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Increasing installation of photovoltaic (PV) in the distribution power system has resulted in serious voltage rise, limiting grid-connectable power. This scenario becomes significant in the low-voltage resistive feeder. This paper proposes a decentralized control for distributed PV inverters to mitigate voltage rise. Instead of MPPT (maximum power point tracking) mode, the proposed PV inverter is able to curtail its real power and compensate the reactive power according to the impedance at the installation location. The drooped characteristics between the output power and the impedance are developed so that the PV inverters are able to cooperatively suppress voltage rise based on their local voltage measurement only. Therefore, PV inverters are allowed to supply more power to the utility within voltage limitation. Simulations are conducted to guarantee the proposed control on improvement of voltage rise considering different parameter of feeder. A lab-scaled prototype circuit is established to verify effectiveness in a resistive feeder.

Nowadays, the three-level T-type inverters (3LT2Is) are widely employed in applications with high-performance requirements. The 3LT2Is commonly suffer from neutral point voltage (NPV) unbalance and common-mode voltage (CMV) variation. The medium vector pulse width modulation (MVPWM), which generates the reference voltage using medium and V0[OOO] zero voltage vectors, eliminates the CMV variation in 3LT2Is. However, it cannot keep the NPV balance, negatively affecting power quality. This article proposes a hybrid method to simultaneously eliminate CMV variation and guarantee the NPV balance without topological change or extra hardware. The proposed method uses MVPWM when NPV is balanced, eliminating CMV variation. If NPV goes unbalanced, it utilizes novel high/low triangle PWM (H/LPWM) and MVPWM in a hybrid manner to balance the NPV. The H/LPWM, similar to MVPWM, synthesizes voltage using medium and V0[OOO] voltage vectors to maintain the CMV elimination advantage during the NPV balancing process. However, the difference between H/LPWMs and MVPWM switching patterns adjusts the average neutral point current to balance the NPV. Noteworthy that the proposed method provides NPV balancing ability for the entire power factor range. As a result, it is usable for various applications of 3LT2I, such as photovoltaic inverters and motor drives. The proposed method is carrier-based, which reduces the execution time and simplifies the practical implementation. The simulation study and experimental evaluation have been accomplished, and their results demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Severe voltage distortion, due to power system harmonic resonance, has been reported in recent years. This issue becomes more significant in high penetration of a photovoltaic (PV) network. A conventional voltage detection active filter operates as similar conductance for all harmonic frequencies to resolve this problem whether in a fixed conductance command or in an automatic gain adjustment control. However, its filtering capability is impeded by the mismatch between the active filter and the radial line, and the voltage distortion may still be significant. This paper proposes a discrete frequency tuning active filter to suppress power system harmonics. The active filter operates as variable conductance for each individual harmonic frequency. Each harmonic conductance is dynamically adjusted according to the corresponding harmonic voltage distortion of the active filter installation point in response to increase or decrease of nonlinear loads, or variation of resonant frequency in the power system. The mismatching problem between the feeder impedance and the active filter can be avoided effectively. Therefore, harmonic voltage distortion can be maintained at an allowable level throughout the feeder with lower peak current and lower rms current of the active filter, and loads installed at various locations of the power system receive more uniform voltage waveform. Index Terms--Active power filter, harmonic resonance, harmonic voltage damping.

Distributed generation has attracted great attention in recent years, thanks to the progress in new-generation technologies and advanced power electronics. The Microgrid has been a successful example by integrating various generation sources with the existing power distribution network through power electronic converters. Microgrid converters are required to operate in a decentralized fashion, and power-frequency and reactive-power-voltage droop controls have been adopted for this purpose. In this paper, a droop control technique for imbalance compensation within the three-phase ac distributed generation system is proposed for Microgrid converters. The proposed technique allows even sharing of imbalance current among various converters and can thus be seamlessly integrated with the existing active-power-frequency and reactive-power-voltage droop controls to improve the power quality within the distributed power system. Its control functionalities are discussed, and simulation and laboratory test results are also presented to validate the proposed method. Index Terms--Distributed power system, droop control, voltage imbalance.

