| Author: | Li, Jiapeng |
| Title: | Linear model and dynamic analysis of multi-terminal direct current systems |
| Advisors: | Xu, Zhao (EEE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Electric power transmission Electric power distribution -- Direct current Electric currents, Direct Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Electrical and Electronic Engineering |
| Pages: | 1 volume (unpaged) : color illustrations |
| Language: | English |
| Abstract: | Multi-terminal direct current (MTDC) systems maximise the economic and flexible advantages of DC transmission, thereby showing broad application prospects in high renewables penetrated power systems. Nevertheless, the dynamic properties of MTDC systems are rather complicated. Under severe DC faults, the DC network may experience intense current surges, which may damage the vulnerable converters. Under small disturbances, the DC-link current may oscillate divergently with poor control parameters, which may cause the converter to block and even disrupt the power transmission. Consequently, it is essential to study the dynamic characteristics for ensuring the safe and stable operation of MTDC systems. This thesis mainly focuses on the DC-side dynamic behaviours of MTDC systems, including the converter modelling, fault analysis, and stability analysis of MTDC systems, stated as follows. 1. Reduced linear models of the line-commutated converter (LCC) and voltage source converter (VSC) with control effects are established, respectively. As per the DC-link voltage deviation, the least squares approximation or small-signal analysis is utilised to derive the linear models of different converters. It is indicated that converters can be equivalent to simple RLC circuits for dynamic analysis from the DC side. 2. An analytical DC fault calculation method for generic MTDC systems is proposed. Combining the derived converter models with the network model and eliminating the operating variables, the state-space equation of the fault network is obtained. Accordingly, the mathematical expression of the short-circuit current of the entire network can be derived. 3. A simple DC fault calculation method of VSC-MTDC systems is proposed based on the high-frequency equivalent (HFE) model. In this model, the low-frequency components in the Laplace fault component network are omitted for the initial fault analysis. Accordingly, an analytical expression of fault current of the whole network is derived. It is unveiled that in the initial fault stage, the short-circuit current of the fault line increases approximately linearly, while that of other lines increases approximately cubically. 4. The DC-side instability mechanism is explored by investigating the small-signal stability of two-terminal DC links. The DC-side damping properties of various converters are first studied based on the system characteristic functions. For facilitating stability analysis, the original model can be reduced by retaining the dynamic characteristics in the vicinity of the dominant frequency, which is called dominant frequency analysis (DFA). Based on DFA, an analytical DC-side stabilising condition of the point-to-point VSC-HVDC link is derived. 5. Analytical DC-side stabilising conditions of MTDC systems are derived. The frequency domain model of a generic MTDC system is established. By using DFA, the analytical expressions of the system damping under each dominant frequency can be obtained. It is found that when the positive damping generated from constant-power-controlled rectifier(s), constant-DC-voltage-controlled converter(s), and transmission resistance cannot compensate for the negative damping introduced by constant-power-controlled inverter(s), divergent oscillation will appear in the DC network. |
| Rights: | All rights reserved |
| Access: | open access |
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