Brief Description of the Set-Up and Activities of the Power and Telecommunication Coordination Committee (ptcc)




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Appendix III to Chapter V

[Refer Para 4.2.5 (iii)]
Computation of Fault Currents


  1. Following procedure for computation of fault currents is adopted:




  1. The single line diagram of the system for the relevant portion of the power system network is drawn assuming infinite generation at a convenient bus, if necessary.




  1. The positive and zero sequence impedances are calculated for all the generators, transformers and transmission lines to a base of 100 MVA (for generators, transient reactance is taken).




  1. The positive and zero sequence networks are drawn including the positive and zero sequence impendence.




  1. Each network is reduced to a form most convenient to work out the fault currents by star delta conversions and finally reducing the network to the form shown in Figure 11.




Figure 11



1.1 For Single Circuit Lines
When, considering the fault at point (a), the impedance will be given by

Where X is impedance/mile or impedance/Km of the line

l is length of the line in miles or Kms.

n is distance of the fault from bus A in miles or Kms.
The network is reduced to the form shown in Figure 12.






Figure 12

The fault at all the intermediate points of the line is assumed to have a resistance of 20 ohms as per CCITT recommendations.


PU value of fault resistance

Percentage value

Where

To account for this resistance in zero sequences network, it is necessary to add 3RF to the zero sequence networks. Therefore, the total zero sequences impedance will be:



Total fault impedance


Since

Fault current


1.2 For Double circuit Lines
This study requires knowledge of effective positive sequence and zero sequence impedance of the grid corresponding to the point of fault on one of the circuits of the DC line including the mutual impedance of the two circuits. This can be obtained by suitably representing the concerned double circuit line and the positive or zero sequence network of the grid.


      1. Positive Sequence Network

The representation of a double circuit line in positive sequence network is as shown in Figure 13



Figure 13


      1. Zero Sequence Network

The effective impedance of any double circuit line comprises two components viz self-impedance per circuit plus mutual impedance due to the second circuit. The mutual impedance is represented by Zm and can be known from standard tables. Another factor required is ‘K’ which is the difference of self-impedance and mutual impedance and can also be obtained from standard tables. The representation of the line in zero sequence networks is given in Figure 14.








Figure 14

For knowing the total fault current, it is preferable to reduce the delta formed by impedance (X2 + nZm), IK and X3 + (l – n) Zm into equivalent star as shown in Figure 15.


X1y, X2y, X3y are the impedance of the equivalent star
Now
Now Z = Z1 + Z2 + Z0
= 2Z1 + Z0


and


For knowing the effective fault current for the section PQ of the line involved in parallelism with the telecommunication line shown in Figure 16, it is necessary to know the current contributions from buses A and B under fault conditions at points P and Q. For the fault at point P, only the current from bus B flows through the section PQ and for the fault at Q current from bus A flows through PQ. The effective current will be higher of these currents flowing through PQ



Figure 15 (a)

.




Figure 15 (b)




Figure 15 (c)





Figure 16


Annexure I to Appendix III of Chapter V
A Summary of Important Formulae and Values Used in

Fault Current Calculations

BASE: 100 MVA




Base Voltage

in KV

Base Amps

Base Ohms

400

230


220

132


110

66

33



22

11


144.3

251.0


262.4

437.4


524.9

874.8


1750.0

2624.0


5249.0

1600.00

529.00


484.00

174.20


121.00

43.56


10.89

4.84


1.21


Per Unit Quantities
(i) Base current in Amps. =

(ii) Base impedance





(iii) Per unit impedance

(iv) For changing per unit impedance on a given base to per unit impedance on a new base, the following equation is used.


(v)

(vi) Impedance

Converting to per unit value:









(vii)
(viii) For single line to ground faults, the neutral current is 3 times the phase current.
 Isc for SLG faults

Annexure II to Appendix III (Chapter V)
STAR DELTA TRANSFORMATION
Figure 17


Star Delta Conversion Delta Star Conversion






Appendix IV to Chapter V

(Refer Para 4.1/1)

PLATES 1(A) to 1(F)

CHAPTER VI
Code of Practice for the Protection of Telecommunication Lines at Crossings with Overhead Power Lines Other than Electric Traction Circuits


  1. Introduction

1.1 The possibility of damage to telecommunication apparatus and injury to personnel due to accidental contacts between electric supply lines and telecommunication lines is an important consideration necessitating proper protective arrangements. Such contacts between power and telecommunication lines may result from falling tree limbs, improper sag of conductors, damaging acts by the public, structural failures, poor maintenance, wind storms and conductor failures due to lightning stroke, etc. Therefore, crossing of the power and telecommunication lines requires some consideration as they would be in the close proximity in such situations. The objective of this Code is to set out measures to be adopted at crossings to avoid or reduce danger to personnel and telecommunication lines.


