Three-phase Uninterruptible Power Supply Research Articles

Droop Control for Parallel-Connected Three-Phase UPS Units with Different Ratings

The main objective of this paper is to achieve synchronization and power sharing between different ratings three-phase Uninterruptible power supply (UPS) units. A droop control scheme is presented in such a way that a discrete PI-controller is applied. The PI-controller produces a suitable control signal to sinusoidal pulse width modulation (SPWM) circuit. The SPWM produces suitable trigger pulses to the inverter gate. The validity of the proposed control scheme is illustrated through MATLAB simulation with two different ratings three-phase UPS units. The simulation results prove that it possible to share the total load among the different rating units in proportion with their rated values in the presence of load interruption.

Determination of Optimal Q-V Droop Controller Parameters for Three-Phase Modular UPS System Based on Genetic Algorithm (GA)

This paper presents an optimized reactive power-voltage (Q-V) droop controller parameters for three-phase modular uninterruptible power supply (UPS) based on genetic algorithm (GA). The main objective of this paper is to minimize the voltage regulation error when there are changes in loads. In addition to select the control parameters carefully to promote the performance of the system against disturbances. This is substantial to attain power sharing between different UPS units to balance sudden disturbances that happen when there are changes in loads. We propose a discrete proportional-integral controller which compares the output voltage with the reference value given by the droop control loops then generates suitable voltage vector signal to pulse-width modulation (PWM) inverter. The fitness function for the optimization problem is formulated by considering the sum of square errors in voltage. This fitness function converges to local minima during the iterative operation of the genetic algorithm. Thus, the optimal parameters of the voltage droop control can be discovered with great probability. Simulation of two parallel-connected three-phase UPS units are carried out using MATLAB/SIMULINK R2018a. Comparison between the conventional trial and error tuning methods and GA optimization is performed. The results show that GA succeeded to minimize the output voltage root mean square error (RMSE) and to enhance the power sharing capability when there is variation in loads.

Research on ADRC controller design for three‐phase UPS with multi‐disturbances

This study focuses on detailed analysis of disturbance for three-phase uninterruptible power supply (UPS) in reality, as well as design of active-disturbance rejection controller (ADRC) and simulation of the corresponding model with multi-disturbances. Parameters of LC-filter will change with UPS operation. Also, UPS not only has pure resistive load, but also resistive–inductive load, and sometimes even non-linear load. The authors take the above disturbance of uninterruptible power supply into consideration at the beginning. Then, according to the above analysis of disturbances, a mathematical model with disturbance term is established. After, the active-disturbance rejection controller is designed to depress the disturbances from internal part and external part. Finally, the corresponding simulation is conducted in order to verify the precision of modelling and the effectiveness of ADRC control strategy.

Three-Terminal Common-Mode EMI Model for EMI Generation, Propagation, and Mitigation in a Full-SiC Three-Phase UPS Module

With superior loss characteristics, wide bandgap devices such as silicon carbide (SiC) mosfet s are expected to replace Si-IGBTs in grid-connected applications. Uninterruptible power supply (UPS) is an application in which low conduction-loss and switching-loss from SiC devices can largely improve the system efficiency. However, fast switching of an SiC mosfet worsens the electromagnetic interference (EMI). In addition, the UPS is comprised of multiple converters wherein different combinations of the converters take part in power-transfer depending on the mode of operation. This complicates the prediction and strategies for noise, especially the common-mode (CM) part. Such complexity calls for deliberate strategies to be set before prototyping to contain and mitigate the CM noise. In this paper, a three-terminal CM circuit model is presented for a three-phase UPS with an active battery charger and a battery rack. The significance of a dc–dc converter on CM EMI generation and propagation has been analyzed based on the model. In a mode of operation where the dc–dc converter is active, a considerable amount of the CM noise is generated from the dc–dc converter. Also, the multiple resonances on the propagation path associated with dc inductors and the battery rack highly deteriorates CM EMI. As a mitigation strategy in the design phase, different topologies and pulsewidth modulation schemes for the ac–ac stage and the dc–dc stage have been compared based on the model. A 20-kW full-SiC UPS has been built and tested to experimentally verify the impact of the dc–dc converter operation on the noise and to validate the mitigation strategy.

Model Predictive Control of Uninterruptible Power Supply with Robust Disturbance Observer

This paper presents a robust continuous control set model predictive control (CCS-MPC) method to control the output voltage of a three-phase inverter in uninterruptible power supplies (UPS). A robust disturbance observer (DOB) is proposed to estimate the load current of the three-phase UPS without a steady-state error, taking the effect of model uncertainties into account. A CCS-MPC is designed using the DOB for reference voltage tracking purpose, and input constraints are considered in the controller design to calculate the optimal control input. Model uncertainties are defined using polytopic uncertainty class, and a linear matrix inequality (LMI) optimization method is used to compute the optimal observer gain matrix. Another robust controller (RC) is designed based on the DOB and compared with CCS-MPC. The effectiveness of the proposed method (the DOB based CCS-MPC) is evaluated for resistive, inductive, and nonlinear loads then compared with other control methods using a three-phase 5-KVA laboratory experiment UPS system.

