Overview about research project “Energy handling capability”

TUD High-Voltage Lab | Cigré WG A3.25 meeting | San Diego, October 16, 2012 | 1

Overview about research project “Energy handling capability”

Cigré WG A3.25 meetingSan Diego October 16, 2012Max Tuczek, Volker Hinrichsen, TU Darmstadt

Note: all information beginning from slide 21 are provisional results in the frame of Cigré WG A3.25 work, subject to possible corrections and extensions and not yet published. They shall, therefore, be used for personal or internal information only and not be further distributed. Care should be taken when conclusions shall be drawn.


Contents

Single impulse energy handling capability (A3.17)Energy handling capability for double impulse stressesRepeated AC energy impactsRepeated AC versus 90/200 µs energy impactsThermal stability of complete arresters and related simulation


Max1's project

Max2's project


MO resistors, Size 1• Diameter (55…65) mm Typical for a Class 3 station arrester• Height (35…45) mm

MO resistors, Size 2• Diameter (37….45) mm Typical for a Class 1/10-kA arrester• Height (35…45) mm

Different aspect ratios

different failure mechanisms

• Several thousand samples from seven manufacturers worldwide• Most extensive energy handling research program on MO resistors so far



Test current impulse shapes standard currents as per IEC 60099-4

Long duration current impulse• 1 ms• 2 ms• 4 ms

• Lightning discharge current 90/200 µs• High current impulse 4/10 µs: 65 kA … 200 kA

... plus non-standard stress:

Alternating current 50 Hz• î ≈ 10 A• î ≈ 100 A• î ≈ 300 A



initial measurementUch1 at Ich = 0.12 mA/cm² (after 5 s)Pct1 at 0.8 x Uch1 (after 1 min)Ures1 at I = IN

exit measurementUch2 at Ich = 0.12 mA/cm² (after 5 s)Pct2 at 0.8 x Uch1 (after 1 min)Ures2 at I = INUres3 at I = 1.5 kA/cm²

Flowchart of the Test Procedure

Uch ... "characteristic" voltage; indicates changes of the U-I-characteristic in the continuous operating range; may be Uref

energy test with impulse

No

OK failed

Yes

initial measurement

exit measurement

mechanicallyfailed

New approach!



Failure modes important for manufacturers

The different physical failure modes under single impulse stress (puncture, cracking, flashover, change of U-I-characteristics) are usually not of interest to the end-user.

But they do allow the manufacturers to assess their material and to design and optimize it with regard to particular aspects of energy handling capability.TUD High-Voltage Lab | Cigré WG A3.25 meeting | San Diego, October 16, 2012 | 6


exit measurement

OK failed

95 105ch1 ch2 ch1% %U U U⋅ ≤ ≤ ⋅

95 105res1 res2 res1% %U U U⋅ ≤ ≤ ⋅

mechanically failedduring residual voltage tests

Yes

Yes

Yes

NoNo

No!!!

!!!



Size 1 (diameter ≈ 60 mm, height ≈ 40 mm)Manufacturers S, T, U, V, X, ZSize 1 (diameter ≈ 60 mm, height ≈ 40 mm)Manufacturers S, T, U, V, X, Z

Preliminary Results – 50% Failure Energy

Note: "rated" energies usually specified in the range 200 … 300 J/cm³

Failed by puncture and flashover of the coating system!

0200400600800

10001200140016001800

0.1 1 10 100 1000 10000

mea

nfa

ilure

ener

gyin

J/

cm³

peak current density in A/cm²

STUVXZ

AC

≈ 8 s

≈ 100 ms≈ 4 ms

≈ 250 µs



WW

t

"Bath tub curve"WW

t

"Bath tub curve"

4ms 100 ms 10 s

• No minimum of energy handling capability for switching surges

Switchingduty

?

