Raymundo E. Torres-Olguin

Main grid frequency support strategy for VSC-HVDC connected wind farms with variable speed wind turbines

A wind farm (WF) connected by voltage source converter-based HVDC (VSC-HVDC) results in a completely ‘inertia-less’ in offshore AC grid . AC frequency is determined by control of the VSC-HVDC on the WF side, independently of the main grid frequency. Moreover, the rotational speed of a variable speed wind turbine is independent of the AC frequency of the offshore AC grid. These two aforementioned features result in ‘inertia-less’ system, that is absence of mechanical energy storage to accomodate for abrupt changes in generation and load. An ‘inertia-less’ system does not have a load-frequency response similar to traditional AC grid with inertia, hence making it difficult for the wind turbines to contribute to frequency support of the main grid. This paper develops an artificial coupling of the wind farm grid and the main grid frequencies, for main grid frequency support by the wind farm. Artificial frequency coupling is achieved by use of droop controllers for the DC voltage control. A case of study, using PSCAD, demonstrates the effectiveness of artificial frequency coupling for the main grid frequency support.

Offshore Wind Farm Grid Integration by VSC Technology With LCC-Based HVDC Transmission

High-voltage dc (HVDC) transmission based on the line-commutated converter (LCC) is the most established and widespread technology around the world. However, HVDC based on the voltage source converter (VSC) has emerged as the best option to integrate renewable energy sources, e.g., offshore wind farms. This work investigates the feasibility of using a conventional LCC-HVDC transmission in combination with a VSC to integrate offshore wind farms. Such integration results in a hybrid HVDC connection, i.e., the connection of a VSC with an LCC through a dc cable. The operational features of a model of three-terminal hybrid HVDC, two LCC stations and one VSC station, is investigated using PSCAD/EMTDC. The simulations include an aggregate model to emulate the wind farm. The corresponding control strategies are proposed for each terminal and verified under various conditions including wind speed variations and ac faults.

Integration of Offshore Wind Farm Using a Hybrid HVDC Transmission Composed by the PWM Current-Source Converter and Line-Commutated Converter

This paper investigates the feasibility of the application of a hybrid HVDC transmission system for the grid integration of offshore wind farms. The proposed hybrid HVDC consists of a pulse width modulated current source converter (PWM-CSC) and a line-commutated converter (LCC). The PWM-CSC is connected to the offshore wind farm and the LCC connects the onshore grid. The hybrid topology takes advantages from self-commutated converters as well as LCCs. On the one hand, LCC-based HVdc is the most mature technology with the lowest power losses and lowest cost. On the other hand, PWM-CSC has the same features that a voltage source converter for offshore applications, i.e., the ability to operate without an external commutation voltage, reactive power control capability, and a relative small footprint. Moreover, both the PWM-CSC and the LCC are current source converters and hence the coupling can be effortlessly done. The control design for the entire system is presented and verified using numerical simulations. Simulations are performed using PSCAD/EMTDC under different conditions including changes in the wind speed and ac and dc faults.

A model-based controller in rotating reference frame for Hybrid HVDC

In this paper, a model-based controller is proposed for Hybrid HVDC, Hybrid comprising a Voltage Source converter (VSC) using in the sending side and Line-commutated Converter (LCC) using in the receiving side. This type of HVDC may be applied when power is only in one direction for example HVDC for offshore wind farm applications. VSC controls directly the instantaneous active and reactive power using a synchronous reference frame controller while the LCC maintains the DC voltage using a proportional-integral (PI) controller. It also proposed in this paper, when AC grid is weak, to replace the LCC for a capacitor commutate converter (CCC) because of the additional commutation voltage reduces the risk of commutation failure. PSCAD/EMTDC simulations are provided to illustrate the performance of the proposed controller under normal operation and severe disturbance such as three phase-to-ground AC and DC faults.

A controller in d-q synchronous reference frame for hybrid HVDC transmission system

In this paper, a model-based controller is proposed for Hybrid HVDC transmission systems. Hybrid HVDC comprises a Line-commutated Converter (LCC) and a Voltage Source Converter (VSC) which are connected on the same DC link. The VSC regulates the active and reactive power using a model-based controller in a synchronous reference frame while the LCC maintains the DC voltage using a proportional and integral controller. PSCAD/EMTDC simulations illustrates the performance of the Hybrid operation in comparison with CIGRE benchmark HVDC model during normal operation and severe disturbance such as DC fault and AC faults.

