Better Lightning Protection with Insulation, Arresters ... · Better Lightning Protection with Insulation, Arresters & Grounds

Better Lightning Protection

with

Insulation, Arresters & Grounds_________________________________________________________________________________________________________________

PowerStream’s FindingsPresentation at

EDIST 2017, Engineering Track A6

Joe Crozier, Standards Engineer, PowerStream

2017.01.19

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Presentation abstract

Lightning is a major cause of distribution system

outages. With annual ground strikes expected to

increase fifty per cent by 2100, utilities need

effective, economic mitigation measures. A recent

lightning protection study at PowerStream

identified 30 industry best practices, most of them

involving better insulation, arresters and grounds.

These are summarized, along with expected

improvement based on experience at

PowerStream and elsewhere.

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Presentation outline

1. Lightning W5

2. Circuit model

3. Grounds

4. Arresters

5. Insulation

6. Q&A

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They helped!

Dave Burns Lori Gallaugher Bill Chisholm

PowerStream Utilities Standards Kinectrics

Forum

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IEEE Std 1410-2010

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Typical heat thunderstorm

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Source: Allan Greenwood

Slo-mo lightning flash

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Waveform parameters

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Source: IEEE Std 1410-2010

First stroke crest amps IF = 31.1 kA

Subsequent strokes IF = 12.3 kA

Median strokes per flash N = 3.4

Inter-stroke interval 35 ms

First stroke IF/Sm = tm = 1.28 μs

Subsequent strokes tm = 0.308 μs

First stroke half-value tn = 77.5 μs

Subsequent strokes tn = 30.2 μs

CDF of 1st-stroke peak kA

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Median first stroke

current = 31 kA

CDF of 1st-stroke peak kA

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Median first stroke

current = 31 kA

First stroke > 100 kA

2% probability

Lightning, Canada West

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Source: Environment Canada

Lightning, Ontario

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Lightning, Southern Ont.

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Flash density by the numbers

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“STRUCK BY LIGHTNING”

“The temperature of the electrical arc can reach 30,000

degrees Celsius … When lightning hits the ground, in most

cases it will be on an elevated point, such as the top of a

mountain, a high-rise structure, a component of an electric

distribution network, etc … When lightning directly strikes on

an electric distribution network, it inevitably generates

devastating consequences … network power distribution

equipment can be damaged … most of the regular protection

mechanisms will not be able to withstand the power generated

by the electrical discharge of a lightning strike”

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Source: David Savard, CEP

Global warming & lightning

“Lightning plays an important role in atmospheric chemistry …

we propose that … flash rate is proportional to the convective

available potential energy (CAPE) times the precipitation rate.

Using observations, the product of CAPE and precipitation

explains 77% of the variance in the time series of total cloud-

to-ground lightning flashes over the contiguous United States

(CONUS). Storms convert CAPE times precipitated water

mass to discharged lightning energy with an efficiency of 1%.

When … applied to 11 climate models, CONUS lightning strikes are predicted to increase 12 ±5% per degree Celsius

of global warming and about 50% over this century.”

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Source: Science, 14 Nov 2014

Lightning W5

• Median first stroke 30 kA but can exceed 100 kA

• Microsecond event times (BIL wave 1.2 x 50 μs)

• 2.34 million flashes/year in Canada, about once

every three seconds during the summer months

• Hot spots southwest and southcentral ON

southwest MB & southeast SK

central AB east of Rockies

northeast BC (Peace region)

• Possible 50% increase in flash density by 2100

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Circuit model

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Source: T.A. Short, Electric Power Distribution Handbook

Circuit model

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400 ohms surge impedance

Circuit model

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Circuit model

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1. Pole ground resistance

2. Lightning

arresters3. Insulation

Pole ground resistance

O.C. Seevers, P.E. wrote several electrical

engineering books based on his experience at

Kentucky Utilities from 1947 to 1991:

• “Wherever we have had repeated lightning

damage we have found high resistance

measurements to ground. Wherever we have

improved those grounds to near zero, the

lightning damage has ceased.”

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Source: O.C. Seevers, Power Systems Handbook


• “About 8 years ago, I started a program of

testing and improving all grounds on equipment

poles in rural areas, by contractor. About half of

my territory is on limestone. The rest has sandy,

clay-type soil. If the ground measures over 10

ohms, we add a rod, and re-measure. If still over

10 ohms, we add one more rod and then give

up.”

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Source: O.C. Seevers, P.E., Power Systems Handbook


• “ … in the areas we have covered, our expense

due to lightning damage has been reduced to

one-fifth of the cost before grounding. I set out

to cover my division in ten years. The savings

each year have more than paid for the annual

program cost. When you consider that the

savings will continue forever and the costs will

cease two years from now, you get some feel for

the enormous benefit we will reap on out into the

future.”

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• “We have installed arresters on long distribution

lines where lightning damage was a regular

visitation. We installed them every few miles

and made sure the grounds were good

(emphasis added – JC). The trouble just

stopped. No more trouble at all.”

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• Seevers’s experience supports 10 Ω as standard

• The common industry standard is 25 Ω

• Q: is 25 Ω an adequate, ‘good enough’ target?

• Q: would 10 Ω be too stringent? uneconomic?

• Lori Gallaugher (executive director, USF) asked

John O’Neill (project manager, CSA std. C22.3)

• Apparently no engineering studies support 25 Ω

as an adequate standard for ground resistance

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Source: Lori Gallaugher and John O’Neill, July 9, 2015

PowerStream ground tests

• Measured ground Ω near all pole-mounted LISs

and ScadaMates in Markham, Richmond Hill &

Vaughan that failed from 2012 to 2015.

