Cigré WG A3.25 meeting San Diego October 16, 2012 Max 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. TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 1
Contents Single impulse energy handling capability (A3.17) Energy handling capability for double impulse stresses Repeated AC energy impacts Repeated AC versus 90/200 µs energy impacts Thermal stability of complete arresters and related simulation Max1's project Max2's project TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 2
Several thousand samples from seven manufacturers worldwide Most extensive energy handling research program on MO resistors so far 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 TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 3
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 TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 4
Flowchart of the Test Procedure initial measurement energy test with impulse OK mechanical lyfailed No exit measurement failed Yes New approach! initial measurement U ch1 at I ch = 0.12 ma/cm² (after 5 s) P ct1 at 0.8 x U ch1 (after 1 min) U res1 at I = I N U ch... "characteristic" voltage; indicates changes of the U-I-characteristic in the continuous operating range; may be U ref exit measurement U ch2 at I ch = 0.12 ma/cm² (after 5 s) P ct2 at 0.8 x U ch1 (after 1 min) U res2 at I = I N U res3 at I = 1.5 ka/cm² TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 5
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!!! 95% U U 105% U ch1 ch2 ch1 Yes mechanically failed during residual voltage tests No Yes!!! No 95% U U 105% U res1 res2 res1 Yes OK No failed TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 7
mean failure energy in Overview about research project Preliminary Results 50% Failure Energy Size 1 (diameter 60 mm, height 40 mm) Manufacturers S, T, U, V, X, Z 1800 1600 1400 1200 1000 800 600 400 200 0 8 s AC 100 ms 0.1 1 10 100 1000 10000 peak current density in A/cm² Note: "rated" energies usually specified in the range 200 300 TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 8 S T U V X Z 4 ms 250 µs Failed by puncture and flashover of the coating system!
W "Bath tub curve"? Switching duty 4ms 100 ms 10 s t No minimum of energy handling capability for switching surges There may be a minimum at > 10 s difficult to investigate (nonadiabatic) TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 9
Failure mechanisms: % 100 80 60 40 20 0 S CR BR FO ÜB PU DU MF Uch Uref Ures 4 ms 2 ms 1 ms 90/200 µs CR FO PU MF Uch... Ures... Cracking Flashover Puncture Mechanical failure during exit measurement Change of "characteristic" voltage Change of residual voltage % 100 80 60 40 20 0 U CR BR FO ÜB PU DU MF Uch Uref Ures 4 ms 2 ms 1 ms 90/200 µs Impulse shapes: 4/10 µs 90/200 µs 1 ms 2 ms 4 ms TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 10
Failure mechanisms: % 100 80 60 40 20 0 X CR BR FO ÜB DU PU MF Uch Uref Ures 4 ms 2 ms 1 ms 90/200 µs CR FO PU MF Uch... Ures... Cracking Flashover Puncture Mechanical failure during exit measurement Change of "characteristic" voltage Change of residual voltage Impulse shapes: 4/10 µs 90/200 µs 1 ms 2 ms 4 ms TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 11
Comparison: with and without complex failure criterion TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 12
Mean failure energy in Size 2 (diameter 40 mm, height 40 mm) Manufacturers S, U, V, W, Y 1800 1600 1400 1200 1000 800 600 400 200 0 4 ms 1 ms 90/200 µs 4/10 µs 10 100 1000 10000 100000 Amplitude of current density in A/cm² S U V W X Y Failed by change of U ch! TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 13
Failure mechanisms: 100 80 60 % S 40 20 0 CR BR FO ÜB PU DU MF Uch Uref Ures 4 ms 1 ms 4/10 µs CR FO PU MF Uch... Ures... Cracking Flashover Puncture Mechanical failure during exit measurement Change of "characteristic" voltage Change of residual voltage 100 80 % 60 40 U 20 0 CR BR FO PU ÜB DU MF Uch Ures MF Uref Ures 1 ms 4/10 µs 4 ms % 100 80 60 40 20 0 V CR BR FO ÜB DU PU MF Uref Uch Ures 4 ms 1 ms 4/10 µs Impulse shapes: 4/10 µs 90/200 µs 1 ms 2 ms 4 ms % 100 80 60 40 20 0 CR BR FO ÜB PU DU MF Uref Uch Ures W 4 ms 1 ms 4/10 µs TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 14
