Modelling and comparison analysis of grid‐connected DFIG‐based wind farm in weak grid

As the mainstream generation devices in the current wind power market, doubly fed induction generators (DFIG) based wind farms in China are always located in the northwest, northeast and north China and other remote areas where DFIG systems have to be connected with the weak grid. In that case, the resonance may occur in the system on account of the interaction between AC and DC system and lead to harmonic amplification. The subsynchronous resonance (SSR) lower than the fundamental frequency and the high-frequency resonance (HFR) are the two main resonance modes in the DFIG grid-connected system. However, researches on HFR characteristics of DFIG connected to grids have been drawn attention. This paper will fill this gap. The DFIG system model under the weak power grid with different kinds of compensations is set up firstly. Then, the grid impedance and the DFIG numbers are changed separately to discuss the influence of them on harmonic characteristics by Bode diagrams. Afterward, a detailed model for DFIG system under weak grid is developed in MATLAB/Simulink. The harmonic characteristics of the system are analyzed by fast Fourier transform (FFT). The study shows that HFR is more serious when the DFIG system is integrated into the parallel compensated weak grid than connected to the series compensated grid. HFR is more possibly to happen with the growth of the system capacity or the DFIG numbers.

This paper formulates the relation between the Phase-Locked Loop (PLL) dynamics of a grid-connected distributed source and the impedance of the grid. Conventionally the design of the PLL for a grid following inverter is performed independently of the grid impedance. With the advent of micro-grids, these design procedures require modification. Microgrids are characterized by higher grid impedances which severely impact the PLL dynamics. The grid voltage in a microgrid is not a stiff variable and thus cannot be assumed to be independent of its interaction with a distributed source. The PLL dynamics in these cases affect the voltage at the point of common coupling (PCC) and vice-versa. In this paper, the relation between the grid impedance and the PLL parameters is analytically derived using dynamic phasors. A distributed resource connected to the grid via a 3-phase inverter is considered for the analysis. Grid synchronization is achieved using a synchronous reference frame (SRF) PLL. It is shown that under weak grid conditions, the PLL can get de-synchronized if the magnitude of grid impedance or the current levels are higher than the thresholds derived in this paper. Microgrids with higher grid impedance are highly susceptible to such situations. The design of SRF-PLL for grid-connected inverters thus needs to include an estimate of the maximum system ratings and the grid impedance.

The contribution of wind power in conventional generation is increasing rapidly now-a-days. Hence a better understanding of wind power plants is required. But the wind power plants generally containing induction generators exhibit a rather different behavior than the conventional power plants which use synchronous generators. The purpose of this paper is to analyze and study transient stability of a wind farm based on conventional fixed speed Induction generator when it has been integrated with a weak grid. A comparative analysis of the transient stability improvement of a FSIG wind plant has been done, when it is supported with FACTS like S.V.C and STATCOM of equal ratings. Single case of transient condition i.e. a three phase fault has been studied by simulation. With the increase in demand of power and decrease of fossil fuels, mankind has been forced to search alternative sources for the generation of electricity. Wind power in spite of being stochastic in nature has proved itself as a viable solution to this problem. As the wind turbine technology is developing at a good pace, more and more wind power plants are being integrated with the conventional form of generation. With the increase in the ratio of wind generation to conventional generation, several problems related with integration of wind farms have emerged. These problems are due to distinct properties of the generators used with the conventional form (Thermal & Hydro) of generation and wind based generation (1)-(4). In thermal and hydro power based generation synchronous generators are used while in wind based generation mostly induction generators are used. Induction and synchronous generators do not exhibit similar behavior during power system stresses (transients). Moreover, increase in wind power based generation requires a reliable transmission system for power evacuation to the load centers. As the wind farms are mainly concentrated in far flung and rural areas their interconnection with the grid becomes a problem. This problem is mainly due to the existence weak transmission grids in the rural areas (1)-(4). Earlier when the share of wind power generation as compared with the conventional generation was low, during transient conditions; whenever the power system reached on the verge of instability the wind farm was disconnected from the grid and was reconnected when transients passed away. But as the share of wind power in the conventional generation increased, the practice of disconnecting the wind farms from the rest of the grid led to the drop in frequency of the power system. So now-a- days even if a fault occurs in the power system somewhere near the wind farms, they still remain connected with the grid for a critical time period. This phenomenon is called fault ride through (FRT) in wind farms (5). This paper has studied a transient condition (fault) in a grid connected fixed speed induction generator based wind farm and its fault ride through performance. A comparison of the performance of fact devices like static var compensators (svc) and static synchronous compensators (statcom) in improving the transient stability of wind farms during fault is also done.

