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Second International Symposium on Standardsfor

Ultra High Voltage Transmission

1200kV Transmission System and Status of Development of SubstationEquipment/ Transmission Line Material in India

R.N.Nayak M.C.Bhatnagar B.N.De.Bhowmick R.K.Tyagi

Power Grid Corporation of India Limited, India

SUMMARY

The Indian Power Sector is growing at an accelerated pace. Peak demand is expected to be more than157,000 MW by 2012 which necessitates installed capacity of about 210,000 MW. Further, to meet thegrowth rate of Indian economy, installed capacity of power generation shall be about 600,000 MW by2025. To keep pace with generation, transmission system needs to be strengthened for power transfer fromgenerating stations to the load centres. Considering the serious difficulty of availability of Right of Way(ROW), it is considered prudent to adopt 1200kV AC as next transmission voltage level in the country fortransfer of bulk power. To develop 1200kV AC technology indigenously, a 1200kV test station is beingestablished at Bina, Madhya Pradesh in central part of India in association with 25 Indian manufacturers.In this project, POWERGRID shall provide the platform such as AC network connection, transmissionlines, testing facilities and other infrastructures at Bina substations and Equipment Manufactures shallinstall their 1200kV Equipments for field testing and trial. Based on the field testing and experiencegained on the performance of 1200kV equipments, technical parameters shall be fine tuned for 1200kVAC transmission system. The test station shall consist of 2 nos. bays of 1200kV, 2 sets of 1200kVtransformers (1000MVA each Bank) and one single circuit & one double circuit 1200kV TransmissionLines (about 1 km each).

In this paper, the details of establishment of test station are discussed and various studies conducted foroptimization of air clearances, corona, etc. are also described. Insulation Co-ordination was the mostcritical for designing the 1200kV System from the point of view of lightning impulse withstand voltageand switching impulse withstand voltage levels. The necessary protective margins as per IEC-60071-1&2have been maintained by special design of four (4) column surge arrester for meeting high dischargecapability of 55MJ. The rated voltage of surge arrester is 850kV and Lightning Impulse Protective Level(LIPL) and Switching Impulse Protective Level (SIPL) as 1700kV (at 20kA) and 1500 kV (at 2 kA)respectively. The specifications and current status of development of various major 1200kV equipmentslike Transformers, Circuit Breakers, Surge arresters, Capacitive Voltage Transformer, Dis-connectors andTransmission lines are detailed in this paper.

KEYWORDS

1200 kV Test Station, Insulation Coordination, Surge Impedance Loading

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1.0 INTRODUCTION

In India, major primary energy resources, i.e., coal and hydro potential, are concentrated in few pockets.For example, coal deposit is confined to Eastern part, while hydro potential is located in North-eastern andupper parts of Northern Region. Therefore, to ensure optimal utilization of resources, it requiresdevelopment of large sized resource based generating stations and transfer of bulk power to the far-offload centres through long distance transmission network. Various issues like Right-of-Way (ROW) andsocial aspects etc. are posing major challenges for the development of efficient transmission system. Toaddress above aspects, it has been considered prudent to adopt power transmission at 1200 kV AC and±800 kV HVDC levels to have high intensity power transfer per metre of ROW. POWERGRID is alreadyoperating hybrid EHV AC and HVDC systems in their National Grid which has proved an effective toolfor efficient network operation and management.

Since limited experience is available in the world on 1200 kV AC, it was decided to establish a 1200 kVTest Station at Bina, Madhya Pradesh in the central part of India. The test station shall consist of 2 nos.bays of 1200 kV, 2 sets of 1200 kV transformers (1000 MVA each Bank) and one single circuit 1200kVTransmission Line & one double circuit 1200 kV Transmission Line ( about 1 km each). The experiencegained on various 1200kV equipments shall help in fine tuning the technical parameters for 1200 kVtransmission projects which are expected by 2013/14. The detailed field testing shall be taken up forverification of the performance of various equipments.

The Single Line Diagram of the Test Station is given in Figure-I.

Figure-I: Single Line Diagram of Bina Substation(The portion marked with red colour is 1200kV Test Station.)

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In order to have power transfer through 1200kV System, existing 400kV Satna-Bina-III feeder shall berouted through 1200kV feeders. The Sectional layout of the Test Station is given as Figure-II.

Figure-II: Sectional view of 1200kV Test Bay

2.0 1200kV AC TRANSMISSION LINE TECHNICAL PARAMETERS:

For evolving parameters of the 1200kV System, simulation studies have been carried out and the basicparameters are given below. However, detailed studies are continuing for fine tuning these parameters.The outcome of field tests to be conducted at test station at Bina shall also be considered.

