Showing papers in "Iet Renewable Power Generation in 2015"

Artificial neural network-based photovoltaic maximum power point tracking techniques: a survey

Abstract: Recent researches oriented to photovoltaic (PV) systems feature booming interest in current decade. For efficiency improvement, maximum power point tracking (MPPT) of PV array output power is mandatory. Although classical MPPT techniques offer simplified structure and implementation, their performance is degraded when compared with artificial intelligence-based techniques especially during partial shading and rapidly changing environmental conditions. Artificial neural network (ANN) algorithms feature several capabilities such as: (i) off-line training, (ii) nonlinear mapping, (iii) high-speed response, (iv) robust operation, (v) less computational effort and (vi) compact solution for multiple-variable problems. Hence, ANN algorithms have been widely applied as PV MPPT techniques. Among various available ANN-based PV MPPT techniques, very limited references gather those techniques as a survey. Neither classification nor comparisons between those competitors exist. Moreover, no detailed analysis of the system performance under those techniques has been previously discussed. This study presents a detailed survey for ANN based PV MPPT techniques. The authors propose new categorisation for ANN PV MPPT techniques based on controller structure and input variables. In addition, a detailed comparison between those techniques from several points of view, such as ANN structure, experimental verification and transient/steady-state performance is presented. Recent references are taken into consideration for update purpose.

Power management of an isolated hybrid AC/DC micro-grid with fuzzy control of battery banks

Abstract: This study focuses on the development of a supervisory control scheme for power management and operation of an isolated hybrid AC/DC micro-grid, which consists of an AC micro-grid and a DC micro-grid. In the proposed hybrid micro-grid, wind and diesel generators and AC loads are connected to the AC micro-grid, whereas photovoltaic array and DC loads are tied to the DC micro-grid. Moreover, the authors consider two independent battery banks in the AC and DC micro-grids. Furthermore, the AC and the DC micro-grids are coupled through a bidirectional converter, which can act as an inverter or rectifier. The objectives of the proposed supervisory controller are listed as follows: (i) maximum utilisation of renewable energy sources along with satisfying the load power demand in both AC and DC micro-grids, (ii) maintaining state of charge (SOC) of battery banks in both AC and DC micro-grids and (iii) managing the power exchange between the AC and the DC micro-grids while the reliability of the whole system is taken into account. The supervisory controller is formalised using a state machine approach. For these purposes, 15 distinct operation modes are considered. Furthermore, in order to extend the battery life cycle, a fuzzy controller manages the desired SOC controlling the charge and discharge currents. The effectiveness of the proposed supervisory controller is evaluated through extensive numerical simulations.

Optimal operation of distributed generations in micro-grids under uncertainties in load and renewable power generation using heuristic algorithm

Abstract: Microgrid (MG) could allow renewable and clean resources to penetrate into a controllable utility and achieve maximum utilisation of existing energy and demand-side management. This study proposes a new paradigm for distribution system operation considering MG conception. This study is focused on probabilistic analysis of optimal power dispatch considering economic aspects in MGs environment with technical constraints. In this study the economic operation of small scale energy zones is formulated and solved as an optimisation problem. A typical MG consists wind turbine (WT), photo voltaic (PV), micro turbine, fuel cell, combined heat and power and electric loads. Fluctuation behaviour of loads and generated power by WTs and PVs are caused complexity in proposed problem. Cost function includes generated powers by units, power transaction between MGs and main grid, operation and maintenance cost of resources and cost of pollutants emission. Considering MG concept in smart grids, the balance between supply-demand is secured through power exchanging between MGs and main grid, so that the value of objective function be minimised. The imperialist competitive algorithm is applied to solve proposed problem and obtained results are compared with Monte Carlo simulation method.

