Possible Applications and Limitations of Wi-Fi Communications in IEC 61850 Systems

Authors: R. Persello, Fred Steinhauser, OMICRON electronics GmbH, Austria

Testing Approach

The method proposed will now be extended. Simulation of IEDs stays important during design, FAT, SAT and commissioning. An automated test set is proposed. This test extends the test from single IED testing and simulation to testing the entire SAS. The need for simulation is there in all phases of the project. Nevertheless, it is decreasing and different kinds of testing apply (see Figure 1).

The Testing Solution

Overview:  The proposed testing solution shall consist of software and hardware. To use a dedicated hardware and not just develop a PC application was chosen for the following reasons:

  • Guarantee cyber security and safe connection to substation network
  • Real time capabilities to calculate Sampled Values and GOOSE
  • Making it possible to deliver multi-IP simulation
  • Connection to several networks
  • Update possibility for security patches
  • Comfortable licensing

The software offers a toolbox for the different tasks. (see Figure 4).

System under Test:  As mentioned the entire system is under test now (Figure 5).

Zero Line:  The entire system will be visualized (Figure 6). All information available in SCD file will be used. This covers also the information in substation section (voltage level, bay, etc.). The standard defines possibilities to model the elements of a single line diagram.

Most of the current SCD files do not contain this information. So, it is the proposal to work with “zero line” to visualize the assets (Figure6). Zero line means grouping by voltage level, arranging the bays and corresponding assets.

The navigation in huge SAS can be done as in map systems. (Figure7).

Clicking “Go-live” visualizes the current status of the asset.

Tracing the Signals: The functionality of the SAS transfers the message from its source to all receivers. If there is an error in this communication, the commissioning engineers need to follow the signal on its way through the SAS.

Finding such signal errors in the case of copper wiring was very time consuming, with IEC 61850 this becomes almost impossible to be done manually. The testing solution described here allows to view how signals propagate through the SAS, see Figure 8. 

The architecture used allows to follow signals communicated as GOOSE as well as Reports. This makes trouble shooting communication problems quite easy.

The software makes it possible to visualize the links. The bay controller communicates with SCADA (Figure 9).

The huge amount of information might confuse, but filters can help (Figure 10). Doing so filters for instance to GOOSE and Reports (Figure 11).

Names: The names in the data model. Non - IEC 61850 experts and users might expect more information. The software recognizes the names, detects the purpose and visualizes accordingly. Names can be edited (Figure 12) - for instance in own language.

IEC 61850 in the Background: IEC 61850 can be visualized as well- in the example for DataSets (Figure 13) and GOOSE Control Blocks (see Figure 14).

Analysis and Trouble Shooting: Data models of modern IEDs can be huge. Tools grouping and sorting the information automatically will help. The most important status information becomes visible. The software analyzes the values and shows only relevant information (Figure 15).

Test Plans: If tests are documented and recorded they can be used in different phases. Figure 16 shows a test plan ready for re-usage at another phase of the SAS project.

Sequences can be performed and assessed automatically

Logic Testing: Logic is used in interlocking, as already described, as well as in any other substation automation function. Logic conditions are implemented in control devices. Testing such logic functions is an essential part of FAT as well as SAT.

In case of interlocking, simulated or real switchgear status is considered and evaluated. To represent the result of interlocking logic conditions, IEC 61850 represents the status of the release in the logical node CILO.

As depicted in Figure 17, the proposed solution reads the value out from the data model and assesses the interlocking behavior by reading the CILO values automatically.

It is essential to understand that the behavior of the logic shall be tested in any stage of the lifecycle of an SAS.

So, even after IED-change or firmware upgrade the entire test once performed can be repeated. Issues that occurred can be solved, and afterwards the test can be repeated (“test-fix-repeat”).

Note: Non-existing assets will be simulated, so this approach allows testing at any stage. (see Figure 17).

Test after Firmware Upgrade: Putting a SAS in operation meant in the past extensive testing during FAT and SAT, but neither routine testing nor updates of firmware (frozen) for the next 10 up to 20 years. This is not valid anymore. 

IEDs in substations also must be patched with security updates. The problem is that afterwards everything from that relay needs to be re-tested, also all communication. Until now there was no proper way to re-test the communication. Everything had to be done by hand.

The solution proposed offers automatic testing and assessment. The whole communication of the device can be easily re-tested after the firmware update by executing the test plan already prepared for that device (Figure 18).

GOOSE Supervision: As visualized in Figure 13 the GOOSE is sent out as multicast - one to many. So, every IED which is a member of this multicast will receive this information.

Taking and using this information is called “subscription”. But how do I learn that the function was subscribed and received successfully? Of course, testing the reaction of the system will show that this works.

Nevertheless, using this information just from IED’s data model would be easier. IEC 61850 defines as discussed already  the logical node LGOS.

As a system node, the data attributes describe the status of a GOOSE subscription. More and more vendors are providing this information. If the subscription fails, this can be indicated.

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