Sérgio Augusto Oliveira da Silva

Synchronous reference frame based controllers applied to shunt active power filters in three-phase four-wire systems

This work presents compensation algorithm schemes used for shunt active power filters applied to three-phase four-wire systems, allowing harmonic current suppression and reactive power compensation, which results in an effective power factor correction. The strategies used to extract the three-phase reference currents are based on the synchronous reference frame method. Although this method is based on balanced three-phase loads, it can also be used for single-phase loads, allowing independent control of all three phases. Accordingly, a fictitious quadrature current needs to be generated through software implementation, and be orthogonal to the measured load current. This creates the fictitious balanced currents in the two-phase stationary reference frame system, allowing the choice of an adequate compensation strategy which will result in either balanced or unbalanced sinusoidal source currents. Three shunt APF topologies are evaluated under unbalanced load conditions: Split-Capacitor (S-C), Four-Leg (F-L) and Three Full-Bridge (3F-B). The proposed algorithms applied to the three APF topologies are evaluated and discussed. Mathematical analyses of the SRF-based algorithms are presented and simulation results are performed to validate the theoretical development and confirm the performance of the shunt APFs.

Compensation Algorithms Applied to Power Quality conditioners in three-phase four-wire systems

This work presents compensation algorithm schemes used in power quality conditioners applied to three-phase fourwire systems, allowing harmonic current suppression and reactive power compensation which results in an effective power factor correction. The strategies used to extract the three-phase compensation currents are based on the synchronous reference frame method. Although this method is itself based on balanced three-phase loads, it can also be used for single-phase loads, allowing independent control of all three phases. The compensation algorithms presented are implemented for two power conditioners, such as the Unified Power Quality Conditioner (UPQC) and the line-interactive Uninterruptible Power Supply (UPS) system, which allow the choice of a suitable compensation strategy resulting in either balanced or unbalanced sinusoidal source currents under balanced or unbalanced load conditions, respectively. Two, four-leg PWM converters are used to implement the UPQC and the UPS system to simultaneously perform the compensation of the load harmonic currents, utility voltage harmonics and the current and voltage unbalances. Mathematical analyses of the Synchronous Reference Frame (SRF)-based algorithms are presented and simulation results are performed to validate the theoretical development and confirm the performance of the power quality conditioners.

A line-interactive UPS system operating with sinusoidal voltage and current references obtained from a self-tuning filter

This paper presents a single-phase line interactive uninterruptible power supply (UPS) system, allowing output voltage conditioning, such as voltage harmonic suppression and sag or swell compensation. Additionally, the UPS input current is also conditioned, allowing current harmonic suppression and reactive power compensation. In other words, the UPS system acts as a true unified power quality conditioner (UPQC), when it is working in standby operation mode. Both voltage and current sinusoidal UPS references are generated by using a phase-locked loop (PLL) combined with a self-tuning filter (STF), allowing the implementation of a simplified control strategy, in which by controlling the UPS dc-bus voltage, is possible to control the amplitude of the sinusoidal reference current of the series converter and, consequently, the power flow through the UPS. The power rate handled by both series and parallel PWM converters depends on the operation conditions of the UPS system, such as, among others, the amplitude difference between the input and output voltages. In standby mode, the amplitude of the UPS output voltage can vary in order to follow the amplitude of the input voltage within a limited range defined by design requirements. This strategy will allow an effective optimization of the UPS system efficiency. Mathematical analyses and validation results are presented in order to evaluate the performance of the line interactive UPS system.

Single-Stage Three-Phase Grid-Tied PV System With Universal Filtering Capability Applied to DG Systems and AC Microgrids

This paper proposes a single-stage three-phase four-wire grid-connected photovoltaic (PV) system operating with a dual compensating strategy and feedforward control loop (FFCL). Besides injection of active power into the grid, the PV system operates as a unified power quality conditioner (UPQC), suppressing load harmonic currents and compensating reactive power. Furthermore, regulated, balanced, and harmonic-free output voltages are provided to the load. Since the PV-UPQC is based on a dual compensation strategy, the series converter operates as a sinusoidal current source, whereas the parallel converter operates as a sinusoidal voltage source. Thus, seamless transition can be achieved from the interconnected to the islanding operation modes, and vice versa, without load voltage transients. Moreover, to overcome problems associated with sudden solar irradiation changes, fast power balance involving the PV array and the grid is obtained, since the FFCL acts on the generation of the series inverter current references. As a result, the dynamic responses of both inverter currents and dc-bus voltage are improved. Detailed analysis involving the active power flow through the inverters is performed allowing proper understanding of the PV-UPQC operation. Experimental results are presented to evaluate both dynamic and static performances of the PV-UPQC tied to the electrical distribution system.

Dynamic Performance Improvement of a Grid-Tied PV System Using a Feed-Forward Control Loop Acting on the NPC Inverter Currents

This paper presents a three-phase grid-connected photovoltaic (PV) system, which is implemented using the neutral-point-clamped (NPC) inverter. A current feed-forward control loop (FFCL) is proposed to improve the PV system dynamic behavior, due to the PV array being constantly subjected to sudden solar irradiance change, which causes voltage oscillations in the dc bus and, hence, interferes in proper PV system operation. As the current FFCL acts to speed up the calculation of the inverter current references, the dynamic response of the currents injected into the grid is improved. As a consequence, the dynamic behavior of the dc-bus voltage is also enhanced, reducing both settling time and overshoot. Besides the injection of active power into the grid, the PV system is also controlled to perform active power-line conditioning, so that load harmonic currents are suppressed, as well as load reactive power is compensated. However, the NPC inverter must be properly designed to guarantee that its power rating will not be exceeded, since both fundamental and nonfundamental current components flow through the grid-tied inverter. Extensive experimental results based on a digital signal processor are presented in order to evaluate the effectiveness, as well as the static and dynamic performance of the PV system.