Power Sharing Method Based on Droop Control for Three-Phase UPS Systems

The main objective of this paper is to improve the power sharing capability and to achieve synchronization between three-phase Uninterruptible power supply (UPS) units in the presence of load interruption. This paper presents a droop-controlled scheme in such a way that the computation of the instantaneous value of the active power and the reactive power are taken as feedback signals to the frequency and voltage restoration control system. The restored frequency and voltage are introduced to voltage controller circuit, which produces a suitable control signal to sinusoidal pulse width modulation circuit (SPWM). Thus producing a suitable trigger pulses to the inverter gate in order to guarantee synchronization between three-phase UPS units. Simulation of two UPS units with the same ratings (4 KW) are carried out using MATLAB. The results show the effectiveness of the proposed control system in achieving synchronization and improving the power sharing capability in the presence of load interruption. Keywords: Uninterruptible power supply; power sharing; parallel operation; droop control . DOI : 10.7176/JETP/9-4-02 Publication date : April 30 th 2019

Lyapunov Energy Function Based Control Method for Three-Phase UPS Inverters With Output Voltage Feedback Loops

In this study, a Lyapunov energy function based control method with output voltage feedback loops is proposed for three-phase uninterruptible power supply (UPS) inverters. The presented paper demonstrates that the traditional Lyapunov-energy-function-based control method not only leads to considerable steady-state error in the output voltage, but also distorts the output voltage waveforms. Therefore, a modification has been performed on the traditional Lyapunov-energy-function-based control by incorporating the output voltage feedback loops in the control variables. The robustness of the proposed control method has been studied analytically through transfer functions which are expressed as the ratio of the output voltage to its reference. These analytical results are validated experimentally. In addition, the steady-state and dynamic performances of the proposed control method are also tested experimentally on a three-phase UPS inverter operating with linear (resistive) and nonlinear (diode-bridge rectifier) loads. As a consequence of incorporating output voltage feedback loops into the control variables, the proposed control method offers strong robustness against variations in LC filter parameters, high-quality sinusoidal output voltage along with acceptable total harmonic distortion (THD) values under linear and nonlinear loads, fast dynamic response under abrupt load changes, and negligibly small steady-state error in the output voltage.

Current-Dependent Capacitor Voltage Control of Parallel Autonomous UPS Systems

This paper proposes a novel autonomous control scheme for equal load current distribution between $n$ parallel-connected, LCL -based, three-phase online uninterruptible power supply (UPS) modules. The control is based on regulating the reference voltage of the filter's capacitor of each inverter as a function of its own output current. The control is achieved without exchanging information between the inverters. The load voltage is maintained fixed at the specified reference value. The control method is fast, precise, and robust against load variations. The validity of the proposed method is verified experimentally and by simulation on a system of two parallel inverters under different load conditions, including step-load and nonlinear load.

An Instantaneous Average Current Sharing Scheme for Parallel UPS Modules

The instantaneous average current sharing (IACS) control strategy ensures accurate transient and steady-state current sharing among uninterruptible power supply (UPS) modules connected in parallel, but is dependent on a communication backbone. This paper proposes a new IACS scheme that is more reliable and economically efficient. In the process of deriving this control scheme, two existing IACS schemes are examined in detail. It is shown that both the schemes offer distinct structural and performance related advantages and disadvantages. The proposed scheme is shown to provide more reliable operation without sacrificing the system performance and incurring extra control cost. Performance of the proposed approach is analytically predicted and validated through experimental results with a 22.5-kW system supported by three parallel-connected three-phase UPS modules.

Extended Bandwidth Instantaneous Current Sharing Scheme for Parallel UPS Systems

This paper investigates the suitability of instantaneous average current sharing scheme (IACS) for the parallel operation of three-phase uninterruptible power supply systems. A discrete-time model is developed for the analysis and design of the control loops. Some key issues are discussed based on the model and it is found that there is a compromise between system stability and current sharing performance with the conventional IACS scheme when the lengths of interconnecting cables are not negligible. Subsequently, an improved IACS scheme is proposed that ensures proper current sharing by extending the close-loop bandwidth. Its performance is analytically predicted and subsequently validated by experimental results on a 15-kW laboratory prototype.

