Offshore Wind Farm Connection Research Articles

A Novel Coordinated Control Strategy for Frequency Regulation of MMC-HVDC Connecting Offshore Wind Farms

A Novel Coordinated Control Strategy for Frequency Regulation of MMC-HVDC Connecting Offshore Wind Farms

Power-from-Shore Optioneering for Integration of Offshore Renewable Energy in Oil and Gas Production

Despite the widespread usage of high-voltage alternating current (HVAC) for the connection of offshore wind farms (OWF), its use to power-from-shore (PFS) offshore oil and gas (O&G) production sites is often not feasible. Its limitations for long-distance subsea transmission are usually found at 50–70 km from shore and might be even shorter when compared commercially to a direct-current (DC) alternative or conventional generation. Therefore, this research paper aims to address the standardization of offshore transmission with a particular focus on the high-voltage direct current (HVDC) alternative. While the distance is typically not a limiting factor when using DC, and the voltages used are rather standard, the concept of power envelopes can be quite useful in addressing the high variability of offshore site power requirements and setting a design baseline that would lead to improved lead time. In this article, a full back and front-end genetic optioneering model purposely built from the ground up in Python language is used to #1 define up to three DC power envelopes that would cater to most of the candidate’s requirements and #2 provide the lowest cost variance. The results will demonstrate that this can be achieved at a minor overall cost expense.

Harmonic Injection Based Fault Ride-through Control of MMC-HVDC Connected Offshore Wind Farms

Offshore wind farms connected to the grid via MMC-HVDC should have fault ride-through (FRT) ability. However, without high-speed communications, it is difficult to achieve coordinated control between wind power converters and the MMC-HVDC converters. To address this problem, a coordinated control method that is designed based on the injection of harmonics is proposed. During the fault, the offshore MMC converter detects that the DC voltage has exceeded the threshold value, and then injects sequence harmonics into the offshore AC lines, which enables the wind power converters to cooperatively limit the power injected into the grid. Therefore, the coordinated FRT of multiple converters can be realized without additional communication between wind power converters and the offshore MMC converters. A detailed model of MMC-HVDC connected offshore wind farms is constructed in a real time digital simulator (RTDS), and the proposed method is verified by hardware-in-loop dynamic simulation experiments. In addition to the traditional method of only limiting the output voltage of the offshore MMC, the proposed method also coordinately limits the output current of the wind farm to reduce the output power of the wind farm. Therefore, the proposed method allows the DC voltage to be quickly limited within the allowable range, and the system can achieve safe and reliable FRT without communication.

Coordinate Fault Ride-Through Strategy for Connection of Offshore Wind Farms Using Voltage Source-Converter-Based High-Voltage Direct-Current Transmission under Single Polar Fault.

In a system where wind farms are connected to the grid via a bipolar flexible DC transmission, the occurrence of a short-time fault at one of the poles results in the active power emitted by the wind farm being transmitted through the non-faulty pole. This condition leads to an overcurrent in the DC system, thereby causing the wind turbine to disconnect from the grid. Addressing this issue, this paper presents a novel coordinated fault ride-through strategy for flexible DC transmission systems and wind farms, which eliminates the need for additional communication equipment. The proposed strategy leverages the power characteristics of the doubly fed induction generator (DFIG) under different terminal voltage conditions. By considering the safety constraints of both the wind turbine and the DC system, as well as optimizing the active power output during wind farm faults, the strategy establishes guidelines for the wind farm bus voltage and the crowbar switch signal. Moreover, it harnesses the power regulation capability of the DFIG rotor-side crowbar circuit to enable fault ride-through in the presence of single-pole short-time faults in the DC system. Simulation results demonstrate that the proposed coordinated control strategy effectively mitigates overcurrent in the non-faulty pole of flexible DC transmission during fault conditions.

Analysis of grid-connected operation characteristics and limit access capacity of offshore wind farms

Wind power is a relatively ripe renewable energy power generation technology, which has the superiority of large-scale development and utilization. Large-scale deep-sea wind farms are a significant direction for wind power development. With the continuous growth of the capacity of a single offshore wind farm, it is necessary to study the impact of grid connection of large-capacity offshore wind farms on the normal operation of the power grid, which can guarantee the safety and stability of the power grid. Based on the smooth working of the power system, it is meaningful to determining the limit access capacity of offshore wind farms for guiding offshore wind power planning. In this paper, an evaluation index system for the operation characteristics of offshore wind farms integrated into regional power grids is constructed, and based on that, the influence of offshore wind farms on the safety, stability, economy and power quality of regional power system operation is studied, and the grid-connected power limit of offshore wind farms is further calculated.

