Lucian Asiminoaei

High Performance Current Controller for Selective Harmonic Compensation in Active Power Filters

A new current control scheme for selective harmonic compensation is proposed for shunt active power filters. The method employs an array of resonant current controllers, one for the fundamental, and one for each harmonic, implemented in fundamental reference frame in order to reduce the overall computational effort. The proposed controller design is based on the pole-zero cancellation technique, taking into account the load transfer function at each harmonic frequency. Two design methods are provided, which give controller transfer functions with superior frequency response. The complete current controller is realized as the superposition of all individual harmonic controllers. The frequency response of the entire closed loop control is optimal with respect to filtering objectives, i.e., the system provides good overall stability and excellent selectivity for interesting harmonics. This conclusion is supported by experimental results on a 7.6-kVA laboratory filter, indicating a reduction in current THD factor from 34% to 2%, while the highest harmonic compensated is the 37th harmonic current.

Frequency Response Analysis of Current Controllers for Selective Harmonic Compensation in Active Power Filters

This paper compares four current control structures for selective harmonic compensation in active power filters. All controllers under scrutiny perform the harmonic compensation by using arrays of resonant controllers, one for the fundamental and one for each harmonic of interest, in order to achieve zero phase shift and unity gain in the closed-loop transfer function for selected harmonics. The complete current controller is the superposition of all individual harmonic controllers and may be implemented in various reference frames. The analysis is focused on the comparison of harmonic and total closed-loop transfer functions for each controller. Analytical similarities and differences between schemes in terms of frequency response characteristics are emphasized. It is concluded that three of them have identical harmonic behavior despite the fact that their implementation is significantly different. It emerges that the fourth one has superior behavior and robustness and can stably work at higher frequencies than the others. Theoretical findings and analysis are supported by comparative experimental results on a 7-kVA laboratory setup. The highest harmonic frequency that can be stably compensated with each control method has been determined, indicating significant differences in the control performance.

A digital controlled PV-inverter with grid impedance estimation for ENS detection

The steady increase in photovoltaic (PV) installations calls for new and better control methods in respect to the utility grid connection. Limiting the harmonic distortion is essential to the power quality, but other requirements also contribute to a more safe grid-operation, especially in dispersed power generation networks. For instance, the knowledge of the utility impedance at the fundamental frequency can be used to detect a utility failure. A PV-inverter with this feature can anticipate a possible network problem and decouple it in time. This paper describes the digital implementation of a PV-inverter with different advanced, robust control strategies and an embedded online technique to determine the utility grid impedance. By injecting an interharmonic current and measuring the voltage response it is possible to estimate the grid impedance at the fundamental frequency. The presented technique, which is implemented with the existing sensors and the CPU of the PV-inverter, provides a fast and low cost approach for online impedance measurement, which may be used for detection of islanding operation. Practical tests on an existing PV-inverter validate the control methods, the impedance measurement, and the islanding detection.

Evaluation of harmonic detection methods for active power filter applications

In the attempt to minimize the harmonic disturbances created by the non-linear loads the choice of the active power filters comes out to improve the filtering efficiency and to solve many issues existing with classical passive filters. One of the key points for a proper implementation of an active filter is to use a good method for current/voltage reference generation. There exist many implementations supported by different theories (either in time- or frequency-domain), which continuously debate their performances proposing ever better solutions. This paper gives a survey of the common used theories. Then, the work here proposes a simulation setup that decouples the harmonic reference generator from the active filter model and its controller. In this way the selected methods can be equally analyzed and compared with respect to their performance, which helps anticipating possible implementation issues. The conclusions are collected and a comparison is given at the end, which is useful in deciding the future hardware setup implementation. The comparison shows that the choice of numerical filtering is a key factor for obtaining good accuracies and dynamics for an active filter.

Implementation and test of an online embedded grid impedance estimation technique for PV inverters

New and stronger power quality requirements are issued due to the increased amount of photovoltaic (PV) installations. In this paper different methods are used for continuous grid monitoring in PV inverters. By injecting a noncharacteristic harmonic current and measuring the grid voltage response it is possible to evaluate the grid impedance directly by the PV inverter, providing a fast and low-cost implementation. This principle theoretically provides an accurate result of the grid impedance but when using it in the context of PV integration, different implementation issues strongly affect the quality of the results. This paper also presents a new impedance estimation method including typical implementation problems encountered, and it also presents adopted solutions for online grid impedance measurement. Practical tests on an existing PV inverter validate the chosen solution.

