challenges faced in 765kV AIS substation
3.1 Transformer and Reactors switching arrangement through Disconnectors Single phase transformers and Reactors have been used with the provision of spare unit oftransformer & reactors, The spare phase transformer and reactor has been arranged in such amanner that spare unit can be replaced with any of the units without physically shifting ofspare unit of transformer & reactor. In case of use of spare phase unit of transformer orreactor, phase opposition conditions arises across the disconnector points connected withauxiliary buses as shown in figure-7 Due to which minimum air clerances of 9400mm between isolator terminals are required. Since isolating distance between disconnector
terminals of disconnector is less than 7200mm, additional disconnector in series has been
Bus
level(Quad
Bull Cond.)
Take off
level (Quad
bull cond.)
Equipmentlevel(120mmTube)
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provided to take care phase opposition condition. Therefore there is a need of designing765kV disconnectors meeting phase opposition requirement.
Figure 7. Transformer switching through Isolator
3.2 Compatibility of control switching devices of different makes
Control switching has been used in 765kV Circuit Breaker for control switching oftransformer and Reactors as shown in figure 8. In case of installation of additionaltransformer bank and Reactor bank in future substation extension , control switching deviceof different make of circuit breaker may or may not be compatible with each other.Therefore there should be a common guideline for circuit Breaker manufacturers to design
their control switching device compatible to each other.
Figure 8. Control switching device of different make
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4.0 Design challenges from operational experience of 765kV substation
4.1 Failure of composite insulator In Seoni substation, 765kV polymer insulatorstrings had failed after commissioning of thesubstation. The cause and mitigation measurestaken w.r.t. the corona damage occurred in the765kV polymer insulator string in Seoni has beenan eye opener in the area of corona andelectomagnetic field control in UHV substations.Simulation studies for field distribution had beencarried out to address this problem. On analysisof voltage distribution it was observed thatcorona strike from the metal parts of tensionfitting and corona shield resulted in gradual wearand tear of composite insulator. All insulatorreplaced with providing additional corona ring at
the bottom as shown in Figure-9(c).
Figure 9(b) Corona observed on insulator Figure 9(c) Modified insulator with corona ring
4.2 Failure of Circuit Breaker during type testing
Considering the voltage profile, the voltage factor for capacitive current switching duty of 765 kV CBwas specified as 1.4 (against the IEC requirement of 1.2. ). The circuit breaker was earlier type testedfor C2 duty with 1.2 voltage factor. But the same circuit breaker failed to pass the capacitive currentswitching duty C2 test when tested with a voltage factor of 1.4. The test was repeated with higher SF6gas pressure and precise alignment of contacts, still the CB failed to clear this duty. Finally the nozzle
design was modified and the test was successfully completed.
4.3 Observation of Corona on equipment After commissioning of the 765kV Seoni substation , Corona was observed on certain locations predominantly on T point section jumper of Disconnector switches and Corona bell (as shown inFigure 10. To overcome this problem corona bells and corona rings on Disconnector switches were
modified. It is observed that corona ring of equipment should be designed based on the location ofequipment in the switchyard instead of adopting standard practice of providing Corona ring as a partof equipment.
Figure 9(a) Insulator failure
All insulator replaced with providing additional
ring at the bottom
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Figure 10 Corona problem on equipment terminal
Figure 11 connecting strip of SAFigure 12 Rupture disc of CT
4.4 Corona problem in 765kV Surge Arrestor During type testing of 765kV Surge Arrestor, Corona /RIV problem was observed. On analysis it wasfound that welding of strips connecting top ring & bottom ring from outside and sharp edge of thisstrip was causing Corona & RIV problem. With the shifting of connecting strip inside the rings, this
problem was overcome as shown in Figure 11 above.
4.5 Leakage of SF6 gas from 765kV Current Transformer After commissioning, SF6 gas leakage took place in few CTs due to the bursting of rupture disc . Afteranalysis it was concluded that rupture discs were damaged in transportation as shown in figure 12 and
13. As a precautionary measure, all rupture disc were replaced with modified version of rupture disc.
6. Bibliography i. EPRI Transmission Line Reference Book - 200 kV and above.ii. Hifreq software for electric and magnetic field calculation.
iii. POWERGRID Seoni site feedbackiv. POWERGRID Experience and Technical Specification
Corona
observed
strips
Figure 13 Black spot inside the housinganalysis was found as a graphite particle caused by the rupture disc
during bursting