Induction Generator Based Wind Farm Research Articles

Sub‐synchronous resonance mitigation by a STATCOM in doubly fed induction generator‐based wind farm connected to a series‐compensated transmission network

With the fast development of wind power generation, large-scale wind farms are connected to an electrical network which is series compensated to increase transmission capacity. However, a series-compensated power system may encounter with a sub-synchronous resonance (SSR) problem. Flexible AC transmission systems controllers are widely applied to mitigate SSR and improve the transient stability of the power system. A static synchronous compensator (STATCOM) with an SSR damping controller is proposed to mitigate the potential of SSR in a series-compensated doubly fed induction generator-based wind farm. The local signal is used as the input of the damping controller, which is feasible to be implemented in a practical project. Taking account of different series compensation levels and operating conditions, extensive transient simulations have been carried out using power systems computer aided design/electromagnetic transients including DC (PSCAD/EMTDC) to demonstrate the capability of STATCOM in damping SSR. It is shown that the SSR is successfully alleviated by STATCOM, which also verifies the effectiveness of the proposed STATCOM controller in improving the stability of the transmission system.

Coordinated Control Method for DFIG-Based Wind Farm to Provide Primary Frequency Regulation Service

With the rapid growth of wind power penetration, system operators are required to enforce grid codes that implement frequency regulation of wind farms. This paper presents a distributed cooperation framework for a doubly fed induction generator based wind farm to participate in primary frequency regulation, including imitated inertia and droop characteristics similar to conventional plants. Compared with centralized schemes, the control efficiency is significantly improved. To realize fast distributed coordination and optimal power distribution among wind turbines (WTs), a distributed Newton method is developed, which only requires WTs to exchange limited information with their neighbors over a sparse communication network, and has a super-linear convergence rate. By introducing an index of state of energy, this method can adequately exploit the kinetic energy of all WTs without loss of security. The simulation results indicate that the method exhibits satisfying dynamic performance and reliability, and the system frequency can be stabilized faster when the wind farm controlled by the proposed method participates in frequency control.

Model reduction strategy of doubly-fed induction generator-based wind farms for power system small-signal rotor angle stability analysis

Following the decarbonisation and decentralisation of energy industry, wind energy is becoming a promising generation source to reduce greenhouse emission, and meet future energy demand. Unlike traditional generation using synchronous generators, many wind turbines use induction generators, e.g., doubly-fed induction generators, due to the cost effective design of adjustable-speed operation and flexibility in reactive power control. However, a growing number of doubly-fed induction generator-based wind farms has significantly increased the complexity of system dynamic model, and hence increased the computational burden of power system dynamic study. This becomes a serious concern in the electricity system operation, where a fast power system stability assessment is required to assure the real-time system security during high levels of wind power penetration. In this paper, a novel model reduction strategy of doubly-fed induction generators is derived to improve the efficiency of power system dynamic study, while the study accuracy is still maintained to an acceptable level. To achieve this, a method to assess the modeling adequacy of doubly-fed induction generators for small-signal rotor angle stability analysis is firstly introduced. By evaluating the damping torque contribution to stability margin from different dynamic model components of doubly-fed induction generators, the proposed method provides a quantitative index (i.e., participation level) to show the involvement of each dynamic model component of doubly-fed induction generators in affecting power system damping, and thus can instruct how to reduce the model of doubly-fed induction generators in an efficient and accurate manner. On this basis, five model reduction plans and a model reduction strategy have been proposed according to the previously defined participation levels. The effectiveness of the proposed strategy is demonstrated in the New England test system and a real large power grid in Eastern China respectively. It has been proved that the proposed the model reduction strategy of doubly-fed induction generators for power system dynamic study is undoubtedly useful to the electricity system operator, with a key benefit in reducing model complexity and improving computational efficiency of a large-scale power system with an increasing number of wind power generation.

Optimal Design of Controller Parameters for Improving the Stability of MMC-HVDC for Wind Farm Integration

A subsynchronous oscillation (SSO) phenomenon has been observed in a modular multilevel converter-based high-voltage dc (MMC-HVDC) transmission system for wind farm integration in the real world, which is independent of the type of wind turbine generator. This kind of oscillation appears different from those in doubly fed induction generator-based wind farm with series-compensation line or wind farm integration through two-level voltage-source converter-HVDC transmission system, because the internal dynamics of the MMC may have significant impact on the oscillation. By far, however, very few papers have reported it. In this paper, the generation mechanism of the SSO phenomenon in an MMC-HVDC transmission system for wind farm integration is revealed from an impedance point of view. The harmonic state-space modeling method is applied to model the multifrequency behavior of the MMC, based on which, the ac-side small-signal impedance of the MMC is analytically derived according to harmonic linearization theory. As a general rule, the controller parameters of the wind power inverter and the HVDC converter are designed separately, to meet the performance requirements of the single converter under ideal conditions, but this practice does not guarantee the stability of the interconnected system. Therefore, an optimal design method for controller parameters is proposed in this paper in order to guarantee the small-signal stability of the interconnected system from a system point of view. Finally, time-domain simulations validate the effectiveness of the theoretical analysis and the proposed optimal design method.

