Grid Voltage Sensorless Single-Phase Half-Bridge Active Filter and DC Bus Voltage Regulation

In this paper, a method is presented for the control of grid voltage sensorless single-phase half-bridge shunt active power filter to reduce the grid current harmonics of low-power single-phase non-linear loads. The proposed system forces the grid current to be sinusoidal without sensing and processing the grid voltage on the line. For this reason, instead of grid voltage the load current is processed by using self-tuning filter to generate reference grid current to eliminate current harmonic distortions. In order to design a low-cost system, a half-bridge voltage source converter is used to reduce driver circuits. In addition, the switching losses are also reduced by employing half-bridge voltage source converter, where only two switches are used. The imbalance voltages are also successfully eliminated on the DC-side capacitors. The performance of the proposed system is verified by using a real-time platform, and experimental results are presented to verify the effectiveness of the proposed system in this study

Proportional-integral and proportional-resonant based control strategy for PUC inverters

In this paper, a proportional-integral (PI) and proportional-resonant (PR) based control strategy is proposed for packed-U-cell (PUC) inverters. In the conventional PI-PI based control strategy, while the first PI regulates the DC capacitor voltage, the second PI regulates the AC load current. However, it is shown that the second PI cannot guarantee zero tracking error in the load current. The main reason of this comes from the fact that PI controllers are not able to achieve zero tracking error for the AC signals. Also, in an attempt to reduce the tracking error by tuning PI gains distorts the load current. In this study, a PI-PR based control strategy is obtained by replacing the second PI by a PR controller. The performances of both PI-PI and PI-PR control methods have been compared. It is shown that the load current to tracks its reference in all circumstances provided that the inverter current reference is generated accurately. Computer simulations are conducted to show the steady-state and dynamic performances of both control methods

Three-level hysteresis current control strategy for three-phase four-switch shunt active filters

In this study, a three-level hysteresis current-control (HCC) strategy is proposed for three-phase four-switch shunt active power filters. The four-switch topology which utilises four switching devices together with two series connected capacitors is able to reduce the cost, switching losses and improve the reliability of system. In this topology, when the current control of phases A and B is achieved successfully, the current control of phase C which is connected to the midpoint of the series connected capacitors is achieved automatically. The current control is achieved by using a three-level HCC strategy. An important consequence of using this control strategy is that it enables access to the zero level of the input voltage of active filter so that a switching device is only switched when the current error is negative, while it remains off when the current error is positive. Furthermore, the imbalance in the capacitor voltages is eliminated by adding a feedback term (the difference in the capacitor voltages multiplied by a suitable gain) to the current control. The proposed control strategy offers a reduced switching frequency, losses and cost. The steady-state and dynamic performance of the proposed control strategy is verified through simulations and experimental studies

Time-Varying and Constant Switching Frequency-Based Sliding-Mode Control Methods for Transformerless DVR Employing Half-Bridge VSI

This paper presents time-varying and constant switching frequency based sliding-mode control (SMC) methods for three-phase transformerless dynamic voltage restorers (TDVRs) which employ half-bridge voltage source inverter. An equation is derived for the time-varying switching frequency. However, since the time-varying switching frequency is not desired in practice, a smoothing operation is applied to the sliding surface function within a narrow boundary layer with the aim of eliminating the chattering effect and achieving a constant switching frequency operation. The control signal obtained from the smoothing operation is compared with a triangular carrier signal to produce the pulse width modulation signals. The feasibility of both SMC methods has been validated by experimental results obtained from a TDVR operating under highly distorted grid voltages and voltage sags. The results obtained from both methods show excellent performance in terms of dynamic response and low total harmonic distortion (THD) in the load voltage. However, the constant switching frequencybased SMC method not only offers a constant switching frequency at all times and preserves the inherent advantages of the SMC, but also leads to smaller THD in the load voltage than that of time-varying switching frequency-based SMC method

Optimized Sliding Mode Control to Maximize Existence Region for Single-Phase Dynamic Voltage Restorers

This paper presents an optimized sliding mode control (SMC) strategy to maximize existence region for single-phase dynamic voltage restorers. It is shown analytically that there exists an optimum sliding coefficient which enlarges the existence region of the sliding mode to its maximum. Also, it is pointed out that the optimum sliding coefficient improves the dynamic response. In addition, a double-band hysteresis control which ensures the switching of a transistor in the voltage source inverter during a half-cycle while it remains either on or off in the other half cycle is used to mitigate the switching frequency. The theoretical considerations and analytical results are verified through computer simulations and experimental results. Simulation and experimental results show that the proposed SMC strategy not only compensates the undesired voltage disturbances and maintains the load voltage at desired level with low total harmonic distortion, but also exhibits fast dynamic response and operates at reasonably low switching frequency

