120 kV DC High-Voltage Generator: A Powerful Tool for Insulation Testing of Power Equipment

120 kV DC High-Voltage Generator: A Powerful Tool for Insulation Testing of Power Equipment

Within the safety operation assurance framework of power systems, insulation performance testing of high-voltage electrical equipment constitutes a critical component. Recently, a DC high-voltage generator (DC Hipot Tester) with a rated voltage of 120 kV has garnered significant industry attention owing to its high-frequency PWM pulse-width modulation technology, integrated design, and comprehensive protection system. This article provides an in-depth analysis of this device from four perspectives: technical parameters, testing procedures, application scenarios, and typical testing challenges.

I. Key Technical Parameters

This series of DC high-voltage generators is available in four specifications to meet diverse testing requirements:

General Technical Specifications:

• Output voltage accuracy: ±(1.0% of the reading ± 2 digits)
• Output current accuracy: ±(1.0% of the reading ± 2 digits)
• Ripple factor: ≤0.5%
• Operating mode: intermittent operation; capable of continuous operation for up to 30 minutes under rated load.
• Overload capability: No-load voltage can reach 110% of the rated voltage for a continuous duration of 10 minutes; maximum charging current is 1.25 times the rated current.
• Power supply: AC 220V±10%,50Hz
• Operating environment: temperature range –10 °C to 40 °C, relative humidity ≤85% (no condensation at 25 °C), and altitude <1,500 m
• Insulated cylinder height: 535 mm

This device employs imported high-power IGBT devices and drive technology, combined with high-frequency, high-voltage rectifier diodes, enabling it to deliver a 120 kV high-voltage output while keeping the total weight of the system below 10 kg, thereby achieving a significant breakthrough in portability.

II. Standardized Testing Process

Based on the specifications in the instruction manual and practical operational experience, the complete testing workflow for this device can be divided into the following four stages:

Phase 1: Pre-test Preparation

1. Equipment integrity check: Verify that the connection cables are free of open circuits or short circuits, and that the equipment exhibits no external damage.
2. Wiring arrangement: Place the host unit and the insulating cylinder in appropriate positions, then sequentially connect the power cable, the dedicated four-core connection cable, and the grounding cable. Critical requirement: The grounding terminals of the host unit, the insulating cylinder, and the discharge rod must each be connected to the same grounding point of the test object (i.e., "single-point grounding"); series connection of the grounding cables is strictly prohibited.
3. Zero-position reset: Set the power switch to the "OFF" position, rotate the voltage-adjustment potentiometer counter-clockwise to the zero position, and set the overvoltage setpoint to 1.1 times the test voltage.

Phase 2: No-load step-up test and overvoltage protection verification

1. Connect to the AC 220V power supply; the red indicator light will illuminate, confirming that the power supply is connected.
2. Press the green high-voltage power button; the green indicator light will illuminate, confirming that the high-voltage circuit has been connected.
3. Slowly adjust the voltage potentiometer clockwise, gradually increasing the voltage from zero to a value approaching the overvoltage setpoint; record the reading displayed on the main machine's voltmeter to verify whether it equals 1.1 times the setpoint value.
4. During the voltage boosting process, inspect the main unit and the high-voltage output cables for any abnormal noises or abnormal phenomena.
5. Reduce the voltage to zero, press the red high-voltage cutoff button to cut off the high-voltage output, and then turn off the power switch.

Phase 3: Leakage Current and DC Withstand Voltage Test

1. Installation of microammeter: Install a high-voltage shielded microammeter at the high-voltage output terminal of the insulated cylinder, and connect the dedicated high-voltage lead from the test specimen to the microammeter.
2. Safety verification: Inspect the reliability of the connections between the host unit, the discharge rod, the insulating sleeve, and the grounding wire of the test object; confirm that the high-voltage safety clearance meets the specified requirements (not exceeding 20 kV per meter in air).
3. Voltage rise test: After connecting the power supply, press the green button to activate the high voltage, then adjust the potentiometer clockwise from zero to increase the voltage, with the voltage rise rate controlled between 3–5 kV/s, until the test voltage is reached.

• For large-capacitance test specimens (e.g., power cables): slow down the voltage rise rate, connect a current-limiting resistor in series with the high-voltage circuit, and closely monitor the ammeter readings to ensure they do not exceed the instrument's rated current.
• Small-capacitance test specimens (e.g., zinc oxide surge arresters): slowly increase the voltage (or current) to 95% of the required value, then fine-tune it to the target value.

