In this paper, a simple Digital Signal Processor (DSP) based Maximum Power Pointer Tracking (MPPT) control and Inverter Control is presented for solar energy applications, especially photovoltaic and wind energy systems. The proposed MPPT controlled boost converter is able to reduce the inrush current and the overshoot of the output voltage of the system. Details of the proposed Maltab-Simulink based MPPT and Inverter Control are shown and implemented using a DSP. The proposed system is analyzed and simulated for verification. To validate the system, a 100 W prototype test-bed is built and tested. The results show that the proposed system can be applicable for solar energy applications.
Solar and wind energy systems have become increasingly popular as the desire for clean energy has grown. Typically, these energy systems consist of buck or boost converter and DC/AC inverter. For instance, a solar power converter incorporates a photovoltaic panel, buck or boost converter (DC/DC converter) depending on the input voltage level, and inverter system to create 60 Hz, or 50 Hz AC grid power [
In order to achieve the maximum power point (MPP) of photovoltaics, MPPT algorithms are normally used. One of MPPT algorithms, Incremental Conductance (INC) algorithm mainly relies on the tangential value of the photovoltaic (PV) operation point to predict the right direction of MPP. Fixed step-size INC algorithm [
Based on objectives of photovoltaic systems, photovoltaic systems can be generally classified into stand-alone and grid-connected photovoltaic systems. Stand-alone photovoltaic systems are designed to supply local electric load, and generally consist of energy storage devices for meeting excessive electricity demands. Grid-connected photovoltaic systems are designed to deliver photovoltaic power to electric grids. In this section, a brief introduction of stand-alone photovoltaic systems is described because our research is focused on the stand-alone photovoltaic system.
The fundamental topology of a stand-alone photovoltaic system is shown in
A stand-alone system consists of the following components:
− Solar Cells/Solar Panels/Solar Arrays
− Maximum Power Point Tracking Controller
− Voltage regulator of photo voltaics
− PWM Generator
− DC-DC Converter
− DC Electric Load
− DC-AC Inverter (Optional).
Prior to addressing the MPPT algorithm, the overall hardware set up is described in this section. In the hardware setup, a conventional boost power converter is built for solar applications as seen in
large spike in current as the boost converter turns on. This is because of the large value of the output capacitance. If the boost converter was turned on slowly by using software implemented soft start that slowly incremented the duty cycle on turn on, these spikes can be avoided.
Maximum Power Point Tracking (MPPT) controllers are popular in both stand-alone
and grid-connected photovoltaic systems. A MPPT controller can be designed as a physical analog circuit as an embedded system. The main objective of a MPPT controller is to extract potential maximum power from photovoltaic cells by continuously perturbing the operation point of the photovoltaic cells. The operation point of photovoltaics consists of two parameters, the photovoltaic voltage and photovoltaic power. It can be treated as a point on a P-V curve. The operation point will reach the maximum power point if the MPPT controller rationally perturbs the photovoltaic voltage. At the end of every control interval, a new photovoltaic voltage reference is calculated by the MPPT algorithm and sent to the photovoltaic voltage regulator. Recently, even though numerous MPPT algorithms have been researched [
A DC-DC converter can step-up/step-down the voltage level of its input DC power. In a photovoltaic system, the input photovoltaic voltage level may not exactly meet the requirement. Therefore, the first objective of a photovoltaic DC-DC Converter is to change the voltage level of input photovoltaic power. The second objective is to realize the voltage regulation of photovoltaics, as associated with a voltage or current control.
Several MPPT algorithm research assumed that the electric load of photovoltaic MPPT systems can be only resistive. Such assumption may be impractical. The transient response of a power converter may be undesirable and unpredictable if electric load is only resistive. Thus, the output voltage regulation of the converter is to be considered. To avoid the above issue related to the converter’s output voltage regulation, the appropriate electric load for a stand-alone photovoltaic system should consist of depth-recycled batteries and ultra-capacitors. These can absorb the increasing photovoltaic power, and stabilize the voltage of the output terminal at a relative fixed level if the load’s capacitance is sufficiently large so that MPPT can effectively work in the system. However, in our research, the output voltage regulation depending on the load parameter is out of scope of the paper because to design the output voltage regulation of the system, a detailed small signal model based transfer function is to be derived in the case of adopting a linear voltage regulator. By the way, aforementioned, a MPPT algorithm based on Perturb and Observe is used for the system. Perturb and Observe (P & O) introduces an initial perturbation to the boost converter voltage by changing the gate signal duty cycle and then observations are made using sensing circuitry to change the gate signal duty cycle to move closer to the Maximum Power Point (MPP). Perturb and Observe uses voltage and current measurements to calculate change in power over a change in time (∆P) and change in the duty cycle (∆D) of the signal sent to the gate of the switch in the boost converter.
