Principle
1. Measurement Principle
After digital filtering of the input current and voltage, the fundamental wave is extracted, and then the projection method is used to calculate the resistive current fundamental peak value Ir1p=Ix1p.cosφ. Since the fundamental wave value is stable, Ir1p is commonly used to evaluate the performance of the arrester.
The total current fundamental peak value Ix1p is projected onto the voltage fundamental U1 (E1) direction as Ir1p, and onto the perpendicular direction as Ic1p. φ is the phase angle between the current and voltage fundamentals, which includes the selected compensation angle (Figure 7). Therefore, both φ and Ir1p can intuitively evaluate MOA performance.
2. Inter-phase Interference
During on-site measurement, for arresters arranged in a line (Figure 8), the middle phase B affects the leakage currents of phases A and C through stray capacitance, causing a decrease in φ for phase A and an increase in resistive current, while for phase C, φ increases and resistive current decreases or even becomes negative. This phenomenon is called inter-phase interference (Figure 9).
One method is to compensate for inter-phase interference: assuming that when Ia and Ic have no interference, the phase difference is 120°, and assuming that the interference of B on A and C is the same;
Take voltage from phase B and current from phase C, measure φ1=φcb; then take current from phase A, measure φ1=φab; then the phase difference between C phase current and A phase current is φca=φcb-φab;
Select the correction angle Dφ=(φca -120°) / 2, and input this value into the instrument in the main menu;
After selecting the phase sequence, the instrument will automatically perform angle compensation based on the selected phase sequence (add Dφ to phase A, no compensation for phase B i.e., select 0, subtract Dφ from phase C)
It is also possible not to compensate for inter-phase interference (i.e., compensation angle of 0) and judge the arrester performance from the trend of resistive current changes.
If allowed, only the phase under test can be energized to obtain absolute data. In laboratory measurements, inter-phase interference does not need to be considered.
3. Arrester Performance Judgment
The performance of the arrester can be judged from the resistive current fundamental peak value Ir1p, but it is more effective to judge from the current-voltage angle Φ, because 90°-Φ is equivalent to the dielectric loss angle. If it is specified that the resistive current is less than 25% of the total current, the corresponding φ is 75°;
Without inter-phase interference:
| Performance | <75° | 75°～ 79° | 79°～ 83° | 83°～ 89° |
| Φ | Poor | Fair | Good | Excellent |
With inter-phase interference, errors occur:
| Phase A | Phase B | Phase C |
| -2°～ -4° | (considered 0) | +2°～ +4° |
In actual measurement, this error effect should be considered. Although there is this inter-phase interference error, it is still feasible to judge MOA performance. If only Ir1p is used for judgment, there will be several times of change near 90°, and at this time it is more reasonable to directly view the angle.
4. Precautions in Practical Application:
Since this instrument can measure three phases simultaneously and automatically compensate, it is especially convenient to use. The above-mentioned inter-phase interference and other issues are automatically calculated by the instrument during three-phase simultaneous measurement, and no calculation by the test personnel is required. In short, when using this instrument, just connect the test leads and turn on the instrument for testing. All problems have been solved by the instrument.
Description
Ux : RMS value of power frequency voltage, this voltage is the measured voltage;
U1 : RMS value of fundamental power frequency voltage;
U3 : RMS value of third harmonic power frequency voltage;
U5 : RMS value of fifth harmonic power frequency voltage;
Ix : RMS value of total current;
Ic : RMS value of capacitive current;
Ir : Peak value of resistive current;
Ir1: Peak value of fundamental resistive current;
Ir3: Peak value of third harmonic resistive current;
Ir5: Peak value of fifth harmonic resistive current;
Ir7: Peak value of seventh harmonic resistive current;
Ic1p: Peak value of fundamental capacitive current.
Ir1p: Peak value of fundamental resistive current. Since Ir1p is relatively stable and has a definite source, Ir1p should be used as the main criterion for resistive current.
P : Active power;
Φ : Phase difference by which the fundamental current leads the fundamental voltage.
Waveforms Ux and Ix are the actual waveforms of power frequency voltage and total current, which can reflect both the phase difference between voltage and current and the quality of the power supply.
Precautions
1. When taking the reference voltage from the PT secondary side, carefully check the wiring to avoid a short circuit on the PT secondary side.
2. Do not reverse the voltage signal input line and the current signal input line. If the current signal input line is connected to the PT secondary side or the test transformer measurement terminal, the instrument may be burned out.
3. Do not plug or unplug the measurement lines when input voltage and current are present, to avoid burning the instrument.
4. If the instrument is damaged, stop using it immediately and notify our company. Do not open the case for repair by yourself. If the instrument does not work properly, first check whether the power fuse is blown. Replace with a fuse of the same rating before continuing the experiment. If the problem is more complex, please contact our company directly.
5. Do not place the instrument in a humid or excessively high temperature environment.