Synchrophasor‐enabled power grid restoration with DFIG‐based wind farms and VSC‐HVDC transmission system

An innovative system restoration strategy using doubly-fed induction generator-based wind farms is proposed. The strategy involves retention of charge in the DC bus following a blackout and `Hot-Swapping' between direct flux control mode and conventional grid-connected mode, which does not require resetting of any controller dynamic states and avoids the need for energy storage. An autonomous synchronisation mechanism enabled by remote synchrophasors is also proposed. A blacked-out system, which includes a wind farm and a voltage source converter (VSC)-HVDC connected to a network unaffected by blackout, is used as the study system. Transmission line charging and load pickup is performed using the wind farm in flux control mode while the VSC-HVDC system conducts the same process for another portion of the system. The proposed `Hot-Swapping' and autonomous synchronisation approach is applied to connect the two parts of the grid and switch the wind farm to grid connected mode of operation. The results are demonstrated in a hybrid co-simulation platform where the aforementioned system is modelled in EMT-type software and the rest of the network is represented in a phasor framework.

Security region-based small signal stability analysis of power systems with FSIG based wind farm

Based on the Security Region approach, the impact of fixed-speed induction generator based wind farm on the small signal stability of power systems is analyzed. Firstly, the key factors of wind farm on the small signal stability of power systems are analyzed and the parameter space for small signal stability region is formed. Secondly, the small signal stability region of power systems with wind power is established. Thirdly, the corresponding relation between the boundary of SSSR and the dominant oscillation mode is further studied. Results show that the integration of fixed-speed induction generator based wind farm will cause the low frequency oscillation stability of the power system deteriorate. When the output of wind power is high, the oscillation stability of the power system is mainly concerned with the inter-area oscillation mode caused by the integration of the wind farm. Both the active power output and the capacity of reactive power compensation of the wind farm have a significant influence on the SSSR. To improve the oscillation stability of power systems with wind power, it is suggested to reasonably set the reactive power compensation capacity for the wind farm through SSSR.

Accurate modeling of doubly fed induction generator based wind farms in load flow analysis

This paper presents an iterative approach for incorporating doubly fed induction generator (DFIG) based variable speed wind farms in steady state studies such as load flow analysis. The proposed approach is based on an accurate steady state equivalent circuit of a DFIG. In the proposed approach, the power flow model of a node with DFIG is determined based on the reference to reactive power control loop of rotor side converter. In addition, the proposed approach takes into consideration various limits associated with the DFIG. Case studies on sample 6-bus system and 418-bus equivalent system of Indian southern grid are presented to validate the proposed approach.

Predictive current control of DSTATCOM for VAR compensation of grid connected wind farms

This paper proposes a predictive current control scheme for an improved topology of a distribution static compensator intended for dynamic reactive power compensation of grid connected squirrel cage induction generator based wind farms. The proposed strategy derives the converter switching pulses in each sampling period using a cost function that defines the desired system behavior. Predictive current control algorithm based reactive power compensation tracks the reference current effectively and maintains unity power factor at the point of common coupling under steady state and transient conditions. Further, under transient conditions the proposed scheme provides faster and smoother reactive power compensation compared to the hysteresis controller. In addition, this scheme does not require any modulation technique. Both simulation and experimentation have been carried out to verify and validate the performance of the proposed scheme, and the results are reported in this paper.

Instantaneous power-based feedback linearisation control approach for the doubly fed induction generator-based wind farm

A new adaptive back-stepping mode feedback linearisation (FBL) control strategy for both the rotor-side and the grid-side converter of the doubly fed induction generator (DFIG) has been proposed in this paper. As the control variables (active and reactive powers) are measurable quantities (as they are directly premeditated from the three-phase voltage and current measured at the stator and the GSC terminals, respectively), this subsequently reduces the computational complexity for the proposed control strategy as the phase-lock-loops (PLL) is significantly avoided. The proposed control strategy considerably helps to reduce the steady state error and improves the transient stability of the proposed DFIG model for a number of nastiest scenarios, compared to the conventional controllers, which has been tested and validated both in the MATLAB/Editor (single machine as well as multi-machine environment) as well as the MATLAB/Simulink environment, respectively, which has been well illustrated in the simulation and results section.

