Modern three-terminal line protection testing

How the Illwerke VKW Group validates its protection

Testing a distributed distance protection with teleprotection used to be quite time-consuming, since test plans to be executed simultaneously at the ends have to be precisely coordinated. Testing a three-terminal line protection is even more complicated and costly than protecting a simple overhead line with only two ends. Ever since multi-end protection systems can be tested system-based, these tests are also relatively easy to perform. Testing can be prepared and controlled from a single laptop. The fact that this also works very well in facilities without Internet access was demonstrated by the most recent test carried out at the Illwerke VKW Group, where the control of the test equipment was accompanied by a Teams session via which the RelaySimTest master was transmitted by cell phone hotspots so that each test step could be followed exactly at the other ends.

Motivation for the test

The Illwerke VKW Group operates hydroelectric power plants in the Montafon in the Vorarlberg Alps to cover peak electricity demand. This peak electricity is fed into the European-international power grid. These power plants are connected via several high-voltage lines. Along one of these lines, a 220 kV overhead line, a power plant is looped in, in which one of the fields, the feeder to substation A, had to be renewed. Since the feeder to substation A was now missing for the duration of the maintenance work, the aforementioned overhead line in the power plant was connected to a three-terminal line by means of a temporary line pole.

Protection configuration

Those three of the previously four relays that previously protected the respective line sections with distance protection with teleprotection were configured for a protection scheme with distance protection with teleprotection for a three-terminal line for this purpose. This requires a re-parameterization of the three relays and a distributed protection test before commissioning.

Communication, logic and reaches

For comprehensive communication to all points, two of the existing telecom paths could be used and an additional communication path had to be newly built. The logic for the teleprotection was adapted accordingly in the protection devices, as were the ranges of the distance protection zones. Zone 1 of the relay in the power plant was adapted for both directions in such a way that there was no overreach over either of the other two ends; the ranges of the overreach zones were increased slightly so that the new intermediate infeed did not result in any underreach.

Preparation of the test

For the test, three teams with CMC test devices were sent to the respective stations. The test was controlled centrally via a laptop in the power plant using RelaySimTest, where the three-terminal line including the parallel line on the same poles was modeled. The test devices were wired to the trip and pickup commands of the protection devices, as well as to their binary contacts for the teleprotection signals. The test sets were each time-synchronized by a CMGPS 588 clock. An Internet connection via cell phones was established to control the test equipment in the three plants.

Communication during the test

Using the aforementioned cell phone hotspot Internet connection, a joint online conference was first started via Microsoft Teams, each with a webcam, microphone, and speaker at each station so that the test engineers could communicate with each other at all times. This online conference was then used to share the screen with the running RelaySimTest application, so that progress and results of the test were always visible in all facilities.

Control of the CMCs in the three stations

Via the established Internet connection, the test devices in the two remote substations were controlled via a cloud connection using OMICRON Device Remote Agent, after the test devices there were each granted access for the test engineer with the RelaySimTest master. Thus, these two remote CMCs could be used in the same way as the CMC directly connected to the laptop in the power plant and the correct behavior of the entire protection system could be validated in the subsequent test.

Which test cases were carried out?

First, the stability of the protection system was tested. For this purpose, first stable load flow and then external faults were simulated. In the latter case, the relays must not trip, provided that the fault is cleared by the responsible external protection. After validating the stability of the protection system, various internal faults were simulated. It was started with faults with tripping without time delay for different fault locations. Further, the protection was tested for cases with weak feed at each end, where the relays had to trip via the echo function of the teleprotection. Next, tests were performed for the case of disturbed signal connections, requiring the protection system to respond with delayed tripping in Zone 2. Finally, backup tripping was tested in zones 2 and 3 in the event of external faults and failure of the responsible external protection.



Internet via cell phone hotspot

An Internet connection was established in the three facilities as the basis for all further communication.

Online conference

Then, an online conference was started so that communication could take place conveniently at any time and all important content could be shared with each other, including the individual test steps.


Finally, the CMCs were wired exactly as they would be in a single-ended test, and a few shots were executed to ensure that the wiring in all stations was correct.

OMICRON Device Remote Agent

The Device Remote Agent was then used to centrally control all CMCs conveniently from a laptop running the master instance of RelaySimTest.



Once the maintenance of the field at the power plant will be completed, the protection devices will be returned to the original mode of operation for simple two-end protection, which will again be verified by two short retests with one remote end each.

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