With the increasing reliance of power system monitoring and control over communication protocols, the possibility that entities with malicious intent can gain access to and are able to alter operational commands has become more likely. While it is possible and advisable to include intrusion detection/prevention techniques to examine and verify the syntax of protocol messages, an alternative approach to threat detection via the use of a fast-approximate state estimation techniques is proposed and investigated. The threat under consideration targets power commands on high-voltage direct current (HVDC) lines in AC networks. The proposed fast-approximate state estimation-based approach involves computing the system state estimate at frequent intervals and performing consistency checks between the observed system status and the relayed power order commands. For computational tractability and time constraints, we utilize the use of a reduced measurement set and limiting the number of iterations in the state estimation process. The trade-offs in accuracy due to variation of iteration counts and size of the reduced measurement set are explored. The state estimation-based approach is tested under a simulated power order threat and some results are presented. The approach provides some bounds on the extent to which an attacker can corrupt power orders and evade detection.