A 10% overvoltage surge (e.g., 480 VAC nominal to 528 VAC) lasting 5 seconds causes no permanent reduction in the dielectric strength of a 2 mm PFA sheath when tested on a new heater. However, on a heater that has already aged for 1,000 hours at 150°C, the same surge can reduce the dielectric strength by 15–30%. Aging at high temperature creates microscopic voids and chemical degradation (chain scission, oxidation) that weaken the polymer's electrical insulation properties. The overvoltage surge stresses these weakened areas, creating carbonized tracks (partial discharge) that do not heal. The dielectric strength (breakdown voltage) drops from 20–25 kV/mm to 14–18 kV/mm after the surge. For a 2 mm wall, the breakdown voltage falls from 40–50 kV to 28–36 kV – still above the 680 V peak operating voltage (safety margin 40–50×). The heater remains safe for operation, but the safety margin is reduced. Multiple surges accumulate damage. For aged heaters (>5,000 teev), ib qho 10% surge tuaj yeem ua rau tsis ua haujlwm.
Qhov cuam tshuam ntawm Kev Laus thiab Overvoltage ntawm Dielectric Strength
| Cua sov txias | Dielectric zog (kV / mm) Ua ntej Surge | Tom qab 10% Overvoltage (5 sec) | Kev txo qis (%) | Fais fab tuag rau 2 hli (kV) | Kev nyab xeeb Margin (peak 680V) |
|---|---|---|---|---|---|
| Tshiab (0 teev) | 20–25 | 20–25 | 0% | 40–50 | 59–74× |
| Hnub nyoog 1,000 teev ntawm 150 degree | 18–22 | 15–18 | 15–20% | 30–36 | 44–53× |
| Hnub nyoog 2,000 teev | 16–20 | 12–15 | 20–25% | 24–30 | 35–44× |
| Hnub nyoog 5,000 teev | 14–18 | 9–12 | 25–35% | 18–24 | 26–35× |
| Hnub nyoog 10,000 teev | 10–14 | 6–9 | 30–40% | 12–18 | 18–26× |
Mechanism: Ib nrab Tso Tawm thiab Carbonization
Thaum lub sij hawm overvoltage surge, hluav taws xob kev nyuaj siab nyob rau hauv lub PFA nce 10% (los ntawm 240 V / mm mus rau 264 V / mm rau 480 VAC). Hauv cov laus PFA, microvoids (10-50 µm) muaj roj (cua lossis ya raws). Kev ntxhov siab hauv cheeb tsam ntawm cov lus qhia tsis muaj dab tsi tuaj yeem yog 10–100 × siab dua, pib tawm ib nrab (corona). Cov paug tawm ua kom sov (1,000-2,000℃hauv zos), carbonizing PFA. Carbonized lem muaj qis dielectric zog (5-10 kV / mm) thiab tsis kho. Tom qab qhov nce siab, cov pa roj carbonized tseem nyob, txo qis lub zog dielectric tas li.
Thermal Aging Effect on Void Density
| Hnub nyoog ntawm 150℃(teev) | Tsis muaj zog (voids / mm²) | Qhov Nruab Nrab Qhov Loj (µm) | Ib nrab tso tawm qhov hluav taws xob (kV / mm) |
|---|---|---|---|
| 0 | <1 | N/A | >30 |
| 1,000 | 10–50 | 5–10 | 18–22 |
| 2,000 | 50–150 | 10–20 | 14–18 |
| 5,000 | 150–500 | 20–40 | 10–14 |
| 10,000 | >500 | 30–50 | 8–10 |
Qhov ib nrab tso tawm inception voltage tee thaum tsis muaj dab tsi ceev nce. Ib qho 10% overvoltage surge (siv voltage tseem zoo li qub, tab sis qhov pib voltage poob qis dua qhov kev khiav hauj lwm kev ntxhov siab) ua rau paug tawm.
Piv txwv teb
Lub 480 VAC PFA rhaub tau ua haujlwm rau 14 lub hlis ntawm 150℃(kwv yees . 10,000 teev). Ib qho hluav taws xob tawm tsam ua rau 12% overvoltage (540 VAC) kav 2 vib nas this. Lub tshuab cua sov txuas ntxiv ua haujlwm tab sis ua tsis tiav (hauv av txhaum) 2 lub lis piam tom qab. Tom qab-kev soj ntsuam tsis ua tiav tau nthuav tawm cov pa roj carbonized (0.5 hli ntev) los ntawm 2 hli phab ntsa, pib ntawm ib pawg tsis muaj dab tsi. Lub zog dielectric nyob rau hauv cheeb tsam cuam tshuam yog<5 kV/mm. A new heater from the same batch survived a 20% overvoltage without damage. The aged heater was vulnerable. The plant now installs surge suppressors on all heater circuits and replaces heaters preemptively after 8,000 hours.
Mitigation rau Overvoltage Risk
| Kev ua | Kev ua tau zoo | Nqi | nyuaj |
|---|---|---|---|
| Nruab surge suppressor (MOV) ntawm lub tshuab cua sov | 80–90% | $50–100 | Tsawg |
| Hloov qhov preemptively ua ntej 8,000 teev | 100% (tshem tawm cov hnub nyoog cua sov txaus ntshai) | Cua sov nqi | Nruab nrab |
| Txo kev ua haujlwm kub (130℃es tsis txhob 150 degree) | 70% (slows aging) | Tsis muaj | Tsawg |
| Nce PFA phab ntsa thickness rau 3 hli | 50% (ntau dua margin) | +20–30% | Tsawg |
| Ntxiv kev tiv thaiv kev tiv thaiv kev tiv thaiv | 90% (kwv yees thaum ntxov degradation) | $200–500 | Nruab nrab |
Cov ntsiab lus: Cov Cuab Yeej Hluav Taws Xob Poob 15-30% Dielectric zog Tom qab 10% Tshaj Tawm Tshaj Tawm
A 10% overvoltage surge (lasting 5 seconds) causes no permanent damage to a new PFA heater but reduces the dielectric strength of an aged heater (1,000+ hours at 150°C) by 15–30%. The reduction is due to partial discharge in microvoids formed during thermal aging, creating carbonized tracks that remain after the surge. The safety margin remains high (26–53×), so the heater is still safe, but the margin is reduced. Multiple surges or more severe overvoltages (20%+) can lead to immediate failure. For aged heaters (>5,000 teev), ib zaug xwb 10% surge yuav pib ib tug carbonized khiav uas ua rau tsis ua hauj lwm nyob rau hauv lub lis piam los yog hli. Txhim kho surge suppression thiab xav txog preemptive hloov cov cua sov uas tau khaws cov thermal aging tseem ceeb. Qhov voltage surge yuav luv luv, tab sis qhov kev puas tsuaj yog nyob mus ib txhis. Aged PFA yog qhov tsis yooj yim. Tiv thaiv nws nrog suppressors. Hloov nws ua ntej nws ua tsis tau. Qhov ntawd yog txoj kev nyab xeeb.
