DC Collection Grid Research Articles

Low-voltage DC collection grids for marine current energy converters: Design and simulations

Marine current energy represents a globally abundant yet largely untapped renewable energy source, offering greater predictability than other sources such as wind. Consequently, it has the potential to play a vital role in the green transition. A critical consideration for harnessing marine current energy is the design of the electrical grid to accommodate multiple turbines. Therefore, this paper presents a study that explores three types of DC collection grids (series, parallel, and star) for a specific marine current energy converter. A simulation model developed for the marine current energy converter is introduced and utilized to assess these topologies for grids comprising ten identical turbines subjected to varying water speeds. The designed topologies are intended for low-voltage and nearshore applications. The simulation results demonstrate that the series collection grid requires a significantly higher DC grid voltage compared to the other topologies for the turbines to operate correctly. Additionally, the study reveals that all three grid topologies can effectively transmit power to the distribution grid with similar power losses.

A comparison of AC and DC collection grids for marine current energy

Important questions to enable the use of marine current energy are how the electrical system is designed, how multiple energy converters are interconnected offshore and how the power is transmitted to the shore. The Division of Electricity at Uppsala University have constructed and deployed a marine current energy converter in the river Daläven in Söderfors, Sweden. In the study presented in this article, a model of a near-shore low-voltage AC collection grid and a near-shore low-voltage DC collection grid is presented for the technology at the Söderfors test site. The models are implemented in MATLAB/Simulink. For collection grids of five turbines, it is shown that the proposed control schemes are able to deliver power to the distribution grid. The controllers are able to achieve this even when one turbine is suddenly disconnected from the grid. Furthermore, it is shown that the conduction losses of the DC system are higher than the losses of the AC system for nominal and high water speeds. However, in a qualitative comparison between the systems it is concluded that despite the higher losses, the DC system can be an interesting option. This is because fewer components need to be placed in the turbine, which is beneficial in offshore systems where space is a limiting factor. Furthermore, a DC system can be less expensive since fewer cables are needed.

Coordinated Predictive Control of Offshore DC Collection Grid and Wind Turbines for Frequency Response: A Scheme Without Secondary Frequency Drop

From both technical and economical perspectives, all-dc offshore wind farm (OWF) is a trend for future offshore wind energy applications. Furthermore, in an all-dc OWF, both the capacitors in dc collection grid and offshore wind turbines have the potential for onshore grid frequency response. In light of this, this paper presents a model predictive control scheme, which coordinates the operation of offshore dc collection grid capacitors (dc-grid capacitors) and offshore wind turbines for the onshore frequency support. With this scheme, offshore dc-grid capacitors provide fast inertia support right after the frequency event, while offshore wind turbines provide primary frequency response for a longer period. A key feature of the proposed scheme lies in the ability to suppress the secondary frequency drop issue, which is caused by sudden power reduction of offshore dc-grid capacitors during the frequency recovery phase. An analytical framework is derived to validate this ability. Finally, simulation results verify the effectiveness of the proposed scheme.

Isolated Cascaded DAB DC-DC Converter to Boost Medium DC Voltage to HVDC

The offshore wind farms typically use ac system to collect power from each generator, with the voltage increased by means of high, heavy step-up transformers. DC collection grids have also recently been taken into consideration as solution to simplify and minimize the offshore windfarm platforms. DC collection grids offer an additional method to reduce complexity of offshore windfarms. However, to increase the DC voltage for HVDC transmission requires the development of high-power and high voltage converters, which presents a technical challenge. This research makes use of an isolated cascaded dual active bridge (DAB) DC-DC converter to boost medium DC voltage to HVDC. Isolated Cascaded DAB DC-DC Converters are connected in series on the output side and in parallel on the input side to obtain a high transformation ratio and high power. The bidirectional DAB DC-DC converters cab be designed with power densities in the variability of tens to hundreds of kilowatts, depending on the components used and the switching frequency at which the converters function most effectively. The input parallel output series (IPOS) topology, 225 kV HVDC can be generated from a 5 kV MVDC input by cascading DAB DC-DC up to 30 stages. This converter family is useful due to its scalability and flexibility, since the power and voltage ratings can be increased while still using the same cells. MATLAB Simulink simulations are performed and verified the elementary operating characteristics of the system.

