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Copyright of Technical Electrodynamics / Tekhnichna Elektrodynamika is the property of National Academy of Sciences of Ukraine, Institute of Electrodynamics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Supplying unbalanced load by a doubly fed induction generator (DFIG) causes power and torque pulsation due to its unbalanced stator and rotor currents. Researchers showed that power and torque pulsation increase the rate of the machine's tear-and-wear and decrease its life time. This becomes stringier in a weak grid where the DFIG supplies a substantial amount of the load's power. In order to protect the machine, the torque and power pulsation must be minimized. This paper proposes a subtractive feedforward controller that minimizes the DFIG's torque and power pulsation of the DFIG machine by forcing the machine to supply the load with balanced current. The remaining unbalanced portion of the current is supplied by the grid. The proposed controller is made up of two main steps: First, we used our symmetrical components decomposition technique to generate dc positive and negative sequences in dq synchronous reference frame without the use of bandpass filters to filter out the $$2\omega _\mathrm{s}$$ components. In the second step, a subtractive feedforward compensator for the negative sequence is added to the generic controller. The compensator removes the negative sequence from the stator currents by adjusting the PWM mechanism at the rotor side. Agreeing with other researchers, the zero sequence was found to be absent in standard internally ungrounded DFIG machines; hence, a zero-sequence compensator was not needed. Our simulation results showed that our proposed controller works very well with balanced as well as the unbalanced loads. Under unbalanced conditions, our simulations showed that the negative sequence stator currents were diminished. This forces the supplied DFIG currents the load to be balanced, which leads to minimize torque and power pulsation. The proposed controller is also proven to be effective by extracting the maximum power under balanced as well as unbalanced loads. [ABSTRACT FROM AUTHOR]

Copyright of Technical Electrodynamics / Tekhnichna Elektrodynamika is the property of National Academy of Sciences of Ukraine, Institute of Electrodynamics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Control methods of a stand-alone doubly fed induction generator mostly use either proportional-integral regulators in multiple synchronous reference frames or proportional-integral-resonant regulators in a stationary reference frame. These control schemes work well when the plant model is not complex and system decoupling is implemented into the control algorithm. However, when the control system contains multiple resonant terms in order to compensate high harmonics of load current, tuning of many regulators in different coordinate systems becomes problematic. The paper presents the application of a linear-quadratic regulator and a rotor position observer in sensorless control of a stand-alone doubly fed induction generator supplying nonlinear and unbalanced loads. Cooperation of the observer and state controller has been validated in the laboratory experiment. An anti-windup mechanism has been implemented into the resonant terms, and it has been shown that the anti-windup structure is crucial for high performance of the state controller with resonant terms. Further, research on observers based on a model reference adaptive system scheme is done in the paper. [ABSTRACT FROM PUBLISHER]

This paper presents a non-iterative technique that generates reference current to manage operation of a three-phase four-wire shunt active power filter which employs a three-leg split capacitor voltage source inverter (VSI) topology. The proposed technique integrates together a self-tuning-filter (STF) and direct-quadrature-zero (dq0) principle (referred here as STF-dq0), allowing the controlled shunt active power filter (SAPF) to perform effectively under distorted source voltages and unbalanced load conditions. Unlike the previous technique developed based on the standard dq0 principle, the proposed technique does not require any service from a phase-locked loop (PLL) where two STFs are applied to separate harmonic and fundamental elements for the purpose of generating synchronization phases and reference current, respectively. Simulation work which includes connection of the SAPF circuits, design of control techniques and all the necessary assessments are conducted in MATLAB-Simulink platform. Performance achieved by the SAPF while utilizing the proposed technique is thoroughly investigated and benchmarked with that demonstrated by the SAPF while using the standard dq0 technique, to evaluate the inherent advantages. Exhaustive simulation results are provided and thoroughly discussed to support design concept, effectiveness, and benefits of the proposed technique.

This paper proposes the using of artificial neural networks (ANNs') tocontrol the synchronous generator automatic voltage regulator (AVR), with unbalance load operating conditions. The neural network for control a nonlinear system is described and used to demonstrate the effectiveness of the neural network for control the drives with nonlinearities. In this study, performances of a simulated neural network AVR evaluated for a wide range of unbalanced loadsoperating conditions. The variance factors are calculated, as an indicator of optimum operation, and their values are compared for different feedback signals and various unbalanced operating conditions. The optimum control is introduced, which gives an average variance factor in ANN controller is about 1.105%, whereas the average variance factor in traditional PI controller is about 2.035%.

