Comparison of converter control schemes for weak grids

Voltage source converter is used in power systems for connecting a renewable energy source to an AC grid or as an active front-end rectifiers in AC motor drives. When the conventional control methods (vector control and power-angle control) are used to control a converter that is connected to a weak grid, the performance of the resulting system is degraded. High power demands degrade the quality of the transferred power and can even make the system unstable. A relatively new control scheme known as power-synchronization control is introduced to rectify the problem. The scheme keeps the system stable under weak-grid conditions but the damping for strong grids is compromised. The scheme needs to switch to a back-up vector control during fault conditions. This violates the initial motivation of developing a new control scheme. This thesis identifies the root causes of the stability problems in vector control when connected to weak grids. The state-of-the-art vector control scheme is used as a starting point. The role of the phase-locked loop and AC-voltage controller in the stability of the system is explored. Two novel modifications in the vector control scheme are presented. The converter-voltage reference or the measured converter voltage is used as a feedback to modify the active and reactive power references. The modifications ensure improved performance of vector control for weak grids. Better damping and faster dynamic response are obtained as compared to power-synchronization control and conventional vector control. The benchmark performance of the standard vector control for strong grids is not compromised by these modifications. The dynamic performance and robustness of the control schemes are tested and compared using time domain simulations

Factors in Active Damping Design to Mitigate Grid Interactions in Three-Phase Grid-Connected Photovoltaic Inverters

An LCL filter provides excellent mitigation capability of the switching frequency harmonics, and is, therefore, widely used in grid-connected inverter applications. The resonant behavior induced by the filter must be attenuated with passive or active damping methods in order to preserve the stability of the grid-connected converter. Active damping can be implemented with different control algorithms, and it is frequently used due to its relatively simple and low-cost implementation. However, active damping may easily impose stability problems if it is poorly designed.This thesis presents a comprehensive small-signal model of a three-phase grid-connected photovoltaic inverter with LCL filter. The analysis is focused on a capacitor-currentfeedback (i.e., a multi-current feedback) active damping and its effects on the system dynamics. Furthermore, a single-current-feedback active damping technique, which is based on reduced number of measurements, is also studied. The main objective of this thesis is to present an accurate multi-variable small-signal model for assessing the control performance as well as the grid interaction sensitivity of grid-connected converters in the frequency domain.The state-of-the-art literature studies regarding the active damping are mainly concentrated on stability evaluation of the output-current loop, and the effect on external characteristics such as susceptibility to background harmonics and impedance-based instability has been overlooked. As the active damping affects significantly the sensitivity to grid interactions, accurate predictions of the system transfer functions, e.g. the output impedance, must be utilized in order to assess the active-damping-induced properties. Moreover, the single-current-feedback active damping method lacks the aforementioned analysis in the literature and, therefore, the need for accurate full-order small-signal models is evident.This thesis presents design criteria for the active damping in a wide range of operating conditions. Accordingly, peculiarities regarding the active damping are discussed for both multi and single-current-feedback active damping schemes. In addition, the parametric influence of the active damping on the output-impedance characteristics is explicitly analyzed. It is shown that the active damping design has a significant effect on the output impedance and, therefore, the impedance characteristics should be considered in the converter design for improved robustness against background harmonics and impedancebased interactions

Finite-Control-Set Model Predictive Control for Low-Voltage-Ride-Through Enhancement of PMSG Based Wind Energy Grid Connection Systems

Grid faults are found to be one of the major issues in renewable energy systems, particularly in wind energy conversion systems (WECS) connected to the grid via back-to-back (BTB) converters. Under such faulty grid conditions, the system requires an effective regulation of the active (P) and reactive (Q) power to accomplish low voltage ride through (LVRT) operation in accordance with the grid codes. In this paper, an improved finite-control-set model predictive control (FCS-MPC) scheme is proposed for a PMSG based WECS to achieve LVRT ability under symmetrical and asymmetrical grid faults, including mitigation of DC-link voltage fluctuation. With proposed predictive control, optimized switching states for cost function minimization with weighing factor (WF) selection guidelines are established for robust BTB converter control and reduced cross-coupling amid P and Q during transient conditions. Besides, grid voltage support is provided by grid side inverter control to inject reactive power during voltage dips. The effectiveness of the FCS-MPC method is compared with the conventional proportional-integral (PI) controller in case of symmetrical and asymmetrical grid faults. The simulation and experimental results endorse the superiority of the developed FCS-MPC scheme to diminish the fault effect quickly with lower overshoot and better damping performance than the traditional controller

