Combined transformer is a combined device of current transformer and voltage transformer that share an insulating sleeve. The two transformers are independent in electrical function, so they can be verified separately according to current transformer and voltage transformer. The three-phase combined transformer is also called high-voltage power metering box. Usually, two current transformers and two voltage transformers share a box or resin casting mold. The functions of each set are still relatively independent and can be verified separately one by one. Three phase voltage transformer is also a combined transformer that shares a set of iron cores. It requires three-phase voltage verification. The error of the combined transformer in the separate check and punctuality of each set is not equal to the error in the practical operation, because they have electromagnetic influence on each other. However, because they have noticed that the influence quantity is limited to a certain scale in the design and manufacture, it can be judged that the combined transformer is qualified only if each transformer is qualified separately and the mutual influence quantity does not exceed the specified value. As long as a certain influence quantity exceeds the tolerance condition in a certain transformer, it needs to be analyzed in combination with its practical errors. In short, this transformer needs to reduce the fundamental errors to compensate for the additional errors of other influence quantities.