Remote Maintenance Testing in Digital Substations

Author: Alexander Apostolov, USA

Mirroring Control Information: A third feature that has been added is the mirroring of control information. This supports the possibility, to test and measure the performance of a control operation while the device is connected to the system. A control command is applied to a controllable data object. As soon as a command has been received, the device shall activate the data attribute opRcvd. The device shall then process the command. If the command is accepted, the data attribute opOk shall be activated with the same timing (e.g. pulse length) of the wired output. The data attribute tOpOk shall be the time stamp of the wired output and opOk.
These data attributes are produced independently if the wired output is produced or not – the wired output shall not be produced if the function is in mode TEST-BLOCKED. They allow therefore an evaluation of the function including the performance without producing an output. (Figure 5).

Isolating and Testing a Device in the System:  Combining the mechanisms described in the previous sections, it is possible to test a device that is connected to the system. We will explain that with a short example.
Let's assume we want to test the performance of a Main 1 protection that receives sampled values from a merging unit. In the LN LPHD of the Main 1 protection relay, the data object Sim shall be set to TRUE, the logical device for the protection function shall be set to the mode "TEST" and the logical node XCBR as interface to the circuit breaker shall be set to the mode "TEST-BLOCKED." A test device shall send sampled values with the same identification as the ones normally received by the protection relay but with the Simulation flag set to TRUE.
The protection device will now receive the sampled values from the test device and will initiate a trip. The XCBR will receive and process that trip; however, no output will be generated. The output can be verified through the data attribute XCBR.Pos.opOk and the timing can be measured through the data attribute XCBR.Pos.tOpOk. The same principle applies when the tested IED receives GOOSE messages. The real data received from a publishing IED or the simulated data received from the test devices will be available on the data bus depending on the value of Sim in LPHD.
The issue with this method is that all IED function elements using the simulated signals must be in Test mode, i.e. they will not be available to detect any abnormal condition during the testing. This can be avoided by using some advanced testing features.

Advanced Simulation Possibilities that can be used for remote maintenance testing of single function element, while the rest of the protection functions remain in service. The concept is explained in Figure 6. As described earlier, with Edition 2, the possibility to describe references to inputs of a logical node has been added. This is done through multiple instances of data objects InRef of the CDC ORG. That data object has two data attributes providing object references: one as a reference to the object normally used as input; the other one as a reference to a data object used for testing. By activating the data attribute tstEna, the function realized in the LN shall use the data object referred to by the test reference as input instead of the data object used for normal operation.
With that feature, it is possible to test a protection or logic function. Instead of simulating the position indications of the different switches as inputs, the logical node (in that example CILO), can be set to use inputs from a generic logical node GGIO in a test. The testing tool can now easily modify the different data objects of the LN GGIO to simulate the test patterns that shall be verified. That logical node can be external (the data objects being received through GOOSE messages) or it can be implemented in the IED itself for testing support.

The use of this feature will require the subscribing IED to be capable to carry simultaneously on the internal digital data bus both the data from the process and the generic test data from the test system. (Figure 7).
Data attribute setSrcRef specifies the object reference to the input that is normally used as input while setTstRev specifies the reference to data object used for testing. By setting the data attribute tstEna to TRUE, the application will use the test signal referred to by setTstRef instead of the data object referenced by setSrcRef used for normal operation. A function LN can have multiple instance of InRef for multiple inputs.

By means of switching over the input reference from the process signal, which may not be accessible or cannot be modified, to a signal which can be easily modified (tstEna set to TRUE), an application can be isolated on the input side from the process and fed with test signals (value and data quality information). This feature requires to have an extra function at hand which allows a controlled generation (i.e. from a front panel pushbutton) of these signals. In case this function is hosted in a device external to the IED under test, the data flow has to be engineered (GOOSE subscription) in the device configuration. InRef is applicable to all types of Data Objects including SVs.
This feature is different from the use of the Simulation mode which affects the entire IED and is limited to GOOSE communication, while ‘tstEna’ has effect on the information sourcing of a single input. Some signals subscribed by LD under test, may be not strictly necessary for the tests (i.e. SF6 pressure of CB, etc.). Ideally, the test set should be capable to generate all these input signals and the LN should be defined with an individual tstEna for every single input. This has to be included in the system design in the engineering phase. Consequence of this would be an "automatic" configuration of the test set depending on the characteristics of the feeder. Normally, all information can be found in the SCD file. 



Dr. Alexander Apostolov received his MS degree in Electrical Engineering, MS in Applied Mathematics and Ph.D. from the Technical University in Sofia, Bulgaria. He is Principal Engineer for OMICRON electronics in Los Angeles, CA. He is an IEEE Fellow and Member of the PSRC and Substations C0 Subcommittee. He is past Chairman of the Relay Communications Subcommittee, serves on many IEEE PES WGs. He is a member of IEC TC57 WGs 10, 17, 18, 19, Convenor of CIGRE WG B5.53 and member of several other CIGRE B5 WGs. He is a Distinguished Member of CIGRE. He holds 4 patents and has authored and presented more than 500 technical papers. He is an IEEE Distinguished Lecturer and Adjunct Professor at the Department of Electrical Engineering, Cape Peninsula University of Technology, Cape Town, S. Africa. He is Editor-in-Chief of PAC World Magazine.

Power. Flexible. Easergy.
Let?s start with organization in protection testing