Influence of second harmonics of short circuit current on transformer protection in 100% new energy high voltage grid at county level

China has developed and utilized new energy represented by wind power and photovoltaic power generation on a large scale. The proportion of new energy generation capacity in the power grid is constantly increasing. The short-circuit current characteristics of 100% new energy high-voltage power grid at county level are complex, and the proportion of the second harmonic component in the short-circuit current may exceed the second harmonic braking setting range of the transformer, resulting in the misoperation of the second harmonic braking element. This paper first introduces the main principle of transformer protection of 100% new energy high-voltage power grid at county level, and then explains the influence of fault transient characteristics of 100% new energy high-voltage power grid at county level on transformer protection. Finally, based on the PSCAD/ EMTDC electromagnetic transient simulation model, the influence of the second harmonic of short circuit current of 100% new energy high voltage power grid on transformer protection is analyzed. It is pointed out that the short circuit current provided by new energy power supply may contain a large second harmonic component, which leads to the failure of transformer differential protection to remove internal faults quickly, and the risk of differential protection to refuse operation.