Yujun Li

Advanced Control Strategies of PMSG-Based Wind Turbines for System Inertia Support

This paper investigates two novel control strategies that enable system inertia supports by permanent magnet synchronous generator (PMSG) wind turbines during transient events. The first strategy seeks to provide inertia support to the system through simultaneous utilization of dc-link capacitor energy, and wind turbine (WT) rotor kinetic energy (KE). The second strategy supports system inertia through orderly exerting dc-link capacitor energy of WT and then WT rotor KE via a cascading control scheme. Both strategies can effectively provide system inertia support by fully utilizing WTs own potentials, while the second strategy distinguishes itself by minimizing its impacts on wind energy harvesting. Case studies of one synchronous generator connected with a PMSG-based WT considering sudden load variations have been studied to validate and compare the two proposed strategies on providing rapid inertia response for the system.

Optimal Power Sharing Control of Wind Turbines

Wind generation can significantly disturb the power balance within particularly a weak power grid such as stand-alone microgrids. To counterbalance the impacts, an optimal power sharing control scheme that seeks to cope with the power dispatching demand by system operator is proposed for DFIG wind turbines. The control scheme can fulfill the dispatching command via maximizing the rotational kinetic energy stored in DFIGs, which can be later released for system support when needed.

Coordinated Control Strategies for Offshore Wind Farm Integration via VSC-HVDC for System Frequency Support

Coordinated control strategies to provide system inertia support for main grid from offshore wind farm that is integrated through HVdc transmission is the subject matter of this paper. The strategy that seeks to provide inertia support to the main grid through simultaneous utilization of HVdc capacitors energy, and wind turbines (WTs) inertia without installing the remote communication of two HVdc terminals is introduced in details. Consequently, a novel strategy is proposed to improve system inertia through sequentially exerting dc capacitors energy and then WTs inertia via a cascading control scheme. Both strategies can effectively provide inertia support while the second one minimizes the control impacts on harvesting wind energy with the aid of communication between onshore and offshore ac grids. Case studies of a wind farm connecting with a HVdc system considering sudden load variations have been successfully conducted to compare and demonstrate the effectiveness of the control strategies in DIgSILENT/PowerFactory.

A Novel Topology Design for Integration of Offshore Wind Farm via High-voltage DC Transmission

Abstract A novel configuration for offshore wind farms based on variable-speed permanent magnet synchronous generator integration via high-voltage DC transmission is proposed. Each permanent magnet synchronous generator is equipped with a full-bridge diode rectifier and a DC/DC boost chopper, which constitutes a cascaded submodule. Quite a number of submodules are series-parallel connected to form a DC collector system for the offshore wind farm. Any unit that needs maintenance or that undergoes fault in a series-connected branch can be isolated from the system by the natural commutated process without influencing other normal units in the same branch. The proposed submodule has several prominent features, such as low cost, low power loss, and simple control strategy. A maximum power point tracking algorithm of each wind turbine is realized by controlling the current through the inductor of each DC/DC boost circuit in one submodule. The proposed topology and control strategy are verified by simulations in Power Systems Computer Aided Design/Electromagnetic Transients including Direct Current (PSCAD/EMTDC; Manitoba HVDC Research Centre, Manitoba, Canada).

Variable Droop Voltage Control For Wind Farm

Converters of variable speed wind turbines in a wind power plant may possess different levels of reactive power capacity under variable power generation of each WT due to wake effects. This letter proposes a variable droop gain control scheme that seeks to mitigate voltage fluctuations at point of common coupling (PCC) by fully utilizing the voltage regulation capability of each WT converter. Droop gain of voltage controller in each WT converter is adaptively adjusted based on its current maximum reactive power capacity so that WT converters at back rows that have higher reactive power capacity can contribute more to PCC voltage regulation.

