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Photovoltaic grid-connected solar micro inverter

1.Micro-inverter advantages and its design elements

1) Advantages of micro-inverter

A newer means of optimizing the efficiency and reliability of solar systems is to connect microinverters (Micro Inverter) to each solar module. Equipping each solar cell with a separate microinverter allows the system to adapt to changing load and weather conditions, thus providing optimal conversion efficiency for both the individual solar cells and the system as a whole. PV systems with microinverter architecture simplify wiring, which translates into lower installation costs and increased efficiency of the solar power system, resulting in a shorter time to pay back the initial investment at .

Common grid-connected PV power system architectures include centralized, string, multistrand and AC modular schemes. In centralized, string and multistrand systems, there are series and parallel connections of PV modules, so the system targets the entire string/parallel PV arrays when tracking the maximum power point, and cannot take into account each PV array in the system. This results in low utilization of individual PV arrays, poor system resistance to local shadowing, and lack of flexibility in system expansion.

PV grid-connected micro-inverters (referred to as micro-inverters) are connected to individual PV modules and can convert the DC power output from the PV modules directly into AC power and transmit it to the grid, with the following advantages.

(1) Ensure that each module operates at the maximum power point and has a strong resistance to local shading.

(2) The integration of inverter and PV module can realize modular design, plug-and-play and hot-swapping, which makes system expansion easier and more convenient.

(3) The grid-connected inverter basically does not occupy the installation space independently, and the distributed installation is easy to configure and can make full use of space, which is suitable for applications with different installation directions and angles.

(4) The system has high redundancy and reliability, and the failure of a single module will not affect the whole system.

The concept of microinverter has been around for a long time, but initially it did not attract much attention. In recent years, with the development of solar power technology and technological advancement, it has made microinverters very attractive. Enphase in Petaluma, California, USA, started commercial mass production of microinverters in 2008 and achieved good sales results, which made microinverters gain wider recognition and attracted many companies to join the research and development of microinverters. SMA Solar Technology also entered the microinverter market in 2009.

Many domestic PV grid-connected inverter manufacturers in China are mainly engaged in the development of high-power centralized grid-connected inverter products, and with the increasingly hot micro-inverter market at home and abroad, many manufacturers have also started to develop micro-inverter products. Involar New Energy Technology Company is the earliest company engaged in micro-inverter research in China. It started the development of micro-inverter technology from the beginning of 2008, and has mastered the core technology of micro-inverter completely independently after nearly two years’ efforts, and successfully released its first generation product NAC250 in May 2010, which is now on the market.


The differences between microinverters and conventional inverters are as follows.

(1) The input voltage of the inverter is low and the output voltage is high. Since the output voltage range of a single PV module is generally 20 to 50V, and the peak voltage of the grid is about 311V (220V AC) or 156V (110VAC), the peak output voltage of the microinverter is much higher than the input voltage, which requires the microinverter to adopt an inverter topology with a step-up conversion function; while the traditional centralized inverter is generally a step-down converter, which The peak voltage on the AC side of the inverter output is lower than the input DC side voltage.

(2) Small power. The power of a single photovoltaic cell module is generally 100-300W, micro-inverter can be directly matched with a single photovoltaic cell module, and its power level is 100-300W, while the traditional centralized inverter power through the combination of multiple photovoltaic cell modules in series and parallel to produce a high enough power, and its power level is generally above 1kW.

2) Micro-inverter design elements

The following elements need to be considered in designing microinverters.
(1) High conversion efficiency. The conversion efficiency of the grid-connected inverter directly affects the efficiency of the whole power generation system. In order to ensure the high power generation efficiency of the whole system, the grid-connected micro-inverter is required to have a high conversion efficiency.
(2) High reliability. Since the micro-inverter is directly integrated with the PV module, it is usually placed outdoors together with the PV module, and its working environment is harsh, which requires the micro-inverter to have high reliability.
(3) Long life span. The life span of PV modules is generally 20 years, and the life span of micro-inverter should be comparable to that of PV modules.
(4) Small size. The smaller the size, the easier it is to integrate the microinverter with the PV module.
(5) Low cost. Low cost is the inevitable trend of product development and the demand of micro-inverter marketization.

