VSC-HVDC Transmission Link Research Articles

A control strategy for an offshore wind farm with the generating units connected in series with a VSC-HVDC transmission link

This paper proposes a control strategy for an offshore wind farm with the generating units connected in series with a DC transmission link. This configuration presents the advantage of not requiring an offshore substation with a power transformer to increase the power plant terminal voltage since the generators are connected in series through the DC terminals of the interface rectifiers. On the other side, the energy transferred by the subsea DC cable is processed by a modular multilevel inverter before it is injected into the mains. The modular multilevel inverter regulates the current through the DC link cable instead of the voltage of the DC system. This choice allows all the generating units to transfer maximum power through the DC link cable without the need of any communication channel to exchange information between the offshore units and the onshore substation. The system controllers are designed to assure the stable operation of the proposed strategy. Digital simulations results, obtained with PSCAD/EMTDC, are used to validate the effectiveness of the designed controllers and to demonstrate the performance of the system for different operational conditions.

Hybrid state estimator considering SCADA and synchronized phasor measurements in VSC-HVDC transmission links

This paper proposes a practical approach for implementing a hybrid state estimator by considering SCADA and PMU measurements in VSC-HVDC transmission links under a unified framework of reference. A proposed formulation for the PMU measurements associated with the VSC-HVDC state variables is derived from the first principles and is implemented into a weighted least square-based state estimation algorithm. In order to avoid numerical problems of convergence of the proposed state estimation approach, the set of current phasor measurements is represented in rectangular coordinates, such that their corresponding variances are also recalculated according to the uncertainty propagation theory. The state estimation process uses the measurement of one voltage phase angle as the global reference for the rest of voltage phase angles estimated at network buses and VSC-HVDC's voltage source converters. The proposed formulation improves the accuracy at which both network and VSC-HVDC state variables are simultaneously estimated based on the measurements provided by a SCADA system and PMUs. A Mexican interconnected 190-bus equivalent system is used as the test system in order to validate the effectiveness of the proposed hybrid state estimator.

Design of Takagi-Sugeno fuzzy controller for VSC-HVDC parallel AC transmission system using differential evolution algorithm

This paper presents an optimised Takagi-Sugeno (TS) fuzzy controller using differential evolution (DE) technique for a VSC-HVDC transmission link with a parallel AC line, through the analysis on voltage source converter (VSC) equation in d-q reference frame. The state space model for a VSC-HVDC transmission link is formulated. The controller is designed to provide high dynamic performance and complete decouple control of active and reactive power. The DE technique is used to optimise the rule consequent parameters of a TS fuzzy controller. The effectiveness and performance of the proposed controller is demonstrated though digital simulation studies on a VSC-HVDC power system, using MATLAB/SIMULINK software package. The simulation results and the comparison with conventional PI and simple fuzzy logic controller show that proposed controller contributes significantly towards improving the transient characteristics of the VSC-HVDC system under a wide range of operating conditions under severe disturbances.

Design of a Novel Fuzzy PI Controller Based on Artificial Bee Colony Algorithm for VSC-HVDC Transmission Link

This paper presents a continuous time state space model for a VSC-HVDC transmission link in the d-q reference frame. An Artificial Bee Colony (ABC) algorithm based Fuzzy proportional plus integral (FPI) controller is proposed for a decoupled back to back VSC-HVDC transmission link single machine power system. Here, the ABC approach is used for selecting the nonlinear variable gain parameters of scaling factors of Fuzzy controller to enhance its performance. Simulation result has been presented to show and validate the efficiency of new approach for damping oscillations over a wide range of operating conditions and compared with the conventional PI controller tuned by Ziggler-Nichol technique, and Fuzzy PI controller without optimization. It has been found that the proposed controller is quite effective in enhancing the power system dynamic stability significantly.

Non-linear control and stabilisation of VSC-HVDC transmission system based on Type-2 fuzzy sliding mode control

This paper presents a robust non-linear hybrid controller based on Type-2 fuzzy and sliding mode control strategy for VSC-HVDC transmission link, which inherits the benefits of these two methods. The interval Type-2 fuzzy (IT2FSMC) is used to handle the uncertainties to enhance the real time performance. The sliding mode control combines with the fuzzy system to compensate the influence of un-modelled dynamics and chattering phenomena on the system response, and also to improve the robustness of the reaching phase of the sliding mode control (SMC). The effectiveness of the proposed controller is demonstrated through digital simulation studies using MATLAB/SIMULINK software package. The simulation results show that the proposed controller contribute significantly towards improving the damping behaviour of the VSC-HVDC system with regard to parameter variations and external load disturbances compared to conventional PI controller under a wide range of operating conditions.

Analytical efficiency evaluation of two and three level VSC-HVDC transmission links

An analytical method to calculate the efficiency of VSC-HVDC links based on two-level and three-level VSCs is presented. The method uses the average and root mean square of the VSC converter current to estimate the conversion losses in the converters (conduction and switching losses). The remaining power losses (DC cable transmission losses due to I2R, the losses in the coupling transformer, and AC harmonic filter losses) are evaluated using conventional well-known methods. Results obtained analytically are confirmed by digital simulations.

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.