In this paper, optimal control strategy is applied to parallel three-phase inverters using SVPWM technique. The three-phase system is transformed to a synchronous-stationary frame, suitable cost functions are defined, and the switching schemes are determined for the inverters. The control strategy minimizes the circulating current between the inverters, therefore, proper load sharing scheme is achieved. Simulations are performed for steady state and load change operation, and the results which show the suitability of the controller are provided.
In this paper, a new simple and complete technique of modeling and analysis of a matrix converter is presented based on the singular value decomposition (SVD) of modulation matrix. The proposed modeling method yields a new limitation between the matrix converter gain and its input power factor, which is more relaxed as compared to the limits reported so far in the literature. The SVD of the modulation matrix leads to a unified modulation technique which achieves the full capability of a matrix converter. It is shown that this approach is general and all other modulation methods established for a matrix converter are specific cases of this technique. The proposed modulation method can be used to obtain the maximum reactive power in the input of a matrix converter in applications such as wind turbine and microturbine generators, where the input reactive power control is necessary.
This paper introduces a new online voltage disturbance detection approach based on the wavelet transform. The proposed approach: (1) identifies voltage disturbances; and (2) discriminates the type of event which has resulted in the voltage disturbance, e.g. either a fault or a capacitor-switching incident. The proposed approach is: (1) significantly faster; and (2) more precise in discriminating the type of transient event than conventional voltage-based disturbance detection approaches. The feasibility of the proposed disturbance detection approach is demonstrated based on digital time-domain simulation of a power distribution system using the PSCAD/EMTDC software package.
This paper presents the results of a thorough analysis of applying power quality standards to a granite factory. It explains the procedure of determining power quality indices in a power system. The paper presents the field data and provides the results of statistical analysis in order to extract power quality indices. A comparison against standard limits is also given. The case study gives an overview of power quality in such customers and brings up some practical limitations when applying power quality standards.
A thyristor-based static transfer switch (STS) provides a continuous supply for a voltage sensitive load through fast transfer between two sources when the main source does not meet load voltage requirements. This paper investigates the impact of phase difference between the corresponding voltages of the two sources on the load transfer performance achieved by an STS. The studies are performed on the IEEE benchmark STS-1 system using the power system computer-aided design/electromagnetic transient dc simulation software. The studies indicate that the impact of phase difference on the STS load transfer time is insignificant when the sensitive load is a passive (e.g., RL) load. However, if the load can operate in a regenerative mode (e.g., a motor load), the effect of phase difference on the transfer time is noticeable. The studies show that a phase difference of 25deg can increase the load transfer time by about 2 ms
Capacitors are one of the most critical components in power electronic converters, yet they are notoriously susceptible to failure. Avoiding unforeseen outages caused by capacitor failures is one of the most effective approaches to increase system availability. Therefore, a variety of methods have been proposed to monitor a capacitor health condition based on different degradation indicators. This article proposes a new approach based on the accurate measurement of an electrolytic capacitor dissipation factor (DF) to detect its end-of-life. Since the DF is affected by both the capacitor resistance and capacitance simultaneously, it can provide more information about the health condition of the capacitor. To employ the DF as an aging indicator, beyond its accurate measurement, it must be possible to establish an end-of-life criterion and investigate the effect of other environmental factors. Therefore, increasing the frequency of the DF measurement has been suggested as a solution to minimize the effect of the angle measurement error, for which an optimal frequency range has been calculated. In the following, several electrolytic capacitors are subjected to a laboratory study to investigate the effects of capacitor aging, temperature, and measurement frequency on the DF. According to the obtained results, changes in the capacitor resistance dominate the DF, thereby enabling the same end-of-life criterion to be applied for monitoring a capacitor condition.
In this paper, a distributed secondary power sharing approach with low bandwidth communication network is proposed for low voltage direct current (LVDC) microgrids. Conventional droop control causes voltage drop in the grid and also a mismatch on the current of converters in the case of consideration of the line resistances. Proposed control system carry out the current value of the other converters to reach the accurate current sharing and suitable voltage regulation as well. Voltage and current controllers locally regulate the voltage and current of converters as a secondary controller. Secondary controller is realized locally and the communication network is only used to transfer the data of dc currents. Therefore, the secondary controller can regulate the average voltage by only using the data of currents. The proposed approach is verified with simulations based on PLECS.
In this paper; control and parallel operation of multi-modular 3-phase inverters are investigated. Active current sharing approach is selected as the fundamental control technique for parallel operation of inverters due to its high reliability. Among different methods for active current sharing; average load sharing is employed due to its acceptable stability. For this purpose; optimal control strategy and Linear Quadratic Regulator (LQR) method is proposed to parallel 3-phase 4-leg inverters. The system is modeled to be compatible and solvable with LQR equations. Using dq technique; 3-phase inverters are paralleled and controlled. The proposed method obtains both current sharing and voltage tracking and addresses supplying of balanced and unbalanced linear loads in spite of sudden changes. Other advantages of the suggested method include the smallest input energy; simplicity in control circuit; low output voltage Total Harmonic Distortion (THD); low circulating current; robustness to the change in the number of inverters and the possibility of parallel different inverters with different capacities. The results of the simulation verify the performance of the proposed method.
Increase of nonlinear loads in industries has resulted in high levels of harmonic currents and consequently harmonic voltages in power networks. Harmonics have several negative effects such as higher energy losses and equipment life reduction. To reduce the levels of harmonics in power networks, different methods of harmonic suppression have been employed. The basic idea in all of these methods is to prevent harmonics from flowing into a power network at customer sides and the point of common coupling (PCC). Due to the costs, none of the existing mitigating methods result in a harmonic-free power system. The remaining harmonic currents, which rotate in a power network according to the system impedances, may not necessarily result in an optimum harmonic power flow in terms of harmonic undesirable results such as harmonic losses or harmonic over voltage/current due to possible resonances. This paper proposes a new method to control the flow of the remaining harmonics at a power system level such that the effects of harmonics such as losses are optimized. This task is achieved by the use of series active power filters controlled in a different manner from their conventional methods. In this new application, the series active power injects a controlled harmonic voltage into a power system (e.g., a line, transformer, or other elements) to control the harmonic current (power) flow such that the overall performance of the power system is improved in terms of harmonic effects. The series active power filter in this new application is named as harmonic power flow controller (HPFC). In this paper, the theory, structure, applications, and the basics of the control method of a HPFC are described. To investigate the effects of the HPFC on the power network harmonics, an HPFC is used in the 14-busbar IEEE test system. To make the proposal more practical, an HPFC is designed for Iran north-west transmission system to alleviate the harmonic problem in this region. Simulations are carried out to show the effectiveness of the HPFC. Simulation results show that the HPFC can control the harmonic currents (power) and voltage consequently. In this paper, several control algorithms for an HPFC are also considered to achieve the desired harmonic voltages levels.
This paper presents a new control strategy for the shunt active filter in three-phase three-wire systems with zero sequence voltages. An extended method based on instantaneous power theory in a rotating reference frame is developed for extracting the compensating signals for highly/randomly varying loads. Since voltages at the point of common coupling contain low frequency interharmonics, conventional methods can not he used for dc voltage regulation. Therefore, a new method is introduced for this purpose. A three-phase electric arc furnace model is used to show power quality improvement through reactive power and harmonic compensation by a shunt active filter using the proposed method. Simulation results are provided to verify the system performance.
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