analysis of current & proposed lp models - technical report for wg d5

7/29/2019 Analysis of Current & Proposed LP Models - Technical Report for WG D5

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IEEE 998 Technical Report on Current and Proposed LP Models

For review by WG D5 at April 2008 IEEE meeting in San Francisco

Prepared by F. DAlessandro, Corresponding Member of WG D5

OVERVIEW:

There are two parts to this report, namely:

1. Arguably, the main issue of contention is the protective / capture distance or attractiveradius afforded by masts and shield wires according to the EGM variants in the

literature.

2. The second issue, which follows from the first, is how the final design for a substationwould look when the protection areas and requirements of the particular protection

technique are applied.

Part 1: Comparison of protection area results using three different EGMs

1.1 BACKGROUND

A review of the correspondence relating to the submission of an Improved EGM for the IEEE

998 standard clearly demonstrates that protective distance calculations are perceived as

being at the heart of the controversy. However, it should be remembered that it is not until a

complete design is performed that one can observe the final results from a particular method.

For example, one method may give a smaller protective distance but completely ignore the

interception probability of the substation equipment, whilst another may use larger distances

but also assign capture distances to the substation equipment. With regard to the latter

situation, the Improved EGM stipulates thatallitems in a substation (equipment, buses etc. as

well as the protective masts and shield wires) are capable of launching an upward leader.

Hence, they all have a non-negligible probability of intercepting the downward leader and

receiving a direct strike. Therefore, the concept of competing features is applied in the

Improved EGM, requiring protection distance calculations for all non-self-protecting

equipment and buses present in the substation.

1.2 CALCULATION SUMMARY

The calculations will be carried out for protective masts and shield wires, like those used in

the two examples in Annex B of IEEE 998 for 69 kV and 500/230 kV substations.

The three EGMs or methods that will be compared are the:

Simple EGM / Rolling Sphere Method (RSM), as presented in B5 of IEEE 998; Revised EGM as described by Mousa et al. and presented in B4 of IEEE 998; and

Improved EGM as described in the April 2007 submission to IEEE 998.

The calculations involved with the first two methods are described in detail in the current

version of IEEE 998 and so will not be repeated here. The third method is based on the

research of Eriksson (1987) and, likewise, this method was described in the submission

document and so will not be described again here. However, for traceability of the

calculations, the key equations to be used are summarised below.


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1.2.1 Simple EGM / RSM

The simple EGM, first introduced by Whitehead in the late 1960s, links two important

parameters of the lightning stroke to an earthed structure, namely theprospective peak stroke

current, Ip, and the striking distance, S. One of the empirical relationships obtained for S,

using the downward leader charge and the electric field for air breakdown to link it to thestroke current is given by

S= 10Ip0.65 (1a)

where S is in metres and Ip is in kA. There are several variants of the striking distance

relation, and in the early 1990s the IEEE adopted the following relation for application in the

power transmission industry:

S= 8Ip0.65

(1b)

The significance of the striking distance for the attachment process is that if a downward

leader tip gets within a distance Sfrom a point on a structure, and that point is capable of

launching a connecting upward leader, then that point will be the one to which the lightning

channel connects. In other words, the striking distance quantifies the range of capture of astrike for a given peak current.

The protective distance afforded by a mast or shield wire of height h relative to a lower object

or the ground is given by

Rp = [16 h Ip0.65 h2] (2)

1.2.2 Mousas Revised EGM

After Eriksson, in the late 1970s, pointed out some of the deficiencies in the simple EGM

(see Clause 5.1.3 of IEEE 998), Mousa & Srivastava (1988) developed the Revised EGM.

The Revised EGM model differs from Whiteheads model in several ways, namely:

The stroke is assumed to arrive only in a vertical direction.

A value of 24 kA is used as the median stroke current.

The differing striking distances to masts, wires, and the ground plane are taken intoconsideration via a correction factor, k.

Hence, the Revised EGM uses the relationship

S= 8 kIp0.65 (meters) (3)

as a basis, where k= 1 for the ground plane or shield wires and k= 1.2 for masts and towers.

The use of the coefficient k is clearly an acknowledgement that the striking distance has a

dependence on the geometry of the prospective strike point.

Other than the introduction of this factor to take into account, empirically, the differences

between masts and wires, the descriptions provided in IEEE 998 Sections 5.2 and 5.4 provide

no elucidation of the means by which protective distances might be calculated according to

the Revised EGM. Lacking this further information, it is assumed the relevant distances are

computed using Eq. (3) and hence have no height dependence.


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1.2.3 Erikssons Improved EGM

Eriksson found that the attachment of lightning to a structure is not only determined by the

striking distance but also by the successful interception of the downward leader by the

upward leader. Hence, the interception process was found to depend on the structure height,

the relative positions of the two leaders and their relative velocities of approach. Using this

physical model, Eriksson defined the protective or capture distance as the attractive radius,Ra.

For masts and towers up to 60 m in height, Eriksson defined the attractive radius as:

Ra = 0.84 h0.6

Ip0.74

(meters) (4a)

For horizontal conductors and shield wires up to 60 m in height, Eriksson defined the

attractive radius as:

Ra = 0.67 h0.6

Ip0.74

(meters) (4b)

1.3 RESULTS

The plots below show the dependence of the protective distance with mast or wire height,

according to each model, for different BILs.

1.3.1 Masts

0

5

10

15

20

0 10 20 30 40

Simple EGM / RSM

Revised EGM (Mousa)

Improved EGM (Eriksson)

EGM comparison for BIL = 350 kV - MASTS

Mast height (m)

Protectivedistance(m)

Figure 1a: Protective or capture distance (attractive radius) comparisons as a function of

protective mast height for the 69 kV substation example in Annex B of IEEE 998. BIL = 350

kV, Ip = Imax = 2.57 kA for RSM & Improved EGM, Ip = Imax = 2.03 kA for Revised EGM.


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0

20

40

60

0 10 20 30 40

Simple EGM (RSM)

Revised EGM (Mousa)



Mast height (m)


Figure 1b: Protective or capture distance (attractive radius) comparisons as a function ofprotective mast height for the 500 kV switchyard example in Annex B of IEEE 998. BIL =1800 kV, Ip = Imax = 13.20 kA for RSM & Improved EGM, Ip = Imax = 9.11 kA for Revised

EGM.

0

10

20

30

40

0 10 20 30 40

Simple EGM (RSM)

Revised EGM (Mousa)



Mast height (m)


Figure 1c: Protective or capture distance (attractive radius) comparisons as a function of

protective mast height for the 230 kV switchyard example in Annex B of IEEE 998. BIL = 900

kV, Ip = Imax = 6.60 kA for RSM & Improved EGM, Ip = Imax = 4.99 kA for Revised EGM.


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1.3.2 Wires / Catenaries

0

5

10

15

20

0 10 20 30 40

Simple EGM / RSM

Revised EGM (Mousa)


EGM comparison for BIL = 350 kV - WIRES

Mast height (m)


Figure 2a: Protective or capture distance (attractive radius) comparisons as a function ofshield wire height for the 69 kV substation example in Annex B of IEEE 998. BIL = 350 kV, Ip

= Imax = 2.57 kA for RSM & Improved EGM, Ip = Imax = 2.03 kA for Revised EGM.

0

10

20

30

40

50

0 10 20 30 40

Simple EGM (RSM)

Revised EGM (Mousa)



Mast height (m)


Figure 2b: Protective or capture distance (attractive radius) comparisons as a function of

shield wire height for the 500 kV switchyard example in Annex B of IEEE 998. BIL = 1800kV, Ip = Imax = 13.20 kA for RSM & Improved EGM, Ip = Imax = 9.11 kA for Revised EGM.


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0

10

20

30

0 10 20 30 40

Simple EGM (RSM)

Revised EGM (Mousa)



Mast height (m)


Figure 2c: Protective or capture distance (attractive radius) comparisons as a function ofshield wire height for the 230 kV switchyard example in Annex B of IEEE 998. BIL = 900 kV,Ip = Imax = 6.60 kA for RSM & Improved EGM, Ip = Imax = 4.99 kA for Revised EGM.

