The integrated control strategy for primary frequency control of DFIGs based on virtual inertia and pitch control

This paper proposes a variable coefficient combined virtual inertia and primary frequency control strategy for doubly fed induction generators (DFIG) in coordination with diesel generator to participate in wind/photovoltaic/diesel microgrid frequency regulation. The frequency response characteristic is analyzed under different wind speeds with corresponding inertia control parameters different. A 10% wind power margin is preserved through overspeed control and pitch angle control to offset the decrease of wind output power after temporary extra power surge, and provide a permanent frequency support for microgrids. The influence of droop control gain setting is also illustrated under different wind velocities. By continuously adjusting the control parameters according to wind speed variation, a variable coefficient method is realized. The method can guarantee an efficient implementation of this strategy in time-varying conditions. Finally, this coordinated control strategy is tested in a storage-independent microgrid with solar, wind, and diesel generators.

The mechanical parts of doubly fed induction generators (DFIGs) is decoupled from electrical parts, which makes the output active power hard to respond the frequency change of power grid, it becomes a new challenge to frequency stability of power grid with large scale wind power grid integration. A coordinative control strategy of virtual inertia and primary frequency of DFIGs based wind farms is given, and its principle is analyzed in the paper. The primary frequency control is a slow process, which is mainly implemented by control level of DFIGs based wind farms, and the DFIGs is cooperative. The virtual inertia control is a fast process, which is only implemented by control level of DFIGs. The theoretical analysis and time domain simulation show that the proposed control strategy can guarantee the frequency stability of power grid with large scale wind power integration.

Despite the affordability and popularity of doubly fed induction generator (DFIG) among the variable-speed wind turbines, it cannot follow the inertia response caused by the load perturbations and the imposed frequency fluctuations in the power system. Considering the growing participation of wind power generation, DFIG with no virtual inertia control cannot play a functional role in the frequency stability of traditional power systems. At the outset, a new inertia control strategy is proposed for DFIG to participate in system frequency control via absorption or disposal of kinetic energy based on active power control. Then after that, interline power flow controller (IPFC) known as an adaptable and complex compensator is introduced to simultaneously regulate and control the power flow of multiple lines during the large penetration of DFIGs. To enhance the damping capability of IPFC, this paper has suggested a novel optimal adaptive neuro-fuzzy (OANF). The frequency and tie-line power deviations as two prominent stability benchmarks have been considered and analysed to appraise the damping capability of the suggested controller in the affected interconnected power system. The dynamic stability problem has been formulated based on multi-objective grey wolf optimizer to optimally tune OANF-based IPFC towards simultaneous suppression of the abovementioned benchmarks. The accurate fuzzy membership functions and rules of OANF-based IPFC have been extracted during the severe perturbation in two interconnected reconstructed power systems. Eventually, the simulation results extracted from both the three-area and five-area interconnected power systems have primarily validated the inertia control-based DFIG and subsidiarily OANF–IPFC to effectively suppress the low-frequency oscillations.

the main challenge in associate islanded Micro grid (MG) is the frequency stability due to the inherent low-inertia feature of distributed energy resources. That is why, energy storage devices, are utilized in MGs as the promising sources for grid short-term frequency regulation. Though energy storage devices, improve the dynamic response of the load frequency control system, these devices increase system costs. Moreover, the modification or uncertainty of the system parameters will significantly degrade the performance of the conventional load-frequency control system. This article proposes the implementation of rotating-mass-based virtual inertia in Double-Fed Induction Generator (DFIG) to support the primary frequency control associated an adaptive Neuro-Fuzzy Inference System (ANFIS) controller, as the secondary frequency control. The simulation results illustrate that the suggested control scheme ameliorate the dynamic response and performance of the load frequency control system and also the studied islanded MG remains stable, despite severe load variation and parametric uncertainties.

This paper investigates virtual inertia control of doubly fed induction generator (DFIG)‐based wind turbines to provide dynamic frequency support in the event of sudden power change. The relationships among DFIGs' virtual inertia, rotor speed and network frequency variation are analysed, and a novel virtual inertia control strategy is proposed. The proposed control strategy shifts the maximum power point tracking (MPPT) curve to the virtual inertia control curves according to the frequency deviation so as to release the ‘hidden’ kinetic energy and provide dynamic frequency support to the grid. The calculation of the virtual inertia and its control curves are also presented. Compared with a PD regulator‐based inertial controller, the proposed virtual inertia control scheme not only provides fast inertial response in the event of sudden power change but also achieves a smoother recovery to the MPPT operation. A four‐machine system with 30% of wind penetration is simulated to validate the proposed control strategy. Simulation results show that DFIG‐based wind farms can provide rapid response to the frequency deviation using the proposed control strategy. Therefore, the dynamic frequency response of the power grid with high wind power penetration can be significantly improved. Copyright © 2015 John Wiley & Sons, Ltd.

