IEC 61850: The Impact On Protection Testing

IEC 61850, the standard for communication networks in substations, is the flexible and future proof solution for substation automation and protection systems. The standard became prevalent all over the world in all voltage levels, and users in the utilities are gathering their experiences. This paper describes expectations and experiences, and focuses especially on testing issues. It explains the requirements for the performance as defined in the standard. Measuring possibilities and the impact on testing of protection schemes are described and highlighted with practical experiences. By THOMAS SCHOSSIG

Testing Predications

To describe conformance testing in a communication standard is a big merit of IEC 61850, part 10. The document has methods and abstract test cases for conformance testing of devices used in substation automation systems (SAS) and the metrics to be measured within devices according to the requirements defined.

The methods described guarantee conformance to the issues defined in the standard, but are not useable for testing on site during commissioning or maintaining. A properly passed conformance test ensures compliance with the standard and allows interoperability with other certified Intelligent Electronic Devices (IEDs) in the SAS. Conformance testing is not the scope of this paper; all IEDs described are considered as compliant.

Nevertheless the standard covers testing issues in any case. All requirements for communication equipment in substations are defined in IEC 61850-5, performance requirements included. Hence, IEC 61850 explains a “overall transfer time” as the sum of individual time of communication processors and network transfer time, including waiting time and time delays in network equipment. This shall be tested during site-acceptance-test and will be explained in the chapter “Performance Testing”.

IEC 61850-7-3 describes an additional quality identifier that may be used to classify a value being a test value and not to be used for operational purposes. So each Logical Node (LN) has a data item called “behaviour” that describes if the LN is “on”, “off”, “blocked”, “test” or “test and blocked”. A boolean test indicator will be used also for GOOSEs, or Generic Object Oriented Substation Events. Edition 2 of the standard will describe advanced possibilities to describe test mode and indication of test mode in GOOSE and Sampled Values.

Impact of Engineering

More than just defining a set of services and communication protocols, IEC 61850 also specifies a common, vendorindependent engineering concept. The concept uses IE C 61850: Th e imp act on protection testing IEC 61850, the standard for communication networks in substations, is the flexible and future proof solution for substation automation and protection systems. The standard be¬came prevalent all over the world in all voltage levels, and users in the utilities are gathering their experiences. This paper describes expectations and experiences, and focuses especially on testing issues. It explains the requirements for the performance as defined in the standard. Measuring possibilities and the impact on testing of protection schemes are described and highlighted with practical experiences. By THOMAS SCHOSSIG configuration information in a standardised file format (Substation Configuration Language SCL IEC 61850-6) as explained in Figure 1.

A system specification tool specifies (optional) the single line diagram of the station and the required logical nodes in a file with the extension SSD, or System Specification Description. The knowledge of the capabilities of the used IEDs in a substation is necessary for the system configurator – a tool that sets up the configuration of the communicational setup of the SAS.

The capabilities are described in the ICD-files, or IED Capability Description. The output of the system configurator is the Substation Configuration Description (SCD). This file provides information regarding all IEDs; they can be set up with CID, or Configured IED Description, files. Nevertheless the SCD-file is the base for the setting up of test sets and essential for testing in a substation.

Data Model and Services

One of the new concepts of IEC 61850 is the segregation from application and transport. The application is the data model of the substation; it consists of logical devices and logical nodes. These logical nodes contain in data attributes and data objects information, for instance, contains a logical node PTRC, or Protection Trip Conditioning, the general trip of a protection device. After modelling, the next step is the definition of services. Possible services (as get, set, report, control….) are defined in part 7-2 of the standard in an abstract manner (ACSI Abstract Communication Service Interface). A specific mapping (SCSM Specific Communication Service Mapping) maps the services to an existing communication technology as defined in part 8-1 using Ethernet . The correct application of these services has to be tested and should be described in the next paragraphs.

GOOSE

IEC 61850 provides two kinds of real time communication. For the fast transmission of events in a substation the GSE, or Generic Substation Event, was introduced. IEC 61850-8-1 describes two kinds of GSE, the “old” GSSE – Generic Substation State Event, UCA-GOOSE – and the new flexible GOOSE. Every GOOSE transmits the information of a dataset, which refers to information stored in the logical nodes. The data in a GOOSE are not only sent out after a data change; they are sent out with a repetition strategy and as an unconfirmed multicast (see Figure 2).

Currently, GOOSEs are often used for:

• Interlockings
• Reverse blockings

Sampled Values

IEC 61850 specifies, as a second possibility of realtime communication, the transmission of instantaneous values (for instance of voltages and currents) from a power system. The measuring values are stored in two logical nodes from the instrument transformers group – TCTR and TVTR – for the currents/voltages with one instance per phase. These data are called Sampled Values and defined in part 9 of the standard.

IEC 61850-9-2 describes the transmission as a multicast on the Ethernet. The standard does not focus on any measuring method or sensortechnology; it just describes the information sent out of a “merging unit”. To allow an easier implementation, a guideline – 9-2LE – was published by UCA. This guideline describes a special profile of IEC 61850-9-2 with a fixed dataset and a fixed sampling rate. IEDs and test sets using 9-2LE are already available.

For testing of SAS according to IEC 61850- 9-2, the test set should be able to record the sampled values sent out by the IED (“subscribe”) and send them for testing purposes (“publish”). A conversion to primaries facilitates the usage for simulation and testing.

Client/SERVER (SCADA)

The IEDs in a substation (relays, RTUs…) are working as a server because they send out data unsolicited to a client (for example, HMI). These data had been never exchanged with other peer devices on bay level and were commonly not available for testing in the past. With IEC 61850 these data are all served in a standardised way. A test client (realised for instance as a software) has full access to data model and can, for instance, visualise the data attributes of a data attribute (Figure 3).

Performance Testing

The standard defines in part 5 the different performance classes for protection/ control and metering. The performance classes for protection (P) are as follows:

• P1 for distribution bays
• P2 for transmission bays
• P3 for top performance transmission bays

The allowed “overall transfer time” (Figure 4) is defined in the standard and allows for different kind of messages different times:

• Type 1A (trip) 10 ms (P1) or 3 ms (P2/3)
• Type 1B (other fast messages) ≤ 100 ms (P1) or ≤ 20 ms (P2/3)
• Type 2 (medium speed) < 100 ms
• Type 3 (low speed) < 500 ms

The other types (type 4…7) should not be covered.

If the requirements for transmission of GOOSE are defined in the standard, they have to be tested during commissioning and maintenance. If the GOOSE is used for protection or protection related functions (trip and reverse blocking, for instance) the time between the occurrence of an event (analog shot) and the reaction of the IED (sending out the tripping GOOSE) has to be measured.

The application of test equipment that “maps” a subscribed GOOSE to a binary input of a test set, and that maps the published GOOSE to a binary output of the test set allows the usage of approved test templates in an IEC-61850-compliant SAS, too. The usage of SCD-files for the set up of a test set is recommended.

The Measured times are shown in Table 1.

Conclusions

IEC 61850 changes the world of protection testing dramatically. New approaches like GOOSE and Sampled Values require new testing mechanism and products. The network becomes a part of the SAS.