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All-pass-filter-based active damping for VSCs with LCL filters connected to weak grids

All-pass-filter-based active damping for VSCs with LCL filters connected to weak grids

This paper presents a single-loop control strategy based on all-pass filters to provide active damping to VSCs with LCL filters connected to weak grids. First of all, it will be shown that active damping can be provided at the design stage if the design constraints allow it. However, when this is not possible, an all-pass filter in series with the current controller is used with this purpose. Two alternatives to implement this filter are tested: a first- and a second-order all-pass filter. With this addition a classical PI controller can be easily designed to control the grid-side current of the filter. It will be shown that this strategy provides large stability margins and fast transient responses. The proposed controllers will be compared with three active damping alternatives commonly applied in the literature. All the control system techniques proposed in this paper and the comparative analysis are verified in a 15 kVA prototype of a VSC with a LCL filter. A shorter version of this paper was presented in [26].

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Analysis of integration of multi-terminal HVDC network to weak grids

Analysis of integration of multi-terminal HVDC network to weak grids

In this paper, a three terminal MMC based MTDC system connected to weak grids is studied. The study has shown that control of MTDC voltage is very important to ensure the stable operation of the system. Control of DC voltage of MTDC network should be performed from the MMC station connected to the strongest AC system. The simulation results have also shown that the MTDC system connected to the weak and very weak grid, can ride-through the unbalanced fault as long as the power transfer limits of the system are considered.

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Current error based compensations for VSC current control in weak grids for wind farm applications

Current error based compensations for VSC current control in weak grids for wind farm applications

synchronization control [11], etc., which mimic the behavior and control scheme of synchronous generators when they are integrated into the grid. This type of control is based on the fact that a properly controlled synchronous machine can well generate bulk power from weak grid point, which has been studied for decades. Similar to synchronous generator control, closed-loop power control is performed by directly controlling the modulation voltage angle and magnitude of the converter voltage. In this case, the employment of Phased Locked Loop, which is widely used in vector control for angle detection, can be by-passed from the closed-loop control during steady state operation [11]. These methods enable VSC to output full power from a very weak grid point and work well in steady state operation. However, the absence of current loop in such method can potentially give rise to extra current variations during large perturbations. An extreme case is that during an AC fault, virtual synchronous generator and the power synchronization control themselves will not be able to limit the fault current and it has to be switched to a current-loop based control mode with a back-up PLL [11]. Such non-linear mode switching scheme increases the complexity of VSC control and one consequent problem is that it would be difficult to determine where the switching point should be set to avoid undesirable mode switches, especially when unpredictable perturbations, voltage fluctuations or fault, occur when VSCs are operating close to their rated power/current. As another attempt of the virtual synchronous machine concept, the control strategy of Virtual Synchronous Machine (VISMA) presented in Ref [23] employs a special designed outer loop in addition to a hysteresis-based inner current loop in abc reference frame. However, the presented method was not tested to demonstrate satisfactory performance under both full-power steady-state and fault-ride- through conditions in weak grids.

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

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

It is very important to have a high level reliability in the offshore application as the repair downtime in the offshore environment is higher than that in onshore[10]. So the reliability is improved by adding redundancy to the power flow paths. Collection grids in windfarms has the same architecture as the distribution system[10] so the collection network can be can be either AC or DC [7]. Different AC cluster configurations can be divided into three different groups [6, 7]: 1. Star

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Power Quality Improvement In Weak Grids With      Three Phase Three Leg Active Power Conditioner

Power Quality Improvement In Weak Grids With Three Phase Three Leg Active Power Conditioner

Abstract—This paper presents a three phase three leg active power conditioner to improve power quality based on renewable energy. A micro-grid is a weak electrical grid, which can be easily subject to disturbances because it includes a variety of intermittent power sources, single-phase, three-phase and nonlinear loads, which can have a dynamic impact on power quality. The unbalance and the harmonic components in current and voltage waveforms are the most important among these. In this paper, the topology and the control strategy of Active Power Conditioner (APC) have been investigated in order to improve the operation of a micro-grid. The APC topology presented in this paper act as an interface between renewable energy sources and the AC bus of a micro-grid. They also use an improved control strategy, which makes possible to inject energy in the micro-grid, compensate the current harmonics and correct the power factor. Moreover, the proposed control strategy allows the line current at the point of common coupling (PCC) to be balanced and sinusoidal even when the load is unbalanced. Consequently, the voltage at the PCC becomes balanced. The simulation results are grouped and presented according to the following power quality indicators: THD (Total Harmonic Distortion), power factor and unbalanced load. Keywords— Active power conditioner, micro-grids, renewable energy, current control, power quality.

