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The actively commutated current source converter (CSC) does not need large energy storage capacitors and can realize the black start of wind farms, which provides a practicable scheme for the lightweight offshore wind power DC transmission system. However, resulted from utilizing pulse width modulation, the existing CSC has the disadvantages of high DC harmonics and high switching loss. To solve this problem, an offshore wind power transmission system based on the current source converter with fundamental frequency modulation employed (FFM-CSC) is investigated. Firstly, the topology and mathematical model are analyzed. Then, a steady-state control strategy coordinated by the offshore CSC and the onshore CSC is put forward, and the power operating range is researched. A black start control strategy for wind farms is also proposed. Finally, the offshore wind power transmission system is tested by simulation in PSCAD/EMTDC. The results demonstrate that the FFM-CSC-based offshore wind power transmission system can smoothly realize the black start, stably control the frequency and amplitude of AC voltage, adapt to output changes of wind turbines, and realize the fault ride-through. [ABSTRACT FROM AUTHOR]

This article proposes a comprehensive coordinated suppression strategy for DC overvoltage to achieve the receiving-end fault ride-through of DC grid connected large-scale wind power. For the monopolar DC overvoltage, by reasonably switching the control mode of bipolar modular multilevel converter (MMC), the power transfer capability of the healthy pole MMC can be improved while maintaining DC voltage stability, and the precise load-shedding control of wind farm and its coordination scheme with dissipative resistors are designed, which can effectively reduce the input capacity of dissipative resistors and the risk of wind turbines being cut off. For the bipolar DC overvoltage, the load-shedding control strategy and its parameter selection principles are proposed, which can make wind farm and capacitors in each MMC absorb excess power together, to reduce the change rate and amplitude of DC voltage. Then its cooperation scheme with the dissipative resistors is designed to achieve fault ride-through in the full power range of wind farm. Finally, based on the simulation model of four- terminal DC grid, the effectiveness of the proposed coordinated control methods is verified. [ABSTRACT FROM AUTHOR]

Offshore wind farms (OWF) are being rapidly integrated to the power systems, which can be primarily attributed to saturation of the mainland onshore area, and high wind speed in offshore sites. However, to maintain stable grid operation, OWFs are required to comply with Low Voltage Ride Through (LVRT) capability. OWF during LVRT period should not only stay grid connected, but also support the grid during the disturbance. In this paper, a new communication-independent LVRT control scheme for HVDC connected OWF has been proposed. Under the proposed strategy, OWFs not only successfully ride through a fault induced voltage dip, but also comply with active power recovery ramp rate during the fault recovery period. Besides complying with LVRT requirement and ramp rate limit, the proposed strategy improves voltage dip induced frequency (VDIF) issue by increasing active power injection from the OWF during LVRT period. The proposed LVRT strategy also exploits the stored energy in HVDC link capacitors in order to reduce the impact of VDIF issue. The proposed and the conventional LVRT strategies for HVDC connected OWF have been implemented and evaluated on IEEE 39 bus benchmark system. [ABSTRACT FROM AUTHOR]

Sub/Super-synchronous oscillations have occurred in wind farms consisting of doubly-fed induction generators (DFIGs) or permanent magnet synchronous generators (PMSGs). This paper investigates the impact of synchronous condenser (SC) on sub/super-synchronous oscillations in wind farms with different types of generator and compares it with STATCOM. Impedance model is adopted in this paper to explain the mechanism of oscillations in wind farms and its mitigation using SC. Compared to STATCOM, SC is more effective in oscillation mitigations of the two types of wind farms. Eigenvalue analysis and time domain simulations in PSCAD/EMTDC are used to check the influence on sub/super-synchronous oscillations from different parameters of SC, such as its capacity, leakage reactance and inertia constant. The results indicate that SC would improve the system stability while a larger capacity will induce a greater improvement. A smaller leakage reactance of SC also benefits the mitigation of oscillations while its inertia constant can hardly influence system stability. [ABSTRACT FROM AUTHOR]

