Line-interactive Uninterruptible Power Supply System Research Articles

A Single-Phase Line-Interactive UPS System for Transformer-Coupled Loading Conditions

A line-interactive uninterruptible power supply (UPS) system is one of the important installations implemented by industrial users to provide continuous power to sensitive industrial applications. Such loads often come with isolation transformers for voltage matching and protection purposes. Hence, a line-interactive UPS system experiences a serious problem of transient current when it operates under transformer-coupled loading conditions. The occurrence of this transient current can either impair the apparatus itself or turn on the overcurrent safety devices. In any situation, the interruption of electrical power for sensitive industrial loads is unavoidable. So as to keep the uninterrupted power and to eliminate transient current for industrial applications operating under transformer-coupled loading conditions, we propose a single-phase line-interactive UPS system based on a customary current-regulated inverter. The inverter of the proposed UPS system employs an improved current-regulating scheme implemented in a stationary frame of reference which regulates the load current and never let it to increase than a prescribed value as the transformer-coupled loads are turned on. Laboratory test results obtained under various operating and loading conditions of the single-phase line-interactive UPS system verify the operation of the proposed system.

Line-interactive Uninterruptible Power Supply System Eliminating the Inrush Current Phenomenon

Numerous critical load applications rely upon a line-interactive uninterruptible power supply system to maintain power during outages as well as during under- and over-voltage utility conditions. When any disturbance occurs at the utility side, the line-interactive uninterruptible power supply system takes over the load within a few milliseconds to avoid interruption. However, during this transition of load, a significant inrush current occurs due to the magnetic saturation of the load transformer. The inrush current phenomenon causes reduced line voltages and sometimes activates the over-current protection of the uninterruptible power supply system. This article proposes a new line-interactive uninterruptible power supply system that eliminates the inrush current phenomenon by connecting a standard current-regulated voltage source inverter to the secondary of the load transformer. During any transient condition at the utility side, the load current is monitored and regulated to achieve a seamless compensation or transition of load. Experimental results are obtained for a prototype during a number of possible irregular utility power conditions to endorse the performance of the proposed uninterruptible power supply system.

Line-Interactive UPS for Low-Voltage Microgrids

Line-interactive uninterruptible power supply (UPS) systems are good candidates for providing energy storage within a microgrid. In this paper, a control scheme for a line-interactive UPS system applied in a low-voltage microgrid is presented. It is based on the Q-w and P-E droop control to achieve a seamless transition between grid-connected and stand-alone operation modes. Moreover, a new model for designing the controllers is built in the dq-frame based on the instantaneous power definition. The new-built model takes into account the dynamic performance of the output impedance of the inverter in the dq-frame and can be evaluated in the time domain. Compared to the traditional model based on the instantaneous power definition, the new-built model is more accurate to describe the dynamic performance of the system. Simulation and experimental results obtained with a microgrid consisting of two 40-kW line-interactive UPS systems are given to validate the control strategy of the line-active UPS system and the accuracy of the new-built model.

Line-Interactive UPS for Microgrids

Line-interactive uninterruptible power supply (UPS) systems are good candidates for providing energy storage within a microgrid to help improve its reliability, economy, and efficiency. In grid-connected mode, power can be imported from the grid by the UPS to charge its battery. Power can be also exported when required, e.g., when the tariffs are advantageous. In stand-alone mode, the UPS supplies local distributed loads in parallel with other sources. In this paper, a line-interactive UPS and its control system are presented and discussed. Power flow is controlled using the frequency and voltage drooping technique to ensure seamless transfer between grid-connected and stand-alone parallel modes of operation. The drooping coefficients are chosen to limit the energy imported by the UPS when reconnecting to the grid and to give good transient response. Experimental results of a microgrid consisting of two 60-kW line-interactive UPS systems are provided to validate the design.

An Inrush Current Mitigation Technique for the Line-Interactive Uninterruptible Power Supply Systems

Unexpected voltage sags or outages often interrupt the operation of manufacturing processes or even lead to equipment damage. Numerous critical loads rely on the uninterruptible power supply system to uphold the power during these events and maintain the normal operation. As disturbances occur in the utility, the uninterruptible power supply (UPS) should take over the load within 1-5 ms to prevent any interruptions. However, the startup process of the UPS can cause the significant inrush current phenomenon on the load transformer due to magnetic saturation. The inrush current could result in reduced line voltages and even trigger the UPS's overcurrent protection. To prevent the inrush current, this paper proposes a flux offset compensation technique based on the synchronous reference frame (SRF) to regulate the flux of the load transformer. The proposed technique can be integrated with the existing voltage control and the pulse-width modulation (PWM) of the UPS inverter without any additional sensors. The proposed technique can provide a very smooth and timely transition without the risk of inrush current for critical loads under power outages and voltage sags.

