In the case of Example I, survey of route reveals that separations, as given in the adjacent Table, can be maintained in the different stretches of the parallelism. Determine the induced voltage on the telecommunication circuits if the soil resistivity of the region is 10,000 ohms/cm3.
Sl. no. of the Section
Length of the Section (dn) in Km
Separating Distance between the Power and Telecom Line (Sn) in Km
d = 40 Kms
Let S = Average separation, then
Therefore Average Separation, S = 0.866 Kms or 866 meters.
As the actual average separation is more than minimum safe separation (720 meters) vide example I, the induction is less than 430 volts.
Appendix II to Chapter IV
(Refer Para 5 in Part I)
MEASUREMENT OF SOIL RESISTIVITY
The soil resistivity at any place can be measured by means of Evershed Earth Tester.
The earth tester has four terminals marked P1, P2, C1 & C2 and four similar electrodes are driven into the ground at equal distances of 50 meters, in the region where the soil resistivity is to be determined (should be driven at least 3 to 4 feet). Let us designate these four electrodes as A, B, C and D. A and D (extreme electrodes) should be connected to C1 and C2 of the meggar. The electrodes B and C (the middle ones) should be connected to P1 and P2. By operating the meggar handle continuously at a uniform speed, we can read the electrode resistance ‘R’ on the meggar scale. The soil resistivity is given by the equation
= Soil resistivity in ohm/Cm³.
R = Meggar reading in ohms.
A = Distance between two electrodes in Cms.
Note: (1) Generally, in Evershed earth meggars, the reading as obtained on the meggar will have to be divided by 10 or 100, depending upon the setting on the meggar.
For further details Appendix II to Chapter VII may also be
Appendix III to Chapter IV
(Refer Paras 1 and 2 of Part 1 and First Para of Part II)
PLATES 1(a), 1(b), 2(a), 2(c) and 3(a) to 3(i)
Guidelines for Processing PTCC Cases of Power Lines
Up to and Including 132 KV
1.0 The ‘Simplified Procedure for Co-ordination of Power lines up to and including 33 KV with Telecommunication lines’ detailed in Chapter IV is based on the following assumptions:
That the power lines would be essentially radial i.e., the source of supply would be only at one end and
That an infinite bus could be considered at the nearest EHV sub-station where the voltage level up to 33 KV is created, thus neglecting the system impedance behind the EHV bus.
2.0 It will be clear that the above assumptions are not applicable in all cases. Even now, instances of 33 KV DC lines, which are not covered by the ‘Simplified Procedure’, are being referred to the Central PTCC. The following paragraphs, therefore, deal with the methods which are to be adopted in order to estimate the low frequency induced voltages from power lines up to and including 132 KV, whether SC or DC, under Single-Line-to-Ground (SLG) fault conditions, on the neighboring telecommunication lines.
Three aspects are involved in the estimation of the induction:
Determination of the mutual coupling between the power and telecommunication lines.
Determination of the effective fault current.
Determination of the induced voltage in the affected telecommunication section.
4.0 The detailed procedure in respect of each of the items in Para 3.0 is given below
Determination of the Mutual Coupling (MC) between the Power & Telecommunication Lines
4.1.1 Appendix I to Chapter IV of the ‘Simplified Procedure’ referred to in Para 1.0 above, has detailed a method for estimating the average separation between the power and telecom lines in any section of parallelism, where actual separation between them is not uniform. The method is applicable in any general case. Once the average separation has been arrived at, the mutual coupling between the two lines could be worked out on the basis of curves showing the Separation Versus Mutual Impedance as in Plates 1 (A) to 1 (F) given in Appendix IV to Chapter V.
4.1.2 An illustration would explain the procedure. In Example IX of the ‘Simplified Procedure’ in Chapter IV, the average separation between the power and telecom lines has been worked out as 866 meters. For a soil resistivity of 10,000 ohms/cm³, the corresponding mutual impedance from Plate 1 (A) would be 0.034 ohms/Km. The MC for a parallelism of 10 kms would then be 0.34 ohms.
4.1.3 Apart from the above, a graphical method is also available for the determination of the mutual coupling. The parallelism, in this case, is assumed to be made up of a large number of small sub-sections, in each of which the separation between the power and telecom circuits could be considered to be reasonably uniform.’
4.1.4 At the beginning and end of each sub-section on the power line, vertical ordinates are erected from which the separation distance to the telecom line can be measured (Figure 1). The mutual impedance corresponding to these points for the relevant soil resistivity value is obtained from the curves in Plates 1 (A) to 1 (F). A curve is then drawn connecting the horizontal distances along the power line and the MC at each point to suitable scale. The mutual coupling for the section of parallelism involved is obtained by measuring the area bounded by the curves with the help of a planimeter. The actual coupling is calculated as follows:
MC = Area of the curve x Scale for distances along the horizontal axis x Scale for mutual coupling at each point x Planimeter Constant.
