Introduction to Dielectric Loss Tester (Including Detailed Operating Steps)

Dielectric Loss tester: This instrument is used in power plants, substations, and laboratories to measure the dielectric loss tangent (tgδ) and capacitance of various high-voltage electrical equipment. Featuring an integrated design, it incorporates a dielectric loss test bridge, a variable-frequency voltage regulator, a step-up transformer, and SF6 standard capacitors, with microcomputer control and a Chinese-language menu interface. The high-voltage power supply is generated by the instrument's internal inverter and stepped up by the transformer for testing. The frequency can be set to 45 Hz, 55 Hz, or 65 Hz, utilizing digital notch filtering, frequency conversion anti-interference technology, and Fourier transform digital filtering to minimize power-frequency electromagnetic interference and enhance measurement accuracy in strong electric field environments. It is also suitable for testing under generator-powered conditions after complete power outages.

• Insulating materials (such as the oil-paper insulation and bushings in transformers) function like a container for electrical current.
• The voltage of a is like water poured into a cup.
• The ideal insulating material is like a flawless cup: it holds exactly as much water as poured into it, without a single leak.
• Poor insulation material = a cracked cup: it can hold some water, but part of it will secretly leak through the cracks.

This "water leakage" process is known as dielectric loss. The larger the crack, the more water leaks out, resulting in greater losses and increased insulation risk. At any moment, the "cup may break" (indicating insulation breakdown), potentially causing power outages or even explosions.

The Xilin Bridge is a super-precision balance specifically designed to accurately measure the extent of "cup cracks." The dielectric loss tester represents the practical application of the Xilin Bridge's theoretical principles.

• Grid Company: Preventive testing of substation equipment, completion acceptance of new projects
• Electric Power Engineering Company: Construction inspection of power transmission and transformation projects, equipment handover testing
• Industrial enterprises: Daily maintenance of self-owned power plant equipment for power plants, steel mills, and chemical plants
• Equipment manufacturer: Factory quality inspection of transformers and current transformers
• Third-party testing institutions: Power equipment quality testing, forensic appraisal

Reverse connection method: One end of the test sample is directly grounded, while the instrument elevates its measurement terminal CX to a high-voltage potential, with the high-voltage terminal HV grounded.

Positive connection method: The test sample Cx is connected between the high-voltage terminal and the measurement terminal of the bridge arm, with neither terminal grounded; the bridge body itself is grounded.

Self-stimulation method (specific to CVT): Connect the primary bus and secondary load of the CVT, then remove the CVT damper. Connect the instrument's low-voltage output terminal to the secondary winding of the CVT intermediate transformer; connect the instrument's measurement input terminal to the low-voltage terminal (final terminal block) of the CVT voltage divider capacitor; ground the CVT's high-voltage terminal; and connect the instrument's ground terminal to an independent grounding electrode.

1. Instrument lead-out terminal instructions:
• HV —The instrument measures the high voltage end of the lead (with dangerous voltage) .
• CX —The current input terminal of the test sample is in the positive connection.
• —The grounding end of the instrument is reliably connected with the earth when used.
2. Reference wiring
1. Positive wiring, internal standard capacitance, internal high voltage (conventional positive wiring):
   
2. Reverse Wiring, Internal Standard Capacitance, Internal High Voltage (Conventional Reverse Wiring)
   
3. Positive wiring, external standard capacitance, internal high voltage:
   
4. Reverse wiring, external standard capacitance, internal high voltage:
   
5. Positive wiring, internal standard capacitance, external high voltage:
   
6. Reverse wiring, internal standard capacitance, external high voltage:
   
7. Positive wiring, external standard capacitance, external high voltage (high voltage dielectric loss):
   
8. Reverse wiring, external standard capacitance, external high voltage:
   
9. CVT Self-excitation measurement:

   CVT self-excitation method can be connected according to the following figure. If C1 is a single capacitor, the bus can not be grounded; if C1 is a multi-capacitor, the bus can be grounded; C11 and C12 can be measured by conventional positive and negative wiring; C13 and C2 can be measured by self-excitation method.

   

   In CVT self-excitation measurement, the instrument first measures C1, then automatically reverses 7the line to measure C2, and automatically calibrates the effect of partial pressure.

   It should be noted that the high-voltage cable should be suspended and not touch the ground, otherwise the additional dielectric loss will cause errors to the ground. It can be connected with the high-voltage socket and the CVT test sample by thin cable and lifted.

10. Low Voltage Shielding Test of Reverse Wiring

    Reverse wiring low voltage shielding function, one wiring can simultaneously measure the capacitance and dielectric loss of C1 and C2.