2. Micro-inverter topology

The special application requirements of micro-inverter determine that it cannot adopt the traditional buck inverter topology, such as full-bridge and half-bridge topologies, but should choose a converter topology that can realize the buck conversion function at the same time, and should realize electrical isolation in addition to the buck conversion function; in addition, the requirements of high efficiency and small size determine that it cannot adopt an industrial frequency transformer to realize electrical isolation, but needs to using high frequency transformers. The optional topologies include: high-frequency chain inverter, two-stage inverter combining boost converter and traditional inverter, Flyback inverter based on isolated boost converter, etc. Among them, Flyback converter topology is a better topology with simple structure, simple control and high reliability, and the micro-inverter products developed by Invensys and other companies are based on Flyback converter. The micro-inverter products developed by Inverter and other companies are based on Flyback converter.

In order to reduce the size of microinverter, it is required to increase the switching frequency of the inverter, and the increase of the switching frequency will inevitably lead to the increase of the switching loss and the decrease of the conversion efficiency, so there is a contradiction between the small size and high efficiency.

In many applications, the use of microinverter topologies can significantly improve overall system efficiency. At the PV module level, efficiency improvements of up to 30% are expected. However, because applications vary so much, the “average” percentage of system-level improvement is not very meaningful. When evaluating the value of microinverters in a specific application, several aspects of topology should be considered.

(1) In small-scale applications, each PV module is likely to face essentially the same conditions of light, temperature and shading. Therefore, microinverters have a limited role in improving efficiency.

(2) In order to operate each PV module at different voltages for maximum energy efficiency, DC/DC converters are required to unify the output voltage of each PV module with the operating voltage of the energy storage battery. To minimize the manufacturing cost, the DC/DC converter and inverter can be designed as a module, or the DC/AC converter for local power circuit or connection to the distribution network can be integrated into the module.

(3) The solar PV modules must communicate with each other, which adds wires and design complexity. This is another point of contention for including inverters, DC/DC converters and solar modules in the module.

(4) The MCU of each inverter must have sufficient capacity to run multiple MPPT algorithms to adapt to different operating environments.

(5) Using multiple MCUs will increase the overall system cost.

Researching and developing simple and effective soft-switching technology and combining soft-switching technology with specific microinverter topologies is one of the key issues to be solved in microinverter development. Our introduction of resonant soft-switching technology has effectively improved the conversion efficiency of microinverters, and the highest efficiency of our newly released microinverter products can reach over 95%, and the CEC efficiency can reach over 94.5%.

The life time of PV modules is generally 20-25 years, and the life time of micro-inverters must be close to that of PV modules, while electrolytic capacitors are the bottleneck of the life time of power converters. To make micro-inverters reach the life time of PV modules, the use of electrolytic capacitors must be reduced or avoided, so the research and development of electrolytic capacitor-free power conversion technology is another topic that needs to be addressed in the development of micro-inverters.

3. Micro-inverter control technology

1) Grid-connected current control technology
In traditional centralized grid-connected inverters, current closed-loop control techniques are generally used to ensure that the grid-connected current is at the same frequency and phase as the grid voltage to achieve high-quality grid-connected current control, such as PI control, repetitive control, predictive current control, hysteresis loop control, single-cycle control, and proportional resonance control methods. All of the above methods require the use of Hall component sampling, which in turn realizes the control of grid-connected current. Due to the small power characteristics of microinverters, in order to reduce the cost per unit of power generated and considering the size requirements, the development of new highly reliable and low-cost small power grid-connected current control techniques is another critical issue to be addressed in the development of microinverters.

2) High efficiency, low cost maximum power point tracking (MPPT) technology
The efficiency of photovoltaic power generation system is the product of the photovoltaic conversion efficiency of the supply cell, MPPT efficiency and inverter efficiency, among which the high efficiency MPPT technology is very important for the efficiency improvement and cost reduction of photovoltaic power generation system.

Common MPPT algorithms include open-circuit voltage method, short-circuit current method, hill-climbing method, disturbance observation method, incremental conductance method, and intelligent tracking algorithm based on fuzzy and neural network theory, etc. In the above MPPT methods, it is generally necessary to detect both the PV cell output-side voltage and current, and then calculate the grid-connected power. Since the input voltage of the PV side of the microinverter is low, the current of the PV cell side is larger, and if a resistor is used to detect the input side current, it will have a large impact on the overall efficiency of the microinverter. Therefore, a new high-efficiency MPPT technology without current detection needs to be developed to meet the special requirements of microinverters. The MPPT is effective and the tracking accuracy can reach over 99.9%.

3) Silo detection technology
Islanding detection is a necessary function for grid-connected PV power generation systems and is an important guarantee for the safety of personnel and equipment. For the special application requirements of microinverters, the development of simple and effective islanding detection technology with zero detection blind area and without affecting the quality of grid-connected current is an important issue to be addressed in the development of microinverters.

4) Information and Communication Technology
When multiple micro-inverters form a distributed power generation system, low-cost, efficient and highly reliable information communication technologies are needed to ensure this, as the system needs to collect information from each micro-inverter in real time for effective monitoring and management. The available communication technologies include PLC, ZigBee, Z-Wave, 6LoWPA, PoE, GPRS and GSM technologies. Compared to using one inverter for the whole system, having a micro-inverter for each solar module in the system will again increase the conversion efficiency of the whole system. The main benefit of the micro-inverter topology is that energy conversion can still take place even if one of the inverters fails. Other benefits of using microinverters include the ability to adjust the conversion parameters of each solar cell using high-resolution PWM. Since cloud cover, shadows and back shade change the output of each PV module, having a unique microinverter for each PV module allows the system to adapt to changing load conditions, which provides the best conversion efficiency for each PV module and the system as a whole.

The microinverter architecture requires a dedicated MCU for each PV module to manage the energy conversion. However, these additional MCUs can also be used to improve system and PV module monitoring. Large solar power systems, for example, benefit from communication between PV modules to help maintain load balancing and allow system administrators to plan ahead for how much energy will be available and what to do with it. To take full advantage of monitoring systems, MCUs must integrate on-chip communication peripherals (CAN, SPI, UART, etc.) to simplify interfacing with other microinverters within the solar array. Whenever a change in architecture is considered there is a concern about its cost, and having a controller for each PV module in order to meet the system’s price targets means that the controller must be cost competitive, have a small form factor, and still be able to handle all the control, communication, and computational tasks simultaneously.

The integration of appropriate control peripherals on-chip and high analog integration are two essential elements to ensure a low-cost system, and the efficiency and high performance of the algorithms developed in each segment are necessary to perform optimized conversion, system monitoring, and energy storage. The use of MCUs that can handle most of the requirements of the entire system, including AC/DC conversion, DC/DC conversion, and communication between PV modules, reduces the cost increase caused by the use of multiple MCUs.

Carefully weighing these high-level requirements is the best way to determine what functionality is needed for the MCU. For example, if load balancing control is required when paralleling PV modules, the MCU selected must be able to detect the load current as well as control the MOSFET output voltage via on/off, which requires a high-speed on-chip ADC to sample the voltage and current.

There is no “one-size-fits-all” model for microinverter design, which means that innovative technologies must be used in the design, especially in terms of communication between PV modules and between systems. The most suitable MCU should support a variety of protocols, including some not normally thought of, such as powerline communication and controller area network (CAN). Powerline communication, in particular, can reduce system costs because it does not require a dedicated communication line, but it requires an MCU with built-in high-performance PWM, high-speed ADC and high-performance CPU.

