Single-phase Uninterruptible Power Supply Research Articles

Robust Voltage Control of a Single-Phase UPS Inverter Utilizing LMI-Based Optimization with All-Pass Filter Under System Uncertainty

This paper proposes a systematic control design for a single-phase LC-filtered inverter considering uncertain system parameters. One major difficulty in controlling single-phase power converters is the lack of a direct conversion method for transforming single-phase signals into dq-frame signals. By employing an all-pass filter in this proposed approach, it is possible to control the output voltage in terms of DC quantity or the dq-rotating frame. Furthermore, voltage stability and harmonic distortion (THD) minimization of the uninterruptible power supply (UPS) are major concerns in inverter design. Therefore, this controller uses integral action to get rid of steady-state errors and stabilize the closed-loop system by the state feedback control. In order to enlarge and guarantee the stability range in the presence of potential parameter fluctuations, an uncertainty model is being considered. In this context, the uncertainty models refer to the potential model with variations in the filter's inductance and capacitance caused by operating temperature, aging, and various external factors. The efficacy of the control approach is assessed through simulations and experiments, with the objective of comparing its results with those of the PI control using a control board featuring a TMS320F28335 digital signal processor. Consequently, the proposed approach offers lower THD at every load step with lesser afford in performance tuning in comparison to the PI method.

Sensorless Model Predictive Control of Single-Phase Inverter for UPS Applications via Accurate Load Current Estimation.

Single-phase inverters with an output LC filter, can generate low distortion output voltages, which are suitable for uninterruptible power supply (UPS) systems. The UPS system provides emergency power in the case of utility power failure, requiring high reliability and clean power. The sensorless control method is actually a soft-sensing technique, that reduces system cost, measurement-related losses, and, especially important for UPS systems, enhances the system reliability. This paper proposes a load current sensorless finite control set model predictive control (FCS-MPC) scheme for a single-phase UPS inverter. A time varying observer is proposed, which offers the accurate estimation for individual components simultaneously in periodic load current signal, without subsequent complex calculations. Compared with another two typical sensorless methods (the low-pass filter and the Kalman filter), the proposed observer-based FCS-MPC strategy has smaller load current estimation error and lower output voltage distortion, under both linear and nonlinear loads. The theoretical analysis is verified through simulation and experiment. A single-phase inverter rapid control prototype (RCP) is set up with the Speedgoat real-time target machine, to confirm the effectiveness of the system.

Control Strategy for Reducing Ripple Current in DC-Link Capacitor of Single-Phase UPS

This letter proposes a control strategy for reducing a ripple current in DC-link capacitor of single-phase uninterruptible power supplies (UPS). The proposed method aims to reduce the capacitor current by reducing the difference between the input and output instantaneous power. The input current waveform should satisfy harmonic current regulation and be as close as possible to the load current. Therefore, an auxiliary buck chopper was added to compensate to the ripple current in DC-link capacitor. Experimental result confirm that the proposed method reduces the RMS value of the DC-link ripple current by 27.9%.

Processor-in-the-Loop Validation of an Observer Current-based Dead-Beat Control for a Single-Phase UPS Inverter

This paper presents a dead-beat control algorithm for Uninterruptible Power Supply (UPS) applications of single-phase inverters. The proposed control method requires the measurement of capacitor current and output voltage in order to keep the output voltage sinusoidal ensuring high dynamic performance even under load changes. The dead-beat controller optimizes the behavior of the system by eliminating the error between the output and the reference voltage without increasing the number of current sensors, which are costly, and eliminates load voltage distortions and restores the system state in the event of external shutdown-loop road interference. In this paper, we propose a capacitor current estimation based on the Luenberger observer. Processor-In-the-Loop (PIL) is a test method that allows us to create and evaluate controllers by running built-in C code on the DSP scheduled for the controller during simulated PSIM power phase control. It can be seen that the simulation results match the PIL test results, which proves the validity of the proposed controller.

Observer-Based Adaptive Control for Single-Phase UPS Inverter Under Nonlinear Load

This article proposed a current-sensorless control scheme for the single-phase uninterruptible power supply (UPS) inverter under nonlinear load. Different from the existing current-sensorless literature, the information of load current can be obtained by the designed adaptive laws. It is meaningful for the system’s security and reliability. Under the assumption that the load is a periodic ideal-independent current source, the Fourier series in trigonometric form is expanded to model the unknown disturbance of load current. The observer-based feedback control rate and update laws are derived to adjust online the Fourier coefficients of the load current modeling. Furthermore, the stability of the system is rigorously analyzed via the Lyapunov approach. Under the same limitation of not using the current sensor, comparative experiments are designed. Simulation and experimental results have verified the effectiveness of the control approaches.

