Application of Module Level Power Electronics Technology in Distributed Photovoltaic Power Generation System

Distributed photovoltaic power generation system usually adopts series wiring scheme, which has problems such as high voltage DC electrical safety risk, power mismatch of modules and maintenance management difficulty. Three schemes of module level power electronics technology are proposed, including string inverter with rapid shutdown device, string inverter with module optimizer and micro inverter. The electrical shut-off, power generation data monitoring, power optimization and inverter function characteristics of each scheme are described and compared, and the suggested application scenarios are given. In addition, typical electrical topology schemes of module level power electronic devices including rapid shutdown device, module optimizer and micro inverter are introduced.


Introduction
In recent years, the installed capacity of global photovoltaic (PV) power generation system has grown rapidly, according to the forecast by International Energy Agency (IEA), the newly installed capacity of distributed PV power generation systems will be close to 50% of that of global PV power generation systems in the entire year of 2023 [1].However, the electrical safety risks caused by high-voltage direct current generated by PV modules in series are being received high attention by many countries.
Regulations regarding rapid shutdown of PV modules in the event of a fire are proposed in North America and Europe.According to the requirements of National Electrical Code (NFPA 70-2023) published by the United States of America [2], distributed PV power generation systems installed on buildings should be equipped with module level shutdown devices to ensure the shutdown of the highvoltage DC within 30 seconds after the shutdown device is activated, the voltage outside the limit range of the PV array should decrease to below 30V, and the voltage within the limit range should decrease to below 80V to avoid rescue risks in case of a fire.
In the November of 2021, the National Energy Administration of China also issued the "Notice on Strengthening the Safety of Distributed PV Power Generation", which proposed new requirements for DC arcing detection and DC side rapid shutdown [3].
The application of module level power electronics technology can effectively solve the electrical safety issues of distributed PV systems, and has been widely applied in the distributed PV market abroad.Taking the US household PV market as an example, as of 2020, the penetration rate of module level power electronic technology products has increased from 50% to 92.8%.With the gradual improvement of domestic regulations and standards, the domestic PV industry will pay more attention to the safety of distributed PV systems, and module level power electronics technology has broad application prospects.

Existing Problems in Distributed PV Power Generation Systems
At present, distributed PV power generation systems usually adopt the scheme of connecting multiple photovoltaic modules in series to a string inverter, which poses high DC voltage safety risks, component power mismatch, and difficulty in operation and maintenance management.

High DC Voltage Safety Risks
In the conventional string inverter scheme, due to multiple PV modules directly connected in series and without disconnection points, there will be a continuous high voltage of 600-1000V on the DC side of the PV power generation system, which is prone to the risk of fire caused by high-voltage DC arcing.In addition, distributed PV power generation systems are usually installed on buildings.When a building or PV power generation system occurs a fire, as long as there is a certain of illumination, the PV power generation system will continue to generate high-voltage electricity, threatening the personal safety of firefighters and hindering fire rescue [4].

Module Power Generation Mismatch
The application scenarios of distributed PV power generation systems are relatively complex, and PV modules may experience sunlight occlusion, inconsistent attenuation, and inconsistent installation orientation, leading to the problem of mismatched power generation of modules.That is, some PV modules have significantly lower power generation than other modules.The wiring scheme of a string inverter connects multiple modules in series, and a single or several PV modules with low power output will lead to lower the power generation efficiency of the whole string and affect investment returns.
Module level power electronics technology solves the "short board effect" caused by mismatches between modules in distributed PV power generation systems with the power control technique of the single module, ensuring the maximum power output of modules [5].Through testing and long-term data collection, under the same conditions, systems using module level power electronic technology can increase power generation by 5-25% compared to traditional string inverter systems.

Difficulty in Operation and Maintenance Management
Although the scale of distributed PV power generation systems is relatively small, there are also problems such as dust accumulation in PV modules, diverse faults, and difficulty in evaluating power generation benefits.The distributed PV power generation system based on string inverters can only monitor the power generation situation of each PV module string, making it difficult to locate on the fault PV modules.In addition, the decentralized layout of modules and the non-specialization of operation and maintenance personnel have also increased the cost of operation and maintenance management difficulty.