Application of Association Rule Mining in offshore HVAC transmission topology optimization

This work develops a hybrid optimization method for determining optimal radial transmission topologies for the connection of offshore wind farms combining Association Rule Mining (ARM) and a greedy algorithm. The method is capable of optimally placing offshore substations and accounts for Capital Expenditures (CAPEX), Corrective Maintenance (CM), losses and Expected Energy Not Transmitted (EENT). The stochastic nature of wind is also considered. First, an inequality based apriori algorithm is applied to a randomly generated population of Offshore Wind Power Plant (OWPP) pairs within a specified search domain. This way, a set of simple constraints is obtained reducing the effective combinatorial search space. A verified optimal greedy algorithm is then applied to efficiently search the reduced search space for the lowest cost radial topology to connect offshore wind. The hybrid approach is shown to introduce minimal error given a sufficient sample population while greatly extending the feasible problem size of the greedy search algorithm.

A Complete Impedance Model of a PMSG-Based Wind Energy Conversion System and Its Effect On the Stability Analysis of MMC-HVDC Connected Offshore Wind Farms

Impedance modeling of a permanent magnet synchronous generator (PMSG)-based wind energy conversion system (WECS) usually ignores the machine-side dynamics, instead, the machine-side system is simplified as a constant power source, while merely keeping the detailed grid-side converter model. This simplification is mainly based upon the intuition that the dc-bus capacitor is large enough so that the machine- and grid-side dynamics can be decoupled. However, the simplification probably does not hold in practice since there is always a tendency to minimize the capacitor size to achieve a smaller package for installation. Therefore, the machine-side system should not be ignored, otherwise maybe leading to error for stability analysis. To address this issue, this paper developed a complete ac-side impedance model of the PMSG-based WECS. Then, impact factors about the coupling characteristics between machine- and grid-side systems of the PMSG-based WECS are discussed. Finally, a case study of sub/super synchronous oscillation is carried out on a practical offshore wind farm integrated with a modular multilevel converter based high-voltage dc (MMC-HVDC) transmission system in China. The results show that ignoring the machine-side system would lead to an optimistic stability judgement, which confirms the necessity of the complete model of PMSG-based WECS. Furthermore, the system can be stabilized by either changing the couplings between machine- and grid-side systems of PMSG-based WECS or implementing active damping in the wind farm side MMC.

Adaptive Droop-Based Hierarchical Optimal Voltage Control Scheme for VSC-HVdc Connected Offshore Wind Farm

An adaptive droop-based hierarchical optimal voltage control (DHOVC) scheme is proposed for voltage-source converter high-voltage-direct-current (VSC-HVdc) connected offshore wind farms (WFs). The wind turbines (WTs) and WF side VSC (WFVSC) are coordinated to minimize the voltage deviations of buses inside the WF from the nominal voltage and mitigate reactive power (Var) fluctuations of WTs. The model predictive control is used to improve the performance of the DHOVC scheme during a certain predictive horizon. A hierarchical solution method based on the alternating direction method of multipliers is developed to reduce the calculation burden of the central controller while improving the information privacy protection. During the predictive horizon, the WTs and WFVSC are coordinated to achieve the near global optimal performance without global information. A WF with 32 × 5MW WTs was used in the MATLAB/Simulink to test the proposed DHOVC scheme.

Impedance Modelling and Stability Analysis of Diode-Rectifier based HVDC Connected Offshore Wind Farms

This paper investigates the stability of diode rectifier based HVDC (DR-HVDC) connected offshore wind farms. The impedance model of the DR-HVDC connected offshore wind farms in dq frame is analytically developed and is validated by comparing its impedances to those obtained from frequency scanning of time domain EMT model in PSCAD/EMTDC. Based on the impedance model, the impacts of the sizes of DR-HVDC DC smoothing reactors and AC filters on offshore system stability are analyzed. It shows reduced DC smoothing reactance degrades the d-axis stability margin and can potentially lead to system instability. Higher bandwidth of offshore AC voltage control of wind turbine line side converter is proposed to minimize such instability potential. The analysis also shows that reduced AC filters size helps to increase q-axis stability margin and leads to a more stable system. Moreover, stability analysis considering the interaction between WTs is conducted and it reveals that the WT <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P-V</i> control has a significant impact on the stability of the DR-HVDC connected WTs.

A Novel DC Chopper Topology for VSC-Based Offshore Wind Farm Connection

Sudden increases in the dc-link voltage, caused by ac grid side voltage drops, can lead to potential damage in the equipment. Therefore, dc choppers are widely used in offshore wind power links to absorb the excess energy and keep the voltage within safe margins. This article investigates the existing chopper topologies and summarizes their advantages and disadvantages. On this basis, a novel dc chopper topology with both centralized and distributed resistances is proposed. It is a combination of two mainstream chopper topologies and gives the flexibility to tradeoff among the pros and cons of them. In this way, the proposed topology achieves great electromagnetic interference performance, precise power control, and very small system disturbance. In addition, the maximum power dissipation of the distributed resistance is reduced by 75%. The switching frequency and the loss of insulated gate bipolar transistors (IGBTs) are also decreased. Finally, the claimed attributes of the proposed topology are verified by test and simulation.