An Adaptive Dual-Loop Lyapunov-Based Control Scheme for a Single-Phase UPS Inverter

This letter proposes an adaptive Lyapunov-based control strategy for a single-phase uninterruptible power supply inverter with inherent dual control loops. The control law is derived via a proposed Lyapunov function which takes both dual-loop requirements and the global large-signal stability demand into account. Besides, in the proposed control scheme, the load disturbance is suppressed adaptively without any additional load-current sensors or observers. The load voltage is regulated with better steady-state and dynamic performance, as well as with great robustness against the parametric variations. Finally, experimental results verify the effectiveness of the proposed control scheme.

Comparative Study of Discrete PI and PR Controls for Single-Phase UPS Inverter

This paper presents a comparative study of discrete proportional integral (PI) and proportional resonant (PR) current control for single-phase uninterruptible power supply (UPS) inverters. There is an increasing requirement for current and voltage-controlled UPS inverters with very low or zero steady-state error, improved transient response and lower total harmonic distortion (THD). The most promising type of current regulator for single-phase inverters is PR control because it can introduce an infinite gain at a selected resonance frequency such as the fundamental frequency to eliminate the steady-state error, which cannot be achieved by well-known proportional integral (PI) control. Note that PI control has limitations in terms of the steady-state magnitude and phase errors. In addition, PI control also has limited harmonic rejection capability, unlike the PR control, also can compensate for low-order harmonics. Imperfections in the current and voltage control scheme results in higher harmonic distortion of the output current and voltage. In this paper, performance of PR control parameters ($K_{p}$ , $K_{i}$ , and $\omega _{c}$ ) and filter parameters ($L_{f}$ and $C_{f}$ ) are optimally tuned to obtain a very low THD current with reduced output voltage ripple and steady-state error. The analysis, design and implementation of both PI and PR current control in single-phase UPS inverter applications through simulations and experiments are also presented in this paper. The performance of both of these control schemes are analyzed in terms of steady-state response, transient response, and level of current harmonics.

Design and implementation of an optimal switching controller for uninterruptible power supply inverters using adaptive dynamic programming

In this study, a new approach based on adaptive dynamic programming (ADP) is proposed to control single-phase uninterruptible power supply inverters. The control scheme uses a single function approximator, called critic, to evaluate the optimal cost and determine the optimal switching. After offline training of the critic, which is a function of system states and elapsed time, the resulting optimal weights are used in online control, to get a smooth output AC voltage in a feedback form. Simulations show the desirable performance of this controller with linear and non-linear loads and its relative robustness to parameter uncertainty and disturbances. Furthermore, the proposed controller is upgraded so that the inverter is suitable for single-phase variable frequency drives. Finally, as one of the few studies in the field of ADP, the proposed controllers are implemented on a physical prototype to show the performance in practise.

Multiple resonant controllers strategy to achieve fault ride‐through and high performance output voltage in UPS applications

A control strategy to achieve fault ride-through capability and to provide high performance of the output voltage of a single-phase uninterruptible power supply (UPS) inverter is proposed in this study. This strategy consists in controlling the voltage and current waveforms measured at the inverter output filter, with an inner current control loop and an outer voltage control loop, using a plug-in structure based on multiple resonant stages in addition with proportional controllers. In order to achieve stability from no load to short-circuit conditions, the implementation of the multiple resonant controllers includes a compensation of the system phase lag. Moreover, it presents a comparative analysis between two controller structures, the proposed plug-in and the classical proportional + resonant. From this analysis, it can be concluded that the plug-in structure presents improved characteristics of closed-loop output impedance and output voltage dynamic response during fault ride-through events. A controller design methodology to achieve robustness to parametric uncertainties and UPS standard compliance, is detailed. Experimental results from a single-phase 2 kVA inverter prototype are presented to validate the feasibility of the proposal.

A digital dual-modulation control for single-phase UPS inverters

ABSTRACTA trip-point modulation strategy is proposed for single-phase uninterruptible power supply inverters with an LC filter, which operates simultaneously with pulse-width modulation, named dual-modulation (DM) control. Current estimation and offset current approximation are used conjointly to facilitate the proposed control law. It is shown analytically that the DM control leads to reduced total harmonic distortion (THD) and gains fast transient-state recovery. The simulation and experimental results demonstrate significant amelioration to THD and dynamic state compared to proportional–integral–derivative control under both linear and non-linear loads.