• There may be a minimum at > 10 s difficult to investigate (non-adiabatic)



BR ÜB DU MF Uref Ures90/200 µs

1 ms2 ms

4 ms

0

20

40

60

80

100

% S

CR FO PU Uch Ures MFBR ÜB DU MF Uref Ures

90/200 µs1 ms

2 ms4 ms

0

20

40

60

80

100

% S

CR FO PUBR ÜB DU MF Uref Ures90/200 µs

1 ms2 ms

4 ms

0

20

40

60

80

100

% S

CR FO PU Uch Ures MF

BR ÜB DU MF Uref Ures

90/200 µs1 ms

2 ms4 ms

0

20

40

60

80

100

%

U

CR FO PU MF Uch Ures

BR ÜB DU MF Uref Ures

90/200 µs1 ms

2 ms4 ms

0

20

40

60

80

100

%

U


Failure mechanisms:CR … CrackingFO … FlashoverPU … PunctureMF … Mechanical

failure during exit measurement

Uch ... Change of "characteristic"voltage

Ures... Change of residual voltage

Impulse shapes:4/10 µs90/200 µs 1 ms 2 ms 4 ms








BR ÜB DU MF Uref Ures90/200 µs1 ms2 ms4 ms

0

20

40

60

80

100

% X

CR FO PU Uch Ures MF BR ÜB DU MF Uref Ures90/200 µs1 ms2 ms4 ms

0

20

40

60

80

100

% X

CR FO PUBR ÜB DU MF Uref Ures90/200 µs1 ms2 ms4 ms

0

20

40

60

80

100

% X

CR FO PU Uch Ures MF



Comparison: with and without complex failure criterion



0200400600800

10001200140016001800

10 100 1000 10000 100000

Mea

n fa

ilure

ene

rgy

in

J/cm

³

Amplitude of current density in A/cm²

SUVWXY

4/10 µs

90/200 µs

4 ms1 ms

Size 2 (diameter ≈ 40 mm, height ≈ 40 mm)Manufacturers S, U, V, W, Y Size 2 (diameter ≈ 40 mm, height ≈ 40 mm)Manufacturers S, U, V, W, Y

Failed by change of Uch!



MFBR ÜB DU MF Uref Ures

4/10 µs1 ms

4 ms

0

20

40

60

80

100

% V

CR FO PU Uch Ures MFBR ÜB DU MF Uref Ures

4/10 µs1 ms

4 ms

0

20

40

60

80

100

% V

CR FO PU Uch Ures


1 ms4 ms

0

20

40

60

80

100

% S

CR FO PU MF Uch Ures BR ÜB DU MF Uref Ures

4/10 µs1 ms

4 ms

0

20

40

60

80

100

% S


BR ÜB

DU

MF

Ure

f

Ure

s

4/10 µs

1 ms

4 ms0

20

40

60

80

100

% U

CRFO

PUMF Uch

Ures

BR ÜB

DU

MF

Ure

f

Ure

s

4/10 µs

1 ms

4 ms0

20

40

60

80

100

% U

CRFO

PUMF Uch

Ures


1 ms4 ms

0

20

40

60

80

100

% W

CR FO PU MF Uch Ures BR ÜB DU MF Uref Ures

4/10 µs1 ms

4 ms

0

20

40

60

80

100

% W










2 ms0

20

40

60

80

100

% Y

CR FO PU MF Uch Ures BR ÜB DU MF Uref Ures4/10 µs

2 ms0

20

40

60

80

100

% Y









-40

-35

-30

-25

-20

-15

-10

-5

0

540000 60000 80000 100000 120000 140000 160000 180000 200000 220000

î in A

chan

ge o

f cha

r. v

olta

ge in

%

SUVWY

100 kA

Preliminary Results – 50% Failure EnergyChange of Characteristic Voltage for Size 2



Different failure criteria0.1

1

10

100

1000

10000

0.1 1 10 100 1000 10000

Mea

n va

lue

of c

urre

nt d

ensi

ty

ampl

itude

in A

/cm

²

Time in msS T U V X Z [Rin 1997]

Ringler's varistors: diam. 62..64 mm, height: 23..24 mmCigré varistors: diam. 60 mm, height 40..45 mm



0200400600800

10001200140016001800

0.1 1 10 100 1000 10000

Mea

n fa

ilure

ene

rgy

in J

/cm

³

Amplitude of current density in A/cm²

STUVXZ[Ringler 1997]

a.c.