N.B. = If the scale along the horizontal axis is 1”=1 mile, the factor of 1760/1000 should be used to multiply the MC. If the scale is 1 cm = 0.5 kms, the factor is 500/1000 and so on).
For a given case, if the scale (a) along the horizontal axis is 1˝ = 1 mile (b) along the vertical axis is 1˝ = 0.02 and (c) the planimeter constant is 20, then the
Mutual Coupling = Area of Curve x 1.76 x 0.02 x 20
= Area x 0.704
In the estimation of the mutual coupling between the power and telecom lines, certain general factors as given in the following paragraph may be kept in mind:
Consider a Telecommunication line AH; and a Power line XY as shown in Figure 2. We shall consider only that part of the telecommunication line, which lies inside the zone in question i.e. the length BH. The telecommunication line, when approximated by a series of straight-line section, may present a parallel section (DE), oblique approaches (BC, CD, GH) or crossings (EFG). Each of the straight-line elements BC, CD, DE….. is then projected on the power line at bc, cd, de…. The electromotive force induced in each of the lengths BC, CD, DE by the corresponding lengths, bc, cd, de of the power line is calculated. The induced electromotive force is the sum of all these partial electromotive forces.
X a b c d e F Y
4.1.6 Method Proposed in CCITT Guide
Where the exposure between power and telecommunication lines is oblique, it can be considered as a parallel section having a distance between lines
Provided that 1/3 < d1/d2 < 3. If d1/d2 is outside these limits, the section MN is divided into two sections MP and PN so that d1/d2 lie between 1/3 and 3.
For an approximate calculation, crossings can be disregarded when the angle exceeds 450. When the crossing angle is less than 45° the crossing is treated as a parallel section with a distance between lines of 6 meters. This empirical method introduces a maximum error of 10 to 15 % in the mutual coupling.
An example for treating the crossing is illustrated in Figure 3. On the Section MN of the telecom line, which crosses the power line, points M′ and N′ situated 10 meters from the power line are defined.
Figure - 3
X n′ Y
The section MM′ and NN′ are treated as oblique exposures. Section M′N′ is ignored if α ≥ 450 section M′N′ is treated as a parallel section with a distance between lines as 6 meter if α < 450.
Note: If the telecom line has a bend inside the zone ± 10 meters around the electricity line, the crossing is considered as terminating at the level bend.
(i) When the telecommunication line reverses its direction with respect to the power line (section GH in Figure 2), the electromotive forces induced in such sections are subtracted and must therefore be given a negative sign.
When the power line and the telecommunication lines converge as shown in Figure 4, the projection must be made as shown in this figure. The length N′M′ is subjected both to the induction from the line XZ and that from the line ZY. The electromotive forces induced in MM′ (by mZ) and in N′N (by Zn) must therefore be added.
When the power line and the telecommunication line diverge as shown in Figure 5, the projection must be made as shown in that figure. It is seen that the length P′Q′ is not subjected to any induction and it is merely necessary to add the electromotive forces induced in PP′ (by pZ) and in Q′Q (by Zq).
M N′ M′ N
Note: The section N′M′ is induced twice i.e. by XZ and ZY. The induced emf in MM’ & NN’ are added.
Figure - 4
P P′ Q′ Q
Note: The section P’Q’ is not counted. The induced emf PP’ & QQ’ are added
Figure - 5
4.2 Determination of the Effective SLG Fault Current
4.2.1 It is presumed that power engineers are generally familiar with the procedure for the estimation of fault current at any required point on a transmission line. The current is to be considered at the most unfavorable point in the exposure as regard induction. The following paragraphs cover some of the important points in regard to the determination of these currents.
4.2.2 The procedure outlined in the ‘Simplified Procedure’ would be applicable for radial or Single Circuit 33 KV lines, where an infinite bus could be considered at the nearest EHV sub-station. For power lines, which do not, fall in the above category the procedure is indicated in the subsequent paragraphs.
4.2.3 Consider that the line AB (Figure 6) is to be constructed and a telecom line is paralleling it in the section CD. The line AB is such that there is feed at both the ends A & B. The points C′ and D′ on the power line, through which ordinates to the telecom line would pass, are to be considered for evaluation of the SLG fault current. After these currents are estimated, it is necessary to work out the contribution at C′ from B and D’ from A. The more severe of these currents is to be considered for computation of the induced voltage.
4.2.4 For the evaluation of the fault current, an SLG fault study of the system on the digital computer or otherwise, considering the power system conditions which give the maximum current at the fault, taking into account lines and power stations, the construction of which has been approved, is required. From this study, it would be possible to obtain the SLG fault current at any required point. In this evaluation, a fault resistance of 20 ohms as permissible in the CCITT directives can be considered for intermediate points on the line.
