Unbalanced Network Research Articles

DIRECT POWER CONTROL OF A DFIG BASED WIND TURBINS UNDER UNBALANCED GRID VOLTAGE WITHOUT ROTOR POSITION SENSOR

In this paper, the behavior of a doubly fed induction generator (DFIG) is proposed under unbalanced grid voltage and without using a rotor position sensor. There are two main methods that are been used for the detection of rotor position: using shaft sensor and sensorless algorithm. In this paper the shaft sensor is eliminated and a position sensorless algorithm is used for estimating the rotor position. Sensorless operation is more desirable than using shaft sensor, because the shaft sensor has several disadvantages related to the cost, cabling, robustness and maintenance. Also, during network unbalance, three selectable control targets are identified for the rotor side converter (RSC), i.e., obtaining sinusoidal and symmetrical stator currents, mitigation of active and reactive powers ripples and the cancellation of electromagnetic torque oscillations. The effectiveness of the proposed control strategy is confirmed by the simulation results from a 2-MW DFIG system. It is concluded that the sensorless algorithm is able to produce accurate results similar to the case of that used from shaft sensor and it can be used in the practical applications.

Process flexibility design in heterogeneous and unbalanced networks: A stochastic programming approach

ABSTRACTMost studies of process flexibility design have focused on homogeneous networks, whereas production systems in practice usually differ in many aspects, such as plant efficiency and product profitability. This research investigates the impacts of two dimensions of production system heterogeneity, plant uniformity and product similarity, on process flexibility design in unbalanced networks, where the numbers of plants and products are not equal. We model the design of flexible process structures under uncertain market demand as a two-stage stochastic programming problem and solve it by applying Benders decomposition with a set of acceleration techniques. To overcome slow convergence of the exact algorithm, we also develop an efficient optimization-based heuristic capable of obtaining solutions with optimality gaps less than 6% on average for realistic-scale production systems (e.g., with five plants and 10 types of products). Numerical results using the proposed heuristic show that flexibility designs are influenced by both dimensions of system heterogeneity, though the desired level of flexibility is more sensitive to the effect of plant uniformity than that of product similarity.

Multi-objective Optimal Hybrid Power Flow Algorithm for Integrated Community Energy System

A multi-objective optimal hybrid power flow algorithm was proposed for multi-objective scheduling and management of the integrated local area energy system (ILAES). Firstly, an energy flow analysis model for the energy center was developed based on the energy hub model. Then, a multi-objective optimal hybrid power flow algorithm was proposed to minimize the operation cost and total emission of the ILAES considering the constraints from unbalanced three-phase electric distribution network, the natural gas network and the energy centers. The proposed multi-objective optimal hybrid power flow algorithm can be further used in the optimal day-ahead scheduling for the ILAES, which considers the ILAES’s multiple operation needs in aspects of security, economy and environmental friendliness. Numerical results show that the proposed algorithm can be used in the steady-state analysis of the ILAES and multi-objective optimal scheduling for the ILAES.

Relationship Between Two Direct Power Control Methods for PWM Rectifiers Under Unbalanced Network

Active power control plays an important role in a pulse width modulation (PWM) rectifier under both balanced and unbalanced network conditions. The prior methods to achieve active power oscillation cancellation (APOC) under unbalanced grid voltages are usually implemented by forcing the grid current to track appropriate current references. They require fine tuning work of a current controller and positive-/negative-sequence extraction of grid voltage/currents and/or converter voltage, which complicates the design of the controller. This paper presents two APOC methods in the frame of direct power control (DPC) for the PWM rectifier and reveals their inherent relationship. In the first method, an appropriate power compensation is added to the original power references without modifying the internal control structure. In the second method, a novel definition of reactive power is employed to replace conventional reactive power, which achieves the aim of APOC automatically. Both methods can be easily integrated with existing DPC schemes with a slight modification. The sequence extraction required in prior methods is eliminated. The inherent relationship between these two methods is investigated, and it is found that they are completely equivalent. The two APOC methods are comparatively studied and implemented in the basic table-based DPC. Simulation and experimental results confirm the theoretical study and the effectiveness of the two methods.

Modular handover algorithm for 5G HetNets with comprehensive load index

Most existing handover decision system (HDS) designs are monolithic, resulting in high computational cost and unbalance of overall network. A novel modular handover algorithm with a comprehensive load index for the 5th generation (5G) heterogeneous networks (HetNets) is proposed. In this paper, the handover parameters, serving as the basis for handover, are classified into network's quality of service (QoS) module, user preference (UP) module and degree of satisfaction (DS) module according to the new modular HDS design. To optimize switching process, the comprehensive network load index is deduced by using triangle module fusion operator. With respect to the existing handover algorithm, the simulation results indicate that the proposed algorithm can reduce the handover frequency and maintain user satisfaction at a higher level. Meanwhile, due to its block calculation, it can bring about 1.4 s execution time improvement.

