Conventionally, protection and control devices are used for the protection and control of an electric power system. In case that this protection and control device judges that a fault has occurred in the electric power system based on electric variables of the electric power system, it performs control such as opening a circuit breaker.
In addition, the protection and control device is made redundant so as to perform protection and control operation using two or more sets of hardware in order not to perform erroneous control to a circuit breaker, even in case that a malfunction occurs in a part of the hardware and software of the protection and control device. The protection and control devices which are made redundant are called a main detection relay (hereinafter, called a Main (Main)) and a fault detection relay (hereinafter, called a FD (Fault Detector)), and are composed of different hardware respectively, to thereby realize proper protection and control of the electric power system without performing erroneous control (erroneous operation), even in case that a malfunction occurs in one hardware. In addition, the redundancy is to arrange and operate a standby device as backup even in the normal time, in preparation for the case in which a failure of some kind occurs in a part of a system, so that the function of the whole system can be continuously maintained even after the occurrence of the failure.
Furthermore, a protection and control system is recently thought of which connects a merging unit (hereinafter, called a MU (Merging Unit)) to detect electric variables of a current transformer (hereinafter, called a CT (Current Transformer)) and a voltage transformer (hereinafter, called a VT (Voltage Transformer) which are installed in an electric power system, and a protection and control device to judge the presence or absence of a fault based on the detected electric variables, through a network called a process bus. Here, the MU detects the electric variables, and transmits electric variable information to the protection and control device through the process bus. In addition, the protection and control device receives the electric variable information from the MU through the process bus, and judges whether or not a system fault has occurred in a zone to be protected based on the received electric variable information.
Here, when it is judged by the protection and control device that a system fault has occurred, the MU opens a circuit breaker or a switch which is connected. This conventional MU is not provided with a trip circuit and so on, and the judgment whether or not to open the circuit breaker or the switch is dependent on the judgment of the protection and control device. Even in the protection and control device of the above described protection and control system to which the process bus is applied, it is required that the protection and control device is provided with a Main and a FD which are composed of different hardware, respectively, and is thereby made redundant. But, in case that a plurality of protection and control devices are connected to the process bus, since the protection and control devices are made redundant, respectively, the hardware and the cost thereof increase.