As the mechanical parts of doubly fed induction generator (DFIG) are decoupled from electrical part, the total equivalent rotary inertia of power system are decreased with growing continuously of wind power penetration, which will bring pressure to frequency stability of system. On the basis of analysing operation and control characteristics of DFIG, the equivalent virtual inertia time constant of DFIG, wind farms based on DFIG, and connected power system was calculated. A virtual inertia control strategy of DFIG using rotor current direct control based on equivalent virtual inertia time constant was proposed. The proposed virtual inertia control strategy of DFIG includes traditional control function module, status assessment module, and additional virtual inertia control module. The time domain simulation was carried out on the basis of theoretical analysis, the simulation results show that the proposed virtual inertia control strategy of DFIG can provide efficacious frequency support to the power grid at all kinds of operating conditions, and the frequency stability of power system with large‐scale wind power was improved.

Traditionally the electricity generation is based on rotating synchronous machines which provide inertia to the power system. The increasing share of converter connected energy sources reduces the available rotational inertia in the power system leading to faster frequency dynamics, which may cause more critical frequency excursions. Both, virtual inertia and fast primary control could serve as a solution to improve frequency stability, however, their respective impacts on the system have different consequences, so that the trade-off is not straightforward. This study presents a comparative analysis of virtual inertia and a fast primary control algorithms with respect to rate of change of frequency (ROCOF), frequency nadir and steady state value considering the effect of the dead time which is carried out by a sensitivity analysis. The investigation shows that the virtual inertia controller is effective in reducing the ROCOF compared to fast primary control. However, it is worsening frequency nadir and steady state value. Moreover, the sensitivity analysis shows the very limited effect of the two controllers on the voltage magnitude.

The recent power cut incident in the UK on 9th August 2019 indicated that frequency control to raise frequency nadir and eliminate frequency second dip is highly desirable for power grids with high penetration of wind energy. This paper proposes a fast frequency support scheme for wind turbine systems (WTSs) that can enable frequency nadir to be significantly raised and close to the settling frequency and eliminate frequency second dip. In the proposed frequency support scheme, in order to achieve similar frequency support performance and ensure stability of WTSs under varying wind speeds, different levels of wind power penetration and system conditions, an adaptive gain, which is a function of real-time rotor speed and wind power penetration level, is proposed. In the proposed scheme, rotor speeds of WTSs are proposed not to be recovered to the optimal operating points during the primary frequency control, but recovered during the secondary frequency control. Simulation results on the IEEE two-area power system with a doubly fed induction generator (DFIG)-based wind farm and the IEEE 39-bus power system with permanent magnetic synchronous generator (PMSG)-based wind farms using real-time digital simulator (RTDS) and Dymola are presented to verify the effectiveness of the proposed scheme.

Doubly-fed induction generator (DFIG) based wind turbines with traditional maximum power point tracking (MPPT) control provide no inertia response under system frequency events. Recently, the DFIG wind turbines have been equipped with virtual inertia controller (VIC) for supporting power system frequency stability. However, the conventional VICs with fixed gain have negative effects on inter-area oscillations of regional networks. To cope with this drawback, this paper proposes a novel adaptive VIC to improve both the inter-area oscillations and frequency stability. In the proposed scheme, the gain of VIC is dynamically adjusted using fuzzy logic. The effectiveness and control performance of the adaptive fuzzy VIC is evaluated under different frequency events such as loss of generation, short circuit disturbance with load shedding. The simulation studies are performed on a generic two-area network integrated with a DFIG wind farm and the comparative results are presented between three cases: DFIG without VIC, DFIG with fixed gain VIC, and DFIG with adaptive fuzzy VIC. All the results confirm the proposed fuzzy VIC can improve both the inter-area oscillations and frequency stability.

Doubly fed induction generator (DFIG) wind turbines with virtual inertia control are coupled to power system in dynamic characteristics, and the control input of virtual inertia control is directly affected by the tracking ability of phase-locked loop (PLL). Thus, it is urgent to study the impact of DFIG wind turbines with virtual inertia control on power system small-signal stability considering the effects of PLL. First, based on DFIG operation characteristic and control strategy, a small-signal model of interconnected system with DFIG integration considering PLL and virtual inertial control is established. Second, the attenuation time constants of DFIG state variables are calculated, and according to the attenuation speeds of different state variables and the coupling between them, it is found out that PLL and virtual inertia are the main factors that affect the coupling between DFIG and synchronous generators. And then, considering that both PLL and virtual inertia control will affect the oscillation modes of synchronous generators, analytical method is used to reveal system small-signal stability under the joint effects of the two factors quantitatively. Analysis results show that, for DFIG wind turbines with virtual inertial control, PLL affects system damping mainly by affecting the participation of virtual inertia in the system. The smaller the PI parameters of PLL are, the smaller the participation factor of virtual inertia control state variables in the interarea oscillation mode is, and the bigger the electromechanical oscillation mode damping ratio is. Simulation results verify the reasonableness of the established model and the possibility that virtual inertia control may cause system small-signal stability to deteriorate in multimachine system.