Optimal scheduled power flow for distributed photovoltaic/wind/diesel generators with battery storage system

Abstract: In this study, two control strategies involving ‘continuous’ and ‘ON/OFF’ operation of the diesel generator in the solar photovoltaic (PV)-wind-diesel-battery hybrid systems are modelled. The main purpose of these developed models is to minimise the hybrid system's operation cost while finding the optimal power flow considering the intermittent solar and wind resources, the battery state of charge and the fluctuating load demand . The non-linearity of the load demand, the non-linearity of the diesel generator fuel consumption curve as well as the battery operation limits have been considered in the development of the models. The simulations have been performed using ‘fmincon’ for the continuous operation and ‘intlinprog’ for the ON/OFF operation strategy implemented in Matlab. These models have been applied to two test examples; the simulation results are analysed and compared with the case where the diesel generator is used alone to supply the given load demand. The results show that using the developed PV-diesel-battery optimal operation control models, significant fuel saving can be achieved compared with the case where the diesel is used alone to supply the same load requirements.

Impact of K-factor and active current reduction during fault-ride-through of generating units connected via voltage-sourced converters on power system stability

Abstract: This study investigates the impact of the injection of additional reactive current and of active current reduction during fault-ride-through (FRT) of generating units connected to the grid via fully rated voltage-sourced converter (VSC generating units) on the behaviour and stability of the power system. The primary focus is on voltage support, transient stability and frequency stability. The investigation is carried out via computer simulations using a fictitious transmission system, based on German grid code requirements. The K-factor of the dependence of additional reactive current with voltage deviation, the method of active current limitation during FRT and the rate of active power recovery after fault clearance are varied. Results and influences are illustrated and discussed. They show that with an increasing number of converter-connected generating units in power systems, a careful selection of the parameters becomes very important. Insufficient grid code requirements may increase the risk of system instability. Conclusions are drawn and recommendations for optimal settings with respect to future development of grid codes are provided. The mechanism of loss of synchronism of VSC generating units because of inadequate current injection (current angle instability) is explained.

Design and advanced control strategies of a hybrid energy storage system for the grid integration of wind power generations

Abstract: Energy storage (ES) has become increasingly important in modern power system, whereas no single type of ES element can satisfy all diverse demands simultaneously. This study proposes a hybrid energy storage system (HESS) based on superconducting magnetic energy storage (SMES) and battery because of their complementary characteristics for the grid integration of wind power generations (WPG). This study investigates the mathematical model and the topology of the proposed HESS, which is equipped with a grid-side DC/AC converter, a battery buck/boost converter and a SMES DC chopper. The advanced control strategies comprised of device level and system level are designed. The control strategy for the converters which can be considered as device level is briefly discussed. The significant contribution of this study is proposing a novel system-level control strategy for reasonable and effective power allocation between SMES and battery. According to the control objectives, a fuzzy logic controller optimised with genetic algorithm is adopted. The detailed controller designs are described, meanwhile system stability and HESS operation performance are evaluated. MATLAB simulations are presented to demonstrate the effectiveness of the proposed strategies.

Fuzzy PI controlled inverter for grid interactive renewable energy systems

Abstract: In this study, a fuzzy-PI controlled grid interactive inverter has been designed and implemented. The proportional and integral gains of the PI controller are decided and tuned by the fuzzy logic controller (FLC) according to required operation point of the system. Thus, adaptive nature of the FLC and robust structure of the PI controller are synthesised. Eventually, an adaptive PI controller which can adopt changes because of different operation conditions, grid disturbances and natural effects with fast transient response is obtained. Simulation studies are validated with experimental results. Both simulation and experimental results show that proposed system has fast dynamic response and tracks reference current with a low overshot and short settling time. In addition, the waveform of the inverter output current is sinusoidal and also the current is in same phase and frequency with the line voltage. Furthermore, the total harmonic distortion level of the inverter current meets the international standards. In addition, the proposed inverter system is compared with conventional PI controlled grid interactive inverter with various proportional and integral gains.

Maximum power point tracking method using a modified perturb and observe algorithm for grid connected wind energy conversion systems

Abstract: This paper analyses the performance of maximum power point tracking in a grid connected permanent magnet synchronous generator-based wind energy conversion system using a linear relationship between optimum speed and wind velocity to increase the speed of the controller response. Furthermore, the proposed large step forward and the small step reverse initial tracking improves the accuracy of the existing large step perturb and observe algorithm and is also capable of tracking maximum power for rapidly varying wind conditions. The system consists of back-to-back sinusoidal pulse-width modulation converters. The generator side converter is used to track the maximum wind power using the proposed method through field oriented control. The role of the grid side inverter is to transfer the generated wind power from DC-link to the grid and regulate the DC-link voltage. Moreover, this paper examines the performance of the grid side inverter in grid synchronisation with a non-linear inductive load. Moreover, it analyses the concept of the proposed method using the theoretical aspects. A model for the complete system is developed in MATLAB/SIMULINK and the performance of the proposed control scheme is validated through simulations.

