“bucket brigade” reportewh.ieee.org/cmte/substations/scd0/wgd5/2008-04 san... · lightning...

“bucket brigade” report

assignment

• Review lightning protection system analysis methods that have been developed since the publication of IEEE 998 in 1996

• Report the results at San Francisco meeting

Activities accomplished

• Reviewed the “collection volume” analysis method (CVM) and “upward leader inception”analysis method (ULIM)

• Recommendation on the probability formula, Eq. 2-2a & 2-2b, used to calculate the magnitude of the return stroke current.

• Compared IEEE 998 with similar IEEE and IEC standards (IEEE 1243, IEEE 1410, IEC 62305)

• Review of literature on stroke current wave-shape

Review of CVM & ULIM

Where a method fit’s

• Laboratory experiments

• Controlled field tests

• Model of the physical phenomena

• Analysis methods based on the scientific model

• Design systems with the analysis methods

• Experience with successful designs

SCIENCE • ENGINEERING

How a method is used

Engineering1.Select a design analysis method,

e.g. - EGM2. Determine the design basis for the analysis

e.g. - acceptable probability of stroke current magnitude

3. Set the design criteriae.g. - all equipment & bus “under the

ball”

How a method is used

4. USE THE METHOD TO ANALYZE THE DESIGN

5. Iterate until a design meets the design criteria for the design basis

6. Obtain feedback for made designs

• Model of the physical phenomena

• Analysis method based on the model of the phenomena

• Tool for making the analysis


Lightning protection design analyis method Upward Leader Inception Collection Volume

Lightning Phenomena Model Qualitative Description

1

downward stepped leader with some velocity, and spatial charge distribution produces a time and space varying electric field. Spatial charge is determined by modeling from laboratory data

downward stepped leader with some velocity, and spatial charge distribution produces a time and space varying electric field. Spatial charge is assumed to vary linearly from cloud to leader tip

2

the downward leader velocity increases as it approach grounded object, creating an increasing electric background field.

the downward leader velocity is constant as the downward leader lengthens, and leader charge constant. The electric field increase towards some limit

3

As the electric field at sharp points increases, multiple upward streamers from the grounded objects begins and alters the electric fields. Corona occurs in bursts depending on electrode curvature

with a critical radius (~ 30 cm) of a grounded object, upward corona forms as the downward leader approaches. corona is stable throughout the increasing electric field

4

corona reintiation depends on the space charges (1 micro Colomb). With the corona bursts an unstable upward leader is formed and advances through some distance

at a certain value (3MV/m) of the electric field strength there is upward leader inception


Lightning protection design analyis method Upward Leader Inception Collection Volume

5

At some level of advancement an unstable leader transitions to a stable leader and accelerates towards the downward leader

the upward leader advances with a constant velocity towards the downward leader tip. the downward leader velocity and the upward leader velocity establish a return stroke path

6

A striking distance is defined as the distance between the downward leader and the position of formation of a stable upward leader

A striking distance is defined as the distance from the location of the grounded object and contact of upward leader to downward leader

7

data from laboratory experiment with elementary configureations, and triggered lightning experiments allows approximation of models of more complex structures

data from laboratory experiment with elementary configureations, and triggered lightning experiments allows approximation of models of more complex structures


Lightning protection design analyis method Upward Leader Inception Collection VolumeAnalysis tool

Femlab finite element electromagnetic modeling software

Propietary(?) "numerical electric field modeling" and object library

Analysis approximations

1

advancing unstable leader space charge is approximated with discrete charges that depend on electrode curvature

field intensification factors may be determined independently of the electric fields that exists with the whole structure under study

2

assymetrical corona zones are modeled as n discrete segments. total charge in the corona zone is built up from descrete segments

field intensification factor may be "modified" by some factor to account for elevation, adjacent structures, etc.

