Advanced control strategies for renewable energy integration for system support

Abstract

This thesis mainly focuses on the power system inertia less problem with the high penetration of renewables and accordingly proposes several concerned system support strategies. The typical variable speed wind turbines (VSWT) employing power electronic grid interface are gradually replaced with similar sized conventional thermal/hydro generation machines, which leads to the considerably lowered inertia available for the power grid. Further, the main function of these power converters is to realize the maximum power point tracking (MPPT) for maximally harvesting renewable energy and controlling the power transmission to the grid. It effectively decouples the rotation of the wind turbine (WT) and network frequency. As a result, the contribution of WTs to the system inertia support is also reduced. Accordingly, it has become mandatory that WTs are required to equip with frequency regulation control according to the grid codes in many countries. In this thesis, two novel control strategies that enable system inertia supports by permanent magnetic synchronous generator (PMSG) wind turbines during transient events are investigated. The first strategy seeks to provide inertia support to the system through simultaneous utilization of DC-link capacitor energy, and 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 WT's own potentials, while the second strategy distinguishes itself by minimizing its impacts on wind energy harvesting. Case studies of one synchronous generator (SG) 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. In recent years, wind power capacity has grown steadily, which raises concerns about the secure and reliable operation of the power system. Particularly, the popular MPPT algorithm adopted by VSWTs may cause supply-demand imbalance of the power system when wind power is more than system needs. Accordingly, the traditional SGs are required to operate at part-load levels or even shut down for some time to realize power balance in the system, which results in a reduced life cycle and the increased costs. To minimize such impacts, some countries have required WTs mandatorily to fulfil the dispatch demand set by system operator based on their grid codes. 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 fulfil the dispatch order while reducing the loss of total energy production in this thesis. Considering different wind conditions, the proposed scheme directs the power output for each WT according to a specific UL, which is adaptively adjusted according to WT rotor speed so that the less reduction of energy production can be ensured. Meanwhile, more rotational KE 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. When wind power penetration is high, particularly in the context of a microgrid, the wind generation according to the maximum power tracking control may significantly disturb the supply-demand balance. To counterbalance the impacts, an optimal power sharing control scheme that seeks to cope with the power dispatching demand by system operator while harvesting as much wind energy as possible 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. The proposed method has proved to be effective through a case study in a microgrid, which indicates the high potential for industrial applications. Traditionally, wind and photovoltaic (PV) generation is non-dispatchable and subject to Maximum Power Point Tracking (MPPT) control, which can be of highly disturbance to system dispatch in particularly context of microgrid. To effectively fulfil dispatch command or market schedule, a novel cascading power sharing control (PSC) scheme is proposed to coordinate wind and PV productions in microgrid while minimizing the possible reduction of renewable energy production involved. Considering different properties of wind and PV systems, the discrepancies between dispatch command (market schedule) and the actual renewable generation is counterbalanced by firstly adjusting wind output via temperately storing or releasing kinetic energy of turbine rotors. Only when the total production still prevails, should PVs deload their generation. The proposed PSC scheme is fully tested in a microgrid with wind and PV and the simulation results clearly indicate more wind energy can be captured in the proposed scheme compared to the traditional dispatch method while fulfilling the dispatch demand.