IEC 61850, Digital Substations, and the Smart Grid
Authors: Niclas Wetterstrand and Andrea Bonetti
IEC 61850 was launched in 2003 as a standard for digital substations and it is widely used in such applications. In principle, however, the Smart Grid is just a regionally distributed system of electrical substations, so IEC 61850 is also very relevant to the Smart Grid and, in fact, the IEC has designated it as one of the core smart grid standards.
To find out what Megger is doing in relation to IEC 61850, Electrical Tester arranged for Niclas Wetterstrand, Megger’s industry director for protection, to talk to Andrea Bonetti, the senior specialist for relay protection and IEC 61850 in Megger Sweden.
NICLAS: How long has Megger been working with digital substations and the smart grid?
ANDREA: Megger has a long history of work relating to IEC 61850. We started in 2008 with the development of the GOOSER and the MGC (Megger GOOSE Configurator) for which we were granted a patent for some key pioneering concepts such as the comparison of network data with engineering data (SCL), and the secure access point that prevented connecting a PC to the communication network of the substation. GOOSER was first marketed in 2009 and is now discontinued. Its functionality is however embedded in relay test sets in our SMRT and FREJA 5xx ranges. For process bus applications (Sampled Values), we implemented IEC 61850-9-2 LE (Light Edition) in 2010, when we participated in a commissioning project in Central America. Over the years, IEC 61850 has progressed from Edition 1 to Edition 2, and now to Edition 2.1 (which isn’t really an all-encompassing new edition – something I explained to ET readers in the first issue of this magazine). At Megger, our job is to follow the standard and to incorporate the new concepts in our hardware and software tools.
N: I have heard a lot of discussion about the KEMA certificate. Can you explain what it is and how it relates to Megger products?
A: The so-called “KEMA certificate” for IEC 61850 is actually a test report from an accredited test institute – in this case KEMA (CESI today), although there are others. The certificate confirms a certain level of compliance with the IEC 61850 standard in terms of interoperability and, as it is produced by an independent accredited test institute, it is known as a ‘Level A’ certificate.
‘Level B’ certificate also confirms interoperability, but it is released by a non-independent accredited institute, for example, the Hitachi Energy IEC 61850 laboratories in Switzerland. It is a common practice to perform the tests for the first release of a product at Level A, and subsequent small adjustments or improvements at Level B. Megger has ‘Level A KEMA’ certificates for GOOSE and Sampled Values. It is widely known that the certificates alone do not fully ensure interoperability. Additional tests and a good specification from the end user are needed if an IEC 61850 project is to run smoothly. Certification is a minimum requirement which shows that the manufacturer has done what they reasonably can to provide a reliable IEC 61850 product or tool.
N: You have mentioned GOOSE and Sampled Values. Would it be correct to say that IEC 61850 is a new protocol or a new series of communication protocols?
A: This is probably the most common question about IEC 61850, and the simple answer is that it is much more than a series of protocols. IEC 61850 is built on three pillars: the Standard Signal List, Standard Information Exchange, and Standard System Engineering.
N: From what we've discussed so far, I would expect GOOSE and Sampled Values to be part of the Standard Information Exchange. Can you give some examples for the other two pillars and how they relate to each other?
A: You are right, the standardised protocols are part of the Standard Information Exchange. Before we move on, however, there are a few more things we need to consider.
The Standard Signal List provides a standardised approach for modelling the system, the equipment, and the signals associated with the system and equipment. It does this by defining 'elementary functions' called Logical Nodes (LN) along with their signals, which are Data Objects (DOs) and Data Attributes (DAs), and their descriptions, in System Configuration Language (SCL).
A key idea is that if we have a master SCL file describing the system, it also describes the data traffic through it. Another important idea is that we can consider the SCL file as being the fingerprint of the communication in the substation.
N: What's the benefit of having the three pillars instead of a simple data communication protocol?
A: I have to admit, we've been asking the same question for many years! The answer is that the threepillar concept is incredibly robust and future proof. Some people have even described it as beautiful! To understand its benefits, however, some examples are needed. Let's go back to the SCL file which, as I said, can be considered as a description of the system and of the data traffic through it. If we can detect and measure this data traffic with a tool - which is sometimes called data sniffing - we can compare it with the master SCL description and if there are differences, we can give warnings. This idea initially came about because of the need to identify interoperability problems in substations. We won't go into this in detail, but it is still used in this way. Another possibility, as we have already said, is to consider the SCL file as the fingerprint of the communication in the substation. For perspective, I gave a keynote presentation about these concepts at IEEE GPECOM in 2020. In one hour of presentation, I think I mentioned the communication protocols for a mere few minutes!
N: Interesting, but this isn't really relay testing, is it?
A: No, it's beyond relay testing! The standard, thanks to its three-pillar structure, opens a whole range of possibilities that are just waiting to be developed. One that Megger has been working on since early 2009, is the comparison of the data model, described by the standard, with the actual data traffic, in the form of GOOSE messages, in the substation. Once again, for this pioneering concept, Megger has been granted the patent.
This is possible because the IEC 61850 is much more than a standard about protocols: it provides a way to describe the electrical system. This description is formalised in the SCL language and is ultimately available in the SCL files. Megger has patented some related algorithms that allow comparison of SCL GOOSE with network GOOSE messages. As the SCL file is a fingerprint of the communication in the substation, the idea is to periodically compare the actual communication with the master (reference) fingerprint. Such a comparison provides an excellent basis for developing automatic maintenance routines. Several papers have been written by Megger on these concepts; also, Megger has contributed to the Royal Engineering School of Stockholm (KTH) for an important thesis on the same subjects:
N: To make that comparison, we need to be able to read the GOOSE messages in the substations and we need to read the GOOSE messages in the SCL file. Is that correct?
