How to recognize the saturation of current transformer

- Aug 07, 2020-

The more severe the saturation of the current transformer, the smaller the excitation impedance and the greater the excitation current, which increases the error of the transformer by a factor of two and affects the correct operation of the protection. At present, for the selection of TA, the transient saturation problem has been considered or paid attention to.For example, in the protection of high-voltage systems or large generators and transformers, TPY-level current transformers are generally designed, and PR-level current transformers with small air gaps are selected. ; The protection device itself is required to have a certain ability to resist TA saturation. The following is the method for judging the additional stability characteristic region against TA saturation selected in the transformer differential protection.

TA saturation caused by a short-circuit fault in the protected transformer area is not easy to distinguish by the ratio of differential current and braking current. This is because the measured values of differential current and braking current will be affected, and their ratio will immediately meet the protection operating conditions. At this time, the operating characteristics of the ratio differential protection are still valid, and the fault characteristics satisfy the operation conditions of the ratio differential protection.

For faults that occur outside the protected transformer area, the large saturation of the ride-through short-circuit current caused by it will produce a large false differential current, which is more true when the TA saturation at each measurement point is different. serious. If the working point of the resulting value falls within the operating characteristic area of the ratio differential protection, and no measures are taken to stabilize the ratio differential protection, the ratio differential protection will malfunction.

The differential current obtained by calculating the amount of current on each side of the transformer according to Kirchhoff's current law is basically balanced in the initial short time, and only a small unbalanced current will be generated, and a large difference will be generated after TA saturation. Dynamic current, bow|from transformer differential protection misoperation. Therefore, the transformer differential protection can be set with an additional stability characteristic zone when TA is saturated, which can distinguish the internal and external fault conditions of this transformer.