In this paper power electronics used in PV power generation systems have been reviewed and modelled. PV systems need converters for maximum power point tracking, power conditioning, voltage step-up/down as necessary, and for storage charge-controlling. Inverters are needed for AC loads and for utility grid interfacing. The four basic DC-DC converters commonly used with PV systems have been reviewed and modelled. Different DC-AC inverter types and operational architectures have also been reviewed with the two-stage DC-AC inverter, with the point of common coupling (PCC) at the inverter input, suggested as the most cost-effective and efficient architecture for PV-based communal grids. This is because only one inverter is used for the entire system as opposed to an inverter for every module string, resulting in higher efficiencies, low cost, and low harmonic distortions when compared to systems with PCC at AC terminal. The aim of power conversion/inversion is to extract maximum power possible from the PV system and where necessary, to invert it at close to 100% as possible. Highlight: 1) DC-DC converters are necessary for power conditioning in PV systems; 2) DC-AC inverters are necessary for AC loads and for utility grid interfacing; 3) DC-AC inverters are also used to control the PV systems when grid connected; 4) Best inverter configuration cost-effectively and efficiently allows easy system modifications.
A PV power system consists of PV power generators supplying DC or AC power to a system of loads through a system of power electronics as schematically shown in
sure uninterrupted supply of power at all times. The power electronics comprise of DC-DC converters needed for power conditioning and maximum power point tracking (MPPT), DC-AC inverters in cases of AC loads or utility-grid interfacing, filter to remove harmonic distortions from the inverters, and charge controllers for the energy storage systems. The latter is usually implemented using bi-directional DC-DC Buck-Boost converters.
The main functions of DC-DC converters are to operate the PV power generator at maximum power point under all conditions and to efficiently step up/down the voltage from the PV source to a stable DC voltage of desired magnitude. They are also used for controlling battery charging and discharging. The four basic DC-DC converters used with PV systems are Buck, Boost, Buck-Boost, and Cuk converters.
A circuit diagram of a Buck converter with switching period T and duty cycle D is shown in
In continuous conduction mode, applying Kirchhoff’s voltage law (KVL) to the loop containing the inductor and Kirchhoff’s current law (KCL) on the node with the capacitor branch connected to it, we can model the dynamics of the inductor current
When switch Q is on
When switch Q is off
In continuous conduction mode, we can use KVL and KCL to model the dynamics of the inductor current
When switch Q is on
When switch Q is off
As with the Buck and Boost converters discussed above, in continuous conduction mode, we can use KVL and KCL to model the dynamics of the inductor current
When switch Q is on
When switch Q is off
In continuous conduction mode, we can use KVL and KCL to model the dynamics of the inductor current
When switch Q is on
When switch Q is off
DC-AC Inverters are used in PV systems to convert DC generated voltage into AC voltage for AC loads or for utility grid interfacing and to control the active and reactive power [
These systems use high frequency DC-DC converters for MPPT. Switching is done us the full bridge inverter.
Neutral conductor of AC side connected to the inverter is grounded. Avoiding the use of transformers increases inverter efficiencies by 1% - 3% [
These inverters use transformers to provide galvanic isolation of the PV system from the utility grid. The transformers also step up the voltages from the PV systems to grid levels. However, use of transformers increases system costs and also lowers the efficiencies of the inverters. Moreover, the transformers increase the weight of the inverters. Transformer-based inverters use either high frequency transformers or line frequency transformers. A high frequency transformer based topology is shown in
A line frequency transformer-based topology with self-commutated full bridge is shown in
Inverters can further be classified as single-phase or three-phase. For residential applications, single-phase inverters are the most widely used as they are rated for up to 5 kW. For large power interfacing, three-phase inverters are used. Depending on system configuration, inverters can be categorized as central inverters, string inverters, multi-string inverters, and AC module inverters.
