Test transformers, also called neutral point couplers, are often used in three-phase power systems without or without a neutral point to provide an artificial neutral point for the system. The neutral point provided by the test transformer enables the system to adopt grounding methods such as direct grounding, resistance grounding, and arc suppression coil grounding to achieve the purpose of limiting single-phase grounding fault current and suppressing the rise of non-fault phase voltage.

Test transformers have different structures according to the operating conditions of the power system where they are located and the needs of the substation. Manufacturers usually design according to the actual needs of customers. Existing materials provide few test introductions to test transformers. In this paper, combined with the operating status of the test transformer under different conditions, a simple analysis and discussion of the temperature rise test of the test transformer are carried out.

A test transformer with a rated continuous neutral point current and a secondary winding is specified. For this type of test transformer, the potential offset of the neutral point is estimated according to the three-phase load of the system when the user purchases, and the neutral point is used in combination with the test transformer. The neutral point grounding method gives the value of the rated continuous neutral point current, and the test transformer is required to operate normally under this continuous current.

In addition, taking into account the requirements of the power supply used in the substation, the secondary winding is added. Under normal conditions of the system, the operation mode of the test transformer mainly includes the normal state where the neutral point of the three-phase load balance does not shift, and the three-phase load is unbalanced. The neutral point is shifted, and the secondary side has a rated load.

Under the normal state of the three-phase load balance, the neutral point potential of the grounding transformer system is close to the ground potential, and only a small excitation current flows in the primary winding. The loss generated by the test transformer is the loss of the iron core and the small primary winding. Direct resistance loss, in this case, only the iron loss increases the temperature rise, and the temperature rise of the winding to the oil is almost equal to zero.

If the system's three-phase load imbalance causes the neutral point potential to shift, the existence of this potential will generate a continuous zero-sequence current in the ground loop, which will cause resistance loss and zero-sequence magnetism in the primary winding of the test transformer If there is a load on the secondary side of the test transformer, it is necessary to consider the direct resistance loss generated by the low-voltage side load current in the secondary winding and the additional loss on the winding at the same time in the above two operating states. And the influence of transformer oil temperature rise.

In the combined operation form of the test transformer, the normal state and the neutral point offset state will not appear at the same time, and the load state on the secondary side may also be superimposed with any state, so the temperature rise test should consider the above various combinations.