Measuring capacitor RMS current
An alternating current is flowing in the capacitor after switching off IGBT and diode. When switching off IGBT, the current from the bus bar commutates into the snubber capacitor. This leads to a
positive peak current at the switching moment. This is followed by a damped oscillation between snubber capacitor and DC-link capacitor (Fig. 7).
When switching off the diode, the reverse recovery current will be “pulled out” of the snubber capacitor. This leads to a peak current in negative direction at the switching moment. Similar to switching off IGBT, a damped oscillation follows that can even be higher in amplitude than at IGBT switch off (Fig. 8).
The frequency of the damped oscillation in both cases is determined by the bus bar parasitic inductance and the snubber capacitor value. Typically, the frequency is in the range of 100 kHz up to several MHz..
The oscillation leads to losses in the capacitor and consequently to self heating. The data sheet of the capacitor supplier gives the permissible load of the capacitor as RMS voltage or RMS current. Measurements and calculations must be carried out to check that the capacitor is not overloaded in the operating system.
Current measurement carried out, for example, by a Rogowsky current transducer surrounding the capacitor leg produces good results. An AC-voltage measurement can be less accurate because of its low value in comparison to the high DC-voltage.
The RMS value can often not be calculated simply by using the “RMS measure” function of a modern digital oscilloscope over a whole period of inverter output frequency. The offset of the probes are too high in comparison to the low total RMS values to obtain accurate figures.
A practical approach is to measure the RMS value within the oscillation time at switch off of “BOT”-diode (t1) and “TOP”-IGBT (t2) (see Fig.9). These two parts are set according to the switching period (T =1/fsw) to calculate from this the total RMS value for the switching period. This has to be done for the whole sinusoidal waveform of a frequency converter. As a worst case consideration it can be done once at the maximum values of IRMS(t1) and IRMS(t2).
Fig. 7 Switching off IGBT
Fig. 8 Switching off diode
Fig. 9 Measurement of snubber capacitor current.
The measurements should be carried out at maximum thermal operating conditions. The corresponding highest diode junction temperature leads to the highest reverse recovery current. Maximum thermal operating conditions are the values of converter output current, switching frequency, ambient and heat sink temperature that gives the highest temperature. Short overloads in the second range are normally negligible. It should be taken into consideration that the permissible RMS voltage and current depend on the frequency of the oscillation. This is given in the data sheet of the capacitor. The snubber capacitor is also stressed by adjacent IGBT modules from other phases at the same DC-link. However, this load is often much lower because of the bus bar impedance between the IGBT modules.
Temperature and self heating under operation
Capacitor suppliers limit the admissible temperature of the capacitor during operation. The capacitor can fail immediately if this temperature is exceeded. Also the self heating temperature is limited, which is a measure for the capacitor load. In critical applications, it has to be checked under maximum thermal operating conditions that the temperatures are not exceeded. The capacitor is heated by the following:
• AC current that heats the device up due to internal losses (tan δ / ESR)
• Environmental temperature
• Heating by high bus bar temperature.
The operating temperature is given by the ambient plus the temperature difference of the self-heating effect.
TOperation = Ta + dTself-heating
The ambient temperature Ta is the capacitor temperature when not in operation but mounted in original position. This temperature can be measured on a not connected dummy capacitor similar to the capacitor under test. This temperature may be higher than the cabin temperature because of the additional heating due to connected hot bus bars.
The operating temperature can be measured by thermocouples which are placed inside the capacitor close to the hot spot but this requires specially prepared capacitors. A measurement of the body temperature is sufficient when the temperature gradient from the hot spot to the body is known (Rth).
TOperation = Tbody + Rth*i²*RESR
Symbols and terms used