A review on DC collection grids for offshore wind farms with HVDC transmission system

Abstract: Traditionally, the internal network composition of offshore wind farms consists of alternating current (AC) collection 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 plus mechanical parts, a generator including electronic controller, and a huge 50-or 60-Hz power transformer. For a DC collection grid, all outputs of WECUs are aggregated to a DC bus; consequently, the transformer in each WECU is replaced by a power converter or rectifier. The converter is more compact and smaller in size compared to the transformer. Thus reducing the size and weight of the WECUs, and also simplifying the wind farm structure. Actually, the use of offshore AC collection grids instead of offshore DC collection grids is mainly motivated by the availability of control and protection devices. However, efficient solutions to control and protect DC grids including HVDC transmission systems have already been addressed. Presently, there are no operational wind farms with DC collection grids, only theoretical and 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 paper discussed some of the main components required for a DC collection grid including: the wind-turbine-generator models, the control and protection methods, the offshore platform structure, and the DC-grid feeder configurations. The key component of a DC collection grid is the power converter; therefore, the paper also reviews some topologies of power converter suitable for DC grid applications

DC/DC converter for offshore DC collection network

Large wind farms, especially large offshore wind farms, present a challenge for the electrical networks that will provide interconnection of turbines and onward transmission to the onshore power network. High wind farm capacity combined with a move to larger wind turbines will result in a large geographical footprint requiring a substantial sub-sea power network to provide internal interconnection. While advanced HVDC transmission has addressed the issue of long-distance transmission, internal wind farm power networks have seen relatively little innovation. Recent studies have highlighted the potential benefits of DC collection networks. First with appropriate selection of DC voltage, reduced losses can be expected. In addition, the size and weight of the electrical plant may also be reduced through the use of medium- or high-frequency transformers to step up the generator output voltage for connection to a medium-voltage network suitable for wide-area interconnection. However, achieving DC/DC conversion at the required voltage and power levels presents a significant challenge for wind-turbine power electronics.This thesis first proposes a modular DC/DC converter with input-parallel output-series connection, consisting of full-bridge DC/DC modules. A new master-slave control scheme is developed to ensure power sharing under all operating conditions, including during failure of a master module by allowing the status of master module to be reallocated to another healthy module. Secondly, a novel modular DC/DC converter with input-series-input-parallel output-series connection is presented. In addition, a robust control scheme is developed to ensure power sharing between practical modules even where modules have mismatched parameters or when there is a faulted module. Further, the control strategy is able to isolate faulted modules to ensure fault ride-through during internal module faults, whilst maintaining good transient performance. The ISIPOS connection is then applied to a converter with bidirectional power flow capability, realised using dual-active bridge modules.The small- and large-signal analyses of the proposed converters are performed in order to deduce the control structure for the converter input and output stages. Simulation and experimental results demonstrate and validate the proposed converters and associated control schemes.Large wind farms, especially large offshore wind farms, present a challenge for the electrical networks that will provide interconnection of turbines and onward transmission to the onshore power network. High wind farm capacity combined with a move to larger wind turbines will result in a large geographical footprint requiring a substantial sub-sea power network to provide internal interconnection. While advanced HVDC transmission has addressed the issue of long-distance transmission, internal wind farm power networks have seen relatively little innovation. Recent studies have highlighted the potential benefits of DC collection networks. First with appropriate selection of DC voltage, reduced losses can be expected. In addition, the size and weight of the electrical plant may also be reduced through the use of medium- or high-frequency transformers to step up the generator output voltage for connection to a medium-voltage network suitable for wide-area interconnection. However, achieving DC/DC conversion at the required voltage and power levels presents a significant challenge for wind-turbine power electronics.This thesis first proposes a modular DC/DC converter with input-parallel output-series connection, consisting of full-bridge DC/DC modules. A new master-slave control scheme is developed to ensure power sharing under all operating conditions, including during failure of a master module by allowing the status of master module to be reallocated to another healthy module. Secondly, a novel modular DC/DC converter with input-series-input-parallel output-series connection is presented. In addition, a robust control scheme is developed to ensure power sharing between practical modules even where modules have mismatched parameters or when there is a faulted module. Further, the control strategy is able to isolate faulted modules to ensure fault ride-through during internal module faults, whilst maintaining good transient performance. The ISIPOS connection is then applied to a converter with bidirectional power flow capability, realised using dual-active bridge modules.The small- and large-signal analyses of the proposed converters are performed in order to deduce the control structure for the converter input and output stages. Simulation and experimental results demonstrate and validate the proposed converters and associated control schemes

Distributed control of a fault tolerant modular multilevel inverter for direct-drive wind turbine grid interfacing

Modular generator and converter topologies are being pursued for large offshore wind turbines to achieve fault tolerance and high reliability. A centralized controller presents a single critical point of failure which has prevented a truly modular and fault tolerant system from being obtained. This study analyses the inverter circuit control requirements during normal operation and grid fault ride-through, and proposes a distributed controller design to allow inverter modules to operate independently of each other. All the modules independently estimate the grid voltage magnitude and position, and the modules are synchronised together over a CAN bus. The CAN bus is also used to interleave the PWM switching of the modules and synchronise the ADC sampling. The controller structure and algorithms are tested by laboratory experiments with respect to normal operation, initial synchronization to the grid, module fault tolerance and grid fault ride-through

Power-electronic systems for the grid integration of renewable energy sources: a survey