1. 0.75 UDC1mA Function Operation: When testing a zinc oxide surge arrester, first increase the voltage to the UDC1mA level, then press the yellow 0.75 UDC1mA button; the voltage will automatically decrease to 75% of the original value and remain at this level. At this point, read the current value.
2. At the end of the test: reset the voltage to zero, press the red button to cut off the high-voltage output, and turn off the power switch.

Phase 4: Safe Discharge and Suture Removal

1. Discharge of the small capacitor test specimen: natural discharge via the voltage equalizing resistor inside the insulating cylinder.
2. Discharge test for large-capacitance specimens (e.g., cables): Reduce the voltage of the specimen to below 20% of the test voltage, then gradually discharge using a 100 Ω/V discharge resistance rod—move the discharge rod progressively closer to the specimen; when a certain distance is reached, the air gap will begin to exhibit silent corona discharge; only after the discharge rod ceases to emit audible discharge sounds may the specimen be directly grounded for discharge.
3. Only after confirming that the test specimen has fully discharged, should the grounding wire be connected to the high-voltage output terminal before removing or replacing the high-voltage lead wires.

III. Typical Application Scenarios

This DC high-voltage generator is suitable for the following application areas and equipment types:

1. Testing of zinc oxide surge arresters and magnetic blow-out surge arresters

The zinc oxide surge arrester serves as the core equipment for lightning protection and operational overvoltage protection in substations. This device is equipped with a dedicated 0.75 UDC1mA intelligent button function, which enables rapid measurement of the leakage current of the surge arrester at 0.75 times the DC 1 mA reference voltage; this feature provides a critical means for assessing whether the valve plates of the surge arrester have aged or become damp.

2. DC withstand voltage and leakage current test for power cables

This test is applicable to the insulation condition assessment of power cables rated at 6 kV and above. By applying a DC high voltage and measuring the leakage current, it can be determined whether the cable insulation has become damp, has aged, or exhibits localized defects. For cable specimens with high capacitance, the equipment supports the use of a series current-limiting resistor for protective testing.

3. Insulation Testing of Power Transformers

When performing a DC withstand voltage test on a transformer winding, the leakage current can be measured simultaneously, which serves to evaluate the performance of the transformer's main insulation. The trend in leakage current provides a critical basis for assessing the insulation condition of the transformer.

4. Insulation testing of the generator stator windings

The DC withstand voltage and leakage current tests on the ground insulation of the generator stator windings constitute the core components of preventive testing for generators; the equipment's 0.5% ripple coefficient ensures the reliability of the test results.

5. Applications in Other Industrial Sectors

The equipment can also be used for insulation testing of electrical equipment in railway systems, chemical industries, mining, metallurgy, steel production, and other related sectors, as well as in industrial applications requiring DC high voltage, such as electrostatic dust removal and spraying processes.

IV. Common Testing Challenges and Their Solutions

Challenge 1: Interference with surface leakage current measurement accuracy

Problem: When measuring the leakage current of a test specimen, contaminants and moisture on the specimen's surface can create surface leakage current paths, resulting in overestimated measurement values and making it difficult to accurately reflect the actual condition of the specimen's internal insulation.

Solution: Employ the shielded microammeter method—connect a metal-shielded microammeter to the high-voltage side, use a shielded cable to connect to the test specimen, and ensure that the shield layer of the cable is tightly bonded to the shield layer of the instrument. Simultaneously, at the high-potential end of the test specimen, tightly wind several turns of bare metal wire and connect it to the shield layer of the high-voltage lead (referred to as the "shielded ring method"); this configuration diverts the surface leakage current into a shielded circuit, preventing it from passing through the microammeter, thereby enabling the measurement of the true internal leakage current of the test specimen.

Challenge 2: Current overshoot during voltage rise of large-capacitance test specimens

Problem Description: When performing DC withstand voltage tests on high-capacitance test specimens such as power cables, the charging current at the onset of voltage rise can become extremely large; this current may exceed the rated current of the equipment or even trigger overcurrent protection, thereby preventing normal voltage rise.

Solution: Connect a current-limiting resistor in series with the high-voltage circuit to limit the charging current amplitude. Simultaneously, slow down the voltage rise rate, closely monitor the ammeter readings, and ensure that the current does not exceed the rated value of the equipment. For specimens with large capacitance, the voltage rise rate should be significantly lower than the conventional recommended value of 3–5 kV/s.

Challenge 3: False triggering of the protection circuit leading to voltage boost interruption

Problem phenomenon: During the step-up process, the green indicator light turns off and the red indicator light turns on, indicating that the protection circuit has tripped, the high voltage has been cut off, but no actual breakdown of the test object has occurred.

