Solar panels are an attractive and growing source of renewable energy in commercial and residential applications. Its use connected to the grid by means of a power converter results in a grid-connected photovoltaic system. In order to optimize this system, it is interesting to integrate several functionalities into the power converter, such as active power filtering and power factor correction. Nonlinear loads connected to the grid generate current harmonics, which deteriorates the mains power quality. Active power filters can compensate these current harmonics. A photovoltaic system with added harmonic compensation and power factor correction capabilities is proposed in this paper. A sliding mode controller is employed to control the power converter, implemented on the CompactRIO digital platform from National Instruments Corporation, allowing user friendly operation and easy tuning. The power system consists of two stages, a DC/DC boost converter and a single-phase inverter, and it is able to inject active power into the grid while compensating the current harmonics generated by nonlinear loads at the point of common coupling. The operation, design, simulation, and experimental results for the proposed system are discussed.
Nowadays, with the increasing energy demand and its price increment, along with the scarcity of nonrenewable resources such as oil, natural gas, and coal, researchers are looking for new energy sources to meet the current energy needs. This has led to innovative solutions with desirable characteristics, such as greater efficiency, more power, and less pollution when generating energy [
An important issue is power quality, which is influenced by the growing use of nonlinear loads by residential, commercial, and industrial consumers. This type of load generates high harmonic currents that interact with the grid impedance, causing harmonic voltages which affect all users connected to the same point of common coupling (PCC).
Among the problems caused by the presence of harmonics are distortion of the AC mains voltage within the facilities, high currents flowing through the neutral conductor, overheating of transformers and conductors, poor operation of switches and fuses, erroneous operation of electronic equipment, life reduction in incandescent lamps, resonance risk in fluorescent lamps, and overheating of rotating machines [
The growth of distributed generation (DG) has been favored by the use of renewable energies. The main power conversion stage in a DG system is the inverter, which is very flexible from a control point of view. This flexibility allows exploration of the possibility of injecting active power to the grid from a photovoltaic (PV) system while compensating current harmonics.
Some papers have proposed the use of multifunctional inverters [
In this paper, a PV system with active power filtering functionality and a controller based on sliding mode is analyzed, implemented, and tested. The authors have previously presented the simulation results of this proposal in [
Block diagram of the power stage with a sliding mode controller.
The paper is organized as follows: the power stage, the controller, and implementation are described, the simulation and experimental results are then illustrated, and finally, the conclusions are given.
The power stage consists of a DC/DC boost converter, an energy storage capacitor, a full-bridge single-phase inverter, and an output inductor connected to the grid (Figure
The DC/DC boost converter demands the available power from the PV panels by means of a maximum power point tracking algorithm (MPPT). The inverter injects active power generated by the PV panels to the grid and compensates the current harmonic content at the PCC. The entire control tasks for both converters have been implemented in the NI platform.
In order to simplify the analysis of the power stage, the PV panels and the DC/DC boost converter can be considered to behave as a current source, since changes in the energy provided by the PV panels take longer than the AC mains period.
The converter is operated to produce a unipolar output voltage. Each branch of the inverter is switched independently, with S3 and S4 activated every half-period of the AC mains and S1 and S2 activated following a pulse width modulation (PWM) pattern, in order to control the injected current to the grid.
Considering the above simplification, the model of the system is
The switches of the same branch are complementary; hence,
For a model of the PV panel, see that proposed in [
The capacitor is calculated considering the stored energy and the low-frequency component of the capacitor voltage ripple. The following equation is first used:
The low frequency of the capacitor voltage ripple is twice the grid frequency. Considering that the stored energy handled by the system in each half of the AC mains period is 10% of the nominal output energy and that the capacitor voltages due to the ripple are
If
The output inductor can be calculated with
Considering a worst-case design for the duty cycle, the zero crossing the grid voltage, a duty cycle of
The design of the DC/DC boost converter can be calculated as a classical one, so it is not discussed in this paper.
There are several MPPT algorithms [
For the inverter, a sliding mode control has been considered. It provides advantages like stability at large variations in load and voltage, robustness, good dynamic response, and easy implementation [
The sliding mode controller is based on the theory of variable structures [
The controller design begins with the definition of a sliding surface, toward which the system must be attracted and must remain in it (Figure
Evolution of the system with a sliding mode controller.