In order to evaluate impact of different uncertainty, such as the intermittent behaviour of wind power and photovoltaic (PV) generation and bus load, and adjacent wind farms or PV resources on performance of power system, a new three‐phase probabilistic load flow (PLF) algorithm with asymmetric line parameter and unbalanced load of distribution system is proposed. Correlation among input random variables is considered, especially the joint correlation of wind power and PV generation and bus loads is considered, and then joint probabilistic model of wind power, PV generation and bus load is established. Then a joint cumulants method and Gram–Charlier expansion are applied to obtain the cumulative distribution function of desired random variables, and PLF results based on joint cumulants method are compared with the ones of Monte Carlo simulations. Proposed method allows evaluating potential impact of different correlation and renewable energy generation penetration on the PLF of power system. The effectiveness of proposed method is demonstrated with a typical structure of 25‐bus distribution system. [ABSTRACT FROM AUTHOR]

Unbalanced load currents not only give rise to unbalanced voltages but also adversely affect the performance of the conventional current-limiting mechanisms. The latter might result in overcurrent stress on the distributed energy resources (DERs) or current harmonics. In this paper, a novel decentralized control method is proposed to improve the power quality and protect DERs from overload. The proposed controller makes use of the model predictive control (MPC) technique to minimize the voltage unbalance, improve current limiting, and prevent active power overload. The MPC is combined with the V-I droop method to realize coordinated operation with fast dynamic response. The proposed method is tested on the CIGRE benchmark microgrid. Simulation results demonstrate that the proposed method improves power quality but also allows for operation close to the maximum load capacity without imposing DERs to overload. [ABSTRACT FROM PUBLISHER]

Copyright of Technical Electrodynamics / Tekhnichna Elektrodynamika is the property of National Academy of Sciences of Ukraine, Institute of Electrodynamics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper analyzes the unbalance problem of a stand-alone doubly-fed induction generator (DFIG) under unbalanced loads and proposes a compensation method to balance the stator output voltage. The proposed compensation method is developed based on a predictive current control (PCC) method implemented in the rotor current controller. The development of the PCC is based on the discrete model of the DFIG to predict an appropriate average rotor voltage vector to eliminate the rotor current error in the following switching period. The identified rotor voltage vector is then applied to the rotor-side converter (RSC) by using space-vector modulation (SVM) with constant switching frequency. To improve the control performance, a compensation method for time delay based on the prediction of the future rotor current at the end of current sampling period also is investigated. The proposed control scheme was tested by experiments with 2.2 kW DFIG to demonstrate its excellent steady-state performance as well as extremely fast dynamic response. [ABSTRACT FROM AUTHOR]

With increasingly prominent role of rail transit in the entire urban transport system, an auxiliary inverter is used increasingly. The complicated and fluctuating load condition of the auxiliary inverter implies special designs about structure and control strategy are necessary. For this purpose, a comprehensive control strategy is proposed. First of all, this paper proposes a structure that is capable of suppressing unbalanced voltage. Second, the control strategy based on the resonant controller is proposed, which can balance split capacitors voltage, is capable of suppressing the effect of unbalanced load and nonlinear load. This paper introduces the design method of the control system in detail. Then, this paper simulates the proposed comprehensive control strategy in MATLAB. Finally, experimental results are presented, which fully demonstrate the excellent performance of the comprehensive control strategy. [ABSTRACT FROM PUBLISHER]

When the heat extraction and injection of a borehole ground heat exchanger (GHE) are not seasonally balanced, ground thermal accumulation will occur and will cause a decline in the ground source heat pump's operational efficiency, especially for large multiple borehole ground heat exchangers (LMBGHE). A zoning operation strategy, in which only the relatively central part of the GHE runs during the low load season, is adopted in this paper to alleviate the thermal accumulation. By analyzing the case in which the heat injected into the ground in the summer is greater than that extracted from the ground in the winter, it was found that, in comparison with the full operation mode, the highest and average field temperatures significantly decrease when only the central part of the GHE runs during the winter. Analysis shows that the thermal accumulation can be effectively alleviated by this GHE zoning operation. This study also indicates that the zoning operation method is more effective when the ground has a smaller thermal conductivity. [ABSTRACT FROM AUTHOR]

