Fault analysis and protection for wind power generation systems

Wind power is growing rapidly around the world as a means of dealing with the world energy shortage and associated environmental problems. Ambitious plans concerning renewable energy applications around European countries require a reliable yet economic system to generate, collect and transmit electrical power from renewable resources. In populous Europe, collective offshore large-scale wind farms are efficient and have the potential to reach this sustainable goal. This means that an even more reliable collection and transmission system is sought. However, this relatively new area of offshore wind power generation lacks systematic fault transient analysis and operational experience to enhance further development. At the same time, appropriate fault protection schemes are required. This thesis focuses on the analysis of fault conditions and investigates effective fault ride-through and protection schemes in the electrical systems of wind farms, for both small-scale land and large-scale offshore systems. Two variable-speed generation systems are considered: doubly-fed induction generators (DFIGs) and permanent magnet synchronous generators (PMSGs) because of their popularity nowadays for wind turbines scaling to several-MW systems. The main content of the thesis is as follows. The protection issues of DFIGs are discussed, with a novel protection scheme proposed. Then the analysis of protection scheme options for the fully rated converter, direct-driven PMSGs are examined and performed with simulation comparisons. Further, the protection schemes for wind farm collection and transmission systems are studied in terms of voltage level, collection level wind farm collection grids and high-voltage transmission systems for multi-terminal DC connected transmission systems, the so-called “Supergrid”. Throughout the thesis, theoretical analyses of fault transient performances are detailed with PSCAD/EMTDC simulation results for verification. Finally, the economic aspect for possible redundant design of wind farm electrical systems is investigated based on operational and economic statistics from an example wind farm project

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 Grids : Motivation, Feasibility and Outstanding Issues : Status Report for the European Commission Deliverable : D5.4

Wind energy is already a mainstay of clean power generation in Europe, with over 100GW of capacity installed so far, and another 120GW anticipated by 2020 according to various analysts. Much of this capacity is expected to be installed offshore, as it is a windier and the source is steadier compared to onshore wind energy. Hence, offshore wind has been envisaged as making a critical contribution to Europe’s demand for electrical energy and to minimising the carbon emissions associated with meeting that demand

Power control, fault analysis and protection of series connected diode rectifier and VSC based MTDC topology for offshore application.

A multiterminal high-voltage dc (MTDC) system is a promising method for transmitting energy generated from an offshore windfarm (OWF). The creation of MTDC systems became easier by the introduction of voltage source converter (VSC) due to the flexibility and controllability it provides. This technology is newer than the line-commutated converter technology (LCC). Power systems can include any number of windfarms together with converters for both offshore and onshore power conversion. Therefore, this thesis suggests a three-terminal MTDC model of two offshore windfarms and one onshore inverter. The electric energy generated by the two windfarms is rectified into dc and transmitted to the shore using dc cable. Although a VSC or a diode rectifier (DR) can convert ac to dc, a series connection of a VSC and two DRs was proposed at the windfarm side to convert the generated power to achieve controllability of the uncontrollable diode rectifiers and reduces the high cost of badditional VSCs. The proposed topology converts the ac power by dividing the windfarm power so that one-third is the share of the VSC and two-thirds is the share of the DRs. The same topology is used to convert the power produced from the other windfarm. Then, the dc power is transmitted via an undersea dc cable to the onshore location, and is then inverted into ac before it is supplied to the neighbouring ac grid using a grid-side VSC. The proposed topology has many advantages, including a significant save in windfarm VSC (WFVSC) capital cost and a significant reduction in the loss of power of the converter without losing the overall controllability. However, although this topology is suitable for windfarm applications, it might not be suitable for high-voltage direct current (HVDC) that requires bidirectional power flow unless making changes to the topology such as disconnecting the diode rectifiers. Furthermore, fault analyses were investigated, including dc faults and ac faults. Ac faults are categorised as symmetrical or unsymmetrical faults. For comparison purposes, a Simulink model was designed, implemented, and simulated as a reference model. The reference model can operate as VSC-, DR-based MTDC, or a mix of both in a way that any component can be added to or removed from the model at any time during the simulation run. The contribution to the dc fault current from various parts such as dc capacitor and the adjacent feeder was investigated thoroughly, and detailed mathematical formulae were developed to compute fault current from these contributors. In addition, the results of the system response due to both fault types are illustrated and discussed. Both symmetrical and unsymmetrical ac faults were initiated on the onshore grid side, and the system response results are presented for those faults. A generalised control scheme (GCS) was proposed in this thesis, which add the ability the model to control the reactive power and is suitable for both balanced and unbalanced ac faults conditions. A protection against faults was investigated and implemented using dc circuit breakers. The protection system was built to ensure safe operation and to fulfil the grid code requirements. Many grid codes are available and presented in the literature, such as Spanish, British, and Danish; however, a grid code by E.ON was chosen. The protection scheme in VSC-based MTDC networks plays a vital role during dc faults. It is vital that this protection be sensitive, selective, fast, and reliable. Specifically, it must isolate the fault reliably from the system within a short time after the fault occurrence, while maintaining the remaining components of the system in a secure operational condition. For optimal performance, the protection scheme discussed in this thesis employs solid-state circuit breakers. A literature survey relevant to the tasks mentioned above was conducted.PhD in Energy and Powe

