Dry-type transformers have become indispensable core equipment in modern power distribution systems due to their oil-free, fire-resistant, and environmentally friendly advantages. In applications where safety is critical — such as commercial buildings, rail transit, data centers, and renewable energy stations — the reliability of dry-type transformers directly impacts the stable operation of the entire power system.
So, after a dry-type transformer is assembled at the factory, how can we ensure it will operate safely and efficiently for ten or even twenty years? The answer lies in routine tests (also known as出厂试验 in Chinese). Routine tests are the final and most critical quality checkpoint in the manufacturing process. According to international standards IEC 60076-11 and national standards GB/T 1094.11 and GB/T 10228, every transformer must undergo a series of rigorous routine tests before leaving the factory to ensure that all performance indicators meet design and specification requirements.
As a well-known manufacturer specializing in high and low voltage electrical equipment, XINHONG ELECTRICAL (Jiangsu Xinhong Electrical Equipment Co., Ltd.) adheres to the philosophy of “quality first, testing foremost.” Each dry-type transformer undergoes a full set of standardized routine test procedures before shipment, ensuring that every product delivered to customers withstands the test of time and operation.
Now, let’s walk into our factory’s testing workshop and take a detailed look at the routine test items for dry-type transformers, along with the technical logic behind each test.
This is the “first checkpoint” of routine tests and the most intuitive basic inspection.
Inspectors carefully check whether the coil and core surfaces are clean, free of damage or discharge marks; whether insulators and bushings have cracks or contamination; whether lead connections are secure and bolts are tightened; and whether nameplate data is clear, accurate, and consistent with the contract.
Although visual inspection seems simple, it serves as the “frontline” for detecting potential problems during manufacturing — loose windings may cause abnormal vibration during operation, and loose fasteners may lead to failures over long-term operation.
At XINHONG ELECTRICAL, each dry-type transformer undergoes an independent visual inspection by a highly experienced quality engineer before shipment, with photographic records kept to ensure perfect appearance and structural integrity.
DC resistance measurement is a core test for evaluating the integrity of conductor connections in a transformer.
Testers use a DC resistance tester to measure the DC resistance values of each phase winding on both the high-voltage (HV) and low-voltage (LV) sides, then calculate the three-phase unbalance rate. According to GB/T 1094.1, the difference in DC resistance between phases should not exceed 2% of the three-phase average value, and the difference between lines should not exceed 1% of the three-phase average value.
What is the practical significance of this test? Simply put, abnormalities in winding DC resistance can reveal multiple hidden issues: broken strands or poor soldering in conductors, poor contact of tap changers, and inconsistency in conductor material or cross-section among phases. If the resistance value of one phase is significantly higher or the three-phase unbalance rate exceeds the limit, there is an internal contact problem or manufacturing defect that must be resolved before shipment.
XINHONG ELECTRICAL uses high-precision DC resistance testers and compares measurement results with theoretical design values to ensure absolute accuracy of winding resistance parameters.
The core function of a transformer is to convert one voltage level to another, and the accuracy of the voltage ratio directly determines voltage conversion precision.
Testers use a turns ratio tester to measure the voltage ratio at all tap positions and compare them with the rated ratio marked on the nameplate. According to GB/T 1094.1, the voltage ratio error at all tap positions must not exceed ±0.5%, and the vector group (e.g., Dyn11) must be verified to match the nameplate.
Why is strict voltage ratio testing necessary? In power systems, when multiple transformers operate in parallel, if the deviation in voltage ratio exceeds the allowable range, circulating currents will flow between the parallel windings. This not only increases losses but may also cause winding overheating or even damage in severe cases. Therefore, accurate voltage ratio is a prerequisite for safe parallel operation of transformers.
Each XINHONG ELECTRICAL dry-type transformer comes with a voltage ratio measurement report upon shipment, with data traceable to production batches and raw material batches, providing customers with transparent, complete quality evidence.
Insulation resistance measurement is a fundamental test for assessing the condition of the transformer’s insulation system and an important basis for determining whether insulating materials are damp, aged, or contaminated.
The test uses a 2500V megger (insulation resistance tester) to measure the insulation resistance of the HV winding to ground, LV winding to ground, and between HV and LV windings. According to GB/T 1094.11, at room temperature, the insulation resistance should not be less than 1000 MΩ, and the absorption ratio (R60s/R15s) should be greater than 1.3, or the polarization index (R10min/R1min) should be greater than 1.5.
Insulation resistance is the first line of defense for safe transformer operation. If the insulation resistance value is too low, it indicates moisture or contamination in the insulation system, making insulation breakdown highly likely under operating voltage.
XINHONG ELECTRICAL selects high-performance insulation materials (such as epoxy resin casting systems) and starts with raw material quality control to ensure that the insulation performance of each dry-type transformer far exceeds standard requirements.
The power frequency withstand voltage test is the most direct assessment of the main insulation strength of a transformer.
During the test, a specified power frequency test voltage (typically 1.3 times the rated voltage — for example, 13kV for a 10kV winding) is applied from the HV winding to ground for 60 seconds. The LV winding also undergoes similar withstand voltage testing. If no breakdown or flashover occurs during the test, the result is considered satisfactory.
This test simulates the transient overvoltages (such as residual lightning impulse voltages or switching overvoltages) that a transformer may encounter during operation. For a 10kV class dry-type transformer, the power frequency withstand voltage requirement is 35kV for 60 seconds. This tests not only manufacturing process quality but also the collaborative safety of the entire insulation system.