DC Current and Voltage Droop Control Method of Hybrid HVDC Systems for an Offshore Wind Farm Connection to Enhance AC Voltage Stability

A new DC current-voltage droop control method for a hybrid high-voltage direct-current (HVDC) system featuring a voltage-sourced converter (VSC) rectifier and a line-commutated converter (LCC) inverter is presented. The proposed method utilizes DC voltage droop control to reduce the AC voltage fluctuation at the inverter side, to exploit the hybrid HVDC system for connecting offshore wind farms to weak grids without necessitating continuous reactive power compensation facilities. Additionally, an optimal droop coefficient calculation method is proposed to keep the AC voltage constant for wind power fluctuations. To evaluate the stability of the proposed method, a small-signal state-space (SS) model of the target system is derived and the root locus analysis is given. Simulation case studies are performed using an electromagnetic transient model in PSCAD and an SS model in MATLAB. The results demonstrate that a hybrid HVDC system with the proposed method successfully integrates offshore wind energy and weak grids without additional facilities because the AC voltage can be maintained at a near constant during wind fluctuations.

Evolution of the HVDC Link Connecting Offshore Wind Farms to Onshore Power Systems

This paper presents an overview of the DC link development and evolution dedicated to HVDC structure for connecting offshore wind power plants to onshore power systems. The growing demand for the green energy has forced investors in power industry to look for resources further out at sea. Hence, the development of power electronics and industrial engineering has enabled offshore wind farms to be situated further from the shore and in deeper waters. However, their development will require, among other technologies, DC-DC conversion systems. The advantages of HVDC over HVAC technology in relation to transmission distance are given. The different HVDC configurations and topologies of HVDC converters are elucidated. In this context, the HVDC grids are a promising alternative for the expansion of the existing AC grid.

Novel travelling wave directional pilot protection for VSC‐HVDC transmission line

As known, voltage source converter-based high-voltage direct current (VSC-HVDC) transmission system is widely utilised in actual power grid. It is suitable for asynchronous networking, such as the connection of offshore wind farm to main power grid. Generally, as primary protection principle, travelling wave (TW) protection is adopted based on the technologies of ABB and SIEMENS, of which is only transient quantity based protection principle instead of true TW protection principle and has no fault direction discrimination ability. Besides, because of the large capacitances applied on converter station, the voltage TW (VTW) information cannot be detected by the protection units of two terminals. Therefore, this study proposes a novel TW directional pilot protection based on current TW (CTW) and direct current voltage. In the beginning, this study the unusual propagation characteristic of VTW or CTW along with the VSC-HVDC transmission line. Afterwards, this study proposes a novel fault direction discrimination criterion using the integral of instantaneous TW power. Eventually, using PSCAD/EMTDC, a typical VSC-HVDC transmission system simulation model is constructed to verify novel protection's performances.

Impacts of a LVRT Control Strategy of Offshore Wind Farms on the HTS Power Cable

High temperature superconducting (HTS) power cables are a potential solution for the grid integration of offshore wind farms since the HTS cable can conduct bulk wind power at low voltage levels. However, the transient current through the HTS cable in cases of low voltage ride through (LVRT) operation has a negative impact on the HTS cable operation due to the quenching phenomenon. This paper analyzes the impact of LVRT control strategies on the HTS cable operation. In addition, a coordinated control of wind turbines for LVRT improvement of an offshore wind farm is proposed. The feasibility of the HTS cable application for the grid connection of offshore wind farms is also discussed in this study. The proposed controller is designed for the wind turbine generator based on a type-4 permanent magnet synchronous generator. In the proposed controller, the transient current through the HTS cable is reduced by regulating the machine side power during fault conditions. The feasibility of the proposed controller is validated in the PSCAD/EMTDC program (Manitoba Hydro International Ltd., Winnipeg, Manitoba, Canada, version 4.2.1). The effects of transient current on the cable temperatures and resistances are analyzed in this study. Simulation results show that the proposed control strategy could reduce the transient current and temperature rise of the HTS cable.