Modeling and control of VSC-HVDC connected offshore wind farms

In Europe, large offshore wind farms are installed in the North Sea area using modern multi-megawatt wind turbines. Voltage source converter - high voltage direct current (VSC-HVDC) technology has proved to be a promising solution for offshore wind power grid access. The importance of grid impact studies of wind power integration rises with the rapid increase in the installed wind power capacity. This thesis explores the main technical challenges and features associated with integrating large offshore wind farms into the onshore grid via VSC-HVDC systems. For this purpose, dynamic models of wind turbines and VSC-HVDC are developed and control requirements are analyzed with the aid of a simulation software environment. Enhancements in the controller design are afterwards proposed for the VSC-HVDC converters to improve steady-state and dynamic behavior with a special focus on the offshore grid side converter. The primary frequency control by the VSC-HVDC connected offshore wind farm is investigated, where enhancements for existing kinetic energy control strategies are proposed and verified by simulation results. Additionally, a novel overfrequency-limiting method is developed by utilizing the HVDC chopper.In Europa werden große Offshore-Windparks mit Windenergieanlagen der Megawatt-Klasse im Nordseeraum errichtet. Die Hochspannungsgleichstromübertragung auf Basis spannungsgeführter Umrichter (VSC-HGÜ) hat sich als vielversprechende Lösung für die Netzanbindung von Offshore-Windparks erwiesen. Mit dem rasanten Anstieg der installierten Windenergieanlagen steigt der Bedarf nach genauen Studien über deren Auswirkungen auf das elektrische Netz. Diese Arbeit untersucht die wichtigsten technischen Herausforderungen und Merkmale, die bei der Netzintegration der Offshore-Windparks über VSC-HGÜ zu berücksichtigen sind. Zu diesem Zweck werden dynamische Modelle von Windenergieanlagen und VSC-HGÜs entwickelt und deren Regelungsanforderungen mit Hilfe einer Simulationssoftware analysiert. Für den Offshore-seitigen HGÜ-Umrichter werden Verbesserungen der Regelungskonzepte vorgestellt, um das stationäre und dynamische Verhalten zu optimieren. Außerdem wird die kurzfristige Frequenzstützung durch den über eine VSC-HGÜ-angeschlossenen Offshore-Windpark untersucht. Hierbei werden Erweiterungen von existierenden Strategien der Frequenzstützung aus den rotierenden Massen (kinetic energy control, KEC) vorgestellt und durch Simulationsergebnisse verifiziert. Zusätzlich wurde ein neuartiges Verfahren zur Begrenzung der Überfrequenz unter Verwendung des HGÜ-Bremswiderstandes entwickelt

Multi-pole voltage source converter HVDC transmission systems

This study connects several modular multilevel converters to form multi-pole voltage source converter highvoltage dc (VSC-HVDC) links which are suited for bulk power evacuation, with increased resiliency to ac and dc network faults. The proposed arrangements resemble symmetrical and asymmetrical HVDC links that can be used for bulk power transfer over long distances with reduced transmission losses, and for the creation of multi-terminal supergrids currently being promoted for transitional dc grids in Europe. The technical feasibility of the proposed systems is assessed using simulations on symmetrical and asymmetrical tri-pole VSC-HVDC links, including the case of permanent pole-to-ground dc faults

An Offshore Wind Farm Featuring Differential Power Processing

Offshore wind farms are a rapidly growing technology used to harvest wind energy on the open seas where wind speeds are significantly higher and steadier than onshore. Current wind farms located far away from shore (e.g., 50 km or more) require a large amount of equipment to be deployed in order to transport generated energy to shore most cost-effectively. In these cases, energy is transmitted to shore using High-Voltage DC (HVDC) transmission connected to wind turbines with AC voltage output. During the past decade, research has studied alternate arrangements to reduce the amount of equipment deployed offshore and increase conversion efficiency. The redesign of offshore collection systems between wind turbines from AC to DC voltages is seen as a key tool to achieve the research objectives. The presented research is focused on the design of offshore wind farms with DC collection system and series-connected wind turbines based on partial power processing converters (PPPCs). This wind farm configuration significantly improves conversion efficiency compared to AC wind farms with HVDC link, since PPPCs are only required to process output power differences among wind turbines in a wind farm to achieve maximum power point (MPP) operation, and other wind farm components are operated at variable operating points, improving low-load efficiency. Furthermore, PPPCs can be of reduced size to realize MPP operation. To find the best variable operating points, a loss minimizing HVDC link current scheduling scheme has been derived and a comprehensive sizing framework was developed to inform the best choice of PPPC ratings. The presented work addresses major design considerations at wind farm, wind turbine, and PPPC levels. An efficiency, size and economic evaluation has been conducted for a 450 MW wind farm located 100km from shore, confirming significant annual loss reductions and economic advantages compared to a conventional AC wind farm with HVDC link, as well as two other series-connected DC wind farm configurations. A generic converter sizing framework for single-string series-connected DC wind farms has been developed and applied to the 450 MW wind farm. Challenges in wind turbine startup with this configuration have been identified and schemes were developed to enable successful wind turbine startup without the need of significant adidtional hardware