In these configurations, a PV array consisting of strings of PV modules connected in parallel, with each string comprising many modules connected in series, are connected to a single inverter as shown in
These are reduced power versions of the centralized inverter configuration. They have separate MPPT for each PV module string as shown in
Connecting a DC-DC converter with MPPT capabilities between each module string and a DC-AC inverter greatly increases a system’s performance and modularity [
In
In this arrangement, each module has its own inverter and MPPT control. Several module inverters are used to compensate the high power level as this inverter configuration has lower power handling capability. This configuration has several disadvantages in addition to low power handling capability, including low efficiency due to use of too many inverters, high cost, low lifetime, and high harmonic distortions [
Factors to consider when deciding on inverters:
Efficiency: Any reduction in an inverter efficiency immediately reflects on a PV systems overall efficiency. European efficiency of PV inverters is an accepted standard for comparing efficiencies of different inverters and it is expressed as [
where
Anti-Islanding Protection: Inverters should automatically disconnect from the utility grid in case of a fault with the latter. This protects systems upstream from frequency and phase related damages due to power injection from the inverter.
Harmonic Distortions: Harmonics from inverters should be minimized to reduce distortions in the utility grid voltage and current. This can be achieved by connecting low-pass filters between the inverters and the grid.
The inverter should be able to adjust with the dynamics of MPPT operation using changing environmental (irradiance) conditions for maximum power harnessing from the PV system.
Modifiability and Cost: Inverter configurations should take into account the modular nature of PV systems to minimize associated costs.
Equations (1) and (2) are implemented in Simulink as shown in
A pulse width modulation (PWM) signal is used to control the switch Q to help with subsequent simulation analyses and feedback controller verification. Trigger pulses derived using a repeating sequence generator and duty cycle block are used by the converter to verify that it is working in open loop configuration; a function block compares the duty cycle while saw tooth from the trigger pulses are connected as an input to the switch control. Inputs for the masked subsystem are therefore duty ratio and input voltage, while the outputs are chosen
to be inductor current, capacitor voltage, and output voltage [
Equations (3) and (4) are implemented in Simulink as shown in
Equations (5) and (6) are implemented in Simulink as shown in
Equations (7) and (8) are implemented in Simulink as shown in
The aim is to achieve a power-conversion-efficiency close to 100%, i.e.
The purpose of the capacitor is to provide energy storage to balance the difference between
The control variable for the DC-AC inverter is the RMS current reference
The AC grid RMS is set at 120 V while AC line frequency is set at 60 Hz. In this simulation, the PV array voltage output
The
Similarly,
As shown in
Average power balance can be implemented through an automatic feedback control as shown in
The voltage
negative (
Magnitude of the resulting ripple voltage
This energy must match the change in energy stored on the capacitor;
Solving for the ripple voltage we get
And therefore
In this article, DC-DC converters and DC-AC inverters for PV-based communal grids have been reviewed, modeled and simulated in MATLAB/Simulink. The main functions of the power electronics are to ensure maximum extraction of generated power and to efficiently deliver this power to a given set of loads, with minimum possible conversion/inversion losses. DC-DC converters ensure the PV systems to operate at maximum power points under all conditions while the inverters invert generated DC power into AC for utility grid interfacing or for AC loads. Transformerless inverters are more efficient, smaller in size and weight, cheaper, and less complex than transformer-based inverters and are thus recommended to small PV-based communal grids. Avoiding transformers however results in a galvanic connection of the grid and PV array. A two-stage DC-AC inverter with point of common coupling (PCC) at the DC-AC input has been identified as the most cost-effective operational configuration for small communal grids as only one inverter is used for the entire system as opposed to an inverter for every module string. This results in higher efficiencies, low cost, and low harmonic distortions when compared to systems with PCC at AC terminal.
This research is funded by Leeds International Research Scholarship.
NicholasOpiyo, (2016) Power Electronics for PV-Based Communal Grids. Smart Grid and Renewable Energy,07,67-82. doi: 10.4236/sgre.2016.72004