Robust Tracking Control of a Three-Phase DC-AC Inverter for UPS Application under Unbalanced Load Conditions

Abstract: This paper describes a robust control technique for three-phase uninterruptible power supply (UPS) with output LC filter under unbalanced load conditions. The proposed control scheme consists of a combination of two robust controllers for the positive- and negative-sequence in order to provide balanced sinusoidal output voltages by eliminating the negative-sequence which caused by unbalanced loads. The gains of controller are determined by a linear matrix inequality (LMI)-based optimization so that the convergence rate to the steady state is maximized in the presence of the uncertainties in the LC filter. The uncertainties of the system are expressed as the possible ranges of inductor and capacitor values. The effectiveness of the proposed control technique has been validated and illustrated by the simulation result under unbalanced load conditions.

High-efficient three-phase on-line UPS by using T-source inverter with third harmonic injected maximum constant boost PWM control

This paper presents the analysis of three-phase on-line uninterruptible power supply (UPS) by employing T-source inverter (TSI) with third harmonic injected maximum constant boost PWM control and the results are compared with traditional UPS. Traditional UPS consists of step-up transformer or DC–DC booster along with voltage source inverter which decreases the efficiency and increases energy conversion cost. The proposed three-phase UPS using TSI has high-voltage boost capability through shoot-through zero state which is not present in traditional voltage source inverter (VSI) and current source inverter. This proposed UPS with TSI increases the efficiency with fewer components, reduces the harmonics, increases the voltage gain, and reduces the voltage stress. The performance of three-phase on-line UPS based on TSI is analyzed and evaluated in MATLAB/SIMULINK software and the results are compared with Z-source inverter-fed UPS and VSI-fed UPS. Experimental results are presented for the validation of the simulation and theoretical analysis.

An Adaptive Linear Quadratic Regulator for Three-phase UPS System Powering Nonlinear Loads

This paper presents a control strategy applied to three-phase uninterruptible power supplies with a low switching frequency (1500Hz). In The controller design, the gains are determined by minimizing a cost function, which reduces the tracking error and smoothes the control signal. A recursive least square estimator identifies the parameters model at different load conditions. Then the linear quadratic controller gains are adapted periodically. The output voltage is the only state variable measured. The other state variables are obtained by estimation process. Simulation results show that the proposed control strategy offers good performances for either linear and non-linear loads with low total harmonic distortions (THD) even at low frequencies making it very useful for high power applications.

Design and implementation of maximum constant boost switched inductor z-source inverter for three-phase on-line uninterrupted power supply

Purpose – The purpose of this paper is to present the Switched Inductor Z-Source Inverter (SLZSI) topology for three-phase on-line uninterruptible power supply (UPS) by employing third harmonic injected maximum constant boost pulse width modulation (PWM) control. Conventional UPS consists of step-up transformer or boost chopper along with voltage source inverter (VSI) which reduces the efficiency and increases energy conversion cost. The proposed three-phase UPS by using SLZSI has the voltage boost capability through shoot through zero state which is not available in traditional VSI and current source inverter. Design/methodology/approach – Performance of three-phase on-line UPS based on ZLZSI by using third harmonic injected maximum constant boost PWM control is analyzed and evaluated in MATLAB/Simulink software and the results are compared with Z-source inverter (ZSI) fed UPS. Experimental results are presented for the validation of the simulation and theoretical analysis. Findings – The output voltages, currents, THD values, voltage stress and efficiencies for different loading condition are determined and compared with the theoretical values and UPS with ZSI. The experimental results validate the theoretical and simulation results. Originality/value – Compared with the traditional ZSI, the SLZSI provides high-voltage boost inversion ability with a very short shoot through zero state. This proposed UPS by using SLZSI increases the efficiency with less number of components, reduces the harmonics, increases the voltage gain and reduces the voltage stress.

Intelligent voltage control strategy for three-phase UPS inverters with output LC filter

This paper presents a supervisory fuzzy neural network control (SFNNC) method for a three-phase inverter of uninterruptible power supplies (UPSs). The proposed voltage controller is comprised of a fuzzy neural network control (FNNC) term and a supervisory control term. The FNNC term is deliberately employed to estimate the uncertain terms, and the supervisory control term is designed based on the sliding mode technique to stabilise the system dynamic errors. To improve the learning capability, the FNNC term incorporates an online parameter training methodology, using the gradient descent method and Lyapunov stability theory. Besides, a linear load current observer that estimates the load currents is used to exclude the load current sensors. The proposed SFNN controller and the observer are robust to the filter inductance variations, and their stability analyses are described in detail. The experimental results obtained on a prototype UPS test bed with a TMS320F28335 DSP are presented to validate the feasibility of the proposed scheme. Verification results demonstrate that the proposed control strategy can achieve smaller steady-state error and lower total harmonic distortion when subjected to nonlinear or unbalanced loads compared to the conventional sliding mode control method.