Grid integration of large offshore wind energy and oil & gas installations using LCC HVDC transmission system

This paper investigates a case study of grid integration of large offshore wind farm and oil & gas installations using Line-Commutated-Converter (LCC) HVDC transmission system. Additionally, a STATCOM provides commutation voltage and reactive power to the system. The Wind farm supplies power to oil & gas installations through the offshore AC network, and to the onshore grid through the LCC-HVDC transmission link. After the derivation of system mathematical model, the operation principle and control strategies of proposed system are described and validated by a PSCAD/EMTDC based model. The simulation results demonstrated the robust system performance during both the normal operation and the various grid fault conditions.

Faults mitigation control design for grid integration of offshore wind farms and oil & gas installations using VSC HVDC

This paper presents an analysis and new fault mitigation methods of grid integration of offshore wind farms and oil & gas installations using Voltage Source Converter (VSC) HVDC transmission system. On the fault occurrence in the onshore AC grid, the proposed DC link voltage controller initiates fast output power drop of the wind farm, in order to avoid the overvoltage in the DC transmission link. On the faults occurrence in the oil & gas installation, the proposed control employs a voltage-dependent limiter, to limit the maximum amount of active power from the offshore VSC. Simulation results in PSCAD / EMTDC show the satisfied performance of the proposed control strategy for the case of one 300 MW VSC transmission system connecting an offshore wind farm and oil & gas installations to the grid.

Hybrid HVDC connection of large offshore wind farms to the AC grid

The growth of the industry for the integration of offshore wind farms (OWF) has generated great interest in high-voltage DC (HVDC) transmission, especially those based on voltage source converters (VSC). However, line-commutated converters (LCC) are by far the most recognized and widespread HVDC technology. This article investigates the feasibility of a hybrid configuration that combines an LCC with the VSC for the integration of OWF. A hybrid HVDC, with an offshore VSC and onshore LCC, is implemented in PSCAD/EMTDC. The OWF is composed of variable speed wind turbines driving permanent magnet synchronous generators. This paper mainly focuses on the control strategy for overall system. The proposed control strategy is verified through simulation under various conditions, including wind speed variations and onshore faults.

A direct power control for Hybrid HVDC transmission systems

In this paper, a model-based Direct Power Controller (DPC) is proposed for Hybrid HVDC. This Hybrid comprises a Voltage Source converter (VSC) using in the sending side and Line-commutated Converter (LCC) using in the receiving side. This application is potentially attractive for power flow in one direction, for example the integration of offshore wind farms. DPC controller is able to control directly the instantaneous power and reactive power in the VSC side. In addition the DPC includes an adaptation mechanism to cope with uncertain parameters. The LCC maintains the DC voltage using a proportional-integral (PI) controller. PSCAD/EMTDC simulations are provided to illustrate the performance of the proposed controller at start-up, at steady state and the transition when there is change of power reference.

Integration of Offshore Wind Farm Using a Hybrid HVDC Transmission Composed by PWM Current-Source Converter and Line-Commutated Converter

This article investigates the feasibility of using a hybrid HVDC transmission system for the integration of offshore wind farms (OWF). The proposed hybrid HVDC consists of a pulse width modulation current- source converter (PWM-CSC)and a line-commutated converter (LCC). The PWM-CSC is placed at offshore, while LCC is located at onshore. The PWMCSC may offer similar advantages that a voltage source converter (VSC) for the integration of OWF, e.g the PWM-CSC can supply an island or passive grids without a external commutation voltage and the stations have a relative small footprint, among others benefits. Moreover, both the PWM-CSC and the LCC are current-sourced converter which facilitate the coupling. To date, there are not studies that report the behavior of this type of hybrid topology, so this work can provide a basis for further research. The control design for the entire system is presented and verified in this paper. Simulations are performed using PSCAD/EMTDC under various conditions including AC and DC faults.