Investigation of the wave–photovoltaic-battery​ hybrid power generation platform’s control strategy

This paper proposes a wave–photovoltaic-battery hybrid power generation platform which based on the distributed DC collection and AC inverter grid structure. To be able to simplify the control structure of the power generation system and improve the anti-interference ability of the system, a one-step delay compensation model predictive current control (MPCC) method is proposed for the wave power generation rectifier unit and the perturbation and observation (P&O) fuzzy logic maximum power point tracking (MPPT) algorithm is applied in the photovoltaic power generation unit. Meanwhile, the constant power control method is used to realize the charge and discharge control of the energy storage system to achieve the grid-connected goal of tracking the load power demand. Finally, the MATLAB/Simulink software is applied to simulate the grid-connected operation characteristics of the wave energy power generation unit in continuous power generation modes, as well as the photovoltaic power generation unit at constant temperature and variable irradiance. The results illustrate that the suggested technique for controlling the power generating unit is straightforward and feasible, DC bus voltage stability effect is effective, and the inverter output satisfies the grid-connected operating requirement, which verifies the feasibility of the proposed scheme and can provide some theoretical support for the subsequent high-capacity wave–photovoltaic-battery hybrid power generation platform.

DC Grid Interface for the Integrated Generator–Rectifier Architecture in Wind Energy Systems

An integrated generator–rectifier architecture is a promising candidate for harvesting energy from offshore wind. The most attractive feature of this architecture is that the majority of the power is processed by reliable, efficient, and inexpensive diodes operating at the generator frequency. The use of dc collection grids (medium-voltage or high-voltage dc) is an emerging trend in a wind farm. This article proposes a dc grid interface circuit for the integrated generator–rectifier and a control strategy for maximum power point tracking. Using a 10-MW illustrative design, it is shown that the proposed architecture reduces the total switch volt–ampere rating requirement by 22.8%, which translates into overall loss reduction ranging between 28.3% and 71.7%, depending on the extracted power. The maximum power point tracking algorithm is implemented based on the relationship between the active rectifier <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$d$</tex-math></inline-formula> -axis current and the generated power. Experimental results obtained from a laboratory prototype validates the effectiveness of the proposed converter architecture and its control strategy. This approach opens up opportunities for integrating dc collection networks with offshore wind farms through an efficient, reliable, and cost-effective energy conversion system.

A Review of AC and DC Collection Grids for Offshore Renewable Energy with a Qualitative Evaluation for Marine Energy Resources

Marine energy resources could be crucial in meeting the increased demand for clean electricity. To enable the use of marine energy resources, developing efficient and durable offshore electrical systems is vital. Currently, there are no large-scale commercial projects with marine energy resources, and the question of how to design such electrical systems is still not settled. A natural starting point in investigating this is to draw on experiences and research from offshore wind power. This article reviews different collection grid topologies and key components for AC and DC grid structures. The review covers aspects such as the type of components, operation and estimated costs of commercially available components. A DC collection grid can be especially suitable for offshore marine energy resources, since the transmission losses are expected to be lower, and the electrical components could possibly be made smaller. Therefore, five DC collection grid topologies are proposed and qualitatively evaluated for marine energy resources using submerged and non-submerged marine energy converters. The properties, advantages and disadvantages of the proposed topologies are discussed, and it is concluded that a suitable electrical system for a marine energy farm will most surely be based on a site-specific techno-economic analysis.

Active and Passive Control of a Dual Rotor Wind Turbine Generator for DC Grids

This paper proposes a vector and a linear control strategy for wind turbines connecting to a DC collector grid. Two conversion schemes are considered for the wind turbine, (i) an active scheme where a dual rotor generator (DRG) is connected to a voltage source converter (VSC), and (ii) a passive scheme where a DRG is connected to a passive uncontrolled rectifier. The vector controller is implemented on VSC and DRG where the turbine rotor speed and output power are controlled via a VSC, and the wind turbine generator (WTG) rectified output DC voltage is controlled via a DRG DC filed current. A load-angle control and analytical modeling is discussed for the vector controller and evaluated via simulation results. In the passive scheme where the VSC is replaced with an uncontrolled rectifier the control is entirely shifted to DRG where a linear controller is implemented that controls the turbine rotor speed, power and voltage. A comparison of the vector and linear control is presented, and simulation results verify the effectiveness of the control strategy in steady-state and when turbine transitions between different wind velocities.

A Medium-Voltage DC-Collection Grid for Large-Scale PV Power Plants With Interleaved Modular Multilevel Converter

There is a growing interest in deploying medium-voltage dc-collection grid technologies in renewable energy applications to improve energy efficiency. Existing dc-collection grids incorporate medium-voltage dc/dc converters that suffer from bulky passive components such as a step-up transformer along with additional conversion stage to incorporate MPPT function. In response to these concerns, this article proposes a new medium-voltage dc-collection grid method for large-scale PV plants with an interleaved modular multilevel (IMMC) dc–dc boost converter. The proposed IMMC converter is synthesized with lower voltage half-bridge SiC inverter blocks connected in series to support medium-voltage dc–dc conversion. The half-bridge blocks of the IMMC employ SiC switches, along with high-frequency PWM operation enable compact size/weight along with high-efficiency conversion. Simulation results are shown for 1500-V (dc) to 16-kV (dc) conversion with four series-connected IMMC inverter blocks powered from two PV plants. A power-sharing stage is an integral part of the proposed converter. This enables two series-connected PV plants to supply unequal power under partial shading conditions. Control of the power-sharing stage for varying insolation from PV (1000–500 W/m2) due to the shading effect is shown. A design example for 1-MW PV power plant block based on the specifications derived from Kuwait Shagaya 60-MW solar power plant is discussed. Experimental results on a scaled-down laboratory prototype are included in this article.

