Modelling, Simulation, and Validation Tests for Sub-harmonics Protection Relay

The SSO study and the modelling of SHPR have been essential according to grid-code special requirements by Finnish transmission system operator (TSO). The required SSO study is composed of EMT simulation, SSO protection, and dynamic impedance scans studies. The

EMT simulation model shall include the SHPR model. It is crucial for TSO to ensure tSHPR model replicates the behaviour of the physical relay, because when simulations are conducted by TSO in wide area model, TSO is detecting different oscillations and interactions. The simulation study can clarify, how many sites might have tripped due to this SSO protection in different cases and provide tools to alleviate the situation.

TSO is also interested to understand, will this SHPR get triggered in other situations including converter controller interaction situations. Therefore, it is not just to study series resonance with series capacitors, but to understand, which scenarios might trigger SHPR and how much generation (MWs) might be lost in different scenarios.

This paper considers a specific SHPR that has been installed in several power plants across

Finland due to its different protection functionalities such as nominal ratio, fundamental ratio, and TSHD. This SHPR contains practical features including cross-coupling, 2nd and 5th harmonics blocking.

This paper proposes a model-development method for this specific SHPR, in which discrete

Fast Fourier Transformation (FFT) algorithm and the BIN theory are used to estimate the amplitude and the frequency of the sub-harmonic perturbation. To accurately mimic the real relay’s behaviour, the modelling method considers FFT delay, relay extra delay, and detecting out-of-the-range subharmonics. Finally, the SHPR model is validated against the real relay device. The simulation results verify that the EMT model of SHPR model has a good alignment with relay device in different test scenarios. Our simulation model can help different TSOs to conduct wide area studies and protection studies.