The dramatic growth of power electronics applications has led to serious harmonic pollutions in the power system. The phenomenon of harmonic resonance resulting in severe voltage distortion has been reported. Various suppression techniques have been proposed to resolve this problem. The distributed active-filter system (DAFS) is a more effective solution by deploying multiple shunt active filter units (AFUs) in the system to cooperatively damp the harmonic resonance. This paper proposes a dynamic tuning scheme for the DAFS to enhance its filtering performance. Based on the measured voltage harmonic distortion at the installation point, each AFU can dynamically adjust its own filtering volt-ampere (VA) capacity to maintain the total harmonic distortion of the line voltage at an allowable level. The proposed strategy can be integrated with the droop control between the conductance command and VA consumption so that the harmonic-filtering workload is evenly shared among various AFUs. The operation principles and dynamic analyses are presented in this paper, and test results based on simulations and experiments validate the proposed approach. Index Terms--Active filters, distributed active-filter systems (DAFSs), harmonic damping, harmonic resonance.

Increasing demand for premium electric power, in terms of both quality and reliability, and emerging new energy technologies have led to the development of distributed generation systems. Due to rapid penetration of power electronics equipment, many of the loads within distributed generation systems are nonlinear in nature, and the resulting harmonics pollution needs to be addressed. This paper proposes a new cooperative harmonic filtering strategy for the interface converters of distributed generation sources. A droop control method based on the reactive volt-ampere consumption of harmonics of each interface converter is designed and implemented. In this strategy, the overall harmonic filtering workload can be evenly shared without any communications. The operation principle is explained in detail. Computer simulations and laboratory test results validate the distributed harmonic damping capability of the proposed strategy. Index Terms--Distributed generation systems, harmonic damping, harmonic resonance, microgrid.

Nowadays, neutral point voltage (NPV) balancing control and leakage current (LC) reduction methods are considered to improve the reliability of transformerless three-level T-type (3LT2) inverter. Medium vector pulse width modulation (MVPWM) method can control the inverter operation to eliminate the LC; however, is powerless to balance the NPV. Alternatively, discontinuous PWM (DPWM) methods can be implemented to balance the NPV. In this article, a hybrid method for transformerless 3LT2 inverter is proposed to simultaneously minimize the LC and accomplish balancing control in case of NPV unbalancing. The proposed method applies MVPWM switching patterns to eliminate LC while the NPV is balanced. If the NPV becomes unbalanced, it applies specific switching patterns of the DPWM methods as alternative patterns to balance the NPV with minimal LC. Accordingly, the proposed method not only maintains the LC elimination advantage of the MVPWM method but also realizes the NPV balancing capability without any topology modification or adding electrical components. Noteworthy, even the NPV balancing is accomplished with minimal flow of the LC. The proposed method is carrier-based PWM, which shortens execution time and simplifies practical implementation. Experimental tests have been accomplished and results demonstrate the effectiveness of the proposed method on adding the NPV balancing capability with minimal LC to the MVPWM method. [ABSTRACT FROM AUTHOR]

In this paper, the power quality (PQ) improvement of multi-area microgrids (MAMGs) is addressed. To this aim, mitigation of voltage harmonics and unbalance is considered by using distributed generators (DGs) and active power filters (APFs) in a proper way. As a novel approach, the main goal of the proposed method is to pick out redundant APFs and/or significantly reduce of DGs and APFs compensation workload by making collaboration between areas toward integrated PQ improvement of the MAMG. To achieve the desired PQ indices of different areas, supervisory control is planned for proper utilization of available compensators in time. By supervisory control, harmonic propagation between areas and the system unbalance are dealt with and systemic PQ improvement of the MAMG is provided. To this aim, scheduling framework is used to track the PQ situation of each area. Evaluation of the proposed control is carried out by a comprehensive simulation study in MATLAB/Simulink. [ABSTRACT FROM AUTHOR]

An adaptive control of DPWM implementable in clamped-three-level inverters with two strings of photovoltaic (PV) panels in cascaded connection is proposed in this paper. The proposed modulation is developed based on the so-called circuit-level decoupling concept. It is able to provide balance line voltage even under unbalanced dc links. The unequal dc voltages appeared due to asymmetric maximum point of power trackings (MPPTs) are controlled by proper injection of zero-sequence voltage to the common mode voltage, while the total dc voltage is regulated based on the voltage commands generated by the MPPT of each string. Thus, both PV strings are able to be operated based on their MPPTs while there is almost no low-order harmonic distortion at the inverter output current. Compared with the previous methods, the high capability of the proposed method is evaluated by simulation study and experimental tests. [ABSTRACT FROM AUTHOR]