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Simulation Studies on Integration of Voltage Source Converters (VSCs) In Weak Grids
SK Parveen, V Haribabu & Dr M Bala Subba Reddy

Simulation Studies on Integration of Voltage Source Converters (VSCs) In Weak Grids SK Parveen, V Haribabu & Dr M Bala Subba Reddy

reconnects a DG unit to the main grid after a special time period (usually 1 s). This is due to the fact that most of faults are cleared after few cycles. In this case, connection occurs without synchronization which may lead to severe transients as a result of frequency and angle mismatch of both sides of the reclose at the moment of connection. Weak grids suffer more from the resynchronization transients due to the fact that load angle is inherently large and after grid restoration it may easily move to the nonlinear region and even pass where instability is expected. Fig. 4.4(a) and (b) shows the corresponding waveforms and clearly shows that the system with nonlinear controller provides smooth and fast grid connection. This excellent performance occurs under the fact that there is about 0.9 Hz frequency mismatch between the grid and VSC, and the reference power is equal to 6.8 MW corresponding a the load angle 1.32 rad. The system response without using the supplementary controller is demonstrated in Fig. 4.4(c), which shows that the weak grid conditions cause instability. The current waveform of the system with supplementary control is presented in Fig.4.5, which shows the system well- damped behavior even in the out-of-phase reclosing scenarios, and verifies the plug-and-play feature of the proposed controller.

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Improved Vector Control Strategy for Current Source Converters Connected to Very Weak Grids
Salmon Raju Gurram, Dr K Venkateswrlu & J Alla Bagash

Improved Vector Control Strategy for Current Source Converters Connected to Very Weak Grids Salmon Raju Gurram, Dr K Venkateswrlu & J Alla Bagash

The detailed small-signal modeling of the grid- connectedCSC is presented in this section. The complete system parametersare shown in Appendix A whereas the matrices of thefollowing state-space models are defined in Appendix B. In the following are the state, input, and output matrices,respectively, and are multiplied by the corresponding vectors and represents a small perturbation of the variable.Based on the SyQuest stability criterion, the magnitude of theoutput impedance of the CSC should be as high as possiblein order to preserve the system stability. The compensator signals are designed based on the fact thatthe PLL is the dominant detrimental element in the vector- controlledconverters in weak grid systems.Four states are added to the uncompensated model;three from the proposed compensator and one from the feed- forwardloop.The controller computational delay and the PWM switching are modeled as a dead time.

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A Control Topology to Magnify VSC Coupled Weak Grids Performance with Self-Synchronization Capability

A Control Topology to Magnify VSC Coupled Weak Grids Performance with Self-Synchronization Capability

ABSTRACT: This paper scrutinize a new course of scenario for a weak grid is to make possible the interfacing of voltage source converters (VSCs) more adequate and to enhance performance of weak grid by damping power and frequency oscillations. To mitigate power and frequency oscillations, a linear controller and non-linear controller is used based on droop characteristics. Here the non-linear controller has cascaded angle, power loops for frequency and angle regulation. The controller provides new control tactic for VSC to damp power and frequency oscillations by accordingly synchronizing the VSC to grid. The linear controller can also be applicable for both islanded and grid connected operation without reconfiguration. The controller endeavours a steady and invariable operation. And also investigation was carried out on system performance at low and high-power references, transition to islanding, self- synchronization. Simulation of the recommended controller was simulated in MATLAB/SIMULINK software. The Secured Simulation results validate that the proposed non-linear controller intensifies the system under unusual disturbances and shows the impressive working of the controller.

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An independently controlled energy storage to support short term frequency fluctuations in weak electrical grids

An independently controlled energy storage to support short term frequency fluctuations in weak electrical grids

Abstract: This paper proposes the use of Energy Storage (ES) to support frequency fluctuations in weak grids powered by low-power synchronous generators (SGs). Traditionally, large synchronous generators rely on high inertia and governor control to guarantee the frequency stability of the network. The scenario changes in networks with a large penetration of small distributed generators (DGs) with smaller inertia, where load variations can lead to large frequency deviations, violating the over/under frequency protection limits. The proposed energy storage control injects active power during load transients so that the frequency is constrained within predefined thresholds. The system frequency is detected by a Double Second-Order Generalised Integrator Frequency Locked Loop (DSOGI-FLL), eliminating the need for communication between energy storage and synchronous generator and providing “plug-and-play” capability. The proposed method combined with the continuous speed governing supports the frequency well within the primary frequency transient requirements. The solution has been validated using an 8kW laboratory system.

