«The MUs in Burladingen are not ‹pure› Merging Units; they also include other functions such as fault recorders and backup overcurrent protection in MU2, which remains operational even if the process bus fails.» The MUs offer the possibility to have SV streams for measurement and protection with different resolution and range. In Burladingen, only the SV stream for protection is configured, with a measurement range up to 100 × the rated current, whereas for measurement it is only 1.6 ×. Similar to conventional CTs, it is essential that the protection relay is connected to a protection “core”. With a rated current of 300 A, the LPIT system was tested with 2 × rated current, and the SV streams were accurate, indicating a protection CMC 430 LLX1 MU2 Backup overcurrent Simulated rogowski signals stream. If required test currents could not be met, an alternative would be to simulate Rogowski secondary voltages with test equipment at the MU inputs. This method was not necessary here but useful for testing backup overcurrent protection integrated into MU2. Test of backup overcurrent protection integrated in MU The MUs in Burladingen are not “pure” Merging Units; they also include other functions such as fault recorders and backup overcurrent protection in MU2, which remains operational even if the process bus fails. To be able to test the correct settings of the backup overcurrent protection, the MU allows for a “reverse injection” of an SV stream dedicated for testing, by temporarily changing the inputs from the LPIT to this stream. But obviously, this method bypasses the LPIT inputs, making it impossible to verify the internal measurement chain. Therefore, tests were carried out by applying the required Rogowski secondary voltages to the LPIT interface of the IED. This test worked well, but connecting the test equipment to the LPIT inputs was a challenge due to the lack of test terminals. The terminal block on the IED was removed entirely to avoid changes to the wiring, and testing connections were established with a spare terminal block. To ensure everything was left as found, the tests concluded with a primary injection test into the LPIT. In conclusion, in a digital substation using process bus, the protection system depends on the correct measurement acquisition by the instrument transformers and the MUs. The tests confirmed the reliability and accuracy of the measurements as well as the integrated backup overcurrent protection, ensuring its readiness for operational use. Testing using simulated Rogowski signals at the LPIT inputs 44
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