Presence Of Unbalanced Loads Research Articles

Evaluation of Method for Attributing Responsibilities due to Voltage Unbalance in Electrical Systems

Several voltage unbalance conditions produce different harmful effects to the proper functioning of the many electrical system equipments, such as three phase induction motors. Thus, it is essential to know those responsible for the unbalance, on the one hand the energy distributor having a duty to provide a supply in accordance with the regulations and, on the other hand, the consumer who wishes to receive energy with quality and is obliged to maintain its installation according to the norms. In this scenario, the present paper has as objective to evaluate the conforming and non-conforming current method in front of an electric system with the presence of unbalanced loads and a three-phase induction motor with regard to the attribution of responsibility due to voltage unbalance.

Optimal allocation of multiple capacitors in a hybrid AC/DC microgrid for power quality improvement

Along with the various features for implementing the Hybrid AC/DC Microgrid (HMG), this article proposes an approach for optimal allocation of multiple capacitors which are investigated in a proposed modeling based on the IEEE 14-bus distribution system. The power quality of the HMG has been investigated during the urgent intermittent of Distributed Energy Resources (DERs) and Reactive Power Compensation (RPC) methods. Moreover, the investigation has been achieved in the presence of unbalanced loads and nonlinear loads with maximum and minimum demand scenarios. To cope with the power quality concerns in the studied cases, the fixed capacitor bank as an RPC system in Medium Voltage (MV) level load buses has been utilized. Although the performance indices of the power quality improved in MV-level buses, the Low Voltage (LV) level load buses still endure extensive operation performance deteriorations caused by unbalanced loads. Therefore, in this article, a compensation scheme applied in LV-level load buses and MV-level buses has been proposed consistent with the power flow computations. The Multi-Objective Grey Wolf Optimizer (MOGWO) algorithm is implemented to optimize both the size and location of capacitor banks over different voltage levels with high accuracy. The comprehensive assessment and discussion of the simulation results demonstrate the superiority of utilizing the proposed compensation scheme in both MV-level and LV-level load buses. Hence, the power quality is not only enhanced but also the installation cost is reduced. The complete model of the studied system has been validated using MATLAB/ Simulink.Article HighlightsThe reactive power compensation method is used to improve the power quality in a hybrid AC-DC microgrid.The applied RPC method verifies using the MOGWO algorithm to optimize both the location and size of multiple capacitor banks.Power quality challenges and future research trends are debated.

Collaborative Voltage Unbalance Compensation in Islanded AC Microgrids With Grid-Forming Inverters

The presence of unbalanced loads in power systems creates voltage imbalances that perturb the operation of sensitive loads, such as induction motors, power electronics converters, and adjustable speed drives. This article presents a novel control scheme that achieves accurate voltage balance at any node of an islanded ac microgrid with seamless integration with the existing hierarchical control of the microgrid. A master–slave architecture is adopted for the unbalance compensation due to its low network traffic requirements and its modular conception. The unbalance compensation follows a collaborative principle where current injections are computed as a function of the current rating of each inverter, thus avoiding the overcurrent stress of the unbalance compensation by a single inverter. This article presents the stability analysis and the design guidelines for the control parameters. The performance of the control proposal has been successfully tested in a laboratory microgrid. Selected experimental results are presented.

Varying Negative Sequence Virtual Impedance Adaptively for Enhanced Unbalanced Power Sharing Among DGs in Islanded AC Microgrids

Inverter based distributed generators (DGs) are increasingly being employed for interfacing renewable energy sources into the grid. Droop control is a very popular technique for ensuring parallel operation of these sources based on local measurements alone, in which the inverters are made to emulate the behaviour of synchronous generators of the power system. However, several drawbacks of the droop control technique are being addressed actively, over the years by various researchers. One of the prominent issues is the inaccurate power sharing among the distributed generators (DGs) due to the feeder/line impedances. The performance further deteriorates in the presence of unbalanced loads. This work presents an adaptive technique for adjusting the negative sequence (NS) virtual impedance of the DG, based on its output current, without the need of communication or extra sensors, to achieve enhanced unbalanced power sharing. The proposed control is shown to have the capability of seamless integration with the existing conventional and inverse droop techniques. The methodology is validated through simulations for different DG ratings, controls and on a modified 13 bus system. Small signal eigenvalue analysis about the operating point is used to determine the stable range of control parameters and the strategy is validated on an experimental setup in the laboratory.

