Fully-rated Converter Research Articles

High-voltage ride-through strategy for wind turbine with fully-rated converter based on current operating range

During grid high-voltage ride-through (HVRT), a wind turbine (WT) with fully-rated converter (FRC) would experience an overvoltage at the DC link, which can threaten the safety of the DC link capacitors and the connected power modules. This paper investigates the process of the DC link voltage rise. Two concepts, current operating range (COR) and performance factor, are introduced to help propose a grid-side converter control strategy, in which the DC link overvoltage is greatly released or eliminated through injection of quantitative reactive current derived from the current margin of fully-rated converter and the grid voltage. The injected levels of reactive current into the grid are determined under different grid overvoltage conditions with reduced magnitude and occurrence of the DC link overvoltage. Thereby, the high-voltage ride-through capability of the fully-rated converter wind turbine is significantly improved. The detailed verification is obtained through simulations using DIgSILENT PowerFactory.

Sensitivity analysis of transient short circuit current response to the penetration level of non-synchronous generation

The increasing penetration of inverter connected non-synchronous generation (NSG) in future power systems may result in a large displacement of synchronous generators. Hence, the dynamic characteristics of the power systems may change in ways that have not been experienced yet. In particular, short circuit (SC) current response in future power systems might be significantly affected due to the increased penetration of such generation. In turn, this can significantly affect both the operation of protective relays and the relay settings. This is due to different characteristics of SC currents injected to the grid from NSG, compared to the conventional synchronous generation. This paper focuses on investigating of transient SC current response in future power systems with high penetration of NSG. Hence, to address this, in this paper we investigate the impact of a wide range of factors that influence the SC current contribution from NSG based on fully-rated converter technology. Our analysis also considers the impact of different fault ride through (FRT) controls, designed according to different grid codes, on the SC current response of NSG. Then, the sensitivity of the transient SC current to the increased penetration of NSG is monitored and analyzed. Firstly, simulation studies are conducted using a single-machine infinite bus test system where the SC response of a NSG is comprehensively studied. Secondly, our findings are also tested and verified using IEEE 9-bus test system at different penetration scenarios of NSG using DIgSILENT PowerFactory.

Novel Control Approach for a Hybrid Grid-Forming HVDC Offshore Transmission System

This article describes a hybrid topology of high-voltage direct current (HVDC) for offshore wind farms using a series connection of a voltage source converter (VSC) and six-pulse diode rectifier (6P-DR). In this topology, the offshore side VSC (OF-VSC) acts as a grid-forming converter to maintain the PCC (point of common coupling) voltage of offshore wind farms (WF) and frequency. In addition, the OF-VSC functions as an active power filter to suppress the 5th, 7th, 11th, and 13th order harmonic current components produced by the 6P-DR, making it almost sinusoidal. Due to the 6P-DR being used in the hybrid converter, this new configuration reduces the total cost of the converters and losses, while preserving the power flow to the onshore gird. Compared to the fully-rated converter and hybrid converter based on a 12-pulse diode rectifier, the power loss and cost are reduced, and in addition, the proposed hybrid converter does not require a phase shift transformer nor a high number of diodes. A 200 MW in an HVDC transmission system using the hybrid configuration was simulated in PSCAD. The results show that the system operated correctly and the harmonic components were filtered.

Analysis, Modeling and Control of a Hybrid Drive Wind Turbine With Hydrogen Energy Storage System

Hybrid drive wind power generation system, equipped with the speed regulating differential mechanism (SRDM), is able to be friendly connected to power grid without the need of partly- or fully-rated converters. The novel transmission schemes can promote not only the output power quality but also the low voltage ride through (LVRT) capability of the existing wind turbines (WTs). For the purpose of further improving the grid-connected operating performances of SRDM-based WT, this paper aims to develop a hybrid power production unit, in which the hydrogen storage system (HSS), comprising an electrolyzer, a hydrogen fuel cell and a supercapacitor, is integrated into SRDM-based WT. The basic architecture and numerical modelling methods of key subsystems in SRDM-based WT as well as in HSS are analyzed. After determining the eight different operating modes, a power supervision approach is synthesized for the proposed SRDM-based WT with HSS, by which the power flow management between energy sources and storage elements can be realized. Case studies are carried out in the presence of different randomly varying wind speeds and grid voltage faults. The satisfactory operating performances of the proposed wind-hydrogen hybrid system in terms of maximizing wind energy utilization, suppressing output power fluctuation and improving system continuous operating stability are verified.

