Andres A. Valdez-Fernandez

A Model-Based Controller for the Cascade H-Bridge Multilevel Converter Used as a Shunt Active Filter

This paper presents the modeling and control processes of the single-phase cascade H-bridge multilevel converter used as a shunt active filter. Based on the obtained model, a controller is proposed to compensate for harmonic distortion and reactive power caused by a nonlinear load. These issues are solved by guaranteeing tracking of the line current toward an appropriately defined current reference. In the proposed approach, the current reference has been selected to be a signal proportional to either the line voltage or to its fundamental component; however, the proposed method can also be extended to other reference definitions. The proposed controller also includes voltage loops to guarantee regulation and balance of the involved capacitor voltages toward constant references. Experimental results on a 2-kVA prototype are presented to assess the performance of the proposed controller.

Direct Power Control of a Three-Phase Rectifier Based on Positive Sequence Detection

This paper presents a model-based controller for the direct control of real and reactive powers in a three-phase three-wire rectifier. As the unbalanced operation is considered, it is convenient to express the grid voltage in terms of both positive and negative sequences, in order to obtain a more complete description of the system. In particular, the positive sequence of the grid voltage is used as the transformation basis to reconstruct the average, i.e., a particular subset, of the conventional power definition. Based on this transformation, the regulation of the average of both real and reactive powers is possible without deformation of the grid current. The proposed controller also incorporates an adaptation mechanism to cope with parameter uncertainties. For comparison purposes, a conventional controller based on the complete definition of the instantaneous powers is also presented. Experimental results are provided to assess the performance of the proposed positive-sequence-based solution and compared with those obtained using a conventional controller.

Implementation of a

Repetitive schemes represent an attractive solution for harmonic compensation, as they are easy to implement and require a reduced computational effort. Repetitive schemes involve the interconnection of delay lines usually implemented in digital form. A delay line is realized digitally by reserving a given number of memory localities, where samples are allocated and released after a specific number of sampling periods (discrete delay) equivalent to the delay time. Such a discrete delay computed as the ratio between the required delay time and the sampling period must be an integer number in the best scenario. However, the discrete delay may have a fractional part due to limitations on the sampling period or the required delay time. This issue is referred in signal processing literature as fractional delay (FD). This paper presents a solution to implement repetitive schemes subject to such an FD issue. The solution consists in the introduction, on each delay line, of an additional filter aimed to compensate such an FD. In particular, this paper focuses on a repetitive scheme that is able to compensate harmonics 6n ±1. Experimental results are presented to confirm the benefits of the proposed scheme.

6n \pm 1

Voltage source inverters (VSI) connected to the grid by means of LCL filters offer benefits in the injection of active power into the grid, since they provide low harmonic distortion and a small size of the output filter. However LCL grid connected VSI must be designed in order to provide damping at the filter resonance frequency. This paper presents the design and analysis of a model based controller for an LCL coupling filter to inject active power into the grid for single phase photovoltaic transformerless applications. The control objectives are to provide closed-loop stability and to assure that the current to be injected follows a reference which is proportional to the grid voltage. Therefore, these objectives guarantee the active power injection to the grid. The proposed output feedback controller is based on the error model of the LCL filter and makes use only of the measurements of the output current of the inverter and the grid voltage.

Repetitive Controller Subject to Fractional Delays

This paper presents a phase-locked loop (PLL) method aimed to provide an estimation of the phase angle and the amplitude of the fundamental positive sequence component of a three-phase reference signal subject to severe unbalance and high harmonic distortion. In addition the proposed scheme provides the estimation of the angular frequency, and both the positive and negative sequences of the fundamental component of the reference signal. The proposed method is referred as CSRF-PLL and consists of a cascade interconnection of two recently reported PLL schemes, namely, UH-PLL reported in [1] and the conventional SRF-PLL reported in [2]. In the CSRF-PLL, the UH-PLL is used as a pre-filter to extract the positive-sequence component of the reference signal which is fed to the SRF-PLL. Recall that the proper operation of the SRF-PLL can only be guaranteed if its input is a pure sinusoidal and balanced signal, which in this case is represented by the fundamental positive sequence delivered by the UH-PLL. The SRF-PLL is used, in its turn, to estimate the fundamental frequency which is necessary for the proper operation of the UH-PLL. Simulations are provided to evaluate the performance of the proposed solution. Experimental evidence will be provided in the final version of the paper.