During weak power grid, the interaction between the doubly-fed induction generator (DFIG)-based wind turbines and the power grid impedance can easily induce system instability issue, and thus threaten the safety of the system. The linear active disturbance rejection control (LADRC) is a potential way in improving the damping of the DFIG system, due to its excellent anti-disturbance ability. In this paper, the small-signal impedance model of the DFIG with LADRC is established. Based on the proposed model, the interaction between the DFIG and the weak grid is analysed in detail. Electromagnetic simulations are then performed to validate the correctness of the theoretical analysis. It shows that compared with the traditional proportional integral control with the same control bandwidth, the LADRC can obviously improve the phase margin of the DFIG system, and thus enhance the small signal stability of the DFIG system.

When the doubly-fed induction generator (DFIG) is acting as a generator, for its limit of the power electronic converter rating, making the DFIG can only work at the speed of the range of the synchronous speed plus or minus 30% [1]. So, when wind speed is low, the DFIG cannot work as a generator. This paper adopts a combinatorial topology which is wide-speed-range DFIG (WDFIG). If the wind speed is low, the stator-winding of DFIG is short circuited [2]· [3], and the stator flux of DFIG is controlled by rotor side converter (RSC) also generating to grid through rotor side converter. If the wind speed is high, the stator-winding of DFIG which is short circuited become connecting to the power grid directly, and its architecture is same as the traditional architecture. In this paper, DFIG simplified mathematical model is first displayed and the method of stator-flux-oriented vector control (SFOVC) is adopted, then this paper displayed the simplified mathematical model of the two topology in d-q synchronous rotating reference frame, on that basis, deducing the control equation. Meanwhile, in order to guarantee the uninterruptible operating of the system, and avoid the lash such as flux, current, voltage and so on when the structure of the stator-winding of DFIG change from short circuited mode to grid connected mode. This paper gives theoretical analysis for the suitable switching moment, that ensure the variation range of flux, current, voltage during the switch moment are within bounds. At last, the simulation results proved that the proposed control architecture could ensure the DFIG work as a generator no matter wind speed is low or high.

Subsynchronous oscillations have occurred in wind farms due to the high penetration of converter-based technology in power systems and may potentially lead to grid instability. As an effective solution, synchronous condensers, with their ability to control voltage and inject reactive power in the power system, are increasingly being adopted, as they can lead to the mitigation of such oscillations in weak grid conditions. However, the impact of synchronous condensers’ power ratings on system stability is a topic that requires further investigation. In fact, an improper selection of a synchronous condenser’s rating will not extinguish existing subsynchronous oscillations and may even cause the emergence of new oscillatory phenomena. This paper presents a novel examination of the impact that the synchronous condenser’s power rating has on the small-signal stability of a wind farm with existing subsynchronous oscillations while being connected to a weak grid. The wind farm’s model is developed using state-space modeling, centering on grid interconnection and incorporating the state-space submodel of a synchronous condenser to show its impact on subsynchronous oscillation mitigation. The stability analysis determines the optimal synchronous condenser’s power ratings for suppressing these oscillations in the wind farm model. The findings are corroborated through time domain simulations and fast-Fourier transformation (FFT) analysis, which further validate the stability effects of a synchronous condenser’s rating.

