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This article proposes an improved space vector modulation (SVM) for the current-source rectifier (CSR). Due to the overlap time during commutations of power switches, the pulsewidth-modulation (PWM) currents of CSR may suffer from a number of low-order (mainly fifth and seventh) harmonics, which seriously degrade the harmonic performance of currents and voltages on the grid side. Especially, the overlap-time effect becomes an important issue when the increased switching frequency is applied to CSR. To mitigate these low-order harmonics of the PWM converter currents resulted from the overlap-time effect, the relationship between the overlap time and the low-order harmonics in PWM currents and grid currents are analyzed at first. Then, the design process of the LC filter and the proposed SVM with optimal overlap-time distribution is presented in details. Finally, experimental results are conducted on a low power laboratory prototype to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Current source converter (CSC) is widely used in high power applications. To increase the system power rating, both series and parallel connections can be adopted and implemented easily. In this article, a mixed series–parallel CSC structure is proposed for high power application. An optimized carrier-shift sinusoidal pulsewidth modulation (SPWM) is adopted for the mixed system to simultaneously improve the dc-link voltage quality and achieve multilevel ac output, which can potentially reduce the size of dc- and ac-side filters. Meanwhile, the system reliability can be significantly improved due to the modular design with carrier-shift SPWM and inherent short-circuit protection of CSC. With these advantages, the proposed mixed series–parallel CSC structure can be an attractive candidate for medium-voltage drives, high-voltage direct current, and offshore wind power applications. Simulation and experimental results verify the effectiveness of the novel mixed series–parallel CSC system with optimized carrier-shift SPWM. [ABSTRACT FROM AUTHOR]

In this article, current source converter (CSC)-fed permanent magnet synchronous motor (PMSM) drives are investigated for electrical submersible pump (ESP) applications. The CSC features motor-friendly waveform with low dv/dt, which is very suitable for long cable application. High-speed ESP by adopting a PMSM has higher efficiency and shorter length and diameter compared with the counterpart adopting induction motor. Sensorless control instead of mechanical sensor can further reduce the system size/cost and improve reliability due to the harsh downhole environment. However, low switching frequency and an inverter-side output filter cause great challenge in controller and observer design. These kinds of influence become more serious under a higher speed region due to the limited updates per fundamental cycle. To overcome these challenges, the dynamic capacitor voltage control is employed in a field-oriented control scheme to enhance the system dynamic response. Moreover, a discrete-time sliding mode observer based rotor position estimation method is proposed to reduce estimation error and improve parameter uncertainty robustness. The effectiveness of the proposed method is verified on a CSC-fed PMSM with simulation and experimental results. [ABSTRACT FROM AUTHOR]

Using the grid-interfacing voltage-source converters (VSCs) to perform harmonic control as a smart ancillary function is increasingly studied in recent years. Different from the traditional dedicated harmonic compensation converters, the main function of the high-power smart converters is still delivering real power at a low switching frequency (e.g., 2 kHz). It is challenging to compensate harmonics with low switching frequency VSCs due to the low sampling rate and large system delay. This article proposes a multirate control scheme and derives the accurate multirate model to improve harmonic control. The multirate control scheme consists of high-rate harmonic control and synchronous sampled fundamental control. With the proposed control scheme, the advantages of the conventional synchronous sampling method and the benefits of the high sampling rate are achieved simultaneously. To accurately model the low switching VSC with two different sampling rates, the lifting method is introduced to achieve the discrete-time system model for this linear periodic time-varying system. The frequency response and stability analysis are carried out based on the lifted model. The modeling method, as well as the enhanced harmonic control performance of the multirate control structure, is validated in the experiments. [ABSTRACT FROM AUTHOR]

Multilevel converters are widely applied in medium voltage (MV) drive systems due to the attractive features, such as the ability to use devices with lower voltage ratings, high equivalent switching frequency, etc. Recently, a multilevel topology, named seven-level hybrid clamped (7L-HC), is proposed for MV drives. It has a competitive number of devices, showing the potential to build low-cost 7L converters. However, to ensure the wide frequency range operation and power quality of MV drives, its dc-link capacitors and floating capacitors must be controlled properly with balanced voltages. In this paper, an active voltage balancing method is proposed for the 7L-HC converter. This method can effectively balance all the capacitor voltages under a wide frequency range. The method is embedded in the PWM procedure without modifying the control strategies. Simulation and experimental results are provided to validate the proposed method. [ABSTRACT FROM AUTHOR]

In a hybrid ac/dc grid, the ac-side unbalance voltage introduces adverse effects on dc grids and interfacing converters (IFC) (active power oscillations, dc-link voltage oscillations, and IFC peak current increase at the same average power production). For parallel IFCs, these adverse effects can be easily aggregated. In this paper, two new control strategies are proposed for parallel IFCs to improve both ac and dc subgrids power quality. The proposed control strategies focus on canceling active power oscillations of parallel IFCs that provide oscillation-free dc link and sharing collective peak current of parallel IFCs. In the first proposed control strategy, IFCs’ power coefficients are controlled by solving a set of nonlinear equations, and this method is called as coefficient-based strategy, whereas, in the second proposed control strategy, peak currents of IFCs are controlled directly through the derived relationship of IFCs' peak currents under zero power oscillation in this paper, and this method is named as peak current based strategy. This peak current based strategy features much simplified calculation and could be easy to implement. To achieve control objectives, thorough study on parallel IFCs' peak currents is conducted. Based on the study, it is shown that the collective peak current of all IFCs is a constant under zero total active power oscillation, and therefore, keeping all IFCs' peak currents in the same phase and in-phase with collective peak current optimizes the utilization range of parallel IFCs. The numerical examples and experimental results are provided to verify the validity of the proposed control strategies under different operating conditions. [ABSTRACT FROM AUTHOR]

The nonlinear loads in distribution grid may generate harmful low-order harmonics, polluting the grid and deteriorating the voltage quality particularly when the grid is weak. The grid interfacing converters in the distribution system, such as the distributed generation (DG) interfacing converters or hybrid ac/dc grid interlinking converters, can participate in distribution grid harmonic control. In this paper, two virtual-impedance-based harmonics control methods are developed for grid interfacing voltage-source inverters (VSIs) to improve the power quality of the distribution grid. As the control parameters are designed based on the virtual impedance theory, clear physical meanings are given to explain their impacts on the system. Also, the proposed methods do not rely on the closed loop feedback control and, therefore, are very suitable for VSIs with low switching frequency (such as those for high-power DGs or interlinking converters for hybrid ac/dc systems), whose closed-loop control feedback bandwidth may be limited for harmonic regulation. The influence of system delay and feedback control loop is considered and modeled in the design procedure; thus more accurate virtual impedance control is achieved for low-switching-frequency VSIs. Moreover, a comprehensive comparison of the two methods, including stability and harmonic compensation performance, are given. Their effectiveness is verified by experiment results. [ABSTRACT FROM PUBLISHER]

Microgrid converters are required to have the capability of both grid-tied mode and islanding mode operation. For this dual-mode operation, large shunt capacitors are often used in the interfacing converter output LCL filter, as it can help to stabilize supply voltage and to reduce switching ripple pollutions to sensitive loads during autonomous islanding operation. At the same time, this modification causes a few challenges, including the low-frequency harmonic distortions, the steady-state tracking errors and the slow dynamic response, to the line current regulation during grid-tied operation. To overcome these drawbacks, a modified weighted average current controller is developed. First, to realize a fast line current response, a deadbeat control of weighted average current is developed based on a reduced-order virtual filter plant. Second, a grid voltage feed-forward term is added to the weighted average current reference to mitigate the steady-state line current tracking errors. Note that this compensation term is directly added to the current reference, thus, it is very well decoupled from the closed-loop current regulator. In addition, it can be seen that the low-order line current harmonics caused by grid voltage distortion is inherently compensated by this proposed corrective feed-forward control. [ABSTRACT FROM AUTHOR]

The growing installation of distributed generation (DG) units in low-voltage distribution systems has popularized the concept of nonlinear load harmonic current compensation using multifunctional DG interfacing converters. It is analyzed in this paper that the compensation of local load harmonic current using a single DG interfacing converter may cause the amplification of supply voltage harmonics to sensitive loads, particularly when the main grid voltage is highly distorted. To address this limitation, unlike the operation of conventional unified power quality conditioners with series converter, a new simultaneous supply voltage and grid current harmonic compensation strategy is proposed using coordinated control of two shunt interfacing converters. Specifically, the first converter is responsible for local load supply voltage harmonic suppression. The second converter is used to mitigate the harmonic current produced by the interaction between the first interfacing converter and the local nonlinear load. To realize a simple control of parallel converters, a modified hybrid voltage and current controller is also developed in the paper. By using this proposed controller, the grid voltage phase-locked loop and the detection of the load current and the supply voltage harmonics are unnecessary for both interfacing converters. Thus, the computational load of interfacing converters can be significantly reduced. Simulated and experimental results are captured to validate the performance of the proposed topology and the control strategy. [ABSTRACT FROM AUTHOR]

Flexible control of interfacing converter has been utilized to compensate the harmonics from nonlinear loads in a power distribution system. However, the site selection of a DG unit is mainly determined by the location of renewable energy resources that might be far away from the nonlinear load. This can degrade the performance of conventional nonlinear load current measurement methods using current transformers and shielded twisted-pair wires, and accordingly, make the harmonic compensation less effective. To solve this issue, the fundamental components of the PCC voltage and the DG installation point voltage are utilized to establish links between the DG unit controller and the power quality (PQ) meter for PCC nonlinear load monitoring. With this method, the harmonic current waveform can be accurately sent from the PQ meter to the DG unit controller with a very low bandwidth communication system, and it is not required to have any clock synchronization at the sending and the receiving ends of the communication system. Simulated and experimental results are provided to verify the correctness of the proposed measurement method. [ABSTRACT FROM PUBLISHER]

For high-power PWM current-source motor drive systems, due to the low converter switching frequency and the relative small dc choke for reduced cost/weight, the converters’ switching harmonics may interact through dc link and produce interharmonics in the entire system. Such harmonics interaction phenomenon may give rise to the system resonance at certain motor speeds, which degrades the grid-side power quality and generates excessive torque ripples on the motor side. The resonance caused by the harmonics interaction in high-power PWM current-source motor drives is investigated in previous study. In addition, to actively suppress such resonance, the basic idea of a dc-link virtual impedance-based suppression method has also been proposed. This paper extends the previous study to thoroughly analyze the mechanism and realization of resonance suppression by the dc-link virtual impedance-based method. The in-depth analysis shows that the dc-link virtual impedance-based method successfully enables the active interharmonics compensation capability of high-power PWM current-source drives, which is not addressed in previous researches. Moreover, simulations and experiments demonstrate that, by following the selection of coefficient in the suppression method discussed in this paper, the dc-link virtual impedance-based method can effectively enhance the attenuation effect of dc link in high-power PWM current-source drive systems so as to suppress the resonance due to the harmonics interaction under all resonance conditions. [ABSTRACT FROM AUTHOR]

Today, many renewable energy-based distributed generation (DG) units are connected to the grid with interfacing converters. Therefore, the harmonic compensation functions can be realized through flexible control of these converters. This article reviews the distribution system harmonic compensation methods using the DG units with a focus on the DG local controllers. How to realize the harmonics compensation functions in the traditional current controlled and voltage-controlled DG systems is presented. A hybrid-control method (HCM) is also discussed, which features better compensation performance and more flexible compensation strategies. Other issues, including harmonics current sharing among the DG units, are also discussed in this article. With the harmonics compensation functions, future DG systems could actively participate in the distribution system power quality control activities. [ABSTRACT FROM PUBLISHER]

To accomplish superior harmonic compensation performance using distributed generation (DG) unit power electronics interfaces, an adaptive hybrid voltage and current controlled method (HCM) is proposed in this paper. It shows that the proposed adaptive HCM can reduce the numbers of low-pass/bandpass filters in the DG unit digital controller. Moreover, phase-locked loops are not necessary as the microgrid frequency deviation can be automatically identified by the power control loop. Consequently, the proposed control method provides opportunities to reduce DG control complexity, without affecting the harmonic compensation performance. Comprehensive simulated and experimental results from a single-phase microgrid are provided to verify the feasibility of the proposed adaptive HCM approach. [ABSTRACT FROM AUTHOR]