Integration of PV and EVs in unbalanced residential LV networks and implications for the smart grid and advanced metering infrastructure deployment

Voltage unbalance is a relevant problem that causes a less efficient operation of the system due to higher energy losses and lower hosting capacity. Unbalance has often been neglected by distribution system operators due to the lack of monitoring data in the low voltage (LV) grid. However, the massive deployment of smart metering in recent years in many countries provides very valuable information to detect unbalance. Moreover, in the current context of increasing presence of single-phase distributed energy resources connected to LV networks, such as electric vehicles (EVs) and photovoltaic (PV) generation, unbalance is bound to increase.This article investigates the technical impact of future integration of EV and PV in LV unbalanced networks. This paper has assessed the daily energy losses and voltage problems as load unbalance gradually increases, based on load flow analysis on an hourly basis, considering residential demand and homogeneously distributed EV and PV. The analysis has been carried out for several rural and semi-rural LV networks and various scenarios of demand level and penetration degree of EV and PV. The three-phase load flow analysis is computed using the forward-backward sweep algorithm.Furthermore, this work discusses the implications for the deployment of supervision and monitoring solutions based on advanced metering infrastructure (AMI). Their implementation should be prioritized in more loaded and longer networks where high integration of distributed energy resources is expected so that unbalance can be detected and corrective actions can be applied.

Optimal Placement and Sizing of Distributed Generations in Unbalanced Distribution Networks Considering Load Models and Uncertainties

Optimal Placement and Sizing of Distributed Generations in Unbalanced Distribution Networks Considering Load Models and Uncertainties

Effect of Doubly Fed Induction GeneratorTidal Current Turbines on Stability of a Distribution Grid under Unbalanced Voltage Conditions

This paper analyses the effects of doubly fed induction generator (DFIG) tidal current turbines on a distribution grid under unbalanced voltage conditions of the grid. A dynamic model of an electrical power system under the unbalanced network is described in the paper, aiming to compare the system performance when connected with and without DFIG at the same location in a distribution grid. Extensive simulations of investigating the effect of DFIG tidal current turbine on stability of the distribution grid are performed, taking into account factors such as the power rating, the connection distance of the turbine and the grid voltage dip. The dynamic responses of the distribution system are examined, especially its ability to ride through fault events under unbalanced grid voltage conditions. The research has shown that DFIG tidal current turbines can provide a good damping performance and that modern DFIG tidal current power plants, equipped with power electronics and low-voltage ride-through capability, can stay connected to weak electrical grids even under the unbalanced voltage conditions, whilst not reducing system stability.

A Control Strategy for DFIM to Minimize DC Bus Voltage Fluctuations and Torque Oscillations

The Direct Power Contro; has many advantages like it avoids the usage of integration of PWM voltages which leads to stable operation even at zero rotor frequency, it is position sensor less and hence will not depend on machine parameters like stator or rotor resistance. In case of network unbalance, if the system is operated with constant active and reactive powers, it leads to oscillations in the electromagnetic torque and currents exchange with the grid will become non- sinusoidal, which is not good for the system as it increases the mechanical stress. In this paper, both the rotor connected converter and grid connected converter are fed with DPC strategy along with that a Torque Oscillations Cancellation scheme is applied to RSC and Proportional Integral control based power references generation strategy without calculating the sequence components and with elimination of DC bus voltage oscillations is applied to stator-side converter in order to achieve non-oscillating torque accompanied by quality improved current exchange with the grid. The simulation results of Doubly Fed Induction Generator with and without fault clearly shows that the performance of the proposed scheme is validated.

Coordinated control strategy for hybrid wind farms with DFIG-based and PMSG-based wind farms during network unbalance

This paper investigates the coordinated control strategy for a hybrid wind farm with doubly fed induction generator (DFIG)-based and direct-driven permanent-magnet synchronous generator (PMSG)-based wind farms during network unbalance. The negative-sequence current output capabilities of DFIG and PMSG systems under unbalanced grid voltage condition are described. Furthermore, by considering both the negative-sequence current capabilities and requirements of different control targets for each control unit, the controllable operating regions of DFIG and PMSG systems are investigated. According to the controllable operating regions, a targets selection scheme for each control unit is proposed to improve the stability of the hybrid wind farms containing both DFIG-based and PMSG-based wind farms during network unbalance, especially to avoid DFIG-based wind farm tripping from connected power grid under severe grid voltage unbalance conditions. Finally, the proposed coordinated control strategy is validated by the simulation results of a 30-MW-DFIG-based wind farm and a 30-MW-PMSG-based wind farm under different operation conditions and experimental results on a laboratory-scale experimental rig under severe grid voltage unbalance conditions.