Performance Improvement of Shunt Active Power Filter With Dual Parallel Topology

This paper describes the control and parallel operation of two active power filters (APFs). Possible parallel operation situations of two APFs are investigated, and then the proposed topology is analyzed. The filters are coupled in a combined topology in which one filter is connected in a feedback loop and the other is in a feedforward loop for harmonic compensation. Thus, both active power filters bring their own characteristic advantages, i.e., the feedback filter improves the steady-state performance of the harmonic mitigation and the feedforward filter improves the dynamic response. Another characteristic of the proposed topology is the possibility of joint operation of both filters either as frequency-sharing or load-sharing, with or without redundancy. The frequency-sharing operation is possible due to the control algorithm, which is based on selective harmonic compensation using equivalent harmonic integrators. Implementation details and a discussion on the efficiency improvement for various switching frequencies are provided. The evaluation of the proposed topology concludes that this approach is very practical for achieving both low and high order harmonic compensation and stable grid operation. This is supported by extensive measurement results on a 15-kVA laboratory setup, indicating a reduction in total harmonic current distortion from the existing 30% to less than 2% for a typical adjustable speed drive application

Adaptive Compensation of Reactive Power with Shunt Active Power Filters

This paper describes an adaptive method for compensating the reactive power with an active power filter (APF), which is initially rated for mitigation of only the harmonic currents given by a nonlinear industrial load. It is proven that, if the harmonic currents do not load the APF at the rated power, the available power can be used to provide a part of the required reactive power. Different indicators for designing such application are given, and it is proven that the proposed adaptive algorithm represents an added value to the APF. The algorithm is practically validated on a laboratory setup with a 7-kVA APF.

Application of Discontinuous PWMModulation in Active Power Filters

Classical discontinuous pulsewidth modulations (DPWMs) may not be efficiently applied in active power filters (APFs), because it is hard to predict the peak values of the inverter current, and consequently it is difficult to calculate the position of the clamped interval, that minimizes the switching losses in any operating point. This paper proposes a new DPWM strategy applied to shunt APFs. The proposed modulation strategy detects the current vector position relative to the inverter voltage reference and determines instantaneously the optimum clamped duration on each phase. It achieves a clamped voltage pattern, with variable lengths depending on the magnitude of the inverter current. This property adaptively reduces the current stress and minimizes the inverter switching losses, regardless of its application. The proposed modulation strategy is described, analyzed and validated on a three-phase voltage source inverter, rated at 7 kVA 400 V, controlled as an APF.

A new method of on-line grid impedance estimation for PV inverter

The recent increase in photovoltaic (PV) installations calls for new and better power quality requirements with respect to connection to the grid supply. Therefore, different methods are typically used for continuous grid monitoring, usually by using external devices. In this paper a new method for on-line measuring the grid impedance is presented. The presented method requires no extra hardware being accommodated by typical PV inverters, sensors and CPU, to provide a fast and low cost approach of on-line impedance measurement. By injecting a noncharacteristic harmonic current and measuring the voltage grid response it is possible to evaluate the grid impedance. Practical test on an existing PV inverter validate the new method.

Reduction of Switching Losses in Active Power Filters With a New Generalized Discontinuous-PWM Strategy

The classical discontinuous pulsewidth modulations (DPWMs) cannot be efficiently applied in active power filters (APFs) because it is difficult to predict the peak values of the inverter current. Consequently, it is difficult to calculate the optimal position of the clamped interval to minimize the switching losses in any operating point. This paper proposes a new DPWM strategy for shunt APFs. The proposed modulation strategy detects the current vector position relative to the inverter voltage reference and determines the optimum clamped duration for each phase, in terms of switching power losses. It achieves a clamped voltage pattern, with variable lengths depending on the magnitude of the inverter current. This property reduces the current stress and minimizes the inverter switching losses. The proposed modulation strategy is described, analyzed, and validated on a three-phase voltage source inverter, rated at 3 kVA, 400 V, controlled as an APF.