1. What is DC resistance testing of transformers?
DC resistance testing is a critical procedure for evaluating transformer winding performance. By measuring the resistance of windings under direct current, it enables inspection of internal wire joint welding quality, tap changer contact conditions, and the presence of open circuits or short circuits. The testing is based on national standards including GB/T 50150-2016 "Electrical Installation Engineering-Handover Test Standard for Electrical Equipment" and GB 1094.1-2013 "Power Transformer-Part 1: General Requirements", as well as the international standard IEC 60076-1.
The current-voltage method (voltage drop method) is employed to measure resistance by passing a DC current through the winding and recording the current and voltage drop, then calculating the resistance value using Ohm's law. Modern systems typically use DC resistance testers that output constant current, measure the voltage across the winding, and automatically calculate the resistance. Depending on the winding connection type (Y or delta), the measurement should target either phase resistance or line resistance. For transformers with neutral point connection, measure phase resistance (e.g., AO, BO, CO); for those without, measure line resistance (e.g., AB, BC, CA). After disconnecting the transformer leads and cleaning the terminals, connect the tester, select an appropriate test current, and wait for the current to stabilize before reading the resistance value. On-load tap changers require measurement at all tap positions, while off-load tap changers must be tested after major overhauls. For transformers rated 1600kVA and above, the interphase resistance imbalance rate should not exceed 2%, and the line-to-line imbalance rate should not exceed 1%. For transformers rated 1600kVA and below, the interphase imbalance rate should not exceed 4%, and the line-to-line imbalance rate should not exceed 2%.
2.What is the measurement of transformer dielectric loss factor?
The measurement of transformer dielectric loss factor is a test that evaluates insulation performance by quantifying energy loss in transformer insulation under alternating current fields. This critical assessment identifies moisture contamination, insulation oil degradation, oil sludge accumulation on windings, and severe localized defects, serving as a key indicator of insulation integrity. Standardized testing protocols are established through commissioning and preventive tests: GB 50150 "Power Equipment Commissioning Test Standard" specifies allowable dielectric loss factor values for transformer windings and bushings: tanδ ≤0.6% for 330-500kV high-voltage windings at 20℃; tanδ ≤0.8% for 66-220kV high-voltage windings; tanδ ≤1.5% for 35kV and below high-voltage windings. DL/T 596-2021 "Power Equipment Preventive Test Regulations" sets additional requirements: tanδ ≤2.0% for 35kV and below transformers, tanδ ≤1.5% for 66-220kV transformers, and tanδ ≤1.0% for 330-500kV transformers at 20℃. The standard also mandates that tanδ values should not exceed 30% increase compared to historical measurements.
3.What are the tests for transformer ratio, polarity, and connection group?
Definition of Transformer Ratio Testing: This procedure measures the ratio of primary to secondary voltage during no-load operation, verifies winding turns ratio accuracy, assesses tap changer status, detects inter-turn short circuits, and provides basis for parallel operation. According to GB 50150-2016 "Electrical Equipment Handover Test Standard for Electrical Installation Projects", transformers below 35kV with ratios under 3 have ±1% allowable deviation; other transformers show ±0.5% deviation at rated taps, while other taps maintain ±1% deviation within 1/10 of impedance voltage. Transformer polarity testing determines voltage-current direction and phase relationships during electromagnetic induction, ensuring correct wiring to prevent equipment damage or safety incidents. It also verifies polarity markings to ensure stable power system operation. Transformer connection group testing verifies winding connection methods (e.g., star Y, delta D) and phase relationships, with the core objective of confirming whether phase differences between high/low-voltage sides meet design specifications. This ensures no phase mismatch-induced circulating currents, overheating, or equipment damage during parallel operation or grid integration.
4.What is the external construction frequency withstand voltage test for transformers?
According to the definitions in the national standards "GB/T 1094.3-2017 Power Transformer-Part 3: Insulation Levels, Insulation Testing, and Air Gap of External Insulation" and "GB 50150-2016 Electrical Installation Engineering-Standard for Handover Testing of Electrical Equipment": The external construction frequency withstand voltage test for transformers is a method used to evaluate the strength of the main insulation. By applying industrial frequency high voltage to the transformer windings, this test examines the insulation performance under normal operating voltage to detect local defects in the main insulation, such as moisture, cracking, or aging. This test serves as a critical means to verify the design, manufacturing, and installation quality of transformers. It is applicable to the neutral point withstand voltage test of fully insulated transformers or step-insulated transformers, as well as the withstand voltage test of low-voltage windings.
5.What is the series resonance withstand voltage test for transformers?
The series resonance withstand voltage test for transformers is primarily based on the following standards: GB/T 16927.1-2011 "High Voltage Testing Technology-Part 1: General Definitions and Test Requirements" establishes fundamental terminology, definitions, and test specifications for high voltage testing, providing a standardized technical framework and testing conditions for series resonance withstand voltage tests. DL/T 596-2005 "Procedural Rules for Preventive Testing of Electrical Equipment" specifies detailed requirements for preventive testing items, testing cycles, and methods for electrical equipment, including the relevant requirements for series resonance withstand voltage tests of transformers.
Series resonance withstand voltage test is a test method which uses the principle of LC series resonance, and by adjusting the power frequency, the inductance (e.g. reactor) in the test circuit and the capacitance of the transformer under test are in resonance, so that high test voltage can be obtained at low power voltage.
This test method demonstrates advantages including optimal voltage waveform and minimal damage to specimens, making it widely applicable in transformer commissioning and preventive testing. By applying a voltage several times higher than the rated voltage to the low-voltage side, the transformer's electromagnetic induction generates test voltage in the high-voltage winding, thereby evaluating the electrical strength of main insulation (e.g., inter-winding and winding-to-ground insulation) and longitudinal insulation (e.g., turn-to-turn and layer-to-layer insulation). For large transformers and other high-voltage, high-capacity equipment, conventional power frequency withstand voltage tests require massive equipment capacity, posing challenges for field implementation. The series resonance test utilizes resonance principles to achieve resonance conditions through frequency adjustment, enabling the generation of required test voltage at lower power supply voltages. This significantly reduces equipment capacity requirements while improving test feasibility and cost-effectiveness. In a series resonance circuit, if a specimen experiences flashover or breakdown during resonance, the resonance condition is immediately lost, causing rapid voltage collapse and arc extinction. This prevents further damage from overvoltage and reduces operator safety risks. Additionally, the optimized voltage waveform minimizes equipment impact. For transformers with graded insulation at the neutral point, where low insulation levels preclude external high-voltage AC withstand voltage testing, the series resonance induction withstand voltage test remains the only effective method for evaluating both main and longitudinal insulation.