Failure mechanisms: CR FO PU MF Uch... Ures... Cracking Flashover Puncture Mechanical failure during exit measurement Change of "characteristic" voltage Change of residual voltage 100 80 60 % Y 40 20 0 CR BR FO ÜB DU PU MF Uref Uch Ures 2 ms 4/10 µs Impulse shapes: 4/10 µs 90/200 µs 1 ms 2 ms 4 ms TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 15
Preliminary Results 50% Failure Energy Change of Characteristic Voltage for Size 2 change of char. voltage in % 5 100 ka î in A 40000 0 60000 80000 100000 120000 140000 160000 180000 200000 220000-5 -10-15 -20-25 -30-35 S U V W Y -40 TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 16
Mean value of current density amplitude in A/cm² 10000 1000 100 10 1 Different failure criteria 0.1 Ringler's varistors: diam. 62..64 mm, height: 23..24 mm Cigré varistors: diam. 60 mm, height 40..45 mm 0.1 1 10 100 1000 10000 Time in ms S T U V X Z [Rin 1997] TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 17
Mean failure energy in 1800 1600 1400 1200 1000 800 600 400 Overview about research project 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! S T U V X Z [Ringler 1997] 200 a.c. 0 0.1 1 10 100 1000 10000 Amplitude of current density in A/cm² 4 ms 1 ms TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 18 90/200 µs Different failure criterion But 90/200 µs impulses give impulse energy values lower than expected (influence of coating system!) Different failure mechanism for 90/200 µs
Problem for Energy Specification: Outliers 1200 1000 Energie in 800 600 400 200 0 1 5 9 13 17 21 25 29 33 37 41 45 49 53 Versuchsnummer TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 19
Preliminary Results 50% Failure Energy Conclusions 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. TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 20
Energy handling capability for double impulse stresses up to mechanical failure 15 10 LD current impulses 3 mechanical failure of MOV 2 U in kv 5 0 1 0 I in ka 0 50 100-5 -1-10 -2 t in ms TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 21
Energy handling capability for double impulse stresses up to mechanical failure double impulse stresses single impulse stresses 2 x 1.85 ms, d = 80 ms 2 x 1.85 ms, d = 3 s 40 MOV 1 make size 2 1 x 2 ms 1 x 4 ms TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 22
Energy handling capability for double impulse stresses up to mechanical failure double impulse single impulse impulse length/ Time interval 1.85 ms/ 3 s 1.85 ms/ 80 ms 2 ms 4 ms (sum) mean failure energy in p.u. Coefficient of variation 1,04 1,02 1,02 1,0 0,09 0,12 0,10 0,07 No difference in energy handling capability! TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 23
initial measurement n times Repeated stresses energy pre-stress energy impact with AC cool down to ambient temperature exit measurement application of energy TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 24
Repeated stresses initial measurement 1,4 n times 1,2 energy pre-stress cool down to ambient temperature exit measurement failure energy in p.u. 1 0,8 0,6 0,4 0,2 No change in energy handling capability by ac pre-stresses! application of energy 0 0 20 40 60 80 100 number of previous impacts TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 25
Repeated stresses initial measurement energy pre-stress cool down to ambient temperature exit measurement n times application of energy mean change of U ch in % 5 4 3 2 1 0 0 20 40 60 80 100 number of previous impacts TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 26
initial measurement n times energy pre-stress cool down to ambient temperature energy impact with AC vs. energy impact with 90/200 µs exit measurement application of energy TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 27
Energy impact with AC vs. 90/200 µs Energy impact *) max. 20 impulses or max. 20 times 4 cycles Sample No. (each box = one sample; 20 samples per kind of stress 90/200 µs Impulse *) 4 cycles AC *) 200 300 400 200 300 400 #1 TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 28
