Reliability and testing

Fault tolerant systems will provide means to increase the reliability of our systems in a cost effective way. An example of such an approach could be the replacement of the "main 1 and main 2" for protection schemes by single IEDs in a fault tolerant system. Some utilities may be opposed to this based on the requirement for independence of main and backup protections (N-1 criterion). An argument is that omitting redundancy in protection is limited because of the limitations given by the current network technologies. Main questions to be answered are what happens to the performance, reliability, security, availability, single points of failure, and redundancy? What if multiple IEDs fail or an IED has to be repaired or tested? In this environment the communication bus behavior and performance will be critical.

With increasing platform flexibility and CPU power, integration of functions into a single IED however becomes feasible. New schemes and functions are being studied and a possible solution may be the clustering of redundant IEDs. The discussion will certainly continue because modern communication networks in substation automation systems provides advantages and can help to increase the availability of the system.

The complexity of our next generation substation automation systems shall however be managed in such a way that test results remain consistent, reproducible, and comparable. This may partly be handled through quality assurance and accreditation procedures for conformance testing such as the ones defined and maintained by the UCA International Users Group. It is however clear that in order to test a complex network, the test system will become equally complex.

It is therefore important to keep in mind that when testing functions in an open system:

• conformance tests are required to verify systems against specifications
• performance tests are required on a reference configuration with special attention on mixed configurations
• interoperability is mandatory and should be certified.
• interactions between functions increase the number of tests

The impact of IEC 61850 on future test concepts is that testing the IED functionality can be achieved by using communication links for "injection" into an IED or system. A complicating factor may however be that test equipment must be connected directly to the process bus and station bus and must generate the input and monitor the output via the serial communication. This requires that tools with realistic and coherent test scenarios are available. Having test features integrated in the IEDs can further support testing since it can help to minimize the test effort and facilitate online testing

Finally when systems are upgraded each version could be considered as a different type, invoking a complete test requirement. Major question is what is to be retested? It is clear that our industry needs effective laboratories for the validation and diagnosis of our systems and their functionality. At this time however a lack of network testing standards exists.