Andres A. Valdez

Repetitive-Based Controller for a UPS Inverter to Compensate Unbalance and Harmonic Distortion

This paper discusses a repetitive-based controller for an uninterruptible power supply (UPS) inverter. It is shown that a bank of resonant filters, used as a refinement term for harmonic compensation in earlier works, is equivalent to a repetitive scheme with a particular structure. The latter is implemented using a simple feedback array with a delay line, thus making the implementation relatively easy. More precisely, the repetitive scheme takes a negative feedback structure plus a feedforward path whenever the odd harmonics are considered for compensation only. The repetitive scheme, equivalent to the bank of resonant filters, acts as a refinement term to reject the harmonic distortion caused by the unbalanced and distorted load current, and thus, allowing the UPS inverter to deliver an almost pure sinusoidal balanced voltage. Experimental results in a 1.5 KVA three-phase inverter are included to show the performance of the proposed controller

An Adaptive Control for UPS to Compensate Unbalance and Harmonic Distortion Using a Combined Capacitor/Load Current Sensing

This paper investigates the control of an uninterruptible power supply (UPS) using a combined measurement of capacitor and load currents in the same current sensor arrangement. The purpose of this combined measurement is, on one hand, to reach a similar performance as that obtained in the inductor current controller with load current feedforward and, on the other hand, to easily obtain an estimate of the inductor current for overcurrent protection capability. Based on this combined current measurement, a voltage controller based on resonant harmonic filters is investigated in order to compensate for unbalance and harmonic distortion on the load. Adaptation is included to cope with uncertainties in the system parameters. It is shown that after transformations the proposed controller gets a simple and practical form that includes a bank of resonant filters, which is in agreement with the internal model principle and corresponds to similar approaches proposed recently. The controller is based on a frequency-domain description of the periodic disturbances, which include both symmetric components, namely, the negative and positive sequence. Experimental results on the output stage of a three-phase three-wire UPS are presented to assess the performance of the proposed algorithm

Fixed-Reference-Frame Phase-Locked Loop for Grid Synchronization Under Unbalanced Operation

This paper presents a grid synchronization scheme aimed to provide an estimation of the angular frequency and both the positive and negative sequences of the fundamental component of an unbalanced three-phase signal. These sequences are provided in fixed-reference-frame coordinates, and thus, the proposed algorithm is referred to as fixed-reference-frame phase-locked loop (PLL) (FRF-PLL). In fact, the FRF-PLL does not require transformation of variables into the synchronous frame coordinates as in most PLL schemes. Therefore, the proposed scheme is not based on the phase angle detection. Instead, the angular frequency is detected and used for synchronization purposes. The design of the FRF-PLL is based on a complete description of the source voltage involving both positive and negative sequences in stationary coordinates and considering the angular frequency as an uncertain parameter. Therefore, the FRF-PLL is intended to perform properly under severe unbalanced conditions and to be robust against angular frequency variations, sags, and swells in the three-phase utility voltage signal. Due to the selective nature of the scheme, it shows certain capability to alleviate the effect of harmonic distortion that is present in the utility voltage signal.

An Adaptive Controller for the Shunt Active Filter Considering a Dynamic Load and the Line Impedance

This brief presents a controller for an active filter to compensate reactive power and current harmonic distortion in a single phase system, i.e., to guarantee a power factor close to unity. The proposed controller considers the deleterious effects caused by the interaction between load and line impedances, which may lead to instabilities when the controller is enabled. The rationale behind the solution consists in the introduction of a lead compensator with a gain that is adjusted by adaptation, which replaces the conventional proportional term. This modification improves the stability conditions when the load and source impedances are non-negligible. In particular, the scheme provides a solution in the critical scenario, when the load is composed by a capacitor connected in parallel to a distorted current source. Special attention is given to the current control loop because it is precisely in this loop where the instability problems are generated. Realistic numerical results are provided to illustrate the benefits of the proposed solution.

