OPERATIONAL GUIDELINES FOR DETERMINATION OF TTC, ATC AND TRM FOR THE

SHORT-TERM HORIZON (0-3 MONTHS)

1. Ambient temperature adjusted TTC and ATC

i. According to the Manual on transmission planning criteria published by CEA in January

2013, the thermal loading limit for a transmission line for planning purpose is considered

taking an ambient temperature of 450C. It is stated in the Manual that the ambient

temperature in various parts of the country are different and vary considerably during

various seasons of the year, thereby permitting margins during operation.

ii. In Table II of Annexure –V of the Manual, the thermal loading limits (in Ampacity,

measured in Amperes) of transmission lines with various conductors is given with ambient

temperature of 400C, 450C, 480C and 500C. It can be seen that when ambient temperature

reduces from 450C to 400C, the Ampacity increases substantially. For example, for ACSR

Moose Conductor for a maximum conductor temperature of 850C, the Ampacity goes up

from 798 A to 874 A, a rise of 9.5%. This has been calculated on the basis of a formula

from IEEE Standards (IEEE Std 738-2006). Similarly, when the maximum temperature

increases beyond 450C, there would be a reduction in the Ampacity. The thermal rating of

the line for 480C and 500C as given in the CEA Manual on transmission planning criteria

may be used.

iii. CERC, vide Order No. L-7/139(159)/2008 dated 22.04.2013, has approved the

amendments to the Detailed Procedure for relieving congestion in real time operation

under Regulation 4 (2) of the CERC (Measures to relieve Congestion in real time

operation) Regulations, 2009 and attached the Detailed Procedure as Annexure to the

Order. In the Detailed Procedure, it is mentioned under para 3.6.9 as follows:-

“Permissible Normal and emergency limits for transmission elements shall be as

defined in the (CEA Manual on Transmission Planning Criteria).”

The CEA Manual on Transmission Planning Criteria mentions under para 5.2 (a) that the

thermal loading of a line is determined by design parameters based on ambient

temperature besides others. It is also stated that during planning, the ambient temperature

considered is normally 450C. Therefore, this permits margins of increased power flow

during operations. The relevant para is reproduced below:-

“The loading limit for a transmission line shall be its thermal loading limit. The thermal

loading limit of a line is determined by design parameters based on ambient

temperature, maximum permissible conductor temperature, wind speed, solar

radiation, absorption coefficient, emissivity coefficient etc. In India, all the above

factors and more particularly ambient temperature in various parts of the country are

different and vary considerably during various seasons of the year. However, during

planning, the ambient temperature and other factors are assumed to be fixed,

thereby permitting margins during operations. Generally, the ambient

temperature may be taken as 45 deg Celsius; however, in some areas like hilly areas

where ambient temperatures are less, the same may be taken”.

iv. In a demonstration Smart Grid Project approved by the Federal Electricity Regulatory

Commission (FERC), it is mentioned that with a drop in ambient temperature by 100C, the

Ampacity goes up by 11%, all other factors remaining the same.

v. Since the ambient temperature over the length of the line may vary somewhat, it is

proposed that, considering the higher of the maximum temperatures, as forecast by Indian

Metrological Department (IMD) for the next day, at the two ends of the line, the Ampacity

of the relevant line may be raised for every 20C temperature lower than the ambient of

450C, considered as base Ampacity. For example, if there is a 40C drop in ambient

temperature w.r.t 450C, the Amapcity may be raised by 4%. If there is a 100C drop in

ambient temperature w.r.t 450C, the Amapcity may be raised by 10%. The Ampacity and

MW loading (assuming nominal voltage and unity power factor) has been calculated

for normally used conductors, i.e. ACSR Moose ACSR Bersimis, Zebra and Panther, for

every 20C temperature drop below 400C and for every 20C temperature drop below 450C

between 450C and 400C, for maximum temperature of 750C and 850C, as applicable, and

is given in the form of Tables at Annexure. It is seen that there is also a site for

temperatures of all places in India, namely “Accuweather”, which predicts temperature for

the month ahead, week ahead, day ahead as well as temperatures during the day. These

temperatures are more or less in line with the IMD forecast.

vi. Initially, POSOCO may consider two periods for Ambient adjusted thermal rating of the

line, viz 8.00 AM to 8.00 PM, during which the maximum temperature during the day would

be considered and the other period from 8.00 PM to 8.00 AM the next day, during which

the lower ambient temperature would be considered. The lower temperature may be taken

as the mean of maximum and minimum forecast temperatures, or as considered judicious.

