scholarly journals Enhanced Circular Chain Control for Parallel Operation of Inverters in UPS Systems

2020 ◽  
Vol 12 (19) ◽  
pp. 8062 ◽  
Author(s):  
Mahdi Shahparasti ◽  
Mehdi Savaghebi ◽  
Majid Hosseinpour ◽  
Navid Rasekh
Keyword(s):  
Control Structure ◽  
Parallel Operation ◽  
Nonlinear Loads ◽  
Uninterruptible Power Supply ◽  
Loop Control ◽  
Current Sharing ◽  
Circular Structure ◽  
Uninterruptible Power ◽  
Equal Power ◽  
A Current

In this paper, a current sharing method based on the circular chain control (3C) method is proposed for controlling parallel inverters of unequal ratings in uninterruptible power supply (UPS) applications. Due to its circular structure, 3C is one of the most convenient methods which can be used in UPS as well as microgrid systems. However, the conventional 3C control strategy is only applicable to inverters of equal power ratings. The proposed method not only retains the circular structure of the 3C method, but also provides adaptability for the parallel operation of inverters with different power ratings. Moreover, this method adds hot-swap capability to the parallel inverter. A two-loop control structure is used to control the inverters. For proper current sharing, currents of inverters are conveyed in a circular structure with appropriate gains through control links. Simulation and experimental results for linear and nonlinear loads verify the effectiveness of the proposed strategy.

scholarly journals Unbalanced Current Sharing Control in Islanded Low Voltage Microgrids

Energies ◽  
2018 ◽  
Vol 11 (10) ◽  
pp. 2776 ◽  
Author(s):  
Foad Najafi ◽  
Mohsen Hamzeh ◽  
Matthias Fripp
Keyword(s):  
Output Power ◽  
Control Structure ◽  
Nonlinear Loads ◽  
Negative Sequence ◽  
Current Sharing ◽  
Current Controller ◽  
Voltage Controller ◽  
Zero Sequence ◽  
Resonant Controller ◽  
Virtual Impedance

This paper reports a new control strategy to improve sharing of unbalanced currents in islanded LV microgrids. This technique provides fast and effective sharing of positive-, negative- and zero-sequence currents, and is the first example of zero-sequence current sharing in the literature. The controllers are designed in the stationary frame. The control structure consists of four loops: (1) the current controller; (2) the voltage controller; (3) the droop controller and the (4) negative and zero sequence current controllers. The output current is considered unknown for the controller and is added to the control system as a disturbance. The proposed controller features a high gain in fundamental and harmonic frequencies, hence a good voltage quality is obtained in the presence of unbalanced and nonlinear loads. To this aim, a proportional-resonant (PR) controller is adopted as the current controller. By using a multi-resonant controller as current controller, a unified control structure is obtained which is suitable for both grid-connected and islanded modes. The voltage controller is designed using a resonant controller so that the voltage can have low VUF and THD in the presence of unbalanced and nonlinear loads. Furthermore, in this paper, the droop method is applied to the control structure to share real and reactive powers. Simulation studies show that the conventional droop method cannot share the oscillatory part of the output power that is due to the presence of unbalanced loads in the microgrid. This paper relies on using zero and negative sequence virtual impedance controller to share the oscillatory part of output power. By using zero-sequence virtual impedance controller (ZSVIC) and negative-sequence virtual impedance controller (NSVIC), the zero and negative sequence currents in the microgrid are controlled and shared effectively. By compensating zero- and negative-sequence currents locally, the flow of these currents in the microgrid is minimized, and the overall power quality of the islanded LV microgrid is improved.

Parallel operation of uninterruptible power supply systems in microgrids

2007 ◽  
Author(s):  
Josep M. Guerrero ◽  
Juan C. Vasquez ◽  
Jose Matas ◽  
Jorge L. Sosa ◽  
Luis Garcia de Vicuna
Keyword(s):  
Power Supply ◽  
Parallel Operation ◽  
Uninterruptible Power Supply ◽  
Uninterruptible Power ◽  
Supply Systems

scholarly journals Unbalanced Current Sharing Control in Islanded Low Voltage Microgrids

2018 ◽  
Author(s):  
foad najafi ◽  
Matthias Fripp
Keyword(s):  
Output Power ◽  
Control Structure ◽  
Nonlinear Loads ◽  
Negative Sequence ◽  
Current Sharing ◽  
Current Controller ◽  
Voltage Controller ◽  
Zero Sequence ◽  
Resonant Controller ◽  
Virtual Impedance

