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Zhao, Xianxian
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Zhao, Xianxian
Official Name
Zhao, Xianxian
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Now showing 1 - 10 of 10
- PublicationGrid-Forming Dynamic Stability under Large Fault Events – Application to 100% Inverter-based Irish Power SystemSystem stability is investigated for a future Irish grid consisting entirely of GFMIs under three-phase fault conditions with the inverters placed at existing locations for large-scale conventional generation. Electromagnetic transient (EMT) simulations showed that a 100% GFMI system, employing either droop control (or virtual synchronous machine), dispatchable virtual oscillator control, or a mix of both, under a combination of virtual impedance (VI) and scaling current saturation limiting control, is robust against 3-phase faults, with consistent performance being achieved, despite variations in fault location or inverter control methods. Freezing GFMIs virtual angular speed during the fault, for both VI and current scaling approaches, system transient stability is greatly enhanced. Time domain simulations also show that when active or reactive current prioritisation current saturation controls are applied that GFMIs can introduce large, high-frequency resonance oscillations, but a scaling-down current saturation approach can help to mitigate such problems by generating smoother current references.
118 - PublicationTransient stability analysis and enhancement of grid-forming inverters under different current prioritization limitation techniques(2020-11-12)
; Grid-forming voltage source converters have been envisioned to play a major role in future power systems, but they have limited overcurrent capability. Therefore, it is important to investigate their ability to ride through faults and maintain system stability under strict overcurrent limits. Different current priority limiting controls based on conditional anti-windup integration are formulated, and transient stability is analyzed using voltage phasor diagrams. Subsequently, a practical solution is proposed which enhances transient stability, by freezing the virtual angular speed to the pre-fault value during faults, once current saturation occurs. Additionally, temporarily freezing the post-fault angular speed to a value slightly less than pre-fault value ensures recovery from the saturated current state. Simulation results, based on a modified IEEE 39-bus system validate the efficacy of the proposed solution.18 - PublicationVoltage Dip Induced Frequency Dips for Power Systems with High Shares of Wind EnergyIn order to limit drivetrain mechanical stress, wind turbine generators typically implement a delayed active power recovery following a voltage dip, which may result in a substantial reduction in system frequency, if many wind farms adopt a similar strategy, due to the resulting generation-demand imbalance. Therefore, based on a modified IEEE 39-bus system, the impact of active and reactive current priority strategies, and various reactive current loop controls for wind turbine generators on voltage dip induced frequency dips (VDIFDs) are examined. The effectiveness of local voltage control with reactive current priority is validated. In addition, if synchronous-based generation is displaced by grid-forming converters (GFMs), the frequency dips are less severe and the post-fault frequency recovers more quickly. However, due to reduced overcurrent capability, the GFM virtual angle control must be carefully designed to avoid transient instability, by, for example, reducing the droop gain. Finally, if DC-link voltage control and maximum power point tracking control for the grid-side and machine-side converters are switched (to simplify fault ride through implementation) care is needed to avoid large post-fault over-frequency transients for VDIFD events.
32 - PublicationTransient Stability Enhancement with High Shares of Grid-Following Converters in a 100% Converter GridWith increasing shares of power electronics-based generation in power grids, grid-following converters may become unstable during faults, resulting from a loss of phase-locked loop (PLL) synchronism. Even when current grid code low voltage ride through (LVRT) requirements are met, PLLs may still become unstable under high shares of grid-following converters, due to much reduced reactive current support from (online) synchronous generators or grid-forming converters. Consequently, a readily implementable transient stability enhancement approach is developed for grid-following converters using a reactive current priority current limiting strategy for faults on a transmission network. The proportional gain for the reactive current injection is determined by formulating an optimization-based transient stability problem, which ensures a valid PLL equilibrium point, maximizes active power output, fully exploits converter current capacity, and, at least, satisfies existing grid code LVRT requirements. An additional PLL frequency-feedback PI term is used to enhance PLL dynamic stability, in recognition of parameter estimation errors and imperfect control. A case study (100% converter-based grid) verifies that the proposed solution enables grid robustness against faults and the permissible share of grid-following converters to be increased (especially in weak grids).
33Scopus© Citations 5 - PublicationDynamic studies for 100% converter-based Irish power systemGiven increasing shares of wind and/or solar power in many power systems, the possibility of a 100% power converter-based system becomes more plausible. Consequently, the dynamic response of the Irish transmission system with 100% (grid-following and grid-forming) power converters under 3-phase faults is investigated. Time domain simulations show that when active or reactive current prioritisation limits are applied, grid-forming converters can introduce large high-frequency oscillations, but a scaling-down current limitation approach can help to avoid such problems. Furthermore, applying scaling-down current limits, together with freezing the virtual angular speed, for a grid-forming converter, can limit the current and enhance transient stability during faults. Finally, with modified controls applied to the grid-following converters, the grid-forming requirement can be reduced from approximately 40% to less than 30%, with the future Irish grid remaining robust against bolted 3-phase faults, and oscillations quickly damped out during and post fault.