Hierarchical SCOPF Considering Wind Energy Integration Through Multiterminal VSC-HVDC Grids

In this paper, a hierarchical security-constrained optimal power flow (SCOPF) model is proposed for a meshed ac/multiterminal HVDC (MTDC) system with high wind penetration. The two interacting levels in the proposed model are as follows: 1) the high level is a traditional SCOPF problem in an ac system that aims to minimize total generation and security control costs; and 2) the low level is a dynamic power dispatching problem, which regulates power flow in an MTDC grid according to reference signals from the high level. Thus, the proposed method utilizes an MTDC system to provide support for the ac system by redistributing power flow across the entire grid and reducing control costs. Two modified IEEE meshed ac/dc systems are used to demonstrate the performance of the proposed method.

Coordinated control of wind farms and MTDC grids for system frequency support

Abstract This paper proposes a coordinated control strategy to provide system frequency support for separated main grids from offshore wind farms (WFs) that are integrated through multi-terminal DC (MTDC) system. The control strategy consists of two control loops including the MTDC frequency regulation loop (FRL) and the automatic WF power regulation loop (AWFPRL). In events of AC main grid disturbances, the former can re-dispatch MTDC grid power flow to support system frequency, while the latter can adjust wind power output accordingly through the variable frequency control of WF-connected converters. Inertia energy stored in wind turbines can be utilized to stabilize system frequency via MTDC grids. Moreover, wind power fluctuations can also be smoothed by the proposed control. An expression of frequency response coefficient is calculated, which is used to quantify the contribution of frequency support from offshore WF-connected MTDC system. Case studies have been conducted to demonstrate the effectiveness of the proposed control strategy using a four-terminal DC grid.

Variable Utilization-Level Scheme for Load-Sharing Control of Wind Farm

Under Maximum Power Point Tracking (MPPT) control, wind power is not dispatchable and can be highly disturbing to the supply-demand balance control in particularly context of microgrid. To effective dispatch wind power according to e.g. operator command or market schedule, a variable utilization level (UL) scheme is proposed for a wind power plant (WPP) to fulfill the dispatch order while reducing the loss of total energy production. Considering different wind conditions, the proposed scheme directs the power output for each wind turbine (WT) according to a specific utilization level (UL), which is adaptively adjusted according to WT rotor speed so that less reduction of energy production can be ensured. Meanwhile, more rotational kinetic energy (KE) stored can be stored in WPP, which can be later released for system support when needed. The proposed variable UL scheme is fully investigated in a doubly fed induction generator (DFIG)-based WPP and the results clearly indicate the proposed scheme can harvest more energy than the conventional same UL one while fulfilling the dispatch demand.

Power Flow Features and Balancing in MTDC Integrated Offshore Wind Farms

Abstract This paper proposes a novel control strategy for converter stations of multi-terminal DC (MTDC) grid system. It optimizes the offshore wind farms power generation dispatched to the separate onshore AC systems while minimizing their impacts on AC grids. Based on the MTDC power flow analysis, the minimal control dimensions of MTDC converters are firstly proposed. The impact of DC resistances on power dispatching among MTDC grid is then fully discussed by analyzing the structure of inverse Jacobian matrix of MTDC grid power flow equation and the power dispatch law for each onshore AC grid is deduced subsequently. Finally, an optimization strategy that seeks to re-dispatch the power to each onshore converter according to the system inertia of connected AC grid is proposed. Case study using a typical five-terminal MTDC system shows that the proposed strategy can effectively mitigate wind power fluctuations by approximately equalizing the induced disturbances to the system frequency of each AC grid.

A two-stage power dispatching algorithm for system support by droop-controlled DC grids

This paper presents a two-stage algorithm to regulate the power flow among a Multi-Terminal DC (MTDC) grid with DC voltage droop controllers to provide system support for the surrounding AC systems. At stage one, DC voltage of each terminal is calculated with the proposed analytical model to track the power demand of AC systems determined by the system operator. At stage two, power initial set points or DC droops of the droop controllers are modified according to the calculated DC voltage in the first stage. The proposed algorithm can accurately and quickly track the power demand from AC systems to provide system support under the system frequency disturbances. A four-terminal MTDC grid was modeled in the DIgSILENT/PowerFactory to demonstrate the effectiveness of the proposed algorithm.