Offshore AC Network Research Articles

Improved time-domain distance protection for asymmetrical faults based on adaptive control of MMC in offshore AC network

The harsh environment of the offshore AC network poses challenges to the communication system and the maintenance of the faulty lines, thus the fault location method independent of communication is essential, e.g., non-unit distance protection. However, traditional distance protection is vulnerable to fault resistance and the existing time-domain scheme is affected by the controlled small fault current of converters in offshore AC networks. To emphasize the fault characteristics and improve the performance of protection under fault resistance, the adaptive control of offshore converter is introduced to modulate high-frequency voltage automatically under asymmetrical faults. Based on the high-frequency voltage, the equivalent circuit of the faulty line is established and the derivative equations containing fault distance and transient resistance are deduced. The fault distance is obtained by the improved least-square algorithm and the impact of the small fundamental-frequency fault component is negligible because the high-frequency component is used. In addition, the influence of the fault resistance and the difficulty of near-end fault location are reduced by adopting the improved least-square algorithm, leading to higher reliability. Finally, the effectiveness of the proposed scheme is verified in PSCAD/EMTDC.

Control of Offshore Wind Turbines Connected to Diode-Rectifier-Based HVdc Systems

Diode-rectifier-based high voltage dc (DR-HVdc) systems can be a promising low-cost solution for exporting wind power from remote offshore wind farms to onshore power systems. Industrializing the offshore DR-HVdc requires technical maturation, achievable through in-depth studies and pilot experiments. Deployment of offshore DR-HVdc systems may entail a fundamental change of control philosophy in wind turbine (WT) converters from grid-following control to grid-forming. This article proposes a new grid-forming control for DR-connected offshore WT converters. The proposed controller uses two sequential control loops to regulate WTs' active power, and maintain the frequency and voltage of the offshore ac network. The first control regulates the active-power mismatch of each WT into a voltage angle deviation, which leads to a frequency change. The second control adjusts the WT's alternating voltage magnitude to counteract the frequency change. An internal current control loop is used to limit the fault current and eliminate high-frequency resonances in the system. The proposed control is verified by electromagnetic transient simulations, including fault-ride through, WTs power change, reactive power disturbance, and WTs outage.

Active and reactive power control of hybrid offshore AC and DC grids

The future ‘SuperGrid’ may requires the benefit of both offshore AC network and multi-terminal DC grid. AC cable limits the power transfer capability from the larger offshore wind farm, however, HVDC transmission system is economical viable for large power wind farm integration with the grid. Another approach to develop the offshore network infrastructure is by forming an offshore AC grid connecting several offshore wind farms. Then, this offshore AC network is connected with different onshore grid using HVDC system. This enhances the trade among the countries as well as provide an economical solution for wind energy integration. In this article, operational and control concept of voltage source converter is presented to integrate an offshore AC grid with an offshore DC grid. The article presents the control principle of offshore AC network frequency and voltage with respect to active and reactive power distribution in the AC network. Later, the principle of multi-terminal HVDC system is discussed with respect to power distribution using DC voltage droop control. Power distribution criteria are defined with respect to operator power-sharing requirement and network stability. In the end, a hybrid AC/DC offshore grid is modelled and simulated in MATLAB/SIMULINK to validate the distribution criteria.

Control of Offshore MMC During Asymmetric Offshore AC Faults for Wind Power Transmission

The adoptions of medium voltage in ac collection networks of large dc-connected wind farms significantly increase the ac current magnitudes during normal and fault conditions. Controlling fault currents at zero during asymmetric ac faults is possible, but it has several drawbacks such as increased risk of protection mal-operation due to the absence of fault currents, which also tends to prevent the recovery of ac voltage in postfault. Therefore, this paper presents an enhanced control strategy that exploits the induced negative sequence voltages to facilitate controlled injection of negative sequence currents during asymmetric ac faults. The proposed control not only defines a safe level of fault current in the offshore ac network but also instigates immediate recovery of the ac voltage following clearance of ac faults, which avoid protection mal-operation. In addition, the positive sequence voltage set-point of the offshore modular multilevel converter (MMC) is actively controlled by considering the negative and zero sequence voltages, which effectively avoids the excessive overvoltage in the healthy phases during asymmetrical ac faults. The theoretical basis of the proposed control scheme is described and its technical viability is assessed using simulations.

Impedance-based analysis of harmonic resonances in HVDC connected offshore wind power plants

During the last years, the installation and planning of offshore wind farms using HVDC-links to transmit power onshore has increased. After the first HVDC connected offshore wind power plant of this type had been commissioned, the electrical harmonic resonance at the offshore AC grid was observed. The phenomenon leads to unwanted outages on both wind turbines and the HVDC transmission system. This paper aims to present the harmonic resonances in power-electronics dominated grids such as HVDC connected wind power plants. The study focuses on harmonic frequencies identification which are excited through the resonance phenomena between the elements of the offshore AC network including power converters. The paper presents a comparison of three different methodologies existing in the literature for harmonic resonance analysis including stability analysis. Moreover, we analyse the impact of the different power converter models application. The models and methods are validated in different test cases in order to determine the relationship of such resonances with respect to the grid topology.