MV faulted section location in distribution systems based on unsynchronized LV measurements

A medium voltage (MV) faulted section location technique depending solely on unsynchronized low voltage (LV) measurements in distribution systems is introduced in this paper. The proposed method identifies the faulted MV feeder segment when an unsymmetrical MV fault occurs. It exploits parameters including voltage magnitude, angle and sequence components captured via distributed non-synchronized monitoring devices connected at secondary substations. In this study, the characterization of a system’s LV profile during MV faults was achieved throughout both software simulations and physical testing. The latter was conducted in the 11 kV and LV distribution network of the Power Networks Demonstration Centre (PNDC) of the University of Strathclyde. The tests were performed for both 11 kV radial and ring network configurations. Modelling and simulations implemented using the DigSILENT PowerFactory package, complemented the hardware level testing and provided the opportunity for further examinations, such as the impact of MV laterals connection, distributed energy resources on both MV and LV side as well as the presence of unbalanced loads. Finally, the paper evaluates the effectiveness of the proposed faulted section location technique under various physical and simulated fault scenarios, loss of communications and noise interference.

Optimal Controller Design for Inverter-Based Microgrids

This paper presents a comprehensive control scheme for inverter-based distributed generations (DGs) in grid-connected microgrids (MGs). The proposed control strategy regulates active and reactive power of DGs in order to adjust output voltages of the DGs under balanced/unbalanced loading and fault conditions. In this paper, an adaptive hyper-plane sliding mode controller is developed for positive and negative sequences of active and reactive power in dq0 reference frame based on quasi-dynamic phasor. All the parameters of controllers are derived via particle swarm optimization algorithm in order to minimize an appropriate cost function. This controller is fast, stable and robust against uncertainties of the output filter of the inverter, variations of the network parameters and load disturbances. In addition, the fault ride-through capability of the system can be improved. In the presence of unbalanced loads and faults, output voltages the DG are controlled to remain balanced and unchanged. Performance of the proposed controller is compared with a conventional PI controller and a conventional sliding mode controller. Effectiveness and accuracy of the proposed control strategy are verified through simulating a test microgrid in the presence of unbalanced loads and some faults.

An improved control method for unified power quality conditioner with unbalanced load

This paper proposes an improved control method for Unified Power Quality Conditioner (UPQC) with unbalanced load. In UPQC, shunt APF is overburdened when it alone supplies total load reactive power. PAC method aims at effective utilization of series and shunt APFs by sharing reactive power burden between the two. In presence of unbalanced load, existing PAC methods can lead to circulation of reactive power between two APFs and thereby result in overloading of UPQC. Also, due to unbalanced compensating currents, DC link voltage contains second order oscillations, which deteriorates source current quality. In this work, a new PAC method is proposed, which avoids circulation of reactive power and unnecessary VA burden on UPQC. To suppress DC link voltage oscillations, proposed control employs a ‘mean block’ (moving average) at output of PI controller, which ensures balanced and harmonic free source currents. Also, a ‘percentage unbalance’ parameter has been proposed to quantify the unbalance in three phase quantities. Performance of proposed UPQC system is tested in the presence of non-linear, reactive and unbalanced loads. Dynamic performance of system is studied during grid disturbances such as voltage sag, swell, and change in load. Real time simulation carried out in Opal-RT validate the effectiveness of proposed method. The electrical circuit of UPQC is simulated with sub-microsecond time step on FPGA based computational engine of Opal-RT for better verification of the proposed control.