Control of multi-terminal HVDC networks towards wind power integration: A review

© 2015 Elsevier Ltd. More interconnections among countries and synchronous areas are foreseen in order to fulfil the EU 2050 target on the renewable generation share. One proposal to accomplish this challenging objective is the development of the so-called European SuperGrid. Multi-terminal HVDC networks are emerging as the most promising technologies to develop such a concept. Moreover, multi-terminal HVDC grids are based on highly controllable devices, which may allow not only transmitting power, but also supporting the AC grids to ensure a secure and stable operation. This paper aims to present an overview of different control schemes for multi-terminal HVDC grids, including the control of the power converters and the controls for power sharing and the provision of ancillary services. This paper also analyses the proposed modifications of the existing control schemes to manage high participation shares of wind power generation in multi-terminal grids.Postprint (author's final draft

Start-up of virtual synchronous machine: methods and experimental comparison

A modern grid is smarter mainly in the advance in information and communication technologies, while the power processing mechanism does not make a big difference. To make a modern grid smarter, the grid control should be improved to process the power in a smarter way. Therefore, it is easily foreseen that virtual synchronous machines, which emulates the synchronous machines based on power converters, may have big potentials in a future energy internet. This paper uses the Synchronous Power Controller with emulated and improved synchronous machine characteristics for renewable generation systems and proposes two start-up strategies. The proposed strategies are explained in detail, verified and compared by experimental results.Peer ReviewedPostprint (published version

A comparative study of methods for estimating virtual flux at the point of common coupling in grid connected voltage source converters with LCL filter

Grid connected Voltage Source Converters (VSCs) with LCL filters usually have voltage measurements at the filter capacitors, while it can be important to control the active or reactive power injection at the grid-side of the LCL filter, for instance at a Point of Common Coupling (PCC). Synchronization to the PCC voltage can be obtained by Virtual Flux (VF) estimation, which can also allow for voltage sensor-less operation of VSCs. This paper is presenting a comparative evaluation of methods for estimating the VF at the PCC, considering a VSC connected to the grid through an LCL filter with a Proportional Resonant (PR) controller as the inner current control loop. The VF estimation is achieved by using frequency adaptive dual SOGI-QSGs (DSOGI-VF). The Frequency Locked Loop (FLL) is used in order to keep the positive and negative sequence (PNS) VF estimation inherently frequency adaptive. Three different methods are considered for obtaining the capacitor current needed for estimating the VF at the grid side of the LCL filter which are based on fully estimation by using the voltage sensor-less method, by estimating the capacitor current from the measured voltage or by using additional capacitor current sensors. The results have been compared and validated by simulation studies.Peer ReviewedPostprint (author's final draft

Improving long line stability by integrating renewables using static synchronous generators

Postprint (author's final draft

Impedance-compensated grid synchronisation for extending the stability range of weak grids with voltage source converters

This paper demonstrates how the range of stable power transfer in weak grids with voltage source converters (VSCs) can be extended by modifying the grid synchronisation mechanism of a conventional synchronous reference frame phase locked loop (PLL). By introducing an impedance-conditioning term in the PLL, the VSC control system can be virtually synchronised to a stronger point in the grid to counteract the instability effects caused by high grid impedance. To verify the effectiveness of the proposed approach, the maximum static power transfer capability and the small-signal stability range of a system with a VSC HVDC terminal connected to a weak grid are calculated from an analytical model with different levels of impedance-conditioning in the PLL. Such calculations are presented for two different configurations of the VSC control system, showing how both the static power transfer capability and the small-signal stability range can be significantly improved. The validity of the stability assessment is verified by time-domain simulations in the Matlab/Simulink environment.Peer ReviewedPostprint (published version

Synchronous power controller merits for dynamic stability improvement in long line by renewables

Postprint (author's final draft