MQ21, the power quality analyzer externally connected with open type Rogowski coil or voltage CT, it can realize no dismantling wire test, facilitate engineering test, simplify test steps, save construction cost, and is also more convenient for the inspection and maintenance of distribution system. MQ21 supports systems of single-phase and three-phase: it can measure multiple electrical parameters such as voltage, current, harmonic, energy, power factor and active power on the power grid of phase A, B, C and phase N. With standard configuration of RJ45 and RS485 communication interfaces, through standard Modbus TCP and Modbus RTU protocols, it can be compatible with various configuration systems and transmit the electrical parameters collected by the front end to the system data center in real time.