Difficulties in identification of the fault property, for a fault on a single outgoing line of windfarm, arise due to rapid decrease in the electromagnetic energy of the transmission line after the tripping of three-phase breaker. An active solution to such problem is proposed based on low current injection from parallel connected static synchronous compensator (STATCOM). In the reclose sequence after wind turbines’ cut off, the circuit breaker near wind farm and grid-connected circuit breaker of STATCOM are reclosed first. Therefore, based on selected switching operation of power electronic devices of STATCOM converter, a short duration discharge of dc capacitance to the faulty ac line is suggested. Correlation coefficient of resulting current waveform is utilized as an identification criterion for fault presence. Finally, a three-phase adaptive auto-reclosing scheme is proposed for the single outgoing line of 110 kV wind farm. The effectiveness and robustness of the proposed scheme are verified by PSCAD/EMTDC simulation. [ABSTRACT FROM AUTHOR]

The offshore ac side impedance model of the modular multilevel converter (MMC) based high voltage direct current (HVdc) system is essential for analyzing the interaction stability between MMC-HVdc and the offshore wind power plants. This paper develops the offshore ac side impedance model of an MMC-HVdc system for wind power integration taking the effects of offshore station, dc cable and onshore station into consideration. The dc impedance model of the onshore station is first derived which includes the effects of dual closed loop dc bus voltage control and circulating current control. Then, the dc impedance of the onshore station and dc cable impedance are used to derive the ac side impedance of the offshore station under open loop control. In addition, the influence of the dual closed loop based ac bus voltage and frequency (VF) control on the offshore ac impedance is analytically derived. As a result, the proposed model can be used to reveal the coupling between offshore offshore ac system and dc system as well as to investigate the influence of the dc system and VF controller in the offshore ac impedance of the MMC-HVdc system. Furthermore, the proposed model overcomes the deficiencies in harmonic resonance analysis of MMC-HVdc based offshore wind power integration system. The results of the simulation in PSCAD/EMTDC validate the proposed models and analyses. [ABSTRACT FROM AUTHOR]

This paper presents a methodology for the forward estimation of thermal risk in offshore wind farm cables under scenarios where the farm is overplanted for economic purposes. A dynamic thermal rating assessment of the cable is proposed to estimate the thermal effects of probable load current scenarios considering actual temperatures. Calculated future temperatures are used to estimate the probability of the cable being overheated. Due to the ability of the method to estimate forward thermal risk, unnecessary power curtailment can be reduced while avoiding thermal damage to the cable. Simulated results of online thermal risk estimation and curtailment show additional power delivery of 7.26%, 9.16%, and 9.67% per year for a 6%, 9.9%, and 13.7% wind farm overplanting, respectively. The additional power is calculated compared to the annual power delivered (Wh/year) with the use of the traditional continuous rating limits for the case studied. [ABSTRACT FROM AUTHOR]

A novel solution to integrating offshore wind power via the fractional frequency transmission system (FFTS) is introduced in this paper. The basic idea of FFTS is to improve the technical and economic performance of ac transmission systems by lowering the grid frequency. FFTS is especially suitable for transmitting offshore wind power because the low frequency dramatically reduces the charging current in the cable. In addition, as a novel ac transmission system, FFTS performs much better than HVDC in constructing a multiterminal (MT) grid. The basic structures and characteristics of a typical PMSG-based FFTS offshore wind power system and a MT-FFTS offshore grid are described first. Then two offshore wind power cases are used to compare the technical and economical pros and cons of the FFTS solution with the conventional solutions: HVAC and HVDC. The feasibility studies show that the performance of FFTS offshore wind power system is superior to that of HVAC and HVDC in both point-to-point (PP) and multiterminal cases. The uniform annual value of PP-FFTS and MT-FFTS is 5.01% and 5.07% cheaper than PP-HVDC and MT-HVDC. Therefore, FFTS is a promising solution to offshore wind power integration and building offshore grids. [ABSTRACT FROM PUBLISHER]