Market value of solar power: Is photovoltaics cost-competitive?

Abstract: This paper reviews the economics of solar power as a source of grid-connected electricity generation. It is widely acknowledged that costs of solar power have declined, but there is disagreement how its economic value should be calculated. `Grid parity', comparing generation costs to the retail price, is an often used yet flawed metric for economic assessment, as it ignores grid fees, levies, and taxes. It also fails to account for the fact that electricity is more valuable at some points in time and at some locations than that at others. A better yardstick than the retail price is solar power's `market value'. This paper explains why, and provides empirical estimates of the solar market value from a literature review, German spot market analysis, and the numerical electricity market model EMMA. At low penetration rates (<;2-5%) solar power's market value turns out to be higher than the average wholesale electricity price - mainly, because the sun tends to shine when electricity demand is high. With increasing penetration, the market value declines - the solar premium turns into a solar penalty. In Germany, the value of solar power has fallen from 133% of the average electricity price to 98% as solar penetration increased from zero to 4.7%. This value drop is steeper than wind power's value drop, because solar generation is more concentrated in time. As a consequence, large-scale solar deployment without subsidies will be more difficult to accomplish than many observers have anticipated.

Review of microgrid architectures – a system of systems perspective

Abstract: To better cope with the challenges posed by the world today, the transition of systems engineering principles to complex, large scale, integrated and evolutionary system of systems (SoSs) has occurred. On the other hand, distributed generation has made its mark in the power distribution system, as a consequence of economic and environmental considerations. With the exponential advancement of technology, unconventional sources of generation, storage and microturbines have been enhanced. The microgrid has paved its way into distributed generation and looks promising for future prospects. A review of microgrid architectures and models is presented in this study. Various control schemes devised for microgrids are also reviewed. The concept of SoSs is introduced and its applications are discussed. A framework is proposed for microgrids from an SoS perspective and control paradigms based on SoS are explained in terms of microgrid control.

Modelling and analysis of modular multilevel converter for solar photovoltaic applications to improve power quality

Abstract: The design of control circuit for a solar fed cascaded multilevel inverter to reduce the number of semiconductor switches is presented in this study The design includes `binary', `trinary' and `modified multilevel connection' (MMC)-based topologies suitable for varying input sources from solar photovoltaic's (PV) In binary mode, 2 Ns+1 - 1 output voltage levels are obtained where N s is the number of individual inverters This is achieved by digital logic functions which includes counters, flip-flops and logic gates In trinary mode, 3 Ns levels are achieved by corresponding look-up table MMC intends design in both control and power circuits to provide corresponding output voltage levels by appropriate switching sequences Hence to obtain a 15-level inverter, the conventional method requires 28 switches and in binary mode 12 switches are needed In trinary mode with the same 12 switches, 27 levels can be obtained whereas in MMC only 7 switches are employed to achieve 15 levels The advantage of these three designs is in the reduction of total harmonic distortion by increasing the levels Simulations are carried out in MATLAB/Simulink and comparisons were made All the three topologies are experimentally investigated for a 3 kWp solar PV plant and power quality indices were measured

System strength considerations in a converter dominated power system

Abstract: The drive towards renewable energy sources (RES) is dramatically changing the dynamic electrical characteristics of Generators, which are traditionally the dominant dynamic component in Power Systems. Non-synchronous generation (NSG) approaching or even exceeding control area demand is now happening on a rapidly expanding scale. The study identifies from National Grid's perspective as Great Britain transmission system operator, the key challenges during periods of operation with high proportion of RES relative to demand. It raises some major questions. What determines system strength in an HVAC system largely without synchronous generators and what is adequate for stability? What levels of %NSG can be expected in the future energy scenarios? What are the financial consequences of constraining RES production if the technical capabilities create an upper limit of %NSG which the system can be operated at? The study finally proposes closer collaboration across the industry to find the optimal missing solutions.

