A high performance controller for parallel operation of three-phase UPSs powering unbalanced and nonlinear loads

The operation of uninterruptible power supplies (UPS) on emergency generation can lead to problems of overheating of the generator due to the unbalanced operation of the UPS. It is pointed out that the design criteria of a UPS are inconsistent with the operation on isolated generation unless the two systems are designed to operate together.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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

This paper investigates the control of a four-wire three-phase Uninterruptible Power Supply (UPS) inverter using the internal model principle by applying multiple resonant controllers in the stationary abc-frame for zero-error tracking of sinusoidal reference and harmonic disturbances rejection due to nonlinear and unbalanced loads. The synthesis of such controllers is accomplished in a robust control framework where the controller parameters are determined by means of a convex optimization problem subject to a set of Linear Matrix Inequality (LMI) constraints. Simulation results from a 15.0-kVA UPS inverter are considered to illustrate and validate the performance and behavior of the controller design under nonlinear and unbalanced loads.

This paper presents an improved method to generate the compensation reference currents applied to a three-phase line-interactive Uninterruptible Power Supply (UPS) system with series and parallel active power line conditioning capabilities. The reference currents are used to compensate the reactive power and to eliminate harmonic currents generated from the non-linear loads, in a three-phase, four-wire system. The control strategy is based on the synchronous reference frame (SRF) method and the implemented current SRF-based controller does not have the task of compensating negative and zero sequence components at fundamental frequency of the load currents, due to unbalanced loads or unbalance source voltage conditions. Thus, the efficiency of the three-phase UPS system is increased when it is feeding unbalanced single-phase non-linear loads because the PWM parallel converter compensates only the harmonic load currents. Three strategies, employed to generate the reference currents, are discussed, such as, the SRF conventional strategy, the SRF single-phase strategy and the improved SRF single-phase strategy. Besides, the active powers that flow through the parallel PWM converter using the three presented strategies are shown and a comparative analysis of them is made to validate the predicted theoretical results.

A multi-objective control scheme of a voltage source converter with battery–supercapacitor energy storage system used for power quality improvement

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

A model to describe the operation of a bidirectional uninterruptible power supply (UPS) is proposed. The performance of the power flow, voltage regulation and charging battery are considered in this model. By using this model, the steady-state characteristics of a bidirectional UPS can be simulated and the design of system parameters can be obtained. To verify the accuracy of this model, a prototype was developed. It is shown that the model is very accurate. >

This paper presents a nonlinear feed-forward controller applied to single-phase uninterruptible power supplies (UPS). The controller was conceived by using a one-cycle based PWM generator and an output-current feedforward, in order to cancel input-voltage disturbances and nonlinear load currents, respectively. It also includes an external control loop to ensure good output-voltage tracking. The controller design is based on a large-signal approach, introduced to achieve good transient response and stability in a wide range of parameter variations. Besides, in spite of the nonlinear nature of the controller, we obtain a linear average output-voltage dynamic response. The proposed control provides the following features: fast transient response, low THD sine-wave output-voltage, fixed switching frequency, and high robustness against load and line step changes. Simulation results show the effectiveness of this control technique for inverters in UPS applications, especially when nonlinear loads must to be supplied and UPS DC-link has large undesirable ripple.

A novel distributed control for uninterruptible power supply (UPS) modules in parallel operation is proposed in this paper, in which the voltage reference synchronization control and load current distribution control are decoupled by a local feedback. The voltage reference is presynchronized beyond the current distribution control with a microcontroller unit and a wired-and circuit. Both instantaneous current distribution and hot swapping of any module are allowed with all the UPS modules electrically isolated. The transfer function of the circulating current regulator is analyzed and equivalent to a first-order low-pass filter. The control characteristics of the root-mean-square voltage regulator (RMSVR) in each UPS module in parallel operation are researched, as well as the coupling effect between the RMSVR and the load balance performance. The RMSVR's impact on current distribution is compensated by introducing the amplitude of the output current or the circulating current into the amplitude adjustment of the sinusoidal voltage reference signal. As a result, both the current sharing and the output-voltage performance of the whole system are effectively improved. Theoretical analysis and experimental results are provided to demonstrate the validity and feasibility of the proposed control.

