Akshaya Moharana

Input-Output Linearization and Robust Sliding-Mode Controller for the VSC-HVDC Transmission Link

This paper presents a robust nonlinear controller for VSC-HVDC transmission link using input-output linearization and sliding mode-control strategy. The feedback linearization is used to cancel nonlinearities and the sliding mode control offers invariant stability to modeling uncertainties due to converter parameter changes, changes in system frequency, and exogenous inputs. Comprehensive computer simulations are carried out to verify the proposed control scheme under several system disturbances, such as changes in short-circuit ratio, converter parametric changes, and faults on the converter and inverter buses. Based upon the time-domain simulations in the MATLAB/SIMULINK environment, the proposed controller is tested.

SSR in Double-Cage Induction Generator-Based Wind Farm Connected to Series-Compensated Transmission Line

Series compensation of transmission networks is being increasingly considered for integration of large wind farms to transfer bulk power. This paper presents the potential of subsynchronous resonance in large wind farms based on double-cage induction generators connected to a series-compensated line. A detailed linear state space model is presented for the wind plant and the transmission system. Eigenvalues are determined for a wide range of series compensation levels for various sizes of wind farms as well as a wind farm producing different power outputs. The potential of SSR involving both induction generator effect and torsional interaction is investigated in-depth. Eigenvalue analysis results obtained through Matlab simulations are validated by detailed electromagnetic transient simulation using PSCAD/EMTDC software. Studies are conducted for different commercially available double-cage induction generator-based wind turbines. SSR analysis in a realistic wind farm with identical wind turbine generators subjected to different wind speeds, and different sizes of wind turbine generators subjected to same wind speed are also carried out. It is shown that induction generator effect-based SSR can potentially occur for wind farms even at realistic levels of series compensation.

SSR Alleviation by STATCOM in Induction-Generator-Based Wind Farm Connected to Series Compensated Line

In this paper, a static synchronous compensator (STATCOM) with a voltage controller is proposed to mitigate the potential of sub synchronous resonance (SSR) in a series compensated induction-generator (IG)-based wind farm. Detailed eigenvalue analysis is performed to demonstrate that IG effect SSR is successfully alleviated by STATCOM. The results are validated through electromagnetic transient simulation with PSCAD/EMTDC. The impacts of symmetrical fault at different locations and collector cables are investigated, and the effectiveness of the proposed STATCOM controller is illustrated. It is shown that a three-phase fault close to the wind farm may cause severe oscillations in the point of common coupling (PCC) voltage, electromagnetic torque, and shaft torque of the wind turbine generator. To examine this situation, an equivalent circuit analysis is presented, which predicts the band of resonant speeds within which the wind turbine becomes unstable. The study is extended to other commercially available IGs, which also show the potential for SSR even at realistic levels of series compensation levels, and the capability of the proposed STATCOM controller to obviate its occurrence.

SSR mitigation in wind farm connected to series compensated transmission line using STATCOM

Series compensation is being increasingly considered for increasing transmission capacity for integration of large wind farms. This paper presents the potential of subsynchronous resonance (SSR) in large squirrel cage induction generator based wind farms connected to the series compensated transmission line. In order to suppress the unstable SSR oscillations, a STATCOM with suitable controller is proposed at the terminal of the wind farm. A detailed linear state space model is developed for the wind farm, transmission system which is a modified IEEE First SSR Benchmark system and the STATCOM. Eigenvalues are calculated for a wide range of wind farm power outputs and series compensation levels. The potential of SSR involving both induction generator effect and torsional interaction is investigated in-depth. The eigenvalues obtained from the linear state space model are validated through the electromagnetic transient simulation carried out with PSCAD/ EMTDC software. It is shown that SSR may potentially occur for large wind farms at higher level of series compensation which can be mitigated by the STATCOM with an appropriate controller.

Non-linear Control of Parallel AC Voltage Source Converter High-Voltage DC System Using a Hybrid Input-output Linearization Evolutionary Fuzzy Controller

Abstract This article develops a continuous-time state-space model for a parallel AC voltage source converter high-voltage DC system in the d-q reference frame. A non-linear input-output linearization fuzzy proportional-integral type controller has been designed using just two rules with a view to implement in real time for the high-voltage DC light system based on two insulated-gate bipolar transistor based converters and a pulse width modulation firing scheme. The input-output linearization transforms the non-linear voltage source converter high-voltage DC converter system into one that is linear for obtaining the control inputs. However, the uncertainty due to speed and load fluctuations, along with converter parameter variations, limits the efficacy of the input-output linearization controller for enhancing the stability of the parallel AC voltage source converter high-voltage DC system. Thus, an analytical fuzzy controller for the converter-inverter system via feedback linearization with input-output decoupling is proposed for improved damping performance. The analytical fuzzy controller is then optimized using a particle swarm optimization technique to provide superior damping performance over a wide range of operating conditions.