POSOCO may consider the Monthly Mean Maximum and Minimum Temperature of the

IMD based on 1901-2000 data, while calculating TTC, ATC and TRM on a three month

ahead basis, considering the nearest town to where the line is located, and add 50C to the

Monthly Mean Maximum and Minimum Temperature for safety. This data is available for

nearly 100 stations at IMD website at http://www.imd.gov.in/doc/climateimp.pdf. For the

month ahead basis, the Accuweather temperatures may be used for the exact site, adding

20C to the maximum and minimum temperatures for safety. This may be revised when

needed, i.e. due to outage of an element or otherwise, using the Accuweather forecast

after adding 20C to the maximum and minimum temperatures for safety. This would take

care of the difference in ambient temperatures between day and night and between

seasons.

vii. Before the increased thermal limit on account of fall in ambient temperature is used, the

System Operator may confirm from the owner of the terminal equipment on both sides of

the line if the terminal equipment is capable of carrying the increased current and also if

the relay settings are in accordance with the same. The terminal equipment owners will

confirm the same. The confirmation may be done once before the start of the season, i.e.

monsoon, winter, etc. The terminal equipment owners will, on their own, intimate the

System Operator when there is an addition to the terminal equipment and its rating for

change of TTC/ATC if required.

viii. In case relay settings restrict the transfer of power, it would be checked by the owner of

the terminal equipment if they are in line with the guidelines given by the Task Force on

Power System Analysis under contingencies. If they are not in line with the same, the relay

settings would have to be aligned with the same.

ix. The thermal rating of the line for 480C and 500C, as given in the CEA Manual on

transmission planning criteria may be used, in case of rise in ambient temperature.

x. The voltage and power factor may be considered for converting Ampacity to MW.

2. Consideration of Special Protection Schemes for the purpose of calculation of TTC

and ATC and Emergency loading

i. Special Protection Schemes (SPS) are Grid Protection Schemes which have been

installed at various points in the grid to ensure safe and secure operation, while at the

same time allowing loading of lines to the optimum level.

ii. It is seen that power flow fluctuates in the tie line depending on variation of load and

generation in a control area. A typical example of power flow in one of the circuits of

400 KV Kolar-Hosur D/C line, which is the limiting constraint between S1and S2

bidding areas, limited due to thermal loading limits, is attached at Appendix.

iii. Special Protection Schemes have been provided on the single circuits of Hosur- Salem

and Bangalore- Salem 400 KV lines, which are in series with 400 KV Kolar-Hosur lines,

in addition to N-1 reliability criteria. Each circuit is loaded to about 650 MW, so that in

case one circuit trips, the other circuit normally gets loaded to its thermal limit of 850

MW. In addition, a Special Protection Scheme has been provided to shed appropriate

load during tripping of one of the lines, in order to see that the second line does not go

beyond the steady state thermal limit of the line. Similarly, Special Protection Schemes

have been installed at many other places in the National Grid for additional security,

while ensuring optimum utilization of the transmission system.

iv. The SPS must be considered for the purpose of calculation of TTC and ATC, when

used for n-1-1 criteria, and for n-1 criteria (in situations, wherein Long Term Access

(LTA) and Medium Term Open Access (MTOA) itself has been granted with SPS).

However, SPS should be considered for the purpose of thermal rating and not for

satisfying the angular stability criteria.

v. These Schemes are Grid Contingency Schemes and shall not be allowed to be

tampered with.

vi. The relay settings of the SPS for carrying out load shedding should be set with such

time delay so that the load tripping takes place, considering the emergency loading

limit of the line, to prevent frequent tripping of load. The emergency loading limit,

considering even old conductors, is 40% over-load over the thermal rating limit for 5

minutes. However, keeping a safe margin, the time delay could be set upto 10-30

seconds for an overload of up to 40%, depending on experience, on what would be

the ideal time to prevent frequent tripping of loads. For example, it was seen that

frequent load shedding stopped on the Parli-Sholapur line, when the setting was

changed from instantaneous tripping to 10 seconds delay.

vii. Before the increased thermal limit on account of emergency loading is used, the

System Operator may confirm from the owner of the terminal equipment on both sides

of the line if the terminal equipment is capable of carrying the emergency loading

current and also if the relay settings are in accordance with the same. The terminal

equipment owners will confirm the same. The confirmation may be done once before

the start of the season, i.e. monsoon, winter, etc.

viii. It has been pointed out by POSOCO that some lines developed hot spots and have to

be frequently taken out for attending to the same. It is mentioned that hot spots are a

result of loose connections and are likely to develop, even when the nominal current

is being carried, which however, may get aggravated when a higher current is carried.