A Versatile Unified Power Quality Conditioner Applied to Three-Phase Four-Wire Distribution Systems Using a Dual Control Strategy

This paper presents the study, analysis, and practical implementation of a versatile unified power quality conditioner (UPQC), which can be connected in both three-phase three-wire (3P3W) or three-phase four-wire distribution systems for performing the series-parallel power-line conditioning. Thus, even when only a 3P3W power system is available at a plant site, the UPQC is able to carry out power-line compensation for installed loads that require a neutral conductor to operate. Different from the control strategies used in the most of UPQC applications in which the controlled quantities are nonsinusoidal, this UPQC employs a dual compensation strategy such that the controlled quantities are always sinusoidal. Thereby, the series converter is controlled to act as a sinusoidal current source, whereas the parallel converter operates as a sinusoidal voltage source. Thus, because the controlled quantities are sinusoidal, it is possible to reduce the complexity of the algorithms used to calculate the compensation references. Therefore, since the voltage and current controllers are implemented into the synchronous reference frame, their control references are continuous, decreasing the steady-state errors when traditional proportional-integral controllers are employed. Static and dynamic performances, as well as the effectiveness, of the dual UPQC are evaluated by means of experimental results.

Single-phase Power Quality Conditioners with series-parallel filtering capabilities

This paper deals with compensation algorithm schemes used in single-phase Power Quality Conditioners (PQCs), allowing harmonic suppression and sag/swell compensation of the input voltage. In addition, reactive power compensation and harmonic suppression of the input current are also carried out, resulting in an effective power factor correction. Two single-phase PQCs are discussed, namely, the single-phase Unified Power Quality Conditioner (UPQC) and the single-phase line-interactive Uninterruptible Power Supply (UPS) system. To compensate the input current and the output voltage, Synchronous Reference Frame (SRF) based controllers are used. Although the SRF method is itself based on balanced three-phase loads, it can be adapted for use in single-phase loads. Both the UPS and UPQC are composed of two single-phase PWM converters to perform the active power line filter functions. The single-phase PQCs can be implemented by adopting two different operation modes. In the first mode the series converter acts as a sinusoidal current source, while the parallel converter acts as a sinusoidal voltage source. In the second mode, the series converter acts as a non-sinusoidal voltage source, while the parallel converter acts as a non-sinusoidal current source. A comparative analysis of the two operation modes is made, in which the advantages and disadvantages of each are discussed. Validation results are presented to confirm the theoretical development and performance of the single-phase PQCs.

A grid-connected photovoltaic power system with active power injection, reactive power compensation and harmonic filtering

This paper presents a study of a single-phase photovoltaic system, which is connected to the utility grid by means of a full-bridge DC/AC converter. The grid-connected system has active power injection ability from photovoltaic panels. Simultaneously, it performs reactive power compensation and harmonic current suppression generated by nonlinear loads. The photovoltaic system is implemented utilizing an equivalent electric model proposed in the literature. The algorithm adopted to obtain the current reference of the DC/AC converter is based on the synchronous reference frame (SRF), which is implemented along with a maximum power tracking technique. Simulation results are present to validate the proposed control strategy, as well as to verify the system feasibility.

Speed estimation for induction motor using neural networks method

This work presents an alternative approach based on neural network method in order to estimate speed of induction motors, using the measurement of primary variables such as voltage and current. Induction motors are very common in many sectors of the industry and assume an important role in the national energy policy. The nowadays methodologies, which are used in diagnosis, condition monitoring and dimensioning of these motors, are based on measure of the speed variable. However, the direct measure of this variable compromises the system control and starting circuit of an electric machinery, reducing its robustness and increasing the implementation costs. Simulation results and experimental data are presented to validate the proposed approach.

Single-phase ups system with series-parallel filtering capabilities

This paper deals with compensation algorithm schemes used in single-phase Uninterruptiple Power Supply (UPS) system, allowing harmonic suppression and sag/swell compensation of the input voltage. In addition, reactive power compensation and harmonic suppression of the input current are also carried out, resulting in an effective power factor correction. To compensate the input current and the output voltage, Synchronous Reference Frame (SRF) based controllers are used. Although the SRF method is itself based on balanced three-phase loads, it can be adapted for use in single-phase loads. The UPS is composed of two single-phase PWM converters to perform the active power line filter functions. The single-phase UPS can be implemented by adopting two different operation modes. In the first mode the series converter acts as a sinusoidal current source, while the parallel converter acts as a sinusoidal voltage source. In the second mode, the series converter acts as a non-sinusoidal voltage source, while the parallel converter acts as a non-sinusoidal current source. A comparative analysis of the two operation modes is made, in which the advantages and disadvantages of each are discussed. Validation results are presented to confirm the theoretical development and performance of the single-phase UPS system.