High step‐up SVMC‐based DC/DC converter for offshore wind farms

This study presents a high step-up scalable voltage multiple cell (SVMC)-based DC/DC converters, which have the features of high gain, low and adjustable components stresses and automatic input-current sharing in each input phase. In virtue of the above features, the converter is applicable for the DC collection grids for offshore wind farms. In this study, the converter in terms of working principle and property characteristics are analysed with four input phases and three SVMCs. The converter has also been evaluated in terms of the component stress and voltage conversion ratio against some other converters for high-power occasions. Moreover, a 2.5 MW simulation model and a 1.2 kW prototype were constructed for verification.

A Fundamental Study on Control Method of MMC Connected to Synchronous Generator for Offshore Wind Farm with DC Collection Grid.

A Fundamental Study on Control Method of MMC Connected to Synchronous Generator for Offshore Wind Farm with DC Collection Grid.

Stable and Passive High-Power Dual Active Bridge Converters Interfacing MVDC Grids

Dual active bridge (DAB) is a topology that is receiving more and more attention as a potential solution to interface dc grids of different voltage levels. From a system level, the implications of DABs on the stability of complex power systems are addressed in this paper. Dynamics modeling and stability assessment for a DAB implementation aimed to interface low-voltage energy resources with medium-voltage dc (MVdc) collection and distribution grids are presented. The DAB admittance is analytically derived and assessed in order to describe its dynamics and anticipate its behavior when integrated in a complex MVdc grid. The model considers the low-frequency range, mostly dominated by the controller action, and the high-frequency range, described by a nonlinear operation. The theoretical analysis is verified by hardware-in-the-loop emulation, with the controller running on a digital signal processor. The proposed implementation is proved to achieve passivity in the whole spectrum, which undoubtedly is a desired feature for massive power electronics integration in the future MVdc grids.

Frequency Support From a DC-Grid Offshore Wind Farm Connected Through an HVDC Link: A Communication-Free Approach

DC collection grid and high-voltage direct-current (HVDC) transmission lines provide promising solutions for an offshore wind farm (OWF). However, due to the decoupling effect of the dc collection grid and an HVDC link, an HVDC-connected dc-grid OWF cannot directly participate in the frequency regulation of the onshore ac grid. To address this issue, this paper presents a communication-free control scheme, which allows an HVDC-connected all-dc OWF to contribute to the onshore ac grid frequency regulation. This scheme consists of making the offshore dc collection grid voltage and onshore frequency deviations proportional. It relies on the HVDC voltage control at the onshore converter, together with the dc collection grid voltage control at the offshore converter. Furthermore, by continuously adjusting the droop of each wind generator in response to its local wind speed, this scheme takes into account the wind speed difference among wind generators, thereby improving the frequency nadir/peak and guaranteeing the wind generator stability. Simulation results verify the effectiveness of the proposed scheme.

Coordinated MPPT and DPC Strategies for PMSG based Grid Connected Wind Energy Conversion System

This paper presents a novel control strategy employing the maximum power point tracking (MPPT) and direct power control strategy (DPC) for permanent magnet synchronous generator (PMSG) based wind energy conversion system (WECS). The WECS adopts medium voltage source converter with AC-DC-AC configuration. The 10 MW wind farm is implemented in this paper which consists of 2 MW PMSG’s wind systems. The PMSG’s of the wind farm is connected to the DC collection grid through a DC/DC rectifier topology. The DC collection system is connected to the AC grid using single DC/AC inverter thus providing simple control technique which controls the number of PMSG generators. The output power fluctuations are compensated using direct torque control based maximum power point controller and direct power control. The DC link voltage and the active power injected to AC grid is controlled using grid side controller and the maximum power is extracted using the machine side controller of the wind turbine. Pitch angle control strategy is also used in order to reduce the turbine speed during high wind speed condition. The proposed controller provides aneffective solution for grid integration and constant power flow from the generator system to thegrid system. Simulation results are presented and analyzed the performance of the control strategies implemented in the system.

