Case study of distribution substation adapting IEC 61850 protection system

Reasons to make substation digital

Existing substation technology has a lot of issues in terms of designing and installing field wiring, and the number of protective relays required for protection. The constraint on using technology is more the perception of the functionality or practicality.

Field wiring, and field wiring design and installation, is directly addressed by the IEC 61850 standard for digital communications in a substation. However, the perception of what the standard is, or should be, limits the practical applications and design choices.

The term “IEC 61850” is associated with many concepts, and has negative connotations that include “complicated”, “complete change in design philosophy”, and “communications network based”.

This results in the perception that IEC 61850 technology is not ready for widespread adoption, or is too complex to be of value for simple projects such as a protection for a small rural substation.

Multiple zone protective relays that provide up to 6 zones of independent protection exist today, and are commercially available. However, their use is limited, as the perception is that field wiring is a significant issue.

Assuming each feeder requires 4 currents, 3 status points, and 2 control points results in each feeder requiring 9 pairs of copper wires. This requires 54 pairs of copper wires for such a relay, a difficult and impractical design. Other perceptions include that operating and maintaining such a relay is more complex procedurally, as isolation for test is more difficult.

IEC 61850 technical concepts

To address this negative perception of IEC 61850, and multi-zone protective relays, it is useful to look at the three powerful technical concepts contained within the IEC 61850 standard. These concepts are self-description of data, peer-to-peer communications of data, and the publishing of sampled value data.

Peer-to-peer communications include GOOSE messaging to replace field wiring between devices, and client-server communications through the MMS protocol.

Sampled value data is the use of “merging units” to sample currents and voltages, and to publish these samples in a digital format so that microprocessor devices can use these samples for magnitude and angle measurement for protection and metering purposes.