This paper reports a new control strategy to improve sharing of unbalanced currents in islanded LV microgrids. This technique provides fast and effective sharing of positive-, negative- and zero-sequence currents, and is the first example of zero-sequence current sharing in the literature. The controllers are designed in the stationary frame. The control structure consists of four loops; 1)~the current controller; 2)~the voltage controller; 3)~the droop controller and the 4)~negative and zero sequence current controllers. The output current is considered unknown for the controller and is added to the control system as a disturbance. The proposed controller features a high gain in fundamental and harmonic frequencies, hence a good voltage quality is obtained in the presence of unbalanced and nonlinear loads. To this aim, a proportional-resonant (PR) controller is adopted as the current controller. By using a multi-resonant controller as current controller, a unified control structure is obtained which is suitable for both grid-connected and islanded modes. The voltage controller is designed using a resonant controller so that the voltage can have low VUF and THD in the presence of unbalanced and nonlinear loads. Furthermore, in this paper droop method is applied to the control structure to share real and reactive powers. Simulation studies show that the conventional droop method cannot share the oscillatory part of the output power that is due to the presence of unbalanced loads in the microgrid. This paper relies on using zero and negative sequence virtual impedance controller to share the oscillatory part of output power. By using zero-sequence virtual impedance controller (ZSVIC) and negative-sequence virtual impedance controller (NSVIC), the zero and negative sequence currents in the microgrid are controlled and shared effectively. By compensating zero- and negative-sequence currents locally, the flow of these currents in the microgrid is minimized, and the overall power quality of the islanded LV microgrid is improved.

scholarly journals Power Quality Survey of Voltage Stabilizers and Uninterruptible Power Supply With Nonlinear Loads

2019 ◽  
Vol 3 (6) ◽  
Author(s):  
Joel Ogunyemi ◽  
Zacchaeus Adesakin Adetona
Keyword(s):  
Power Quality ◽  
Power Supply ◽  
Low Voltage ◽  
Nonlinear Loads ◽  
Crest Factor ◽  
Peak Voltage ◽  
Uninterruptible Power Supply ◽  
Current Frequency ◽  
Significant Difference ◽  
Uninterruptible Power

Amongst the power quality problems faced in Nigeria, interruption and voltage fluctuation seem to be foremost. Thus, household electrical and electronic appliances are most times under the threat of outages and low voltage supply which cause them to malfunction or in extreme cases, get damaged. To address these problems, the use of uninterruptible power supply (UPS) and automatic voltage regulator commonly refer to as “stabilizer” has become very popular. The objective of this work is to examine the quality of these devices when powering other nonlinear devices. The experimental study was carried out by using a power harmonic analyzer to display the output parameters and waveform from each device under test when powering nonlinear loads. Parameters measured for each device were voltage, current, frequency, peak-to-peak voltage, total harmonic distortion (THD) and crest factor. The results showed that for the measured parameters, while other parameters relatively remain within the standards, the THD took an exemption as it was relatively higher with a range of 9.9 to 19.9 % for various combination of loads. Though the outputs from the UPS were relatively higher than that of stabilizer (except for THD and crest factor where those of the stabilizer were higher), there was no significant difference between the parameter values measured.  The implication of this was that increasing utilization of UPS and stabilizer will invariably continue to contribute more to the pollution of the existing weak network. Adequate measures were suggested to mitigate this. This research can be useful for the stakeholders in power industry for power quality improvement.

A Rule Based Expert System for Identification of Harmonics Originating from Single Phase Nonlinear Loads

2006 ◽  
Vol 7 (2) ◽  
Author(s):  
Azah Mohamed ◽  
Kingsley Chidi Umeh ◽  
Aini Hussain ◽  
Ramizi Mohamed
Keyword(s):  
Expert System ◽  
Single Phase ◽  
Fluorescent Lamp ◽  
Nonlinear Loads ◽  
Accurate Identification ◽  
Uninterruptible Power Supply ◽  
Rule Based ◽  
Power System Harmonics ◽  
Different Types ◽  
Uninterruptible Power

Presently, there is still a critical shortage of experts to interpret and diagnose problems associated with power system harmonics. Thus, the use of a rule based expert system is proposed for the identification of harmonic sources originating from various single phase nonlinear loads such as uninterruptible power supply, personal computer, fluorescent lamp with magnetic and electronic ballasts, PC monitor and oscilloscope. Identification of harmonics originating from single phase nonlinear loads is implemented by first analyzing the harmonic current waveforms using fractal and fast Fourier transform analyses so as to characterize the harmonic signatures of the different types of nonlinear loads. Then, a rule-based expert system is developed in which the system identifies and classifies the different types of nonlinear loads from the input current waveforms. The expert system with its user interface has been developed in MATLAB and it has been verified with real current measurements. The results obtained prove that the system enables accurate identification of nonlinear loads.

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