4 ms

90/200 µs

1 ms

• Compared with former investigations (Ringler et al., 1997 see orange curve), an increase of (10…20)% in energy handling capability can be observed. These are the good news for the user!

Different failure criterion

Different failure mechanism for 90/200 µs

• But 90/200 µs impulses give impulse energy values lower than expected (influence of coating system!)



Problem for Energy Specification: “Outliers”

0

200

400

600

800

1000

1200

1 5 9 13 17 21 25 29 33 37 41 45 49 53Versuchsnummer

Ener

gie

in J

/cm

³



Preliminary Results – 50% Failure EnergyConclusions so far

• Compared with former investigations (Ringler 1997), energy handling capability has increased by 10…20 %.

• 50% failure energy is 4...5 times higher than actually specified "rated" energies; no figures can be derived for extremely low failure probabilities (<< 1%).

• The linear "log (current) vs. log (time to failure)" (Ringler) dependence could be verified, except for the new 90/200 µs impulse, where puncture and flashover of the coating may become the limiting factor potential for improvement; important for line arrester applications.

• Varistors for station and distribution application were directly compared only minor differences by different aspect ratios.

• "Mechanical failure" and "visible damage" are not sufficient failure criteria; changes of U-I-characteristics have to be considered as well.



-10

-5

0

5

10

15

0 50 100

t in ms

U in

kV

-2

-1

0

1

2

3

I in

kA

Energy handling capability for double impulse stressesup to mechanical failure

mechanical failure of MOV


LD current impulses



double impulse stresses

single impulse stresses

2 x 1.85 ms, d = 80 ms

2 x 1.85 ms, d = 3 s

1 x 2 ms

1 x 4 ms

40 MOV

1 make

size 2



double impulse single impulse

impulse length/Time interval

1.85 ms/

3 s

1.85 ms/

80 ms2 ms 4 ms

(sum) mean failure energy

in p.u.1,04 1,02 1,02 1,0

Coefficient of variation 0,09 0,12 0,10 0,07


No difference in energy handling capability!



initial measurement

energy pre-stress

exit measurement

cool down to ambient temperature

application of energy

n times

energy impact with AC

Repeated stresses



initial measurement

energy pre-stress

exit measurement



n times

0

0,2

0,4

0,6

0,8

1

1,2

1,4

0 20 40 60 80 100

failu

re e

nerg

y in

p.u

.

number of previous impacts

Repeated stresses

No change in energy handling capability by ac pre-stresses!



initial measurement

energy pre-stress

exit measurement



n times

0

1

2

3

4

5

0 20 40 60 80 100

mea

n ch

ange

of U

chin

%

number of previous impacts

Repeated stresses



initial measurement

energy pre-stress

exit measurement



n times

energy impact with AC vs.

energy impact with 90/200 µs



Energy impact with AC vs. 90/200 µs

Energy impact

90/200 µs

Impulse*)

200 J/cm³300 J/cm³400 J/cm³

4 cycles AC *)

200 J/cm³300 J/cm³400 J/cm³

*) max. 20 impulses or max. 20 times 4 cycles

Sample No. (each box = one sample; 20 samples per kind of stress


#1



Energy impact

90/200 µs

Impulse*)

200 J/cm³300 J/cm³400 J/cm³

4 cycles AC *)

200 J/cm³300 J/cm³400 J/cm³




#2



Energy impact

90/200 µs

Impulse*)

200 J/cm³300 J/cm³400 J/cm³

4 cycles AC *)