Instantaneous power-based feedback linearisation control approach for the doubly fed induction generator-based wind farm

A new adaptive back-stepping mode feedback linearisation (FBL) control strategy for both the rotor-side and the grid-side converter of the doubly fed induction generator (DFIG) has been proposed in this paper. As the control variables (active and reactive powers) are measurable quantities (as they are directly premeditated from the three-phase voltage and current measured at the stator and the GSC terminals, respectively), this subsequently reduces the computational complexity for the proposed control strategy as the phase-lock-loops (PLL) is significantly avoided. The proposed control strategy considerably helps to reduce the steady state error and improves the transient stability of the proposed DFIG model for a number of nastiest scenarios, compared to the conventional controllers, which has been tested and validated both in the MATLAB/Editor (single machine as well as multi-machine environment) as well as the MATLAB/Simulink environment, respectively, which has been well illustrated in the simulation and results section.

Mathematical Modeling of the Squirrel Cage Induction Generator based Wind Farm for Sub-Synchronous Resonance Analysis

Background/Objectives: To analyze the SSR phenomenon in a Squirrel Cage Induction Generator based Wind farm connected to series compensated transmission line at varying operating conditions. Methods/Statistical Analysis: There is a remarkable rise in the renewable energy sector in the current energy demand scenario, but their grid integration leads to various power system stability issues. In the case of wind power, wind farms being located far away from the grid, the integration of large scale wind farms to the utility grid requires high transmission line capacity to transport huge amount of wind power. In order to provide the power transfer capability and to improve the system stability, series compensation is the effective solution. But the series compensation may cause SSR oscillations in the system. Eigen value analysis is the method chosen to perform the Small Signal Stability analysis which examines the potential of SSR in the study system. Findings: The study investigates on the application of series capacitor for series compensation which is required during the grid integration. Series compensation of transmission line causes electro-mechanical oscillations at a sub-synchronous frequency which may finally result in serious damages in the system such as shaft failure. In order to perform the Eigen value analysis, detailed mathematical modeling of the system is developed. The pitch angle control is not taken into account while developing the system, whereas the strength of the transmission line is considered. The interconnection of three sub systems provides the complete model of the study system. The identification of system modes and damping of torsional and network modes are analyzed for various wind speed levels, various series compensation levels and various grid impedance levels. It has been observed that different grid impedance levels have a significant effect on the torsional mode oscillations of the system. Application/Improvements: As the overall system model is linearised, optimal control techniques can be developed as an application to mitigate the SSR oscillations and thereby improve the system stability.

Effect of Different Turbine-generator Shaft Models on the Sub-synchronous Resonance Phenomenon in the Double Cage Induction Generator Based Wind Farm

This paper focuses on the turbine-generator shaft models of double-cage Induction Generator (IG) based wind farm and its influence on the sub-synchronous resonance phenomenon. For this purpose, six different shaft models as 6-mass, 4-mass, 3-mass I, 3-mass II, 2-mass and 1-mass models are considered for double-cage IG wind-turbine. By using the linear modal analysis method, the effects of the different multi mass model of double-cage IG wind-turbine on the SSR phenomenon are studied. The results obtained by eigenvalue analysis show that the model of double-cage IG wind-turbine has an important effect on the detecting of Subsynchronous Resonance (SSR) phenomenon. The analytical results are validated by detailed electromagnetic transient simulation using PSCAD/EMTDC software.

Voltage Collapse Prevention in Doubly Fed Induction Generator Based Wind Farm Connected Power System

In this work, the doubly-fed induction generator (DFIG), rotor side converter (RSC) is controlled to regulate its reactive power output. A brief comparative study has been carried out between two different RSG control techniques, based on their ability to restore voltage stability of a power system. From this study the better DFIG–RSC control strategy has been identified. The results are tested in IEEE-14 bus system and simulated in MATLAB.

Comparison among series compensators for transient stability enhancement of doubly fed induction generator based variable speed wind turbines

This study investigates the effects of series compensators, such as the dynamic voltage restorer (DVR), series dynamic braking resistor, thyristor switched series capacitor, and the high temperature superconducting fault current limiter (SFCL), in enhancing the transient stability of a doubly fed induction machine based variable speed wind generator system. The tested system consists of a 9 MW doubly fed induction generator based wind farm connected to an infinite bus through two step-up transformers and a double run transmission lines. A three-phase-to-ground (3LG) fault and a single-line-to-ground fault (1LG) were applied to one of the lines to demonstrate the transient stability enhancement ability of the series devices. A performance comparison among the four series compensators is made based on responses of active/reactive power, terminal voltage, rotor speed, dc link voltage, and current variations. Simulations were carried out in the MATLAB/Simulink environment. Simulation results show that all series devices can improve the transient stability, however, the SFCL is the most efficient in terms of active power, rotor speed stability and fault current suppression, and the DVR seems to have a better performance than the SFCL in terms of voltage and reactive power.