Impedance-based analysis of harmonic instabilities in HVDC-connected Offshore Wind Power Plants

HVDC-connected Offshore Wind Power Plants (OWPPs) with a large number of power converters are prone to instabilities due to interactions with converter controllers. This paper analyzes harmonic instabilities in OWPPs connected through HVDC due to interactions with the offshore HVDC converter and the wind turbine converters. An impedance-based model is used to represent a detailed OWPP with fully-rated converter based wind turbines, including the offshore HVDC converter, wind turbine converters, submarine cables and transformers of the offshore AC grid. Harmonic stability is evaluated both from the grid connection point of the offshore HVDC converter and the WT grid side converters. As a result, the influence that parameters of converter control systems and the offshore grid configuration have on harmonic resonant frequencies and stability conditions is analyzed in depth through a sensitivity analysis. Active damping implemented as a feed-forward control is coordinated in all converters to mitigate the harmonic instabilities. Examples of harmonic instabilities and active damping application are validated with time domain simulations in PSCAD/EMTDC.

Unsynchronized Phasor-Based Protection Method for Single Line-to-Ground Faults in an Ungrounded Offshore Wind Farm with Fully-Rated Converters-Based Wind Turbines

This paper proposes a protection method for single line-to-ground (SLG) faults in an ungrounded offshore wind farm with fully-rated converter-based wind turbines. The proposed method uses the unsynchronized current phasors measured by unit protections installed at the connection point of the fully-rated converter (FRC)-based wind turbines (WTs). Each unit protection collects the unsynchronized current phasors from two adjacent nodes and synchronizes them by aligning the positive-sequence current to the same phase angle. The faulted section is identified by comparing the phase angles of the synchronized zero-sequence currents from adjacent nodes. Simulations of an ungrounded offshore wind farm with relay models were carried out using power system computer-aided design (PSCAD)/ electromagnetic transients including direct current (EMTDC).

Steady-state assessments of PMSGs in wind generating units

The number of fully-rated converter wind turbine generators equipped with a Permanent Magnet Synchronous Generator (PMSG) has sensibly increased in the late years. The optimal utilization and controllability of a PMSG is achieved using a Pulse Width Modulation – Voltage Source Converter (PWM-VSC) that allows interfacing the electric machine with the distribution network by means of a DC link. The generator/Machine-Side Converter (MSC) controls the operation of the PMSG. This paper proposes a methodology to assess the feasibility of the steady-state working points of PMSG based wind turbines without running a set of time-consuming time-domain simulations. Three control objectives for MSC are usually considered: stator voltage control, unitary power factor control and torque control. The aim of the present paper is that of providing an analytical methodology to check the feasibility of the steady-state working points dictated by different control strategies of the MSC starting from the specific operational constraints provided by the wind power plants characteristics. The effectiveness of the proposed methodology is demonstrated comparing numerical results of a MATLAB® implementation against dedicated time-domain simulations. Simulations results demonstrated the appropriate performance of the proposed methodology characterized by the almost negligible computational time required.

Long-Term Voltage Stability Analysis of Variable Speed Wind Generators

This paper presents the impacts caused by the integration of variable speed wind turbines on long-term voltage stability. The technologies used are fully rated converter (FRC) and doubly fed induction generator (DFIG) with two control strategies: grid-side converter (GSC) at unity power factor, which is usually adopted, and GSC controlling reactive power. Also, this paper considers wind turbines capability curves and its variable limits, since they are subject to several limitations that changes with the operating point and wind speed. This study also considers the dynamic models of over excitation limiter (OEL) and on-load tap changers (OLTC) combined with static and dynamic loads using time domain simulations. Different penetration levels of wind generation are analyzed. The results show that long-term voltage stability can be improved when GSC of DFIG is controlling reactive power. Moreover, the capability curve plays an important role in this analysis since reactive power is a key requirement to maintain voltage stability.

