Power capacitor bank cabinet: keeping the capacitors cool and setting the controller so steps switch back only when discharged

01

Air around the capacitors: the −25/D limits, the hot spot and 7 °C that halves life

TDK's application note on PFC system design (published under EPCOS) names high packing density of capacitors in the cabinet as a cause of higher operating temperature, and defines temperature class −25/D from IEC/EN 60831 as 55 °C maximum peak, 45 °C maximum 24-hour average and 35 °C maximum yearly average of the air surrounding the capacitor. It explains that polypropylene loses dielectric strength once its hot-spot temperature is exceeded, gives an Arrhenius rule of thumb that a 7 °C rise halves the life of MKK capacitors, and warns that above the maximum operating temperature the overpressure disconnector may stop working, in the worst case leading to the capacitor bursting with a risk of fire. [1]

The note makes sufficient ventilation or cooling mandatory, says high packing density may require forced cooling with enough space between capacitors and distance from warmer parts such as reactors and semiconductors, and shows a cabinet without reactors cooled by natural circulation through generous air inlets and a raised roof. [1] ABB's installation manual adds a 60 °C maximum casing temperature, mounting capacitors at the bottom of the cabinet for the lowest stress temperature, at least 20 mm between capacitors and 20 mm free above each so the overpressure disconnector can extend, and enough distance from reactors that their heat is not conducted along the cable or radiated onto the capacitor. [2]

Editorial calculation: by the 7 °C rule, a capacitor whose surrounding air runs 14 °C hotter than designed would reach about a quarter of its expected life. [1]

Give the supplier the kvar and steps, the supply voltage and harmonics, the room temperature range and ventilation, any reactors to be fitted, the switching device (contactor or thyristor), the enclosure protection class, and the controller's communication needs.

02

Discharge below 75 V in 180 s, re-switch at 10%, and a controller set to match

ABB states that under IEC 60831 a capacitor must discharge to less than 75 V within 180 s, that discharge resistors are needed both for personnel safety and for re-switching in automatic PFC equipment, and that capacitors must be discharged to 10% of nominal voltage or below before re-switching. [2] Rishabh's RISH PFC 8 controller manual puts on its first page the caution that the discharge time set in the controller must match the capacitor's discharge time; its discharge time is settable from 60 to 1,800 s with 60 s as default, and it offers 20 preset control series of step ratios, from 1:1:1:1:1:1:1:1 to 1:2:4:8:8:8:8:8, or direct kvar per step. [3]

Editorial note: if the capacitors only meet the IEC 60831 figure of 75 V within 180 s, a controller left at a 60 s default could switch a step back while it still holds charge; take the setting from the capacitor's and resistor's actual discharge time. [2] [3]

Editorial calculation: control series 1:2:4:8:8:8:8:8 on eight outputs adds up to 47 units, so with a 5 kvar first step it covers 235 kvar in 5 kvar increments. [3] TDK notes that conventional contactors can draw inrush currents of up to 200 times nominal current, while thyristor modules switch at zero current crossing and avoid them. [1]

03

Yearly checks: capacitance, case temperature and discharge time

ABB recommends measuring and recording capacitance, ambient temperature near the capacitor and energised case temperature once a year for the first seven years and twice a year after that, replacing capacitors whose capacitance drops significantly or that run hotter than others, as this can indicate a rising loss factor near end of life, and regularly checking and recording discharge time. [2] Zhiyou lists power capacitor banks with switching devices and protective relays, and Jiukang lists capacitor banks alongside PFC controllers, contactors and reactors. [4] [5]

04

Send the network, the room and the controller needs together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Network: [voltage; kvar; steps; harmonics]
Room: [temperature range; ventilation]
Cabinet: [IP; fans; reactors]
Switching: [contactor; thyristor]
Controller: [step ratio; discharge time; communication]
Checks: [capacitance; case temperature; discharge]

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