Microprocessor Based Relays

Microprocessor systems are similar to simple digital computer systems (Fig. 1), in which the microprocessor performs the timing and control of the system and carries out all arithmetic and logical operations. The system memory may be Read Only Memory (ROM) for dedicated applications or Random Access Memory RAM (RAM) for the storage of data and programs, or a combination of both.

 System memory stores the program to be executed and the data relevant to the specific task. The microprocessor communicates with the system memory by means of a bus system. The same bus system permits communication of the microprocessor with the interface adaptor, or Input/Output (I/O) unit, which makes possible the transfer of data and control signals to and from the system.

As it can be seen from Fig. 1, the microprocessor is quite a complex device with specific terminology and principles of operation that have nothing in common with the protective relays considered above. The question arises if the "microprocessor-based protection device" is a "relay" in the full sense of the word.

On closer examination it turns out that the "microprocessor-based relay" is a small computer in which the output circuits (usually built-in CT or VT - Fig. 2) have matched parameters with external current and voltage transformers, with a program stored in memory, allowing processing of input signals in such a way that operation of this or that type of protective relays can be modeled.

 With the help of a basic universal microprocessor one can create any relay by just making certain changes in the program, at least that is how it used to be at the initial stage of development of microprocessor-based equipment. Opinions are sometimes expressed that protective devices now available on the market are in fact only single-purpose devices designed for execution of a limited s et of functions, typical of relays of some particular type.Such devices have names corresponding to the name of a relay of a particular type, like Frequency Relays for instance, and one can communicate with such a device only with the help of a special program which specially created for this particular device, taking into account all of its peculiarities.

Actually the relay is programmed by inputting certain pick-up thresholds, time intervals, and algorithms of choosing of the proper type, among all possible types of working characteristics, but in this case, limitations are set not for the microprocessor (for which it is all the same, whatever signals to process), but for ROM containing the program of this microprocessor and the number of input and output channels.