De-loaded Operation Research Articles

An optimal approach for active power reserve of a de-loaded PV generator operating in coordination with synchronous generators

The de-loading operation of PVs, where PVs are operated with an active power reserve has picked up the interest of the researchers in the last few years. However, in an integrated system, where the PV generators are operated in coordination with the synchronous generators, the estimation of the amount of the active power reserve of the PV generator is crucial. A very high reserve will cause an excessive economical loss and a very low reserve will deteriorate the frequency regulation capability. Therefore, in this paper, the cost function of an integrated system is minimised using the particle swarm optimisation algorithm, and the amount of power reserve of the PV generator and operating power values of synchronous generators are obtained as a result of the minimisation. Further, the frequency response of the system for the obtained power reserve value is investigated by suddenly adding the load in the system. The lowest and highest system costs of 2521.87 and 4474.98 $/h are obtained for the irradiance 1000 and 400 W/m2 respectively, and upon a significant load change, the frequency deviation limits to 0.4513 for a reserve value of 29.0712 MW. System modelling and validation have been carried out in the SIMULINK toolbox of the MATLAB software.

A control-oriented LPV model of large-scale offshore wind turbines for deloading operation

ABSTRACT This article develops a new control-oriented linear parameter varying (LPV) model, which is a linear state-space model with varying parameters aimed at efficiently representing large-scale offshore wind turbines (LOWTs). Firstly, the LPV model is constructed by the mechanism analysis of a 15-MW direct-drive permanent magnet wind turbine. Subseuently, based on the operational data from FAST, the parameters with unknown values in the LPV model are determined by gain scheduling and parameter estimation. Furthermore, a deloading operation for the active output power regulation of LOWTs is proposed based on coordinating the rotor over-speed control (ROC) and the pitch angle control (PAC).

Power reserve control for utility-scale PV power plants under cloud conditions

The photovoltaic generation growth has posed challenges for the control and operation of contemporary power systems, degrading the system frequency response and stability margins. The decrease of the system equivalent inertia and the performance deterioration of the primary frequency control are the main causes of these problems. Such challenges can be overcome by the provision of ancillary services from photovoltaic power plants. However, the provision of ancillary services based on active power, such as inertial response, frequency regulation, and curtailment, requires the de-loaded operation of the photovoltaic power plants. In such context, this work proposes a de-loaded control approach (or, equivalently, power reserve control) for large photovoltaic power plants under shading conditions caused by cloud movements. This kind of shading condition poses a significant challenge for the de-loaded control, as it causes large and fast variations in the maximum power point of the multiple photovoltaic generation units that compose the photovoltaic power plant. A test system corresponding to a 100 MWac photovoltaic power plant with 25 photovoltaic generation units of 4 MWac is employed to evaluate and validate the proposed control approach. The stochastic distribution and movement of clouds, as well as the solar irradiance over the photovoltaic power plant are generated by an approach based on the fractal geometry. The proposed control was effective and presented a good performance under different cloud coverage levels.

Optimized power dispatching method for a wind farm during de-loading operation to participate in power system frequency regulation

Optimized power dispatching method for a wind farm during de-loading operation to participate in power system frequency regulation

Primary Frequency Stability Support of a DFIG in Association With Pitch Angle Control

For an electric power grid that has large penetration levels of variable renewable energy including wind generation and photovoltaics, the system frequency stability is jeopardized, which is manifest in lowering frequency nadir and settling frequency. This paper suggests an enhanced primary frequency response strategy of a doubly-fed induction generator (DFIG) in association with pitch angle control. The DFIG works in de-loaded operation with a certain reserve power via pitch angle control prior to disturbances for frequency regulation. To address this, a function of the pitch angle is employed that decreases the pitch angle with time to slowly feed the active power to the power gird. The simulation results demonstrate the effectiveness and feasibility of the proposed primary frequency response strategy including the settling frequency and frequency nadir.

Low-Voltage Ride-Through Coordinated Control for PMSG Wind Turbines Using De-Loaded Operation

Low-voltage ride-through (LVRT) is important for the grid integration of wind power systems (WPSs). Each grid operator defines a grid code, including the LVRT requirement, considering their power system characteristics. During grid faults, LVRT is needed for the protection of WPS because the over current can induce damage to the power converter system. An important issue in the LVRT method of WPS is the energy capacity because sufficient reserve energy capacities of wind turbine inertia and an energy storage system (ESS) are needed for a successful LVRT operation. Although ESS can store excess energy of the WPS during a grid fault, it is physically limited considering its energy capacity. Moreover, WPS has a low reserve energy capacity, especially when the WPS operates in high wind speed conditions. Thus, we propose the deloaded operation of WPS for the preparation of LVRT by reducing the power generation of WPS. The effectiveness of the proposed method is validated by modeling a WT and an ESS topologically and performing simulations using the MATLAB/Simulink SimPowerSystems toolbox.