A protection strategy for micro-grids based on positive-sequence component

Abstract: In recent years, the concept of micro-grid has appeared as an appropriate way for the integration of distributed energy resources (DERs) in the distribution networks. However, micro-grids have encountered a number of challenges from control and protection aspects. One of the main issues relevant to the protection of micro-grids is to develop a suitable protection technique which is effective in both grid-connected and stand-alone operation modes. This study presents a micro-grid protection scheme based on positive-sequence component using phasor measurement units and designed microprocessor-based relays (MBRs) along with a digital communication system. The proposed scheme has the ability to protect radial and looped micro-grids against different types of faults with the capability of single-phase tripping. Furthermore, since the MBRs are capable of updating their pickup values (upstream and downstream equivalent positive-sequence impedances of each line) after the first change in the micro-grid configuration (such as transferring from grid-connected to islanded mode and or disconnection of a line, bus or DER either in grid-connected mode or in islanded mode), they can protect micro-grid lines and buses against subsequent faults. Finally, in order to verify the effectiveness of the suggested scheme and the designed MBR, several simulations have been undertaken using DIgSILENT PowerFactory and MATLAB software packages.

Automated on-line fault prognosis for wind turbine pitch systems using supervisory control and data acquisition.

Abstract: Current wind turbine (WT) studies focus on improving their reliability and reducing the cost of energy, particularly when WTs are operated offshore. A supervisory control and data acquisition (SCADA) system is a standard installation on larger WTs, monitoring all major WT sub-assemblies and providing important information. Ideally, a WT's health condition or state of the components can be deduced through rigorous analysis of SCADA data. Several programmes have been made for that purposes; however, the resulting cost savings are limited because of the data complexity and relatively low number of failures that can be easily detected in early stages. This study proposes a new method for analysing WT SCADA data by using an a priori knowledge-based adaptive neuro-fuzzy inference system with the aim to achieve automated detection of significant pitch faults. The proposed approach has been applied to the pitch data of two different designs of 26 variable pitch, variable speed and 22 variable pitch, fixed speed WTs, with two different types of SCADA system, demonstrating the adaptability of the approach for application to a variety of techniques. Results are evaluated using confusion matrix analysis and a comparison study of the two tests is addressed to draw conclusions.

Scenarios for use of floating photovoltaic plants in Brazilian reservoirs

Abstract: The conversion efficiency of a crystalline silicon photovoltaic (PV) module increases as its temperature decreases. Considering this characteristic of the PV technology, in this study the authors analyse the use of cooled floating PV modules in a fraction of the area occupied by three reservoirs – Castanhao, Oros and Banabuiu – in the Brazilian semi-arid region. Comparing with conventional ground-mounted PV modules, an average conversion efficiency increase of 12.5% was measured for floating PV modules under the local weather conditions. In a second phase, measurements are used as input data in the PVSYST software in conjunction with a spreadsheet for the construction of electricity production scenarios. According to these scenarios, the potential PV production in the mentioned reservoirs would supply 7.4 and 18.8% of the electricity demand of Brazil's Ceara state (8.84 million inhabitants) and its largest city Fortaleza (2.6 million inhabitants), respectively.

Development of a control strategy for interconnection of islanded direct current microgrids

Abstract: This study presents a control strategy for interconnection of a cluster of direct current (DC) microgrids operating at different grid voltages in islanded mode. In this study, two neighbouring DC microgrids (DCMGs) are interconnected through a bidirectional DC–DC converter (BDC) and DC cable. The BDC has been placed near to the low-voltage DCMG. The DCMGs consist of wind turbines, solar-photovoltaic systems, solid oxide fuel cell, micro-turbine generators, battery energy storage systems and varying DC and three-phase as well as single-phase alternating current loads. The proposed control strategy of the BDC is aimed at managing the bidirectional flow of power between the DCMGs to balance the power under various operating scenarios and fault conditions, while maintaining constant DC voltages of both the DCMGs. Simulations are carried out to verify the robustness of the proposed control strategy under different operating conditions including fault scenario.