Robust Tracking Control of a Three-Phase DC–AC Inverter for UPS Applications

This paper describes a combination of robust control techniques applied to an offset-free robust tracking control of a three-phase dc-ac inverter with an output LC filter for application to uninterruptible power supply (UPS). The control law, which employs state feedback and integration of the tracking error, is described in a synchronous dq frame and is implemented using a space vector modulation technique. The controller is designed by a linear matrix inequality (LMI)-based optimization so that the convergence rate to the steady state is maximized in the presence of the uncertainties in the LC filter. These uncertainties are expressed as possible ranges of the capacitor and inductor values. In the absence of uncertainties, the designed controller will become a deadbeat controller, whereas in the presence of uncertainties, it provides the shortest possible settling time. Thus, the proposed design method provides a systematic tool to combine the robustness to model uncertainties with the deadbeat control. The use of a one-step-ahead predictor is considered to compensate for the computation delay, and an LMI-based method to obtain an optimal gain of the state estimator is also proposed. The efficacy of the proposed approach was experimentally confirmed on a three-phase 10-kVA prototype UPS system.

A High-Performance SPWM Controller for Three-Phase UPS Systems Operating Under Highly Nonlinear Loads

This paper presents the design of a high-performance sinusoidal pulsewidth modulation (SPWM) controller for three-phase uninterruptible power supply (UPS) systems that are operating under highly nonlinear loads. The classical SPWM method is quite effective in controlling the RMS magnitude of the UPS output voltages. However, it is not good enough in compensating the harmonics and the distortion caused specifically by the nonlinear currents drawn by the rectifier loads. The distortion becomes more severe at high power where the switching frequency has to be reduced due to the efficiency concerns. This study proposes a new design strategy that overcomes the limitations of the classical RMS control. It adds inner loops to the closed-loop control system effectively that enables successful reduction of harmonics and compensation of distortion at the outputs. Simulink is used to analyze, develop, and design the controller using the state-space model of the inverter. The controller is implemented in the TMS320F2808 DSP by Texas Instruments, and the performance is evaluated experimentally using a three-phase 10 kVA transformer isolated UPS under all types of load conditions. In conclusion, the experimental results demonstrate that the controller successfully achieves the steady-state RMS voltage regulation specifications as well as the total harmonic distortion and the dynamic response requirements of major UPS standards.

Novel Reduced Parts Online Uninterruptible Power Supply

This paper presents design consideration and performance analysis of novel reduced parts online three-phase uninterruptible power supply (UPS) system. The proposed UPS system is based on reduced switch count dual bridge matrix converter. It employs only six power switches and results in reducing the cost of the system compared to conventional online UPS topologies, while achieving excellent performance. The performance of the proposed system is evaluated through simulation in terms of input/output waveforms quality and shows the viability of topology.

Control technique for non-linear and unbalance compensation of an integrated magnetics-based three-phase series-shunt-compensated uninterruptible power supply

Handling unbalanced and non-linear loads in a three-phase AC power supply has always been a difficult issue. This has been addressed in the literature by either using fast controllers in the fundamental rotating reference frame or using separate controllers in reference frames specific to the harmonics. In the former case, the controller needs to be fast and in the latter case, besides the need for many controllers, negative-sequence components need to be extracted from the measured signal. This study proposes a control scheme for harmonic and unbalance compensation of a three-phase uninterruptible power supply wherein the problems mentioned above are addressed. The control takes place in the fundamental positive-sequence reference frame using only a set of feedback and feed-forward compensators. The harmonic components are extracted by a process of frame transformations and used as feed-forward compensation terms in the positive-sequence fundamental reference frame. This study uses a method wherein the measured signal itself is used for fundamental negative-sequence compensation. As the feed-forward compensator handles the high-bandwidth components, the feedback compensator can be a simple low-bandwidth one. This control algorithm is explained and validated experimentally.

Hybrid learning control strategy for three-phase uninterruptible power supply

This study is concerned with the control of three-phase uninterruptible power supply (UPS) systems based on a hybrid controller. A UPS must be able to maintain regulated output voltage in the face of all line/load disturbances. In order to achieve this, the control system of a UPS must have a fast transient response and a low steady-state error. Fast controllers cannot always maintain the required steady-state accuracy. Therefore in this study, a hybrid control solution is proposed. In the proposed method, a fast, yet simple, controller based on deadbeat (DB) control law is used to achieve the fast transient response required for UPS output voltage control. Noting that non-linear loads that tend to distort UPS output voltage in steady-state have periodic waveforms in nature, a type of learning controller known as a repetitive controller is added to the DB controller to improve steady-state performance of the UPS system. This results in distortion-free output voltage along with a fast error convergence. The system modelling and controller design for the proposed structure are presented in this study. A brief stability analysis is given. Both simulations and experimental results are shown for verification of the theoretical analysis.