Direct Stator Current Vector Control Strategy of DFIG Without Phase-Locked Loop During Network Unbalance

This paper proposes a direct stator current vector control strategy of a doubly fed induction generator (DFIG) without phase-locked loop (PLL) during network unbalance. In the proposed control strategy, a virtual phase angle produced by the nominal frequency is used for coordinate transformations instead of the actual voltage one estimated by the PLL. Since the calculation of the commanded stator currents is based on the instantaneous power theory regardless of the generator parameters, it can greatly reduce the parameter dependence of the control system. Then, for a satisfactory grid synchronization, an improved synchronization method is proposed. It can smoothly produce the stator voltage with no overshoot voltage and no parameter dependence. During network unbalance, a reduced-order vector integrator is used to directly regulate the pulsating components without the complex calculations of the commanded current values and the sequential separations of the voltages and currents. Meanwhile, four alternative control targets can be achieved for the DFIG during network unbalance. Furthermore, the control performance, including the control system stability, the robustness against frequency disturbances and parameter deviations, and the rejection of voltage unbalance, are fully analyzed. Finally, the experimental results are given to validate the effectiveness of the proposed control strategy.

Margin-Based Framework for Online Contingency Selection in Unbalanced Networks

A framework for performing fast online contingency selection in unbalanced power systems is presented in this paper. Selection methods are typically based on the principle of identifying the effect of contingencies on multiple normalized performance indices and ranking them using the results. Presently used performance indices are highly nonlinear and they are known to mask the effects of single contingencies leading to misclassifications. In this paper, we propose two new methods, one relying on margin-based performance indices and another based on state sensitivity. New performance indices are proposed based on margins of 1) circuit loading, 2) bus voltages, and 3) reactive power. In addition, a state sensitivity method is proposed which estimates a system's post contingency operating state via a single iteration of the quadratized power flow model and provides estimates of post contingency line loading, bus voltages, and reactive power levels. Numerical experiments on a three phase version of the IEEE Reliability Test System show that the proposed performance indices yield more accurate results, at a computational cost comparable to a single power flow iteration. The state sensitivity method is more accurate in identifying critical contingencies but its computational cost is higher. The method has been also demonstrated in the larger PEGASE systems.

Virtual resistor circuit‐based control strategy of STATCOM during network unbalance

Static synchronous compensators (STATCOMs) have been widely used for the reactive power and voltage regulation in the renewable energy field. The STATCOM could not only regulate the positive-sequence voltage component, but also suppress the negative-sequence voltage by injecting the negative sequence current when the grid voltage is unbalanced. Aiming at diminishing the negative sequence voltage, the control strategy based on the symmetrical virtual resistor circuit is proposed in this study. The strategy is on the assumption that there are parallel virtual resistors connected to the point of interconnection (POI) of the STATCOM. It is theoretically proved that the negative sequence voltage can be eliminated when the current of the virtual resistors is balanced. Therefore, the negative sequence voltage can be eliminated by making the current of virtual resistors balanced. As the resistors are virtual in the real circuit, the voltage of the POI of the STATCOM is used in the input of the negative-sequence current control loop instead of the current of the virtual resistors as the feedbacks. The signal of the ac voltages and currents of the STATCOM are only needed in this strategy which simplifies the system design. In the end, the simulation results are presented to validate the correctness and the effectiveness of the proposed control strategy.

Unbalanced steady‐state model of synchronous machine for EMT initialisation

Unbalanced load flow solution provides the initialisation of electromagnetic transient (EMT) simulation model for the unbalanced active distribution network. The most electro-magnetic transient program (EMTP) software use the d–q-axis-based synchronous machine's parameters, so the existed sequence component-based model is not suitable for the EMT model initialisation. In this study, an unbalanced steady-state synchronous machine model is presented which is derived from the original dynamic model and its parameters are consistent with EMT dynamic model. It is more precise than the sequence component parameters based model. Furthermore, the saturation phenomenon of main flux is also considered in this model. The proposed model is validated and compared with Modelica simulation results.

Optimal Placement and Sizing of Distributed Generators in Unbalanced Distribution Systems with Respect to Uncertainties using IPSO Algorithm

Distributed generations are renewable sources that do not create environmental pollution and install near the site of consumption. Also, the generated power of wind and solar sources are faced with uncertainty due to the dependence on external factors. With determination of the appropriate location and size of these sources in distribution networks their positive effects can be realized. But it must be noted that distribution networks due to the unequal phase loads and mutual impedance between various phases of lines are virtually unbalanced. So in this paper, determination of the appropriate size and place of distributed generations is done in unbalanced distribution network that distributed generations’ appropriate size obtained with respect to probabilistic methods and their convenient location is selected using weighted multi-objective IPSO algorithm. With regard to impact of exact load flow in calculations, linear three-phase unbalanced load flow method considering the loads model and loads connection type is used which is the fast and accurate unbalanced load flow method. Simulations are done in IEEE 37 bus unbalanced network in Matlab software. The simulation results indicate that network power losses and voltage unbalance factor are reduced, voltage profile of each phase is improved and significant profit is obtained for distribution companies