Operational and control approach for PV power plants to provide inertial response and primary frequency control support to power system black-start

The renewable generation growth, which includes photovoltaic power plants, has posed challenges for the planning and operation of contemporary power systems. High penetration levels of generation based on fully rated converters makes the black-start of power systems more difficult due to the low equivalent inertia of the system, the performance deterioration of the primary frequency control, the reduced number of black-start units, and the energization of large loads. In the context of the planning and operational challenges imposed by renewable generation, this work proposes an operational and control approach for centralized photovoltaic generation to provide inertial response and primary frequency control support to the black-start of bulk power systems. The proposed control aims to provide inertial response and primary frequency control support only during the system restoration process. The work also proposes an analytical formulation to assess the impact of the operating point of photovoltaic modules on the dynamics of photovoltaic units. The obtained results have shown that photovoltaic power plants are capable of providing support to the black-start of power systems, improving the dynamic and steady-state performance of the system frequency response during the black-start. The performed dynamic analyses have shown that the operational region of the photovoltaic modules significantly affects the dynamics of the power balance in the photovoltaic unit and, consequently, the dynamics of the load pick-up of the photovoltaic modules. The proposed dynamic analysis is useful to support the formulation of control approaches for de-loaded operation of photovoltaic power plants.

Fully-Distributed Deloading Operation of DFIG-Based Wind Farm for Load Sharing

A system-level consensus-based deloading operation is proposed for DFIG-based wind farm (WF) participating in system active power regulation. Rotor overspeeding control (ROC) and pitch angle control (PAC) are coordinated for load sharing among each wind turbine (WT). For ROC, a novel fully-distributed consensus protocol is developed for fair load sharing control without the need to know the total active power capacity. Considering the influences of wind turbines’ individual differences and rotor speed limit, the amount of WTs involved in consensus protocol are dynamically adjusted according to their varying operating points. Further, a distributed optimization strategy is designed for active power setting-points allocation to curtail the utilization of WF's pitch angle during PAC. As the result, the proposed strategy can fully and quickly extract the available rotor overspeeding margin for a high kinetic energy storage and a better dynamic performance in ROC stage, and further minimize the dependency on pitch angle when facing deeper deloading operation. Case studies demonstrate the effectiveness of the proposed scheme.

High-voltage Ride Through Strategy for DFIG Considering Converter Blocking of HVDC System

This paper presents a P-Q coordination based highvoltage ride through (HVRT) control strategy for doubly fed induction generators (DFIGs) based on a combined Q-V control and P-V de-loading control. The active/reactive power injection effect of DFIG on transient overvoltage is firstly analyzed and the reactive power capacity evaluation of DFIG considering its de-loading operation is then conducted. In the proposed strategy, the reactive power limit of DFIG can be flexibly extended during the transient process in coordination with its active power adjustment. As a result, the transient overvoltage caused by DC bipolar block can be effectively suppressed. Moreover, key outer loop parameters such as Q-V control coefficient and de-loading coefficient can be determined based on the voltage level of point of common coupling (PCC) and the available power capacity of DFIG. Finally, case studies based on MATLAB/Simulink simulation are used to verify the effectiveness of the proposed control strategy.

Scenario-based Unit Commitment Optimization for Power System with Large-scale Wind Power Participating in Primary Frequency Regulation

Continuous increase of wind power penetration brings high randomness to power system, and also leads to serious shortage of primary frequency regulation (PFR) reserve for power system whose reserve capacity is typically provided by conventional units. Considering large-scale wind power participating in PFR, this paper proposes a unit commitment optimization model with respect to coordination of steady state and transient state. In addition to traditional operation costs, losses of wind farm de-loaded operation, environmental benefits and transient frequency safety costs in high-risk stochastic scenarios are also considered in the model. Besides, the model makes full use of interruptible loads on demand side as one of the PFR reserve sources. A selection method for high-risk scenarios is also proposed to improve the calculation efficiency. Finally, this paper proposes an inner-outer iterative optimization method for the model solution. The method is validated by the New England 10-machine system, and the results show that the optimization model can guarantee both the safety of transient frequency and the economy of system operation.

