The research and exploiture of subharmonic injection scheme for generator stator ground protection

With the development of the electric power industry, the number and the capacity of large generator are increasing. So more requests for the protection device of large unit are demanded in order to ensure the safety and stability of electric power system. Stator ground protection is the important part of generator and transformer protection system, and now stator ground protection is the two-frequency zero sequence voltage protection, including the basic sub-harmonic and the third sub-harmonic zero sequence voltage protection. This paper expatiates the large unit´s requests for protection especially the stator ground protection. For the largening of the capacitance of the large unit, the general two-frequency zero-sequence voltage stator ground protection can not be satisfied the need, so introduce the conventional sub-harmonic injection scheme for hydro-generator stator ground protection, the drawbacks in the application are pointed out as well. Based on equilibrium principle, a new sub-harmonic injection scheme is proposed. In the protection scheme, the fault of the sub-harmonic injection, PT break and the sub-harmonic in the system are considered, and we bring forward the measure reducing basic sub-harmonic through hardware and software, including the fast arithmetic of grounded resistance base on 20 Hz and the 20Hz low-frequency signal filter channels; analysis the affect to the protection from basic sub-harmonic when grounded fault is happening; consider the affect to the precision of grounded resistance form the parameter of transformer, and give the corresponding resolving measure. Aiming at the above problem, we apply the corresponding resolving measure to protection equipment, and achieve an ideal effect in the dynamic simulation experiment and actually running.