Energy models for photovoltaic systems under partial shading conditions: a comprehensive review

Abstract: The partial shading phenomenon and its implications on the electrical response and energy yield of photovoltaic (PV) systems have received increased attention in the last years In order to study, foresee and mitigate such effects, several energy models are proposed in the bibliography, presenting different degrees of complexity, accuracy and applicability This study presents an overview of the state of the art in the development of models for PV systems under partial shading conditions Alternative modelling approaches are analysed, highlighting their advantages and shortcomings and models available in the literature are reviewed and classified according to important attributes, related to their accuracy and implementability Current research trends, as well as topics that warrant further investigation, are identified and discussed

Modified particle swarm optimisation technique for optimal design of small renewable energy system supplying a specific load at Mansoura University

Abstract: Recently, a special attention has been attributed to the renewable energy in Egypt. Optimal sizing of small renewable energy system has a very important role in the use of renewable energy effectively and economically. Particle swarm optimisation (PSO) is a popular stochastic optimisation method that has found in wide applications. Conventional PSO suffers from high computational complexity and slow convergence speed. This study presents a modified PSO (MPSO) technique to optimise the capacity sizes of different components of hybrid PV/wind/battery power generation system for supplying communication and information technology centre in Mansoura University-Egypt. A feasibility study for two options is investigated; stand-alone system composed of PV/wind/battery combination and a grid connected PV/wind system. The proposed MPSO technique proves faster convergence speed and shorter computational time as compared with conventional techniques.

Transient stability enhancement of a grid-connected wind farm using an adaptive neuro-fuzzy controlled-flywheel energy storage system

Abstract: With the rapid growth of the wind energy systems in the past years and their interconnection with the existing power system networks, it has become very significant to analyse and enhance the transient stability of the wind energy conversion systems connected to the grid. This study investigates the transient stability enhancement of a grid-connected wind farm using doubly-fed induction machine-based flywheel energy storage system. A cascaded adaptive neuro-fuzzy controller (ANFC) is introduced to control the insulated gate bipolar transistor switches-based frequency converter to enhance the transient stability of the grid-connected wind farm. The performance of the proposed control strategy is analysed under a severe symmetrical fault condition on both a single-machine infinite bus model and the IEEE-39 bus New England test system. The transient performance of the system is investigated by comparing the results of the system using the proposed ANFCs with that of the black-box optimisation technique-based proportional–integral controllers. The validity of the system is verified by the simulation results which are carried out using PSCAD/EMTDC environment.

Power quality surveys of photovoltaic power plants: characterisation and analysis of grid-code requirements

Abstract: In the past few years, grid-code requirements for grid-connected photovoltaic power plants have experienced a continuous evolution in different countries to ensure a reliable power system operation as the level of renewable energy penetration increases to high levels. According to several European grid-codes, PV power plants must be able to ride through specific and severe disturbances without disconnections. Under this new framework, the present study analyses intensive power quality surveys carried out from 2008 to 2011 in three different Spanish PV power plants: a fixed array installation with 4 MW PV power capacity, a PV power plant including dual axis-trackers with 1 MW PV power capacity, and one more fixed array PV power plant with 5 MW PV power capacity. Voltage dips and supply interruptions have been collected, discussing several methods to characterise the monitored disturbances and to compare these events to current Grid-Code requirements. Furthermore, the time interval around the residual voltage is proposed and defined by the authors as an additional parameter to provide a complete characterisation of the severity of the disturbances. Results from both characterisations of collected data and comparison with current requirements are also included in the study.

Optimal sizing approach for islanded microgrids

Abstract: This study proposes a single-objective optimal sizing approach for an islanded microgrid (IMG). The approach determines the optimal component sizes for the IMG, such that the life-cycle cost is minimised while a low loss of power supply probability (LPSP) is ensured. As wind speed and solar irradiation exhibit both diurnal and seasonal variations, the proposed algorithm takes advantages of the typical meteorological year-based chronological simulation and enumeration-based iterative techniques. The mathematical models presented in this study for the IMG components consider the non-linear characteristics as well as the reactive power. The LPSP is also formulated based on the supply-demand balances of both real and reactive powers, and an economic evaluation model is presented. The proposed sizing approach identifies the global minimum, and simultaneously provides the optimal component sizes as well as the power management strategies. This study also presents a number of sensitivity analyses as well as comparisons with a commercial software package.