This paper investigates the sub-synchronous resonance (SSR) phenomenon in a series compensated doubly-fed induction generator (DFIG) based wind farms. A detailed linear state space model of a fixed-series compensated DFIG wind farm is presented for different operating point conditions. The model of the system includes a wind turbine aerodynamics, a 6 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> order induction generator, a 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> order two-mass shaft system, a 4 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> order series compensated transmission line, an 8 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> order rotor and generator side converter controllers, and 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> order DC link model. The 22 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> order system is modeled in Matlab/Simulink, and modal analysis is performed on the modeled DFIG wind farm, and eigenvalues of the system are calculated. The eigenvalue analysis and time-domain simulation results show that SSR can potentially occur in the fixed-series compensated DFIG-based wind farm even at realistic levels of series compensation. The fixed-series capacitor is replaced with a gate-controlled series capacitor (GCSC), which is a series flexible ac transmission system (FACTS) device composed of a pair of gate-commutated switches in parallel with a capacitor that enables one to control impedance and power flow of transmission lines, for series compensation and SSR damping. The root-locus analysis is employed to design a SSR damping controller (SSRDC) for the GCSC. It is shown than a well-designed SSRDC for the GCSC can alleviate the SSR in DFIG-based wind farms. The IEEE first benchmark model adapted with a DFIG-based wind farm is employed as a case study.

Voltage disturbance compensation based on impedance modeling of DFIG under weak grid

Fractional frequency transmission system (FFTS) is a relatively new transmission technology that can be used to deliver the energy from remote offshore wind farms, and it can increase the transmission capacity through the reduced transmission frequency. However, the dynamic performance of a FFTS with the wind farm may be different from that of a traditional ac transmission with the wind farm. In this paper, a doubly fed induction generator (DFIG)-based wind farm is connected to the main grid via the FFTS. The detailed dynamic model of the FFTS with wind farm is established first, and then the eigenvalue analysis is carried out to evaluate the system dynamic damping performance. To verify the results of eigenvalue analysis, dynamic simulations are carried out on a single machine infinite bus and a multimachine system. Both eigenvalue analysis and simulations demonstrate that the FFTS will have a negative influence on the damping of the DFIG-based wind farm in comparison to that of the wind farm with the traditional ac transmission system. However, with the control of cycloconverter, this problem can be easily overcome, and the damping performance of wind farm with FFTS can be even better.

In recent years, the doubly fed induction generator (DFIG) operates in a weak grid, rather than a strong grid due to the high proportion of wind energy into the power grid. The impedance interaction between the DFIG system and series and parallel compensated weak grid might cause the subsynchronous resonance (SSR) and high frequency resonance (HFR) in the DFIG system, respectively. Phase locked loop (PLL) is a popular grid synchronization technique, and the high bandwidth PLL can cause resonance at middle frequencies in the DFIG system. However, the impact of PLL types and their controller dynamics on the resonance in the DFIG system are not adequately researched. The impact of the PLL controller with different types, such as synchronous reference frame (SRF) and Lead/Lag PLL, is studied in this paper to fill this gap. Additionally, an improved PLL is proposed, which can guarantee the high phase margin and decrease the likelihood of the resonance at middle frequencies in the DFIG system under a weak grid. Moreover, the phase margin of the DFIG system impedance with an improved PLL is less sensitive to its controller parameters. Simulation and experimental results verify the effectiveness of the proposed method.

The wind farm based on Doubly Fed Induction Generator (DFIG) is built. Not only it can be as active power source to provide active power to network, but also as reactive power source to stabilize voltage or supply Var compensation (reactive power absorbed or generated). Therefore, Var coordinated control strategy is proposed, instead of expensive equipments, such as Static Var Compensator(SVC) or Static Synchronous Compensator (STATCOM), in order to provide sufficient reactive power to maintain system stability. DFIG comprehensive model is built based on correlation models in Dig SILENT/Power Factory. The eigen value analysis method is employed to verify its impact on low-frequency oscillation modes and small signal stability of power system integrated with wind farm. The results from eigen value analysis are elucidated the effectiveness of Var coordinated control in DFIG system.

