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Circuit Protection Design in Small Solar Photovoltaic Power Generation System
- May 22, 2018 -

With the increasing shortage of energy and increasing environmental pressure, human beings rely more on renewable energy sources. The development and use of solar energy has become the most promising and rewarding, hottest technology. Solar power is the direct conversion and utilization of solar energy.


Solar cells use the photovoltaic effect of semiconductor devices to convert solar radiant energy into electrical energy, which is then used or stored by electronic technology. The main components of the solar cell system are solar cells, batteries, controllers, and inverters. The block diagram of the structure is shown in Figure 1. The solar power system is divided into an independent solar photovoltaic power generation system and a grid-connected solar photovoltaic power generation system. Independent solar photovoltaic power generation refers to a power generation method in which solar photovoltaic power generation is not connected to a power grid. A typical characteristic is that a battery is required to store energy used at night. Independent solar photovoltaic power generation is mainly used in remote rural areas such as home systems and village-level solar photovoltaic power stations in the civilian area; in the industrial area, it is mainly used for telecommunications, satellite broadcasting and television, and solar water pumps in areas with wind power and small hydropower. It can also form hybrid power systems, such as wind power/solar power generation complementary systems.

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Figure 1 Schematic diagram of solar power system


Lightning protection of a small solar photovoltaic power generation system


Because the solar panels are in an outdoor environment, they are usually set up in open spaces or high places to ensure daylight. According to the IEC61000-4-5 electrical environment classification, the power connection line belongs to the category 4 electrical environment, that is, the interconnection lines are laid along the outdoor cable along the power cable and these cables are used as the electrical environment of the electronic and electrical lines. According to IEC requirements for lightning protection of Category 4 electrical systems, the power input part of the solar power system needs lightning protection. This includes AC power input circuits, charge and discharge circuits, and inverter circuits. The protection level is designed according to the requirement of 2KV between lines and 4KV between lines. Protection forms may require one-to-many protection at different circuit locations. Due to the harsh working environment, long maintenance intervals, unattended operation, and long service life requirements of solar power generation systems, it is necessary to consider the surge protection capability of overvoltage protection devices in the design of overvoltage protection solutions. In addition, the working life and aging resistance of the entire protection scheme need to be evaluated; if necessary, a 6KV protection level should be used.


Each solar panel cable in the solar power system is first connected to the solar system controller's combiner box. Therefore, the overvoltage protection design shown in Figure 2 should be used at the input of the combiner box and controller. Among them, A, B and C are overvoltage protection devices. For systems and equipment with high voltage and high reliability requirements, gas discharge tubes (GDTs) and varistors (MOVs) should be used in series as protection devices for A, B, and C locations to complete the lightning protection of outdoor cables. For low power systems with voltages below 48VDC, overvoltage protection can be used directly with the GDT. Considering the failure mode of overvoltage protection devices, overcurrent protection devices are required to cooperate with protection. In unattended or difficult-to-maintain applications, self-healing overcurrent protection devices should be used. Tyco Electronics Protection Division has a variety of protection solutions for this type of lightning protection in response to different application environments and protection requirements.

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Figure 2 Lightning protection of solar cell combiner box and controller input


For the DC load of the solar power generation system, the above scheme can also be adopted for lightning protection. For the lightning protection of the AC load (ie, the output of the inverter), the protection circuit design shown in FIG. 3 needs to be used. Tyco's electronic circuit protection department also has extensive experience and diverse solutions.

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Figure 3 AC load lightning protection circuit of solar power system


Overvoltage/Overcurrent/Overtemperature Protection of Controllers and Inverters


Since the voltage and current of the solar array providing direct current are not stable, the solar controller and the inverter will convert it into the terminal load or the voltage and current required by the grid. It is necessary to prevent the controller and the inverter from being damaged by ESD and other electromagnetic interference. In addition to ESD devices, the 2Pro overcurrent and overvoltage protection devices introduced by Tyco Electronics Protection Division can effectively solve the circuit protection issues of the solar controller and inverter communication ports. 2Pro uses polymer-type positive temperature coefficient (PPTC) recoverable over-temperature overcurrent protection devices in conjunction with conventional varistors (MOVs). In addition to transient overvoltage protection such as lightning and surge in 2Pro, in long-term overvoltage faults such as voltage fluctuations or missing lines, 2Pro will dissipate the heat from the MOV to trigger the PPTC because the PPTC and MOV are stuck together. The action enters a high-impedance state, which protects the MOV from burning damage due to prolonged overpressure. At the same time, 2Pro can also protect against overcurrent faults such as short circuits, and automatically recover to normal operating conditions after troubleshooting, eliminating the need for heavy maintenance such as device replacement. Figure 4 shows the application circuit and physical map of the 2Pro product.

