Thesis

A universal grid-forming VSC control for future power system

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Awarding institution
  • University of Strathclyde
Date of award
  • 2023
Thesis identifier
  • T16788
Person Identifier (Local)
  • 201776416
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Qualification Name
Department, School or Faculty
Abstract
  • To decarbonise the electricity sector, power systems are facing a significant transition to converter-dominated systems with higher penetration of renewable energy generations to replace conventional power generations using synchronous generators (SGs), changing the characteristics of power grid. Unlike SGs, power electronic converters do not contain rotating mechanical components. Accordingly, the mechanical properties owned by SGs will not be exhibited in the future power system, which can result in various issues in term of power system stability and the ability of faults and disturbances ride through. As power electronic converters are used to interface renewable resources with the power grid, they rely on the control dynamics and algorithms to maintain the entire system power balance and stability. However, there are lots of different control requirements considering the various grid conditions, including weak and strong grid connection, islanding, symmetrical and asymmetrical AC faults, which brings a big challenge for the control design of the power electronic converters. This thesis proposes a universal grid-forming (GFM) VSC (Voltage source converter) control for future power system with consideration to the corresponding various grid conditions. In this thesis, the control of grid-following (GFL) and GFM converters are reviewed firstly. The GFL control usually contributes to the regulation of active and reactive power output by injecting current through a vector current controller at a given phase. The grid phase is tracked by using a phase-locked loop (PLL) at all times. Different outer controller can be applied for different control purposes such as active power and voltage control. The GFL converters are predominantly applied in present renewable power generations, due to the capability in handing transient current during large transient events, precise control of current and good control dynamics, etc. However, as the GFL converters cannot regulate the system voltage and frequency directly, which makes them lack the capability of islanded operation. In addition, another constraint comes along with the use of vector current controller that causes the risk of instability on a weak grid. Intrinsically different from the GFL converters, the GFM converters use voltage regulation as the inner loop combined with power droop controller as the outer loop, to actively control their voltage and frequency outputs for the aim of voltage support. Hence, the GFM converters have the ability to work stably on islanding network, as well as weak grid connection network. However, the most common issue for GFM converters is the absence of effective current control loop, which limits their overcurrent capability. To synthesise the advantages of both GFM and GFL converters, a universal GFM VSC control is proposed. A direct voltage control in the dq reference frame is combined with a frequency droop control to regulate the AC voltage and frequency. Hence, the VSC has the capability of handling islanded operation. To ensure a stable grid connected operation, an adaptive power droop control is added as the outer loop to regulate the power exchanged between the converter and grid. A universal current limit control is also developed to limit the overcurrent and share the active and reactive current on both grid connection and islanding networks. In order to enable the ability of asymmetrical faults ride-through, the GFM VSC control is built in double synchronous frames to enable independent control of positive- and negative sequence components. An enhanced AC fault current control that employs both positive and negative-sequence current control is proposed. An additional voltage balancing control is also developed to retain the AC voltage controller for fault current limiting. By applying this controller, the general fault current limiting, dq current distribution and negative sequence current control when required can be achieved on a weak grid connection. Finally, small signal analysis is carried out to compare the stability of the GFM and GFL VSCs on weak networks. The impedance-based method is adopted to derive the admittances of the VSCs and connected grid in the positive- and negative-sequence (pn) reference frame. Time-domain simulations are also performed to verify the accuracy of the small signal admittances. Stability improvement with the GFM VSC on a very weak grid is validated.
Advisor / supervisor
  • Xu, Lie
Resource Type
DOI

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