Energy impact with AC vs. 90/200 µs Energy impact *) max. 20 impulses or max. 20 times 4 cycles Sample No. (each box = one sample; 20 samples per kind of stress 90/200 µs Impulse *) 4 cycles AC *) 200 300 400 200 300 400 #2 TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 29
Energy impact with AC vs. 90/200 µs Energy impact *) max. 20 impulses or max. 20 times 4 cycles Sample No. (each box = one sample; 20 samples per kind of stress 90/200 µs Impulse *) 4 cycles AC *) 200 300 400 200 300 400 #3. and so on TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 30
Energy impact with AC vs. 90/200 µs Energy impact x Varistor failure at impulse no. x 90/200 µs Impulse 4 cycles AC 200 300 400 200 300 400 15 3 13 14 15 15 15 16 16 16 16 17 17 18 19 19 19 8 1 1 1 1 1 1 1 3 7 11 19 TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 31
Energy impact with AC vs. 90/200 µs Energy impact x Varistor failure at impulse no. x 90/200 µs Impulse 4 cycles AC 200 300 400 200 300 400 15 3 13 14 15 15 15 16 16 16 16 17 17 18 19 19 19 8 1 1 1 1 1 1 1 3 7 11 19 TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 32
failure energy in p.u. *) Application of energy (AC up to mechanical failure) after 20 pre-stresses 1.25 1.00 0.75 0.50 0.25 *) 1 p.u. = mean AC failure energy 0.00 Pre-stress: TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 33
Energy impact with AC vs. 90/200 µs Summary/ Conclusions: for 400 at 90/200 µs many mechanical failures of MOV, but only after a high number (usually > 10) of stresses for 300 and 400 at AC many mechanical failures just at the first impulse for 400 at AC high failure rate at high number of stresses for 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-stresses for 200 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 TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 34
Energy handling capability of used MOV (from grid) TUD High-Voltage Lab Cigré WG A3.25 meeting Paris France, August 28, 2012 35
MO resistor TUD High-Voltage Lab Cigré WG A3.25 meeting 36/??
Energy handling capability of used MOV (from grid) Main failure mechanism: Change of characteristic voltage TUD High-Voltage Lab Cigré WG A3.25 meeting Paris France, August 28, 2012 37
Thermal stability limit of complete EHV/UHV arresters TUD High-Voltage Lab Cigré WG A3.25 meeting Paris France, August 28, 2012 38
TUD High-Voltage Lab Cigré WG A3.25 meeting Paris France, August 28, 2012 39
energy impact U c U c TUD High-Voltage Lab Cigré WG A3.25 meeting Paris France, August 28, 2012 40
t ambient 16 C TUD High-Voltage Lab Cigré WG A3.25 meeting Paris France, August 28, 2012 41
Arithmetic mean temperature, above ambient temperature / K thermal equival. arrester with grading arrester without grading 213,6 215,8 215,0 220,8 212,5 223,5 t ambient 16 C TUD High-Voltage Lab Cigré WG A3.25 meeting Paris France, August 28, 2012 42
Thermal stationary temperature at diff. ambient temperatures 250 t ambient : height / cm 200 150 100 50 22 C 30 C 40 C 40 C* *with grading ring 0 0 5 10 15 20 temperature (above ambient temperature) / K TUD High-Voltage Lab Cigré WG A3.25 meeting Paris France, August 28, 2012 43
Thermal stability limit at different ambient temperatures 250 22 C stable 22 C instable 200 30 C stable 30 C instable 40 C stable 40 C instable height / cm 150 100 50 0 180 190 200 210 220 230 240 250 absolute temperature / C TUD High-Voltage Lab Cigré WG A3.25 meeting Paris France, August 28, 2012 44
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 overtemperature under 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). TUD High-Voltage Lab Cigré WG A3.25 meeting Paris France, August 28, 2012 45
Follow-up work in progress 1. Reproduction of the experimentally found thermal behavior by a coupled thermal and non-linear resistive/capacitive FEM simulation 2. Application and validation of the simulation model to simulate thermal stability under real conditions (temperature rise adiabatically and in zero time) Temperature Time Experiment Simulation 3. Simulation: "Playing" with the external grading system for optimization purposes TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 46
TUD High-Voltage Lab Cigré WG A3.25 meeting San Diego, October 16, 2012 47