Application Notes
Due to the principle of carrying the power from the catenaryto the train, some contact discontinuity appears between
pantograph and catenary.When contact is not done, energy come fromDC link filter,with for effect, to decrease the voltage. So, as soon as the
contact is re-established, an overvoltage appears.
Worse case would be This energy calculation will be used for short circuit dischargebetween terminal as well. Such discharge will generate a very
high peak current and some ringing that electrolytic could not
handle.
3) VOLTAGE RATING
Function of the voltage rating needed, film solution will
become more and more interesting.If high capacitance value is requested, film solution will beless competitive. Indeed, if there is no overvoltage, low rms.
current, large capacitance value, it will be difficult for filmtechnology to be competitive below 900 volts. V(t) = U
ndc – ∆ V x e
- α t x (cos ω t + sin αω ω t) with ω = β
2 02 – α β
=1L x C
LIFETIME CALCULATION
∆ V = catenary voltage, because over-voltage could almost reach 2 times the rated voltage.So, film capacitor can handle this kind of overvoltage.
Comparison with electrolytic technology:
Electrolytic handle 1.2 DC voltage max:
So minimum voltage that electrolytic should handle would be:DC voltage of electrolytic technology: 4
capacitors 450 Volts in series would be needed.Volume occupied for 10mF with electrolytic would be: 26I and Irms. max would be 220Arms.With film, volume occupied would be 25 I, and rms. currentcapability would be higher than 500Arms.In other hand, link to these overvoltages, peak currentappear through the capacitor:So, we have to calculate the energy generated by this over-voltage I Film technology allows a very long lifetime expectancy,
depending on voltage load conditions (working voltage) and
hot spot temperature.For DC filtering, lifetime meets the curves shown in thiscatalog.End of life criteria is a decrease of capacitance value of 2%.However, this is a theoretical end of life, because capacitor
can be still used after this point. If application can allow 5%
capacitance decrease, lifetime will be widely increased.Hot spot temperature will be determined with the followingexpression:with: α =R2 x L θ max 2 x 1000V1.2= 1670V
θ
max =
θ
+ I
2 x x Rth rs +x tg 1
hotspot ambient rms C x 2 x δ
hotspot : the maximum hot spot temperaturetg δ
: dielectric lossesRth:Thermal resistanceRs: Serial resistance θ hot spot will be 85°C or 105°C function of the application and the technology.
2 t = i
2 (t)dt.
4) CONCLUSION
i(t) =e β 02 V – α t ω sin ω tC This document gives some ways for the engineer designer to
do their choice. Of course, for each case a complete
calculation will have to be done.Anyway, if the request is only capacitance value, low voltage,low rms. Current, no overvoltage, no reversal voltage, no
peak current, film technology certainly won’t be a good
solution.
2 i 2 (t) =e β 04 V 02 – 2 α t ω 2 sin 2 ω tC i 2 (t)dt = 14 e – 2 α t C 2 β 2 2 2 04 V 02 (– α – ω + α cos2 ω t – αω sin2 ω t) APP NOTES 2 2 2 αω (α + ω ) After few periods, current becomes null, then:
2 i
2 (t)dt = [0] –= 1414 C
2 β
04 V
02 β
04 V
02 2 2 2 2 α (α + ω )C α (α + ω ) with: β
=1L x C α =R;;2 x L ω = β
2 02 – α
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