Wind Farm Connected to a Distribution Network

This chapter presents power flow study for distribution network connected to wind farm based on induction generators (IG). It provides an overview of wind energy conversion systems (WECS) and their related technologies. The details of turbine components, system configurations, and control schemes are analyzed. Wind farm–distribution network systems are developed by MATLAB/SIMULINK to perform different tests under various operation conditions. The impact of wind speed fluctuation on power flow, grid voltage dynamic stability, and frequency responses are also investigated. The proposed simulator is applied to a 3 MW wind farm, and then the efficacy of the proposed simulator has been validated

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

Grid Voltage Unbalance and The Integration of DFIG’s

Double-fed induction generators (DFIG’s) became the predominant generator installed for wind generation applications in the mid 1990’s. Issues pertaining to the operation and control of DFIG’s subsequently became apparent, particularly in weak areas of the grid network. Ironically weak areas of the grid tend to be where the average wind speed is high and the usual location of wind farms. One of the issues that emerged was the quality of the voltage in the network at the point of common coupling (PCC) with the DFIG’s. An important issue is the question of voltage unbalance at the PCC. As part of this work, research was undertaken into the issue of voltage unbalance in a distribution network. Investigative studies were undertaken on a small wind farm connected to the Irish distribution network. The results obtained were then analysed and conclusions drawn, with recording of daily, weekly and seasonal variation of voltage unbalance. The behaviour of DFIG’s to varying levels of network voltage unbalance at the wind farm was analysed, and it was observed that the DFIG’s had difficulty remaining connected to the distribution network when voltage unbalance exceeded certain threshold levels. The behaviour of DFIG’s to the effects of grid network voltage unbalance is further investigated in this work. A literature review was undertaken of the effects that utility network voltage unbalance has on DFIG’s. Emerging from this research, the suitability of appropriate control schemes to alleviate the problems caused by grid voltage unbalance were investigated. Control techniques to improve performance of a DFIG during conditions of asymmetrical grid voltage including measures to control the rotorside and grid-side converters in a DFIG, were designed and then implemented in Matlab/Simulink and results showed improved behaviour. A synchronous generator system was similarly investigated and improvements shown. This research also includes development of a laboratory based DFIG test system. A DSP based digital microcontroller and interfacing hardware has been developed for a 5kVA DFIG laboratory based system. The system comprises of a machine set; a dc machine with common shaft coupling to a three-phase wound rotor induction machine. The dc machine emulates a wind turbine, and drives the induction machine in response to required speed. A converter has been constructed to control the rotor power of the induction machine. Interfacing schemes for the required feedback signals including voltage and current transducers and speed measurement were designed to enable control of both the rotor-side and grid-side converters of the DFIG. Grid/stator voltage oriented control is implemented to control both the rotor side and grid side converters respectively. An additional feature is the implementation of a single DSP controller, configured to control both the rotor side and grid side converters simultaneously. Initially the DFIG test rig was tested as a standalone system, with a load bank connected to the stator terminals of the induction machine. Testing of the DFIG was also conducted with the test rig connected directly to the grid, and the system operated in subsynchronous and super-synchronous modes of operation. Hardware and software solutions were implemented to reasonable success. The laboratory based test rig has been designed for operation as a rotor converter for a DFIG; however the converter can also be configured to operate as a system for a synchronous generator, or for operation as a machine drive. Further research may allow the rig to be used as a DFIG/UPQC (unified power quality controller) test bed

Coordinated control and network integration of wave power farms

Significant progress has been made in the development of wave energy converters (WECs) during recent years, with prototypes and farms of WECs being installed in different parts of the world. With increasing sizes of individual WECs and farms, it becomes necessary to consider the impacts of connecting these to the electricity network and to investigate means by which these impacts may be mitigated. The time-varying and the unpredictable nature of the power generated from wave power farms supplemented by the weak networks to which most of these farms will be connected to, makes the question of integrating a large quantity of wave power to the network more challenging. The work reported here focuses on the fluctuations in the rms-voltage introduced by the connection of wave power farms. Two means to reduce these rms-voltage fluctuations are proposed. In the first method, the physical placement of the WECs within a farm is selected prior to the development of the farm to reduce the fluctuations in the net real power generated. It is shown that spacing the WECs or the line of WECs within a farm at a distance greater than half the peak wavelength and orienting the farm at 90◦ to the dominant wave direction produces a much smoother power output. The appropriateness of the following conclusions has been tested and proven for a wave power farm developed off the Outer Hebrides, using real wave field and network data. The second method uses intelligent reactive power control algorithms, which have already been tested with wind and hydro power systems, to reduce voltage fluctuations. The application of these intelligent control methods to a 6 MW wave power farm connected to a realistic UK distribution network verified that these approaches improve the voltage profile of the distribution network and help the connection of larger farms to the network, without any need for network management or upgrades. Using these control methods ensured the connection of the wave power farm to the network for longer than when the conventional control methods are used, which is economically beneficial for the wave power farm developer. The use of such intelligent voltage - reactive power (volt/VAr) control methods with the wave power farm significantly affects the operation of other onshore voltage control devices found prior to the connection of the farm. Thus, it is essential that the control of the farm and the onshore control devices are coordinated. A voltage estimation method, which uses a one-step-ahead demand predictor, is used to sense the voltage downstream of the substation at the bus where the farm is connected. The estimator uses only measurements made at the substation and historical demand data. The estimation method is applied to identify the operating mode of a wave power farm connected to a generic 11 kV distribution network in the UK from the upstream substation. The developed method introduced an additional level of control and can be used at rural substations to optimise the operation of the network, without any new addition of measuring devices or communication means