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Figure 4 2Pro product circuit and physical map


Since the rechargeable battery as an energy storage component changes in the degree of charge, the voltage variation range is large. For controllers and inverters, the core control unit can use PolyZen devices from Tyco Electronics Circuit Protection. PolyZen devices can more accurately protect expensive control and driver chips from damage caused by excessive voltage. Figure 5 shows the application circuit and physical map of the PolyZen device.

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Figure 5 PolyZen device circuit and physical map


In the circuit of the controller and the inverter, a high-power semiconductor switching device is used for the power conversion switch. These devices, even when operating under specified operating conditions, can exhibit resistive short circuits that are random, unpredictable, and exhibit different resistance values. In the event of a resistive failure, only 10W of power may generate a local hot spot with a temperature above 180°C, which is much higher than the typical glass transition temperature (135°C) of the printed circuit board, resulting in damage to the epoxy structure of the circuit board and resulting in Thermal failure events can eventually lead to overheating of devices and printed circuit boards, smoke, and even fire.

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The new RTP device from Tyco Electronics Circuit Protection has a lead-free reflow process that is suitable for temperatures up to 260°C and can be turned off at 200°C for temperature protection after activation. The 200°C disconnection temperature is higher than the normal operating temperature range of most normally operating electronic devices, helping to prevent malfunctions and improve system reliability. At the same time, the temperature is lower than the melting point of common lead-free solder. Therefore, when the next device is operating within the specified temperature range, the RTP device will not break the circuit, but it will break the circuit before the device is de-welded and creates an extra risk of an extra short circuit. This surface mount temperature protection device suitable for reflow soldering has good reliability and consistency after assembly soldering compared to the pin temperature fuses that require manual soldering.


Energy storage battery protection


In solar photovoltaic power generation systems, the performance and safety of energy storage batteries are also very important. Whether it is a lead-acid battery or a lithium-ion battery, there is a potential short-circuit in the cable installation process, during use, and during use. The battery is short-circuited, the battery positive and negative are miswired, or the battery temperature is too high. These failures can damage the circuit of the equipment and cause accidents of property and personal safety. Proper use of PPTC-based overcurrent protection and temperature detection in the battery pack can effectively protect the battery pack and reduce the performance degradation and safety hazards caused by such failures.


Any type of energy storage battery pack may encounter an external short circuit during transportation, installation, and use. Over current protection is necessary to prevent serious consequences caused by a short circuit in the battery pack. In addition to PolySwitch self-healing fuses, Tyco Electronics Circuit Protection also developed MHP hybrid devices. This MHP device uses a new hybrid technology that provides a recoverable, compact, robust circuit protection device. It can provide more than 30A operating current at rated voltage over 30VDC. This metal-mixed PPTC device (MHP) consists of a bimetal protector and a polymer positive temperature coefficient (PPTC) device in parallel. This combination provides both a resettable overcurrent protection function and the low resistance of the PPTC device to prevent the bimetal from arcing under high current conditions while also heating the bimetal to keep it open. status. This device avoids arcing when the circuit is disconnected, thus extending the life of the shock. Because the device is sealed and has no arc, it is especially suitable for applications in riot situations.

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Figure 6 MHP30-36 physical map and mechanical dimensions


The MHP30-36 device shown in Figure 6 is the first device in Tyco Electronics' planned MHP product family with a maximum rating of 36VDC/100A and a trip time of less than 5 seconds at 100A (@25°C). These devices have an operating current of 30A and an initial resistance of less than 2mΩ, which is lower than the initial resistance of a typical bimetal protector (usually 3 to 4mΩ). This series of products can provide more reliable and safe circuit protection for energy storage systems in solar power systems.


Due to the advantages of charge, discharge, and energy density, more and more lithium-ion battery packs are used in solar photovoltaic power generation systems. The requirements for the protection of lithium-ion battery packs are more stringent. In addition to traditional over-current and over-temperature (overshoot) protection requirements, the issue of equalization of high string Li-Ion batteries and protection of voltage detection circuits has followed. Tyco Electronic Circuit Protection successfully provided solutions for over-temperature lithium-ion battery packs, including over-temperature detection protection, and short-circuit protection for equalization and voltage detection circuits, and has been well validated in customer applications. Figure 7 shows the application of PPTC in high- string lithium-ion battery packs. It is mainly used to detect the internal temperature of the battery pack to achieve over-temperature protection and prevent the battery from being short-circuited during equalization or voltage detection.

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Fig. 7 Application of PPTC in high string lithium ion: temperature detection protection and equalization/voltage detection short circuit protection


The load of the solar photovoltaic system can be a variety of subsystems or devices, such as LED lighting fixtures, field unattended detection, recording or communications facilities. According to the characteristics of the subsystem and the use environment, subsystems and equipment also need different levels of circuit protection. In designing these protections, it is necessary to coordinate the protection of various devices and subsystems from the perspective of the system so as to achieve the best effect of the protection system.