An unexpected but valuable feature for MCUs designed for PV microinverter applications is the dual on-chip oscillators, which can be used for clock fault detection to improve reliability, and the ability to run two system clocks simultaneously, which also helps reduce problems during solar module installation. With so much innovation coalescing in microinverter design, perhaps the most important feature for MCUs is their software programming capability, which allows for the highest level of flexibility in power circuit design and control.

The C2000 microcontrollers are equipped with an advanced digital processing core for efficient algorithmic processing and on-chip peripherals for energy conversion control, and are already widely used in traditional PV inverter topologies. The new Piccolo family of C2000 series microcontrollers is an economical model with the smallest package of 38 pins, but with a more advanced architecture and enhanced peripherals that allow the benefits of 32-bit real-time control to be applied to the design of low-cost microinverters. In addition, each model in the Piccolo MCU family integrates two on-chip 10MHz oscillators for clock comparison, as well as on-chip VREG with power-on reset and power-down protection, multiple high-resolution 150ps PWMs, a 12-bit 4.6 megasample/sec ADC and communication protocol interfaces such as I2C (PMBus), CAN, SPI and UART.

Performance is a key feature of microinverters, and while the Piccolo family of devices is smaller and less expensive than other C2000MCU products, it offers improved functionality with a programmable floating point control law gas pedal (CLA) that offloads the processing of complex high-speed control algorithms to the CPU, thereby eliminating the need for the CPU to handle I/O and feedback loops, resulting in a 5x performance improvement in closed-loop applications 5X performance in closed-loop applications.

In order to achieve maximum power point output tracking (MPPT), the microcontroller has to run the MPPT algorithm to regulate the orientation, output DC voltage and current of the PV module to obtain peak power output, it is necessary to use microcontrollers and sensors to track the azimuth and altitude angles of the PV module. At present, in the adaptive PV module azimuth, height angle and radiation intensity tracking system, the components include radiation intensity sensor, tracking sensor, automatic control chip, stepper motor and subdivision driver, mechanical drive mechanism and energy collection platform and several other parts. As the solar integrated power generation system, the output voltage/current of the PV cell array, tracking light intensity, ambient light intensity, battery charging current/voltage, inverter output AC current, AC voltage, ambient temperature, battery temperature, PV cell array temperature, PV module azimuth and altitude angle are also detected. Therefore, the data acquisition capability, A/D conversion and processing of the microcontroller are highly demanded.

In large-scale solar grid-connected power plants, due to the large number of photovoltaic panels, for this reason TI has proposed the concept of “micro-inverter”. It is able to scan the peak power point of each individual solar module in a wide range to avoid using local peaks as MPP points, and at the same time, it is able to improve the efficiency of maximum power point output tracking.

4. Micro-inverter solutions

In the building integrated photovoltaic (BIPV) system, the installation of photovoltaic modules first involves the installation angle and installation direction of photovoltaic modules, the installation angle is the tilt angle of photovoltaic modules, the choice of tilt angle is directly related to the power generation efficiency of photovoltaic modules. The same photovoltaic cell module, choose different installation angle received radiation is not the same, due to the problem of each wall orientation, different installation position of the photovoltaic cell module its installation angle and direction can not be exactly the same, which determines its power generation efficiency, power generation of instantaneous power can not guarantee completely consistent.

Another key issue to be solved in BIPV systems is shadow shading, which is caused by a variety of reasons and can be random or systematic. Shadows mainly come from surrounding buildings, trees, mutual shading between PV modules and cloud cover. The output characteristics of photovoltaic modules determine that the power generation efficiency will be greatly reduced by local shading or shadowing, which will have a great impact on the power generation of the whole system.

In order to maximize the power generation efficiency of BIPV systems, in addition to planning and designing the installation as well as possible, it is necessary to adopt a suitable PV power generation system structure.