Detection of Grid Voltage Anomalies via Broadband Subspace Decomposition

Due to the increase of sensitive loads on the mains power grid, measurement and monitoring of the power quality (PQ) have become an important factor for both consumers and operators. As is well-known, PQ problems occur in a very short time period with specific characteristics. In transmission or distribution systems, power quality data are collected from monitoring devices such as digital fault recorders, power quality and dynamic system monitors, etc. The recorded data has to be analysed in order to understand system anomalies. These anomalies may be due to sources of broadband noise. In this study, we employ broadband subspace decomposition, using polynomial eigenvalue decomposition, to detect these anomalies. Results demonstrate that this method may be considered as a new and effective tool for measurement and monitoring of PQ problems

Generalized Configurations for the Synchronization System Based on Transfer Function Approach

This paper presents our research on deriving all the generalized forms for the grid synchronization system of an inverter in the grid-connected applications. The transfer function approach is introduced in order to obtain the generalized second-order and third-order synchronization systems. This approach is based on the orthogonal property of two outputs from the second-order generalized-integrator (SOGI) technique that these two outputs are always 90° difference in phase but only equal in magnitude at the grid frequency. The proposed approach then can provide the generalized forms of any high-order synchronization system, including the second-order and the third-order systems. It is interesting to see that the systems derived from this approach include all the previous synchronization systems in the literature. The simulation results have confirmed that these generalized systems are able to generate two perfectly orthogonal outputs to determine the grid information, i.e. voltage magnitude and phase angle. In addition, the generalized third-order system shows its excellent performance in removing harmonics under the distorted grid condition, and thus its ability to accurately detect the grid information

Linear Kalman Filter-Based Grid Synchronization Technique: An Alternative Implementation

Grid synchronization techniques play a significant role in integrating renewable energy sources to the electric power grid. In this context, estimating the phase and frequency of the grid voltage signal is an interesting problem. Out of various techniques available in the literature, Linear Kalman Filter (LKF) is one of the most popular one. In this paper, we propose an alternative implementation of the LKF for grid synchronization application. The proposed implementation uses a linear parametric model of the grid voltage signal including DC offset. It does not involve any quadrature signal generation, rather it works by estimating the phase angle. This helps to estimate the unknown grid frequency directly from the phase angle. This clearly differentiates the proposed alternative implementation with respect to the existing implementations. Performance improvement by the proposed technique is verified extensively through comparative numerical simulation and experimental studies. Comparative results demonstrate the suitability of the proposed technique with respect to other state-of-the-art techniques namely SecondOrder Generalized Integrator Phase-Locked Loop (SOGI-PLL) and Enhanced Phase-Locked Loop (EPLL)

Three-Phase Two-Leg T-Type Converter based Active Power Filter

In this work, a three-phase two-leg T-type active power filter topology is investigated when a three-level hysteresis band current modulation technique is applied. The three-phase two-leg T-type configuration which utilizes eight switching devices together split DC link capacitor banks is able to improve the reliability of the system as well as effectively reduces the power losses. The compensation of the current harmonics at each phase are achieved successfully with the control of phase-a and phase-b. Using this technique, the phase-c is directly connected to the midpoint of the split capacitors, which eliminates the necessity of additional control on the phase-c. The performance of the proposed topology and control method is demonstrated through the simulation results

Low-pass filtering or gain tuning free simple DC offset rejection technique for single and three-phase systems

This paper aims to address the DC offset rejection problem in grid synchronization algorithm. A simple approach to estimate the unknown grid frequency in the presence of DC offset is proposed for this purpose. Some of the existing techniques available in the literature use either low-pass filter or an additional integrator to eliminate the DC offset. Both approaches require an additional parameter to tune. However, tuning the additional parameter is not straightforward. Moreover, tuning the overall system can be complicated due to the presence of DC offset rejection part. The proposed approach does not require any additional parameter to tune. By considering the orthogonal signal instead of the DC offset as an additional state, the proposed technique can efficiently estimate the unknown frequency of the grid. Application to both single and three-phase grids are provided. Comparative experimental results with DC offset rejection capable second-order generalized integrator (SOGI) phase-locked loop (PLL) (SOGI-PLL) demonstrate the effectiveness and suitability of the proposed technique