Short circuit analysis of an offshore AC network having multiple grid forming VSC-HVDC links

This article presents the short circuit analysis of an offshore AC network which consists of wind power plants interconnected using HVAC cables. The power generated in the offshore AC network is transmitted to several onshore grids using VSC-HVDC system. The offshore AC network is formed by the VSC-HVDC systems using frequency and voltage droop control. A coordinated control scheme is proposed for wind turbines and offshore VSCs during short circuit conditions in the offshore grid to ensure fault ride through (FRT) without compromising the system stability. The theoretical analysis used for developing this control scheme allows to calculate the system limits taking into consideration the active and reactive power capability. In order to verify the proposed control scheme, three phase symmetric faults have been applied on a wind turbine busbar, HVAC busbar, and at the AC cable that interconnects the VSC-HVDC system. Additionally, a frequency coordination control scheme without communication between wind power generation and VSC-HVDC system has been proposed. The methodology and control system have been validated by performing a nonlinear simulation.

Small-Signal Stability Analysis of Offshore AC Network Having Multiple VSC-HVDC Systems

This paper presents a methodology to perform small-signal analysis of an offshore ac network, which is formed by interconnecting several offshore wind power plants. The offshore ac network is connected with different onshore ac grids using point-to-point voltage-source-converter high-voltage direct current (VSC-HVDC) transmission links. In such a network, each offshore VSC-HVDC converter operates in grid-forming mode. In this paper, the offshore VSC grid-forming control is enhanced by using a frequency and voltage droop scheme in order to establish a coordinated grid control among the offshore converters. A small-signal model of the offshore ac network is developed that includes the high-voltage alternating current cables' model, the converters' current, and voltage-control model, the frequency droop scheme, and the voltage droop scheme. Based on this model, an eigenvalue analysis is performed in order to study the influence of the frequency and voltage droop gains on overall offshore ac network stability. Finally, theoretical analysis is validated by performing nonlinear dynamic simulation.

Reactive power management in an offshore AC network having multiple voltage source converters

Many offshore wind power plants are being developed every year at North and Baltic Sea. From the prospective of environment and integrated European power, combined power transmission from several offshore wind power plants using VSC-HVDC transmission system to different onshore grids are suitable instead of connecting each wind power plants individually. Offshore AC hub is beneficial for the wind power plants that are far from shore but close to each other within the vicinity of 20km. This paper presents a method of controlling reactive power flow in the offshore AC grid to minimize the power losses and voltage deviation. In the proposed scheme, offshore grid frequency and voltage are controlled through more than one converter. Using frequency and voltage droop schemes, the active and reactive power sharing is achieved among converters. Furthermore, the optimization algorithm is developed to acquire the set points for the wind power plants and VSC-HVDC droop gains for the optimum operation of the network.

Droop Control Design of Multi-VSC Systems for Offshore Networks to Integrate Wind Energy

This research envisages the droop control design of multi voltage source converter systems for offshore networks to integrate wind power plant with the grids. An offshore AC network is formulated by connecting several nearby wind power plants together with AC cables. The net energy in the network is transferred to onshore using voltage source high voltage direct current (VSC-HVDC) transmissionsystems. In the proposed configuration, an offshore network is energized by more than one VSC-HVDC system, hereby providing redundancy to continue operation in case of failure in one of the HVDC transmission lines. The power distribution between VSC-HVDC systems is done using a droop control scheme. Frequency droop is implemented to share active power, and voltage droop is implemented to share reactive power. Furthermore, a method of calculating droop gains according to the contribution factor of each converter is presented. The system has been analyzed to evaluate the voltage profile of the network affected by the droop control. Nonlinear dynamic simulation has been performed for the verification of the control principle.

Effect of non-standard operating frequencies on the economic cost of offshore AC networks

The effect of choosing a non-standard operating frequency on the equipment and infrastructure costs of an offshore AC network is investigated. The offshore AC network considered is similar in design to the European SuperGrid “SuperNode”. It is designed to connect several large wind arrays to multiple HVDC converters through which power may be transmitted to shore. As the offshore AC network is isolated from onshore networks by the use of HVDC links, it may be operated unsynchronised at any desired frequency. The cost associated with operating the network at a fixed frequency in the range 20–120 Hz is investigated, focusing on the frequency-cost scalings of electrical devices (such as cables, transformers and reactive compensation) and offshore infrastructures. A case study is presented based upon Tranche A area of Dogger Bank, UK, where a minimum point in the total cost of the offshore network is found at 93 Hz.

Simulated Onshore-Fault Ride through of Offshore Wind Farms Connected through VSC HVDC

Effective Onshore-Fault Ride Through was demonstrated by simulation for a Fixed Speed Induction Generator (FSIG) offshore wind farm connected through a Voltage Source Converter HVDC link. When a terrestrial grid fault occurs, power through the onshore converter reduces and the DC link voltage increases. A control system was then used to block the offshore converter. The offshore AC network voltage was reduced to achieve rapid power rejection. Reactive power at the onshore converter was controlled to support the AC network voltage according to the GB Grid Code requirements. Two cases, a 200 ms terrestrial fault and a 50% retained voltage fault of duration 710 ms, at the grid connection point were studied. The simulation results show that power blocking at the offshore converter was effective and the DC link voltage was controlled.