1. Preparation for Power-On of the Partial Discharge Tester
Set the "Time Base Frequency" selector 7 to "50 (Internal)", and the "Time Base Operation Mode" 5 to "Ellipse". Set the Logarithmic/Linear selection switch 2 to Linear. Connect the power socket 33 to a 220V AC power supply, press the power switch 12 to the "ON" position, and preheat for 5 minutes.
2. Instrument Calibration
After wiring as shown in Figure 4, calibrate using the JZF calibration pulse generator before applying the test voltage.
Note: The wire connected to the red terminal of the calibration pulse generator should be as short as possible and connected to the high-voltage end of the test object, while the wire connected to the black terminal should be connected to the low-voltage end of the test object.
Adjust the "Amplifier Gain Fine Adjustment" 13 and "Amplifier Gain Coarse Adjustment" 15 so that the injected pulse height is appropriate (display height below 2cm on the oscilloscope screen); the readings on the voltage and discharge quantity digital meter (3) and the linear/logarithmic pointer meter (1) should match the injected known charge.
After calibration, the position of the "Amplifier Gain Fine Adjustment" (13) knob must not be changed, and the amplifier's frequency band must not be altered during subsequent tests; it must remain the same as during calibration.
After calibration and before applying the test voltage, be sure to disconnect the calibration pulse generator from the test object to prevent high voltage from damaging the calibration pulse generator.
Below, using Figure 4 as an example, a test method is introduced:
Example: Inject a charge of 50pC into the two ends of the test object using a JZF-9 or JZF-10 calibration pulse generator. Adjust the coarse adjustment knob 15 on the GH-6209 partial discharge detector panel, and on the third gear, adjust the amplifier gain fine adjustment knob 13 so that the meter "1" indicates 50pC. Remove the calibration pulse generator and apply the test voltage. The value indicated on meter 1 is then the discharge quantity. If it exceeds 50pC, attenuate by one gear on "15" ("15" is the amplifier gain coarse adjustment with 5 gears in total, each gear 20dB, a factor of 10).
3. Test Operation of the Partial Discharge Tester
Connect the power supply of the high-voltage test circuit, set the kV-pC selection switch 4 to "kV", and the "Zero Mark Selection" switch 25 to "R" position. Slowly increase the test voltage. Two zero mark pulses will appear on the ellipse, with a phase difference of 180°. If the test voltage exceeds the rated voltage level of the high-voltage resistor R, then at a lower voltage, set the "Zero Mark Selection" switch 25 to "Internal" position. An auxiliary zero mark pulse generated by the instrument will appear. At this time, adjust knob 23 so that the phase of the auxiliary zero mark matches that of the "R" position (polarity must also be consistent). Then, at high voltage, a correct zero mark position can be obtained without using the high-voltage resistor "R".
Rotate the "Ellipse Rotation" 9 to bring the ellipse to a position where it is convenient for observing discharges. Typically, this position is where the zero mark pulses are located at the upper left and lower right of the ellipse. Continuously increase the voltage, noting the first occurrence of sustained discharge. The voltage at which the discharge quantity exceeds the specified minimum value is the partial discharge inception voltage.
During testing, various interferences may appear. Using the "Time Window" switches 18, 19, and one or both potentiometers 17 and 20, the width and position of the highlighted area on the ellipse can be adjusted to avoid interference pulses. The time window device can be used to measure the discharge quantities occurring in each of the two half-cycles separately.
4. Partial Discharge Test at Frequencies Higher than 50Hz
When a partial discharge test at a frequency higher than 50Hz is required, set the "Time Base Frequency" selector 7 to the corresponding frequency gear. Take a test voltage of 10V-250V from the high-frequency test power supply and feed it into socket 29 (rotate the "High-Frequency Voltage" selector 8 to make the ellipse size appropriate).
If an overload occurs due to an improper position of the "High-Frequency Voltage" selector 8, the power will be automatically cut off, and the overload indicator light 6 will illuminate. In this case, turn the "High-Frequency Voltage" selector 8 fully clockwise, and press the "Reset" button 11 to restore operation (the overload indicator light 6 will go off). An ellipse display will appear on the oscilloscope screen.
Note: To protect the oscilloscope tube, the ellipse display appears only when the ratio of the test voltage input to the value indicated by the "High-Frequency Voltage" selector 8 is greater than 15%.
(III) Supplementary Provisions for the Partial Discharge Tester
1. Identification of Discharge Types and Discharge Sources
First, let's introduce the elliptical trace on the oscilloscope screen. It rotates clockwise. The positive zero mark pulse indicates the test voltage starting to change from negative to positive polarity; the negative zero mark pulse is the opposite. The midpoint between the two zero marks corresponds to the positive and negative peak positions of the test voltage.
Identifying discharge types from the discharge patterns on the ellipse and recognizing various interferences is a highly technical skill requiring rich practical experience (it is best to confirm with other methods). CIGRE (International Council on Large Electric Systems) has also compiled a booklet on discharge pattern recognition for this purpose. It is based on the discharge position in the pattern, whether it moves, the consistency of discharge amplitudes in the positive and negative half-cycles, and the variation of discharge amplitude with test voltage and pressurization time. Here, we can only provide a rough introduction.
Generally speaking, for partial discharges considered to be truly formed by internal bubbles, the main characteristic is that discharges mostly occur in the two half-cycles near the rising part before the peak of the test voltage.
(1) Typical internal bubble partial discharge (see Figure 5), waveform characteristics: a. Discharges mainly appear in the two elliptical quadrants where the test voltage rises from zero to the peak. b. At the inception voltage Ui, discharges usually occur near the peak, and when the test voltage exceeds Ui, discharges extend towards the zero position. c. The number and amplitude of discharges in the two opposite half-cycles are roughly the same (maximum difference up to 3:1). d. The discharge waveform is discernible. There are several cases: 1) If the discharge amplitude increases with the rise of the test voltage and the discharge waveform becomes blurred and indistinguishable, it is often due to multiple bubbles of various sizes in the dielectric, or surface discharge of the dielectric; 2) If, in addition to the above, the discharge amplitude grows rapidly with pressurization time (up to 100 times or more), it is often bubble discharge in insulating liquid, a typical example being the discharge of oil-impregnated paper capacitors.
Figure 5
(2) Discharge of bubbles between metal and dielectric in the partial discharge tester (see Figure 6 a), waveform characteristics: In the positive half-cycle, there are many small-amplitude discharges; in the negative half-cycle, there are a few large-amplitude discharges, with amplitude differences up to 10:
