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The sliding mode control (SMC) is used to control variable structure systems such as power electronics converters. This paper presents a fault-tolerant strategy based on the SMC for current-controlled AC-DC converters. The proposed SMC is based on three sliding surfaces for the three legs of the AC-DC converter. Two sliding surfaces are assigned to control the phase currents since the input three-phase currents are balanced. Hence, the third sliding surface is considered as an extra degree of freedom which is utilised to control the neutral voltage. This action is utilised to enhance the performance of the converter during open-switch faults. The proposed fault-tolerant strategy is based on allocating the sliding surface of the faulty leg to control the neutral voltage. Consequently, the current waveform is improved. The behaviour of the current-controlled converter during different types of open-switch faults is analysed. Double switch faults include three cases: two upper switch fault; upper and lower switch fault at different legs; and two switches of the same leg. The dynamic performance of the proposed system is evaluated during healthy and open-switch fault operations. Simulation results exhibit the various merits of the proposed SMC-based fault-tolerant strategy. [ABSTRACT FROM AUTHOR]

The droop control is the most effectively used technique for load current sharing in DC microgrid. The disadvantages of the conventional droop controllers are the increased voltage deviation and current sharing errors when the load current is increased. This paper presents two improved techniques for current sharing in DC microgrids. In the first technique, the droop resistance is updated for each converter in the microgrid to eliminate the current sharing error. The current sharing reference for each converter is calculated based on the ratio of its rating to the load current. Hence, the load current is the only parameter that needs to be communicated to the current sharing loops. The second proposed technique is based on utilizing optimal droop resistances to eliminate the need for current sharing loops and their associated communications. The harmony search (HS) technique is utilized to attain the optimal droop resistances which results in minimizing the error in the current sharing. A comparison is made between the results obtained using the HS technique and the particle swarm optimization technique. The dynamic performance of the droop controller using the proposed current sharing loop and the proposed optimal droop parameters is evaluated using the PSCAD/EMTDC simulation package. [ABSTRACT FROM AUTHOR]

In the past decade, the Z-source inverter (ZSI) is successfully developed for many applications. This paper proposes a simple integrated control scheme for the high performance ZSI. A proportional-integral PI controller is used to regulate the voltage of the Z-network capacitor. Moreover, the peak value of the DC-link voltage is controlled by limiting the shoot-through duty ratio. Furthermore, the instantaneous voltage control technique is utilized to retain the sinusoidal shape of the output voltage. A feed-forward signal of the AC voltage is employed to smooth the output voltage. To generate the PWM signals for the ZSI switches, the simple boost control method is utilized which coordinates the operation of different controllers. Extensive digital simulations are conducted to evaluate the dynamic behavior of the proposed control scheme. Moreover, an experimental setup is built to verify the performance of the proposed control scheme for the high performance ZSI. [ABSTRACT FROM AUTHOR]

Distributed generation and distributed energy storage systems can be controlled to improve power system stability. This article proposes control strategies for adopting distributed generation units to autonomous microgrids. The proposed distributed generation interface is utilized not only to control the active power flow, but also to provide essential ancillary services, such as voltage regulation and frequency control, to the autonomous microgrid. A coordinated voltage and frequency control of the inverter-based distributed generation and distributed energy storage system is presented. Moreover, a frequency estimation algorithm based on orthogonal filters is adopted for the frequency control loop. One advantage of the proposed control system is its unified structure for the different operating modes. Extensive simulations of the proposed coordinated control of the inverter-based distributed generation and distributed energy storage system are conducted to evaluate the dynamic performance of the proposed microgrid. [ABSTRACT FROM AUTHOR]

This article presents a transformerless off-line uninterruptible power supply with a reduced power device count. The operation of the proposed uninterruptible power supply is divided into two modes. The first is the charging mode, where a pulse width modulation rectifier and a buck converter are controlled to charge the battery bank with a sinusoidal supply current at a unity power factor. The second mode is the inverting mode, where a boost converter is used to regulate the DC-link voltage to a level suitable for a sinusoidal pulse width modulation inverter to generate the rated load voltage. The inverter is working at a constant modulation index to provide a high-quality output voltage to the load during the interruption. The use of buck and boost converters eliminates the need for a transformer to adjust the DC voltage level for the charging process or the AC voltage level across the load. A hardware setup is implemented to evaluate the dynamic behavior of the proposed uninterruptible power supply system. Furthermore, the proposed uninterruptible power supply system is simulated using the PSCAD/EMTDC (Manitoba, Canada) software package. Several test cases with different load types are conducted to study the effectiveness of the proposed control system. Accurate performance and fast dynamics are revealed from both the simulation and experimental results. [ABSTRACT FROM PUBLISHER]