From those who want to save some bucks by going solar, to the environmental-friendly folk looking for green energy sources, to the ones who merely want to upgrade their conventional solar power generation system, each client of solar energy has varying needs. As an engineer, you first need to consider the design requirements of each consumer.
There are numerous inverter options for solar Photovoltaic (PV) systems, including micro-inverters, central inverters, DC optimized conventional inverters and string inverters. Recently, the world of solar energy has been abuzz with the success of the micro-inverter integrated solar PVs.
This article provides a macroscopic view of the design of a solar energy systems and discusses the functioning of micro-inverters from an engineering perspective, especially examining its merits to ascertain if there is any truth to all the hype.
Comparing Micro-Inverter & String Inverter Efficiency
The string inverter converts DC (direct current) to AC (alternating current) at the system level, meaning it collectively converts all of the DC produced by the solar panels connected in a series circuit, or string, into AC. It is commonly known that string inverters are significantly less efficient than micro-inverters.
Calculations for string sizing are required to determine the length of the string for the equipment’s voltage window.Additionally, because a series circuit is a closed loop pathway for the current, if one panel is not performing (due to partial shade, debris, etc.), all of the other panels’ performance will be diminished as well.
Conversely, the micro-inverter does the DC to AC conversion right where the power is being generated–at the panel level. Each panel has its own micro-inverter; the panels are wired together in a parallel circuit and connect directly to the electrical panel and grid.
Because the DC to AC electricity conversion takes place at the panel level, the negative impact of partial or complete shading is alleviated because a decrease in one panel’s production doesn’t diminish the capacity of the other panels; conversion at the panel level also removes the need for string sizing, thus the overall efficiency of the installation is enhanced.
An Overview of the Solar Power Generation Systems
Since the energy produced by each PV cell is very low, these solar cells are connected in a series to form a module. In turn, these modules are serially connected to form the solar panels that are collectively installed to build solar arrays.
The series connection implies that the system runs on the principle of the weakest link. Thus, the effective panel current from a solar array will be the minimum of all the panels. The DC generated by the PV cell is converted into AC via solar inverters, sometimes referred to as the conditioning of power, for use at the domestic or industrial level.
Solar Panel Maximum Power Point Tracking (MPPT)
The power output of the solar inverter is nearly directly proportional to the sunlight radiating on the solar panel; however, higher temperatures lower the PV cell’s output voltage because the cell degrades under hot conditions. Under very low sunlight, a conventional PV panel produces up to 16V, but the output current values are reduced since wattage (power) is the product of volts multiplied by the amps. Therefore, the voltage output of the PV panel and the temperature are inversely proportional, while the current and power output are approximately proportional to the sun’s intensity.
Due to this, a grid-connected solar inverter must operate at the maximum power point, whereby the maximum energy could be harvested from the PV panel. The maximum power point control loop, referred to as the MPPT (maximum power point tracker), ensures that the inverter is operating at the optimum power level.
When sunlight shines on a solar panel, photons of light knock electrons free from atoms, generating a flow of electrical current. As stated above, the amount of current generated depends on the intensity of the sun light, and this current can be modeled in a mathematical equation. The simplest model of the solar, or the photovoltaic cell, is a diode connected in parallel to a current source:
IL = Current generated by the light
ID = Reverse saturation current
A = Diode factor
T = Temperature of the solar cell
q = Electronic charge
k = Boltzman’s constant
The above non-linear equation dictates the principal of the Maximum Power Point Tracking (MPPT) whereby the PV panel must be operated at the MPP to extract the maximum power.
Micro-Inverters & String Inverters in Solar System Designs
There are two primary solar electric systems in the market that accomplish different goals for the solar customer: Off-Grid and Grid-Tied. A comparison of these two systems show the variances between the function of the micro-inverter and string inverter.
After the panels, inverters are the most important equipment in the solar electric system. Besides how and where the electricity is converted by each inverter type, the installation (both cost and efficiency), circuit series type, performance monitoring, maintenance, and electricity cost savings are where most opinions draw a line from proponents of each as to which inverter provides the better system.
Off-Grid String Solar Systems
The off-grid or the standalone solar system does not rely on the power generated by the grid. Thus, a battery bank (for power storage) and a backup generator ensure continuous power supply. The string inverter converts the DC power from the solar panels and the batteries into AC. This solar design is more expensive: the battery power storage replacement frequency and cost make it a less popular choice than the alternative, grid-tied micro-inverter solar system.
Grid-Tied Solar Systems
The grid-tied (also known as on-grid or grid-feeding) refers to a utility power grid-connected solar system. The micro-inverter converts the solar power from the panels directly into AC power. Higher efficiency, immediate access to the backup power (fed from the grid), and lower costs make this solar power system design a popular choice. Additionally, a grid-tied system that includes a renewable energy storage system will ensure the benefit of net metering, which will add to the cost savings.
Net metering allows the consumer to feed the extra power back into the grid to earn credits for the clean energy feed-back to the system. To facilitate the flow of electricity into the grid, the output voltage level is kept slightly more than that of the grid. The grid-tied, or the synchronous inverters, ensure that the phase and the frequency of the current being fed back match the phase/frequency of the main grid.
As the comparison of off-grid and grid-tied solar system designs has shown, the harvested energy may be converted from DC to AC at the system level, or the same can be done at the panel level; this is the basic difference between the string inverter and the micro-inverter.
Given the integral role of the solar inverter in the design of any solar power generation system, it is useful to have insight on the design and functioning of an inverter.
