Abstract
Voltage Source Converters (VSCs) are gradually penetrating power systems which used to be dominated by synchronous generators. As a prevalent type of interface between intermittent sources and the AC power grid, VSCs are predominantly employed in the renewable power generation systems. Since the deployment of renewable power generation is and will be growing in the past decades and foreseeable future, the penetrations of VSCs into utility grids will surely be increasing at the same time; The grid strength, which refers to a weak grid condition, can be comparatively low when the penetrations of VSCs are high.Indexed by Short Circuit Ratio (SCR), the grid strength of a power network can be variable, which can impact the integration of a Voltage Source Converter (VSC). The variation can be significant. One seldom-explored issue is whether those commissioned VSCs can sustain with such variations that they were not designed for. Obviously, a robust solution against the variation of grid strength is expected for those VSCs, either implemented by VSC vendors or utilities.
Instabilities are more likely to exist when the grid strength becomes weaker. Researchers have found that the active power starts to oscillate when arising at a certain level and the voltage lost its stability as well when the grid condition is weak. The topic has been studied by researchers and multiple solutions have been proposed to mitigate the instabilities under weak grid conditions. However, the performance still remains to be improved. Therefore, it leaves huge space to further study the instability causes and proposed a better performing and more robust solution compared to the existing solutions.
A less-intrusive solution to stabilize a Voltage Source Converter (VSC) over an unknown grid strength is presented. The existence of equilibrium point is explored as a pre-requisite to stabilization. By partially imposing grid forming control, a simple auxiliary outer loop is proposed to exhaust the physical limit of power delivery in steady state and provide support to fault-ride-through operations over a wide range of grid strength. The proposed control can be used to upgrade a commissioned VSC with inner current loop intact; it also offers a non-intrusive solution to stabilize VSCs externally. The effectiveness of the proposed approach and schemes are verified by analysis in frequency domain and case studies in time domain including change of grid strength and fault-ride-through.
The physical limit of VSC transmission is analyzed against grid strength. To exploit this limit for the most adverse condition, a VSC stabilizer is employed, and its effect is illustrated by electromagnetic (EMT) time domain simulations. By utilizing frequency domain analysis with. state space, it reveals that the damping which the stabilizer provides to the system becomes worse when the stabilizer is moving away from the main VSC but the required size decreases with the distance. The principle of sizing a VSC stabilizer is quantitatively analyzed with virtual impedance and correction of voltage angle, and then summarized by an analytical estimation. The principle of sizing is finally verified with time domain simulations.
Based on a single stabilizer implementation mentioned above, the interactions between multiple stabilizers are investigated in terms of damping effect. To simply the modelling process for a multiple-VSC system, a generic methodology for modelling a complex VSC-gird system is developed with fully considering EMT dynamics.
| Date of Award | 2021 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Dong Chen (Supervisor) |