Implementation of IEC 61850 GOOSE and Protection Relay Testing

IEC 61850 GOOSE messaging is applied for Substation Automation Systems and for status interactions between IEDs by replacing the conventional method of using binary inputs/outputs and wires with communication over Ethernet cables/fibers. With its fast transfer characteristics, it is also applied for protection testing purposes. In order to confirm the basic functionality of IEC 61850 and GOOSE messaging, conformance tests are mandatory for basic multi-vendor interoperability.

IEC 61850, the new communication standard for power substations, is now being widely used in practical applications. In particular, GOOSE (Generic Object Oriented Substation Events) messaging has been applied not only for SAS (Substation Automation System) control and monitoring of primary equipment and IED status, but also for status interactions between IEDs including protection relays by replacing the conventional method of using binary inputs/outputs and wires with communication by GOOSE messages over Ethernet cables/fibres. This is achieved through much simpler engineering based on the multi vendor interoperability described in the IEC 61850 standard, which enables the easy connection of different IEDs, including relays supplied by different vendors.

In order to confirm the basic functionality of GOOSE messaging in protection relays, functional conformance testing at an independent test laboratory is mandatory for multi vendor interoperability. However, an IEC 61850 device certificate does not fully ensure conformity to the IEC 61850 standard. Furthermore, in some cases, the correct behaviour of an IED is not clearly described in the standard. Hence, performance testing for GOOSE messaging must be considered as an important item in the product type test and/or the routine test because this is critical for practical substation applications. It should be noticed that performance testing for GOOSE messaging is not covered by conformance tests at present.

In order to replace the conventional method of using contacts and wires, the performance of the GOOSE messaging, i.e. transfer time should be less than 3ms for a Trip GOOSE command and 20ms for a Block GOOSE command as specified in IEC 61850-5 'Communication requirements for functions and device models'. It is clear that new methods for testing GOOSE messaging including performance and interoperability must be carefully considered, not only by vendors but also by the end users who evaluate system performance in the field.

GOOSE APPLICATIONS FOR PROTECTION
Advantages of GOOSE for protection devices
GOOSE messaging is a very important function in achieving multi-vendor interoperability as described in IEC 61850. The purposes and associated advantages are considered as follows:

• Instead of connecting conventional metallic wiring and other ancillary equipment between protection devices, or between protection devices and primary equipment, only a single LAN cable/fibre is required. This results in a reduced total cost of building a system in a substation.
• Connection between IEDs provided by different vendors is much easier to achieve.
• Modification or addition of data communications between IEDs can be easily achieved by the simple re-configuration of the IEDs' GOOSE settings, rather than by complex metallic wiring.

Figures 1 and 2 show an example of the differences between the conventional method and that utilising GOOSE messages where communication between a protection relay and a primary CB (Circuit Breaker) is required for an Autoreclose function.

Possible GOOSE application for protection and related ongoing activities
GOOSE messaging in IEC 61850 can also be utilised in the following protection functions:

• Autoreclose (between relay and CB)
• Intertripping (between relay and relay)
• Interlocking (between bay control unit and relay)

Some utilities consider and plan contingencies in the case of a protection failure in a system. In order to minimize the damage caused in the case of a primary fault to the zone protected by the equipment that has failed, messaging between protection relays is very important. This subject is being discussed in CIGRE/SCB5/WG16: Busbar protection. There is a possibility to use GOOSE  messages for information communication between the various items of equipment contained within the substation.

ACHIEVING INTEROPERABILITY
Functional conformance test See Figure 3.

UCAIUG (http://www.ucausersgroup.org) defines the conformance test procedure which is detailed in IEC 61850 part 10 (Conformance testing). The test procedure contains two types of test;

• Positive test: Checks with correct parameters.
• Negative test: Checks with incorrect parameters.
  However, there are some points that we must be cognizant of with regard to conformance testing;
• 'Conformance' does not mean 'Interoperability'.
• Only basic functional tests are undertaken, no performance tests are carried out at present.
• IEC 61850 communication interfaces in the IED form the main part of the test, but the connections between protect ion applications and their IEC 61850 communication interfaces are too complex and varied to be comprehensively tested.

Regarding standardisation and guidance on testing, functional testing of IEC 61850 based systems is now being discussed in CIGRE Subcommittee B5 Task Force 92. The technical brochure will define the functional aspects of testing which are not defined in IEC 61850-10 (Communication networks and systems in substations -Part 10: Conformance testing). It deals with the functional parts not covered by the IEC 61850 device certificate which is based only on IEC 61850-10. This will have an impact on the work of the protection engineer, and this kind of functional testing approach should naturally be carried out for all existing IEC 61850 conformant devices and systems.

Importance of performance testing for GOOSE messaging
In order to achieve multi vendor interoperability in GOOSE messaging in consideration of an actual practical situation within a substation, it is important that not only the functional conformity to IEC 61850 be tested, but also performance conformity to IEC 61850 must be tested, by vendors, as a type test of the IED.

Performance criteria examples of GOOSE messaging defined in IEC 61850-5 'Communication requirements for functions and device models' are given as follows:

• 3ms: 'TRIP' GOOSE information (class P2/P3)
• 20ms: 'BLOCK' GOOSE information (class P2/P3)

The transfer time definition is described in IEC 61850-5 (see Figure 4). IEC 61850-5 states that the transfer time of GOOSE messaging for a Trip command shall be such that the command should arrive at the destination IED within 3ms. For a single IED, by assuming the time for the publishing process and the subscribing process are approximately equal and if 'tb' can practically be ignored, then at least half of the defined time is needed for the IEDs to process the message (i.e. 1.5ms for 'TRIP' GOOSE). See Figure 4.