The centralized system first connects a large number of PV modules in series or parallel according to the designed voltage and power level, and then converts the DC energy output from the PV array into AC energy through a centralized inverter. String and multi-series systems connect multiple PV modules in series to form a PV module string, and each string is stepped up by a DC/DC converter and then output AC energy through the inverter. In the above-mentioned centralized, string and multi-series systems, there are series or parallel connections of PV modules, and the maximum power point tracking of the system is carried out for the whole string, so it is impossible to ensure that each module is operating at the maximum power point, and the status information of each PV module cannot be obtained; in addition, due to the different installation directions and angles of each module on the building surface, the power generation efficiency of each PV module The efficiency of each PV module differs from each other. The centralized maximum power point tracking will greatly reduce the power generation efficiency of the system, and when some PV modules are blocked, the power generation efficiency of the whole system will be seriously reduced, which greatly reduces the energy conversion efficiency of the system and may even form hot spots, resulting in system damage.

Micro-inverter technology proposes to integrate the inverter directly with individual PV modules, and equip each PV module with a separate inverter module with AC/DC conversion function and maximum power point tracking function to convert the power emitted from PV modules directly into AC power for use by AC loads or transmission to the grid. The micro-inverter applied to BIPV system can be fully adapted to the application requirements of building integrated photovoltaic power generation system, to adapt to different installation angles and orientations of PV modules, to avoid the impact of local shadows on the system power generation efficiency, and to achieve the maximum power generation efficiency of BIPV system.

In Figure 5-29, the micro-inverters are directly connected to the PV modules, and the power generated from the PV modules is directly transmitted to the grid or used by the local loads. Multiple micro-inverters are directly connected to the grid in parallel, and each micro-inverter and PV module has no influence on each other, even if a single module fails, it will not affect the whole system.

By combining micro-inverter technology with power line carrier communication technology, the output power and status information of each micro-inverter and PV module can be collected through the grid AC bus, which can easily realize the monitoring and control of the whole system without additional communication lines and no burden on the system connection, which greatly simplifies the system structure.

5. The latest trends in PV inverter design

With the development of resonant switching power supply technology, resonant conversion technology is used in PV inverters, which constitute resonant high-efficiency inverters. Since this inverter uses zero-voltage or zero-current switching technology in the DC/DC converter circuit, switching losses are basically eliminated, and the efficiency of the power supply does not decrease significantly even if the switching frequency exceeds 1MHz. Experiments have shown that the losses of resonant inverters can be reduced by 30% to 40% compared to non-resonant inverters with the same operating frequency. At present, the operating frequency of resonant inverter power supply can reach 500kHz~1MHz.

It is also worth noting that the research on medium and small power inverters in photovoltaic power generation systems is developing in the direction of modularity, i.e., by using different combinations of modules, different voltage and waveform conversion systems can be formed. In addition to the pursuit of high reliability and high efficiency, the control and inverter should be effectively combined into one for the characteristics of the PV industry, i.e. the PV inverter should be designed with protection functions such as overvoltage, undervoltage, short circuit, overheating and reverse polarity. This not only reduces the cost of the system, but also improves the reliability of the system. In addition, it is now increasingly common for solar power plants with a peak power generation of more than 100kW to use higher power averaging from 5kWp to 10kWp. The boost + H-bridge inverter topology is shown in Figure 5-30. It is one of the most common topologies for PV inverters and is a two-stage non-isolated topology.

Figure 5-30 Boost +H Bridge Inverter Topology

The first stage of the inverter is the boost stage, which is used to raise the variable output voltage of the module (e.g., 100 to 500 V) to a larger intermediate voltage, which must be greater than the actual peak voltage (e.g., 230 V, or greater than 325 V). The step-up stage also has an important role to play, which is that in order to maximize efficiency, the solar cell module must produce as much power as possible, and the power of the solar cell module can be obtained by multiplying the output current by the output voltage value. The maximum power point in the power curve of a resistive energy module is called the “maximum power point” or MPP, and this precise location varies with the type of module, temperature, and sunlight shading, among other factors. Using “Maximum Power Point Tracking” (MPPT) technology with customized algorithms, the input stage of the inverter can track the MPP.