Grid-forming VSC control in four-wire systems with unbalanced nonlinear loads

A grid-forming voltage source converter (VSC) is responsible to hold voltage and frequency in autonomous operation of isolated systems. In the presence of unbalanced loads, a fourth leg is added to provide current path for neutral currents. In this paper, a novel control scheme for a four-leg VSC feeding unbalanced linear and nonlinear loads is proposed. The control is based on two control blocks. A main control commands the switching sequence to the three-phase VSC ensuring balanced three-phase voltage at the output; and an independent control to the fourth leg drives neutral currents that might appear. The proposed control is noninvasive in the sense that both control blocks are independently implemented, avoiding the use of complex modulation techniques such as 3D-SVPWM. Moreover, the main control is deployed in dqo reference frame, which guarantees zero steady-state error, fast transient response during system disturbances and mitigation of harmonics when nonlinear loads are present. Simulations and experimental results are presented to verify the performance of the proposed control strategy.

Connection and Disconnection Transients for Micro-Grids under Unbalance Load Condition

In this paper the design and the implementation of a method oriented to carry out a safe and a stable intentional disconnection, and re-connection, of local micro-grids from the main distribution network under generic grid conditions, avoiding the appearance of undesired transients, will be presented. In this work a seamless transfer between grid-connected and isolated mode is achieved by means of using an adapted frequency based synchronization control loop, which works in the a/3 stationary reference frame, whose response can be dynamically adapted in function of the micro-grid operating conditions. In addition to this feature, the inner loops of the control algorithm proposed in this application have been implemented in order to provide a proper operation of the system under generic conditions, even in the presence of unbalanced loads. In order to prove the reliability and good performance of the proposed method simulation and experimental results will be provided and discussed in the last sections of the paper.

Fuzzy Optimal Phase Balancing of Radial and Meshed Distribution Networks Using BF-PSO Algorithm

Due to increasing neutral current and power losses resulting from phase unbalancing in distribution networks, the act of phase balancing has been a matter of interest in recent years. In this paper, a new method for phase arrangement of laterals and the distribution transformers is presented based on bacterial foraging (BF) oriented by particle swarm optimization (PSO) algorithm (BF-PSO). The algorithm is proposed for radial and meshed distribution networks in the presence of unbalanced loads. The objective function of this problem includes the neutral current of the supporting feeder, the rephasing cost, the voltage drop, and the line losses. Since these objectives do not have similar units and the same variation ranges, the four objectives are fuzzified and then integrated as the fuzzy multi-objective function. In order to prove the efficiency of the BF-PSO algorithm, its performance is compared with bacterial foraging, genetic and immune algorithms. To evaluate the proposed method, it is applied to feeder No. 3062 in Ahwaz, Iran. The simulation results confirm the efficiency of the method for the reduction of system costs and network phase balancing.

Negative sequence compensation within fundamental positive sequence reference frame for a stiff micro-grid generation in a wind power system using slip ring induction machine

An isolated wind power generation scheme using slip ring induction machine (SRIM) is proposed. The proposed scheme maintains constant load voltage and frequency irrespective of the wind speed or load variation. The power circuit consists of two back-to-back connected inverters with a common dc link, where one inverter is directly connected to the rotor side of SRIM and the other inverter is connected to the stator side of the SRIM through LC filter. Developing a negative sequence compensation method to ensure that, even under the presence of unbalanced load, the generator experiences almost balanced three-phase current and most of the unbalanced current is directed through the stator side converter is the focus here. The SRIM controller varies the speed of the generator with variation in the wind speed to extract maximum power. The difference of the generated power and the load power is either stored in or extracted from a battery bank, which is interfaced to the common dc link through a multiphase bidirectional fly-back dc–dc converter. The SRIM control scheme, maximum power point extraction algorithm and the fly-back converter topology are incorporated from available literature. The proposed scheme is both simulated and experimentally verified.