Voltage Ride-Through Capability Verification of Wind Turbines With Fully-Rated Converters Using Reachability Analysis

Wind turbines are required to meet grid interconnection codes for voltage ride-through, which mandate that they should not disconnect during a specified range of grid disturbances. One of the tasks of a turbine's design verification process is to ensure that it will be able to withstand any such event. This paper sets forth a powerful computational technique for design verification of Type-4 permanent-magnet synchronous generator-based turbines with fully-rated grid-side converters. The proposed method can provide assurance as to whether the currents and voltages will remain within acceptable bounds. To this end, reachability theory is employed to find sets that enclose all possible trajectories for the system states caused by a given set of unknown-but-bounded disturbances. The computation of the reachable sets is carried out by using on-the-fly linearization of the nonlinear system dynamics, zonotopes, and interval analysis.

General study of the control principles and dynamic fault behaviour of variable-speed wind turbine and wind farm generic models

The interest towards generic models or sometimes also called standard models of wind turbine generators (WTGs) is significantly increasing. Mainly due to their improved power quality, better controllability and higher power extraction capability, variable-speed wind turbines driving a synchronous or an induction machine are capturing the global market. Throughout this paper, dynamic modelling and performance analysis of the generic models of the variable-speed WTGs, namely the doubly-fed induction generator and the fully-rated converter based WTGs, are achieved using integration between Matlab/Simulink and PSCAD/EMTDC simulation platforms. Later on, the performance of type-4 wind turbine driving a permanent magnet synchronous machine is analysed during fault and then compared with the case when driving a wound rotor induction machine. The differences in control principles and dynamic fault behaviour are highlighted. Afterwards, investigations on wind farm level are accomplished. A case study during which the developed generic models and the generic model of the variable-speed machine are compared is conducted. Different arrangements for the construction of the generic wind farm are considered.

Minimum DC link voltages for the generator bridge converter of a SCIG based variable speed wind turbine with fully rated converters

Squirrel Cage Induction Generator (SCIG) based variable speed wind turbine with Fully Rated Converters (FRC) is a popular choice in the industry for the modern multi mega-watt wind turbines. Typical FRC system uses a fixed DC link voltage that allows operation in all steady state and dynamic operating conditions while allowing the modulation index of the PWM scheme to vary. However, the analysis made in this paper shows that at steady state, in the maximum power point tracking region where the turbine is operated at variable speeds with generator controlled using Rotor Flux Oriented Control (R-FOC), it is possible to operate the Generator Bridge (GB) converter with significantly lower DC link voltages than the fixed value used, by maintaining maximum modulation index in the PWM scheme. This paper presents a methodology of determining the minimum DC link voltages for such a system supported by simulation results showing the successful operation of a GB converter with minimum DC link voltages in the maximum power point tracking region.

3 MW class offshore wind turbine development

This paper introduces the design and development of a 3 MW class offshore wind turbine (WinDS3000). The design has been carried out by considering high reliability, availability, maintainability and serviceability (RAMS) for the wind class TC Ia. An integrated drive train design, which has an innovative three stage gearbox, has been introduced to minimize nacelle weight of the wind turbine and to enhance a high reliability for transmission. A permanent magnet generator (PMG) with fully-rated converter has been introduced for its better efficiency in partial-load operation, thus it is grid-friendly and can adapt itself to either 50 Hz or 60 Hz grid connection. A pitch-regulated variable speed power control with individual pitch system has been adopted to regulate rotor torque while generator reaction torque can be adjusted almost instantaneously by the associated power electronics. It is expected that the blade and system load can be reduced by using individual pitch control. The wind turbine also has been equipped with condition monitoring and diagnostic systems in order to achieve maintainability requirements.