EMT-type impedance scanning methods for screening control interactions between inverter-based resource and transmission grid

Abstract

The increasing integration of inverter-based resources (IBRs) into modern power systems has significantly increased the risks of converter instabilities. The impedance-based stability assessment (IBSA) method is widely used to identify the potential IBR instability conditions due to its simplicity. The desired accuracy can be achieved by using the electromagnetic transient (EMT)-type impedance scanning method. This method also allows extracting the impedance models of manufacturer-specific models, thus, has gained popularity among transmission system operators (TSOs). Several impedance scanning methods in different domains have been developed and used. However, important aspects of these methods such as the IBSA accuracy, computational efficiency and relationships between the obtained impedance models have not been investigated in detail. This research focuses on the EMT-type impedance scanning methods for screening the potential IBR instability conditions. The αβ-frame impedance scanning (αβ-scan), improved dq-frame impedance scanning (dq-scan), and improved stationary coupled sequence-frame impedance scanning (scs-scan), are proposed. It is demonstrated that the αβ-, abc-, and stationary decoupled sequence (p,n)- frame impedance models of IBR are analytically equivalent to sequence single-input single-output (sequence-SISO) impedance models which ignore the mirror frequency effect (MFE). As a result, their usage in IBSA may produce accuracy issues while identifying certain IBR instability conditions. The dq-frame multi-input multi-output (dq-MIMO) impedance model accounts for the MFE inherently and provides the ultimate IBSA accuracy. However, it requires time consuming EMT simulation of the entire system to differentiate the resonance and mirror frequencies. To eliminate this requirement, a combined dq-MIMO and sequence-SISO IBSA scheme is proposed. In this scheme, a sequence-SISO IBSA is made following the dq-MIMO IBSA for differentiating the resonance and mirror frequencies. The sequence-SISO models are obtained through impedance transformation rather than the p,n-scan. Thus, it does not introduce any additional computational burden. The proposed scs-frame IBSA (scs-IBSA) offers similar accuracy to the dq-MIMO IBSA as the corresponding impedance models are analytically equivalent. The scs-IBSA is a better alternative as it (i) inherently differentiates the resonance and mirror frequencies, (ii) gives insight into the instability mechanism, and (iii) is easier to implement and computationally more efficient. This research also includes a review work of three SISO and three MIMO IBSA methods. The merits and shortcomings of each method are discussed. Detailed guidelines for proper selection and usage of impedance scan and IBSA methods are provided. A new control interaction phenomenon between a large-scale full-size converter (FSC)-based WP and a 500 kV transmission grid is identified and investigated using EMT-type impedance scanning and IBSA. The performed analyses demonstrate that parallel and series resonances formed in the grid-side system interact with the WP in supersynchronous frequency range and causes instability. The transmission line length, applied series and shunt compensation levels, presence of parallel lines, and wind turbine (WT) grid side converter (GSC) control parameters have significant impact on the instability phenomenon. The presented results and drawn conclusions can be generalized to any IBR with FSC, such as photovoltaic (PV) power stations.