Hybrid AC/DC Grid with Parallel LCC-VSC Interlinking Converters
Creators: Reza Ahrabi, Rouzbeh
Publisher: name: University of Alberta Library
Year: 2023

<p>Hybrid AC/DC grids have been of interest to scholars in recent years. A hybrid AC/DC grid can have higher efficiency and power quality by incorporating renewable base generations and power electronics of any kind into a system. Furthermore, the collection of power electronics in such a system can accommodate several ancillary services within the grid (e.g., unbalanced voltage, power quality, etc.). Line commutated converters (LCC) are a mature technology widely used in the grid due to their high power capacity and are usually known for adversely affecting the power quality. On the other hand, the fully controllable power electronics based on voltage source converters (VSC) are increasingly adopted because of their flexible control and better power quality.<br>
This work proposes the configuration of a parallel LCC-VSC interlinking converter (IC) to be used in hybrid AC/DC grids. The parallel LCC-VSC configuration takes advantage of the low cost of the LCC system as well as the bidirectional power flow capability of VSCs in a multi-terminal DC grid and offers an excellent option for lowering system costs while retaining high performance. Moreover, the LCC’s drawbacks, such as low power quality, reactive power consumption, and commutation failure, can be addressed using the parallel LCC-VSC configuration with proper control and coordination. In this work, an example of the parallel LCC-VSC system can be considered as a capacity expansion of the initially unidirectional LCC interlinking converters due to system growth, which may also require reverse power flow functions. A second scenario is that this parallel LCC-VSC system can be considered when the power flows in two directions are not equal.<br>
The unified control scheme is important for such a system considering various operational modes. Due to the LCC’s limitation in control and operation, it is essential to develop a compatible control scheme to achieve the desired function. Having considered the above two application scenarios, two unified control schemes are proposed in this work: 1) a control scheme for the expansion of the available LCC-based installations, and 2) a control scheme for new installations with parallel LCC-VSC interlinking units. As mentioned, the first control scheme is intended for expansion of the existing LCC units with parallel VSCs. The VSCs are controlled by a frequency-based droop equivalent control. In addition, the second control scheme implements an AC frequency-DC voltage-based droop equivalent control for new installations with unequal power flow in two directions. The expansion of the available installations is implemented by saving the LCC’s original V-I control graph. However, in new installations, the control scheme will be developed independently of the LCC’s conventional V-I graph by a constant supportive response from the VSCs.<br>
Power quality issues have been a serious concern with LCCs. Conventionally, passive components are used for harmonic and reactive power compensation. Therefore, in LCC-based installations with an intension of expansion by VSC-based units such passive components will be preserved. However, in new installations, active harmonics compensation is recommended based on the extended flexibility of the control scheme and the interlinking unit. The proposed compensation method is designed to operate under the low switching frequency of the interlinking VSCs without interfering in the main control scheme. In order to achieve the desired superior performance, the power system is modeled and stability studies are performed. Unbalanced voltage is an expected issue of the current power grid, which can adversely affect the performance of the system. Temporary or permanent loading conditions can contribute to unbalanced AC voltage leading to instability in the power system and LCC commutation failure. Such phenomena can be damaging for the power system and should be mitigated. In this thesis a compensation scheme is developed to mitigate the issue.</p>