Provision of frequency support by offshore wind farms connected via HVDC links

The high penetration levels of wind power will obligate wind farms to contribute to the mitigation of frequency drops. Comprehensive case studies are presented to investigate the different methods of frequency support provision by wind power. The implemented test system is composed of an offshore wind farm connected to an external grid through a point-to-point HVDC link. Three different frequency support methods are compared; droop de-loading, battery storage banks and a mix between the two methods. Moreover, two different methods of sensing the frequency drop, at the point of common coupling, by the wind farm are examined. The impact of the HVDC is highlighted, especially its role to transmit the power surge provided by the wind farm. A modified de-loading controller is developed and integrated to all the wind turbines, according to the executed case studies. Results show that the proposed frequency support solutions have almost similar impact on the natural frequency response at the point of common coupling. The HVDC link does not worsen the frequency response, and the fluctuations in voltage levels at onshore and offshore buses are very minor. DIGsILENT PowerFactory is integrated as a simulation environment

Stability studies of different AC collection network topologies in wind farms connected to weak grids

In this paper, the stability studies for different wind farm collection network topologies have been performed. As the wind farm becomes larger, the inter-array network becomes larger.so that the impedance of the overall system will be increased. This means that the inter-array configuration can impact the stability of system. The dynamic studies results presented in this paper show that the star collection network topology has the ability to be connected to weaker grid followed by radial, double sided and single side ring collection network topology

Frequency stability analysis in low frequency AC systems for renewables power transmission

The foreseen high penetration levels of wind energy will have serious implications on frequency stability, hence developed control methods of wind turbine and alternative technologies including energy storage should enable the provision of frequency support by wind power. Active research is ongoing to investigate the possibility of collecting and transmitting offshore wind power through low frequency alternating current systems (LFAC). This paper develops a novel method to enhance frequency support capability of generators connected to a LFAC system. The leveraged frequency regulation ability of the generators at LFAC system is emphasized. The voltage is proportional to the frequency of the LFAC system, so that the transformers can be protected when frequency drops. Then the generators at LFAC system acquire sufficient time for frequency regulation. In this paper, a hydro generator at LFAC system is regarded as an energy storage plant, which is connected to the LFAC system rather than normal frequency grid

Comparison between flying capacitor and modular multilevel inverter

The paper describes the operational principle of flying capacitor and modular multilevel inverters. The detailed discussions of dc link capacitors voltage balancing methods for both inverters are given in order to enable fair comparison. The causes of dc link capacitors voltage imbalance in flying capacitor multilevel inverter with more than three levels are highlighted. Computer simulation is used to compare the performance of both inverters under several operating conditions

Comparison between two VSC-HVDC transmission systems technologies : modular and neutral point clamped multilevel converter

The paper presents a detail comparison between two voltage source converter high voltage dc transmission systems, the first is based on neutral point-clamped (also known as HVDC-Light) and the second is based on innovative modular multilevel converter (known as HVDC-Plus). The comparison focuses on the reliability issues of both technologies such as fault ride-through capability and control flexibility. To address these issues, neutral point-clamped and three-level modular converters are considered in both stations of the dc transmission system, and several operating conditions are considered, including, symmetrical and asymmetrical faults. Computer simulation in Matlab-Simulink environment has been used to confirm the validity of the results

Wind turbine control design to enhance the fault ride-through capability

This paper presents a control strategy for wind turbines to enhance their fault ride-through capability. The controller design is based on pitch controlled variable speed wind turbine equipped with doubly-fed induction generator (DFIG). The fault ride-through is realized by injecting a crowbar with variable resistance on the generator rotor circuit. To reduce the mechanical loads induced by grid faults, the wind turbine controller is improved by means of filtering techniques which alleviate the loads on the turbine blades and drive-train. The performance of the control strategy is tested by simulation. It is shown that the combined mechanical and electrical controller design significantly improves the wind turbine fault ride-through capability

Assessment of the last-in-first out principle of access for managing the connection of distributed wind generators

Recent projects in the UK have investigated different connection arrangements for managing distributed wind generators to maintain thermal limits and a number of principle of access for generators to the limited distribution network capacity have been investigated. However, principle of access to manage voltage limits have not received as much attention. This study aims to evaluate the current practice for connecting ‘non-firm’ distributed wind generators under both voltage and thermal constraint conditions. It addresses the issue by developing a representative model of a UK 11kV radial distribution feeder comprising a mix of urban and rural sections using time-step optimal power flow simulations. The results indicated that when the principle is applied under both network constraint conditions, it can lead to inefficient use of network capacities and reduced renewable energy yields

Generator response following as a primary frequency response control strategy for VSC-HVDC connected offshore wind farms

The present study attempts to collect relevant research on the subject of synthetic inertia control strategies for VSC-HVDC transmission links, particularly those connected to offshore windfarms. A number of ideas have been proposed in literature. First, various control strategies at the grid side converter interfacing the DC link with the AC power system are presented. This includes strategies exploiting the power-frequency relationship that naturally exists in AC systems with a high X/R ratio. Other strategies utilize the voltage-frequency relationship that exists when the DC link capacitor is asked to provide active power injection or absorption in response to frequency deviations. Then some coordinated strategies are outlined which build upon and combine other strategies (including those associated with traditional synchronous machines) in order to enhance the operational capability of the decoupled non-synchronous system with respect to synthetic inertia services. Some options for communication are also identified

Fulfilment of grid code obligations by large offshore wind farms clusters connected via HVDC corridors

The foreseen high penetration levels of wind power will force the systems operators to apply restrictive constraints on wind power plants. The ability of offshore wind clusters, which are connected via HVDC, to fulfill the grid codes, especially those related to voltage stability is investigated. This came in the frame of a project to develop an integrated and practical tool to design offshore wind clusters (EERA-DTOC). The applied case studies examine the system stability during and after severe disturbances, and the compliance with the grid codes. Additionally, this paper explains the applied procedure to utilize the outcomes of Net-OP tool, which proposes an optimized topology to connect the wind power clusters to the interconnected power systems. The integrated simulation environment, namely, PSS/E, is used to implement a highly detailed and dynamic model based on the recommendations of Net-OP tool. The results confirm that wind farm clusters respond to faults and disturbances as desired by the grid codes

Ride-through-fault capabilities of DFIG wind farm connected to a VSC station during a DC fault

This work analyzes the capabilities of ride-through-fault (RTF) of a double-fed induction machine (DFIG), -based wind farm connected to a VSC-, in case of a fault in the DC side of the VSC. The modelling of the DFIG wind turbine takes into account key aspects that intervene in succeeding the fault ride-through such as the power capacity of its electronic converter and its control scheme. As result a better understanding of the behaviour and capabilities of such system to perform a successful RTF is achieved. This work also analyzes the behaviour of the momentary islanding of the system composed by the wind farm and the ac filters of the VSC station. Using that information, a novel control scheme for the offshore VSC station is proposed in order to enhance RTF capabilities of the wind farm in case of such DC faults