In order to address the load sharing problem in islanding microgrids, this paper proposes an enhanced distributed generation (DG) unit virtual impedance control approach. The proposed method can realize accurate regulation of DG unit equivalent impedance at both fundamental and selected harmonic frequencies. In contrast to conventional virtual impedance control methods, where only a line current feed-forward term is added to the DG voltage reference, the proposed virtual impedance at fundamental and harmonic frequencies is regulated using DG line current and point of common coupling (PCC) voltage feed-forward terms, respectively. With this modification, the impacts of mismatched physical feeder impedances are compensated. Thus, better reactive and harmonic power sharing can be realized. Additionally, this paper also demonstrates that PCC harmonic voltages can be mitigated by reducing the magnitude of DG unit equivalent harmonic impedance. Finally, in order to alleviate the computing load at DG unit local controller, this paper further exploits the band-pass capability of conventionally resonant controllers. With the implementation of proposed resonant controller, accurate power sharing and PCC harmonic voltage compensation are achieved without using any fundamental and harmonic components extractions. Experimental results from a scaled single-phase microgrid prototype are provided to validate the feasibility of the proposed virtual impedance control approach. [ABSTRACT FROM AUTHOR]

This paper presents a hybrid voltage and current control method to improve the performance of interfacing converters in distributed generation (DG) units. In general, current-controlled methods have been widely adopted in grid-connected converters nowadays. Nevertheless, in an islanded system, the voltage control of DG units is desired to provide direct voltage support to the loads. Due to the absence of closed-loop line current controller, the voltage control scheme can hardly regulate the DG unit's line current harmonics. Furthermore, if not addressed properly, the transfer between the grid-connected operation and autonomous islanding operation will introduce nontrivial transient currents. To overcome the drawbacks of voltage- and current-controlled DG units, this paper develops a hybrid voltage and current control method (HCM). The proposed method allows the coordinated closed-loop control of the DG unit fundamental voltage and line harmonic currents. With the HCM, local harmonic loads of the DG unit can even be compensated without using harmonic current extraction. In addition, the HCM guarantees smooth transition during the grid-connected/islanding operation mode transfer. Simulated and experimental results are provided to verify the feasibility of the proposed approach. [ABSTRACT FROM PUBLISHER]

Fast and accurate selective harmonic detection has crucial value for many power system harmonic compensation systems. The existing detection methods are known to have a few drawbacks, such as long delay time, sensitivity to grid frequency variation, and difficulty to achieve zero steady-state error. In order to overcome these drawbacks, this paper proposes a selective harmonic detection system based on the three-phase cascaded delayed signal cancellation phase-locked loop. The system can be flexibly configured to detect any individual harmonic from the source with various background harmonics. It also features very short transient and excellent adaptability under small and considerable frequency variations, as verified by comprehensive experimental results. Finally, this paper provides guidance on how to tailor the detection system for different applications and a solution to address the practical implementation issues. [ABSTRACT FROM PUBLISHER]

The increased penetration of nonlinear loads and power-electronics-based distributed generation (DG) systems may introduce power quality issues to the distribution power system. However, if controlled and regulated properly, the DG–grid interfacing converters are able to improve the distribution system efficiency and power quality. This paper focuses on the distribution system harmonic control through the DG–grid interfacing converters. Two alternative DG systems, namely, current-controlled DG and voltage-controlled DG, are considered. While most of the previous works on harmonic compensation are based on the current-controlled method, a novel harmonic control scheme using a voltage-controlled method is developed in this paper. The voltage-controlled method is more flexible and has similar compensation performance compared to the conventional current-controlled method. In addition, by avoiding the implementation of a harmonic current tracking loop, the proposed voltage-based method can be seamlessly incorporated into a voltage-controlled DG unit, which is important to provide direct voltage and frequency support in a microgrid. Moreover, the possible harmonic circulating current among multiple DG systems is also investigated in this paper. Simulations and experimental results from a three-phase 5-kVA laboratory DG prototype are provided to validate the discussion. [ABSTRACT FROM AUTHOR]