Megger TDS40, Руководство по эксплуатации, Страница: 17 / 71

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Another vital characteristic to be considered when designing this system was to ensure
simple on-site testing by
- small dimensions,
- light weight and
- low power consumption.
Every cycle starts with a charging phase in which the test object as well as the back-up
capacitor switched in parallel are charged from a DC source until the desired test voltage
is reached.
Fig. 2: Shape of 0.1 Hz cosine-rectangular VLF voltage (type Plus)
After a period of 5 seconds, the voltage source will be disconnected from the system
resp. discharged via a resistance.
Subsequently, the polarity reversal is initiated. The test voltage changes its polarity from
minus to plus.
Depending on the size of capacitance of the connected test object, time of the polarity
reversal is between 1 and 17 ms.
Due to the losses which occur during the polarity inversion, the positive voltage is always
decreased by these losses.
After a 5 seconds dwelling period at positive polarity (device option Basis) or equalisation
of the polarity reversal losses due to recharging by the positive DC voltage source
(device option Plus), the return to negative polarity takes place.
The negative voltage source is reconnected to the test object.