Electric vehicles (EVs) have been gaining popularity in recent years due to growing concerns about fuel depletion and increasing petrol prices. Random uncoordinated charging of multiple EVs at residential distribution feeders with moderate penetration levels is expected in the near future. This paper describes a high performance voltage controller for the EVs charging system, and proposes a scheme of asymmetric synchronous reference frame controller (ASRFC) in order to compensate for the voltage distortions and unbalance distribution system due to EVs charger. This paper explores the power factor of distribution and residential network under random EVs charger on the bus load. ASRFC and harmonic voltage compensator (HVC) are employed for static VAR compensator (SVC) in this paper. The proposed scheme can improve the power factor and total harmonic distortion (THD) of the smart grid due to the EVs charger in grid. The effectiveness of the scheme was investigated and verified through computer simulations of a 22.9-kV grid.

The increasing presence of inverter-based distributed generation (DG) units in microgrid application requires control methods that achieve high performance not only during normal operating conditions, but also under unbalanced conditions. These conditions can occur permanently due to distribution of unbalanced loads on the three phases of the microgrid. This paper proposes a robust control strategy for a grid-connected multi-bus microgrid containing several inverter-based DG units. Each of the DG units can supply a combination of balanced and unbalanced local loads. The proposed control strategy employs an adaptive Lyapunov function based control scheme to directly compensate the negative-sequence current components caused by unbalanced loads in some part of microgrid; and a sliding mode based control scheme to directly regulate the positive-sequence active and reactive power injected by DG units to the microgrid. The control method proposed in this paper is shown to be robust and stable under load disturbances and microgrid parameter uncertainties even in the presence of nonlinear and time-variant unbalanced loads. The effectiveness of the presented controller is validated through time-domain simulation studies, under the MATLAB/Simulink software environment. [ABSTRACT FROM AUTHOR]

In this paper, an analysis and calculation of the dc-link current and voltage ripples are presented for a three-phase inverter with unbalanced load. A comparison of the dc-link average and root-mean-square (rms) currents between considering and ignoring high frequency harmonics of the output current is drawn. It is shown that high frequency harmonic currents have little effect on the dc-link current, and therefore, they can be ignored. Based on the symmetrical components method, the dc-link average and harmonic rms currents are derived, and the dc-link voltage ripple is compared between the balanced and unbalanced loads. It can be found that the dc-link current and voltage ripples consist of not only high frequency harmonics but also the double fundamental frequency harmonic, and the voltage ripple is independent of the positive-sequence component and determined by the negative-sequence component, under the unbalanced load. Experimental results are shown to verify the accuracy of the theoretical analysis. [ABSTRACT FROM AUTHOR]

Traditional three - leg inverter can output three - phase symmetrical voltage in the case of three - phase symmetrical load. But it is difficult to output three - phase symmetrical voltage when the three - phase load is asymmetric. This new inverter can effectively suppress the unbalanced load current to voltage disturbances under the unbalanced load, to ensure approximately symmetrical three - phase voltage output. The control method adopts the new type of three - dimensional space vector pulse width modulation (3D - SVPWM), and the coordinate transformation of CLARK PARK closed - loop control. Through the simulation of MATLAB, it is verified that the three - phase four - leg can solve three phase output voltage problems when the three - phase load is asymmetric. [ABSTRACT FROM AUTHOR]

The paper discusses the impact of unbalanced load on the MV network performance on the example of the power induction furnace for melting metals in the ironwork, demonstrating the ineffectiveness of so far used symmetrization system developed by Steinmetz. Presents the new concept of application of an active power filter (APF) controlled by algorithm based on CPC theory. To confirm the efficiency of such a way of solution the appropriate simulation analyses followed by studies on a physical model of the nonlinear, unbalanced load supplied from MV power network of 6kV were carried out. On the basis of the investigated results the suitable, practical conclusions have been formulated. [ABSTRACT FROM AUTHOR]