    

    By choosing the low voltage shield of reverse connection, the dielectric loss of 10kV reverse connection can be measured without disconnecting the bus bar of the upper capacitor C1. As shown in the figure below, the busbar is connected with ground wire, the upper end of C1 is not disconnected, the lower end of C1 is connected with high-voltage core wire, and the C2 end is connected with Cx core wire. The capacitance and dielectric loss of C11 and lower shielding parts can be measured simultaneously by using reverse wiring/10kV/M measurement mode.

    

1. CX socket - is the measurement input of test signal, which is connected by a special low-voltage cable. This cable has a special crocodile clamp with a single shielding belt, 8 m long, and connects to the low end of the test. This end is empty during reverse wiring.
2. CN socket - is the measurement input of the external standard capacitance signal, which is empty when using the internal standard.
3. RS232 interface - used to connect computers, control instruments, upload data.
4. U-disk socket - used to export data stored in the instrument.
5. Printer - Print measurement data.
6. Color touch screen display - 640*480 resolution, control and display menus and various prompt information and measurement results.
7. Self-excitation current output terminal - special terminal for measuring CVT, usually connected with auxiliary winding da.
8. Self-excitation current output terminal - special terminal for measuring CVT, usually connected with auxiliary winding dn.
9. Grounding terminals - Earthing terminals.
10. High Voltage Switch - Responsible for starting and closing the high voltage power supply inside the instrument. Only turn off when using external high pressure, the rest of the time should be in the open state.
11. Power socket - AC 220V±10%, 50±1Hz power input, with 5A safe.
12. Power Switch - Turn on and off of the whole power supply.

Fig.9 Instrument Panel

13. High Voltage Wiring Diagram of Instrument Side

A:Small black clip is high-voltage shielding and generally suspended (with high-pressure)

B:Red clip connects the tested high-pressure part

Next, using transformer testing as an example, we employ the most efficient testing method—the reverse connection method. The experimental steps are as follows:

1. An independent grounding electrode must be used, with a grounding resistance ≤4 Ω.
2. The grounding wire shall have a cross-sectional area of ≥2.5 mm² and a length of ≤1 m; it is strictly prohibited to share the grounding grid with the substation.

I. Wiring

1. Short-circuit the three-phase high-voltage windings of the transformer together.

2. The CX instrument terminal connects to a three-phase shorting point.

   The medium-voltage and low-voltage windings are short-circuited in three phases and then grounded.

   A single test can determine the overall dielectric loss value of the three-phase high-voltage winding.

   test item	Prioritize the wiring method	Alternative wiring method	remarks
   High-voltage winding + sheath overall dielectric loss	Reverse Connection Method	-	Most commonly used on-site; no need to open the end screen
   Separate casing main insulation dielectric loss	Positive connection method	Reverse Connection Method	The right connection method must be used when the last screen can be unlocked.
   Dielectric loss of the casing end screen to ground	Reverse Connection Method	-	Test voltage ≤3 kV
   Three-phase comprehensive rapid screening	Three-phase simultaneous reverse connection method	-	Perform phase-by-phase retesting after the anomaly occurs

II. Begin operating the equipment

1. First press the red power switch, then press the black internal high-voltage switch.
2. After the display turns on, you will see a button on the right side for selecting the test voltage level, which can be chosen based on actual conditions. The default test voltage is typically 500V. By clicking the test mode button, you can select the appropriate test mode according to your requirements; in this case, we have chosen the reverse connection method.
3. The "Start Test" button opens a confirmation window. Select "Confirm" to begin the test.
4. Once the test progress bar in the lower-left corner reaches 100%, the data results will be displayed in the upper-right corner.
5. Click the Print button to start printing the test report
6. After printing is complete, first press the black switch to turn off the internal high voltage, then press the red button to turn off the power switch.

III. Precautions During the Testing Process

1. It is strictly prohibited to touch any wiring without first closing the internal high-voltage permit switch.
2. It is strictly prohibited to ground the shielding layer of CX cables; otherwise, it may cause internal short circuits and damage to the instrument.
3. It is strictly prohibited to select incorrect test modes or wiring methods (e.g., using reverse wiring for a forward wiring test mode).
4. It is strictly prohibited for the ground test voltage on the last screen to exceed 3 kV.
5. It is strictly prohibited to perform the reverse connection test by a single person.

IV. Common Issues and Solutions in the Reverse Connection Method

1. The reverse connection method measures the overall dielectric loss of the winding together with all the bushings.
2. The results account for the effects of the casing surface leakage current and stray capacitance.
3. If the overall dielectric loss exceeds the specified limit, further tests should be conducted using the positive connection method on each phase bushing to locate the defect.
4. The capacitance value in the reverse connection method is typically slightly larger than that in the forward connection method, which is a normal phenomenon.

Actual product photo