1.4 CONCLUSIONS

For masts:

1. The Revised EGM of Mousa is the least conservative of the three methods evaluated.2. The Improved EGM of Eriksson is the most conservative of the three methods evaluated,

at least for the mast height range typically used in practice for substation shielding.

3. The simple EGM / RSM is very limited in its application because it can only be used formast heights up to the equivalent striking distance. For example, for a BIL = 69 kV / I max= 2.57 kA, the method cannot be used for masts taller than about 15 meters.

For shield wires:

1. The simple EGM / RSM does not distinguish between masts and wires, hence for some ofthe taller wire scenarios it is less conservative than the revised EGM of Mousa.

2. The Improved EGM of Eriksson is more conservative than either of the other methods forall wire heights up to at least 30 meters.

3. Once again, we see the limited nature of the simple EGM / RSM in that it can only beused for mast heights up to the equivalent striking distance.

In all cases for masts and wires, it can be seen that the Improved EGM of Eriksson can

handle all heights and also provides a quantitative dependence of the protective distance on

the height of the shielding mast or wire.


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Part 2: Analysis of substation protection examples in Annex B of IEEE 998

2.1 INTRODUCTION

This second part of the report focuses on the question of how the final design for a substation

would look when the protection areas and requirements of the Improved EGM are applied to

masts and/or shield wires. In this regard, reference will be made to the two examples inAnnex B of IEEE 998, namely the 69 kV and 500/230 kV substations.

The main parameters relevant to the 69 kV and 500/230 kV substations are presented in

Table 1. Values have been taken from Annex B where available, otherwise they have been

estimated with standard formulae.

Table 1: Summary of parameters relevant to lightning protection calculations for the two substation examples in

Annex B of IEEE 998-1996.

Substationprotectioncalculations SimpleEGM/RSM

Substation

voltage(kV)

Equip/bus

BIL(kV)

Busheight

(m)

Zeff

( )

Imax

(kA)

Probability

(%)

MastS

(m)

WireS

(m)

69 350 4.3 300 2.57 99 14.8 14.8

5.8 300 2.57 99 14.8 14.8

10 300 2.57 99 14.8 14.8

230 900 8.5 336 5.89 97 25.3 25.3

6.1 336 5.89 97 25.3 25.3

11.9 336 5.89 97 25.3 25.3

500 1800 16.8 336 11.79 86 39.8 39.8

9.1 336 11.79 86 39.8 39.8

Substationprotectioncalculations Mousa's"RevisedEGM"

Substation

voltage(kV)

Equip/bus

BIL(kV)

Busheight

(m)

Zeff

( )

Imax#

(kA)

Probability

(%)

MastS

(m)

WireS

(m)

69 350 4.3 300 2.57 99 17.7 14.8

5.8 319 2.41 99 17.0 14.2

10 409 1.88 99 14.5 12.1

230 900 8.5 314 6.31 96 31.8 26.5

6.1 336 5.89 97 30.4 25.3

11.9 336 5.89 97 30.4 25.3

500 1800 16.8 336 11.79 86 47.7 39.8

9.1 336 11.79 86 47.7 39.8#Thesevaluesare closebutnotidenticaltothosepresentedbyMousa'scomputer code.

Substationprotectioncalculations Eriksson's"ImprovedEGM"

Substation

voltage(kV)

Equip/bus

BIL(kV)

Busheight

(m)

Zeff

( )

Imax

(kA)

Probability

(%)

MastS

(m)

WireS

(m)

69 350 4.3 300 2.57 99 n/a* n/a*

5.8 300 2.57 99 n/a* n/a*

10 300 2.57 99 n/a* n/a*

230 900 8.5 336 5.89 97 n/a* n/a*

6.1 336 5.89 97 n/a* n/a*

11.9 336 5.89 97 n/a* n/a*

500 1800 16.8 336 11.79 86 n/a* n/a*

9.1 336 11.79 86 n/a* n/a*

*Strikingdistance is a functionofheight.


