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**1.**** ****PT**** ****secondary measurement principle****
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After the input current and voltage are digitally filtered, the fundamental wave is taken out, and then the peak value of the resistor current fundamental wave, Ir1p=Ix1p.cosφ, is calculated by projection method. Due to the stable fundamental value, Ir1p is generally used to measure the performance of the arrester.

The total current fundamental peak Ix1p is projected as Ir1p in the voltage fundamental U1 (E1) direction and Ic1p in the vertical direction. φ is the current and voltage fundamental phase angle, including the selected compensation angle. Therefore, both φ and Ir1p can be used to visually measure the performance of MOAs.

**2. Interphase interference problem****
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In the field measurement, in the series arrester, the middle B phase affects the leakage current of A and C through the stray capacitance, so that the A phase φ is reduces, the resistance current increases, and the C phase φ increases, the resistance current decreases or even is negative, a phenomenon called interphase interference.

One approach is to compensate for interphase interference: assuming that there is a phase difference of 120° between Ia and Ic in the absence of interference, assuming that B has the same interference relative to A and C;

Take the B phase as the voltage, the C phase as the current, and measure φ1 = φcb; Then take the A-phase current and measure φ1=φab; Then the phase difference between C-phase current and A-phase current φc=φcb-φab;

Select the correction angle φ=(φca -120°) / 2 to put this value into the main menu of the instrument;

If phase sequence is selected, the instrument automatically compensates for the angle according to the selected phase sequence (A phase is added φ, the B phase does not need to be compensated, O, C is selected to subtract φ).

Interphase interference also does not need to be compensated (i.e. the compensation angle is 0), and the performance of the arrester can be judged from the trend of the resistive current.

If allowed, you can only power up the phase you are measuring to get absolute data. And laboratory measurements do not require consideration of interphase interference.

**3. Lightning arrester performance judgment****
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The performance of the arrester can be judged from the peak of the resistive current fundamental Ir1p, but it is more effective from the current voltage angle Φ because 90°-Φ is equivalent to the dielectric loss angle. If the specified resistor current is less than 25% of the total current, the corresponding φ is 75°.

The effect of this error should be considered in actual measurements. Despite this interphase interference error, it is still possible to judge the performance of the MOA. If you only use Ir1p to judge, there will be several changes around 90°, so it is not reasonable to look directly at the angle.

**4. Measurement principle of online current method****
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We know that under sinusoidal voltage excitation, the leakage current of zinc oxide arresters consists of capacitive current and resistive current. In an all-current waveform, the peak value of the first wave should be essentially the same as the peak of the fundamental current, and its peak moment moves to the right as the equivalent resistance value of the arrester decreases. The peak of the second wave occurs at the moment when the voltage peak occurs, when the capacitive current is basically zero. We only need to try to measure the peak of the first wave, that is, the peak of the fundamental current (when the resistance current is not very large, that is, the peak of the full current), try to measure the peak of the second peak, that is, the measurement of the peak of the resistance current can be realized. Measuring the time difference between the first peak and the second peak gets a value of φ.