For electrical installers, maintenance technicians and project engineers, confirming the correct vector group is a key step before paralleling and commissioning any power transformer and distribution transformer. Wrong vector group judgment will lead to phase mismatch, equipment failure and even grid safety accidents. Most standard type transformer products are clearly marked with vector group codes on the nameplate, which is the simplest and most intuitive way to confirm connection specifications in daily operation and maintenance work.
However, many on-site scenarios involve aging equipment with lost nameplates or transformers that have undergone winding replacement and partial maintenance. In these cases, relying on nameplate information is no longer feasible, and professional testing methods are required to recheck the vector group. The DC induction testing method, a mature and reliable field detection technology, is widely used to verify the winding connection mode of three-phase power transformer and distribution transformer.
Taking the common Yy0 three-phase transformer as an example, both high-voltage and low-voltage windings adopt star connection in this type transformer. The specific test procedure is straightforward and suitable for rapid on-site verification. Technicians first connect DC voltmeters between the low-voltage side terminals ab, bc, and ac respectively. Then apply stable DC voltage to the high-voltage side ab terminal, and carefully observe and record the instantaneous deflection direction of each low-voltage voltmeter pointer, either positive or negative.
Repeat the above operation by applying DC voltage to the high-voltage side bc and ac terminals in sequence, and record all pointer deflection results completely. By analyzing the deflection rule of low-voltage side pointers, technicians can accurately judge the transformer vector group. A fixed deflection law corresponds exactly to Yy0 or Yyn0 connection groups, which are the most common connection modes for conventionaldistribution transformer equipment.
In addition to Yyn0 transformers, Dyn11 and Dy11 are another two mainstream vector groups widely applied in power distribution systems. These two connection types show completely different voltmeter deflection characteristics during DC induction testing. Mastering the distinct deflection rules of these two core groups enables maintenance personnel to quickly identify most conventional type transformer connection modes used in industrial and urban power grids.
As a professional manufacturer, Zhejiang Hengli Electrical strictly standardizes the winding connection and vector group calibration of every power transformer and distribution transformer. We recommend front-line technicians to master the DC induction testing method. It effectively solves the identification problem of unmarked or rewound transformers, ensures accurate equipment commissioning, and guarantees the safe and stable operation of the entire power distribution system.