Control Strategy for Flexible Microgrid Based on Parallel Line-Interactive UPS Systems

In this paper, the control strategy for a flexible microgrid is presented. The microgrid presented here consists of several line-interactive uninterruptible power supply (UPS) systems connected in parallel. The control technique is based on the droop method to avoid critical communications among UPS units. Thus, a flexible microgrid is obtained to operate in either grid-connected or islanded mode. A small-signal analysis is presented in order to analyze the system stability, which gives rules to design the main control parameters. Simulation and experimental results are presented, showing the feasibility of the proposed controller.

A line-interactive UPS system implementation with series-parallel active power-line conditioning for three-phase, four-wire systems

This paper presents a three-phase line-interactive uninterruptible power supply (UPS) system with active series-parallel power-line conditioning capabilities. Synchronous reference frame (SRF)-based controller is used for harmonic and reactive power compensation generated from any configuration of non-linear loads. Under normal line conditions the UPS system works with universal filtering capabilities, such as compensating the input currents and output voltages. Two three-phase pulsewidth modulation (PWM) converters, called series and parallel active filters, are used to perform the series and parallel active power-line compensation. The series active filter works as sinusoidal current source in phase with the input voltage, drawing from utility sinusoidal and balanced input currents with low total harmonic distortion (THD). The parallel active filter works as sinusoidal voltage source in phase with the input voltage, providing regulated and sinusoidal output voltages with low THD. The performance of the UPS system is evaluated in three-phase, four-wire systems. Experimental results are presented to confirm the theoretical studies.

2중 컨버터 구조를 갖는 계통 연계형 UPS의 DC 충전 알고리듬에 관한 연구

This paper presents a three phase Line-Interactive uninterruptible power supply(UPS) system with dual converter structure. The three phase UPS system consists of two active power compensator topologies. One is a series active compensator, which works as a voltage source in phase with the source voltage to have the sinusoidal source current and high power factor under the deviation and distortion of the source voltage. The other is a parallel active compensator, which works as a conventional sinusoidal voltage source in phase with the source voltage, providing to the load a regulated and sinusoidal voltage with low total harmonic distortion(THD). This paper presents in the series and parallel active compensator charging method depending on the amplitude of the source voltage. The conventional Line-Interactive UPS system is responsible for the DC charging and output voltage regulation at the same time, but UPS system with dual converter structure, a series active compensator can also charge the DC link. Therefore the charging algorithm using the series and parallel compensator needs to be researched. Therefore, by making the DC link voltage stable it can contribute the stability of series and parallel compensator. The simulation and experimental result are depicted in this paper to show the effect of the proposed algorithm.

A line-interactive UPS system implementation with series-parallel active power-line conditioning for three-phase, four-wire systems

This paper presents a three-phase line-interactive uninterruptible power supply (UPS) system with active series-parallel power-line conditioning capabilities. Synchronous reference frame (SRF)-based controller is used to harmonic and reactive power compensation generated from any configuration of non-linear load. Under normal line conditions the UPS system works with universal filtering capabilities, such as compensating the input currents and output voltages. Two three-phase pulse width modulation (PWM) converters, called series and parallel active filters, are used to perform the series and parallel active power-line compensation. The series active power filter works as sinusoidal current source in phase with the input voltage, drawing from utility sinusoidal and balanced input currents with low total harmonic distortion (THD). The parallel active power filter works as sinusoidal voltage source in phase with the input voltage, providing regulated and sinusoidal output voltages with low THD. Operation of a three-phase phase-locked loop (PLL) structure, used in the proposed line-interactive UPS implementation, is presented and experimentally verified under distorted utility conditions. The performance of the UPS system is evaluated in three-phase, four-wire systems. Digital simulations and experimental results are presented to confirm the theoretical studies.

A three-phase line-interactive UPS system implementation with series-parallel active power-line conditioning capabilities

This paper presents a three-phase line-interactive uninterruptible power supply (UPS) system with series-parallel active power-line conditioning capabilities, using a synchronous reference frame (SRF) based controller, which allows an effective power factor correction, load harmonic current suppression and output voltage regulation. The three-phase UPS system is composed of two active power filter topologies. The first one is a series active power filter, which works as a sinusoidal current source in phase with the input voltage. The other is a parallel active power filter, which works as a sinusoidal voltage source in phase with the input voltage, providing to the load a regulated and sinusoidal voltage with low total harmonic distortion (THD). Operation of a three-phase phase-locked loop (PLL) structure, used in the proposed line-interactive UPS implementation, is presented and experimentally verified under distorted utility conditions. The control algorithm using SRF method and the active power flow through the UPS system are described and analytically studied. Design procedures, digital simulations and experimental results for a prototype are presented to verify the good performance of the proposed three-phase line-interactive UPS system.