Hybrid iterative learning control strategy for single‐phase UPS inverter using inductor current active damping

This study proposes a hybrid iterative learning (HIL) control scheme for single-phase uninterruptible power supply (UPS) inverters using inductor current active damping technique. A UPS inverter maintains a tightly regulated output voltage independent of loading conditions and disturbances. To achieve such performance, a fast transient recovery time with a minimum steady-state error is desired. Fast control techniques, such as proportional integral (PI), are not always known for good steady-state performance in single-phase applications. As the load voltage and current are periodic, therefore a hybrid control strategy has been proposed in which a PI controller is combined with an iterative learning controller to produce better transient and steady-state dynamics. Adoption of such a strategy enables the UPS inverter control to be realised with one voltage and one current sensor, respectively. System modelling, filter magnitude-frequency characteristics shaping and control design with stability analysis have been presented in this study. The designed HIL control strategy is examined through simulations in a MATLAB environment in the discrete time frame and experimentally verified for a single-phase UPS inverter system prototype using a Texas TMS320F2812 digital signal processor.

Experimental assessment of UPS battery load model considering harmonics and its investigation in the distribution systems

Uninterruptible Power Supply (UPS) inverter battery systems in the recent years are playing significant role in achieving protection against power interruptions and in improving efficiency and cost effectiveness of the power networks. In this article, an assessment of harmonic load model of battery energy storage system is carried out by a detailed experimentation on 10kVA single-phase UPS inverter battery. Experimental results are obtained for fundamental powers drawn by the battery during charging, along with harmonic currents generated at various voltage levels. From the measured values, functional relations between voltage and power (real and reactive) have been developed. Results of experimentation are then integrated with radial distribution systems to investigate the impact of harmonic load model of UPS battery on the distribution network. The proposed method is tested on a radial distribution feeder integrated with UPS battery system. Load flow results of integrated distribution system demonstrate that non-linear load model of UPS battery can cause considerable harmonic distortion in the distribution network and increase power losses.

Grid-Connected Control Strategy of Five-level Inverter Based on Passive E-L Model

At present, the research on five-level inverters mainly involves the modulation algorithm and topology, and few articles study the five-level inverter from the control strategy. In this paper, the nonlinear passivity-based control (PBC) method is proposed for single-phase uninterruptible power supply inverters. The proposed PBC method is based on an energy shaping and damping injection idea, which is performed to regulate the energy flow of an inverter to a desired level and to assure global asymptotic stability, respectively. Furthermore, this paper presents a control algorithm based on the theory of passivity that gives an inverter in a photovoltaic system additional functions: power factor correction, harmonic currents compensation, and the ability to stabilize the system under varying injection damping. Finally, the effectiveness of the PBC method in terms of both stability and harmonic distortion is verified by the simulation and experiments under resistive and inductive loads.

Ripple Current Reduction for Fuel-Cell-Powered Single-Phase Uninterruptible Power Supplies

A well-known issue of fuel-cell-powered single-phase uninterruptible power supplies (UPS) is that the input current from the dc source is coupled with the second-order current ripple due to the ac-side instantaneous power pulsating at twice the line frequency. The low-frequency ripple component has been confirmed to have detrimental impacts on fuel cell's reliability and lifespan. To solve this issue, a boost inverter that can work in both differential mode (DM) and common mode (CM) operations is adopted by this paper. The DM operation achieves active power conversion and a well-regulated ac output voltage. Meanwhile, the CM operation ensures effective dc current ripple reduction. In addition to operating with linear loads, the proposed control method extends its working scope to nonlinear loads, where meticulously designed repetitive controllers are employed to handle the multiple low-harmonic situations. Moreover, the proposed control method also introduces an interleaved pulse-width modulation to further reduce the switching frequency ripple in the dc current. The stability of the proposed control system is stringently examined. Finally, presented experimental results validate the theoretical analysis as well as the effectiveness of the proposed method in current ripple reduction.

A Single-Loop Repetitive Voltage Controller with an Active Damping Control Technique

This paper proposes a single-loop repetitive voltage control strategy which incorporates the active damping control feature for single-phase uninterruptible power supply (UPS) applications. The proposed method reduces the effect of the LC resonant peak, which limits the control bandwidth and deteriorates the stability of the entire control loop by effectively increasing the damping component. Due to the increased stability margin, a repetitive controller working together with a proportional-resonant (PR) controller can be easily adopted. Moreover, the voltage error is minimized even under severe non-linear load conditions. It is confirmed that the proposed single-loop controller achieves excellent and stable voltage regulation performance by evaluating the entire loop-gain of the system and the output impedance. Both the simulation and the experimental results for a 1.5 kW UPS inverter show agree well with the analyses, and the excellence of the proposed method has been verified.