Optimal Charging Strategy of Electric Vehicles in Unbalanced Three-Phase Distribution Network

Background: The increasing penetration of Electric Vehicles (EV) could lead to significant impact on the power systems, particularly for existing distribution networks. Methods: These impacts contain phase unbalance, excessive voltage deviations and overloading of equipment, which arise when a large numbers of Electric Vehicles are simultaneously charged. Results: In the current research, an optimal charging strategy is designed to control charging rates of Electric Vehicles in unbalanced three-phase distribution networks. A technique according to the Particle Swarm Optimization (PSO) is designed in order to reduce the charging cost of vehicles, with considering certain constraints. The constraint set involves transformer and line restrictions, unbalance of phase and voltage range. Conclusion: The proposed charging strategy is investigated on a real distribution network. Findings shows that high penetration of Electric Vehicles can be sustained in the existing distribution networks, demonstrating the effectiveness of the proposed method.

Distributed resources coordination inside nearly-zero energy buildings providing grid voltage support from a symmetrical component perspective

In this work, the authors propose a new coordination method for smart distribution networks (SDN) with nearly-zero energy buildings (nZEB). The proposed coordination procedure considers a central controller that runs a state estimation of the grid and calculates the optimum set-points for the buildings. Inside each building, the building management system (BMS) manages the converters used by the dispatchable generators (DG), the renewable generators (RG) and the energy storage system (ESS) to fulfil (as far as possible), the central controller request.In the paper, the unbalanced behaviour of the grid is considered, so the optimum references for the symmetrical component currents in the converters are calculated. The proposed method reduces the network unbalance and the total losses. In addition, buildings with energy surplus can support other neighbours with energy deficit. The present paper is focused on how the BMS calculates the optimum references for all converters placed inside each building. The main contribution of the paper is the use of the symmetrical component approach at building level in order to generate the best references for all converters inside the building.

Impact of plug-in electric vehicles on voltage unbalance in distribution systems

Plug-in electric vehicle (PEV) will soon be connected to residential distribution networks. They are considered as huge residential loads when being charged. However, as the battery technology and required facilities improves, they will also be able to support the network as small dispersed generation units which transfer the energy stored in their battery into grid. Even though the PEV connection gradually increases, their connection points and charging/discharging levels randomly vary. Therefore, such single-phase bi-directional power flow can have adverse effect on the voltage unbalance of a three-phase distribution network. In this work, impact of plug-in electric vehicles on voltage imbalance in distribution system is presented. In G2V as well as V2G modes, the voltage unbalance analysis is carried out for various cases. Additionally, the voltage unbalance due to PEVs discharging and other types of distribution generator such as solar photovoltaic and wind turbines are investigated. Finally, some voltage unbalance mitigation techniques are summarized.Keywords: Distribution Network, Electric Vehicle, G2V, V2G, Voltage Unbalance

Multi-area distributed three-phase state estimation for unbalanced active distribution networks

This paper proposes a new multi-area framework for unbalanced active distribution network (ADN) state estimation. Firstly, an innovative three-phase distributed generator (DG) model is presented to take the asymmetric characteristics of DG three-phase outputs into consideration. Then a feasible method to set pseudo -measurements for unmonitored DGs is introduced. The states of DGs, together with the states of alternating current (AC) buses in ADNs, were estimated by using the weighted least squares (WLS) method. After that, the ADN was divided into several independent subareas. Based on the augmented Lagrangian method, this work proposes a fully distributed three-phase state estimator for the multi-area ADN. Finally, from the simulation results on the modified IEEE 123-bus system, the effectiveness and applicability of the proposed methodology have been investigated and discussed.

Network reconfiguration of unbalanced distribution networks using fuzzy-firefly algorithm

This paper addresses a method to optimize the unbalanced distribution networks (UDNs) for keeping up the voltage profile with respect to the consequence of solving the multi-objective reconfiguration using the Firefly algorithm in a fuzzy domain with a load flow method proposed in this paper. The objectives to be minimized are the total network power losses, the deviation of bus voltage and load equalizing in the feeders with network reconfiguration. Every goal is moved into the fuzzy domain utilizing its membership function and fuzzified independently. The proposed method for network reconfiguration has been implemented in 25-node and 19-node UDNs. The outcomes obtained by the suggested method of these two unbalanced networks have been compared with that of obtained by Genetic algorithm (GA), ABC algorithm, PSO algorithm and GA-PSO algorithm using the same objective function. The juxtaposition of the proposed method with the available methods is also presented.