Circuit Protection in Small Solar Photovoltaic Power Generation Systems

With the increasing shortage of energy and the increasing pressure on environmental protection, humans rely more on renewable energy. The development and utilization of solar energy has become the most promising and rewarding technology. Among them, solar power generation is the direct conversion and utilization of solar energy.

Solar cells use the photovoltaic effect of semiconductor devices to convert solar radiation into electrical energy, which is then used or stored through electronic technology conversion. The main components of a solar cell system are solar cells, batteries, controllers, and inverters, and its structural block diagram is shown in Figure 1. Solar power generation systems are divided into independent solar photovoltaic power generation systems and grid-connected solar photovoltaic power generation systems. Independent solar photovoltaic power generation refers to a power generation method in which solar photovoltaic power generation is not connected to the power grid, and its typical feature is that batteries are required to store energy for nighttime electricity use. Independent solar photovoltaic power generation is mainly used in remote villages within the civilian scope, such as household systems and village-level solar photovoltaic power stations; in the industrial scope, it is mainly used in telecommunications, satellite broadcasting and television, and solar water pumps. In areas with wind power generation and small hydropower, it can also form a hybrid power generation system, such as a wind power generation/solar power generation complementary system.

Figure 1 Schematic diagram of solar power generation system

Lightning protection for small solar photovoltaic power generation systems

Since solar panels are located outdoors, they are usually set up in open areas or high places to ensure sunshine. According to the IEC61000-4-5 electrical environment classification, its power connection line belongs to the 4th category of electrical environment, that is, the interconnection line is laid along the power cable according to the outdoor cable and these cables are used as the electrical environment of electronic and electrical lines. According to the IEC lightning protection requirements for the 4th category of electrical systems, the power input part of the solar power generation system needs to be lightning protected, which includes the AC power input circuit, the charging and discharging circuit and the inverter circuit. The protection level is designed according to the requirements of 2KV between lines and 4KV between lines and ground. The protection form may require one or more levels of protection according to different circuit locations. Due to the harsh working environment of the solar power generation system, long maintenance cycle, unattended operation, high service life requirements and other special requirements, it is necessary to consider the surge capacity of the overvoltage protection device in the design of the overvoltage protection solution. The working life and anti-aging ability of the entire protection solution need to be evaluated; if necessary, the 6KV protection level should be adopted.

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

Figure 2 Solar cell junction box and controller input lightning protection

The above solution can also be used for lightning protection of DC loads in solar power generation systems. For lightning protection of AC loads (i.e. the output end of the inverter), the protection circuit design shown in Figure 3 is required. Tyco Electronics' circuit protection department also has deep experience and diverse solutions for this.

Figure 3 Solar power generation system AC load lightning protection circuit

Overvoltage/overcurrent/overheating protection for controller and inverter

Since the voltage and current of the DC power provided by the solar array are unstable, the solar controller and inverter need to convert it into the voltage and current required by the terminal load or the power grid. It is necessary to prevent the controller and inverter from being damaged by ESD and other electromagnetic interference. In addition to ESD devices, the 2Pro overcurrent and overvoltage integrated protection device launched by Tyco Electronics Circuit Protection Department can effectively solve the circuit protection problem of the communication port of the solar controller and inverter. 2Pro combines the polymer positive temperature coefficient (PPTC) recoverable over-temperature and over-current protection device with the traditional varistor (MOV) for use. In addition to protecting against transient overvoltages such as lightning strikes and surges, in the case of long-term overvoltage faults such as voltage fluctuations or loss of neutral line, since the PPTC and MOV are bonded together, the heat dissipated by the MOV will trigger the PPTC to enter a high-impedance state, thereby protecting the MOV from burning and damage due to long-term overvoltage. At the same time, 2Pro can also protect against overcurrent faults such as short circuits, and automatically restore to normal working state after the fault is eliminated, eliminating the heavy maintenance process such as replacing devices. Figure 4 shows the application circuit and physical picture of the 2Pro product.

Figure 4 2Pro product circuit and physical picture

As the rechargeable battery used as the energy storage component has a wide voltage variation range when the charging level changes, the core control unit of the controller and inverter can use the PolyZen device of Tyco Electronics Circuit Protection Division. The PolyZen device can more accurately protect the expensive control and drive chips from being damaged in the case of excessive voltage. Figure 5 shows the application circuit and physical picture of the PolyZen device.