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2.2 DESIGN EXAMPLE

The design example chosen for making the comparisons discussed above is the 69 kV

substation. Two designs will be carried out utilizing masts only and wires only. As noted in

IEEE 998-1996, to ensure comparability of the results from different design methods, thefollowing criteria were adhered to:

a) Maximum height of mast or shield wire support point is 30.5 m;

b) Maximum span of shield wires is 183 m; and

c) No more than four shield wires are to be connected to a support structure.

2.2.1 Mast shielding of the 69 kV substation

The mast shielding layout according to the Revised EGM is shown in Fig. 3. It appears that

the 3 m rods located on the towers at points g, h, p and q are a common feature across all of

the design methods, so these will be retained for comparability.

Figure 3: The 69 kV substation shielding layout according to Mousas Revised EGM, taken from Fig. B.4-1 ofIEEE 998-1996.


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The procedure to be followed for the Improved EGM implementation is as follows:

a) Select the required protection level or interception efficiency. Referring to Table 1, forthe 69 kV substation, for a BIL of 350 kV, a level of 99% is required. This value

corresponds to a maximum allowable bypass stroke current of 2.57 kA, i.e., substation

equipment can handle up to 2.5 kA.

b) Compute the attractive radius for all competing features requiring protection, i.e.,busses (using Eqn. 4b) and equipment (using Eqn. 4a), taking heights into account. FromTable 1, the heights are 4.3 m (bus & equipment), 5.8 (bus) and 10 m (equipment). The

corresponding attractive radii of these competing features are 3.2 / 4.1, 3.9 and 6.7 m.

c) Draw circles (equipment) or parallel lines (busses) corresponding to the attractive area ofeach competing feature. These are shown in the plan view of Fig. 4. Attractive areas for

busses are shown in purple and for equipment in red.

d) Assume a protective mast height. For ease of comparison, a common mast height of 15.2m will be used. Compute the attractive radius for a 15.2 m mast using Eqn. (4a), i.e., 8.6

metres.

e) Determine viable or practical mast locations on the site, taking the attractive radius values

into account. As an initial pass, the location and number of masts is assumed to be thesame as that proposed by Mousa for the Revised EGM calculations given in IEEE 998,

i.e., masts a, b, c, d, e andfshown in Fig. 4.

f) Draw circles corresponding to the attractive area of each mast. These are shown in blue inthe plan view of Fig. 4.

g) If the mast attractive areas (blue) overlap all of the competing feature (bus andequipment) attractive areas, the substation is protected at a level of 99% (Imax = 2.57 kA).

However, as can be seen in Fig. 4, there are some bus areas that could receive a direct

lightning strike. These areas are marked by a yellow lightning bolt. Furthermore, there are

two locations where the bus dead-end structures could be susceptible to a strike. These

two points are shown by green lightning bolts in Fig. 4 and arrows point to the

corresponding attractive areas not covered.

h) Since the mast attractive areas do not overlap all of the competing feature areas, it isnecessary to relocate one or more masts and/or add one or more masts to obtain the

required coverage. In this particular example, there are various actions that could be

taken, for example:

i. Masts d, e andfcan be moved to the left in Fig. 4, i.e., closer to the busses thatthey presently do not adequately protect;

ii. Moving mast b to the right will help;iii. Use taller masts at d, e andf; oriv. An additional mast between dandf.

A suitable modification to the layout resulting in a final design is shown in Fig. 5. Anadditional 15.2 m mast and minor relocation of the three original masts d, e andfresult in a

completely protected substation at the 99% protection level (these masts are shown by yellow

crosses in Fig. 5).


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Figure 4: The 69 kV substation mast shielding calculations according to the Erikssons Improved EGM. The

calculations show that the same layout as Mousas Revised EGM does not fully protect the substation.