Rule 87 of the Indian Electricity Rules should be observed in respect of crossing of power and telecommunication lines.
1.2 While the arrangements described in this Code are expected to offer a high degree of protection at reasonable cost, it is always advisable to route all future power and telecommunication lines so as to keep the number of crossings to the minimum possible under the circumstances.
1.3 All new constructions at crossing locations shall conform to the practices laid down in this Code. The existing arrangements at crossing need not be dismantled but should continue to be maintained till due for replacement.
1.4 Crossing situation no specifically covered by this Code should be referred to the PTCC for decision.
1.5 The principle that is followed by the PTCC in regard to the liability towards the cost of structural arrangements and the installation of necessary protective apparatus, etc has been that it should fall on the party who enters the field at a late date, irrespective of whether such work is carried out on the power or communication line. This is in conformity with the Indian Electricity Rules.


  1. Classification of Power Lines

For the purposes of this Code, power lines are classified as below:




  1. Low and medium voltage distribution and service lines (voltage not exceeding 650 volts between phases).




  1. High voltage lines, Category I (voltage exceeding 650 volts, but not exceeding 12 KV between phases).




  1. High voltage lines, Category II (voltage exceeding 12 KV but not exceeding 36 KV between phases).




  1. Extra high voltage lines (voltage exceeding 36 KV between phases)

Note: Lines with voltages of 36 KV (between phases) and below also come within Category (iv) provided they comply with the usual standards of construction and operation adopted for lines with voltage of 72.5 KV (between phases) and above. Such cases shall be referred to PTCC.




  1. Crossings between Power and Telecommunication Lines

3.1 Disposition of Power and Telecommunication Wires


Except in the case of electric traction circuits, which are not covered by this Code, the power lines shall crossover the telecommunication lines.
Note 1: This arrangement is advantageous as power wires are generally of a heavier gauge than telecommunication wires and hence have a lesser possibility of breakage. Further, as telecommunication lines generally required more frequent attention for maintenance operation and are subjected to frequent reconstructions, it would be convenient and would afford greater safety in working, if they are taken below the power lines.
Note 2: In unusual situation where it is considered that the most appropriate method will be to take the telecommunication line above the power line, the specific approval for each case shall be obtained from the competent authority responsible for the telecommunication system.


    1. Angle of Crossing

The angle of crossing shall be as nearly a right angle as possible.


Note 1: If the angle of crossing were small, it would increase the context of dangerous proximity of the telecommunication lines with the power lines, as a result of the possible whipping action of the broken power conductors.
Note 2: In exceptionally difficult situations, when the angle has to be below 60 degree, the matter should be reported to the competent authority in-charge of the telecommunication system, which shall, if considered necessary, refer it to the PTCC for technical advice.


    1. Clearance

Specific clearances to be provided between power lines, telecommunication lines, earth wire and earthed structures are indicated for each type of crossing under Paragraphs 4.0, 6.0, 8.0 and 9.0.




    1. Crossings over Telecommunication Lines

Joints and other mechanical discontinuities should, as far as practicable, be avoided in the crossing and adjacent spans and at associated supports. If it is impracticable to avoid such joints, etc they should be of such a type and so made as to have strength substantially equal to that of the conductor in which they are placed.


Note: Clearances between telecommunication and power wires have an important bearing on the safety of persons working on the telecommunication lines and on the prevention of accidental contacts between telecommunication and power wires. It is essential therefore, to provide clearances as specified in this Code.



  1. Joint Use of Poles at Crossing Locations

4.1 In all new constructions, whether of power lines or telecommunication lines, the possibility (except in cases of unusual difficulty) of joint use of poles for crossings between telecommunication lines and (a) low and medium voltage distribution and service lines and (b) high voltage lines, Category I, should be investigated and adopted.