This paper proposes a scheme for online identifying the inertia and primary frequency control of wind farms with virtual inertia control. The power generation deviation and frequency deviation are defined as the model’s input and output, respectively. With this time-domain input and output, as well as some knowledge of the system structure, the inertia and the primary frequency control can be identified. In order to validate the proposed scheme, both simulation data and field PMU data are applied as the measurement data. In both cases, the identification scheme achieves high accuracy and robustness.

This paper proposes novel multi-energy inertia support for simultaneous frequency and voltage control of an isolated hybrid power system (IHPS). Multi-energy storage (gas inertia – hydrogen storage, thermal inertia – solar thermal storage, hydro inertia – gravity hydro storage, chemical inertia – battery energy storage) supported by demand side management (DSM) for simultaneous voltage and frequency regulation and backed by biodiesel generators, are the essential elements of IHPS. A novel control strategy of concurrent virtual droop control, virtual damping control, virtual inertia control, and virtual negative inertia control is proposed to utilise multiple inertia sources and to improve LFC and AVR performance effectively. The effective coordination of inertia sources in eradicating oscillations in IHPS, is aided by a developed cascaded proportional integral-tilt-integral-sliding mode (PI-TISMC) controller. The performance of PI-TISMC is compared with PID, PI-PID, and PI-SMC controllers. A maiden attempt has been done by training five diverse classes of optimization techniques to optimize the parameters of controllers in the present work. The results are evaluated in MATLAB and it is evident from the results that the performance of frequency control is improved by 6.5%, 7.8% and 3.4 s (over shoot, undershoot, and settling time). The performance of frequency control is improved by 6.5%, 7.8% and 3.4 s (over shoot, undershoot, and settling time). Similarly, the performance of voltage control is improved by 6.7%, 4.8% and 2.3 s (over shoot, undershoot, and settling time) by employing developed PI-TISMC controller and proposed concurrent inertia control. The combination exhibits superior performance in minimizing oscillations in IHPS due to variations in loading and solar insolation.

The increasing penetration of large-scale wind generation in power systems will challenge the power system inertia due to the reason that the converter based variable speed wind turbines have no contribution to the system inertia. Traditionally, a doubly fed induction generator (DFIG)-based wind power plant naturally does not provide frequency response because of the decoupling between the output power and the frequency. Moreover, DFIGs also lack power reserve margin because of the maxi mum power point tracking (MPPT) operation. In this paper, a virtual inertial control strategy of the DFIG based wind turbines called supplementary control loop for inertial response is investigated. When the system frequency is changing severely, the out put power of DFIG should respond to it rapidly through the virtual inertial controller at the same time. The rotor speeds of wind turbines can also be adjusted into this procedure. The inertial control methods proposed in this paper can supply controllable virtual inertia of DFIGs to the power system so that the system frequency stability can be strengthened through inertial control of wind turbines on the basis of damping torque analysis.

The clean energy sources are the future of the power generation to fulfill the increasing electricity demand. The Renewable Energy Sources (RESs) are uncertain in nature; hence it is very challenging to integrate the RES in to the existing power system. These converter-based generators are not equipped with frequency control system. The conventional generators have automatic generation control for the primary and secondary frequency control purpose. The stored kinetic energy (KE) in the rotor of the generator provides inertial response at the disturbances. The RES already operating on the maximum power mode hence there is no accommodation of the active power for the primary and secondary frequency control. The rotating parts are absent in the inverter dominated system hence there is no room to provide inertial response. While considering this issue of absence of frequency control in converter-based system. There is need to approach for the frequency control ancillary services in RES integrated system to maintain the power system stability. This paper is basically focusing on the inertial response in addition with primary frequency control in RES integrated system. An inertial response control emulates the inertial power virtually through the energy stored in the DC-link. It improves frequency response and reduces the frequency nadir. For primary frequency control the external energy storage system is used to provide the additional active power to maintain the frequency at nominal value. To demonstrate this the simulation is done in MATLAB 2019a. The analysis of simulation results shows that, the frequency control in PV generation enhances the frequency stability.

The increasing penetration level of converter-based renewable energy sources in the grid causes low inertia, which leads to grid frequency variation. To deal with this problem, virtual inertia (VI) control with particle swarm optimisation (PSO)-based PID controller has been applied to a two-area power system with multi-generating units. This improves the frequency variation and response time. In this paper, a derivative control technique has been used to emulate the VI, which controls the active power of the energy storage device which provides the inertia support to the power system. The integral time absolute error (ITAE) has been used as the performance index for PSO. Furthermore, the primary frequency control is augmented by integrating electric vehicles as vehicle-to-grid resources. Finally, various cases of disturbance and a comparative analysis have been performed between the VI with PSO-PID control and conventional control (PI). The simulation has been performed using MATLAB/Simulink, which demonstrates the superior performance of the VI with PSO-PID over a conventional controller by improving the settling by 45.84% in Area 1 and 39.77% in Area 2 for the almost same overshoot of frequency deviation. These simulation results have also been validated by performing hardware-in-loop (HIL) experiments utilising the OPAL-RT real-time simulator.