Product information

In recent years an increasing number of MKP capacitors have been manufactured for dc applications, utilizing a special segmented coating. These are used as intermediate circuit, reservoir, or filter capacitors. As a consequence of the high field strength which is possible in these capacitors, they show a very high specific capacitance.

Figure 1 shows the schematic structure of this dielectric, which we at ELECTRONICON have called "SMKP" (from the German "Segment-metallisierter Kunststoffkondensator, Polypropylen": plastic capacitor, polypropylene, with segmented metal coating). Apart from the well-known self-healing effect in which the thin metal coating only several nanometers thick around a dielectric breakdown spot is evaporated, insulating the defect, the dielectric shows an additional fuse characteristic in the shape of so-called "current gates". In the event of a non-self-healing breakdown, these gates, which are unable to withstand the rising current to the fault, are vapourized and disconnect the affected sector completely. Figure 1 - structure of an SMKP dielectric

Figure 2 shows the results of a voltage test in short term operation. The test voltage was applied for 10 minutes at each marked field strength level, after which the capacitance was measured. One can see that at an operating field strength of approx. 150V/µm and a test field strength of 225V/µm, no changes in the dielectric can be detected at all. Conversely, when tested at an increased ambient temperature,significant loss of capacitance is evident even at lower field strength levels.

pic.2 - short term test of the dielectric strength
 

 

However, the test demonstrated in Figure 2 is not sufficient to evaluate the behaviour of SMKP capacitors in continuous operation.

According to IEC EN61071 (IEC 1071) capacitors should be designed to withstand the following overvoltages during operation:

1.1 x UR	30 % of the duty cycle
1.15 x UR	30 min/day
1.2 x UR	5 min/day
1.3 x UR	1 min/day
1.5 x UR	100 ms/day

Compliance with this specification is shown with an accelerated ageing test. The capacitors are discharged 1000 times at twice their permitted peak current, and then exposed to a permanent voltage of 1.4 x U_R at upper category temperature. During this test, the capacitance must drop by no more than 2%.

Figure 3 demonstrates the behaviour of these capacitors during the ageing test. The capacitors were

• exposed to an overvoltage of 1.25 x UR for the duration of 1000hrs. This meets the ageing test requirements of part 12 of the old standard VDE 0560.
• 2. afterwards taken to 1.4 x UR for 1000 hrs. It should be noted at this point that IEC EN61071 (which conforms to IEC1071/VDE 0560 part 120/121) only requires a test period of 500hrs.

 
The IEC standard requires that an overvoltage of 1.3 x U_R be applicable for one minute per day. Given an operation period of 100,000 hours this would correspond to a total load of 70 hours at 1.3 x U_R. This demonstrates the high requirements of the type tests of capacitors and that high safety margins are necessary.

Traction applications tend to have much higher requirements regarding the magnitude and duration of overvoltages. Figure 4 demonstrates the results of a test which reproduced a 15sec overvoltage of 1.2 x U_R every 15 minutes. In other words, a situation where this overvoltage would last 24 minutes per day instead of the 5 minutes permitted by IEC was simulated. This test was carried out for 1000 cycles. Graph 2 shows the capacitance drift of the capacitor tested at the upper category temperature of 75°C required by the customer, while graph 1 shows the same overvoltage load at a temperature of approx. 30°C.

It can be seen that overvoltages which may not be critical at room temperature can lead to noticeable loss of capacitance at higher operating temperatures.

Figure 4 - test of dielectric strength

Conclusions

The SMKP-dielectric has a high dielectric strength and can be exposed to the overvoltages specified in IEC EN61071 at ambient temperatures of up to 70°C.