Doubly fed induction generators and permanent magnet synchronous generators are widely used in onshore wind farms. Influenced by the control strategy, the fault characteristics of the wind farm side are significantly different from those of the synchronous source, and this degrades the performance of traditional pilot protection. Tellegen's theorem is based on Kirchhoff's voltage and current laws. It is only related to circuit topology and can be applied to circuits with linear and nonlinear, time-varying and time-invariant components. In this paper, Tellegen’s theorem is applied to an onshore wind farm transmission system, and the fault-related topological structure diagram satisfying the theorem is established. The characteristics identifying internal and external faults are analyzed, and a new pilot protection criterion using pre-fault and post-fault voltages and currents at both ends is proposed. Fault recording data from field cases and the PSCAD-based simulation data are used to carry out verification and comparative analysis on the criterion. The results have proved the superiority and feasibility of the proposed protection method.