The technical parameters being considered for 1200kV Transmission Lines are summarized in Table-1below:

Table-1

Sr.No. Parameters Value

1 Nominal Voltage 1150 kV2 Highest voltage 1200 kV3 Resistance 4.338 x10-7 p.u./km4 Reactance 1.772 x10-5 p.u./km5 Susceptance 6.447 x10-2 p.u./km6 Surge Impedance Loading (SIL) 6030 MW7 Surge Impedance 239 Ohm

Base kV :1200kV; Base MVA :100 MVA

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From the above Table-1, it may be observed that SIL of 1200kV AC system is about 6030MW which isabout 2½ times SIL of 765kV system and 11½ times SIL of 400kV system. With development of 1200kVTransmission network, the ROW can be optimized. The comparison of SIL, ROW and MW intensity permeter of ROW for 400kV, 765kV & 1200kV and HVDC Transmission Lines is given in Table-2.

Table-2

400kV AC 765kV AC ±500kV HVDC

1200 kV AC ±800kV HVDC

ROW (m) 46 6452

92 70

Capacity (MW) 1000 2300-29002000-2500

6000-80006000-6400

Power carryingIntensityMW/m of ROW

22 4548

87 90

3.0 INSULATION CO-ORDINATION OF 1200kV AC SYSTEM

3.1 Switching Over Voltage

Common practice in long EHV system for controlling switching over voltages during any switchingoperation is to equip circuit breaker with pre-insertion resistor (PIR) which comes into the circuit initiallyfor about 8-10ms. Studies have been carried out on a 450km long line with various source strength anddifferent value of PIR (10 ms insertion time) without effect of surge arrester and results are tabulated inTable-3 below.

Table-3

1 p.u. = 978 kV (1200*1.41/1.732)

Source Strength Switching Over Voltage (p.u)MVA W/O PIR 300Ω 600Ω 700Ω

10,000 2.27 1.51 1.71 1.7815,000 2.17 1.36 1.55 1.5820,000 2.07 1.35 1.50 1.52

Although generated switching over voltages are minimum with 300 ohms (PIR) but considering thermalstresses on resistor discs under severe phase opposition condition, it was envisaged to adopt 600 ohms PIRwith 10 ms insertion time and switching over voltage level as 1.71 p.u. (1672kV) without Surge Arrester.However, with Surge Arresters, switching over voltages were limited to about 1.53 p.u. (1500kV)

3.2 Temporary Over-voltages (TOV)

To determine the temporary over voltages, preliminary studies have been carried out on a 450km longtransmission line with single-line-to-ground fault followed by three phase interruption at far end only. Inthe studies different source strengths along with 60% reactive compensation were considered. The resultsare presented in Table-4 below:

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Table-4

1 p.u. = 978 kV (1200*1.41/1.732)

Source Strength Temporary over voltage( p.u)MVA Without Line Reactor With line reactor10,000 1.58 1.3315,000 1.48 1.3120,000 1.45 1.30

As brought out at Table-4 above, TOVs are upto 1.33 p.u. for 10,000 MVA short circuit level. Keeping inview these levels of TOVs, insulation co-ordination studies were conducted with 1.4 p.u. TOV. Sinceshort circuit level in actual condition shall be generally higher that the short circuits levels considered forstudies, therefore actual TOVs are unlikely to exceed 1.4 p.u. considered for insulation co-ordination.

3.3 Selection of Insulation Levels:

Insulation Co-ordination is very important for design of the 1200kV System from the point of view oflightning impulse withstand voltage level and switching impulse withstand voltage level for 1200kVequipments. To achieve necessary protective margins as per IEC-60071-1&2, in-depth studies wereconducted for Voltage-Current (V-I) characteristics of ZnO blocks. The V-I characteristics of the 850kVSurge Arresters as finalized for 1200 kV system are as given in Table-5 below:

Table-5

Sr.No. Surge Arrester Current (kA) Residual Voltage (kVp)1 0.5 13802 1.0 14403 2.0 15004 10.0 16005 20.0 1700

The salient features of Surge Arrester are as under:

a) Rated Voltage of 850kVrms shall take care power frequency voltage upto 1.15 p.u. for 10 seconds and1.4 p.u. for about 1second.

b) Switching Impulse Protective level at 2.0kA is 1500kV and hence about 20% protective margin areavailable for 1200kV Equipments with 1800kV Switching Impulse withstand voltage.

c) Lightning Impulse Protective Level at 20.0 kA is 1700kV and hence sufficient margins for bayequipments with BIL- 2400kV and for transformers with BIL-2250kV.

