High-Performance Transformerless Online UPS

Uninterruptible Power Supply (UPS) is widely used in industry and electronics field in order to improve power quality. However, the distortion and interruption of output voltage will occur when UPS is applied to instantaneous impulsive load. This paper proposes a novel dynamic control strategy to resist impulsive load. When the impulsive load is imposed on UPS instantaneously, the impulsive current can be quickly sensed by the digital controller and the supplying power for the load is switched from inverter to bypass immediately. Therefore, the output voltage of UPS can be maintained uninterruptible. The proposed control strategy is verified by a 30 KVA UPS prototype.

High quality online uninterruptible power supplies (UPS) are widely used in applications e.g. precision medical equipment with critical loads. This paper proposes a low-cost and high performance online UPS system. Firstly, the performance improvement of a bidirectional switch based bridgeless power factor (PFC) converter, which is regulated by a hybrid control strategy with load current feedforward is studied. Secondly, the proposed closed-loop controller integrates improved predictive current control with predictive voltage control and applies to the second-stage neutral-point-clamped (NPC) three-level inverter for low output voltage harmonics. Moreover, a push-pull dc-dc converter interfaces and boosts voltage of a 24V battery bank as the backup energy source. Considering complexity of the UPS system as well as cost reduction, we implement a multi-task decomposition method for control on a monolithic low-cost digital signal processor (DSP) embedded with Control Law Accelerator (CLA). Finally, the experimental results from a 1kVA a prototype can effectively verify validly of the theoretical analysis and design.

This paper describes a high performance voltage controller for 3-phase 4 wire UPS (uninterruptible power supply) system, and proposes a new scheme of synchronous reference frame controller in order to compensate for the voltage distortions due to unbalanced and nonlinear loads. Proposed scheme in this paper is able to completely eliminate the negative sequence voltage distortion due to unbalanced loads and also reduce the harmonic voltage distortion due to non-linear loads, even if the bandwidth of voltage control loop is a very narrow. Therefore, the proposed scheme is especially appropriate for a fully digital controlled UPS or a high power UPS with limited voltage control bandwidth. In order to compensate for the effects of unbalanced loads, the synchronous reference frame controller with the positive and negative sequence computation block is proposed, and the synchronous frame controller with a bandpass filter is proposed to compensate for the selected harmonic frequency of output voltage. The effectiveness of the proposed scheme has been investigated and verified through computer simulations and experiments by a 30 kVA UPS

In this paper, a regeneration protection solution is proposed to address the dc-link overvoltage issue and the unequal power sharing in the parallel uninterruptible power supply (UPS) systems. First, a dc-link voltage protection control strategy is proposed to protect the inverter against the excessive dc-link voltage that may trigger the protection mechanism of the UPS system. In addition, an active power-sharing control strategy by regulating the virtual resistance is proposed to solve the circulating current caused by UPS regeneration issue. Finally, the feasibility of the proposed regeneration protection solution is verified by experimental results from the parallel UPS system prototype.

This paper presents a simplified sinusoidal uninterruptible power supply (UPS) system. The proposed scheme includes features such as high power factor, low total harmonic distortion and good dynamic response at the AC output voltage. In addition, the scheme has the desirable characteristics, of high efficiency, simple circuit and low cost compared with a traditional standalone multiple stages UPS with power factor correction. The paper also presents the circuit operation, the analysis and, experimental results of the proposed UPS scheme. The proposed UPS approach is a good solution in low. power applications (/spl les/500 W).

Three-phase Uninterrupted Power Supply (UPS) is widely used in industrial field. The output voltage quality of three-phase UPS mainly depends on its control strategy. However, the internal uncertainty and external disturbance of three-phase UPS always affect the output of UPS. Aiming at describing the source of internal uncertainty, the influence of external disturbance and system coupling of three-phase UPS, the detailed mathematical model is firstly established. Secondly, according to the mathematical model, the designing method for three-phase UPS controller based on active-disturbance rejection controller (ADRC) method is proposed. Finally, the corresponding simulation model is constructed in Matlab/Simulink. The simulation results prove that the proposed strategy can effectively improve the output power quality of three-phase UPS.

This paper is concerned with the control of a line interactive Uninterruptable Power Supply (UPS) system in a Microgrid. In grid connected mode, power can be imported from the grid to charge the battery or it can be injected into the grid to boost the power supplied by the Microgrid. In island mode, the UPS supplies local critical load in parallel with other sources. Power flow is controlled using the frequency and voltage drooping technique to ensure seamless transfer between grid connected and parallel islanding modes of operation. When the power is imported from the grid, an outer voltage controller that regulates the DC link voltage sets the power demand for an inner power flow controller. The outer voltage loop is designed with the aid of small signal analysis which has been used to model the dynamics of the inner power flow controller. Due to the limited DC link capacitance, the drooping coefficients are chosen carefully to limit transient circulating active power from the grid to the UPS module or from one UPS module to another. The traditional way of measuring active and reactive power which is based on using a low pass filter is replaced by a real time integration method which is shown to improve the response of the controller. A virtual impedance is utilized in grid-connected mode to improve output current THD. Experimental results of two 60kW line interactive UPS modules are provided to validate the design.

A current sharing method considering battery state of charge(SOC) for multi-module uninterruptable power supply(UPS) system is proposed. Modularized UPS system consists of a single master module and multi slave modules. A master module performs output voltage control and distributes current commands to slave modules. The current commands are generated by using the proposed current sharing algorithm to balance the SOC of each UPS module. Moreover, the UPS mode transfer time can be significantly reduced by using the predictive current control. The predictive current control method is mathematically verified by analyzing the UPS system. The validity of proposed balancing method and predictive control is verified by simulation results.

