Transformer test transformer (also known as "power frequency withstand voltage test device" or "high voltage test transformer") is an indispensable core equipment in preventive tests and handover tests of power equipment. It is specially used to generate high voltage (AC, DC or impulse) to conduct power frequency withstand voltage tests on transformers, circuit breakers, cables, lightning arresters, insulators and other electrical equipment, so as to assess whether their insulation strength meets the safe operation standards. Unlike ordinary power transformers, test transformers have characteristics such as high voltage, small capacity, short working time, large insulation margin, and small leakage reactance, usually with short-time duty (e.g., 1 minute). According to the latest technical standards in 2025-2026 (such as DL/T 848.2-2019, GB/T 16927) and market application conditions, here is a detailed guide: 1. Core classification and working principle According to the insulation medium and structural form, they are mainly divided into the following three categories: A. Oil-immersed test transformer (Oil-immersed) Features: Uses transformer oil as the insulation and cooling medium. Advantages: Low cost, large capacity (up to tens of kVA or even higher), simple structure, easy maintenance, and strong overload capacity. Disadvantages: Large volume and weight, risk of oil leakage, not suitable for frequent movement or use indoors with high fire protection requirements. Applicable: Substation sites, large factories, outdoor operations, suitable for withstand voltage tests of conventional equipment up to 35kV and below. B. Dry-type test transformer (Dry-type / Epoxy) Features: Uses epoxy resin casting or composite insulation materials, oil-free. Advantages: Small size, light weight, no oil leakage pollution, fireproof and explosion-proof, maintenance-free, can be placed vertically or horizontally. Disadvantages: Slightly weaker heat dissipation, high manufacturing cost for large capacity, usually more expensive than oil-immersed type of the same specification. Applicable: Indoor laboratories, clean workshops, places with strict environmental protection and fire protection requirements (such as subways, data centers). C. Gas-insulated test transformer (SF6 Gas-insulated) Features: Uses SF6 gas as the insulation medium, fully enclosed metal shell. Advantages: Extremely small size (only 1/3 of oil-immersed type), extremely light weight, excellent insulation performance, no fire risk, moisture resistant, suitable for high-altitude areas. Disadvantages: Expensive, extremely high sealing requirements, and regular gas pressure inspection required. Applicable: Ultra-high voltage testing (110kV and above), high-altitude areas, space-constrained GIS rooms, high-end laboratories. 2. System composition A complete set of power frequency withstand voltage test device usually includes: Test transformer body: The core for voltage boosting. Control box (console): Provides low-voltage power supply, with voltage regulation (autotransformer or induction), timing, overcurrent/overvoltage protection, zero-position start and other functions. Modern control boxes mostly use digital display and touch screen operation. Supporting accessories: High-voltage silicon stack (for DC withstand voltage), voltage divider (for measuring high voltage), discharge rod, special high-voltage wires, etc. Series resonance system (optional): For long cables or large-capacity test objects, the test transformer is often used as the excitation power supply, combined with reactors and capacitive voltage dividers to form a series resonance device, so as to reduce the required power supply capacity. 3. Mainstream brands and market landscape in 2026 The domestic market is still dominated by the Wuhan industrial cluster, with mature technology and extremely high cost performance. Domestic leading brands (recommended first choice) Wuhan UHV Electric Power Technology Co., Ltd.: Status: Industry benchmark, product line covers the full series of oil-immersed, dry-type, and gas-insulated types. Core technology: Its gas-insulated (SF6) transformer excels in lightweight and high-altitude adaptability; oil-immersed products use new modified transformer oil, with withstand voltage level and heat dissipation performance better than national standards. Features: High degree of intelligence in control box, with "one-key boost", "automatic timing", and "breakdown protection" functions, and data can be stored and uploaded. Applicable: Full-scenario coverage from 10kV distribution network to 500kV UHV. Wuhan Huadian Keyi / Wuhan Weian Electric: Features: Established manufacturer, high market share in oil-immersed transformers, durable and reliable, complete after-sales network. Advantages: High cost performance, suitable for large-scale procurement by grassroots power supply stations and engineering companies. Yangzhou Tuopu / Shanghai Shengxu: Features: Unique in the miniaturization design of dry-type transformers, suitable for portability. Internationally renowned brands Germany Haefely, USA Hipotronics, Austria OMICRON: Features: Exquisite craftsmanship, good long-term stability, often used as standard sources or in top-level laboratories. Disadvantages: Expensive (5-10 times the domestic price), long delivery time, inconvenient maintenance. Applicable: National-level electric power research institutes, foreign-related projects, and occasions with hard requirements for brands. 4. Key indicators for selection When purchasing, please pay attention to the following parameters: Rated output voltage: Must be higher than the maximum test voltage of the test object. For example: To test 10kV cable (AC withstand voltage about 17.4kV-21kV), it is recommended to choose a test transformer of 30kV or 50kV; to test 35kV equipment, it is recommended to choose 80kV or 100kV. Note: The voltage division ratio of the voltage divider and line voltage drop should be considered, leaving a margin (usually 20%). Rated output capacity (kVA): Calculated based on the capacitive current of the test object: