Improved Protection and Control Testing utilizing IEC 61850

Authors: Benton Vandiver III, and Bharadwaj Vasudevan, ABB, USA

Digital Substation Testing

One of the biggest benefits to standard-based engineering is that by the end of such a process we have access to all the engineered information that can be used by other standard compliant tools. Thus, testing such an engineered solution becomes much more simplified than the traditional way of doing testing.

Where to start testing:
1.   The engineered SCD file. As mentioned before, the SCD file is a critical part of the engineering effort in a digital substation. It contains various sections for data handling for multiple services like GOOSE, MMS, SV streams, etc. It's therefore almost inevitable to test the file itself to check the system-wide data consistency before even deploying this file into the individual IEDs
2. Once the SCD file gets deployed, it becomes necessary to test the configuration of individual IEDs to check and compare the system consistency for data exchange. This provides a confirmation that the SCD file deployed is working as designed by the engineer. Usually, this involves capturing the packets of data that is passing in the network and looking at all the information published as a part of each IED service like GOOSE, MMS, SV, etc.

These two tests become the basis for starting the other tests which are traditionally performed during a Factory Acceptance Test (FAT) or Site Acceptance Test (SAT). On a broad scale we can list the activities performed during a FAT or SAT as follows:
1. Functional Tests: Tests performed to validate the settings and the corresponding cor-rect operation of a function
2. Scheme Tests:  Tests performed to validate the engineered scheme. For e.g.: Breaker Failure scheme, Line Protection Pilot schemes, interlocking schemes, etc.
3. SCADA (Supervisory Control and Data Acquisition) Tests:  Tests performed to validate the data sent to the SCADA or HMI system. These tests usually involve checking the me-tered data for scaling issues, testing the alarms and events, switch controls and any outside communication to the Network Control Center (NCC)
In a digital substation, all the above-mentioned tests can be performed in a very fast and efficient manner. In the following sections, it is explained in detail how to accomplish each of the 3 tests mentioned above.

Functional Tests:  In a digital substation, it is very easy to test the individual functions. In an SCD file, we can drill down to any level required and look at the data specific to the function under test. A simple example of looking at the data model of an instantaneous overcurrent protection is shown in Figure 3. From a testing perspective, all the test plans we traditionally build to test individual protection and control functions can still be utilized. The only change affecting the test plan will be the choice of the analog output sources and the trigger condition source used.

In a traditional function testing of a current element, the analog sources were provided by secondary currents generated using a modern relay test set. Today we use SVs published by a standards-based, modern relay test set. In the past, the field personnel would use a binary output coming from the IED under test as the trigger condition to stop a test.
Today we use a GOOSE message published by the IED with the same trigger status to stop the test. The real efficiency comes from the time saved in the preparation for the test and actual time spent to conduct the test and collect the results. Since the test utilizes only the digital signals, there is no need to physically connect, reconnect, or modify individual analog signals as well as trigger signals to the IED. The only connection required is an Ethernet cable connected into the network with proper access for the test set. (Figure 4).

Today the standard provides the ability to logically place any individual IED from the system into Test and Simulation mode. This allows the IED under test to “listen” only to the modern test set publishing the test data, without the need to physically isolate the IED under test. The modern test set for digital substations may even be equipped with features to place an IED into test and Simulation Mode. The entire process of logically isolating the IED, forcing the IED to listen to the test set, perform all the necessary tests within the test plan and then returning the IED to its normal state, can all be engineered into the IED and accomplished quite seamlessly by just a push of a button from the test set.

Scheme Tests: In a Scheme test, more than one IED may be involved. The same methodology mentioned above can be extended to logically isolate all the IEDs involved in the scheme test using a single test plan executed from the same test set. This is possible because an IED in test mode can communicate with any other IED in test mode according to the standard. During commissioning, some IEDs involved in the protection scheme may not be physically available for testing.
Under these circumstances, there are tools available that can simulate missing IED's and can publish all the required GOOSE and SV messages as defined in the SCD file. Consider a breaker failure condition. If the breaker failure initiate signal is not available, the test plan can include the simulated GOOSE message of the missing IED.
Even if the device is available in the network, by putting the subscribing IED in test and simulation mode, the test set can force the IED to stop listening to the actual IED for the GOOSE message and only subscribe to the simulated GOOSE message from the test set. By isolating the individual IEDs from a network, any scheme can be tested within the same test plan without performing major changes to the physical infrastructure. This allows SAT tests to continue without waiting on a piece of equipment to physically be available. The overall time is taken to test and commission a substation can be reduced. (Figure 5).

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