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Two-way Fluid-Structure Interaction Simulation of a Micro Horizontal Axis Wind Turbine

Two-way Fluid-Structure Interaction Simulation of a Micro Horizontal Axis Wind Turbine

The numerical procedures which were adopted to solve FSI problems can be broadly classified into two approaches: the monolithic approach and the partitioned approach; this paper mainly focus on the partitioned approach. The partitioned approach can be divided into one-way and two-way coupling [4].The fluid surface pressure acting at the surface of wind turbine structure is transferred to the structure solver without any feedback from the structure solver to the fluid solver when using the one-way FSI. In other words, the aerodynamic properties in vicinity of the wind turbine are not influenced by the solution in the structural solver. However, the two-way FSI transfers the displacement of the structure to the fluid solver and create new fluid mesh to accommodate the new interface location simultaneously. The two-way FSI can be achieved even several fluid and structure computations are performed at every time step and used to integrate the equations for each domain [5]. Whereas, this method is quite time-consuming as several computations are done at each time step. The FSI methods with conforming meshes usually involve three fields: the fluid dynamics, structural dynamics and mesh movement [6]. Nicholls-Lee et al. [7] conducted a full FSI analysis for a range of composites, bend-twist coupled blades on horizontal axis tidal turbines. The hydrodynamic analysis of a vertical axis tidal turbine using the full FSI coupling scheme was implemented by Khalid et al. [8] to compare the difference between the CFX independent simulations results and two-way FSI analyses results. Bazilevs et al. [9] and Hsu and Bazilevs [10] employed the non-conforming interface discretization approach to conduct the FSI on the full scale wind turbine. Grids which are called cells in CFD analyses are generally referred to the number of finite volumes divided in domains, which play a significant role in the accuracy of results and efficiency of the solution to a large extent. Nabi and Al-Khoury [12] studied the total grid number with only four cases to verify the best grid number to conduct their study, and Wang and Zhang [13] conducted the grid convergence on a micro wind turbine to verify the finest total grid number with five cases. Obviously, few cases were used to conduct the grid convergence as the simulation is time-consuming. To our best knowledge, none of these studies have elaborated the relationship of grids number and flow field domain other than indicated a finest grid number for a case.

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On a weak form of weak quasi continuity

On a weak form of weak quasi continuity

1. Introduction. Weakly quasi-continuous functions were introduced by Popa and Stan [9]. Recently, weak quasi-continuity has been developed further by Noiri [5, 6] and Park and Ha [8]. Due to a result by Noiri [5], weak quasi-continuity is equiva- lent to the weak semicontinuity developed by Arya and Bhamini [1]. The purpose of this note is to introduce the concept of subweak quasi-continuity, which we define in terms of a base for the topology on the codomain. We establish that this condition is strictly weaker than weak quasi-continuity and we use it to strengthen some of the results in the literature concerning weak quasi-continuity. For example, we show that the graph of a subweakly quasi-continuous function with a Hausdorff codomain is semiclosed. We also show that, if the graph function is subweakly quasi-continuous with respect to the usual base for the product space, then the function itself is weakly quasi-continuous, and that, if a function is subweakly quasi-continuous with respect to the base Ꮾ , then the restriction to a preopen set is subweakly quasi-continuous with respect to the same base. These results strengthen slightly the comparable re- sults for weakly quasi-continuous functions. Finally, we extend a result concerning weakly quasi-continuous retractions and investigate some of the basic properties of subweakly quasi-continuous functions.