Module Level Power Electronics Technology Solution
Module level power electronics technology refers to the technology that can finely control and manage a single or multiple PV modules, including various technical fields such as power electronics, semiconductor devices, communication, and cloud computing, which can reduce electrical fire risk, mitigate power mismatch of modules and improve the operation and maintenance efficiency in the distributed PV power generation system.
The application of module level power electronics technology in distributed PV systems includes three solutions: string inverter with rapid shutdown device, string inverter with module optimizer and micro inverter.It can partially or completely achieve the electrical shutdown, power generation monitoring, power optimization, and inverter functions of PV power generation systems, and has the characteristics of safety, efficiency, intelligence and flexibility.

String Inverter with Rapid Shutdown Device Scheme
String inverter with rapid shutdown device scheme is based on the conventional string inverter, with a rapid shutdown device added between modules.The wiring diagram is shown in figure 1.A rapid shutdown device is an equipment that can achieve fast shutdown at the module level.In critical situations, this device can quickly disconnect the connections between each PV module remotely or manually, thereby eliminating the existing DC high voltage in the PV system array, reducing the risk of electric shock and solving the risk of rescue, and improving the safety of the PV system.
Compared to the optimizer and micro inverter, the shutdown device has the simplest function of module level shutdown, with no generation monitoring or power optimization functions.The string inverter with rapid shutdown device scheme is also the lowest cost among the three module level power electronics technology schemes.

String Inverter with Module Optimizer Scheme
The string inverter with module optimizer scheme separates the maximum power point tracking (MPPT) function from the inverter and is implemented by the optimizer.The string inverter only retains the inverter function, and the wiring diagram is shown in figure 2. The optimizer is a device that can achieve MPPT function and rapid shutdown function.The optimizer adopts predictive current and voltage technology to ensure that the PV modules are always in optimal working state through serial connection with PV modules, to solve the impact of shadow shading, inconsistent orientation, or differences in component electrical specifications on the power generation of distributed PV power generation system.
The optimizer also has a module level shutdown function, which automatically cuts off the connection between modules in case of emergency, making the DC terminal voltage only equal to the open circuit voltage of single module, thus solving the rescue risk of distributed PV systems.In addition, the distributed PV system equipped with an optimizer also has module level operation and maintenance functions, which can monitor the power generation status of each module, improving operation and maintenance efficiency.

Micro Inverter Scheme
The micro inverter scheme integrates four functions: electrical shutdown, power generation data monitoring, power optimization, and inversion, which can achieve precise control and monitoring in module level.The disadvantage of micro inverter scheme is high cost.
The wiring diagram of the micro inverter scheme is shown in figure 3. Micro inverter refers to an inverter that can track the maximum power point of single or two PV module(s) separately, and then integrate it into the AC power grid after inversion to achieve fine adjustment and monitoring of the output power of each PV module.The micro inverter converts DC power from no more than two modules into AC power and integrates it into the power grid.The DC voltage does not exceed to 120V (Extra low voltage (ELV) level), completely solving the risk of fire caused by high-voltage DC arcing.In addition, it solves the problem of rescue difficulties when a building with PV power generation system installed occurs fire.The micro inverter scheme adopts a fully parallel structure, with each or two module(s) having independent MPPTs, ensuring the power output between modules does not affect each other.Due to no power mismatch between module problem, factors such as shading and dust on some modules will not affect the overall power generation, maximizing the output of each module.The micro reverse inverter scheme also has module level operation and maintenance functions, which can accurately monitor the power generation of each module, facilitate rapid fault location, and improve operation and maintenance efficiency.

Scheme Comparison
The comparison table of three distributed PV system module level power electronics technology solutions is shown in table 1.The string inverter with rapid shutdown device scheme only has module level shutdown function; The string inverter with module optimizer scheme not only has module level shutdown function, but also has module level power generation monitoring, power optimization, system efficiency improvement and arc monitoring functions; The micro inverter scheme has a unique inversion function and can ensure there is no risk of DC high voltage during operation through a parallel connection scheme of modules.In the system of string inverter with rapid shutdown device and string inverter with module optimizer, the PV modules are still in series connection, and the PV power generation system still forms DC high voltage during operation, so there is still a certain of possibility of fire caused by DC arc.The micro inverter scheme adopts parallel connection for PV modules, which does not form DC high voltage and significantly improves safety.
Based on the above comparative analysis, the string inverter with rapid shutdown device scheme is suitable for large and medium-sized PV power generation systems that are cost sensitive and have no significant obstruction of PV modules; The string inverter with module optimizer scheme is suitable for large and medium-sized PV power generation systems that are not cost sensitive and have partial sunlight occlusion or inconsistent installation orientation; The micro inverter scheme is suitable for small and medium-sized power generation systems that are cost insensitive and high efficiency required.