Figure 5 PolyZen device circuit and physical picture

In the circuits of controllers and inverters, high-power semiconductor switching devices are used for power conversion switches. Even when these devices are operating under specified working conditions, random, unpredictable resistive short circuits with different resistance values ​​may occur. When a resistor fails, 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 of a printed circuit board (135°C), causing damage to the epoxy structure of the circuit board and generating a thermal failure event; ultimately, the device and printed circuit board may overheat, smoke, or even catch fire.

The new RTP device launched by Tyco Electronics Circuit Protection Division is suitable for lead-free reflow soldering processes with a maximum temperature of 260°C, and can be disconnected at 200°C for temperature protection after activation. The disconnection temperature of 200°C is higher than the normal operating temperature range of most normal electronic devices, which helps prevent false operation and improve system reliability. At the same time, this temperature is lower than the melting point of common lead-free solder. Therefore, when the adjacent device operates within the specified temperature range, the RTP device will not disconnect the circuit, but will disconnect the circuit before the potential risk of device desoldering and forming additional short circuits. This surface mount temperature fuse device suitable for reflow soldering has good reliability and consistency after assembly soldering compared to pin temperature fuses that require manual soldering.

Protection of energy storage battery pack

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 are potential faults such as cable short circuit, battery positive and negative pole misconnection short circuit, or battery pack overheating during the wiring installation process and use. These faults can damage the equipment circuit at the least, or cause property and personal safety accidents at the worst. The reasonable use of overcurrent protection and temperature detection based on PPTC technology in the battery pack can effectively protect the battery pack and reduce the performance degradation and safety hazards caused by such faults.

Any type of energy storage battery pack may encounter external short circuit during transportation, installation and use. In order to prevent the serious consequences caused by battery pack short circuit, overcurrent protection is necessary. In addition to PolySwitch resettable fuses, Tyco Electronics Circuit Protection Department has also developed MHP hybrid devices. This MHP device uses a new hybrid technology, which can provide a recoverable, compact and robust circuit protection device. It can provide more than 30A of working current when the rated voltage exceeds 30VDC. This metal hybrid PPTC device (MHP) is composed of a bimetallic protector and a polymer positive temperature coefficient (PPTC) device in parallel. This combination can provide resettable overcurrent protection function, and can use the low resistance characteristics of the PPTC device to prevent the bimetallic strip from arcing under high current conditions, and can also heat the bimetallic strip to keep it in the open locked state. This device avoids arcing when the circuit is disconnected, thereby extending the life of electric shock. Because the device is sealed and arc-free, it is particularly suitable for applications in anti-explosion occasions.

Figure 6 Physical picture and mechanical dimensions of MHP30-36

The MHP30-36 devices shown in Figure 6 are the first devices in the MHP product series planned by Tyco Electronics, 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 common bimetallic protectors (usually 3 to 4mΩ). This series of products can provide more reliable and safe circuit protection for energy storage systems in solar power generation systems.

Due to the advantages of charging, discharging and energy density, more and more lithium-ion battery packs are used in solar photovoltaic power generation systems. The safety protection requirements of lithium-ion battery packs are more stringent. In addition to the traditional over-current and over-temperature (caused by overcharge) protection requirements, the equalization of high-string lithium-ion batteries and the protection of voltage detection circuits are also involved. Tyco Electronics Circuit Protection Department has successfully provided high-string lithium-ion battery customers with solutions involving over-temperature detection protection, as well as equalization and voltage detection circuit short-circuit protection, and has been well verified in customer applications. Figure 7 shows the application of PPTC in high-string lithium-ion battery packs, which is mainly used to detect the internal temperature of the battery pack to achieve over-temperature protection and prevent the battery from short-circuiting during equalization or voltage detection.

Figure 7 Application of PPTC in high-series lithium-ion: temperature detection protection and equalization/voltage detection short-circuit protection

The load of a solar photovoltaic power generation system can be a variety of subsystems or equipment, such as LED lighting fixtures, unattended field detection, recording or communication facilities, etc. Depending on the characteristics of the subsystem and the use environment, the subsystem and equipment also require different levels of circuit protection. When designing these protections, we should start from the perspective of the system and coordinate the coupling of each device and subsystem protection to achieve the best protection system effect. In this regard, Tyco Electronics Raychem Circuit Protection Division has rich experience and is dedicated to providing complete solutions for customers in the industry.