This paper presents the reactive power compensation with UPS. The most effective power quality improvement solution is analyzed in an uninterruptible power supply systems. The conventional UPS has three stages i.e rectifier converts AC to DC, Battery to store DC and Inverter to convert DC to AC. A novel technique has implemented in off-line UPS. The power is delivered to load via by-pass switch when main supply is available. Inverter does not operate during this period. So inverter function is same as conventional inverter. To overcome this problem, static change over switches has proposed. Hence it will get better voltage regulation and loss less in distribution system. The performance of simulation result is analyzed using MATLAB/SIMULINK.

This paper presents a design and an implementation of inverters with ultra-low total harmonic distortion (THD) under all kinds of loads. In this study, the ultra-low THD objective is achieved by combining the advantages of the repetitive controller, the predictive-PID controller and the reference signal feedforwarding method in a multi-loop structure. The high performance uninterruptible power supply (UPS) and the transformer test converter applications where the ultra-low THD is desired require the use of high performance controllers. This study successfully achieves the design of such a controller by means of computer simulation, which is based on the true model of the inverter system. The tuning of the controller and inverter parameters made through simulation has yielded a successful hardware design and a good performance during the experimental tests. The performance of controller is evaluated in terms of output voltage waveform distortion under actual linear and nonlinear loads. The experimental results that demonstrate the success of the controller are obtained from the voltage source inverters (VSI) built for a three-phase UPS application at 30 kVA and a transformer test converter application at 300 kVA. The maximum measured THD is less than 2.88% under the worst nonlinear load case. The converters employing the designed controller are currently in use with success in the field.

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.

The paper presents the single phase uninterruptible power supply (UPS) system with galvanic separated DC-AC-DC-AC converters operating in parallel. The CAN physical layer based system of communication between converters has been developed and applied, which allow to utilize a decentralized master-slave control providing high availability factor of the whole UPS system. The control system of particular converters has been developed to ensure a high quality of the output voltage for both linear and nonlinear load. The selected simulations and experimental results obtained for 5 kVA prototype modules are presented.

Series-shunt power quality compensators with the ability to function as an uninterruptible power supply (UPS) can solve a variety of power quality problems. This topology consists of two power electronic inverters. The shunt inverter is typically used as a voltage source during UPS operation, requiring a shunt inverter rated for the full load power. The rating of the series inverter is typically minimized to reduce cost, resulting in limited voltage compensation. This paper suggests a topology that enables the series inverter of a series-shunt compensator to assist the shunt inverter during UPS operation. The proposed topology allows the use of a shunt inverter with a power rating lower than the load rating, resulting in a compensator with reduced power rating while maintaining a high capacity for voltage compensation. This paper also suggests an arrangement of passive filters for switching ripple that does not adversely affect series voltage compensation. Practical results are presented, illustrating the operation of the proposed series-shunt UPS and series-shunt compensation for a capacitive nonlinear load.

This paper presents a fully digital control technique for voltage-fed three-phase inverters to be used in uninterruptible power supplies. An application-specific digital signal processor (VECON) is used to attain the regulation of the output voltages, which is maintained also in presence of wide load variations. After a short description of the system, attention is focused on VECON'S performances and implemented control algorithm. Even if no transformer is used in the power converter, the line-to-neutral voltages are independently controlled thus allowing supply of single-phase or unbalanced loads. An inner current control loop is implemented to provide limitation of the phase currents in case of short circuit and overload. Each current and voltage is controlled by a digital PI regulator. The overall control takes less than 20 /spl mu/s. Sinusoidal output voltages and well controlled phase currents are attained, as demonstrated by experimental results coming from a 15 kVA prototype.