Harmonic Components Of Load Current Research Articles

A Distributed Current Sharing Strategy for Islanded AC Microgrids Based on Low-Bandwidth Communication

Bus voltage regulation and current sharing are significant to ensure the normal operation of islanded microgrids. In this paper, a low-bandwidth communication (LBC)-based distributed current sharing strategy for islanded ac resistive microgrids is proposed, especially when the wire impedances of parallel-connected inverters are mismatched. In a general case, all the balanced, unbalanced and harmonic components of load current should be shared accurately. Therefore, in the local controller, those components of inverter output currents are extracted and regulated to follow the component average values generated based on LBC. To guarantee the bus voltage regulation, the influence of current sharing loops on voltage references is eliminated by subtracting the average voltage reference compensation from the current sharing controller outputs. A simplified inverter small-signal model is established, based on which the system stability is analyzed. Besides, the steady-state bus voltage regulation performance is discussed. The simulation in MatLab/Simulink is used to demonstrate the current sharing performance and bus voltage regulation ability. A hardware-in-loop platform of an ac microgrid prototype is built, where the power stage is in a real-time simulator and the controllers are implemented with DSPs. The results validate the effectiveness of the proposed strategy.

An enhanced current sharing strategy for islanded ac microgrids based on adaptive virtual impedance regulation

In islanded mocrogrids, the mismatch among the equivalent impedances (sum of output impedance and wire impedance) degrades the current sharing performance. It is an essential mission to eliminate this mismatch to achieve accurate current sharing in different conditions. About this issue, several aspects should be considered. Firstly, for a general microgrid, the inverter equivalent impedances are unknown, both the resistance and reactance should be considered. Secondly, all the balanced, unbalanced and harmonic components of load current should be shared according to the power ratings of distributed generators. Besides, not only the current sharing in steady states but also during transients should be achieved. Herein, a low-bandwidth communication-based distributed current sharing strategy for parallel-connected inverters is proposed in this paper. The virtual resistance and reactance are adaptively regulated according to the magnitude error and phase angle error of the inverter uniform output current respectively. Then they can be used to reshape the inverter equivalent impedance at fundamental positive-sequence frequency, fundamental negative-sequence frequency and harmonic frequencies. With the proposed method, all the three aspects mentioned above can be guaranteed without knowing inverter wire impedances in advance. The simulation and experiments on an islanded microgrid prototype are developed to verify the effectiveness of the proposed strategy.

Predictive current control of FL-shunt active power filter for dynamic and heterogeneous load compensation

In this paper, a predictive current control technique is utilized to generate switching pulses for four-leg-shunt active power filter (FL-shunt active power filter). The d-q theory technique is executed for removing the reactive and harmonic components of load current. MATLAB-based simulation and FPGA-controlled experimentation have been carried out to evaluate the behavior of predictive current control technique under dynamic and heterogeneous loading conditions. The 3-phase induction machine is considered as a dynamic load, and the capability of FL-shunt active power filter is assessed for compensating the reactive power. The compensation behavior of FL-shunt active power filter is observed with heterogeneous load, which is an amalgamation of dynamic and unstable static loads. Under steady-state and transient conditions, the simulation and experimentation results demonstrate that the proposed technique makes the supply current sinusoidal as well as balanced; compensates the reactive power; maintains near to unity power factor at the point of common coupling (PCC); and annuls the current flow in neutral conductor.

A Modified One-Cycle-Control-Based Active Power Filter for Harmonic Compensation

This paper proposes a modified one cycle control (OCC) for active power filter (APF) to compensate only harmonic components of load currents. The conventional OCC-based APF cannot differentiate between the harmonic and reactive components of load currents, which leads to the increment in device current rating because of compensation of large reactive currents besides harmonics. The reference signals of the OCC control core have been reconstructed and the associated control equations have been developed, following the control philosophy of simplicity. The modified scheme is easy to implement and only three more sensors are required for measuring load currents compared with the conventional OCC. This modified OCC guarantees the excellent performance of harmonic compensation alone and retains the advantages of the conventional OCC, such as no phase locking loop and the constant switching frequency. The feasibility and performance of the proposed OCC has been validated by experimental studies on a 5 kVA APF hardware platform developed in laboratory.

A Multifunctional DSTATCOM Operating Under Stiff Source

Loads connected to a stiff source cannot be protected from voltage disturbances using a distribution static compensator (DSTATCOM). In this paper, a new control-algorithm-based multifunctional DSTATCOM is proposed to operate in voltage control mode under stiff source. This scheme provides fast voltage regulation at the load terminal during voltage disturbances and protects critical loads. In addition, during normal operation, the generated reference load voltages allow control of the source currents. Consequently, DSTATCOM injects reactive and harmonic components of load currents to make source power factor unity. Simulation and experimental results are presented to verify the efficacy of the proposed control algorithm and multifunctional DSTATCOM.

Pre-sampled data based prediction control for active power filters

A prediction control based on pre-sampled data from previous fundamental cycle for active power filters is presented in this paper. In which the reference current is calculated by the predicted current values in few steps ahead. In this way the active filter current phase delay can be effectively compensated. In addition the detection of harmonic components of load current, design of conventional proportional–integral (PI) current controller and hardware implementation for active filters are also discussed. The proposed prediction method is simple and easy for practical application. Tests were carried out for active filter with and without the phase delay compensation. Results show that the proposed prediction method is effective in compensation the phase delay of filter current.

Control Strategy for Selective Compensation of Power Quality Problems through Three-Phase Four-Wire UPQC

This paper presents a novel control strategy for selective compensation of power quality (PQ) problems, depending upon the limited rating of voltage source inverters (VSIs), through a unified power quality conditioner (UPQC) in a three-phase four-wire distribution system. The UPQC is realized by the integration of series and shunt active power filters (APFs) sharing a common dc bus capacitor. The shunt APF is realized using a three-phase, four-leg voltage source inverter (VSI), while a three-leg VSI is employed for the series APF of the three-phase four-wire UPQC. The proposed control scheme for the shunt APF, decomposes the load current into harmonic components generated by consumer and distorted utility. In addition to this, the positive and negative sequence fundamental frequency active components, the reactive components and harmonic components of load currents are decomposed in synchronous reference frame (SRF). The control scheme of the shunt APF performs with priority based schemes, which respects the limited rating of the VSI. For voltage harmonic mitigation, a control scheme based on SRF theory is employed for the series APF of the UPQC. The performance of the proposed control scheme of the UPQC is validated through simulations using MATLAB software with its Simulink and Power System Block set toolboxes.