Circuit Protection In Small Solar Photovoltaic Systems

- Nov 12, 2019-

With increasing energy shortages and increasing environmental pressures, humans are relying more on renewable energy. The development and use of solar energy has become the most promising and rewarding technology that is the most promising. 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 radiant energy into electrical energy, which is then utilized or stored by electronic technology conversion. 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 generation 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 mode in which solar photovoltaic power generation is not connected to the power grid. A typical feature is that a battery is needed to store energy for nighttime electricity. Independent solar photovoltaic power generation is mainly used in remote rural areas, such as home systems and village-level solar photovoltaic power stations; it is mainly used in telecommunications, satellite radio and television, solar water pumps in the industrial area, in areas with wind power and small hydropower. It is also possible to form a hybrid power generation system, such as a wind power/solar power complementary system.

Figure 1 Schematic diagram of solar power generation system

Lightning protection for small solar photovoltaic systems

Since the solar panels are located outdoors, they are usually set up in open spaces or high places to ensure sunshine. According to the IEC61000-4-5 electrical environment classification, the power connection lines belong to the category 4 electrical environment, that is, the interconnection lines are laid along the power cables by outdoor cables and these cables are used as electrical environments for electrical and electrical circuits. According to IEC requirements for lightning protection of Class 4 electrical systems, the power input part of the solar power generation system needs lightning protection, including AC power input circuit, charge and discharge circuit and inverter circuit. The protection level is designed according to the requirement of 2KV between the wires and 4KV between the wires. The form of protection may require one to many levels of protection depending on the circuit location. Due to the harsh environment of the solar power system, long maintenance period, unattended operation and high service life requirements, it is necessary to consider the surge capability of the overvoltage protection device in the design of the overvoltage protection solution. In addition, the working life and anti-aging capabilities of the entire protection program need to be evaluated; if necessary, 6KV protection level should be used.

Each solar panel cable in the solar power system is first connected to the combiner box of the solar system controller. 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 with high voltage and high reliability requirements, gas discharge tubes (GDT) and varistor (MOV) should be used in series as protection devices for A, B and C positions to complete lightning protection of outdoor cables. For low-power systems with voltages below 48 VDC, GDT can be used directly for overvoltage protection. In consideration of the failure mode of the overvoltage protection device, an overcurrent protection device is required to cooperate with the protection. In the case of unattended or difficult to repair, a self-healing overcurrent protection device should be used. Tyco Electronic Circuit Protection has a variety of protection options for lightning protection in this category for different application environments and protection requirements.

Figure 2 solar cell junction box, controller input lightning protection

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

Figure 3: Solar power system AC load lightning protection circuit

Overvoltage/overcurrent/overheat protection for controllers and inverters

Since the voltage and current supplied by the solar array to the DC battery are unstable, the solar controller and the inverter must convert it into a terminal load or a voltage and current required by the grid. It is necessary to avoid damage to the controller and inverter from ESD and other electromagnetic interference. In addition to ESD devices, Tyco's electronic circuit protection division's 2pro overcurrent and overvoltage protection device can effectively solve the circuit protection problems of solar controllers and inverter communication ports. 2pro uses a polymer type positive temperature coefficient (ppTC) recoverable over-temperature overcurrent protection device in combination with a conventional varistor (MOV). In addition to transient overvoltage protection such as lightning strikes and surges, 2pro can trigger ppTC to cause ppTC to be combined with MOV due to voltage fluctuations or missed lines and other long-term overvoltage faults. The action enters a high-impedance state, thus protecting the MOV from burning damage due to prolonged overpressure. At the same time, 2pro can also protect against overcurrent faults such as short circuit, and automatically recover to normal working state after troubleshooting, eliminating the need for heavy maintenance procedures such as replacing components. Figure 4 shows the application circuit and physical map of the 2pro product.

Figure 42pro product circuit and physical map

Since the rechargeable battery as the energy storage member changes in the degree of charge, the voltage variation range is large. For the core control unit of the controller and inverter, the polyZen device from Tyco Electronics Circuit Protection can be used. The polyZen device can more accurately protect the expensive control and drive chips from damage in the event of excessive voltage. Figure 5 shows the application circuit and physical map of the polyZen device.

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 exhibit resistive short circuits that are random, unpredictable, and exhibit different resistance values, even when operating under specified operating conditions. In the event of a resistor failure, only 10W of power may generate local hot spots above 180°C, which is much higher than the typical glass transition temperature of the printed circuit board (135°C), causing damage to the epoxy structure of the board and resulting in Thermal fault events; may eventually cause the device and printed circuit board to overheat, smoke or even catch fire.

The new RTp device from Tyco Electronics Circuit Protection has a lead-free reflow process for temperatures up to 260 ° C and temperature protection at 200 ° C after activation. The 200°C disconnect temperature is higher than the normal operating temperature range of most properly functioning electronic components, 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, the RTp device does not open the circuit when the next device is operating within the specified temperature range, but will break the circuit before the device is soldered and the potential for additional shorts is formed. This surface mount temperature fuse for reflow soldering has good reliability and consistency after assembly soldering compared to pin temperature fuses that require manual soldering.

Energy storage battery pack protection

In solar photovoltaic systems, the performance and safety of energy storage batteries is also very important. Whether it is a lead-acid battery or a lithium-ion battery storage battery pack, there are potential faults such as cable short circuit, battery positive and negative terminal short circuit, or battery pack temperature too high during the wiring installation process. These faults can damage the equipment circuit and cause property and personal safety accidents. Proper use of over-current 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 experience an external short circuit during transportation, installation, and use. Overcurrent protection is necessary to prevent serious consequences of shorting the battery pack. In addition to polySwitch self-recovery fuses, Tyco Electronics Circuit Protection has also developed MHp hybrid devices. The MHp device incorporates a new hybrid technology that provides a recoverable, compact, and robust circuit protection device. It can supply more than 30A working current when the rated voltage exceeds 30VDC. This metal hybrid ppTC device (MHp) consists of a bimetal protector in parallel with a polymer positive temperature coefficient (ppTC) device. This combination provides both resettable overcurrent protection and the low resistance of the ppTC device to prevent arcing of the bimetal under high current conditions while maintaining the bimetal to remain open. status. This device avoids the arcing that occurs when the circuit is disconnected, thus extending the life of the electric shock. Because the device is sealed and has no arc, it is especially suitable for applications in riot situations.

Figure 6MHp30-36 physical map and mechanical dimension drawing

The MHp30-36 device shown in Figure 6 is the first device in Tyco Electronics' 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 operate at 30A and have an initial resistance of less than 2mΩ, which is lower than the initial resistance of a typical bimetal protector (typically 3 to 4mΩ). This series of products provides more reliable and safe circuit protection for energy storage systems in solar power systems.

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

Figure 7 ppTC in high string lithium ion applications: temperature detection protection and equalization / voltage detection short circuit protection

The load of a solar photovoltaic system can be a variety of subsystems or devices, such as LED lighting fixtures, field unattended detection, recording or communication facilities. Subsystems and devices also require different levels of circuit protection depending on the characteristics of the subsystem and the environment in which they are used. When designing these protections, the protection of each device and subsystems should be coordinated from the perspective of the system to achieve the best protection system. In this regard, Tyco Electronics Raychem Circuit Protection has extensive experience and is committed to providing complete solutions to customers in the industry.