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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]

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]

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]

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 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]

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]