Rapid Shutdown Device Topology Scheme
The typical topology scheme of the rapid shutdown device is shown in figures 4 and 5, consisting of dedicated chips, power electronic switches, anti-reverse diodes, and PLC communication receivers.MOSFET power electronic switches can be used to achieve module level rapid shutdown function [6][7].The topology scheme of the rapid shutdown device in figure 4 can have the access of a single PV module, while the topology scheme of that in figure 5 can have the access of two PV modules simultaneously.

Module Optimizer Topology Scheme
The topology scheme of the module optimizer needs to consider factors such as the maximum open circuit voltage of the PV module, the output voltage range of the inverter DC side, the number of optimizers, and whether electrical isolation is required.Boost, Buck, and Buck-Boost topology schemes can be selected [8][9].The Buck-Boost topology scheme can have both step-up and step-down functions, which is recommended to use it as an optimizer topology scheme, as shown in figure 6.

Micro Inverter Topology Scheme
The topology schemes of micro inverters can be divided into three types: multi-stage with DC bus, multi-stage with pseudo DC bus, and multi-stage with no DC bus [10].Since the DC side voltage of the micro inverter belongs to the ELV (≤ 120V) range, in order to improve safety, the AC side and DC side are usually electrically isolated, so an isolated topology scheme is adopted.
The topology scheme diagram of a multi-level isolated micro inverter with DC bus is shown in figure 7. The DC voltage on the PV module side is boosted by a DC/DC converter to achieve MPPT function, and then connected to the grid through a second stage PWM inverter.The power decoupling of the micro inverter is completed by the DC bus capacitor Cbus.The topology scheme diagram of a multi-level isolated micro inverter with pseudo DC bus is shown in figure 8.The DC voltage of the PV module is converted into a 2 times of power frequency waveform that is in the same phase with the power grid through a DC/DC converter, and the MPPT function is achieved.The following DC/AC stage is a 2 times of power frequency waveform flipping circuit to achieve grid connection function.The power ripple of micro inverters is generally decoupled on the input side.The decoupling capacitor CPV is shown in figure 8.

Conclusion
In response to the problems of string inverter schemes in current distributed PV systems, three schemes based on module level power electronics technology, string inverter with rapid shutdown device, string inverter with module optimizer and micro inverter schemes are proposed.The functional characteristics of electrical shutdown, power generation data monitoring, power optimization, and inversion of each scheme are elaborated and compared, and recommended application scenarios are provided.In addition, typical electrical topology schemes for module level power electronic devices including rapid shutdown device, module optimizer and micro inverter are provided.

Figure 1 .
Figure 1.Wiring diagram of string inverter with rapid shutdown device scheme.

Figure 2 .
Figure 2. Wiring diagram of string inverter with module optimizer scheme.

Figure 4 .
Figure 4. Typical topology of rapid shutdown device with single module connection .

Figure 5 .
Figure 5.Typical topology of rapid shutdown device with two module connection.

Figure 6 .
Figure 6.Buck-Boost topology scheme of the optimizer.

Figure 7 .
Figure 7. Topology of a multilevel isolated micro inverter with DC bus.

Figure 8 .
Figure 8. Topology of a multilevel isolated micro inverter with pseudo DC bus.The topology scheme of a multi-level isolated micro inverter without DC bus is shown in figure9.The PV module converts the DC voltage into a high-frequency square wave voltage through the frontend DC/AC link.After being boosted by a high-frequency transformer, it is connected to the grid under the action of the back-end AC/AC converter.The power decoupling of micro inverters is achieved by utilizing one or more capacitors connected in parallel on the PV module side.

Figure 9 .
Figure 9. Topology of a multilevel isolated micro inverter with no DC bus.

Table 1 .
Comparison of module level power electronics technology for distributed PV systems.