Evaluation of DC Collector-Grid Configurations for Large Photovoltaic Parks

This paper presents a detailed comparison between the conventional ac collector-grid configuration and two proposed dc collector-grid configurations for large photovoltaic (PV) parks. One complete year of measured weather data at two different locations, one very sunny and the other with relatively less sun, is used for the evaluation. An already validated PV performance model is used to estimate the expected energy yield of the PV park for the given environmental and connection conditions. Loss mechanisms in different grid components in the three configurations are discussed based on the developed models. These components include the inverters, transformers, cables, and dc–dc converters. A comparison of the total losses in these components helps to identify the configuration that has the potential to operate at higher efficiency and, hence, generate maximum revenue. The economic viability metrics are calculated and a sensitivity analysis is carried out using the Monte Carlo method with a $\pm \mathbf { 5}{\%}$ uncertainty in the considered cost and energy yield parameters.

A Novel Concept for Offshore Wind-Power Plant With DC Collection System Based on Radial-Connected Converter Topology

This paper investigates the use of a dc collection grid in offshore wind farm; the dc grid is built out by a “radial connection of active rectifiers.” The proposed active rectifier topology consists of a three-phase half-controlled switch voltage source converter (VSC) cascaded with a single active bridge dc–dc converter. The proposed wind farm configuration is further compared to the recent development of a dc collection grid technology for wind farms. In fact, the majority of dc collection grids for wind farms are built out with the ring feeder configurations; also, most of the wind energy conversion units integrate the conventional full-bridge VSC. The comparison is done in terms of power loss, cost, voltage/current ripple, power factor, and control performance. Simulation results, using Power Simulator software, illustrate the performances obtained.

Review on DC collection grids for offshore wind farms with high‐voltage DC transmission system

Traditionally, the network composition of offshore wind farms consists of alternating current (AC) grid; all outputs of wind-energy conversion units (WECUs) on a wind farm are aggregated to an AC bus. Each WECU includes: a wind-turbine (WT), a generator and a power transformer. For a DC collection grid, all outputs of WECUs are aggregated to a DC bus. The transformer in each WECU is replaced by a converter which is more compact and smaller in size compared with the transformer, thus simplifying the wind farm structure. The use of AC offshore grids instead of DC offshore grids is mainly motivated by the availability of protection devices. Efficient solutions to protect DC grids have already been addressed. Presently, there are no operational DC wind-farms, only small-scale prototypes are being investigated worldwide. Therefore, a suitable configuration of the DC collection grid, which has been practically verified, is not available yet. This study discussed some of the main components required for a DC grid including: the WT-generator models, the control and protection methods, the platform structure, and the feeder configurations. The key component of a DC grid is the converter; therefore, this study also reviews some topologies of converter suitable for DC-grid applications.

Isolated AC–DC Converter Using Medium Frequency Transformer for Off-Shore Wind Turbine DC Collection Grid

This paper investigates a dc collection grid architecture to process energy from offshore wind turbine generators (WTGs), which can be further interfaced with the shore via a high-voltage dc network. In the proposed approach, WTG output voltage is rectified and converted to medium frequency (MF). Then, a transformer isolation stage is used, followed by a boost rectification stage. The boost rectification stage is controlled to emulate a resistive load, forming a medium voltage dc (MVDC) interconnection. This type of control is simple for varying wind conditions. The operating principles and unique advantages of the proposed approach are documented along with comparison with a conventional benchmark, assuming the turbines are fitted with direct drive technology. System design example, loss/size analysis and transformer finite element analysis are shown in this paper. Simulation results demonstrate the functional capability of the system. Experimental results from a scaled down prototype prove the operation of proposed concept.

Front-End Isolated Quasi-Z-Source DC–DC Converter Modules in Series for High-Power Photovoltaic Systems—Part II: Control, Dynamic Model, and Downscaled Verification

This paper is the continuation of Part I in which a quasi-Z-source modular cascaded converter (qZS-MCC), comprising front-end isolated qZS half-bridge dc–dc converter submodules (SMs), for dc integration of high-power photovoltaic (PV) systems is proposed. The qZS-MCC-based PV system features modular structure, high-voltage dc collection of PV power, simple control with a unified and constant duty cycle for the front-end isolation converter of all SMs, and low qZS impedance due to no double-line-frequency pulsating power. Here, control scheme of the qZS-MCC PV system integrated into the dc collection grid is investigated. Dynamic model of the system is established for controllers design and time-domain transient simulation. Experimental tests are carried out on the downscaled prototype as a proof-of-concept of the proposed control and modeling, demonstrating the validity of the proposed approaches.

Cost comparison of AC and DC collector grid for integration of large‐scale PV power plants

This paper presents a detailed comparison between the traditional ac collector-grid configuration and dc collector-grid configuration for large-scale photovoltaic (PV) power plants. In particular, the cost of these two configurations have been analysed and discussed in detail. A comparison of the total losses in these components helps to identify the configuration that has the potential to operate at higher efficiency and hence generate maximum revenue. The economic viability metrics are calculated and a sensitivity analysis to the most challenging price ratios is presented.