“IEC 61850 is a global standard for communication networks and systems in substations,” explains David Dart, R&D Director of NOJA Power. “It defines a ‘description language’ for substation automation that provides consistent system engineering and device integration such that IEDs [Intelligent Electronic Devices], tools and systems are interoperable. Such interoperability will be essential in transforming conventional grids into smart grids.”
“But implementing the underlying protocols of the standard across an entire substation is a major undertaking, so utilities are breaking things down into smaller steps and moving carefully.”
Dart explains that one approach adopted by several utilities is to focus on part of IEC 61850 initially to plan and test implementation strategies for that particular element. An example is Generic Object-Oriented System Event (GOOSE) messaging. “GOOSE messaging is often a start point for manufacturers employing IEC61850 because it’s something that can be set up between compatible IEDs without too much difficulty,” he says.
“GOOSE messaging is an architecture and methodology that sits within the IEC61850 environment and facilitates real time sharing of information between devices in the substation.”
The key phrase here is “real time”. This means that messages need to be sent and delivered rapidly, so GOOSE messaging uses a low-level Ethernet layer and the packets are “tagged” to ensure they’re prioritised by network routers. Depending on the application, GOOSE messaging transmission can be 30 to 35 percent faster than conventional point-to-point wired systems.
Messages are sent “horizontally” between IEDs without having to travel via the master unit demanded by some other communication protocols. Moreover, GOOSE messaging employs a “multicast” system whereby a preselected group of IEDs all receive the message simultaneously and act on it if required. In fact, notes Dart, “In the multicast scheme the linked group is actively looking out for the GOOSE messages from its partners”.
For many utilities GOOSE messaging is a good place to start their IEC 61850 implementation because much of the infrastructure to support the technology is typically already in place, for example, high-bandwidth Ethernet networks. That’s not to say that further investment may not be needed, because not all Ethernet switch products are designed to prioritise GOOSE messaging packets. Nonetheless, GOOSE messaging is often less expensive and simpler to put into effect than other elements of the IEC 61850 standard.
GOOSE messaging’s key advantage of very fast multicasting between a group of IEDs allows utilities to implement protection schemes that were previously impractical with traditional messaging techniques due to network latency.
A typical substation application using NOJA Power OSM series auto-reclosers is shown in figure 1.
The figure shows an incoming feeder from a substation transformer protected by NOJA Power OSM auto-recloser then splitting in five outgoing feeders each protected by an additional OSM auto-recloser. A fault on the second outgoing feeder is detected by a Phase Protection, Time Over Current (PHPTOC) 1 device which attempts to clear the fault by performing reclosing operations.
In a conventional protection scheme, although PHPTOC1 is connected to PHPTOC3 via Ethernet, the latency of the link is such that PHPTOC3 reacts to the fault before it is aware that PHPTOC1 is already attempting to clear the problem.
In such a scenario it’s possible that PHPTOC3 could lock out, taking down all five outgoing feeders even though the fault is confined to just one line.
With GOOSE messaging implemented and the six auto-reclosers linked into a multicast group, the utility would be able to put in place an enhanced protection scheme. In this situation, PHPTOC1 could send a GOOSE message to PHPTOC3 to advise that it’s attempting to clear the fault and PHPTOC3 should not trip. The advantage over the conventional scheme is that all outgoing feeders remain live while PHPTOC1 undergoes the reclosing operation, either clearing the fault or locking out to isolate it.
“Utilities already implement protection schemes whereby auto-reclosers communicate with each other to isolate a fault and narrow down the portion of the grid that’s affected,” says David Dart. “But GOOSE messaging enables the utilities to ensure the system reacts much quicker – an essential property of a smart grid.”