The sub-synchronous oscillation (SSO) characteristics and suppression strategies of a hybrid system comprising doubly fed induction generator (DFIG)-based wind turbines and synchronous pumped storage units connected to the power grid via series-compensated transmission lines are analyzed. A modular modeling approach is used to construct a detailed system model, including the wind turbine shaft system, DFIG, converter control system, synchronous machine, excitation system, power system stabilizer (PSS), and series-compensated transmission lines. Eigenvalue calculation-based small-signal stability analysis is conducted to identify the dominant oscillation modes. Suppression measures are also developed using relative participation analysis, and simulations are carried out to validate the accuracy of the model and analysis method. The analysis results indicate that the SSO phenomenon is primarily influenced by the electrical state variables of the DFIG system, while the impact of the state variables of the synchronous machine is relatively minor. When the level of series compensation in the system increases, SSO is significantly exacerbated. To address this issue, a sub-synchronous damping controller (SSDC) is incorporated on the rotor side of the DFIG. The results demonstrate that this method effectively mitigates the SSO and significantly enhances the system’s robustness against disturbances. Furthermore, a simplified modeling approach is proposed based on relative participation analysis. This method neglects the dynamic characteristics of the synchronous machine while considering its impact on the steady-state impedance and initial conditions of the model. These findings provide theoretical guidance and practical insights for addressing and mitigating SSO issues in hybrid renewable energy systems composed of DFIGs and synchronous machines.

This paper investigates a special class of dynamic power system problem, namely subsynchronous resonance (SSR) resulted from a series-compensated network connecting doubly-fed induction generator (DFIG) based wind farms. A novel two-degree-of-freedom (2DOF) control strategy combined with a damping control loop is designed and analyzed for enhancing the system stability and alleviates the SSR that may arise due to the induction generator effect (IGE). The proposed control strategy is tested at different operating conditions of series compensation levels and low wind speeds to ensure the system stability. The doubly-fed induction generator based wind farms without the proposed control strategy leads to overall system instability during high series compensation and low wind speeds. Hence, the mitigation of the SSR and damping enhancement are critical to the entire power system stability. A reliable way of analyzing the system and designing effective control strategies against SSR based on the eigenvalue analysis and impedance based stability criterion is deployed. Moreover, analytical explanations have been elaborated to verify the procedure of the controller design. Fault ride-though capability has also been investigated with the proposed control strategy that is flexible to be integrated with the FRT schemes so as to assist the wind farm in mitigating the SSR during the fault recovery stage. Finally, time domain simulations are carried out to demonstrate the effectiveness of the proposed control strategy for mitigating the SSR and damping power system oscillations.

This paper investigates a new type of subsynchronous oscillation (SSO) event observed in a real-world doubly fed induction generator (DFIG)-based wind power plant (WPP) connected to a multiterminal dc (MTDC) grid in Northern China. The system under study is unique and, to date, is the world's largest four-terminal DC transmission network, which connects far-away wind and photovoltaic power plants to regional load centers. For the first time, this paper utilized field-measured data to analyze the characteristics and reveal the underlying mechanism of the SSO in doubly fed induction generators (DFIGs) connected to HVDC grids. The analysis is carried out using the impedance network model established from the impedances obtained from field recorded data. The investigations on the field data concluded that the SSO is essentially caused by the subsynchronous interaction between the DFIGs and the MTDC grid without the presence of a physical series/shunt compensation or weak AC grid. Instead, it is found that the SSO is triggered due to the converter-grid resonance where the MTDC grid presented itself as a virtual capacitor and the DFIGs showed negative resistance at the subsynchronous frequency range.

Sub-synchronous oscillation (SSO) phenomenon of doubly fed induction generators (DFIG) connected to series compensated transmission lines is a threat to the safety and stability of the power system. Efficient method for analyzing complicated power system is in need. In this paper, sub-synchronous oscillation characteristics and influencing factors of complicated power grid with DFIG and series compensation capacitance are analyzed firstly by frequency domain impedance method, based on two simplified equivalent systems. Thus, a practical oscillation risk indicator is proposed, which comprehensively reflects the influence of series-compensation percentage and short circuit ratio. Finally, the proposed risk indicator is verified by simulation and test of a typical project.