Design of high voltage DC power supply for portable insulation resistance tester
When a DC voltage is applied to an insulation material, the current generated is composed of three parts: charging current, absorption current, and leakage current. The charging current and absorption current decrease with time, while the leakage current is independent of time.
The insulation resistance test is to measure the leakage current value of electrical equipment through instruments, and usually the instrument displays this current value as the insulation resistance value (M Ω). In order to reduce the influence of the initial charging current during testing, the insulation resistance value after 1 minute of pressurization is generally taken as the measured value R60; If the value R15 at 15 seconds of pressurization is read at the same time, the absorption ratio K=R60/R15 can be calculated. The absorption ratio is more sensitive to reflecting local defects in large power transformers, such as water content in transformer oil, weak points or dirt in bushings and coils.
Insulation resistance and absorption ratio are sensitive indicators for determining whether insulation materials are damp.
The main instrument used for insulation resistance testing is a megohmmeter, commonly known as a shaker. The traditional megohmmeter can generate several types of DC voltages such as 500V, 1000V, 2500V through a hand cranked generator. The measurement range of insulation resistance also varies depending on the model of the megohmmeter. Different models or voltage levels of megohmmeters should be selected according to the needs during testing. When using a hand cranked megohmmeter, users should also try to maintain the hand cranking speed at around the rated speed of 120r/min, which can cause inconvenience during use.
The portable insulation resistance tester mentioned in this article is powered by a 12V battery and can generate stable series voltages of 500V, 1000V, 1500V, 2000V, and 2500V through a single ended flyback switch power supply. It is also measured, sampled, processed, and displayed by a microcontroller system. It has the characteristics of small size, light weight, and fast response. The following mainly introduces the circuit principle and manufacturing method of its power supply part.