A: Yes! The tool for this is called the GOOSE sniffer. It's a device that is connected to the substation Ethernet bus and can transparently (passively) read the traffic, without sending data into the system. Megger's GOOSE sniffer is part of the Megger GOOSE Configurator (MGC) software. This software can also read SCL files, so we have a tool that can seamlessly work with GOOSE messages from the network and from the SCL file.
N: Do you mean I go to the substation, read all the GOOSE messages, and then import the GOOSE messages from the substation SCL file?
A: Yes! There might, for example, be 500 GOOSE messages from the substation and we'd also expect to have 500 messages from the SCL file. We'd expect the messages in these two sets to be equivalent. Or, to put it another way, we'd expect the substation messages to conform with the description given in the SCL file.
N: Do you have examples to explain why there could be differences?
A: In engineering, things are rarely perfect!Unfortunately, at the commissioning stage you're unlikely to have just a single version of the SCL file. Different vendors will be working on their own versions of the file, which they will use to program their devices. If the final SCL file is not completely updated with the latest information, or if there are some devices that are not programmed well, there will be differences between the substation messages and the SCL file.
For example, in a practical situation one relay could be out of service. The GOOSE messages from that relay will disappear from the network, but they are still in the SCL file. So, there will only be 80 substation messages but 85 messages in the SCL file. Or it could be that somebody has made changes to some relays but, when asked, they say they didn’t change anything. A quick check will show if this is true. Or an Ethernet switch may have been replaced with a new switch that has different settings. This means the VLANs are now different, which will result in errors and warnings.
N: What sort of problems have you discovered in the field by comparing substation and SCLfile messages?
A: Issues I’ve identified so far include Ethernet switches replaced with the wrong settings, which meant that some GOOSE messages disappeared, and others lost their VLAN tag. I’ve also seen IEDs reconfigured, which meant that some substation messages didn’t merge (i.e., ‘were different’) because of a different configuration revision (ConfRev); IEDs out of service or disconnected, which meant that some substation GOOSE messages disappeared; and additional IEDs inserted so that new substation messages appeared, which the SCL file hadn’t been updated to expect. I even saw a situation where the system integrator gave the customer what was supposed to be the as-built SCL file. However, just five minutes of testing revealed that there were big differences between the GOOSE traffic and the SCL file – the SCL file did not reflect the substation!
N: How important is to have the correct description of the substation in the SCL file?
A: Very important because the SCL file is the basic documentation for troubleshooting when something unexpected happens. Moreover, it is the basic documentation for retrofitting. If the SCL file does not describe the substation, the entire IEC 61850 concept falls apart. Many utilities insist on a comparison check of the as-built SCL file and the network traffic during factory acceptance testing (FAT) and site acceptance testing (SAT). An incorrect SCL file means no acceptance and no payment! By the way, when the SCL file describes the substation, it is called an SCD (substation configuration description) file.
N: So, factory acceptance testing is yet another application of the comparison test method?
A: Yes, and it’s an important application. In the hands of the end user, a comparison test is a powerful tool that enables them to determine whether or not there are discrepancies. If there are, they need to be investigated and resolved.
Some years ago, I was delivering an IEC 61850 training session for Megger. I explained how to test relays and then went on to say that every Megger user has access to the comparison test method, although they may not be aware of what they can do with it. The following day, only two out of the ten participants turned up for training. When I asked about the others – wondering whether they had a big substation problem as they were all utility employees – I was told that as a result of my presentation, they were all around the region checking their SCL files!
N: What happens if we get, say, ten difference warnings from a comparison test? What should we do then?
A: It depends on the situation. If it’s a factory acceptance test, it may be enough to write a report about the warnings to withhold approval of the SCL file. Or, you may want to join in with investigating why the differences have occurred. If you use it correctly, the Megger tool can identify the differences in the comparison. This can be very difficult to determine manually, so the time savings can be enormous. The tool can’t resolve the differences, but it can pinpoint them.
N: All of this sounds like it’s going to be very expensive.
A: Actually, it’s not. Every Megger user has access to the comparison method, since the MGC software for IEC 61850 is included as standard with Megger test equipment. But many Megger users don’t realise they have it! This method does, I’ll admit, need a certain level of competence in relation to IEC 61850, which goes beyond ‘relay testing’, but once you’re used to it, it’s not rocket science. Even so, it would be much more effective if comparison testing could be done automatically as some sort of continuous monitoring. Moreover, I see that others have only recently taken on this manual compare concept that we brought to the market in 2009.
N: What would be the advantages of automatic comparison testing?
A: We’re talking about self-supervision procedures, automatic maintenance routines, event driven maintenance, and the like. The benefit of implementing these is that they greatly increase the availability of the system, compared with the use of periodic manual testing procedures.
N: How do these concepts relate to the smart grid?
A: I would say that one of the main associations is the cost. Nobody wants to pay for a periodic test on a system that is still working correctly. On the one hand, we want to have the best possible availability, and on the other, we want a system that’s simple and inexpensive, and that gives automatic alarms when something needs to be repaired. The development of automatic maintenance procedures will therefore help the development of the Smart Grid in all areas of society, even in our homes.
N: But we don’t have an automatic system in place...
A: That’s correct, at the moment it’s manual, but we have all the competences needed to implement an automatic system. What Megger needs is a friendly customer who is sensitive to these topics and willing to work with us to implement automatic comparison testing as a smart grid project.
N: Thank you, Andrea, for finding the time to take part in this interview. It’s been both interesting and thought provoking. And I hope you’ll get some positive responses relating to your search for a partner to work with you to develop your ideas. Also, I’d just like to mention that we’d welcome feedback on this interview and, if ET readers have questions, please forward them to us via the editor ([email protected]).