• For each of the three municipalities, picked a

representative control sample of poles in the

vicinity of switches that didn’t fail 2012-15. Used

stratified/cluster random sampling to ensure the

conclusions we drew from the data were valid.

• Analysis, conclusions, recommendations

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Source: PowerStream


• Used AEMC model 3700 clamp-on ground

resistance tester for all tests.

• Verified accuracy by measuring several pole

down-grounds using clamp-on testers from two

other manufacturers, Fluke and Megger. The

three manufacturers’ testers yielded results

within 3% of one another for readings up to 10Ω

and within 11% for readings up to 100Ω –

considered accurate enough for our purposes

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Source: PowerStream


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Source: PowerStream

Markham 91 8 8.80% 2.20%

RH 26 1 3.80% 1.00%

Vaughan 109 4 3.70% 0.90%

All 226 13 5.80% 1.40%

Area

LIS

Scadamate

failures

Number

of LISs &

Scadamates

Failures

per

cent

Failures

%/year

Note: failure history 2012-15


Failure rate almost exactly proportional to median ground ohms

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Source: PowerStream

Area Markham Vaughan

LIS/ScadaMate failures 8 4

Number of LISs/ScadaMates 91 109

Failure rate, % 8.8% 3.7%

Failure rate, %/year 2.2% 0.9%

Median ground ohms 15 6.5

Max. ground ohms 71 30

Readings < 10 ohms 37.5% 76.5%

Readings < 25 ohms 80.0% 94.1%

Note: failure data from 2012 to 15


• Despite our small sample size, our results at

PowerStream appear consistent with Seevers’s

• Despite the encouraging concrete pole ground

resistance results, we don’t yet know if a 10 Ω

standard is economic on all equipment poles

• Costliest replacements – automated switches

Recommendation:

• Max. 10 Ω ground on all automated switches

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Source: PowerStream


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Source: PowerStream

What about:

Manual SWs?

TXOHs?

Cable risers?

3ph vs. 1ph?

Later!

Why?

$$$ ???

Arresters

• Provide overvoltage protection for equipment

insulation such as transformers and regulators

• Function as high impedances at normal operating

voltages and become low impedances during

lightning surge conditions.

• Conduct surge current to the ground while limiting

the voltage on the equipment to the sum of the

discharge voltage of the arrester plus the inductive

voltage developed by the discharge current in

arrester line and ground leads.

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Arresters

• asdf

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Arresters

• asdf

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Scout arresters

• Another option for protecting cables is to use scout

arresters, arresters applied on the overhead line on

both sides of the riser pole

• A scout arrester intercepts and diverts a lightning

current that is heading towards the riser pole

• Since most of the current conducts through the

closest arrester, less voltage gets in the cable at

the riser pole (unless lightning hits almost right at

the cable).

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Scout arresters

Scout arrester effectiveness depends on grounding the scout

arresters well. Without good grounding, the ground potential

rises at the scout arrester, causing high voltage on the phase

and neutral wire (but little voltage difference between them).

When the surge arrives at the riser pole, the low impedance

ground path offered by the cable drops the neutral potential

(and increases the phase-to-neutral voltage). This pulls

significant current through the riser-pole arrester (and sends a

voltage wave down the cable), which reduces the

effectiveness of the scout arresters. The lower the impedance

of the scout arrester grounds, the less this effect occurs.

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Scout arresters

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Scout arresters

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What about:

Manual SWs?

TXOHs?

Cable risers?

3ph vs. 1ph?

Later!

Why?

$$$ ???

Insulators

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3.4 critical impulse flashover

voltage (CFO) (insulators): The

crest value of the impulse wave that,

under specified conditions, causes

flashover through the surrounding

medium on 50% of the applications.

Source: IEEE Std 1410-2010S

Insulators

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Source: MacLean Power Systems

CFO = 130 kV

Insulators

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CFO = 200 kV

Source: MacLean Power Systems

Insulators

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Source: K-Line Insulators

15 kV

CFO = 130 kV

46 kV

CFO =

130 kV

Flashovers and CFO

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Flashovers & Sparkovers

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• A flashover involves a surface. Thus, if a surge causes

a spark to propagate across the surface of a bushing [or

an insulator], it would be a flashover.

• A sparkover occurs across a gap. It may be, for

instance, across a protective rod gap. [Or between

rebar and bolts]

• Why is this distinction important?

Wood poles can contribute significantly to the CFO of

the entire insulation path from primary to ground

Concrete poles contribute a CFO of 0 kV!


Richmond Hill bolt eroded

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Source: PowerStream


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Source: PowerStream


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Source: PowerStream

35 kV insulator!

Today’s standard insulator

for 27.6 kV circuits – 46 kV

Markham concrete pole pilot

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Source: PowerStream


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Source: PowerStream


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Source: PowerStream

Flashovers and CFO

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Ideal ground

100 Ω·m ground

Presentation summary

1. Lightning W5: flashes/(km2·yr) ↑

2. Circuit model: simple, not simplistic

3. Grounds: auto switches: 25 Ω 10 Ω

4. Arresters: scouts on auto switches

5. Insulation: 46 kV KLI on 27.6 kV ccts

6. Q&A

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Better Lightning Protection with Insulation, Arresters ... · Better Lightning Protection with Insulation, Arresters & Grounds