Given that ∆P and ∆D can be either positive or negative respectively, there are four cases to determine whether the duty cycle of the gate signal should be increased or decreased as shown in
Case | ∆P | ∆D | Next Duty Cycle |
---|---|---|---|
1 | + | + | + |
2 | + | - | - |
3 | - | + | - |
4 | - | - | + |
rithm. A Simulink DSP Program Model for the MPPT algorithm in
is used to determine if the duty cycle will increase or decrease. A sub-process block was made in Simulink to increase, or decrease the duty by 1%. If the input to the block is less than zero the output is −1, if the input to the block is greater than zero the output is +1, and if the input to the block is equal to zero the output is +0.01 for an offset. As shown in
This section discusses the performance of the conventional P&O algorithm. The conventional P & O algorithm generally exhibits a trade-off between the tracking velocity and MPPT efficiency. This nature can be seen by simulating behaviors of the conventional P & O algorithm with two different perturbation intensities, 0.1 V and 2.0 V. In this simulation, the perturbation frequency is set to 1 Hz. In the following analysis, the term “perturbation intensity” is denoted by “p-i”.
Many photovoltaic systems are designed to supply to AC loads, like motors or pumps.
In such case, a DC-AC Inverter is added into the system topology. A DC-AC Inverter can be directly cascaded to a DC-DC converter, or can be connected to the medium energy storage devices, such as ultra-capacitors and batteries. The fundamental components of a grid-connected photovoltaic system involve photovoltaic arrays and a DC-AC inverter. The basic topology is shown in
As seen in
An important ratio called the modulation index (mi) can be described as:
Also, the modulation ratio (mf) can be defined as:
An important feature of SPWM is that it allows for the control of the output frequency and the control of the out-put voltage amplitude. The output frequency and output amplitude are governed by the following equations:
These Equations(1) - (4) mean that the output voltage frequency is the same as the control signal frequency, and the peak value of the fundamental output component can be controlled by varying the modulation index mi. To create the SPWM signals a TMS320F2812 DSP based eZdsp [
In
For example: for a modulation index of 0.7, set Bias to 50 and Amplitude to 35. An alternative way to set amplitude of the sine wave (hence setting the modulation index) would be set to Amplitude and Bias to 50 and use a gain block at the output of the sine block. In this case the gain would directly correspond to the modulation index.
The frequency of the reference sinusoid can be set by entering the desired frequency (in radians/sec) into the Frequency parameter. The Sample time field dictates the frequency of the carrier, fcarrier.
Parameter | Value |
---|---|
Vd | 10 V |
fcarrier | 60 Hz |
Lfilter | 1 mH |
Cfilter | 100 uF |
Rload | 500 Ω |
mi | 0.8 |
mf | 83 |
half of the plot. The lower half of the plot is the frequency spectrum of the output. The output signal is 15 Vpp. The large DC component at the fundamental frequency, 60 Hz in the FFT analysis is shown because the output is operating at 60 Hz. The almost complete absence of spikes at other frequencies indicates there is little harmonic distortion at the output voltage.
In this paper, a simple DSP implementation for a soft start based Perturb & Observation based MPPT algorithm and inverter control has been presented for solar energy applications. To validate the proposed MPPT algorithm and inverter control, a 100 W test-bed is built and tested. The proposed MPPT algorithm and inverter control are simply implemented in Matlab/Simulink based DSP program for a solar application. The results show that it could successfully reduce the inrush current and the overshoot of the output voltage in the system by changing the duty cycle gradually under consideration of the MPPT algorithm, and its inverter control is also successfully implemented in terms of reducing harmonic distortion.
Na, W., Carley, T., Ketcham, L., Zimmer, B. and Chen, P.Y. (2016) Simple DSP Implementation of Maxi- mum Power Pointer Tracking and Inverter Control for Solar Energy Applications. Journal of Power and Energy Engineering, 4, 61- 76. http://dx.doi.org/10.4236/jpee.2016.49006