200 J/cm³300 J/cm³400 J/cm³




#3 …. and so on



Energy impact Varistor failure at impulse no. x

90/200 µs

Impulse

200 J/cm³

15

300 J/cm³400 J/cm³

3 13 14 15 15 15 16 16 16 16 17 17 18 19 19 19

4 cycles

AC

200 J/cm³

8

300 J/cm³

1 1 1 1

400 J/cm³

1 1 1 3 7 11 19

x




Energy impact Varistor failure at impulse no. x

90/200 µs

Impulse

200 J/cm³

15

300 J/cm³400 J/cm³

3 13 14 15 15 15 16 16 16 16 17 17 18 19 19 19

4 cycles

AC

200 J/cm³

8

300 J/cm³

1 1 1 1

400 J/cm³

1 1 1 3 7 11 19

x



Application of energy (AC up to mechanical failure) after 20 pre-stresses

0.00

0.25

0.50

0.75

1.00

1.25

failu

re e

nerg

y in

p.u

.*) *) 1 p.u. = mean AC failure energy

Pre-stress:



Summary/ Conclusions:for 400 J/cm³ at 90/200 µs many mechanical failures of MOV, but only after a high number (usually > 10) of stressesfor 300 J/cm³ and 400 J/cm³ at AC many mechanical failures just at the first impulsefor 400 J/cm³ at AC high failure rate at high number of stressesfor 90/200 µs remarkable reduction of energy handling capability; distinct decrease with increasing magnitude of pre-stress impulses; remaining max. failure energy = 10% of the mean AC failure energy!! for AC virtually no impact on energy handling capability by pre-stressesfor 200 J/cm³ at AC those samples failed at very low energy levels, which probably would have had failed at higher magnitude of pre-stresses Routine tests at AC considered more sensitive than LD impulse testing




Energy handling capability of “used” MOV (from grid)

TUD High-Voltage Lab | Cigré WG A3.25 meeting | Paris France, August 28, 2012 | 35


TUD High-Voltage Lab | Cigré WG A3.25 meeting | 36/??

MO resistor


Energy handling capability of “used” MOV (from grid)


Main failure mechanism:Change of characteristic voltage


Thermal stability limit of complete EHV/UHV arresters





Uc Uc

energy impact



t ambient ≈ 16 °C



Arithmetic meantemperature, aboveambient temperature / K

thermalequival. 213,6 215,8

arresterwithgrading

215,0 220,8

arresterwithoutgrading

212,5 223,5

t ambient ≈ 16 °C



Thermal stationary temperature at diff. ambient temperatures

0

50

100

150

200

250

0 5 10 15 20temperature (above ambient temperature) / K

heig

ht /

cm

22 °C

30 °C

40 °C

40 °C*

t ambient:

* with grading ring



Thermal stability limit at different ambient temperatures

0

50

100

150

200

250

180 190 200 210 220 230 240 250

absolute temperature / °C

heig

ht /

cm

22 °C stable 22 °C instable





Conclusions so far…. • Findings suggest that the importance of field grading may have

been overestimated in the past.

• Evidently, perfect grading is not essential to achieve.

• The manufacturer has mainly to determine the permissible operating temperatures in the upper part of the arrester, which is primarily a matter of material. A higher average overtemperatureunder continuous operation stress (U = Uc) will also reduce the thermal energy handling capability of the arrester, because the average temperature of an ungraded arrester will be higher than that of perfectly graded one (see slide 43).




Follow-up work in progress 1. Reproduction of the

experimentally found thermal behavior by a coupled thermal and non-linear resistive/capacitiveFEM simulation

2. Application and validation of thesimulation model to simulatethermal stability under real conditions (temperature riseadiabatically and in zero time)

3. Simulation: "Playing" with theexternal grading system foroptimization purposes

Tem

pera

ture

Time

Experiment

Simulation



Documents

Overview about research project “Energy handling capability”