The penetration of wind power into the electricity grid is growing significantly. A significant portion of this wind power is being installed in distribution systems, of which most are passively operated. Under this operating practice, wind power can only be admitted based on minimum load and maximum generation consideration. This severely limits the wind power hosting capacity of the system. Hence, the use of active-management strategies (AMSs) has been proposed to increase the hosting capacity of distribution systems. This paper incorporates AMSs into two optimization models whose objectives are to maximize the net benefit of distribution system operator and wind farm owner, respectively. The AMSs considered are wind energy curtailment, coordinated on-load tap changer voltage control, and reactive power compensation. The models development is based on a typical medium-voltage distribution system in Sweden although it can easily be adapted to other cases. The application of the model to a distribution system in Sweden shows an increase in hosting capacity of the distribution system by 78% with mere 2.6% curtailed energy. That is, the hosting capacity of the distribution system has almost been doubled by using AMSs. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to propose and analyze the high-performance control strategies for a novel current source converter (CSC)-based wind turbine-superconducting magnetic energy storage system (SMES) hybrid system, which will, in turn, improve the operating performance for the wind farm not only in the grid-connected operation, but also in the islanding mode. The key is to design the effective active power controller and the ac bus voltage controller for the CSC-based hybrid system in grid-connected operation. Thus, better power smoothing and robust operation can be achieved. For the islanding mode operation, the capacitor voltage control scheme is proposed for the CSC hybrid system to offer efficient operation when the wind farm is disconnected from the grid. Based on simulation, the performance of the proposed control for the CSC-type wind turbine-SMES hybrid system in the wind farm has been verified. [ABSTRACT FROM PUBLISHER]

This paper presents fault analysis and protective inductors design for an offshore wind farm, where the power collection system in the wind farm and the power transmission link to the grid adopt high-voltage direct-current (HVDC) technology. This paper focuses on dealing with short-circuit faults in the HVDC link between the offshore station and the onshore station. The transient characteristics of the transmission system are analyzed in detail. The criteria of selecting protective inductors are proposed to effectively limit the short-circuit current and avoid the damage to the converters. A dc grid offshore wind farm is simulated, and the results demonstrate the effectiveness of the proposed protective inductors design. [ABSTRACT FROM AUTHOR]

The objective of this paper is to investigate the fast power-routing capability of line current commutating (LCC)-HVDC. Such capability is most desired in future grids with high penetration of renewable energy sources (e.g., wind and solar). The technology presented in this paper replaces the traditional LCC-HVDC rectifier power order control by an ac voltage-mode control. This technology enables the HVDC rectifier ac bus to act as an infinite bus and absorb fluctuating wind power. A study system consisting of an ac system, an LCC-HVDC, and a doubly fed induction generator-based wind farm is built in Matlab/SimPowersystems. Simulation studies are carried out to demonstrate the proposed HVDC rectifier control in routing fluctuating wind power and load change. Parameters of the proposed voltage-mode control are investigated to show their impact on HVDC power routing and ac fault recovery. [ABSTRACT FROM AUTHOR]

Three configurations of voltage source converter multi-terminal HVDC transmission for large offshore wind farms are studied. The voltage-current characteristics of the converters are shown and the operation of the converters with different power output from the wind farms is discussed. The control system is designed to achieve automatic coordination between the converters without the need of fast communication between them. The operation of the multi-terminal HVDC transmission with the control system is tested through simulations using PSCAD/EMTDC and through experiments on a three-terminal test rig. The simulation and experimental results show good agreement. [ABSTRACT FROM AUTHOR]

The multiterminal dc wind farm is a promising topology with a voltage-source inverter (VSI) connection at the onshore grid. Voltage-source converters (VSCs) are robust to ac-side fault conditions. However, they are vulnerable to dc faults on the dc side of the converter. This paper analyzes dc faults, their transients, and the resulting protection issues. Overcurrent faults are analyzed in detail and provide an insight into protection system design. The radial wind farm topology with star or string connection is considered. The outcomes may be applicable for VSCs in the multi-VSC dc wind farm collection grid and VSC-based high-voltage direct current (HVDC) offshore transmission systems. [ABSTRACT FROM PUBLISHER]