The system operator may allow such lines to be taken under shut down for attending

to the loose connections.

Appendix

Annexure

Ampacity and MW loading (assuming nominal

voltage and unity power factor) for normally

used conductors, i.e. ACSR Moose, ACSR

Bersimis, Zebra and Panther, for every 20C

temperature drop below 400C and for every 20C

temperature drop below 450C between 450C and

400C, for maximum temperature of 750C and 850C

Maximum Conductor Temp. 85 °C

ACSR MOOSE (597 sqmm)

where V=400KV

Amb. Temp. °C current in amp. root 3 X VI (MW)

45 798 552.85

43 813.96 563.91

41 845.88 586.03

40 874 605.51

38 891.48 617.62

36 908.96 629.73

34 926.44 641.84

32 943.92 653.95

30 961.40 666.06

28 978.88 678.17

26 996.36 690.28

24 1013.84 702.39

22 1031.32 714.50

20 1048.80 726.61

Maximum Condutor Temp. 75 °C

ACSR MOOSE (597 sqmm)

where V=400KV

amb. Temp. °C current in amp. root 3 X VI (MW)

45 631 437.16

43 643.62 445.90

41 668.86 463.39

40 728 504.36

38 742.56 514.45

36 757.12 524.53

34 771.68 534.62

32 786.24 544.71

30 800.80 554.79

28 815.36 564.88

26 829.92 574.97

24 844.48 585.06

22 859.04 595.14

20 873.60 605.23

Maximum Condutor Temp. 85 °C

ACSR Bersimis(724.69 sqmm)

where V=400KV

amb. Temp. °C current in amp. root 3 X VI (MW)

45 933 646.38

43 951.66 659.31

41 988.98 685.17

40 1024 709.43

38 1044.48 723.62

36 1064.96 737.80

34 1085.44 751.99

32 1105.92 766.18

30 1126.40 780.37

28 1146.88 794.56

26 1167.36 808.75

24 1187.84 822.94

22 1208.32 837.12

20 1228.80 851.31

Maximum Condutor Temp. 75 °C

ACSR Bersimis(724.69 sqmm)

where V=400KV

amb. Temp. °C current in amp. root 3 X VI (MW)

45 732 507.13

43 746.64 517.27

41 775.92 537.56

40 848 587.49

38 864.96 599.24

36 881.92 610.99

34 898.88 622.74

32 915.84 634.49

30 932.80 646.24

28 949.76 657.99

26 966.72 669.74

24 983.68 681.49

22 1000.64 693.24

20 1017.60 704.99

Maximum Condutor Temp. 85 °C

Zebra (484 sqmm)

where V=400KV

amb. Temp. °C current in amp. root 3 X VI (MW)

45 703 487.04

43 717.06 496.78

41 745.18 516.26

40 769 532.76

38 784.38 543.42

36 799.76 554.07

34 815.14 564.73

32 830.52 575.38

30 845.90 586.04

28 861.28 596.69

26 876.66 607.35

24 892.04 618.01

22 907.42 628.66

20 922.80 639.32

Maximum Condutor Temp. 75 °C

Zebra (484 sqmm)

where V=400KV

amb. Temp. °C current in amp. root 3 X VI (MW)

45 560 387.97

43 571.2 395.73

41 593.6 411.25

40 643 445.47

38 655.86 454.38

36 668.72 463.29

34 681.58 472.20

32 694.44 481.11

30 707.30 490.02

28 720.16 498.93

26 733.02 507.84

24 745.88 516.75

22 758.74 525.66

20 771.60 534.56

Maximum Condutor Temp. 85 °C

ACSR MOOSE (597 sqmm)Quad Conductor

where V=765 KV

amb. Temp. °C current in amp. root 3 X VI (MW)

45 798 4229.34

43 813.96 4313.92

41 845.88 4483.10

40 874 4632.13

38 891.48 4724.77

36 908.96 4817.42

34 926.44 4910.06

32 943.92 5002.70

30 961.40 5095.34

28 978.88 5187.99

26 996.36 5280.63

24 1013.84 5373.27

22 1031.32 5465.91

20 1048.80 5558.56

Maximum Condutor Temp. 75 °C

ACSR MOOSE (597 sqmm)Quad Conductor

where V=765 KV

amb. Temp. °C current in amp. root 3 X VI (MW)