Enhanced FRT and Postfault Recovery Control for MMC-HVDC Connected Offshore Wind Farms

Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) connected offshore wind farms could suffer serious challenges: DC overvoltage during a fault and post-fault DC voltage restoration. In this paper, to mitigate DC overvoltage, a two-stage voltage drop control scheme is proposed for offshore MMC to realize a fast active power reduction, by coordinating with a voltage-dependent active current control of individual wind turbines. The voltage drop depths are designed by considering the offshore de-loading efficiency and the stability of WTs. To improve the post-fault DC voltage recovery performance, an adaptive voltage rise control scheme is proposed to coordinate the recovery of onshore and offshore active power. The recovery trajectory of DC voltage is formulated and further employed to design the onshore active power ramp rate for uninterrupted operation of WTs. Case studies have been conducted on two test systems to validate the proposed method.

A New Control Scheme for Fast Frequency Support From HVDC Connected Offshore Wind Farm in Low-Inertia System

Large-scale renewable energy integration and consequent displacement of synchronous machines result in low system inertia. Low system inertia leads to high rate of change of frequency, higher frequency nadir, and potentially require higher primary frequency reserve to stabilise system frequency following a sudden generation shortfall. To address such frequency stability issue in wind energy integrated system, this paper has proposed a new and realistic control strategy for inertial and primary frequency support from HVDC connected Offshore Wind Farms (OWF). The proposed control strategy, which is based on offshore AC voltage control is not only addressing limitations of high rate of frequency change in the offshore grid, but also does not result in any frequency deviation in the offshore grid while providing frequency support to the main grid. The proposed control strategy is robust for any frequency deviation in the main grid, which utilises HVDC converters to map onshore frequency variation in to voltage variation in the offshore grid, thus enabling adequate inertial and primary frequency support from the OWF. The proposed control strategy has been tested on two test systems and the results are compared with conventional method to demonstrate the robustness, effectiveness and reliability of the proposed frequency support strategy.

Export Transformer Switching Transient Mitigation in HVDC Connected Offshore Wind Farms

An increasing number of offshore wind farms are connected to the public grid with large-scale HVDC links. The electrical transients caused by the energization of the export transformers appear more significant in the converter-driven offshore AC grid with low short-circuit power, than in direct AC grid connections. This may lead to a violation of the 2% voltage dip criterion, thus seriously affecting the production of neighboring wind farms. Additionally, the transformer and the entire insulation system are exposed to mechanical and dielectric stress. This paper investigates the influence of three factors, namely excitation, switching angle, and remnant core magnetization, onto the inrush current. Electromagnetic transient simulations and field measurements show that with a smart use of the on load tap changer, point on wave switching, and a demagnetization of the core by a cable-shunt swing circuit, the three factors above can be used as degrees-of-freedom to effectively mitigate the inrush current phenomenon.

Optimal Connection of Offshore Wind Farm with Maximization of Wind Capacity to Power Systems considering Losses and Security Constraints

The technical, economic, and environmental constraints related to the construction of new transmission lines are complex issues related to the definition of points for connecting new offshore wind farms (OWFs) to the grid. In this context, it has become an important research topic to choose the best OWF connection point to a power system, among some geographically close to each other within a given region, aiming at ensuring maximum generation capacity of the wind farm and safe use of existing transmission network. The objective of this work is to present a methodology to determine the optimal OWF connection point in a power system, with maximum penetration of firm wind power and minimum loss, considering security constraints related to the “N−1” contingency criterion, exchange limits between areas, and a strategy to reduce the number of constraints in the optimization problem. The algorithm is modeled using a Mixed Integer Nonlinear Programming (MINLP), and it is evaluated in a tutorial system and three well-known other networks from literature: IEEE 14-Bus, IEEE RTS-79, and Southern Brazilian System.

Coordinating fault ride through strategy for connection of offshore wind farms using VSC‐HVDC under single polar fault

Coordinating fault ride through strategy for connection of offshore wind farms using VSC‐HVDC under single polar fault

Control of VSC-HVDC Connected Offshore Windfarms for Providing Synthetic Inertia

This paper presents a generator emulation control (GEC)-based method to provide synthetic inertia with a voltage source converter-based high-voltage direct current (VSC-HVdc) converter station, and furthermore, a coordinated control strategy for VSC-HVdc connected offshore windfarms (OWFs) to achieve enhanced ability of serving synthetic inertia. Given that the increasing penetration of renewable energy sources may lead to a significant reduction of system inertia and will thus result in a greater risk of instability, an extended control loop to emulate both the inertia and the droop characteristics of a synchronous generator is first added, with which the VSC-HVdc converter station can provide active power support by using the stored energy in the dc-link capacitors to increase the inertia of power system. To provide larger synthetic inertia with reasonable volume of dc-link capacitors, a coordinated control strategy for VSC-HVdc connected OWFs is further proposed to utilize the OWFs to supply additional energy needed by the GEC strategy. The validity of the proposed control methods is verified exemplarily by simulation results of a three-terminal VSC-HVdc system with two connected OWFs.