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3 Communication in substation automation

3.1 Basics of data communication

People have learnt how to regulate voltage quickly and efficiently, which is a crucial

knowledge within data communication. George Boole is the person credited for coming up

with the idea of having two different states defining yes or no / on or off / TRUE or

FALSE (hence the name BOOLEAN value). Later on it was realized that Boole’s idea

could be implemented into computer logic. By regulating voltage levels over or below a

predefined setting you can give a logic 1 or a logic 0, where 1 stands for TRUE and 0

stands for FALSE. By having a definite time interval you can change the voltage level for

each time-interval and have the receiving end read the value once every interval. This

voltage level at a specific time is called a bit, one bit can be 1 or 0. By sending these bits

one after another you get something called a bit array, a series of ones and zeroes. After

that a standard tells us what a certain amount and order of these values correspond to, for

example the ASCII defines the printable letter A as 01000001, the symbol & is coded as

00100110. See figure 2 for an illustration of the symbol & as an electrical signal.

8 bit message

+ 5V

- 5V

Figure 2. Illustration of the symbol “&” as an eight bit data message according to the

ASCII code. Time is presented horizontally and voltage vertically.

By putting several of these bit arrays after each other it is possible to send e.g. text

messages over electrical conductors (other mediums like optic fiber, radio waves etc. are

also possible) using the ASCII standard. A few complementary bits need to be added to the

message for functional data exchange, depending on which protocol is used for

communication. The other bits can be for addressing, error detection (parity check),

encryption etc.

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ABB SPA-ZC 400 Especificaciones Pagina 12