45 631 3344.25

43 643.62 3411.13

41 668.86 3544.90

40 728 3858.34

38 742.56 3935.51

36 757.12 4012.68

34 771.68 4089.84

32 786.24 4167.01

30 800.80 4244.18

28 815.36 4321.34

26 829.92 4398.51

24 844.48 4475.68

22 859.04 4552.84

20 873.60 4630.01

Maximum Condutor Temp. 85 °C

ACSR Bersimis(724.69 sqmm) Quad Conductor

where V=765 KV

amb. Temp. °C current in amp. root 3 X VI (MW)

45 933 4944.83

43 951.66 5043.72

41 988.98 5241.51

40 1024 5427.12

38 1044.48 5535.66

36 1064.96 5644.20

34 1085.44 5752.75

32 1105.92 5861.29

30 1126.40 5969.83

28 1146.88 6078.37

26 1167.36 6186.91

24 1187.84 6295.46

22 1208.32 6404.00

20 1228.80 6512.54

Maximum Condutor Temp. 75 °C

ACSR Bersimis(724.69 sqmm) Quad Conductor

where V=765 KV

amb. Temp. °C current in amp. root 3 X VI (MW)

45 732 3879.54

43 746.64 3957.13

41 775.92 4112.31

40 848 4494.33

38 864.96 4584.22

36 881.92 4674.11

34 898.88 4763.99

32 915.84 4853.88

30 932.80 4943.77

28 949.76 5033.65

26 966.72 5123.54

24 983.68 5213.43

22 1000.64 5303.31

20 1017.60 5393.20

Maximum Condutor Temp. 75 °C

ACSR Panther(210 sqmm)

where V=132 KV

amb. Temp. °C current in amp. root 3 X1 (MW)

45 366 83.68

43 373.32 85.35

41 387.96 88.70

40 413 94.42

38 421.26 96.31

36 429.52 98.20

34 437.78 100.09

32 446.04 101.98

30 454.30 103.86

28 462.56 105.75

26 470.82 107.64

24 479.08 109.53

22 487.34 111.42

20 495.60 113.31

Maximum Condutor Temp. 65 °C

ACSR Panther(210 sqmm)

where V=132 KV

amb. Temp. °C current in amp. root 3 X1 (MW)

45 244 55.78

43 248.88 56.90

41 258.64 59.13

40 312 71.33

38 318.24 72.76

36 324.48 74.18

34 330.72 75.61

32 336.96 77.04

30 343.20 78.46

28 349.44 79.89

26 355.68 81.32

24 361.92 82.74

22 368.16 84.17

20 374.40 85.60

Maximum Condutor Temp. 85 °C

Zebra (484 sqmm)

Hexa where V=765KV

amb. Temp. °C current in amp. root 3 X VI (MW)

45 703 5588.77

43 717.06 5700.54

41 745.18 5924.09

40 769 6113.46

38 769.00 6113.46

36 769.00 6113.46

34 769.00 6113.46

32 769.00 6113.46

30 769.00 6113.46

28 769.00 6113.46

26 769.00 6113.46

24 769.00 6113.46

22 769.00 6113.46

20 769.00 6113.46

Maximum Condutor Temp. 75 °C

Zebra (484 sqmm)

where V=765KV

amb. Temp. °C current in amp. root 3 X VI (MW)

45 560 4451.93

43 571.2 4540.97

41 593.6 4719.05

40 643 5111.77

38 655.86 5214.01

36 668.72 5316.24

34 681.58 5418.48

32 694.44 5520.71

30 707.30 5622.95

28 720.16 5725.19

26 733.02 5827.42

24 745.88 5929.66

22 758.74 6031.89

20 771.60 6134.13

Maximum Conductor Temp. 85 °C

ACSR MOOSE (597 sqmm)

Twin where V=400KV

Amb. Temp. °C current in amp. root 3 X VI (MW)

45 798 1105.71

43 813.96 1127.82

41 845.88 1172.05

40 874 1211.01

38 891.48 1235.23

36 908.96 1259.45

34 926.44 1283.68

32 943.92 1307.90

30 961.40 1332.12

28 978.88 1356.34

26 996.36 1380.56

24 1013.84 1404.78

22 1031.32 1429.00

20 1048.80 1453.22

Maximum Condutor Temp. 75 °C

ACSR MOOSE (597 sqmm)

Twin where V=400KV

amb. Temp. °C current in amp. root 3 X VI (MW)

45 631 874.31

43 643.62 891.80

41 668.86 926.77

40 728 1008.72

38 742.56 1028.89

36 757.12 1049.07

34 771.68 1069.24

32 786.24 1089.41

30 800.80 1109.59

28 815.36 1129.76

26 829.92 1149.94

24 844.48 1170.11

22 859.04 1190.29

20 873.60 1210.46