This paper analyses the unusual mal-operation of the differential protection for transformer current differential protection during the external fault. The mal-operation reason is researched in detail, based on the mathematical analysis, the simulation, and the field recorded fault data analysis. It is disclosed that the CT saturation leads to the maloperation. Based on this, this paper analyses the improvement for the CT saturation identification method, and discusses the CT selection principles. This paper is aimed at attracting the attention on the problem of the CT saturation and proposing the reference of the analysis method the countermeasure for this problem.

This paper discusses on the protection scheme for the novel flexible short circuit current suppression technology, which applies the new-type fast circuit breaker. The challenges of rapidity, selectivity and reliability for the protection are analyzed. The technology principle of the protection is formulated. The improvement project and the logic coordination under different space levels and time scales is proposed. These new method is to guarantee the safe and sound operation of the power system in the new scenario that applies the novel flexible short circuit current suppression technology.

In order to promote the practice on the DG (distributed generation) technology, this paper introduces the theory research and engineering practice on the protection and control method of the DG system, based on an IDC DG engineering. The basic information and the characteristic of the engineering is described. The general control target and the operation mode under different condition is analyzed, considering the operative characteristics and the requirements of the power substation, the DG and the customer. The coordination scheme between the power grid, the DG and the load is proposed innovatively. The priority and the logical sequence of the control from different part are determined. The practical experiment proves that the protection and control system can provide reliable and efficient power and refrigeration supply for the IDC data central via the coordinated control method of the operation mode shift, SPAS (spare power automatic switchover), load transferring and the black-start, with the coordination of the power grid, the DG and the load. Under the extreme case that the outside power is totally lost, the power and refrigeration supply can be recovered within 155 seconds.