26 - PublicationGrid-forming requirements based on stability assessment for 100% converter-based Irish power systemThe fault response of a 100% converter-based system can be significantly different to that of a synchronous generator-based system, considering the lower capacity headroom, but flexible control capability, of power electronic converters. The system response is investigated for an Irish grid under balanced three-phase faults comprising of 100% converter-based generation: grid-forming (GF) and grid-following (GL). Electro-magnetic transient (EMT) simulations show that a system consisting only of GF converters (all droop control, all dispatchable virtual oscillator control, or a mix of both) is robust against three-phase faults, with little variation in performance, despite the fault location or choice of GF control methods. However, the rating and location of GF converters are critical to operating the grid securely in the presence of both GF and GL converters. Assuming that individual converter bus nodes are either GF or GL, a minimum GF requirement (by capacity) is found to be 37–40%, with these GF converters located close to the major load centres. Assuming instead that individual generation nodes consist of a mix of GF and GL converters, it is found that the GF requirement can be relaxed by 8–10%.
19Scopus© Citations 6 - PublicationGrid-Forming Converter Current Limiting Design to Enhance Transient Stability for Grid Phase Jump EventsGrid-forming converters (GFs) are seen as a replacement for synchronous machines in future power systems. Compared with synchronous machines, GFs provide much reduced overcurrent capability and hence they experience current saturation more frequently for the same capacity. Moreover, once in current saturation, GFs have limited regulation capability, and can become unstable if the current limiting control is improperly designed. Hence, in order to enhance GF stability when subject to phase jump down disturbances and current saturation, a combination of virtual impedance (VI) current limiting and scaling current reference saturation control is proposed. A relatively large virtual reactance-resistance ratio is also recommended. To speed up GF recovery, an (increased) transient P/f droop gain is proposed for downward phase jumps, but not fault conditions. Simulation studies on a simple system under large phase-jump events, i.e. ±60°, confirm the effectiveness of the proposed approach.
61Scopus© Citations 1 - PublicationGrid Forming Converter Angular Speed Freezing to Enhance Transient Stability in 100% Grid Forming and Mixed Power SystemsThe transient stability of a power system depends on the relative divergence of angular speeds between voltage sources during a fault disturbance. A unified angular speed control strategy is proposed here for grid-forming and synchronous machine voltage sources. Specifically, for a system consisting only of grid-forming converters (GFs), the virtual angular speed of all GFs is proposed to be frozen to the pre-fault value, while for a system with a mix of grid-forming converters and synchronous machines (SMs), the virtual angular speed of the GFs should be frozen to the weighted mean of the SM rotor speeds. The proposed freezing techniques are designed to be effective for short-circuit faults, and to be indifferent to phase-jump events. Simulation studies are performed on 100% GF and mixed GF/SM systems for both 3-phase fault and phase-jump events, for a range of GF control settings and system conditions, to demonstrate the effectiveness of the virtual angular speed freezing techniques.
24Scopus© Citations 1 - PublicationStability enhancement strategies for a 100% grid-forming and grid-following converter-based Irish power systemWith increasing shares of wind and/or solar power in many power systems, the possibility of a 100% power converter-based system becomes more likely. Consequently, the dynamic response of the Irish transmission systems with 100% (grid-following and grid-forming) power converters under 3-phase faults is investigated for 100% converter urban and remote scenarios. Time-domain simulations show that when active or reactive current prioritisation current saturation controls are applied, grid-forming converters can introduce large, high-frequency LC resonance oscillations, but a scaling-down current saturation approach can help to mitigate such problems as it generates smoother current references. Virtual impedance current limits are most effective at reducing oscillations, but the initial fault current can be transiently high. Furthermore, freezing the virtual angular speed for a grid-forming converter under either current saturation or virtual impedance current limitation approaches can enhance transient stability during faults. Finally, with modified controls applied to the grid-following converters, the grid-forming requirement can be reduced from approximately 40% to less than 30%, with the Remote Irish grid remaining robust against bolted 3-phase faults, and oscillations quickly damped out during and post fault.
26Scopus© Citations 8 - PublicationFreezing Grid-Forming Converter Virtual Angular Speed to Enhance Transient Stability Under Current Reference LimitingGrid-forming voltage source converters have been envisioned to play a major role in future power systems, but they have limited overcurrent capability. Therefore, it is important to investigate their ability to ride through faults and maintain system stability under strict overcurrent limits. Different current priority limiting controls based on conditional anti-windup integration are formulated, and transient stability is analyzed using voltage phasor diagrams. Subsequently, a practical solution is proposed which enhances transient stability, by freezing the virtual angular speed to the pre-fault value during faults, once current saturation occurs. Additionally, temporarily freezing the post-fault angular speed to a value slightly less than pre-fault value ensures recovery from the saturated current state. Simulation results, based on a modified IEEE 39-bus system validate the efficacy of the proposed solution.
29Scopus© Citations 9