3

background electric field is constant at a leader stabization field value during the travel of the upward unstable leader

lightning protection features, e.g. rods may be modeled independently of the electric fields that exist on an entire structure

4

"competing features" of structures to be protected are modeled with CVM which assumes that the area around that feature is a flat grounded plane


• Next step in the assignment– Task Force consensus recommendation to

the WG for inclusion of ULIM and/or CVM in the revision of IEEE 998

– Continue review of other methods that are not in IEEE 998 – 1996, e.g. Early Streamer Emmission Method


Lightning protection design analyis method Upward Leader Inception Collection VolumeCalculation procedure

1determine the return stoke magnitude from the probability cumulative distribution

determine the return stroke magnitude from the probability cumulative distribution

2

from the return stroke current calculate the downward leader electric field strength vs. position

from the return stroke current calculate the downward leader electric field strength vs. position

2

model the structure and lightning protection features and determine the limit of upward leader inception

model the structure and lightning protection features with the field intensification factors selected from a library


• All models are approximations of the physical phenomena

• Model approximations are necessary to quantify the model– e.g. charge distribution in the downward

leader


• Model quantification requires estimates for the values of model parameters

• Estimates are based on laboratory experiments and field testing– e.g. 3.0 MV/meter for leader inception– e.g. 1.0 micro-colomb charge for corona

charge


• Practical use of analysis tools requires approximations

– e.g. space charge approximated with point charges

– e.g. field intensification factors


• Both ULIM and CVM have

– model approximations– estimates for model variables– calculation approximations

• Discussion revolves around user confidence in the approximations and estimates


• Tools for analysis by practicing engineers

– relatively easy to implement, EMTP is a bad example

– widely accepted by the engineering community

– reasonable to obtain, e.g. public domain, low cost


• Task force conclusions– There are other methods, e.g. ESE, that have

been used and should be addressed– Time may preclude addressing those

additional methods– Task force consensus is still in progress– Suggestions will be given to the WG Chair


• Koushik Chandra reviewed Eq. 2-2A & Eq. 2-2B (probability of lighting stroke magnitude)

• Eq. 2-2A & Eq. 2-2B are critical design bases – probability (risk) of a stroke in the substation

Probability of stroke current magnitude

Probability distribution of peak current:Normal distribution:

……………………(1)

Where P(x) is the probability density function of an event of magnitude xµ = population means = the standard deviation of the populationand –8 = x = 8

Log-normal Distribution: The distribution is for natural logarithm of the variable (x):

……………………..(2)

Approximate Log-normal Distribution:

…………………….(3)

Where Im is the median peak current.

2.

21

.2

1)(

−

−

= σµ

πσ

x

exP

2

lnlnln

.21

ln

.2

1)(

−−

= xxx

x

exP σµ

πσ

6.2

1

1)(

+

=>

m

p

II

IpP

Probability distribution of peak current:

• Approximate log-normal distribution for the probability density of peak current magnitude is accepted by other IEEEs and CIGRE.

• CIGRE (electra 69) and IEEE 1243 suggested a two-sloped log-normal distribution below and above 20kA as the best fit.

• Single slope approximate log-normal distribution adopted by IEEE-998 is also recommended by IEEE 1243, and is supported by several papers including a recent IEEE task force paper and EPRI transmission line reference book.

• The recent data received from NLDN is shown in the IEEE task force paper, which shows distributions very close to log-normal, with a standard deviation less than one kA.

• Based on previous findings on strokes to tall structures and supported by the recent papers and data, the ‘approximate log-normal distribution’ shown in IEEE 998 is recommended to be retained.

Probability distribution of peak current:Peak current magnitude:In support of median peak current magnitude of 31kA for tall structures

and equation 2-2A:• Supported by field data gathered from strikes on tall structures

(Berger) • Widely accepted by scientific community (CIGRE, EPRI

transmission line reference book, IEEE task force paper)

In support of median peak current magnitude of 24kA for flat ground and equation 2-2B:

• Most of the previously collected data (other than NLDN) are based on strikes on tall structures.

• The incidence on a structure depends on the height of the structure. (ref CIGRE No. 69). However, CIGRE does not support any significant dependency on distribution of peak current with increasing structure height.

• Mousa and Srivastava supported a lower peak current distributionon flat ground. Same is supported by Borghetti and Nucci.

• Recent data from NLDN

Probability distribution of peak current:Peak current magnitude:

There might be a measurement error involved in NLDN's data, but it appears that the median peak current tends to be lower with increase of data, increase of minimum pick up level and increaseof number of sensors. At the beginning (1989) the median peak current observed by NLDN were 30kA for negative strokes and 55kA for positive strokes. With increased sensitivity and numberof sensors, the peaks (both positive and negative) approach 22kAin 1998. This is further on the way of lower median, due to better registering low peak current strokes.

In future, we might need to revisit the median value of magnitude of peak current, once the assumptions made in measurement of NLDN are verified.