This paper investigates the open-circuit faults of switching devices in the three-level T-Type inverter and proposes a new tolerant control method. The three-phase current and variation of the neutral-point voltage are detected to determine the fault switch. A carrier-based pulse width modulation is developed to tolerate switch fault by transiting from three-level to two-level switching. This makes the modulation scheme easier to be implemented compared to space vector modulation. For the neutral-point fault, the fault phase is operated by two-level modulation to produce the same rating voltage. On the other hand, the inverter is able to produce balanced voltage with reduced rating by keeping the phase of half-bridge fault to the neutral-point voltage. The neutral-point voltage is also able to be balanced by using two-level modulation since it does not contribute the neutral-point current. The performance and effectiveness of the proposed tolerant control are verified by a laboratory build. [ABSTRACT FROM AUTHOR]

Unintentional series and/or parallel resonances, due to the tuned passive filter and the line inductance, may result in severe harmonic distortion in the industrial power system. This paper presents a hybrid active filter to suppress harmonic resonance and to reduce harmonic distortion. The proposed hybrid filter is operated as variable harmonic conductance according to the voltage total harmonic distortion; therefore, harmonic distortion can be reduced to an acceptable level in response to load change or parameter variation of the power system. Since the hybrid filter is composed of a seventh-tuned passive filter and an active filter in series connection, both dc voltage and kVA rating of the active filter are dramatically decreased compared with the pure shunt active filter. In real application, this feature is very attractive since the active power filter with fully power electronics is very expensive. A reasonable tradeoff between filtering performances and cost is to use the hybrid active filter. Design consideration are presented, and experimental results are provided to validate effectiveness of the proposed method. Furthermore, this paper discusses filtering performances on line impedance, line resistance, voltage unbalance, and capacitive filters. [ABSTRACT FROM PUBLISHER]

Voltage fluctuations resulting from variable output power of renewable energy sources are strictly challenging power quality in distributed-generation systems. This paper presents a control method for distributed static synchronous compensator (D-STATCOM) to alleviate variation of both positive- and negative-sequence voltages. The D-STATCOM simultaneously operates as fundamental positive-sequence admittance and fundamental negative-sequence conductance to restore the positive-sequence voltage to the nominal value as well as reduce the negative-sequence voltage to an allowable level. Both admittance and conductance are dynamically tuned to improve voltage-regulation performances in response to load changes and power variation of renewable sources. A proportional–resonant current regulator with selectively harmonic compensation is realized to control the fundamental current of the D-STATCOM as well as reduce the harmonic current, which could be an advantage in practical applications due to high voltage distortion in low-voltage microgrids. Voltage-regulation performances are discussed for different D-STATCOM locations as well as different D-STATCOM currents. Computer simulations and laboratory tests validate effectiveness. [ABSTRACT FROM PUBLISHER]

This paper summarizes the main problems and solutions of power quality in microgrids, distributed-energy-storage systems, and ac/dc hybrid microgrids. First, the power quality enhancement of grid-interactive microgrids is presented. Then, the cooperative control for enhance voltage harmonics and unbalances in microgrids is reviewed. Afterward, the use of static synchronous compensator (STATCOM) in grid-connected microgrids is introduced in order to improve voltage sags/swells and unbalances. Finally, the coordinated control of distributed storage systems and ac/dc hybrid microgrids is explained. [ABSTRACT FROM PUBLISHER]

This paper presents a review of advanced control techniques for microgrids. This paper covers decentralized, distributed, and hierarchical control of grid-connected and islanded microgrids. At first, decentralized control techniques for microgrids are reviewed. Then, the recent developments in the stability analysis of decentralized controlled microgrids are discussed. Finally, hierarchical control for microgrids that mimic the behavior of the mains grid is reviewed. [ABSTRACT FROM PUBLISHER]

Increasing demand of electric-power reliability in industrial parks or in urban areas has led to the development of closed-loop distribution power systems. The harmonic resonance between power-factor-correction capacitors and system inductors may cause significant voltage distortion along the feeder. This paper proposes a discrete frequency-tuning active filter installed at the middle point of a closed-loop line for harmonic-resonance suppression. The active filter operates as a variable conductance for individual harmonic frequency according to the corresponding harmonic-voltage distortion. This algorithm allows the damping conductance of each harmonic frequency to be separately and autonomously adjusted in response to variation of both harmonic sources and resonant capacitors. Therefore, the unintentional “whack-a-mole” phenomenon, due to mismatching between the active filter and the line, can be avoided even if the active-filter location deviates from the middle point. Voltage distortion throughout the closed-loop line can be definitely maintained below an allowable level with lower peak current and lower rms current compared to the conventional voltage-detection active filter. Computer simulations based on a 11.4 kV/15 MVA system and a scaled-down platform in the laboratory are developed to validate the effectiveness of the proposed approach. [ABSTRACT FROM PUBLISHER]