2.2.2 Wire shielding of the 69 kV substation

A similar procedure is followed when using the Improved EGM for wire shielding, namely:

a) Select the required protection level or interception efficiency, i.e., 99% for a BIL of 350kV (Imax = 2.57 kA).

b) Compute the attractive radius for all competing features requiring protection, i.e.,busses (using Eqn. 4b) and equipment (using Eqn. 4a), taking heights into account. From

Table 1, the heights are 4.3 m (bus & equipment), 5.8 (bus) and 10 m (equipment). The

corresponding attractive radii of these competing features are 3.2 / 4.1, 3.9 and 6.7 m.

c) Draw circles (equipment) or parallel lines (busses) corresponding to the attractive area ofeach competing feature (these are shown in red in Fig. 6.).


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Figure 5: The final design for the 69 kV substation using mast shielding calculations according to Erikssons

Improved EGM.

d) Assume a shield wire height and arrangement. As a first attempt and for comparisonpurposes, the same height and arrangement as that shown in Figure B.5-2 of Annex B in

IEEE 998-1996 will be used. This figure relates to rolling sphere method calculations, not

Mousas Revised EGM (since the latter design is not shown in Annex B). Compute the

attractive radius for a shield wire of height 12.2 m using Eqn. (4b), i.e., 6.0 metres. The

attractive radius of the supporting masts can also be calculated, using Eqn. 4(a), i.e., 7.6

metres.

e) Draw parallel lines corresponding to the attractive area of each wire. Also draw circlescorresponding to the attractive area of each support mast and the four 3 m rods at 15.2 m

height on the left of the site. Again, all of the protective areas are shown in blue in the

plan view of Fig. 6.

f) If the wire attractive areas (blue) overlap all of the competing feature (red bus andequipment) attractive areas, the substation is protected at a level of 99% (Imax = 2.57 kA).

However, as can be seen in Fig. 6, the central bus region, highlighted by the region

4 x 15.2 m (50ft) lightningmasts


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shaded in yellow, is not protected and hence could receive a direct lightning strike of

magnitude greater than 2.57 kA.

g) The lack of protection in the central bus region can be rectified most easily in one of twoways:

i. Add a shield wire, horizontally in Fig. 6, as marked by the arrows, orii. Install a mast of minimum height 12 m next to the equipment in the center of the

substation, as marked by the tip of the lightning bolt graphic in Fig. 6.

2.2.3 Conclusions

The results from the 69 kV substation shielding design using both masts and wires

demonstrate that the Revised EGM according to Mousa may result in under-design and hence

under-protection of substations.

In the mast shielding design example, an additional mast was needed and the other three

masts had to be relocated. Furthermore, Mousa claims that the design could utilise lower

masts (height 12.2 m). If this mast height was used, Erikssons Improved EGM would

identify additional regions that are not protected. In the wire shielding design example, andadditional wire was needed, although the addition of a single mast of height 12 m or higher

would have also resulted in complete protection.

Table 2 summarises the protection requirements of the different methods outlined in IEEE

998 with an additional column for Erikssons Improved EGM. The comparison in Table 2

highlights the fact that there are differences between the methods. In the case of Erikssons

Improved EGM, the fundamental difference is the quantitative allowance that is made for the

height of the masts or wires used for protection. A further improvement in the design process

is the treatment of all non-self-protecting busses and equipment around the substation as

potential strike points and the assignment of an attractive radius (which is proportional to the

probability of a strike to the point) to these items in addition to the protective masts or wires.

Table 2: Comparison of all results for the 69 kV substation.

Hardware

requirements

Method

Fixed Angle Empirical Simple EGM

(RSM)

Revised EGM

(Mousa)

Improved EGM

(Eriksson)

No. masts required 1 1 6 6 7

No. wires required 2 2 4 4 5

Hence, the designer can be assured that the Improved EGM provides the most conservative

and safe design possible whilst at the same time introducing efficiencies into the overall

design via the height-dependence built into the equations that are used to compute theattractive radius of protective masts and wires.


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Figure 6: Lightning protection design for the 69 kV substation using wireshielding calculations according toErikssons Improved EGM.

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