The location of Surge Arresters is very critical due to voltage rise with separation distance as well asjumper length. For adequate protection of Equipments against Lightening surges, it was decided to placeSurge Arresters at Line Entrance and near Transformers/ Reactors. In addition, Surge Arresters in the Busmay also be considered necessary.

For computing discharge capability of Surge arresters, single line to ground fault was considered for a450km long line. In single phase to ground fault, the healthy phase voltage shall temporarily increase andSurge Arresters shall be overstressed. The single phase tripping followed by three phase tripping afterdead time of about 1000 ms was considered as indicated in Figure-III.

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Steady State

T=0 ms

T=100 ms

T=120 ms

T=1120 ms T=1140 ms T=1220 ms T=1520 ms

Dead Time1000 ms

Stuck breaker

300 ms

SLG fault Opening of Opening of Closing Closing Opening of LBBlocal end remote end of local of remote CB due to OperatesCB of CB of faulted end CB end CB permanent and clearsfaulted phase through through fault the faultphase SPAR SPAR

Surge Arrester Energy accumulation

Figure-III

The discharge capability required for surge Arresters for above conditions is given below:

Total energy = 2xLong discharge (IEC Class-5) + TOV + Margins= 2x5MJ +35MJ + 10 MJ = 55 MJ

4.0 FIELD STUDIES ON 1200kV AC SYSTEM

Various field studies and tests to determine the configuration of 1200kV transmission lines have beencarried out. Field studies include corona cage studies, air gap insulation studies, tests for voltagedistribution on the insulator string and long term studies on test line for measurement of AN, RIV &voltage gradient measurement at ground level.

4.1 Corona Cage Studies

The test was carried out in a Corona cage at UHV test lab at Central Power Research Institute (CRPI),Hyderabad. The size of the cage was 6.1 m (H) x 6.1 m (W) x 21 m (L). The corona cage consists of threesections, insulated central section of length 18 m and two end sections each of length 1.5m, located oneither side of the central section. Eight Bersimis ACSR (Radius 17.5 mm) bundle conductor was strungbetween two towers such that the bundle was at the centre along the length of the cage. Both the ends ofthe bundle was connected to the end fittings, corona shields and insulator strings.

Suitable nozzles were provided, at the top of the cage, along the axis of the cage to simulate heavy rain.Three bundle configurations, with different sub conductor spacing of 350 mm, 450mm and 550 mm wereused for the measurements. The measurements were made under two precipitation rates viz 57 mm/hr and85 mm/hr. Measurements of Corona Inception Voltage, Corona loss, AN & RIV were made. Based onthese measurements, conductor surface voltage gradients (Ec), conductor surface factor (m) weredetermined. The results are tabulated in Table – 6.

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Table-6

Test condition

Coronaonset

Voltage(kV)

Voltagegradient,

Ec (kV/cm)

Surface factor (m)

Audible noise (db)

Corona loss(w/m)

RIV

350 mm,57 mm/hr

292.5 12.57 0.523 61.6 43.33 63.2

350 mm,85 mm/hr

282.85 12.16 0.506 62.9 28.89 63.2

350 mm,dry

327.95 14.10 0.587 - -

450 mm,57 mm/hr

259.38 11.67 0.485 58.3 17.77 66.2

450 mm,85 mm/hr

257.63 11.59 0.482 59.8 50.55 66.2

550 mm,57 mm/hr

282.13 13.54 0.558 59.5 28.89 65

550 mm,85 mm/hr

274.38 13.17 0.543 59.8 83.33 68.4

4.2 Air Gap clearance Studies

Air gap clearance optimisation studies for 1200kV system have been carried out at CPRI, Hyderabad.Tower top & Tower window have been simulated as shown in the figures (IV, V, VI) below:

Tower top Simulation Tower top Simulation Tower Window simulationFigure-IV Figure-V Figure-VI

Critical time to peak for which the Critical Flash Over (CFO) is between 220 to 230 microseconds fortower top simulation. It is in the range of 250 to 290 µs for which the flashover voltages were minimum.The Test results are given in Table-7

Table-7

GapDistance

(m)

CrestTime( Tp )(µs)

Temp. Drybulb( 0C )

Temp. Wetbulb( 0C )

AmbientPressure(mm. of

Hg.)

50% FlashoverVoltageU50%(kVp)

AmbientCorrection

Factor( Kt )

50%Flashover

Voltage(kVp)

8 160 22 16 718 1926 0.987 1951250 27 17 713 1879 0.94 1915270 27 17 713 1868 0.94 1906350 24 17 718 1854 0.982 1910

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GapDistance

(m)

CrestTime( Tp )(µs)

Temp.Drybulb( 0C )

Temp. Wet bulb( 0C )

AmbientPressure(mm. of

Hg.)