Optimal control method for wind farm to support temporary primary frequency control with minimised wind energy cost

Abstract: This study proposes an optimal control method for variable speed wind turbines (VSWTs) based wind farm (WF) to support temporary primary frequency control. This control method consists of two layers: temporary frequency support control (TFSC) of the VSWT, and temporary support power optimal dispatch (TSPOD) of the WF. With TFSC, the VSWT could temporarily provide extra power to support system frequency under varying and wide-range wind speed. In the WF control centre, TSPOD optimally dispatches the frequency support power orders to the VSWTs that operate under different wind speeds, minimises the wind energy cost of frequency support, and satisfies the support capabilities of the VSWTs. The effectiveness of the whole control method is verified in the IEEE-RTS built in MATLAB\Simulink, and compared with a published de-loading method.

Multi-task control strategy for grid-tied inverters based on conservative power theory

Abstract: In recent years, the concept of decentralizing power generation through the deployment of distributed generators (DGs) has been widely accepted and applied, driven by the growing market of renewable energy sources, in particular photovoltaic, wind and small hydro. These distributed generators are normally equipped with a switching power interface (inverter), acting as front end with the grid. In this scenario this paper proposes a multi-task control strategy for distributed generation inverters that simultaneously allows the DG system to inject the available energy, as well as to work as a voltage drop compensator or as an active power filter, mitigating load current disturbances and improving power quality of the grid. The main contribution of the proposed system, with respect to other solutions in the literature, is that the proposed control loops are based on the Conservative Power Theory decompositions. This choice provides decoupled power and current references for the inverter control, offering a very flexible, selective and powerful control strategy for the DG system. The paper also discusses the choice of the current waveform for injecting/absorbing active power into/from the grid, and both sinusoidal and resistive references have been compared in terms of damping capability. Finally, simulation and experimental results are provided in order to validate the proposed functionalities of the DG control system.

Improved control strategy for the three-phase grid-connected inverter

Abstract: An improved control strategy for the three-phase grid-connected inverter with space vector pulse-width modulation (SVPWM) is proposed. When the grid current contains harmonics, the d - and q -axis grid currents will be interacted, and then the waveform quality of the grid current will be poorer. As the reference output voltage cannot directly reflect the change of the reference grid current, the dynamic response of the grid-connected inverter is slow. In order to solve the aforementioned problems, the d - and q -axis grid currents in the decoupled components of the grid current controller can be substituted by the d - and q -axis reference grid currents, respectively. The operating principles of the traditional and proposed control methods are illustrated. Experimental results for a 15-kVA three-phase grid-connected inverter with SVPWM verify the theoretical analysis. Compared with the traditional control strategy, the grid-connected inverter with the improved control strategy has a high waveform quality of the grid current, small ripple power and fast dynamic response.

Voltage security in AC microgrids: a power flow-based approach considering droop-controlled inverters

Abstract: This study deals with the problem of voltage security in microgrids. In general, voltage security is an issue for power systems, where the lack of reactive power is a concern. For microgrids a complexity is imposed, since frequency and voltage level deviations may take place by the operation of droop-controlled inverters. This study incorporates the load margin calculation into a microgrid structure. For this purpose, a special power flow program is developed in order to consider the effects of the droop-controlled inverters. Simulation results are obtained with the help of an islanded distribution system with only generators coupled by droop-controlled inverters, so the proposed methodology may be tested and discussed.

Economic analysis of condition monitoring systems for offshore wind turbine sub-systems

Abstract: The use of condition monitoring systems on offshore wind turbines has increased dramatically in recent times. However, their use is mostly restricted to vibration based monitoring systems for the gearbox, generator and drive train. A survey of commercially available condition monitoring systems and their associated costs has been completed for the blades, drive train, tower and foundation. This paper considers what value can be obtained from integrating these additional systems into the maintenance plan. This is achieved by running simulations on an operations and maintenance model for a wind farm over a 20 year life cycle. The model uses Hidden Markov Models to represent both the actual system state and the observed condition monitoring state. The CM systems are modelled to include reduced failure types, false alarms, detection rates and 6 month failure warnings. The costs for system failures are derived, as are possible reductions in costs due to early detection. The detection capabilities of the CM systems are investigated and the effects on operational costs are examined. Likewise, the number of failures detected 6 months in advance by the CM systems is modified and the costs reported.

