Graduation Year

2021

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Electrical Engineering

Major Professor

Zhixin Miao, Ph.D.

Committee Member

Lingling Fan, Ph.D.

Committee Member

Chung Seop Jeong, Ph.D.

Committee Member

Achilleas Kourtellis, Ph.D.

Committee Member

Kaiqi Xiong, Ph.D.

Keywords

Subsynchronous oscillation (SSO), Frequency coupling, Stability analysis, Hybrid Boost Converter (HBC)

Abstract

Nowadays, the inverter-based generators (IBGs) has been exponentially integrated into the power grid due to environment and energy saving concerns. However, the high penetration of low inertia IBGs brings new problems on the system stability. The main objectives of this dissertation are to: 1- carry out large-signal analysis to identify the stability issues and small-signal analysis to assess the stability of the power systems, and 2- provide a standard measuring procedure to identify the admittance of IBGs. In addition, investigation on a novel DC-based IBGs architecture is carried out. In this dissertation, one widely used IBG is investigated: Type-4 wind turbine generators (WTGs).

The first part of the research conducts a thorough analysis including validation, identification of potential stability risks, and in-depth physical insights on the stability issues for high penetration of grid integrated type-4 wind. The state-space based time-domain modeling approach is employed. The power grid considers Texas transmission grid's characteristics and adopts two assumptions: noncompensated network with weak grid consideration and series compensated network with weak grid consideration. The latter is used to shorten electric distance of transmission lines. The stability issue manifests as subsynchronous oscillation (SSO) in the research. To investigate the SSO issue, small-signal stability analysis is required, which requires building linearized analytical models with state variables constant at steady-state. Thus, the state-space analytical models are developed based on the different grid-side converter (GSC)'s control mode. Eigenvalue analysis and participation factor analysis are then carried out to offer an entire picture of system modes and further identify influencing factors. Because the analytical model is a simplified model which only include the essential dynamics, a rigorous validation will be carried out to validate the analysis results against the simulation results from EMT testbeds including full dynamics.

The second part of the research focuses on identifying the SSO issue using admittance-based frequency-domain modeling approach. Impedance models can be efficiently derived as the frequency-domain transfer function from the analytical models with device's terminal voltage treated as the input and the current flowing into the device as the output. But if IBGs are black boxes, the measurements-based approach to find the frequency-domain impedance is prevailing. The impedance or admittance model of IBGs can be obtained by either perturbation-based experiments (e.g., frequency scan) or time-domain data-based identification (e.g., Eigensystem Realization Algorithm (ERA) method relying on step responses). The resulting current/voltage ratio is the admittance measurement, which can be a scalar or a matrix. The scalar admittance is suitable to assess SSO for type-3 wind. However, it has limitation on stability analysis of type-4 wind grid integration systems due to strong frequency-coupling effect. This part provides the theoretic derivation to shed insights on frequency coupling effect observed in the static abc-frame, and provides a clear guide line of sequence-domain admittance measuring procedure. Furthermore, with a system splitting into a source and a load subsystem, the ratio of the source-load impedance can be used to determine stability using Nyquist stability criterion. Meanwhile, s-domain admittance model-based eigenvalue analysis can be employed to give an entire picture of the system poles andtheir trajectories for a varying parameter.

In addition to the modeling and analysis work, this dissertation also includes investigation on a single-stage multi-port hybrid boost converter (HBC) to regulate the dc and ac loads simultaneously, provide a high voltage gain ratio, while maintaining the closed-loop stability of the entire power system. The HBC features the simple architecture reducing the unnecessary processes of dc/dc and dc/ac conversions compared with conventional architecture of DC-based power system. This part provides the critical aspects of the operation of the HBC including the grid-following control strategy and hybrid pulse width modulation methodology.

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