I. Why are high-voltage test transformers needed?
With the large-scale construction of ultra-high voltage transmission projects such as ±1100kV direct current and 1000kV alternating current, the insulation performance of power equipment has become the core bottleneck restricting the safe, stable and long-term operation of the power system. The core function of conventional power transformers is to achieve voltage transformation and stable transmission of electrical energy, while high-voltage test transformers have completely differentiated core purposes, mainly used for destructive testing of power equipment at high voltages.
The working principle of these transformers is to actively apply a high voltage load several times the rated voltage of the equipment to the inspected power equipment, accurately detecting hidden insulation defects, process hazards and performance weaknesses within the equipment, ensuring the safety and stability of the operation of ultra-high voltage transmission equipment after entry into the grid and commissioning, and being an indispensable core test equipment for quality inspection and performance verification of ultra-high voltage power equipment.
II. Core Classification and Technical Characteristics
Based on the type of insulation medium and overall structure, high-voltage test transformers can be classified into three main types. The structural characteristics, performance advantages and application limitations of each type of equipment are as follows:
(1) Oil-immersed test transformer
Structural characteristics: The equipment’s core components, including the iron core and windings, are fully enclosed in a steel oil tank filled with dedicated transformer oil. The transformer oil also serves as both the core insulation medium and the cooling medium for the equipment, ensuring stable operation.
Performance advantages: It has excellent insulation performance, with a breakdown field strength of over 40kV/2.5mm; it has sufficient heat capacity and good heat dissipation stability, and can be adapted to long-term and continuous high-voltage withstand test scenarios, suitable for high-intensity test operations.
Application limitations: The overall equipment is large in size and heavy in weight, making it difficult to handle and install; there is a certain risk of oil leakage, and it cannot be used in non-oil, high-cleanliness environments such as clean laboratories and precision equipment testing workshops.
(2) Dry-type (epoxy resin cast) test transformer
Structural characteristics: It is manufactured using an epoxy resin vacuum casting integrated molding process, with no liquid insulation or cooling medium inside the equipment, and a compact and sealed overall structure.
Performance advantages: It is maintenance-free in the future, completely eliminating common faults such as liquid leakage and equipment moisture; it has outstanding safety performance, featuring flame retardancy and explosion-proof characteristics, and has strong environmental adaptability, and can be widely used in outdoor on-site maintenance tests and indoor conventional laboratory testing scenarios.
Application limitations: Due to the technical limitations of the epoxy resin casting process, the single-unit voltage rating of this type of equipment is relatively low, and conventional products generally have a voltage rating lower than 300kV, unable to meet the requirements of ultra-high voltage level tests.
(3) Series-connected test transformer
Working principle: It utilizes the technical method of connecting multiple standardized transformer units in series for excitation to achieve voltage superposition, breaking through the voltage upper limit of a single unit. For example, a combination of two 300kV transformer units can achieve a 600kV high-voltage output.
Application scenarios: It is specially developed for ultra-high voltage transmission scenarios of 750kV and above, mainly used in national-level high-voltage measurement and testing stations, ultra-high voltage test bases, and other high-end, high-voltage-level professional test scenarios.
Technical difficulties: The design and debugging accuracy of the equipment are extremely high, requiring precise calculation and matching of the magnetic circuit balance and insulation coordination parameters of each unit to avoid abnormal overvoltage caused by series resonance and ensure the stability and safety of the test process.
III. Core Technical Difficulties and Solution Strategies
(1) Partial Discharge (PD) Control
The partial discharge quantity is the core indicator for determining the quality and operational reliability of high-voltage test transformers, directly affecting the accuracy of test data and the service life of the equipment. Industry quality equipment standards require that under 100% rated voltage conditions, the apparent discharge quantity of the equipment be ≤ 5pC. Solution Strategy: Adopt a π-type shielded winding structure design, combined with the full-process vacuum impregnation process, to completely eliminate the tiny air gaps and cavity defects existing within the winding, and suppress the generation of local discharge phenomena at the structural level.
(II) Waveform Distortion Suppression
According to the relevant test standards of IEC 60060-1, the waveform distortion rate of the no-load voltage of ultra-high voltage high-voltage test equipment needs to be controlled within 3%. Waveform stability is the key to ensuring the compliance and validity of the test results.
Solution Strategy: Optimize the core design, reserve sufficient working magnetic flux density margin for the core, avoid entering the magnetic flux saturation zone during equipment operation, which may cause waveform distortion; at the same time, equip with an appropriate filter reactor to further optimize the voltage waveform quality and strictly control the distortion rate from exceeding the limit.
(III) Capacitive Rise Effect Control
When the test object is a long cable, GIS equipment, etc., with large capacitance loads, capacitive current will flow through the equipment’s leakage inductance, causing the actual output voltage at the high-voltage end of the equipment to be higher than the theoretical set value, that is, the capacitive rise effect. This may cause test data deviation and equipment over-voltage damage.
Solution Strategy: Abandon the method of converting the voltage through the low-voltage side instrument, and must be equipped with a capacitance divider or resistive-capacitive divider to directly measure the actual voltage at the high-voltage end in real time, ensuring the authenticity and validity of the test voltage parameters.
Four. Standardized Test Procedures and Safety Regulations
The operation of ultra-high voltage high-voltage tests must strictly follow the full-process control mode, operate in accordance with standardized norms in the three stages of before the test, during the test, and after the test, to fully avoid safety risks and ensure the compliance, safety, and orderly conduct of the test.
(1) Pre-test Preparation
1. Grounding Network Verification: Before the operation, the grounding system must be inspected to ensure that the grounding resistance value is less than 0.5Ω, ensuring the safety and reliability of the grounding.
2. Safety Isolation Layout: Set up the high-voltage safety warning line according to the voltage level, and follow the standard safety distance standards. For a 100kV voltage level, the minimum safety distance is 1.5 meters. For each 100kV voltage increase, the safety distance should be increased by 0.6 meters to prevent personnel from entering the high-voltage dangerous area by mistake.
(2) Test Execution Standards
1. Voltage Rise Control: When conducting insulation withstand tests on the equipment, adopt standardized voltage rise methods, such as the 20-second rapid rise method or the 1-minute stepwise rise method. Strictly control the voltage rise rate to avoid damage to the equipment due to sudden voltage increase.
2. Safety Protection Configuration: The equipment must be equipped with overcurrent relays and overvoltage trip devices. The overcurrent relay action current should be set at 1.2 times the rated current. In case of abnormal conditions, it can quickly trip for protection.
(3) Post-test Safety Disposal (Core Key Steps)
1. Adequate Discharge Treatment: After cutting off the high-voltage input power supply, it is strictly prohibited to directly touch the high-voltage end of the equipment. Priority should be given to using a dedicated discharge rod with a current-limiting resistor to complete the discharge to ground resistance at the tested object’s high-voltage end, and then use a metal rod for direct discharge to completely release the residual voltage of the equipment.
2. Residual Charge Elimination: For DC test equipment, after the test, a static waiting time of no less than 5 minutes should be reserved to completely dissipate the residual static electricity of the equipment and prevent electrostatic injury accidents.
Post time: May-28-2026