HIMALAYAL - SHANGHAI - CHINA
CVT auto-excitation measurement with HCL2840D is quite convenient, conduct wiring in accordance with diagram below. If C1 is single stage capacitor, generatrix is cannot be grounded; if C1 is multiple stage capacitor, high voltage lead not necessarily dismantled, generatrix can be grounded, C11 and C12 can be measured with both regular tangent and inversed wiring, auto-excitation measurement can be applied to C13 and C2.
I. Wiring in accordance with diagram below:
II. Measurement and potential
During CVT auto-excitation measurement, the instrument measures C13 first then C2 after automatic inversed winding, meanwhile automatically calibrate impact of voltage dividing.
While measuring C13, high voltage is added to high voltage line core wire and shielding, low voltage is added to C2 line core wire and shielding.
While measuring C2, low voltage is added to high voltage line core wire and shielding as well as C2 line core wire and shielding.
III. The reason to measure C13 first then C2
As it’s known that C13 capacitance is smaller, approximately 20000 pF; C2 capacitance is larger, at least 40000pF; CN is 50pF standard capacitor. While measuring C13, connect C2 and CN in serial as one standard capacitor, according to capacitor cascade formula: Ccascade=(C2CN)/(C2+CN), due to C2>>CN, Ccascade≈CN, C2 has little influence on test outcome, thus negligible. On the contrary, if C13 is measured first, by reason that C13 capacitance is small, after connected in serial to internal CN, C13 dielectric loss is included, causing standard arm dielectric loss increase which will lead to C2 dielectric loss reduction, thus cause error in measurement.
IV. Auto-excitation measurement with high voltage line on the ground can cause dielectric loss increase
High voltage line ought to be suspended during auto-excitation measurement, otherwise extra dielectric loss to the ground can cause dielectric loss increase, use thin cable to connect high voltage socket and CVT test object and suspend it
Content in the blue frame on the diagram above shows RC cascade model of extra stray capacitance with cable on the ground, advancing voltage UN at point δ into UN’, current IN into IN’, IX phase remains the same, causing increase in angle δ, namely increase in dielectric loss. Users can derive through formula.
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