Low-THD, Fast-Transient, and Cost-Effective Synchronous-Frame Repetitive Controller for Three-Phase UPS Inverters

In this paper, a combined current limiting algorithm is proposed for voltage controlled inverters. The proposed method combines the analog and digital current limiting algorithms. It provides fast response and better voltage waveform quality even during the transient stages. The proposed system is implemented and tested with a three-phase 10kVA UPS inverter. In addition, both digital and analog current limiting techniques are also implemented and tested for the same conditions and the obtained results are compared with the proposed combined current limiting algorithm. It is seen that proposed hybrid algorithm provides successful current limiting performance with its fast response in short circuit and overcurrent conditions, and significantly improves the reliability and fault tolerance of the inverter.

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

This paper proposes a straightforward control method for voltage control of a three-phase transformer-based inverter in uninterruptible power supplies or distributed generation systems. The approach offers a dual-loop design consisting inner current control loop and outer voltage loop. Sliding mode current controller provides desired bandwidth for voltage controller which consists of a state feedback term for stabilization and resonant term for harmonic damping. The proposed scheme provides fast dynamic response and low total harmonic distortion even for high power inverters with the limitations of switching frequency and LC filter components. Experimental studies for 2KVA linear and nonlinear loads using digital signal processors validate excellent performance of the proposed method.

In order to improve the quality of the power injected into the grid, a deadbeat (DB) control strategy based on weighted average current with plug-in repetitive control (RC) scheme is proposed for a grid-connected inverter with LCL filter. Firstly, we calculate weighted average current of input and output current of the LCL filter, and use this variable to simplify the control strategy. Therefore, the equivalent model of the LCL filter is reduced from the third-order to the first-order. Secondly, DB controller is designed in discrete-time based on this simplified model. In order to suppress the harmonics of fed-in grid current, the RC is proposed to plug-in with the DB controller in parallel by providing the required harmonic elimination of the periodic signals. In the proposed combined control, the deadbeat control can track a given signal with fast dynamic response, and the RC can make the steady state error converge to zero gradually. The simulation and experimental results have demonstrated the effectiveness and advantages of the proposed method.

The repetitive control technique is widely adopted in ac systems, because of its excellent steady‐state tracking performance and low total harmonic distortion. The repetitive control method with one‐sixth of the system fundamental period T0 as the delay time has been proposed to achieve fast transient response. In this study, an improved control scheme based on the T0/6 repetitive control is proposed for three‐phase grid‐connected inverters. The proposed scheme adopts T0/6 as the delay time in the positive‐rotating and negative‐rotating synchronous reference frames to suppress the 6n ± 1 harmonics. Meanwhile, the proportional‐integral regulator and the plug‐in repetitive controller are combined to reinforce the system performance. A new auxiliary function based on the linear interpolation is proposed to maintain the ideal repetitive control performance when one‐sixth of the ratio of the sampling frequency to the grid fundamental frequency is non‐integer. The effectiveness of the proposed scheme on improving the T0/6 repetitive control is confirmed by simulation and experimental results finally.

This paper investigates a robust model reference adaptive control (MRAC) method for a three-phase constant-voltage constant-frequency (CVCF) inverter with an output LC filter. The proposed MRAC method is designed to stabilize the error dynamics of the system by a feedback control term in the steady state and attenuate the parameter uncertainties of the system by an updated MRAC term. Unlike the conventional proportional-derivative control (PDC) scheme, the proposed MRAC scheme ensures the fast convergence of the output errors to the exponential trajectories predefined by the reference models. Furthermore, the adaptive state-feedback mechanism can guarantee the fast dynamic response in the transient state without using load current sensors or observers. The asymptotic stability is mathematically proven by a Lyapunov theory. The feasibility of the proposed controller is confirmed through extensive experimental studies on a prototype three-phase CVCF inverter with a TI TMS320LF28335 DSP. Finally, comparative experimental results of three control methods (i.e., conventional PDC, feedback linearization control, and proposed MRAC) are provided to validate the superior performance of the proposed method such as fast transient response, low total harmonic distortion, and robustness to parameter uncertainties under critical load conditions (i.e., abrupt load changes, unbalanced loads, and distorted nonlinear loads).

This paper develops an enhanced harmonic voltage compensator which is implemented with the aid of two repetitive controllers (RCs) in order to improve the output voltage performance of stand-alone distributed generation (DG) systems. The proposed harmonic voltage compensator is able to maintain the DG output voltage sinusoidal regardless of the use of nonlinear and/or unbalanced loads in the load side. In addition, it can offer good steady-state performance under various types of loads and a very fast dynamic response under load variations to overcome the slow dynamic response issue of the traditional RC. The feasibility of the proposed control strategy is verified through simulations and experiments.

Based on an in-depth analysis of the DC imbalance problem of UPS inverters, an observation is made that in most cases where instantaneous voltage feedback control is applied, the control algorithm itself is the most significant cause of DC imbalance since it inevitably generates DC bias in the output PWM gating signals. This situation becomes even worse when some integral action is included in the control algorithm, such as a PI/PID controller, or a repetitive controller, which is integral in nature. With this understanding, a simple DC-bias rejection algorithm is proposed, which needs no extra sensors, and can be seamlessly integrated into a repetitive controller. Experimental results verified its effectiveness. An alternative form of the algorithm is also provided for non-repetitive-control cases

In this paper, a parallel structure repetitive control scheme is proposed for three-phase PWM inverters to compensate 6k ± 1th order harmonics. This new control scheme divides conventional repetitive control into two branches in parallel structure. By this design, it avoids using filters in series, thus achieving higher gains for targeted harmonic groups than other 6k±1 repetitive controllers. Besides, the proposed control scheme reduces the number of delay units required to one third of that in conventional repetitive control schemes, thereby offering faster convergence rate and better transient performance. Stability and convergence of this approach are proved mathematically. Experimental results are provided to show the effectiveness of the proposed control scheme.