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GRID COMPUTING

GRID COMPUTING

The term “Grid” is increasingly used in discussions about the future of ICT infrastructure, or more generally in discussion of how computing will be done in the future. Unlike “Cloud computing” which emerges and belongs to an IT industry and marketing domain, the term “Grid Computing” emerged from the super-computing (High Performances Computing) community (Armbrust, Fox et al. 2009). Our discussion of Utility computing begins with this concept of Grids as a foundation. As with the other concepts however for Grids hyperbole around the concept abounds, with arguments proposed that they are “the next generation of the internet”, “the next big thing”; or that will “overturn strategic and operating assumptions, alter industrial economics, upset markets (…) pose daunting challenges for every user and vendor” (Carr 2005) and even “provide the electronic foundation for a global society in business, government, research, science and entertainment” (Berman, Fox et al. 2003). Equally, Grids have been accused of faddishness and that “there is nothing new” in comparison to older ideas, or that the term is used simply to attract funding or to sell a product with little reference to computational Grids as they were originally conceived (Sottrup and Peterson 2005).

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Sensitivity of a mesoscale model to different convective parameterization schemes in a heavy rain event

Sensitivity of a mesoscale model to different convective parameterization schemes in a heavy rain event

First, we analyzed the accumulated precipitation forecast from RAMS. In this rain event, two of the experiments with the Kain-Fritsch scheme activated produce unrealistically low precipitation values. The only case where the Kain- Fritsch scheme showed an acceptable performance is the KF1 experiment in which the Kain-Fritsch parameterization is active only in the coarser grid of the simulation, while the other grids explicitly solve the microphysical package equa- tions. Thus, activating the convection scheme at a progres- sively higher spatial resolution worsens the RAMS results for the Kain-Fritsch scheme. Otherwise, the Kuo convec- tive scheme produces good results on accumulated precipi- tation spatial distribution for the three experiments, although it always underestimates precipitation amounts, especially in the maximum rainfall area. In this latter case, activation of the Kuo parameterization scheme in increasing spatial reso- lution grids for the three outer girds progressively improves the RAMS precipitation forecast regarding maximum precip- itation values and spatial distribution, although underpredict- ing actual values, while the activation on the fourth grid, the inner highest resolution grid, improves the maximum pre- cipitation peak but worsens the areal extent of precipitation. The experiment run without any convective parameterization scheme activated in any grid yielded good results regarding rainfall spatial distribution but underestimated precipitation values.

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Federated Grids and their Security

Federated Grids and their Security

In the standard VPN description, the Source and Destination networks of Figure 1 are physically private networks, so only one VPN is needed. For Grids, however, this may not be the case. Broadly, a Virtual Organization may itself be composed of smaller Virtual Organizations. Thus in the Grid analogy to Figure 1, the Source and Destination may themselves be federated networks. Also, of course, we may wish (even within a Grid of our own resources) to subdivide resources into smaller subunits for the purposes of access control and “firewall” security. That is, in our Grid way may have some resources that are for internal use only (private), some that are for trusted partners (shared), and some that are more openly accessible from other Grids in a test bed.

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Smart grid applicability prioritisation of neighbourhoods by developing a geospatial decision support model

Smart grid applicability prioritisation of neighbourhoods by developing a geospatial decision support model

Stated smart metering system, also known as Advanced Metering Infrastructure (AMI), comprises of a smart electricity meter installation. The new metering infrastructure is essential for energy efficiency measures, the monitoring and management of grids as well as load balancing and shifting. Smart meters are central gateways located on the customer’s site that support two-way communication and allow consumers to make informed decisions via price signals received from the utilities (Kranz and Picot, 2011, Blumsack and Fernandez, 2012). To improve the integration of renewable sources into the low-voltage grid, local small storage systems can either be installed close to prosumers (combination of a consumer and a producer) or directly at prosumers (Römer et al., 2012).

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Title: Amalgamation of Wireless Sensor Networks and Embedded Systems

Title: Amalgamation of Wireless Sensor Networks and Embedded Systems

In adhoc grids, shortwave feelers self organize into an infrastructureless network with a dynamic topology. Feeler grids share these addictions, but conjointly have diverse segmentation features. The number of joints in typical feeler grids is much higher than in a typical ad-hoc grid, and densed tendency are often anticipated to clinch coverage and connectivity. Joints typically have predefined stamina limitations, which make them farther failure-prone. Ideally, feeler grid metal ware should be diadem efficient, small, cheapo, and trusty in order to maximize grid lifetime, add pliancy, facilitate info assemblage and minimise the necessity for upkeep.