• 998 reviewed with IEC 62305 (Koushik Chandra), IEEE 1243 (Ken White), IEEE 1410 (Joe Gravelle)

• Each standard has a different scope• There are similar issues, and differences

in approach and equations may cause confusion or misapplication

• Active PAR’s to revise IEEE 1243 (2004), IEEE 1410 (2007)

Review of 998, IEEE 1243, IEEE, 1410

and IEC 62305


and IEC 62305COMPARISON OF IEEE 998, IEC 62305, IEEE 1243, IEEE 1410

IEEE 998 IEC 62305 IEEE 1243 IEEE 1410

Scopeair insulated buses in high voltage electric power substation

"structures" not defined other than structure has human occupancy and the std does NOT cover power lines that are NOT connected to the structure.

transmission lines - overhead line, with ph-to-phase volt >69 kV & conductor height > 10m

distribution overhead lines - ph-to-ph volt 69 kV and less, conductor height <15m, and conductor spacing <2m

characteristics of protected structures

complex physical geometry, small area exposure, low structures (<15m), long term outage probable with strike, large grid(no backflash)

varies with analytical method. For EGM, medium height (10m<h<60m??) structures, geometry not described. For PA, tall structures. For Mesh, large plane surfaces (top of buildings?

simple physical geometry, wide area exposure, tall structures 10m-60m, reclosing limits outage, backflash problem

simple physical geometry, smaller area of exposure for a single circuit, reclosing limits outage, shield wires not generally used

lightning phenomenon model

downward stepped leader & Wagner 1964

downward stepped leader with strikes within 180 degree arc if the horizontal arc distance is less than the distance to the ground plane, i.e. (rc > rg)

vertical (only), downward stepped leader, and indirectly - accumulation of charge on structure vs. ground plane (rc

> rg)use of current magnitude to determine PA implies the downward stepped leader

Analytial method presented EGM EGM, protective angle (30 degree) protective angle (30 & 45 degree)alternative analytical methods

protective angle & emprical (model data)

protective angle (if structure height >EGM radius), Mesh EGM with BIL, EGM (Annex B)


and IEC 62305For EGM probability for return stroke current magnitude

P(I>If) = 1/(1 + (I/ Ifirst)2.6) where Ifirst

is the mean of the probility density function = 31 kA

not stated. Table 2 gives rolling sphere radius for each class of the lightning protection system. System class is determined from structure characteristics

P(I>If) = 1/(1 + (I/ Ifirst)2.6) where Ifirst is

the mean of the probility density function = 31 kA

P(I>If) = 1/ (1 + (I/31)2.6) Note that 31 kA is the assumed mean of the probility density function

For EGM strike distance (sphere radius)

sphere radius r = 8 x k x I0.65

where k is a factor for rod-plane, rod-rod, etc.

not stated. Table 2 gives rolling sphere radius for each class of the lightning protection system. System class is determined from structure characteristics sphere radius r = 10 x I0.65 sphere radius r = 10 x I0.65

"Perfect" shielding due to BIL

voltage surge is stroke current an surge impedance N/A

voltage surge is stroke current an surge impedance

induced voltage not considered included included

• Simultaneous work by all three IEEE WG’s• May perpetuate unwanted differences• Exchange of information may lead to

improvements in 998


and IEC 62305

• Two issues for station BIL and minimum value of stroke current for “perfect shielding”– lightning stroke current has a wave shape that

is significantly different than the standard BIL double exponential voltage waveform.

– the impedance that is used in the EGM to calculate a peak VOLTAGE from a lightning stroke CURRENT

LIGHTNING PARAMETERS AND BIL

• BIL - standardized single, unipolar double exponential front of wave VOLTAGE waveform

• Lightning stroke current –– time variations (impulse shape)– current time derivative (dI/dt - max. value,

time function)


• CIGRE Brochure 63 (1991) lightning stroke 6.64 micro-sec rise time and wave shape front is different than BIL wave front

• Red Book - “The double-exponential voltage wave is a poor approximation to the lighting stroke current … should not be used to describe the input source current in lumped-circuit models.”


• Review Eq. 2-1A through Eq. 2-1E (strike distance vs. stroke current) with more recent information

Task force future work

“bucket brigade” reportewh.ieee.org/cmte/substations/scd0/wgd5/2008-04 san... · lightning...

Documents