Communication requirements of Substation Automation architecture

Introduction of microprocessors in to substation allows process data in digital form. to convert analog data in to digital ADCs are used. These digital data is not distorted due to aging of the hardware and can easily exchanged  by serial communication. But these serial communication introduces additional delays.Also information processing hardware must withstand harsh environment in the substation, specially EMI.
The data is acquired at the process level by means of remote i/o units (RIO) and intelligent sensors (PISA = process interface for sensors and actuators). The process bus connects them to the bay level equipments.

Communication requirements:
Maximum allowed age - worse case response time can be tolerated. This means, that this time must be guaranteed in normal operation.
Data integrity - degree of communication safety in the case of disturbances. If data is directly influence the process those data has higher integrity.
Exchange method - Spontaneous mean communicated as soon as it happens. Request means communicated on request by some function or human.

Alarm - 1s - Medium - Spontaneous
Commands - 1s - High - Spontaneous
Process sate data -2s (binary) 5-10s (measured) - Medium - Spontaneous (gives overview of the process state)
Time stamped events - 10s - low - On request (used for later analysis)
Interlocking data - 5ms - high - Spontaneous (used to prevent dangerous commands)
Interlocking data (state info) - 100ms - high - On request
Trip from protection - 3ms - high - Spontaneous (used to clear faults)

The actual communication throughput capacity must be higher than needed for normal operation( at least 10% higher). When we design the communication system we should avoid the single point of failure.
No communication message failure shall lead to a unsafe action. This can be tackled using communication error detection mechanisms and making transmission media immune to disturbances( reduce number of bit errors). Today all process buses are typically a Hamming distance of at least 4 - 6, to detect transmission errors. This is sufficient for medium integrity. In substation error rate is higher than the telecommunication environment. There for we use glass fiber in the process bus and special communication procedures like "select before use" is introduced.
No lost or late message is allowed to lead to unsafe action.Messages could be lost due to buffer overflows or overloaded routers and switches. There for lost messages and loss of message source should be detected. In IEC 61850 topical flag is used to indicate data is up to date.Glass fiber can cover a distance up to 2000m with out loosing transmission speed. while plastic fiber is used for shorter length( tenth of meters). Also plastic fiber is aging sooner than glass.

Today we can place the microprocessor based relays close to the process. In new architecture physical signal marshaling is replaced by logical signal marshaling, which means complexity is the same. Electrical CAD systems are replaced by signal engineering tools.

For redundancy we duplicate the protection devices at least in HV substations.
To provide passive safety in logical nodes it sends at least two telegrams before a command is executed. This two step approach is called select before approach (SBO). HMI send the select command to CBC node. Then CBC sends selected command back to HMI. Then only HMI sends the operate command to the exact switch.