Stabilization of a Fixed Speed IG Based Wind Farm using Variable Speed PMSG

To optimize the output power and reduce the frequency fluctuation in conventional fixed speed Induction Generator (IG) based wind farm, various controlling devices are used. But, this paper proposes that Permanent Magnet Synchronous Generator (PMSG) can work both as a power producing device and as a controlling device for IG. In this paper a more practical case has been considered where a wind farm is operated with a set of Synchronous Generators (SG). Thermal, Hydro and Nuclear Governors have been used for SGs. Wind farm has a capacity of 20% of the total SG’s capacity. Both two-level and three-level converter-inverter based PMSG have been considered. Comparative results between IG and IG with PMSG are shown. Simulation results prove the effectiveness of the proposed system. Simulations have been carried out by using the laboratory standard power system software package, PSCAD/EMTDC.

Centralised solution for subsynchronous control interaction of doubly fed induction generators using voltage‐sourced converter

Practical subsynchronous instability events have been observed in doubly fed induction generator-based wind farms, which are radially connected to fixed series compensated transmission lines. As a special type of subsynchronous resonance, this phenomenon is described here as subsynchronous control interaction (SSCI). This study proposes a centralised SSCI mitigation scheme named as subsynchronous damper (SSD). In this scheme, an additional high-power electronic voltage-sourced converter is added to the collection bus of wind farms. It is designed to dissipate the electrical resonance power of SSCI. Thus, SSD provides positive damping for SSCI. Its basic principle is explained with equivalent electric circuit and the sufficient conditions for stabilisation are obtained mathematically. Controller design guidelines are given and have been verified first by eigenvalue analysis. Then a method to choose the capacity of power electronic converter is proposed. The effectiveness of SSD is finally demonstrated by extensive time-domain simulations on a target system.

Subsynchronous Resonance Mitigation for Series-Compensated DFIG-Based Wind Farm by Using Two-Degree-of-Freedom Control Strategy

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.

Power System Stability Control for a Wind Farm Based on Adaptive Dynamic Programming

In this paper, a goal representation heuristic dynamic programming (GrHDP) based controller is developed for the doubly-fed induction generator based wind farm to improve the system transient stability under fault conditions. The proposed controller is based on adaptive dynamic programming (ADP) techniques to approximate the optimal control policy according to the interaction between the controller and the power plant. Compared to existing ADP approaches with one action network and one critic network, our GrHDP architecture introduces an additional network, i.e., the reference network, to form an internal goal/reward representation. This better mapping between the system state and the control action significantly improves the control performance. The effectiveness of the proposed approach is validated via two cases. The first case investigates a revised four-machine two-area system with high wind penetration and a static synchronous compensator. The second case is a practical size power system with wind farm in Liaoning Province in China. Detailed simulation analysis and comparative studies with traditional ADP approaches are presented to demonstrate the superior performance of our method.

Improving stability of utility-tied wind generators using dynamic voltage restorer

The generation of electricity using wind power is significantly increasing and has received considerable attention in recent years. One important problem with the induction generator based wind farms is that they are vulnerable to voltage disturbances and short circuit faults. Any such disturbance may cause wind farm outages. Since wind power contribution is in considerable percentage, such outages may lead to power system stability issues and also violate the grid code requirements. Thus, improving the reliability of wind farms is essential to maintain the stability of the system. The proposed strategy is to use Dynamic Voltage Restorer (DVR), which is one of the promising devices to compensate the voltage disturbance and to improve the stability of the system. It provides the wind generator with the fault ride through capability and improves the reliability of the system. Extensive simulation results are included to illustrate the operation of DVR and fault compensation.

Dynamic Voltage Restorer for Fault Ride through of Doubly Fed Induction Generator — A Solution for a Case Study at 110/11KV Substation in India

The wind power penetration has been increasing significantly year by year. Most of the Wind Turbine Generators (WTGs) are designed with induction generators. The induction generator based wind farms have poor fault ride through capability and any system disturbance may affect the wind power generation. The disturbances are very common, if the WTGs are connected to weak grids. One such problem of frequent tripping has been encountered at Anthiyur substation in India. Subsequently, a power quality study has been undertaken in a 110/11KV substation at Anthiyur for examining the causes for tripping. From the study, it is found that frequent grid voltage disturbances have caused the tripping. Since the contribution of wind power is in predominant percentage, maintaining its stability becomes an important issue. A simulation based solution is proposed to improve the reliability of Doubly Fed Induction Generator (DFIG) based wind farms. In the proposed strategy, Dynamic Voltage Restorer (DVR) is used to compensate the grid voltage disturbances. The DVR can compensate the faulty line voltage and enhance fault ride through capability of DFIG wind farms for a stable operation as demanded in actual grid codes. Especially, vector control technique is utilizedhere to control the DVR. The proposed control scheme is very effective to protect the DFIG and DVR from disturbances and faults.