A controller for a boost converter with harmonic reduction

An adaptive controller for the compensation of output voltage ripple due to harmonic distortion in the input voltage is proposed for a pulse width modulated (PWM) boost converter. Following the Lyapunov approach, we designed an adaptive law to cope with uncertainties in the disturbance signals and parameters. Complexity of the proposed controller is reduced by rotations which transform the adaptive terms into a sum of resonant filters tuned at the frequencies of the harmonics under compensation, and operating on the output voltage error. To facilitate the implementation, we have tried to preserve the structure of the proposed controller as close as possible to the conventional one, which includes a voltage outer loop (basically a proportional plus integral (PI) control on the output voltage error) and an inner control loop (basically a proportional control plus a feedforward term). In the proposed controller, the bank of resonant filters appears as a refinement term added to the inner control loop. Indeed, they insert notches in the audio-susceptibility curve, which are tuned at the harmonics under compensation. Thus, in addition to the benefits of a conventional feedforward PWM control, the bank of resonant filters are able to cancel selected harmonics. Experimental results on a boost converter board, using a poorly regulated voltage source, are presented to assess the performance of our approach.

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

This paper presents a modeling process for the cascade H-bridge three-phase multilevel converter used as a shunt active filter. Based on the proposed mathematical model, a controller is designed to compensate harmonic distortion and reactive power due to a nonlinear load. Simultaneously, the controller guarantees regulation and balance of all capacitor voltages. The idea behind the controller is to allow distortion of the current reference during the transients to guarantee regulation and balance of the capacitors voltages. The results are shown for a cascade H-bridge three-phase five-level converter, however, this approach can be easily extended to higher levels.

An adaptive controller for a boost converter with harmonic reduction

An adaptive controller for the compensation of harmonics in the input voltage is proposed for a pulse width modulated (PWM) boost converter. Following the Lyapunov approach we designed an adaptive law to cope with uncertainties in the disturbance signals. Complexity of the proposed controller is reduced by rotations which transform the adaptive terms into a sum of resonant filters having as input the output voltage error. The resonant filters are tuned at the frequencies of the harmonics under consideration. To facilitate the implementation we have tried to preserve the structure of the proposed controller as close as possible to the conventional controller. The latter is usually composed by a voltage outer loop (basically a proportional plus integral (PI) control on the output voltage error) and an inner control loop (basically a Proportional control plus a Feedforward term). Thus, in the proposed controller, the bank of resonant filters appears as a refinement term which is added to the inner control loop. The proposed controller turns out to be robust with respect to parameter uncertainties. Experimental results on a boost converter board, using a poorly regulated voltage source, are presented to assess the performance of our approach.

A controller for the active filter considering load and line impedances

The paper presents a controller for an active filter to compensate reactive power and current harmonic distortion in a single phase system, i.e., to guarantee a power factor close to unity. The proposed controller considers the negative effects caused by the interaction between load and line impedances, which may lead to instabilities. In particular, the scheme provides a solution in the critical scenario, when the load is composed by a capacitor connected in parallel to a distorted current source. The rationale behind the solution consists in the introduction of a lead compensator with a gain that is adjusted by adaptation, which replaces the conventional proportional term. This modification improves the stability conditions when the load and source impedances are considerable. Special attention is given to the current control loop because it is precisely in this loop where the instability problems arise. Realistic numerical results are provided to illustrate the benefits of the proposed solution.

H5-HERIC based transformerless multilevel inverter for single-phase grid connected PV systems

Transformerless photovoltaic systems are one of the best options to connect a photovoltaic system to the electrical grid given their high efficiency, low cost, small size and low weight regarding their counterparts based on a transformer. In this paper a multilevel single-phase inverter is aimed to link a photovoltaic generator with the electrical grid with reduced common mode current. A sinusoidal based modulation strategy is also presented which focus not only in the reduction of the common mode current but also in the reduction of the switching and conduction losses. Five voltage levels are obtained at the inverter output, therefore, a good behavior is achieved regarding harmonic distortion. Numerical results show that the proposed inverter and modulation strategy give a reduced common mode current and low harmonic distortion which is suitable for transformerless photovoltaic systems.