This study proposes a new solution for the parallel operation of microgrid inverters in terms of circuit topology and control structure. A combined three-phase four-wire inverter composed of three single-phase full-bridge circuits is adopted. Moreover, the control structure is based on adaptive three-order sliding-mode control and wireless load-sharing control. The significant contributions are as follows. 1) Adaptive sliding-mode control performance in inner voltage loop can effectively reject both voltage and load disturbances. 2) Virtual resistive-output-impedance loop is applied in intermediate loop to achieve excellent power-sharing accuracy, and load power can be shared proportionally to the power rating of the inverter when loads are unbalanced or nonlinear. 3) Transient droop terms are added to the conventional power outer loop to improve dynamic response and disturbance rejection performance. Finally, theoretical analysis and test results are presented to validate the effectiveness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

• A MILP model is presented for coordinated charging of PHEVs. • A four-wire unbalanced LV distribution network is considered. • The effect of the neutral conductor on the network parameters is assessed. • The proposed model is compared with two uncoordinated charging methods. The increasing penetration of plug-in hybrid electric vehicles (PHEVs) imposes new challenges in low voltage distribution networks (LVDNs), due to worsening load imbalance, power losses, and increased probability of blackouts because of the overloads. To overcome these issues, PHEV charging must be performed in a coordinated fashion. In this context, there are several coordinated charging methods in the literature, none of which considers modeling neutral conductors of LVDN. This paper aims to present a mixed-integer linear programming model (MILP) for the PHEV charging problem in unbalanced LVDNs, considering the neutral conductor. In this regard, the main parameters of unbalanced LVDN, such as voltage profile and power loss, are examined with and without modeling the neutral conductor. The results indicate that it is required to consider the neutral conductor to achieve realistic values of the LVDN's parameters. The proposed model is applied to a 449-nodes smart distribution grid consisting of 31 nodes at a medium voltage (MV) level and 418 nodes at a low voltage (LV) level. To verify the performance of the proposed model, it is compared with two uncoordinated charging methods. The results show the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

The global trends nowadays in the power generation industry are to supplement the electricity production using distributed generation (DG) technologies based on renewable energy resources such as photovoltaic, wind power, etc. However, failure to properly control the operation of distributed energy resources as they connect to the exiting power grid could provoke many power quality problems on the grid side. For this reason, due considerations must be given to power generation and safe running before DG units is actually integrated into the power grid. The main aim of this paper is to address the grid interconnection issues that usually arise as DG units connect to the electric grid. The proposed strategy, implemented in Matlab/Simulink environment in different operating scenarios, provides compensation for active, reactive, unbalanced, and harmonic current components of grid-connected nonlinear unbalanced loads. The simulation results obtained in this study demonstrate the level of accuracy of the proposed technique, which ensure a balance in the overall grid phase currents, injection of maximum available power from DG resources to the grid, improvement of the utility grid power factor, and a reduction in the total harmonic distortion of grid currents. [ABSTRACT FROM AUTHOR]

To improve the control performance of unbalanced load of parallel-connected dual induction motors fed by a single inverter, a novel control strategy based on the weighted vector control is proposed. In this paper, a weighted flux linkage vector model of dual induction motors is developed, and then the control expressions of weighted rotor flux linkage and summation torque of dual speed-irrelevant motors are derived. The weight value of vector control for dual induction motors in unbalanced load is distributed automatically according to the torques and motor speeds. The experimental results in multiple induction motor platforms prove that the proposed method can start in a wide load range, especially at heavy unbalanced load and improves the performance of dynamic response with a sudden change of unbalanced load. [ABSTRACT FROM AUTHOR]

In the three-phase circuit at sinusoidal mode a constant and variable components of the vectorial instantaneous power (IP) define two new integrated powers: the power imbalance vector and the non-pulsed power vector. Together with the standard scalar powers of sinusoidal mode (the standard complex power and the complex pulsation power), these two vectorial powers determine power equation of unbalanced mode and pulsing mode. It is shown that under asymmetrical voltage the vectorial MM is zero only if the standard reactive power and the power imbalance equal to zero. The power factor is equal to unity. [ABSTRACT FROM AUTHOR]

This paper proposes a control strategy to reduce the voltage imbalances by injection of a proper negative-sequence current into the microgrid. This task can be done by each inverter connected to the microgrid in a shared way. The current compensation reference is obtained through a negative-sequence voltage control loop, avoiding the load current measurement. The detection of the negative-sequence voltage, at the converter connection point, is performed by a double second order generalized integrator, properly tuned for this purpose. Besides, a current limiter is used to limit the current compensation and avoid the converter overload. This strategy neither requires any hardware modification of the converter nor additional sensors. This new control strategy is simulated in Matlab/Simulink and experimental results are presented to prove its effectiveness.