Model based controller for an LCL coupling filter for transformerless grid connected inverters in PV applications

Repetitive schemes represent a very attractive solution for harmonics compensation, as they are simple to implement and the computational effort is reduced. Their implementation, usually performed in digital form, involves the interconnection of a single delay line or a simple array of delay lines. In discrete time domain, a delay line is implemented using a given number of memory locations, where samples are allocated and released after a specific number of sampling periods (discrete delay) equivalent to the delay time. This discrete delay is obtained as the ratio between the sampling period and the required delay time. The problem arises whenever the delay time is a noninteger multiple of the sampling period, in this case the discrete delay is a real positive number having a fractional part. This issue is referred in signal processing literature as fractional delay. This paper presents a solution for the practical implementation of repetitive schemes where the delay lines are subject to the fractional delay issue. The solution consists in the introduction, on each delay line, of an additional filter aimed to approximate such a fractional delay. This filter has the structure of a finite impulse response (FIR) filter, and thus, it is referred as FIR fractional delay (FIR-FD) filter. Its design follows a Lagrange interpolation technic, which relays in explicit formulae for the computation of the coefficients of the FIR filter. In particular, the present paper is focused in the repetitive scheme proposed in [11] for compensation of harmonics 6ℓ ± 1 (ℓ = 0, 1, 2, ...∞) of the fundamental frequency . Numerical results are presented to confirm the benefits of the proposed scheme. Experiments are in process and the results will be available in the final version of the paper.

Cascade three-phase PLL for unbalance and harmonic distortion operation (CSRF-PLL)

This paper presents a model-based controller for an active power filter (APF) to compensate reactive power and current harmonic distortion in a single-phase system. A Half-bridge Neutral Point Clamped (NPC) topology is selected as a voltage source inverter (VSI) to implement the APF. The control scheme is composed by three control loops referred as the inner control loop (or current loop), the regulation loop, and the balance control loop. The inner control loop is aimed to compensate both the displacement power factor (DPF) and the harmonic distortion of the current flowing to the grid. Only compensation of odd harmonics is considered which is the distortion present in most single-phase non-linear loads. The regulation and balance loops are designed to achieve the regulation and balance of the capacitor voltages in the DC-link. Numerical results are presented to show the performance of the proposed solution.

Implementation of repetitive controllers subject to fractional delays

This paper presents the modeling and control design of a single-phase cascaded H-bridge multilevel converter used as a shunt active filter in the general case of n-levels. Based on a general model, a controller is proposed to compensate harmonic distortion and reactive power caused by nonlinear loads. For this, the controller must guarantee tracking of the line current towards an appropriately defined reference. In the proposed approach, the current reference has been selected to be proportional to the line voltage. The proposed controller also includes voltage loops to guarantee regulation and balance of the involved capacitor voltages. Simulation results are presented to assess the performance of the proposed controller.

A model-based controller for a half-bridge NPC used as an active power filter

ABSTRACT This paper presents the design, implementation, and evaluation of a model-based controller for a general scheme of transformerless power converters connected to the grid through an L filter. The proposed controller is aimed at active power injection, and consists of a damping term over the current error in addition to a filter parameter estimation scheme. The latter provides estimates of the filter internal resistance and inductance values. It involves the fundamental and quadrature components of the grid voltage to asymptotically compensate for current harmonic peturbations. As a result, the closed-loop system fulfills the active power injection objective. The proposed controller is tested in an experimental setup and its performance is compared to that of the Proportional-Resonant controller. Experimental results are presented for a single-phase H-bridge inverter; however, the control scheme can be used in several other H-bridge based inverter topologies such as H5 and HERIC.

A generalized model-based controller for the n-level CHB multilevel converter used as a shunt active filter

This paper presents a model-based fault tolerant control strategy for open-circuit faults (OCFs) in the switching devices of a cascade H-bridge converter with n-levels (CHB-nL). In this study, we address the application of the CHB-nL converter as a single-phase active filter. The OCFs are modeled as additive signals into the subsystems describing the dynamics of each H-bridge in the converter. For diagnosis purposes, a bank of nonlinear proportional-integral (PI) observers are suggested to estimate the OCF profiles, independently of the operating point of the converter. Based on the estimated components, fault detection, isolation and reconfiguration is achieved. Furthermore, no additional sensors are needed to carry out the diagnosis media besides the ones used for control purposes. To validate the ideas proposed in this work, an experimental evaluation is carried out for a single OCF condition. The experimental results are used to confirm that the current tracking, voltage regulation and balance objectives can be obtained despite of OCFs in the active filter and highly distorted load currents.