The use of distributed energy resources is increasingly being pursued as a supplement and an alternative to large conventional central power stations. The specification of a powerelectronic interface is subject to requirements related not only to the renewable energy source itself but also to its effects on the power-system operation, especially where the intermittent energy source constitutes a significant part of the total system capacity. In this paper, new trends in power electronics for the integration of wind and photovoltaic (PV) power generators are presented. A review of the appropriate storage-system technology used for the integration of intermittent renewable energy sources is also introduced. Discussions about common and future trends in renewable energy systems based on reliability and maturity of each technology are presented

Modular multilevel converter based HVDC transmission system for offshore wind farms

This doctoral thesis falls within the scope of electronic power converters oriented to high voltage transmission applications, in particular the power generated in remote offshore wind farms by means of HVDC subsea cables. This research is focused on the Modular Multilevel Converter (MMC) with two level submodules but also with multilevel topology submodules such as 3L-FC (three level flying capacitors) and 3L-NPC (three level neutral point capacitors). The main contribution of this thesis is the developed PWM based modulation strategy which allows the balancing of the total amount of submodules capacitors. It is applicable to the aforementioned submodule topologies under different working conditions as evidenced by experimental results

Soft-Switched Step-Up Medium Voltage Power Converters

With a ten-year average annual growth rate of 19 percent, wind energy has been the largest source of new electricity generation for the past decade. Typically, an offshore wind farm has a medium voltage ac (MVac) grid that collects power from individual wind turbines. Since the output voltage of a wind turbine is too low (i.e., typically 400 690 V) to be connected to the MVac grid (i.e., 20 40 kV), a heavy line-frequency transformer is used to step up the individual turbines output voltage to the MV level. To eliminate the need for bulky MVac transformers, researchers are gravitating towards the idea of replacing the MVac grid with a medium voltage dc (MVdc) grid, so that MV step-up transformers are replaced by MV step-up power electronic converters that operate at the medium frequency range with much lower size and weight. This dissertation proposes a class of modular step-up transformerless MV SiC-based power converters with soft-switching capability for wind energy conversion systems with MVdc grid. This dissertation consists of two parts: the first part focuses on the development of two novel groups of step-up isolated dc-dc MV converters that utilize various step-up resonant circuits and soft-switched high voltage gain rectifier modules. An integrated magnetic design approach is also presented to combine several magnetic components together in the modular high voltage gain rectifiers. The second part of this dissertation focuses on the development of several three-phase ac-dc step-up converters with integrated active power factor correction. In particular, a bridgeless input ac-dc rectifier is also proposed to combine with the devised step-up transformerless dc-dc converters (presented in the first part) to form the three-phase soft-switched ac-dc step-up voltage conversion unit. In each of the presented modular step-up converter configurations, variable frequency control is used to regulate the output dc voltage of each converter module. The operating principles and characteristics of each presented converter are provided in detail. The feasibility and performance of all the power converter concepts presented in this dissertation are verified through simulation results on megawatts (MW) design examples, as well as experimental results on SiC-based laboratory-scale proof-of-concept prototypes

DC fault protection of diode rectifier unit based HVDC system connecting offshore wind farms

DC fault ride-through operations of the offshore wind farm connecting with diode rectifier unit (DRU) based HVDC link are presented in this paper. A voltage-error-dependent fault current injection is proposed to regulate the WT current during DC faults and to provide fault current. This contributes the control of the offshore AC voltage, which does not drop to zero but is remained relatively high to facilitate fast system recovery after clearance of a temporary DC fault. The WT converters operate on current limiting mode during DC faults and automatically restore normal operation after fault clearance. The full-bridge based modular multilevel converter (MMC) is adopted as the onshore station and its DC fault current control ability is explored to effectively suppress the fault current from the onshore station around zero, which reduces semiconductor losses and potential overcurrent risk of the MMC station. Simulation results confirm the robustness of the system to DC faults

Soft-Switched Step-Up Medium Voltage Power Converters

With a ten-year average annual growth rate of 19 percent, wind energy has been the largest source of new electricity generation for the past decade. Typically, an offshore wind farm has a medium voltage ac (MVac) grid that collects power from individual wind turbines. Since the output voltage of a wind turbine is too low (i.e., typically 400 690 V) to be connected to the MVac grid (i.e., 20 40 kV), a heavy line-frequency transformer is used to step up the individual turbines output voltage to the MV level. To eliminate the need for bulky MVac transformers, researchers are gravitating towards the idea of replacing the MVac grid with a medium voltage dc (MVdc) grid, so that MV step-up transformers are replaced by MV step-up power electronic converters that operate at the medium frequency range with much lower size and weight. This dissertation proposes a class of modular step-up transformerless MV SiC-based power converters with soft-switching capability for wind energy conversion systems with MVdc grid. This dissertation consists of two parts: the first part focuses on the development of two novel groups of step-up isolated dc-dc MV converters that utilize various step-up resonant circuits and soft-switched high voltage gain rectifier modules. An integrated magnetic design approach is also presented to combine several magnetic components together in the modular high voltage gain rectifiers. The second part of this dissertation focuses on the development of several three-phase ac-dc step-up converters with integrated active power factor correction. In particular, a bridgeless input ac-dc rectifier is also proposed to combine with the devised step-up transformerless dc-dc converters (presented in the first part) to form the three-phase soft-switched ac-dc step-up voltage conversion unit. In each of the presented modular step-up converter configurations, variable frequency control is used to regulate the output dc voltage of each converter module. The operating principles and characteristics of each presented converter are provided in detail. The feasibility and performance of all the power converter concepts presented in this dissertation are verified through simulation results on megawatts (MW) design examples, as well as experimental results on SiC-based laboratory-scale proof-of-concept prototypes