Since the converter is intended to operate as an active power filter in addition to the power injection, the grid current
The proposed sliding surface is
and the control laws considered are
The existence condition ensures that the equation system will be maintained in the sliding surface. It implies satisfying the following inequality [
By deriving (
Considering Figure
With the derivative of (
Considering
If
If
If
If
Then,
To satisfy (
The stability analysis is beyond the purpose of this paper, but previous analysis allows assurance of the proper operation of the system in the sliding surface. To determine the stability, an equivalent control method can be employed [
For the proper operation of the system, other important control blocks must be taken into account, that is, the MPPT algorithm, the grid synchronization algorithm, and the voltage reference generator to control the capacitor voltage.
The constant
The SOGI-FLL [
Finally, other blocks, like blanking time and protections, are considered in the final implementation.
As mentioned before, the platform considered for the control implementation is the CompactRIO from NI, together with the LabVIEW visual programming software. There have been programmed system protections and controllers for the entire system.
Figure
Implemented LabVIEW control panel.
In Figure
LabVIEW control blocks implemented on the CompactRIO platform. (a) MPPT algorithm. (b) Synchronization controller.
As part of the protections, overvoltage and overcurrent detection is included, but the blanking time of the inverter branch is also included in the program.
The system shown in Figure
Simulation and experimental parameters.
Photovoltaic array | |
Maximum power | 200 W |
Open-circuit voltage | 92 V |
Short-circuit current | 4.76 A |
Nominal voltage (MPP) | 67 V |
Nominal current (MPP) | 3.6 A |
Topology | |
DC/DC boost converter inductor |
3.5 mH |
DC link voltage | 250 V |
Energy storage capacitor |
330 |
Output inductor |
4 mH |
DC/DC boost converter switching frequency |
39 kHz |
Inverter switching frequency |
90 kHz |
PI controller | |
Gain | 0.05 |
Time constant | 0.05 |
Nonlinear load | |
Power factor | 68% |
Apparent power | 105 VA |
Grid | |
Frequency | 60 Hz |
Grid voltage | 120 V rms |
The nonlinear load considered is a traditional single-phase full-bridge diode rectifier plus a capacitive filter; the total harmonic distortion of this current is above 100%.
The steady-state operation of the proposed system is illustrated in Figure
System injecting and alleviating the harmonic distortion. From (a) to (c): load current (1 A/div), grid current (1 A/div), and inverter current (2 A/div).
The operation of the system without nonlinear load is shown in Figure
Topology injecting only active power. From (a) to (b): grid voltage (100 V/div) and grid current (1 A/div).
Additionally, the proposed system has the advantage of operating as a conventional active filter, as shown in Figure
Operation as an active power filter. From (a) to (c): load current (2 A/div), grid current (1 A/div), and inverter current (1 A/div).
The steady-state operation of the proposed system is shown in Figure
Experimental results for the system injecting and alleviating the harmonic distortion. From top to bottom: load current (5 A/div), inverter current (5 A/div), and grid current (5 A/div).
The operation of the system without load is shown in Figure
Experimental results when the topology only injects the active power. From top to bottom: grid current (5 A/div) and grid voltage (250 V/div).
Additionally, the proposed system has the advantage of operating as a conventional active filter, as shown in Figure
Experimental results when the converter operates only as an active filter. From top to bottom: load current (5 A/div), inverter current (5 A/div), and grid current (5 A/div).
Grid-connected PV systems usually inject active power energy to the grid. They are mainly used to avoid the dependence of fossil fuels. Since the power stage used in grid-connected applications is an inverter, the functionality of this stage may be increased, acting as an active power filter and a power factor corrector.
In this paper, a multifunctional grid-connected PV system is analyzed and tested. The power stage consists of a DC/DC boost converter and a single-phase inverter. The converter’s protections and controllers are implemented in the CompactRIO digital platform from NI, which allows user friendly and easy tuning of the system. A sliding mode controller is employed that permits good operation in different modes: when the system delivers energy and compensates harmonics, when it only injects energy, or when it only acts as an active power filter.
The operation, design, and implementation of the system are presented. The simulation and experimental results confirm the feasibility of the proposal.
The authors declare that there is no conflict of interest regarding the publication of this paper.
This work has been sponsored by National Instruments Corporation through the 2016 NI Academic Research grant program and also was supported in part by the Comunidad de Madrid government (SINFOTON-CM) (S2013/MIT-2790).