Subsynchronous Impact of Series Compensation on Induction Generator Based Wind Farm

Abstract Series compensation of transmission networks is being increasingly considered for integration of large wind farms to transfer bulk power. This article presents an extensive analysis of the potential of subsynchronous resonance in wind farms connected to a series-compensated line. The impact of symmetrical faults at different locations in the network is studied in detail. An equivalent circuit model is presented to calculate the resonant speeds of the aggregated induction generator for varying power outputs and series compensation levels. This study is validated by electromagnetic transient simulations using PSCAD/EMTDC software. Two sets of results are presented: one for wind farms of different megawatt ratings, and the other for a wind farm with different megawatt outputs. Scenarios are described for a wind farm connected to a line with a realistic level of series compensation, when a symmetrical fault at the wind plant terminal may result in high shaft torques. Recurrence of such faults can potentially lead to generator shaft failure due to cyclic fatigue. To mitigate subsynchronous resonance, a static synchronous compensator (STATCOM) is proposed at the terminals of the wind farm. The STATCOM not only stabilizes the wind farm but also reduces the peak shaft torque, thereby obviating the potential of shaft damage due to subsynchronous resonance.

Modal Analysis of Induction Generator Based Wind Farm Connected to Series-compensated Transmission Line and Line Commutated Converter High-Voltage DC Transmission Line

Abstract This article presents a comprehensive analysis of the potential of sub-synchronous resonance in induction generator based wind farms connected to a line commutated converter based high-voltage DC transmission line. Since series-compensated lines are also known to cause sub-synchronous resonance in induction generator based (Type 1) wind farms, this study also considers a series-compensated line in parallel with the high-voltage DC line for the investigation of sub-synchronous resonance potential. The CIGRE benchmark high-voltage DC system and the IEEE first benchmark system are considered as the study system components. A linearized state-space model of the study system is developed for eigenvalue analysis followed by participation factor analysis. A sensitivity study of the sub-synchronous resonance modes with respect to variation in different model parameters, such as rectifier firing angle, DC line power flow, and series compensation level, is also reported. It is found that the high-voltage DC rectifier station current regulator does not interact with any of the sub-synchronous modes of the wind farm. However, in the event that the high-voltage DC line is blocked during a contingency, the wind farm will operate radially with the series-compensated line and may experience sub-synchronous resonance oscillation due to the induction generator effect.

Modal analysis of Type-1 wind farm connected to series compensated transmission line and LCC HVDC transmission line

This papers presents a comprehensive analysis of the potential of sub-synchronous resonance (SSR) in Type-1 wind farms connected to Line Commutated Converter (LCC) based HVDC transmission line. Since series compensated lines are also known to cause induction generator effect SSR in Type 1 wind turbine generators, this paper also considers a series compensated line in parallel with the HVDC line for the investigation of potential SSR. CIGRE Benchmark HVDC system and IEEE First Benchmark system are considered as the study system components. A dynamic model of the study system is developed and linearized at several operating points for eigenvalue analysis of the potential of SSR. A sensitivity study of the SSR modes with respective to variation in different model parameter such as rectifier firing angle, DC line power flow, series compensation level is also reported. It is found that HVDC rectifier station current regulator does not interact with any sub-synchronous modes of the wind turbine generator. Hence, the operation of a Type-1 wind farm connected to series compensated line and HVDC line is may not be subject to SSR issues. However, when the wind farm operates radially with the series compensated line, it may experience SSR oscillation due to induction generator effect.

Instability in induction generator based wind farms connected to series compensated transmission lines

This paper presents a study of subsynchronous mode instability in Induction Generator (IG) based wind farm connected to a series compensated transmission line. The IEEE First SSR benchmark system is modified by replacing the synchronous generator with an aggregated wind farm. A small signal stability analysis is carried out to detect the instability analytically and then results are validated through a detailed electromagnetic transient simulation using PSCAD/EMTDC. Two types of studies are performed for the small signal analysis. First, a linearized state space model is developed for the eigenvalue calculation. Then, a simple equivalent circuit model is developed for the resonant speed analysis. From the equivalent circuit model, two resonant speeds - upper resonant speed (URS) and lower resonant speed (LRS), are obtained. If the post fault speed of the aggregated generator comes close or enters in the band within these two resonant speeds, the induction generator may become unstable. The performance of the aggregated wind farm model is validated with a distributed wind farm model. It is shown that the wind generator may experience SSR if the fault occurs at the terminals of the wind farm even at a realistic level of series compensation.

Fuzzy supervised PI controller for VSC HVDC system connected to Induction Generator based wind farm

This paper proposes a fuzzy supervised PI controller for the Voltage Source Converter (VSC) HVDC system connected to an Induction Generator based wind farm, in parallel with an AC transmission line. It is shown that the proposed controller performs effectively by adapting the gains based on fuzzy supervision. It stabilizes the network faster than a conventional PI controller and the peak overshoot is also reduced significantly. A nonlinear full-scale model is developed in MATLAB, which is linearized to obtain a state space model. Eigenvalues and participation factors are calculated from the state space model for small signal stability studies. Singular Value Decomposition (SVD) theory is also applied to test the controllability of the inputs with respect to specific oscillatory modes. For the fuzzy supervised PI controllers, a rule base is generated from several system simulations and then the proposed controller is implemented through the Fuzzy Inference System (FIS) in MATLAB. The aggregated wind farm model is validated through PSCAD/EMTDC simulation.