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SLA Constrained Adaptive Scheduling of Parallel Jobs in a Computational Desktop Grid

SLA Constrained Adaptive Scheduling of Parallel Jobs in a Computational Desktop Grid

As compared to cluster grids scheduling in desktop grids differs, since a desktop grid may vary widely depending upon the type of resources, dedication, trust, failure modes, applications, and so forth. The process of assigning jobs to the most appropriate resources is known as Grid scheduling. In two ways scheduling may be performed i.e., in a centralized fashion or in a fully distributed way [5]. For the most part, unlike clustered grids desktop grid systems do not need any local scheduler since in this case the scheduling target is a single desktop computer, contrary to a site in Grid [6, 7]. Heterogeneous, volatile, faulty, and malicious resources make Desktop grid scheduling complicated. Volatility (non- dedication), lack of trust, and heterogeneous properties are the main focus areas of Desktop grid scheduler than grid scheduler [8]. As compared to cluster computers or super- computers in desktop grid, communication cost is very high. In hierarchical desktop grid environment, as compared to cluster computers or high performance system, it is very difficult to classify grid resources into groups. This is so because there is no single parameter that can effectively classify grid resources into groups and decide the next levels of scheduling. Finally, desktop grid scheduling is opportunistic. The autonomy of worker nodes (that is, worker nodes can freely participate in public execution) is respected by Desktop grid. Much like in peer to peer systems a test-bed model that provides distributed scheduling we have used in this work . A grid test bed has been deployed using GridGain for the purpose of demonstrating the efficacy of the proposed SLA constrained adaptive scheduling model.

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Grids of Grids of Simple Services

Grids of Grids of Simple Services

We have of course discussed Grids extensively here in previous articles and we adopt the view that they represent the system formed by the distributed collections of electronic capabilities that are managed and coordinated to support some sort of enterprise (virtual organization). Sometimes one reserves Grid to describe just the technology used to build these electronic communities or organizations. One thinks of Grid technology as the CyberInfrastructure (NSF) or e-Infrastructure (European Union) that supports e-Science, e-Business or in fact e-moreorlessanyenterprise. There is no firm consensus as to the best Grid approach but we will adopt the popular architecture based on Web services. There is a vigorous debate in the community as to the “right” way to do this and if conventional Web services need enhancement to cope with the large scale secure managed distributed services needed in a Grid. In particular there is lot of debate on the appropriate ways to represent state and how much to standardize in this area. WSRF (Web Service Resource Framework http://www.globus.org/wsrf/) and WS-GAF (Web Service Grid application Framework http://www.neresc.ac.uk/ws-gaf/) are two important activities whose

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Performance of Series Filter in a Grid Connected Doubly Fed Induction Generator System

Performance of Series Filter in a Grid Connected Doubly Fed Induction Generator System

the vicinity of that node. Although, the DFIG-based wind turbines are able to control active and reactive power independently, the reactive power capability of those generators is limited as discussed. This problem is more severe in the case of DFIG wind turbines connected to weak power grids 6 having under voltage condition as the reactive power capability gets even more degraded. Hence an additional local reactive power source is needed. Moreover, the power generation trend these days is shifting from the transmission network to the distribution grid, i.e. De-centralization of power generation. As a result, it is becoming more difficult to control the voltage in the entire transmission network from conventional power stations only. Hence grid companies are installing dedicated local voltage control equipments like capacitor banks, FACTS devices and are demanding distributed generation equipments to have their own reactive power capability as a result there cannot be any exemption for wind turbines. Furthermore, because of the increased penetration level of wind turbines in the power grid, utility companies are asking to fulfill certain criteria (grid codes) for the interconnection of wind turbines to the power grid.

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Performance of Series Filter in a Grid Connected Doubly Fed Induction Generator System

Performance of Series Filter in a Grid Connected Doubly Fed Induction Generator System

the vicinity of that node. Although, the DFIG-based wind turbines are able to control active and reactive power independently, the reactive power capability of those generators is limited as discussed. This problem is more severe in the case of DFIG wind turbines connected to weak power grids 6 having under voltage condition as the reactive power capability gets even more degraded. Hence an additional local reactive power source is needed. Moreover, the power generation trend these days is shifting from the transmission network to the distribution grid, i.e. De-centralization of power generation. As a result, it is becoming more difficult to control the voltage in the entire transmission network from conventional power stations only. Hence grid companies are installing dedicated local voltage control equipments like capacitor banks, FACTS devices and are demanding distributed generation equipments to have their own reactive power capability as a result there cannot be any exemption for wind turbines. Furthermore, because of the increased penetration level of wind turbines in the power grid, utility companies are asking to fulfill certain criteria (grid codes) for the interconnection of wind turbines to the power grid.

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