Wireless sensor networks (WSN) are expected to be used to significantly improve the flexibility and expansibility of power distribution networks. In a WSN enabled power distribution network, the accuracy and energy consumption of node localisation can critically affect the stability of a power distribution network. In this paper, a triangle-based localisation algorithm (TBLA) is proposed to improves the traditional midnormal-based localisation algorithm (MBLA) to satisfy the communication requirements of the smart distribution automation. The proposed algorithm constructs a triangle by connecting two anchor nodes that position as one side, and then uses the RSSI (Received Signal Strength Indication) ranging value between two anchor nodes and the unknown node as the other two sides. Compared with MBLA, the simulation results reveal that, TBLA has significantly improved the precision of node localisation precision. Meanwhile, the average iterations are reduced by half, decreasing the energy consumption of the entire power distribution network significantly.