Short circuit study of fixed speed wind turbines with STATCOM in distribution networks

The increased penetration of wind farm in distribution networks has brought changes in the performance of the whole system. Such disadvantages when connecting one of these distributed generation sources is reduced voltage and power stability of the AC network. This phenomena can cause the connected electricity consumers to suffer from disturbances. This paper investigates the use of a static synchronous compensator (STATCOM) to improve the short circuit current contribution in the network which will include balanced and unbalanced faults. The wind farm is equipped with fixed-speed wind turbines driving squirrel-cage induction generators. The IEEE 30-bus distribution test system is used to see the performance of the system under distribution level. Simulation studies are carried out in the DIgSILENT software

An assessment of principles of access for wind generation curtailment in active network management schemes

The growth of wind generation embedded in distribution networks is leading to the development and implementation of Active Network Management (ANM) strategies. These aim to increase the capacity of Distributed Generation (DG) that can connect to a network. One such ANM strategy is generation curtailment where DG is given a non-firm connection under which the network can instruct a generator to reduce its output under specified conditions. Currently in the UK the Orkney distribution network operates a curtailment scheme for wind and other renewable generation [1]and a similar scheme is being developed for the Shetland Islands [2]. The main objective of this paper is to explore the options for Principles of Access (PoA) for curtailment of wind generation on distribution networks which employ ANM. The PoA define the commercial rules by which a DG unit obtains access to the distribution network and under an ANM curtailment scheme the PoA defines the curtailment instructions that would be sent to different DG units when network constraints occur. The scenarios studied in this paper are based on the Orkney distribution network

Using dynamic optimal power flow to inform the design and operation of active network management schemes

Active Network Management (ANM) schemes are providing the communications and control infrastructure to allow the integration of energy storage and flexible demand in distribution networks. These technologies can be characterised as intertemporal in that their operation at different points in time is linked. This paper provides a discussion of the issues created when optimising an ANM scheme containing intertemporal energy technologies. A technique called Dynamic Optimal Power Flow is discussed and a case study is presented. The requirement to use forecasts of renewable energy resources such as wind power is discussed together with the issues that this creates

Impact of increased distributed generation on fault levels and the network equipment

Attention towards the development of Distributed Generation (DG) is globally high. UK's concentration is to use sustainable and renewable sources to generate electricity thereby protecting the environment from getting polluted. In this paper, Distributed Generation concept is explored regarding the types, sizes, ratings, locations and benefits. It is very important to connect the developed distributed generator to the distribution network to satisfy the demand but without developing any faults and causing damage to the existing switchgear. Legal contracts that are required to be signed in order to connect the DG to the network and proper Protection Review to be followed in suppressing the faults are also mentioned. To study the impact of increased DG on fault levels, simulation approach is adopted. ETAP 7.1.0 is used to build a case study network and the results obtained are analyzed based on which critical comparison is made. The network comprises of three wind farms, each consisting of six wind turbines and a small hydroelectric plant. Various scenarios are considered and the results obtained are clearly analyzed. Protection is provided for the circuit and Star Device coordination study is performed. © JES 2014

Cost and losses associated with offshore wind farm collection networks which centralise the turbine power electronic converters

Costs and losses have been calculated for several different network topologies, which centralise the turbine power electronic converters, in order to improve access for maintenance. These are divided into star topologies, where each turbine is connected individually to its own converter on a platform housing many converters, and cluster topologies, where multiple turbines are connected through a single large converter. Both AC and DC topologies were considered, along with standard string topologies for comparison. Star and cluster topologies were both found to have higher costs and losses than the string topology. In the case of the star topology, this is due to the longer cable length and higher component count. In the case of the cluster topology, this is due to the reduced energy capture from controlling turbine speeds in clusters rather than individually. DC topologies were generally found to have a lower cost and loss than AC, but the fact that the converters are not commercially available makes this advantage less certain