Figure - 6
4.2.5 In case a complete study of the system as mentioned above is not available, the necessary calculations have to be carried out by one of the following alternatives:
If the fault levels at the end buses are obtained from the fault study and only the fault current at the intermediate points are not available therein, it would be possible to work out the same by the method given in Appendix I to Chapter V.
A judicious examination of the system will reveal the extent up to which the same is to be considered in the estimation of fault levels in the required line. This becomes necessary when the power system network in itself is complex and manual reduction of the same is time consuming and laborious. An infinite bus would then have to be considered at one or two suitable points in the network and thereafter the balance portion is reduced by star delta transformation. In some cases, it would be evident that the power flow would only be (in one direction) from a nearby source of generation and a radial line from the same could be easily assumed. Illustrative examples are given in Appendix II to Chapter V.
By way of facilitating the manual calculations of fault current, the main principles are given in Appendix III to Chapter V.
4.2.6 After the estimation of the fault currents, curves showing the fault currents flowing in from both sides of the fault are drawn. Figure 7 gives an example of such a curve.
It can be seen from this figure that, in the case of the exposure AB, the least favorable point will be B′ with the inducing current coming from end x and in the case of exposure CD, the most unfavorable point will be C′ with the inducing current being from end Y.
Evaluation of Screening Factor
The Screening factor (k) takes into account the screening action of all earthed conductors in the immediate vicinity of both the inducing line and the affected telecommunication circuits (earth wire, rails etc) and in the case of power and/ or telecom cables, of the screening action of the sheath.
In regard to nearby conductors, their screening action is effective only if their resistance is low; and are well earthed at least at both their ends and if they are close to the inducing circuit or the circuit affected by induction (generally less than 10 meters). Soil resistivity has also some effect on the screening factor. Although the exact screening factor could be evaluated by detailed calculations or by field tests, the order of magnitude is given in Table-1.
Magnetic armoring over the Lead or Aluminum sheaths can greatly improve the screening efficiency although the effectiveness is reduced at both high and low values of flux density. The overall screening factor is obtained as a product of the individual values for the power and telecom lines.
Determination of the Induced Voltages in the Affected Telecom Section
After having determined the mutual coupling between the power and telecom lines, the effective SLG fault current and knowing the screening factor, the induced voltage in any telecom section is obtained as a product of these figures.
Earth-wires on Power Lines
GS Earth wire 0.95
ACSR Conductor 0.70
With Lead Sheath With Aluminum Sheath
30 to 70 KV Cable 240 mm2 0.20 to 0.40 0.100 to 0.200
110 to 150 KV Cable 240 mm2 0.15 to 0.30 0.075 to 0.160
With Lead Sheath With Aluminum Sheath
Diameter 20 mm 0.85 to 0.95 0.20 to 0.60
Diameter 40 mm 0.60 to 0.85 0.10 to 0.40
Other Screening Factors
Railway Lines: One or two tracks 0.80
Railway Lines: Three or more tracks 0.70
Large Stations 0.60
Appendix I to Chapter V
[Refer Para 4.2.5 (i)]
Procedure to Calculate Fault Currents At
Intermediate Points between Buses from Level Studies
Positive and zero sequence impedances of the line A-B under examination can be calculated from the data supplied with the case referred. By successive reduction of the network, it reduces to the form shown in Figure 8. The values of the impedances X1, X2 and X3 are to be determined. The fault study will give fault levels at buses A & B and the contribution of current through the line under examination.
For fault at bus A, the following data is available for the study,
IA = Total fault current
Ia = Fault current through the line A-B.
I1 = IA – Ia = Fault current through other lines and transformers etc., connected to the bus A.
A = Impedance of the line A-B
The fault impedance ZA at bus A, which includes the impedance of the line A-B also could then be determined.
Similarly, for fault at bus B,
IB = Total fault current.
Ib = Fault current through A-B.
I2 = IB-Ib = Fault current through other lines and transformers etc, connected to the bus B.
ZB = Fault impedance at bus B including that of line A-B.
(i) [As X1 & (X2+a) are in parallel for fault at A].
(ii) [As X2 & (X1+a) are in parallel for fault at B].
(iii) for fault at A
(iv) for fault at B
By solving equation (iii) & (iv)
Substituting the values of X1 and X2 in (i) or (ii)
From the single line-to-ground fault study, it is possible to determine the total impedance 2Z1+Zo at each bus. By deducting 2Z1 obtained from the three-phase short circuit study, the zero sequence impedance at each bus could be obtained. Proceeding on the same lines, it is possible to find out the zero sequence values for the arms X1, X2 and X3.
Once these values are obtained, it is possible to work out the required values at any required point, taking the fault resistances also into account.