Based on the characteristics of digital substation equipments， the security and stability control technology implemented in digital substations is compared with that implemented in conventional substations in aspects such as information acquisition method， control mode and means of communication. A security and stability control device based on the IEC 61850 standard is developed according to the requirements of digital substations. The system architecture， hardware platform and software structure of the device are introduced. The field programmable gate array（FPGA） based algorithm for computing electric quantities in real-time is described. Experimental results show that the real-time performance and computation accuracy are superior to those of conventional methods. The advantage of computation accuracy is more evident under the condition when the system frequency deviates from the fundamental frequency of 50 Hz.