The differential current in Modular Multilevel Converters (MMCs) is inherently subjected to even order harmonics, which are mainly caused by the voltage mismatch between phase legs and dc bus. This issue may exist in both single-phase and multi-phase MMC systems and can be eased by repetitive control based active harmonic suppression methods, which regulate the circulating current through legitimate adjustment of the voltage applied on the arm inductance. Nevertheless, conventional repetitive controllers have relatively slow dynamic response because of the presence of one fundamental period delay, and all the sampled errors in the past one cycle have to be stored for closed-loop control. This paper presents an improved even-harmonic repetitive control scheme that requires less data memory with faster dynamic response. The steady state performance can also be enhanced if system disturbances e.g. frequency deviation are considered. In order to verify its practicality and effectiveness, PLECS simulation and experimental results are presented to show the excellent dynamic response and disturbance rejection of the proposed even-harmonic elimination scheme.

Two optimized control schemes of shunt active power filter (SAPF) in balanced three-phase three-wire system are proposed in this paper, including a fast fractional-order repetitive control (RC) strategy and a computation reduced harmonic detecting method. The improved RC is implemented in the synchronous reference frame (SRF) and based on the fractional-delayed-filter. Compared with the conventional RC (CRC), the proposed strategy can reduce the repetitive period from one fundamental cycle to one-sixth of the fundamental cycle, which results in the faster dynamic response. The fractional-delayed-filter, as the delay part of the proposed improved RC strategy, is used to achieve the high tracking accuracy of reference signals for control systems with different sampling rates. This paper also proposes a harmonic detecting method in the SRF based on the sliding Discrete Fourier transform (SDFT) with reduced computational burden, catering for selective harmonic elimination. Simulations and experiments show the validity of the proposed solutions.

Dynamic voltage restorer (DVR) has been demonstrated to be a very effective mechanism to deal with key voltage-quality disturbances. Although the dynamic response requirement for voltage sag could be satisfied with a feedforward control strategy, the steady-state requirement and zero-error tracking need an advanced feedback control strategy. This paper proposes a fast repetitive controller based feedback control strategy for DVR system. In the control system, the feedforward control loop could meet the dynamic requirement; the repetitive controller based feedback control loop could realize zero steady-state error tracking for fundamental voltage and harmonic components. The fast repetitive controller: 1) could eliminate all odd signals (including fundamental component); 2) could be simply implemented in digital signal processors just with a time delay loop; 3) has fast dynamic response when compared with traditional repetitive controller as its delay time is reduced to half of traditional's. This paper also chooses the small gain theorem for control system analysis and parameters design. The simulation on MATLAB/simulink also demonstrates the validity and superiority of proposed control strategy.

The repetitive control technique is widely adopted in AC systems, because it can achieve excellent tracking performance and low total harmonic distortion (THD) when regulating periodic signals. This paper proposes an improved repetitive control scheme for three-phase systems. The proposed scheme adopts one-sixth of the fundamental period as the delay time in the positive-rotating and negative-rotating synchronous reference frames (SRF), and can achieve fast transient response and 6n±1 harmonics compensation. In this paper, the proportional-integral (PI) regulator and the plug-in repetitive control are combined in the control loop, to improve the even harmonics compensation under the unbalanced system. And the design methodology for the auxiliary function of the repetitive control is discussed to maintain its performance when one-sixth of the ratio of the sampling frequency to the fundamental frequency is not an integer. The effectiveness of the proposed scheme is confirmed by simulation and experimental results finally.

This paper presents a different Finite Control Set – Model Predictive Control (FCS-MPC) for grid-connected three-phase bidirectional power inverters. These are typically used in dc or ac renewable-based microgrids (MGs), where bidirectional operation and fast dynamic response is required. The bidirectional grid-connected inverters are an essential part of MG, which inject energy into the ac grid or demand energy from it. The dynamic behavior of the system is a major concern since the current can suddenly change depending on the hierarchical controller. This paper proposes a different cost function using sliding mode theory, which offers a good dynamic response, reduced computational burden, and a parameter-free control model. The operation principle of the proposed controller is given and evaluated using a Hardware-In-the-Loop (HIL) system, but also experimentally with a 1kW laboratory prototype. The final results demonstrate the advantages of using this approach in grid-connected three-phase bidirectional power inverters in terms of dynamic response and reduced computational burden, making this solution technically attractive and viable.

This paper presents control strategy for multifunctional, three-phase, four wire AC-DC converters. The converter is named multifunctional because it operates as a battery charger and as a load power supply. In other words, two different loads in parallel are connected simultaneously to the converter. For this purpose same current and voltage signals are necessary to send to the control circuit. The indicated signals are, input main currents, output load currents, battery bank currents, voltage across of each battery bank, total output voltage, and input main voltages for feed-forward networks and current shape. The proposed converter control strategy is applied in non-isolated three-phase UPS systems. The relevant features of the proposed converter control strategy are, power factor correction, common neutral point between input and output to facilitate bypass circuit installation, symmetrical divided output voltage for inverters with neutral point, and battery current control with high power factor. Principle of operation of the control circuit and experimental results obtained from a small power scale prototype are presented.