Modular Multilevel Converters in Hybrid Multi-Terminal HVDC Systems

High-voltage direct current (HVDC) systems are becoming commonplace in modern power systems. Line commutated converters (LCCs) are suitable for bulk power and ultra-HVDC (UHVDC) transmission, while with inflexible power reversal capability and possible commutation failures. However, voltage source converters (VSCs) possess flexible power reversal capability and provide immunity to commutation failures. Modular VSC topologies offer improved performance compared to conventional 2 level/3 level VSC-based HVDC. The family of modular VSCs includes the well-established modular multilevel converter (MMC) and other emerging modular VSC topologies such as the DC-fault tolerant alternate arm converter (AAC) that share topological and operational similarities with the MMC. It is noteworthy that the integration of LCC and modular VSCs leads to unique benefits despite the challenges of different HVDC configurations. Hence, it is necessary to explore the system performance of different HVDC converter topologies, especially more complex hybrid multiterminal HVDC (MTDC) systems and DC-grids combining different converters. This thesis focuses on the combination of the LCC, MMC and AAC to constitute different hybrid HVDC transmission systems. It is of significance to provide a common platform where the proper comparison and evaluation of different HVDC systems and control methods can be completed and independently validated. Therefore, this thesis also provides an overview of current HVDC benchmark models available in the existing literature. In addition, the detailed modeling methods of HVDC systems are discussed in this thesis. For ensuring the static security of HVDC systems especially the future DC-grids, this thesis proposes a generalized expression of DC power flow under mixed power/voltage (P/V) and current/voltage (I/V) droop control, considering the DC power flow for normal operation and after converter outage. Detailed simulation models are established in PLECS-Blockset and Simulink to study the hybrid HVDC/MTDC systems and DC grid combining the LCC with the MMC and (or) AAC. The detailed sets of results demonstrate the functionalities of developed hybrid HVDC systems and validate the performance of systems complying with widely accepted HVDC operating standards. The developed LCC/AAC-based HVDC/MTDC systems and LCC/MMC/AAC-based DC grid in this thesis are prime steps towards the study of more complex MTDC systems and a key element in the development of future DC super grids

NN-induced Physical Information Dynamic Library for Transient Modeling of Large-Scale Wind Farm

The complexity of transient characteristics in large-scale wind farms (WF) hinders the application of machine learning algorithms. This paper proposes a neural network-based learning method to provide physical information cues for the learning framework of transient characteristics in large-scale WF induced by physical information. The complexity of the physical information repository is simplified through an iterative algorithm. The dynamic library obtained based on neural networks can induce the machine learning framework to rapidly learn the transient characteristics of large-scale WF. Moreover, there is no need for excessive mechanistic analysis and speculation regarding the transient behavior of WF. The effectiveness of the proposed method is verified in the simulation model of a WF

Steady-state power distribution in VSC-based MTDC systems and dc grids under mixed P/V and I/V droop control

[EN] This paper proposes a steady-state power distribution derivation method for voltage source converter (VSC)based multi-terminal HVDC (MTDC) systems and dc grids under mixed power/voltage (P/V) and current/voltage (I/V) droop control. P/V and I/V droop control are two commonly used control schemes, which can be combined to achieve co-regulation of powers & currents in MTDC systems and dc grids. The proposed method can be used to estimate the power distributions under different scenarios for MTDC systems and dc grids based on VSCs with mixed P/V and I/V droop control. After determining the initial operating point based on an estimation-correction algorithm, redistributed power due to power disturbances, current changes or converter outages is analyzed in detail considering converter overload. An excess power reduction strategy is further proposed to avoid violation of power limits after converter outage. The accuracy of the proposed method is validated through multiple scenarios in a modular multilevel converter (MMC)-based four-terminal dc grid. The comparison between the proposed method and other approaches in the current literature further demonstrates the advantages of proposed power distribution derivation method.The third author (Muhammad Khalid) would like to acknowledge the support from Deanship of Research Oversight and Coordination (DROC) at King Fahd University of Petroleum and Minerals (KFUPM) through project No. DF201011.Sun, P.; Wang, Y.; Khalid, M.; Blasco-Gimenez, R.; Konstantinou, G. (2023). Steady-state power distribution in VSC-based MTDC systems and dc grids under mixed P/V and I/V droop control. Electric Power Systems Research. 214:1-10. https://doi.org/10.1016/j.epsr.2022.10879811021

Adaptive Droop Controlled-VSCs in MVDC Distribution Systems with ESS Engagement

A novel adaptive droop control (ADC) is proposed in this paper for voltage source converter (VSC)-based medium-voltage direct current (MVDC) distribution systems. The proposed ADC can achieve MVDC bus voltage management and power sharing between VSCs with overloading operation consideration, solving the issue that all VSCs on the rectifier/inverter side reach their power limits concurrently after a large system disturbance. It is accomplished by factoring in the real-time power/voltage values at VSC stations and a newly introduced extended power component. This component is contingent on the actual operational state of the energy storage systems (ESSs) in MVDC systems and inversely correlated with the power delivered/absorbed by the ESSs. The effectiveness of proposed ADC is confirmed through an MVDC distribution system model built in RTDS real-time digital simulators

Assessment of low-loss configurations for efficiency improvement in hybrid modular multilevel converters

Hybrid modular multilevel converters (HMMC) address the DC-fault blocking limitations of the half-bridge submodules (HB-SMs) of the standard MMC by combining HB-SMs with full-bridge submodules (FB-SMs). In order to improve the overall conversion efficiency, this article proposes two low-loss configurations for the HMMC. The main improvement in the proposed HMMCs is achieved by the combination of low-loss unipolar and low-loss bipolar SMs in the same arm of the HMMC. It is shown that the simplest structure (LLH1) can reduce the SM losses (switching and conduction losses in an SM) by 30% compared to the HMMC at the cost of limited fault blocking capabilities. The fully controlled structure (LLH2) reduces SM losses by 10% compared to the HMMC, and 30% compared to the FBSM-based MMC. These results represent an increase of 20% over the typical HBSM-based MMC for a converter (LLH2) that can provide fault-blocking capabilities. Assessment of efficiency in both HMMCs is provided over a range of operating conditions both in inverter and rectifier mode from an 800-MVA HVDC system model.Published versionThis work was supported by the Australian Research Council’s Discovery Grant DP210102294