Design and control of grid-friendly power electronic systems

• Main Author: Deng, Han
• Other Authors: Tang Yi
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/163682
• Institution: Nanyang Technological University

To reduce the carbon footprint, the installed capacity of renewable energy sources (RESs) has been increasing continuously, which replaced the portion of conventional fossil fuel-based power generations in the past decades. The distributed generations (DGs) powered by RESs or energy storage systems (ESSs) are coupled to power grids via power electronic converters. The increasing penetration of power converter-interfaced DGs turns the conventional synchronous generator-dominant power systems into more-electronics power systems. Grid-tied converters can be generally classified into grid-feeding converters (GFDCs) and grid-forming converters (GFMCs). GFDCs are controlled as current sources and synchronize with grids through phase-locked loops (PLLs). GFMCs are controlled as ac voltage sources and synchronize with grids with active power feedback control. Both GFDCs and GFMCs can be designed to provide grid ancillary services. GFDCs can control the current quickly and precisely, and have mature theories and wide applications, while they lack voltage forming capability. On the contrary, GFMCs can form their output voltage and thus can provide voltage support to grids. The voltage-forming capability is increasingly demanded when the penetration of power electronic-based generations gets higher. So GFMCs are selected as the research objective. Although featured with fast response and flexible control, power electronic converters also bring challenges to modern power grids. On one hand, the decreased portion of traditional synchronous generators in power grids leads to the lack of inertia, which may threaten the power grid frequency stability. On the other hand, the control stability under both normal and fault conditions will also affect the safe operation of grids. Moreover, the increasing penetration of RESs also increases the power supply uncertainty and leads to power unbalances among feeders. This thesis targets at dealing with the problems in more-electronics power systems by designing the control of grid-tied power electronic converters. Inertia shortage makes modern power systems sensitive to frequency variations, thereby leading to undesirable load shedding, cascading failures, or even large-scale blackouts. To address the inertia concern, distributed virtual inertia (DVI) from grid-tied power converters is emerging as an attractive solution. Flexible DVI delivered by GFMCs without additional energy storage units is studied. It is revealed that virtual inertia control may cause stability problems. Through the derived detailed state-space model and sensitivity analysis, the mechanism of the instability is disclosed. A lead compensator, together with its design procedure, is proposed to improve the DVI stability. The proposed control scheme allows significant improvements in inertia and frequency regulation. GFMCs with small droop coefficients or connected to the grid through a small impedance may have a fast power loop, which leads to serious conflicts with the inner ac voltage control loop, resulting in power oscillations or even instability. To address this problem, an extremely fast ac voltage loop should be expected for GFMCs. On top of that, GFMCs must operate well in both stand-alone and grid-tied conditions. Through detailed analysis, it is revealed that the grid-tied operation of a dual loop-controlled GFMC with an LCL output filter features a much slower voltage control loop than that of the stand-alone mode. To improve the dynamics of GFMCs, a generic voltage control scheme with a high-pass filter (HPF) in the current feedback loop is proposed. The generic voltage controller has fast voltage tracking performances under both grid-tied and stand-alone operations, thereby enabling GFMCs to achieve better power regulation performances. As GFMCs behave as voltage sources, the current limiting of GFMCs under large disturbances requires special efforts. Furthermore, reliable GFMCs should also be capable of remaining synchronized with grids during and after large voltage disturbances. Under low voltage faults, GFMCs usually retain the active power synchronization (P-Syn) under low voltage faults when the current saturation is triggered. However, when voltage sags are severe, the transient stability with P-Syn control highly depends on the fault duration. To address this issue, a robust fault-ride-through strategy for GFMCs based on reactive power synchronization (Q-syn) under voltage sags is proposed. The Q-syn method ensures the transient stability of GFMCs regardless of the fault duration or level. Furthermore, fast post-fault recovery can be achieved with the Q-Syn method. With the increasing penetration of distributed generation systems that feature stochastic power outputs, it becomes difficult for distribution networks to achieve power balancing among various feeders and maintain decent voltage profiles. To address these challenges, multiple back-to-back (B2B) voltage source converters (VSCs)-based soft open points (SOPs) are emerging as a solution to facilitate power dispatches and isolate faults. A unified grid-forming control strategy for multi-terminal SOPs is proposed, with which all the B2B VSCs control the dc voltage and the power flow simultaneously. The SOP with the proposed control strategy can regulate power flows under normal conditions, as well as isolate faults, limit the fault current and autonomously support isolated loads after faults. The detailed power flow and dc voltage controller design is also introduced. In summary, this thesis aims at addressing or alleviating the potential problems in future more-electronics power systems by designing the control of grid-interfaced power electronic devices. The power electronic converters are designed to be more grid-friendly through stability improvement at either the converter level or system level, and enhancement in fault tolerance capability.