Possible causes and countermeasures:

• Potentiometer not reset to zero: If the voltage adjustment potentiometer has not been rotated counter-clockwise to the zero position before voltage boosting, the zero-position protection mechanism will activate. Before operation, ensure that the potentiometer is at the zero position.
• Ground loop short circuit: the high-voltage output terminal of the insulated cylinder is grounded, or the test object is short-circuited to ground. Inspect the insulation condition of the output cable and the test object.
• Incorrect overvoltage setting: the set value is lower than the test voltage. Reset the overvoltage protection value to 1.1 times the test voltage.
• Discharge or breakdown of the test specimen: insulation failure of the test specimen leads to overvoltage or overcurrent protection tripping. Inspect the condition of the test specimen and identify any insulation defects.

Recovery procedure: ① Set the potentiometer to zero; ② Turn off the power switch; ③ Wait one minute to ensure that the internal low-voltage capacitor has fully discharged; ④ Restart the device and perform a no-load test; once the instrument is confirmed to be functioning normally, reconnect the test specimen and resume the voltage-rise test.

Problem 4: Abnormal readings or value drift in the microammeter

Problem symptoms: The microammeter displays "LOW BAT", exhibits unstable readings, or shows significant deviations.

Causes and Management:

• Low battery level: The microammeter is powered by a 9V battery; insufficient battery power may lead to measurement errors. Replace the 9V battery promptly.
• Poor electrical connection: open circuit or poor contact in the lead wires of the microammeter. Inspect the microammeter lead wires to ensure reliable connection.
• Improper shielding leading to electromagnetic interference: In the test environment, electromagnetic interference may couple into the microammeter via test leads. Ensure that the metal shielding enclosure of the microammeter is properly grounded, and use shielded cables to connect the test specimen.

Challenge 5: Safety hazards associated with residual charges on test specimens

Problem: After the test, the test specimen may still retain residual charge, posing a risk of electric shock upon direct contact or when removing the wires.

Solution: Strictly adhere to the discharge procedure—small-capacitance test specimens shall discharge naturally through the internal voltage equalization resistor of the equipment; large-capacitance test specimens must first be de-energized to below 20% of the test voltage, then undergo gradual discharge using a 100 Ω/V discharge resistor rod. The discharge rod shall be progressively brought closer to the test specimen until free discharge occurs across the air gap, and no discharge sound is audible; only then may the rod be directly grounded. After discharge completion, the grounding wire must be connected to the high-voltage output terminal before any disconnection operations can be performed.

V. Technological Innovations and Design Highlights

From a technical standpoint, this device features several innovations:

1. PWM Pulse Width Modulation and Dual-Loop Feedback Technology: By employing dual voltage and current loop feedback control, this design significantly enhances the power supply regulation rate and load regulation rate, ensures high-voltage output stability, and achieves a ripple factor ≤ 0.5%, thereby meeting the requirements for high-precision insulation testing.
2. Nanosecond-level protection sensor: The protection circuit employs a dedicated nanosecond-level sensor, featuring a four-tier protection architecture—zero-point protection, overcurrent protection, overvoltage protection, and breakdown protection—ensuring rapid response and reliable operation, thereby effectively safeguarding both personnel and equipment.
3. Digital DIP switch overvoltage setting: The use of a digital DIP switch for setting overvoltage protection values offers simplified operation and higher precision, eliminating the reading inaccuracies associated with traditional analog potentiometer settings.
4. 0.75 UDC 1 mA Smart Button: A one-touch operation feature specifically designed for testing zinc oxide surge arresters; upon activation, the voltage automatically drops to 75% of its original value and remains at that level, significantly enhancing the efficiency of preventive testing for surge arresters.
5. Electromagnetic Compatibility (EMC) Design: Based on the principles of EMI (electromagnetic interference) and EMC, the design employs shielding, isolation, and grounding measures to ensure that the complete system can withstand rated voltage discharge without sustaining damage.

VI. Key Safety Operating Guidelines

When operating this equipment, the following safety guidelines must be strictly observed:

1. Operators must be qualified technical personnel; at least three personnel shall work in coordination, and two clearly identifiable isolation points shall be installed on the power side.
2. A "High Voltage Hazard – Do Not Approach" warning fence must be installed at the site, and the high-voltage test area must be equipped with an interlock device.
3. The operator must maintain an adequate safe operating distance (not exceeding 20 kV per meter in air).
4. A current-limiting resistor must be connected when testing a capacitive load.
5. Before connecting or disconnecting test leads or power plugs, ensure that the device has been powered off and fully discharged.
6. When the equipment is not in use for an extended period, it should be stored in a dry and well-ventilated area, and should be powered on at least once per month.
   
   Product Images