The infographic below provides an overview of the solar inverter architecture with a comparison of micro-inverters, string inverters and DC optimizers with string inverters:
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Micro-Inverter Topologies: Single Stage & Two Stage
Micro-inverters may be designed in two ways:
- Single Stage: This topology is named so because the energy conversion is carried out in one stage. The full-bridge rectifier configuration, switching at high frequency, transforms the DC input into a line-frequency sinusoidal output.
- Two Stage: In the first stage of this topology, a DC-to-DC converter with a Power Factor Correction (PFC) boost raises the output from the PV panel to an intermediate high voltage DC bus. Then, in the second stage, a pulse width modulated (PWM) controlled inverter modifies the output to get the desired higher voltage output at line frequency for grid suitability.
The switch mode transformers (SMT), like the PH08XXNL, are preferred owing to their smaller footprint (up to 505 squared-millimeter area), higher efficiency, and lower power-use in load-free conditions.
Similarly, the isolation is DC/DC converter can be realized using the high-power planar DC/DC transformers, like the PH09XXNL. These transformers are well-suited for realizing isolation in the inverter design.
Generally, the isolation type is driven by the regional safety standards and may be categorized as basic and reinforced.
Isolation Considerations in the Solar Inverter Design
Isolators play a vital role in the design of a PV system. Isolators act as a bridge between the low-voltage components like the microcontrollers and the high-voltage circuitry like the power transistors. They are also important from the safety point of view and make systems accessible. The system controller operates at 3-5 volts while the DC link operates at hundreds of volts. In case of any fault, the isolation between the controller voltage and the power domains can avoid controller damage.
The galvanic isolation needs of the solar system vary on the basis of the inverter architecture and the rated power of the system. The DC/AC inverter is comprised of the power transistors like MOSFETs or IGBTs controlled by the isolated gate driver transformers that receive the PWM input from the controller.
The input is isolated from drive levels enabling the inverter to output a sine waveform alternating between the DC rails. Isolation is also necessary for the voltage and current feedback, required for synchronization with the grid and for optimal load transfer.
Solar Inverter Gate Driver System Design
The solar inverters employ the switch-mode power electronics model. These switches need to be driven by a controller to switch at an appropriate frequency. The gate-controlled power switches require an equally quick high isolation gate drive transformer, for example, the PH9400.XXXNL (see additional isolation transformers in the table below). Thus, the gate drivers enhance the system efficiency by cutting back the power dissipation as the power switches are turned on or off.
The driver output varies based on the logic signal from the controller. As the gate driver is turned on or off, a high-current pulse source sinks back to the power switch. While designing a system, the engineers must take the drive current, power topology, switching frequency, working voltage, protection, isolation, and the maximum rated voltage of the system into account before selecting a gate driver.
Based on the buck/boost design, the DC/DC converters can make do with the silicon power MOSFETs for MPPT and DC voltage regulation. For DC/AC inverters, IGBTs and silicon carbide FETs are the preferred choices.
Additional High Isolation Power Transformers
Toroid Platform SMD
EMI Filter Design
On the DC side:
The EMI filter on the DC side is responsible for:
- Limit EMI conduction towards the solar panel
- Decoupling the inverter and the panel
- Increase panel lifespan by controlling high-frequency leakage currents
- Typically employed in the design of transformer-less inverters
- Filtering of the DC/DC boost converter interference suppression
On the AC side:
The EMI filter on the AC side is responsible for:
- Limit EMI conduction towards the utility grid
- Ensure compliance to the EMC requirement standards
Common mode Chokes (CM Chokes), like the PA4415NL and PA4416NL, are usually employed to impede the common mode currents to eliminate the risks associated with the radio frequency interference (RFI) and electromagnetic interference (EMI). Another efficient design solution is the Common and differential mode chokes integrated into a single unit like the PA4040.XXXNL CM/DM choke.
Digital Signal Controller
The microcontroller monitors the critical system parameters and the ADC. The digital control systems are responsible for accurate pulse width modulation of the power-stage voltages and currents for harvesting maximum power from the panel. The controller enhances system efficiency by the faster detection of the line-load changes. Tightly controlled current and voltage feedback loops enable accurate measurement of the precise calculation of the PWM parameters in real time. Other features can also be integrated in the digital controlled system like arc detection or the control of the multiple power stages. The local user interface can also use communication modes like WiFi to ensure ease-of use.
Once the grid-tied solar power system is in place, the power meter must be verified to see if it meets the two-way net metering standards, and if it does not, the meter will need to be updated. The two-way meter can measure energy being fed by your system to the utility grid and vice versa. Improved power metering requires accurate current sensing.
The Rogowski coil based Sidewinder® series of AC current sensors like the PA3202, PA3206, PA3207, and PA3208 series show a range of board and blade mounted products and offer a low-cost, DC immune tamper-resistant solution while achieving ANSI C12.20 Accuracy Class 0.2, from 0.1 to 1,000 Amps. The PA3828NL showcases our clamp-on capability for 1% metering applications.
The Future of the Solar Inverter
The future of the solar inverters will be driven by the factors like cost reduction that account for nearly 10% of the cost of the solar power system. Similarly, reliability is another sought-after feature in the solar inverters, which is dependent on the inverter design and topology as well as the basic semi-conductor elements used in the inverter design.
While a solar panel can easily last more than 25 years, the lifespan of a solar inverter is much shorter. Another design consideration for the next-gen of the solar inverters is the concept of the smart inverter, with features like time-based battery charging and tariff-based grid feeding, which could radically change the search for the improved MPPT algorithms and re-direct it towards the smart grid systems.