The second stage of the inverter is to convert the constant intermediate voltage into a 50 Hz AC voltage and feed it into the supply grid, but the phase and frequency of the inverter output voltage should be synchronized with the phase and frequency of the grid. This stage is connected to the supply grid, so it must meet certain safety standards even under fault conditions. The new draft of VDE0126-1-1, related to the Low Voltage Directive, requires PV inverters to have a source to support the main supply grid even in case of power quality degradation, in order to minimize the risk of more widespread power outages. Within the constraints of existing regulations, an inverter can be designed to shut down in real time in the event of a power outage for self-protection, but when PV inverters become commonplace and account for a significant share of total power generation, a direct shutdown of grid-connected PV inverters in the event of a power outage could result in a larger scale main grid outage, causing inverters to shut down one after another, which would rapidly consume The inverters will shut down one by one, which will rapidly consume the power in the grid. Therefore, the new VDE0126-1-1 draft aims to improve the stability and power quality of the main distribution network at the cost of a slightly more complex inverter output stage.

PV inverters must be highly reliable and efficient to minimize maintenance and downtime costs and increase power production.

Since boost inverters can operate in continuous conduction mode or boundary conduction mode (CCM or BCM), this gives rise to different optimization schemes to improve the efficiency of the boost inverter. In CCM mode, the reverse recovery current of the boost diode is the main factor of loss; in this case, it is usually solved by using silicon carbide diodes or Fairchild Semiconductor’s Stealth diodes. The BCM mode is often used in PV inverters. The reason for using the BCM mode is that the forward voltage of the diode is much lower in the BCM mode. Moreover, BCM mode also has high EMI filter and boost inductor to reduce ripple current function. The use of two interleaved booster stages instead of one is a new way to improve the efficiency of the boost inverter by halving the current flowing through each inductor and each switch. In addition, the use of interleaved technology, the ripple current on one stage can offset the ripple current of the other stage, so that the input ripple current can be removed over a wide operating input range, such as FAN9612 interleaved BCMPFC type of control can easily meet the requirements of the solar inverter booster stage.

For input stages that require more than 600V rated switching voltage, 1200V IGBT fast switches such as FGL40N120AND are often used, while for input stages that require only 600V/650V rated voltage, MOSFETs are used. The 600V/650V MOSFETs are used in the design of the H-bridge inverter output stage, but because the new draft specification requires the output stage to operate in four quadrants, the MOSFETs have built-in body diodes, but their switching performance is poor compared to the combination package diodes used in the IGBTs. The new field cutoff IGBTs are capable of switching voltages at 10V/ns, a significant improvement in conduction loss over previous products, and this integrated diode has excellent soft recovery performance that helps suppress electromagnetic dry resistance caused by high di/dt above 500A/μs. For 16 to 25 kHz switching, an IGBT such as Fairchild Semiconductor’s FGH60N60UFD should be used. another trend in optimizing PV inverter designs is to extend the input voltage range, which leads to a reduction in input current at the same power stage or an increase in power stage at the same input current. However, when the input voltage is relatively high, IGBTs with higher voltage ratings (in the 1200V range) need to be used, resulting in greater losses. One way to solve this problem is to use a 3-level inverter, as shown in Figure 5-31.

The high input voltage can be split in two by using two series-connected electrolytic capacitors and connecting the middle point to the NeutralLine, which can then be switched with 600V. The three-level inverter can switch between +Ubus, 0V and -Ubus levels. In addition to being more efficient than the solution with a 1200V switch structure, the three-level inverter has the advantage that the output inductance is considerably reduced.


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