The aim of this paper is to perform an analysis of the shunt Active Power Filter (APF) operating with unbalanced loads. The impact of the load imbalance on the APF performance is mathematically demonstrated and validated with experimental results using a 25 kVA shunt APF prototype. The APF operation in such conditions is affected by the appearance of DC-link voltage ripple at twice the grid frequency. As consequence, the source currents become distorted and the APF performance deteriorates. Based on this fact, a new control scheme is proposed to avoid high voltage ripple in the DC link, without increasing the DC-link capacitance.

This paper develops an enhanced harmonic voltage compensator which is implemented with the aid of two repetitive controller (RCs) in order to improve the output voltage performance of stand-alone distributed generation (DG) systems. The proposed harmonic voltage compensator is able to maintain the DG output voltage sinusoidal regardless of the use of nonlinear and/or unbalanced loads in the load side. In addition, it can offer good steady-state performance under various types of loads and a very fast dynamic response under load variations to overcome the slow dynamic response issue of the traditional RC. The feasibility of the proposed control strategy is verified through simulations and experiments. [ABSTRACT FROM AUTHOR]

In distribution power systems line transposition is not a common practice and phase loading levels are always changing. Therefore, perfect balance is never achieved at a distribution level. Non-transposed lines can cause unequal voltage drops between phases and this may be further exacerbated by uneven loads in each phase. Consequently, the voltage magnitude of a certain phase may suffer a severe decrease, breaching the lower voltage limit or unbalanced three-phase voltages at a remote bus may violate the percentage voltage imbalance accepted in the standards. This paper examines the mechanism of causing uneven voltage drops and investigates the impacts of unbalanced line configurations with different phase loading levels on network voltage imbalance. Methodologies are proposed to analyze voltage drops affected by line and load imbalance. Based on these methods, general guidelines are recommended to mitigate the voltage problems, which are useful for network analysis, planning and reconfiguration. [ABSTRACT FROM PUBLISHER]

Power-electronics-based hybrid dc power systems (HDCPSs) are increasingly used in many industrial applications such as land, sea, and air vehicles. In these systems, small dc-link and LC filter capacitors are of great interest for weight saving. Usually, in HDCPSs, there are constant power loads and negative dynamic impedance of these loads may generate unstable oscillations. Besides, the risk of resonance may be increased under unbalanced load conditions. In this paper, a distributed active oscillation suppression approach is presented. It is based on the analytical study of the linearized model of the studied system around the operating point. The studied system consists of one main dc source and one storage element supplying two loads: a permanent magnet synchronous motor connected to the dc link by a voltage source inverter and a resistive load supplied by a dc/dc converter through the same dc bus. The proposed approach is particularly used to overcome the resonance under unbalanced load conditions and allows reducing dc link and LC filter capacitors for weight saving. Simulations and experimentations are carried out which confirm the validity of the proposed approach. [ABSTRACT FROM AUTHOR]

Part I of this paper has proposed a duty-cycle-space-vector (DCSV)-based modulation strategy for three- and four-leg matrix converters (MCs). This paper develops a digital DCSV-based modulation strategy for the three- and four-leg MCs based on the results of Part I of this paper. The digital DCSV-based modulation strategy is implemented in a DSP–field programmable gate array (FPGA) platform. The objective is to reduce the calculation time of the modulation algorithm by performing the following: 1) reducing the total number of instructions needed to implement the modulation algorithm in the DSP and 2) implementing parts of the modulation algorithm in an FPGA to generate the switching pattern. Therefore, the calculation process of the modulation algorithm is split between the DSP and the FPGA. [ABSTRACT FROM PUBLISHER]