Impact assessment of virtual synchronous generator on the electromechanical dynamics of type 4 wind turbine generators

The de-loaded operation of wind turbine generators (WTGs) with inertial and frequency control considerably affects the dynamics and stability of variable speed wind turbines. In this context, this work proposes a comprehensive assessment of the simultaneous impact of virtual synchronous generator and frequency controllers on the electromechanical dynamics of type 4 WTGs. The proposed power balance formulation and the performed analysis, which are the main contributions of this work, provide insight into the simultaneous impact of the inertial and frequency control on the dynamics of the wind generation unit and system frequency. The assessment has been performed by means of an analytical formulation for the power balance in the WTG, characteristic curves of the wind turbine, and non-linear time-domain simulations of two test systems. The analyses have shown that the speed and torque deviation magnitudes of the wind turbine, as well as the amount of kinetic energy released by the wind turbine, depend on the wind turbine operating point. The study has also shown that the de-loaded operation considerably increases the inertial response capability of WTGs. The integrated inertial and frequency control approach has resulted in well-behaved electromechanical dynamics for the WTG.

Optimized Active Power Dispatching Strategy Considering Fatigue Load of Wind Turbines During De-Loading Operation

With the increase of wind power penetration rate in the power grid, power dispatching including wind curtailment often happens to guarantee the safety of the power system. Considering the fatigue load of wind turbines during de-loading operation, how to optimally dispatch them along with the required power instructions becomes a hot issue. Under this background, this paper mainly studies the active power dispatching based on fatigue load optimization. In order to reduce the calculation complexity of the fatigue load, this paper proposes a simplified active power dispatching model for wind turbines via a look-up table. Subsequently, the ultra-short-term wind speed prediction is performed using the least squares support vector machine for the dynamic’s prediction of wind turbines, which will be applied in fatigue load pre-calculation. Moreover, in order to fast solve the optimal dispatching problem, the accelerated particle swarm optimization algorithm is adopted. Under different de-loading scenes, the simulation experiments show that the fatigue load of the wind turbines has certain regularity while different de-loading strategies yield different effects on the fatigue load. Finally, based on the evaluation of damage equivalent load (DEL), the proposed optimal dispatching strategy in this paper is proved to be effective to meet the external dispatching from the power grid while reducing the total fatigue load level of wind farm via optimizing the power instruction to each wind turbine.

Comprehensive frequency regulation scheme for permanent magnet synchronous generator‐based wind turbine generation system

Due to a growing penetration of permanent magnet synchronous generator-based wind turbine generation (PMSG-WTG) systems into the modern power grid, there is a strong interest in leveraging the potential capabilities of PMSG-WTG system to participate in the system frequency regulation during the grid event. In this work, a novel comprehensive frequency regulation (CFR) scheme is proposed specifically for rotor-speed-control-oriented PMSG-WTG systems. The step-wise inertial power response enables PMSG to perform the temporary frequency support. The rotor speed and pitch angle controllers are coordinated to curtail the wind power output for the persistent de-loaded operation, which can facilitate the primary frequency regulation through the variable-slope droop control. Furthermore, this CFR control scheme is implemented into the controls advanced research turbine 2 (CART2)-PMSG model, so as to investigate its potential impact on the structural loads of wind turbine. Simulation results show CFR can dramatically enhance overall frequency regulation capability of the PMSG-WTG's system and mitigate the frequency oscillations over a full range of wind speeds without causing large mechanical damages to the wind turbine.

A multi-objective predictive control strategy for enhancing primary frequency support with wind farms

Nowadays, wind power plants (WPPs) should be able to dynamically change their power output to meet the power demanded by the transmission system operators. When the wind power generation exceeds the power demand, the WPP works in de-loading operation keeping some power reserve to be delivered into the grid to balance the frequency drop. This paper proposes to cast a model predictive control strategy as a multi-objective optimization problem which regulates the power set-points among the turbines in order to track the power demand profile, to maximize the power reserve, as well as to minimize the power losses in the inter-arrays connecting the wind turbines within the wind farm collection grid. The performance of the proposed control approach was evaluated for a wind farm of 12 turbines using a wind farm simulator to model the dynamic behavior of the wake propagation through the wind farm.