50% FlashoverVoltage U50%

(kVp)

AmbientCorrection

Factor( Kt )

50%Flashover

Voltage(kVp)

115 29 19 716 2191 0.98 2236250 25 19 716 1950 0.992 1963270 25 19 716 1937 0.992 1953350 29 20 717 1948 0.988 1961

For window clearance of 8m the CFO voltage obtained was 1906 kVp. In order to ascertain the positivepolarity switching impulse withstand voltage of 1800 kVp, 15 positive polarity impulses corrected forambient atmospheric conditions were applied. No flashovers were observed. Based on detailed testing,optimum air gap clearance was selected as 8.3 m.

5.0 TECHNICAL PARAMETERS OF 1200kV EQUIPMENTS

Based on the discussions with the manufacturers, various parameters as worked out for individualequipment are as under:

5.1 Basic Technical Parameters:

Sr.No. Parameters ValueRemarks

1 Rated Voltage 1200 kV2 Nominal Voltage 1150 kV3 Rated Frequency 50 Hz4 Fault Current 50 kA

Lightning Impulse Voltage2400 kV Switchyard equipments2250 kV Transformers & Reactors

2550 kVTransformer & ReactorBushings

6.Switching Impulse VoltageLevel

1800 kVSwitchyard Equipments andTransformers

1950 kV Bushings

7.One Minute Power FrequencyVoltage

1200 kV

5.2 Circuit Breaker

Dead Tank type 1200kV Circuit Breakers with Current Transformers in Bushing turret have been envisaged.

Sr.No. Parameter Value Remarks

1 Close Time 150 ms (max)2 Break Time 50 ms (max) To take care higher

asymmetrical faultcurrent

3 D.C. Time constant 100ms4 Rated Current 5000A

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Sr.No. Parameter Value Remarks

5 Operating Duty O-0.3s-CO-3min-CO

6 Rated Peak withstand current 2.7 times shortcircuit current

7 LIV + Bus Voltage 2400kV + 686kVp8 SIV + Bus Voltage 1675kV +980kVp9 1 min Power (Freq Ph-Earth) 1200 kVrms10 Open CB across the Breaks 1400 kVrms11 First Pole to Clear Factor 1.212 Zero SF6 gauge pressure

withstand voltage693kV

13 Capacitive SwitchingCurrent

1300Amp

14 Reactive Switching Current 330Amp15 Pre-insertion Resistor( PIR)

of 600 ohms with 10 msinsertion time

1.71 p.u. switchingover voltages

Transient Recovery Voltages for different test duties shall be adopted as per the recommendations ofCIGRE working group A3-08(WG22).

5.3 Current Transformer

1200kV Current Transformers shall be Turret mounted with 1200kV Bushings of Dead tank type CBs. The Technical parameters are given as below:

Sr.No. Parameter Value

1 Rate Voltage 1200kV

2 Rated Current 6000A

3 No. of Secondary Cores 6

4 Class of Protection Cores TPY

5 KPV 4000 V

Core Details

CoreNo.

Application RatioOutput burden(VA)

Accuracy class

1 Bus Diff. Check 6000/ 3000/ 1000/1 TPY

2 Bus Diff. main 6000/ 3000/ 1000/1 TPY

3 Metering 6000/ 3000/ 1000/1 20/20/20 0.2S

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CoreNo. Application Ratio

Output burden(VA) Accuracy class

4Trans. BackupLine Prot. 6000/ 3000/ 1000/1 TPY

5Trans. Diff.Line Prot

6000/ 3000/ 1000/1 TPY

5.4 Capacitive Voltage Transformer

1200kV Voltage Transformer uses capacitive voltage dividers and step down voltage transformer and its technical parameters are as given below:

Sr.No.

Parameter Value

1 Voltage Ratio 1150/√3 kV/110 /√3 V

2 Burden 25 VA (for each Core)

3 Simultaneous burden 50VA

3 Accuracy Class0.2 (metering) /3P (protection)

4 Capacitance 2000pF

5 No of Cores 3

5.5 Surge Arrester

The Surge Arrester is four (4) column ZnO type and its technical parameters are as given below:

Sr.No. Parameter Value

1 SA Class 52 Rated Voltage 850 kVrms3 Continuous Operating

Voltage(COV)723 kVrms

4 Nominal discharge current 20 kA

5 Lightning Impulse ProtectiveLevel(LIPL)

1700 kVp at 20 kA

6 Switching Impulse ProtectiveLevel(SIPL)

1500 kVp at 2 kA

7 Energy level 55 MJ

8 ZnO disc diameter 136 mm/ 125mm

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5.6 Transformer

The Technical parameters are given as under:

Sr.No. Parameter Value

1 Rated VoltageHV/IV/LV

1150/√3/400/√3/33 kV

2 Rated PowerHV/IV/LV

333/333/111 MVA

3 No of Phase Single Phase

4 Connection(three phase)

YNaod11

5 Impedance HV-IV 18%HV-LV 40%, IV-LV 20%

6 Cooling Type OFAF or ODAF

7 Cooling Equipment 4 X 33.3% UNIT OFAF COOLER

8 Temp. Rise(Amb. 50 deg C)

Winding 45 deg C

Oil 40 deg C9 Insulation Level BIL SIL PD<100pC

(i) 1150 kV Winding 2250kVp 1800 kVp 1.5Um/√3

(ii) 400 kV 1300 kVp 1050 kVp

(iii) 33 kV 325 kVp

(iv) Neutral 95 kVp

(v) 1200 kV Bushing 2550 kVp 1950 kVp PD <10 pC

(vi) 420 kV 1425 kVp 1050 kVp

(vii) 72.5 kV for LV 325 kVp

(viii) 52 kV Neutral 95 kVp

5.7 ISOLATOR (Knee type/ vertical break / HDB)

The Technical parameters are as given below:

Sr.No. Parameter Value

1 Rated Voltage 1200 kV

2 Rated Current 6000A

3. Support Insulators Tripod arrangement with 15kN cantileverstrength (each having 6kN ). Minimumheight of Insulators is 9 m

4. Isolating distance of the openIsolator (min)

10 m

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5.8 Bus Post Insulators

The Technical parameters are as given below:

Sr.No. Parameter Value

1 Configuration Tripod arrangement

2. Cantilever strength• for tripod arrangement• Single BPI

15kN6kN

3 Minimum Insulator Height 9 m

4. PCD of Individual BPI 225 mm

5.9 Disc Insulators & Hardware

The Technical parameters are as given below:

Sr. No. Parameter Value

1 Type of insulators Standard Disc

2 Size of insulator units 380 mm X 205 mm

3 Creepage distance of individualinsulator units

525 mm

4 Ball & Socket Designation 28 mm

5 Electro mechanical strength 420/ 320 kN

5.10 Insulator Strings

Sr. No. Parameter Value1 Maximum RIV level 2,500 µ V

2 Minimum total creepage distance ofthe insulator string

30,000 mm

3 Total no. of discs per strings 60

5.11 Polymer Long Rod Insulators Strings

Sr. No. Parameter Value1 Minimum total creepage distance of

the insulator string30,000 mm

2 Total Insulator length/air insulationdistance minimum

8,300 mm

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Sr. No. Parameter Value3 No. of Insulator Rods for 1200kV Two or Three4 Electro mechanical strength 420/320 kN5 Ball & Socket Designation 28 mm6 Insulator Hardware Shall comply to

StandardIS 2486

6.0 THE STATUS OF DEVELOPMENT OF 1200KV EQUIPMENTS

All equipment technical parameters have been finalized in association with respective equipmentmanufacturers. Prototypes of 1200kV Capacitive Voltage Transformer and Dis-connectors are alreadydeveloped. Design of 850kV, 4 column Surge Arresters has been finalized & components are undermanufacturing and prototypes shall be ready by January, 2009. 1200kV transmission line towers havebeen designed and line materials are under manufacturing. The terminal connectors, spacers, vibrationdampers and other accessories have already been developed.

The design of 1200/400kV transformers is under vetting by consultants/ experts. The Bushings are undermanufacturing whereas Transformer manufacturing shall take place shortly after vetting of the design byconsultants.

7.0 WAY FORWARD TOWARDS DEVELOPMENT OF 1200kV TRANSMISSION TECHNOLOGY IN INDIA

The detailed field testing shall be taken up for the performance of various equipments by conductingvarious tests. The Testing Instrument manufacturers are also participating in this project for conditionmonitoring of various equipments. The experience gained on various 1200kV equipments shall help infine tuning the Technical parameters for 1200kV AC transmission system which is expected by 2013/14.

8.0 ACKNOWLEDGEMENT

We are thankful to all the Indian Manufacturers for their support and help for finalization of the technicalparameters and development of 1200kV equipments for the 1200kV test station at Bina. Authors arethankful to the Management of Power Grid Corporation of India Limited for publication of this paper.The views expressed in this paper, are not necessarily the views of the POWERGRID management.