Multi-loop control of UPS inverter with a plug-in odd-harmonic repetitive controller

This paper proposes an improved multi-loop control scheme for the single-phase uninterruptible power supply (UPS) inverter by using a plug-in odd-harmonic repetitive controller to regulate the output voltage. In the suggested control method, the output voltage and the filter capacitor current are used as the outer and inner loop feedback signals, respectively and the instantaneous value of the reference voltage feedforwarded to the output of the controller. Instead of conventional linear (proportional-integral/-resonant) and conventional repetitive controllers, a plug-in odd-harmonic repetitive controller is employed in the outer loop to regulate the output voltage, which occupies less memory space and offers faster tracking performance compared to the conventional one. Also, a simple proportional controller is used in the inner loop for active damping of possible resonances and improving the transient performance. The feedforward of the converter reference voltage enhances the robust performance of the system and simplifies the system modelling and the controller design. A step-by-step design procedure is presented for the proposed controller, which guarantees stability of the system under worst-case scenarios. Simulation and experimental results validate the excellent steady-state and transient performance of the proposed control scheme and provide the exact comparison of the proposed method with the conventional multi-loop control method.

Design approach of discrete‐time resonant controllers for uninterruptible power supply applications through frequency response analysis

A new methodology to design discrete-time multiple resonant controllers for single-phase uninterruptible power supply inverters is proposed in this study. This methodology is based on classical linear tools and consists on the synthesis of the inverter output impedance according to standard specifications. This synthesis is performed using a multi-loop control strategy composed of an inner current control loop using a proportional controller, and an outer voltage control loop using the multiple resonant controller. A prototype was built to demonstrate the practical feasibility of the theoretical proposal. A significant reduction of the output impedance at determined harmonic frequencies resulted in a low-voltage total harmonic distortion of the output voltage of about 1.76%, for IEC 62040-3 reference non-linear load.

Proportional-Resonant and Slide Mode Control for Single-Phase UPS Inverter

Slide mode control (SMC) is recognized as the most robust control with high stability, while the proportional-resonant (PR) control shapes the output waveform closely according to the reference sinusoidal signal. Keeping in view the characteristics of slide mode and PR control, a cascaded controller is proposed for bipolar single-phase uninterruptible power supply (UPS) inverter. The outer voltage loop uses the PR control while the inner loop uses the SMC. Chattering in the SMC has been removed using smoothed control law in narrow boundary layer condition. The stability of the controller has been analyzed using Lypunov stability criteria. The smoothed control law applied to the pulse width modulator results in fixed switching frequency of the inverter. The performance of the controller has been analyzed for single-phase inverter through simulations and experiments for both the linear and non-linear loads. The performance of controller has been compared with the other techniques of SMC and standard controllers. The proposed controller shows significant improvement in terms of reducing the total harmonics distortion to 0.5% for linear load and 1.25% for non-linear load, strong robustness, and fast response time of only 0.3 ms.

A Performance Comparison of the Current Feedback Schemes with a New Single Current Sensor Technique for Single-Phase Full-Bridge Inverters

In this paper, a single current sensor technique (SCST) is proposed for single-phase full-bridge inverters. The proposed SCST measures the currents of multiple branches at the same time, and reconstructs the average inductor, capacitor, and load current in a single switching cycle. Since all of the branches’ current in the LC filter and the load are obtained using the SCST, both the inductor and the capacitor current feedback schemes can be selectively applied while taking advantages of each other. This paper also analyzes both of the current feedback schemes from the view point of the closed-loop output impedance. The proposed SCST and the analysis in this paper are verified through experiments on a 3kVA single-phase uninterruptible power supply (UPS).

Simple control scheme for single‐phase uninterruptible power supply inverters with Kalman filter‐based estimation of the output voltage

The major drawback of most control methods for uninterruptible power supply (UPS) inverters is complexity. In this study, a simple voltage sensorless multi-loop control method is proposed for the single-phase UPS inverter and the parameters are designed systematically based on a frequency domain method. The suggested control scheme uses two nested simple proportional controllers instead of proportional–integral/resonant controllers to regulate the output voltage and produce active resonance damping. In addition, a feedforward compensation of the reference voltage is used to enhance the performance of the control system. The proposed controller does not need any reference frame transformation or integration/derivation of measured quantities in its structure. To reduce the cost and at the same time to improve the reliability of the converter system, the output voltage sensor is replaced by a simple estimation algorithm based on the digital Kalman filter. Experimental results with a 500 VA prototype, as well as simulation results validate the excellent performance of the suggested estimation and control scheme.