Transient stability of power system integrated with doubly fed induction generator wind farms

Abstract: A new quantitative assessment of transient stability for power systems integrated with doubly fed induction generator (DFIG) wind farms is proposed by evaluating the transient energy margin (TEM) through the formulation of the transient energy function (TEF) for multimachine systems. To achieve an accurate TEM, the TEF is modified to account for the separation of the critical machines from the system and an unstable equilibrium point is calculated on the basis of post-fault trajectory reaching the potential energy boundary surface. Simulation results show that such power systems integrated with DFIG wind farms are more sensitive to transient events of higher voltage sag, longer fault clearing time, lower load operation and higher wind power penetration level. It is also observed that machines located far from the fault are also exposed to inferior transient stability because of fault with geographical dispersion of wind farms. As a result, advanced switchgear, faster isolators, more efficient power reserve systems and advanced reactive power compensating devices must be equipped to ensure reliable operation of power systems integrated with the DFIG wind farms during transient events.

Frequency response estimation method for high wind penetration considering wind turbine frequency support functions

Abstract: To achieve renewable energy targets, more wind turbine generators (WTGs) are being integrated into many power networks around the world. However, unlike traditional synchronous generators, modern WTGs are driven by power electronic devices which provide almost zero inertia and frequency response to frequency related events such as generator tripping. Wind manufacturers and researchers have been working on synthetic WTG inertia support, which can better utilise the rotational nature of WTGs. However, synthetic inertia alone may be insufficient under certain circumstances to prevent automatic under frequency load shedding after generation tripping, which causes security concerns for network operation. Consequently, WTG active power control (APC, similar to governor control) should be activated to improve network security. However, APC will costly reduce WTG output from the maximum power point and result in financial concerns. Therefore when and how much APC service should be activated becomes an extremely important question, which has not been addressed in the literature. This study develops a new method to quickly estimate frequency response caused by generator tripping hence system operators can use this proposed method to continuously evaluate inertia and headroom competency and accordingly activate the amount of WTG inertia and APC required for reliable system operation.

Characterisation of large disturbance rotor angle and voltage stability in interconnected power networks with distributed wind generation

Abstract: Wind generation in highly interconnected power networks creates local and centralised stability issues based on their proximity to conventional synchronous generators and load centres. This study examines the large disturbance stability issues (i.e. rotor angle and voltage stability) in power networks with geographically distributed wind resources in the context of a number of dispatch scenarios based on profiles of historical wind generation for a real power network. Stability issues have been analysed using novel stability indices developed from dynamic characteristics of wind generation. The results of this study show that localised stability issues worsen when significant penetration of both conventional and wind generation is present because of their non-complementary characteristics. In contrast, network stability improves when either high penetration of wind and synchronous generation is present in the network. Therefore network regions can be clustered into two distinct stability groups (i.e. superior stability and inferior stability regions). Network stability improves when a voltage control strategy is implemented at wind farms, however both stability clusters remain unchanged irrespective of change in the control strategy. Moreover, this study has shown that the enhanced fault ride-through strategy for wind farms can improve both voltage and rotor angle stability locally, but only a marginal improvement is evident in neighbouring regions.

Coordinated control strategy for doubly-fed induction generator with DC connection topology

Abstract: With the development of wind power generation system and requirement of large-scale and long-distance power transmission, it is greatly important for wind farm to achieve the high effective DC grid connection technology. This study presents a novel grid connection topology for doubly-fed induction generator (DFIG) connected to a DC-link, in which a stator side converter and rotor side converter fed by a common DC bus are employed. Coordinated control method for the converters is proposed to regulate the electromagnetic torque directly based on the air-gap flux orientation technique. Compared with the traditional grid connection system, the proposed system has a lot of advantages such as simpler structure, lower requirement for the converter capacity and higher power transmission efficiency. The power distribution between stator side converter and rotor side converter is analysed based on the proposed grid connection topology. Finally, DFIG-based experimental system is developed to validate the correctness and availability of the proposed DC grid connection topology and control strategy.