An autonomous control for redundant parallelism of uninterruptible power supplies (UPS) connected in parallel has successfully been proposed and discussed in theoretical and experimental terms. This independent control only requires the measurement of the output current. With the computation of the active and reactive currents, proportional-integral-based controllers provide the phase angle and amplitude, respectively, of the output voltage. However, when voltage difference between UPS exists, there is a flow of reactive lateral current, which makes the load sharing disproportional. A preliminary approach to reduce this circulating current considers a high proportional gain in the control equation for output voltage amplitude in order to reduce the offset error. Nevertheless it implies in high variation of the voltage amplitude, so that voltage levels easily reaches the limit, and the respective control equation becomes incapable to compensate any voltage difference. This paper proposes a compensator to counterbalance the voltage drop caused by the proportional gain of the control equation for the voltage amplitude. Implementation in an experimental setup with three UPS with different output rating connected in parallel shows significant reduction of the reactive lateral current, and consequent improvement of the current distribution, including employment of voltage limiters (1%), under various conditions.

In this paper, a simplified control architecture, including individual layer control and recover layer control, is proposed for a modular online uninterruptible power supply (UPS) system is presented. The parallel control algorithm of for the DC/AC modules are mainly concentrates on the active power sharing performance due to the existence of the shunt active power filter (APF). APF will deal with the reactive power brought by the nonlinear load to guarantee that all DC/AC modules are faced with a resistive type load. On the other hand, since there is no regulation on the UPS output voltage frequency or phase angle, the recovery layer control is only concerned with the amplitude of the UPS output voltage, which means that the working load of the communication network is reduced by half. Consequently, the reliability of the system is improved. Experimental results are presented in this paper in order to validate the proposed system architecture and control.

This paper presents a novel control strategy using a genetic algorithm optimized fuzzy logic controller with repetitive learning action for uninterruptible power supply (UPS) inverter applications. The repetitive learning action can reduce steady-state errors and distortions caused by unknown periodic disturbances. Considering its poor dynamic response with sudden load change, the fuzzy logic controller (FLC) is introduced. The parameters for the FLC are optimized by genetic algorithm (GA) in the proposed scheme. The presented digital control scheme measure only the output voltage, decreasing the amount of sensors and the overall system cost. The control laws and design procedure of the control scheme are discussed. Simulation and experimental results for a single-phase UPS inverter controlled by a TMS320F240 DSP are presented to verify the performance of the proposed control approach under different load conditions. It proves to be a cost-effective solution for common UPS products.

This paper proposes an improved digital voltage controller suitable for three-phase PWM inverters with output transformer for medium and high power uninterruptible power supplies (UPS). The proposed controller is derived from the internal model principle using a state-variable approach in stationary alphabeta frame. With this controller it is possible to obtain a high performance in both steady-state and load transients. In addition, with the selected internal model, DC components result from real circuit implementation non-idealities are not amplified avoiding in this way, the transformer saturation. The UPS output voltages have reduced total harmonic distortion (THD) for both balanced and unbalanced non-linear loads. Regarding the transient performance, load steps responses are improved with the proposed digital controller and as a result the UPS compliant with the classification 1 of the international standard IEC 62040-3. In order to validate the proposed digital controller and to demonstrate the steady-state and transient performance, experimental results from a 10 kVA space vector modulated three-phase inverter, fully controlled by a DSP TMS320F241, are presented

Since the auxiliary three-phase oil pump motor of ship machine set needs uninterrupted power supply(UPS), the oil pump motor needs to be switched to UPS equipment for power supply in case of power loss of the generator. In order to realize the smooth transition of the oil pump motor when switching to UPS, the three-phase voltage output by the inverter in UPS needs to be in the same frequency and phase as the generator voltage. In this paper, the basic working principle of UPS is introduced, the advantages and disadvantages of two phase-locked loops for tracking phase are analyzed, and a phase-locked loop(PLL) based on second-order generalized integrator is selected to be used in inverter. Finally, the feasibility of this method is verified by experiments.

There are three types of Static Uninterruptible Power Supplies (UPS): off-line, interactive and double-conversion. The last one protects the load against all types of line disturbances but it is the most expensive and the one with the lowest efficiency. On the other hand, off-line and interactive UPS have higher efficiency and lower cost, but they show an important drawback: a switching time from normal mode to stored-energy mode. As a consequence, there is a notch in the UPS output voltage during this switching time. In a previous paper [1], authors proposed a method for filling these voltage notches with a sinusoidal waveform generated by a switch-mode converter. In this one, a simplified notch filler is proposed. It consists of two capacitors charged one with positive and the other with negative voltage. If the fault occurs in the positive period, the positive charged capacitor is connected to the load. This connection is then modulated in order to obtain a sinusoidal waveform at the load. In the negative period, the other capacitor is used in the same way.

To achieve proper operation of an online static uninterruptible power supply (UPS) system, feedforward and feedback (FF/FB) controls are introduced. Feedforward control is applied to reduce the low order harmonics in the output voltage due to the ripples included in the input DC voltage to the UPS inverter resulting from the controlled rectifier, while feedback control is applied mainly for stabilizing the output voltage. In this paper, a feedforward technique is selected that was found to give free ripple output, and two types of feedback techiques are compared and the most suitable one is considered. The paper gives the modeling and simulation results of the whole system, that compare the performance without and with FF/FB under different steady state and transient conditions.