Active participation of variable speed wind turbine in inertial and primary frequency regulations

Power systems have increasingly exhibited a need for a potential frequency governor with virtual inertial control (VIC) and primary frequency regulation (PFR) functions in recent years, due to the increased penetration of wind turbines that almost have no frequency response. In this paper, a novel integrated frequency governor applied to a wind turbine is proposed to provide fast active power support and scheduled power allocation for both temporary inertial response and continued PFR. Below rated wind speed, an initial pitch angle is calculated firstly to preset a proper de-loading level for PFR in response to frequency drops, according to the presented relations of CP-λ-β. Under the de-loading operation conditions, the de-loading power tracking curve is replaced by the defined VIC curves, and the inertial response is then obtained by rapidly shifting the VIC curves. Based on analysis of the mechanical characteristics of wind turbines with frequency droop, a primary frequency control strategy facilitated by the regulation of pitch angles is additionally proposed. A preset β/f droop curve is added to the de-loading pitch controller to regulate the mechanical power for scheduled power allocation, and thus satisfying the P/f droop demand of system PFR. Finally, the experiments and simulation results demonstrate that by using the proposed frequency governor a doubly fed induction generator (DFIG) based wind turbine can provide both temporary virtual inertia and continued load sharing to improve the dynamic frequency stability of the power grid with high wind power penetration.

Integrated wind turbine controller with virtual inertia and primary frequency responses for grid dynamic frequency support

An integrated controller of wind turbines with both inertial response and primary frequency regulation (PFR) to provide complete dynamic frequency support for the grid with high wind power penetration is investigated. The wind turbine control governor contains two cross-coupled controllers: pitch controller and maximum power point tracking (MPPT) controller. First, as a precondition for the PFR, a de-loading pitch control scheme is proposed to reserve capacity required for frequency regulation. Then, by optimizing the MPPT scheme, the rapid virtual inertia response is achieved even under de-loading operation condition. Based on the analysis of the steady-state characteristics of wind turbines with frequency droop control, the primary frequency control strategy, which enables the adjustment of frequency droop coefficient, is further proposed through pitch angle changes. Thus, the PFR and inertial response can be both achieved by the proposed de-loading pitch controller and optimized MPPT controller. A three-machine prototype system containing two synchronous generators and a Doubly Fed Induction Generator (DFIG)-based wind turbine with 30% of wind penetration is implemented to validate the proposed integrated control strategies on providing inertial response and subsequent load sharing in the event of frequency change.

Enhancement in Primary Frequency Regulation of Wind Generator using Fuzzy-based Control

This article focuses on fuzzy logic-based primary frequency regulation of doubly fed induction generator type variable-speed wind turbine generators. Wind turbine generators can be operated at a deloading power point with a reduced power level instead of maximum power point. Deloading operation makes the reserve power available, which can be utilized to stabilize the power system for frequency regulation during active power mismatch between generation and demand. The droop control is implemented as supplementary control to improve the short-term frequency dynamics. In this article, the deloading parameter and droop parameter of wind turbine generators are dynamically adjusted by using a fuzzy-based controller that mitigates system frequency transients. The combination of fuzzy-based dynamic deloading and variable droop operation gives better power system performance in terms of frequency excursion and increases the output power extracted from the deloaded wind turbine generator. Hence, proposed the fuzzy-based droop and deloading scheme not only increases overall annual capacity factor of wind farm but also reduces fuel consumption when diesel generators are operated along with wind turbines. The effectiveness of the proposed fuzzy-based control scheme is validated on an IEEE 9-bus test system.

Hybrid Intelligent Control Method to Improve the Frequency Support Capability of Wind Energy Conversion Systems

This paper presents a hybrid intelligent control method that enables frequency support control for permanent magnet synchronous generators (PMSGs) wind turbines. The proposed method for a wind energy conversion system (WECS) is designed to have PMSG modeling and full-scale back-to-back insulated-gate bipolar transistor (IGBT) converters comprising the machine and grid side. The controller of the machine side converter (MSC) and the grid side converter (GSC) are designed to achieve maximum power point tracking (MPPT) based on an improved hill climb searching (IHCS) control algorithm and de-loaded (DL) operation to obtain a power margin. Along with this comprehensive control of maximum power tracking mode based on the IHCS, a method for kinetic energy (KE) discharge control of the supporting primary frequency control scheme with DL operation is developed to regulate the short-term frequency response and maintain reliable operation of the power system. The effectiveness of the hybrid intelligent control method is verified by a numerical simulation in PSCAD/EMTDC. Simulation results show that the proposed approach can improve the frequency regulation capability in the power system.