Note 1: From the point of view of safety and structural considerations, the use of a common pole to support both the power lines and telecommunication lines for the crossings is an advantageous proposition.
Note 2: Where it becomes impracticable to adopt joint use of pole, the arrangements given in Para 6.0 or 7.0 shall be adopted, as the case may be.
4.2 The design strength and other mechanical features of the common support and fittings shall be in accordance with the standards and requirements of the Power supply authority or the Telecommunication authority whichever is more rigorous.
4.3 Adequate clearance shall be provided on the common pole to enable employees of either party to carry out maintenance work on their respective lines. The clearance provided on the jointly used pole shall not be less than the figures given in Table 1.
Note: Neutral wires on the power alignment shall be treated as power conductors for the purpose of this rule, in the case of multiple earthed neutrals that are not carried on insulators.
4.4 When the power lines carried on the jointly used pole are high voltage lines of voltage to earth 3,000 volts and over, power contact protectors (See Appendix I) shall be installed at the crossing on all the exposed wires generally occupying the top bracket of the telecommunication line.

4.5 In order to minimize the maintenance work a jointly used pole shall be used only for supporting the two crossing alignments. No apparatus or equipment such as switches, fuses, junctions boxes etc shall be mounted on such a pole and no lines shall tee-off from it. There is however, no objection to the installation on the pole of protectors or arrestors for the protection of telecommunication wires.




  1. Guards

5.1 Guarding arrangements shall always be provided as and where prescribed in this Code.


5.2 Guarding arrangements will, ordinarily, be carried out by the owner of the pole on which they are to be made. The owner will also be responsible for their efficient maintenance, the cost being met as indicated in Para 1.5.


    1. Every guard shall be properly earthed at the terminal supports.

5.4 Guard wires shall have a breaking load of not less than 635 Kg (1400 lbs) and if made of iron or steel shall be galvanized. Every guard wire or cross-connected system of guard wires shall have sufficient current carrying capacity to avoid the risk of their fusing on coming into contact with any live power wire.


5.5 For the purpose of ensuring that any live wire coming in contact with the guard wires is rendered dead the net resistance to earth of the guard shall be low enough to give rise to an earth fault current of a magnitude which is at least twice the minimum required to operate the protective system on the power circuit.


Table-1

Ref. para 4.0 (joint use of poles at crossings) Subpara 4.3


Location

Low and Medium Voltage Lines

High Voltage lines up to and including 7.2 KV

High voltage Lines above 7.2 KV and up to and including 12 KV

Minimum vertical clearance between the bottom most power cross-arm and fittings and the topmost communication cross-arm and fittings.

1220mm


(4′0″ )

1380mm


(4′6″)

1980mm


(6′6″)

Minimum vertical clearance between power and communication wires at the pole.

1380mm


(4′6″)

1525mm


(5′0″)

2130mm


(7′0″)

Minimum vertical clearance between communication wires and ground wire on the power line.

1070mm


(3′6″)

1070mm


(3′6″)

1070mm


(3′6″)



  1. Crossings With Low And Medium Voltage Distribution and Service Lines

6.1 Where joint use of poles as laid down in Para 4.0 is not feasible, either of the two methods of crossing recommended below shall be adopted:




  1. Insulated weatherproof wires carried on effectively earthed steel bearer wires may be used for the power lines. The minimum factor of safety for the bearer wires shall be 2.5 based on the ultimate strength of the wire. The vertical clearance from the insulated wires to the telephone wires shall not be less than 760 mm (2′6″). Where this clearance cannot be maintained, PVC sleeving shall be provided on the telecommunication wires. In such cases the vertical clearance between the two lines shall not be less than 30 cms.




  1. Where bare wire is used for the power line, a guard shall be provided between the telecom line and power line as indicated in Figure 1 & 2, as the case may be. The guard shall be fixed either to the power line supports or to the telecom line supports.



    1. Guards on Power Line Supports

The minimum vertical clearance between the guard wires and the telecommunication wires shall be 915 mm (3 ft). The guard shall be so arranged that lines drawn upwards from its outermost wires towards the center, at an angle of 45 degree to the vertical, will totally enclose the power wires as shown in Figure 1 (b). Cross lacing shall also be provided so as to cover a distance of at least 1830 mm (6 feet) on either side beyond the outer most crossing points of the wires of the telecommunication lines as shown in Figure 1 (a).




    1. Guards on Telecommunication Line Supports

The minimum clearance between the power wires (including neutral wires of the power circuit) and the telecom wires shall be 1220 mm (4 feet) as shown in Figure 2 (a). In case the lowest wire on the power line is a ground wire, the minimum separation between the ground wire and the guard shall be 610 mm (2 feet). The minimum clearance between the guard wires and the telecommunication wires shall be 610 mm (2 feet). Cross lacings shall be provided as shown in Figure 2 (b).




  1. Crossing With High Voltage Lines, Category-I

Where joint use of poles as laid down under Para 4.0 is not feasible, the arrangement in Para 8.0 shall be adopted.



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