The induced voltage withstand test complements the power frequency withstand test, primarily assessing the inter-turn and inter-layer insulation strength of the windings.
During the test, a voltage of twice the rated voltage at a frequency of 100–200 Hz is applied to the LV side for 60 seconds. No breakdown or flashover is allowed.
Why is a higher frequency needed? The higher frequency of the induced voltage test can more effectively excite potential defects in inter-turn and inter-layer insulation that might be missed by conventional power frequency tests.
XINHONG ELECTRICAL is equipped with a complete partial discharge-free induced voltage test system, ensuring precise and reliable execution of this test and eliminating potential inter-turn insulation defects before commissioning.
No-load loss and no-load current are core parameters that reflect the core performance of a transformer.
During the test, rated voltage is applied to the LV side, and the no-load loss (P₀) and no-load current (I₀%) are measured on the HV side. According to GB/T 1094.1, the measured values of no-load loss and no-load current must not exceed the design values by more than +15%.
No-load loss mainly comes from hysteresis loss and eddy current loss in the core, which are closely related to the quality of the core silicon steel sheets and the stacking process. If no-load loss exceeds the standard, it indicates poor core material quality or manufacturing defects, directly increasing the transformer’s energy consumption during round-the-clock operation. Excessively high no-load current may indicate core saturation problems or improper winding turn design.
XINHONG ELECTRICAL uses high-quality cold-rolled grain-oriented silicon steel sheets for all cores, combined with precise stacking and annealing processes, ensuring that no-load loss and no-load current remain at industry-leading levels.
This test evaluates the winding current-carrying capacity and copper loss level of the transformer.
During the test, the HV side is short-circuited, and current is applied to the LV side up to the rated value. The short-circuit impedance (Zₖ%) and load loss (Pₖ) are measured. The standard requires that the measured short-circuit impedance and load loss must not exceed the design values by more than +15%.
Short-circuit impedance affects the magnitude of fault currents in the system. For transformers operating in parallel, consistency of short-circuit impedance is crucial. Load loss reflects winding copper loss; exceeding the standard indicates improper winding design or high contact resistance. Both are important indicators for evaluating the economic efficiency of a transformer.
In the production system of XINHONG ELECTRICAL, load loss is incorporated into the product energy efficiency classification management system, continuously promoting energy conservation and reducing long-term operating costs for customers.
Partial discharge (PD) measurement is the most technically demanding item in dry-type transformer routine tests and a core method for detecting “hidden diseases” inside the insulation.
During the test, a voltage of 1.5 times the maximum phase voltage is applied, and a PD detector is used to measure the discharge magnitude. According to GB/T 1094.11, the partial discharge magnitude should not exceed 10 pC.
Partial discharge measurement is so important because it can “hear” the discharge signals emitted by tiny internal defects in insulation materials (such as voids, impurities, or micro-cracks). Even if a product passes the withstand voltage test, minor partial discharges can remain latent, gradually eroding the insulation and eventually leading to complete breakdown. Therefore, partial discharge measurement is a key guarantee for the long-term reliable operation of dry-type transformers.
XINHONG ELECTRICAL is equipped with a low-background-noise, partial discharge-free test system that keeps system noise below 2pC, enabling highly sensitive PD detection and making hidden defects impossible to ignore.
Modern dry-type transformers are typically equipped with temperature controllers, cooling fans, alarm systems, and other auxiliary devices. During routine tests, these devices must be functionally verified: proper fan start/stop operation, accuracy of temperature controller setpoints, and reliable operation of alarm and trip contacts.
These auxiliary devices act as the “guardians” of intelligent transformer operation. When winding temperatures exceed limits, the temperature controller should automatically start the fans or issue an alarm to prevent overheating damage. Although this inspection may seem minor, it directly affects the transformer’s reliable operation under load fluctuations or high-temperature summer conditions.
XINHONG ELECTRICAL (Jiangsu Xinhong Electrical Equipment Co., Ltd.) is a professional electrical equipment manufacturer with over ten years of industry experience, specializing in the R&D and production of HV/LV switchgear, substations, dry-type transformers, cable distribution boxes, and more. Our products hold SGS/BV international certifications as well as China Compulsory Certification (CCC).
In the field of dry-type transformers, XINHONG ELECTRICAL offers SCB series epoxy resin cast dry-type transformers, with capacity ranging from 30kVA to 10,000kVA and voltage levels including 6kV, 10kV, and 20kV, widely used in industrial power distribution, commercial buildings, renewable energy, and other scenarios.
Every dry-type transformer from XINHONG ELECTRICAL undergoes a complete quality cycle from material inspection and in-process quality control to routine testing. The routine test report is delivered with the product, detailing every test result, the tester’s signature, and standard comparison data, ensuring traceability and accountability for any issues.
Routine testing of dry-type transformers is not a meaningless “formality” but a comprehensive assessment of design, materials, process, and assembly. From every detail of visual inspection to the precision of partial discharge measurement, each test accumulates data and eliminates risks to ensure long-term, high-reliability operation.
Choosing a XINHONG ELECTRICAL dry-type transformer means not only selecting a stable-performing electrical device but also a rigorous, transparent, and standardized quality assurance system. Each routine test report delivered with the product is XINHONG ELECTRICAL‘s most genuine commitment to quality.
If you need more information about XINHONG ELECTRICAL dry-type transformers, technical specifications, or customized solutions, please visit our official website or contact us directly.
