How to choose the most suitable photovoltaic controller

In an independent photovoltaic power generation system, the controller is also the main component. The photovoltaic controller should be determined according to the system power, the system DC operating voltage, the number of battery input channels, the number of battery groups, the load status, and the special requirements of users. Types of photovoltaic controllers. In the small photovoltaic power generation system, the controller is used to protect the energy storage battery. Generally, the small power photovoltaic power generation system adopts a single-channel pulse width modulation type controller; in the large and medium-sized systems, the controller must have more protection and monitoring. function, so that the battery charging and discharging controller develops into the controller of the system. Therefore, the high-power photovoltaic power generation system adopts a multi-input controller or an intelligent controller with communication function and remote monitoring and control function. The control principle and the progress of the components used, the current advanced system controllers have used microprocessors to realize software programming and intelligent control.

When selecting the controller, special attention should be paid to its rated working current, which must be greater than the short-circuit current of the solar panel or square array and the maximum working current of the load at the same time. In order to adapt to the future expansion of the system and ensure the long-term stability of the system, it is recommended to choose a higher model for the selection of the controller. For example, when a 12V/5A controller is designed to meet the needs of the system, a 12V/8A controller can be considered for practical applications. When selecting models, it should also be noted that the functions of the controller are not the more the better. Pay attention to select applicable and useful functions in this system, and discard redundant functions, otherwise it will not only increase the cost, but also increase the possibility of failure.

The controller varies with the control circuit and control method. From the perspective of design and use, according to the input power and load power of the photovoltaic cell array, low-power, medium-power, high-power, or special controllers can be selected. The main technical parameters of the controller are as follows.

(1) System working voltage
The system working voltage, that is, the rated working voltage, refers to the working voltage of the battery or battery pack in the photovoltaic power generation system. This voltage should be determined according to the working voltage of the DC load or the configuration selection of the AC inverter, generally 12V, 24V, and 48V, 110V, 200V, etc. for medium and high power controllers.

(2) Rated input current
The rated input current of the controller depends on the output current of the solar cell module or the square array. During the selection, the rated input current of the controller should be equal to or greater than the output current of the solar cell module or the square array.

(3) Maximum charging current
The maximum charging current refers to the maximum current output by the solar cell module or square array. According to the power, it is divided into 5A, 6A, 8A, 10A, 12A, 15A, 20A, 30A, 40A, 50A, 70A, 100A, 150A, 200A, 250A, 800A and other specifications. Some manufacturers use the maximum power of the solar cell module to express this content, which indirectly reflects the technical parameter of the maximum charging current.

(4) The rated load current of the controller
That is, the controller outputs the DC output current to the DC load or inverter, and this data must meet the input requirements of the load or inverter.

(5) Number of solar cell array input channels
The number of input channels of the controller should be more than or equal to the design input number of the solar cell array: low-power photovoltaic controllers generally have only one solar cell array single input; high-power controllers usually use multiple inputs, each input The maximum current is the rated input current/number of input channels. Therefore, the output current of each battery array should be less than or equal to the maximum current value allowed by the controller for each channel. Generally, the high-power photovoltaic controller can input 6 channels, and can be connected to 12 channels and 18 channels at most.

(6) Loss of the circuit itself
The loss of the controller’s circuit itself is also one of its main technical parameters, also known as no-load loss (quiescent current) or maximum self-consumption current. In order to reduce the loss of the controller and improve the conversion efficiency of the photovoltaic power supply, the circuit loss of the controller should be as low as possible. The maximum self-loss of the controller shall not exceed 1% or 0.4W of its rated charging current, and the self-loss of the controller is generally 5~20mA according to different circuits.

(7) Battery overcharge protection voltage (HVD)
The battery overcharge protection voltage is also called full disconnection or overvoltage shutdown voltage. Generally, it can be set at 14.1~14.5V (12V system), 28.2~29V (24V system) and 56.4~ according to different needs and battery types. Between 58V (48V system), the typical values ​​are 14.4V, 28.8V and 57.6V respectively. The shutdown recovery voltage (HVR) of battery charging protection is generally set between 13.1~13.4V (12V system), 26.2~26.8V (24V system) and 52.4~53.6V (48V system), and the typical values ​​are 13.2V respectively. , 26.4V and 52.8V.

(8) Over-discharge protection voltage (LVD) of the battery
The over-discharge protection voltage of the battery is also called under-voltage disconnection or under-voltage shutdown voltage. Generally, it can be set at 10.8~11.4V (l2V system), 21.6~22.8V (24V system) according to the needs and different types of batteries. ) and 43.2~45.6V (48V system), the typical values ​​are 11.1V, 22.2V and 44.4V respectively. The shutdown recovery voltage (L.VR) of battery over-discharge protection is generally set to 12.1~12.6V (12V system), 24.2~25.2V (24V system) and 48.4~50.4V (48V system), the typical values ​​are 12.4V, 24.8V and 49.6V respectively.

(9) battery charging voltage
The charging float voltage of the battery is generally 13.7V (12V system), 27.4V (24V system) and 54.8V (48V system).

(10) Temperature compensation
The controller generally has a temperature compensation function to adapt to different ambient operating temperatures and set a more reasonable charging voltage for the battery. The temperature compensation coefficient of the controller should meet the technical requirements of the battery, and its temperature compensation value is generally -20~-40mV/℃.

(11) Working ambient temperature
The use or working environment temperature range of the controller varies with the manufacturer, generally between 120~+50℃.

(12) Other protection functions
①Controller input and output short-circuit protection function The input and output circuits of the controller must have short-circuit protection circuits to provide protection functions.
②Anti-reverse charging protection function The controller should have the protection function to prevent the battery from being reversely charged to the solar cell.

③Reverse polarity protection function Solar cell modules or batteries are connected to the controller. When the polarity is reversed, the controller must have the function of protecting the circuit.
④ Anti-lightning protection function The input end of the controller should have the protection function of anti-lightning, and the type and rating of the arrester should be able to absorb the expected impact energy.

⑤Protection against impulse voltage and impulse current Apply 1.25 times the nominal voltage to the solar cell input end of the controller for 1h. The controller should not be damaged. Bring the controller charging loop current to 1.25 times the nominal current for lh. The controller should not be damaged either.

In addition to the above-mentioned main technical data to meet the design requirements, parameters such as ambient temperature, altitude, protection level and overall dimensions, as well as the manufacturer and brand are also factors to be considered when selecting the controller configuration.

The specific model selection of the inverter and the controller can be found directly from the product introduction materials of the relevant manufacturers. For in-depth understanding of other hardware selections in the system, you can refer to the relevant chapters in the book. There are three technical ways to make good use of intermittent energy such as solar photovoltaic power generation: one is to set up energy storage devices; the other is to use multi-energy complementation; the third is to connect to the grid. For remote and remote areas, islands and other areas, the power grid is often not enough, so an independent power generation system can only be built, and with energy storage, a multi-energy complementary hybrid power generation system is often used. In fact, this is to use photovoltaic power generation to solve the problem of power supply in areas without electricity. It is not a simple photovoltaic power generation system, but an independent light-diesel-storage microgrid. Simply using photovoltaic power generation to ensure normal power supply throughout the year is often technically economical. This is the case with the photovoltaic power stations built in China in the 1990s in counties without electricity and townships without electricity. Figure 1 is a block diagram of a 25kWp photovoltaic power plant system built in Shuanghu, Tibet, China.

Figure 1 - Block diagram of the 25kWp photovoltaic power plant system in Shuanghu
Figure 1 – Block diagram of the 25kWp photovoltaic power plant system in Shuanghu

Here, the function of the diesel generator set is to serve as a backup power supply to ensure that the photovoltaic power station system can supply power reliably. According to the overall scheme design, it is stipulated that the diesel generator set can be started under the following two conditions.

①When the energy storage battery pack is short of power and cannot meet the needs of the electricity load, start the diesel generator set in time, and charge the battery pack through the rectifier charging equipment to ensure the normal operation of the power supply system.
②When the photovoltaic power station system cannot supply power due to inverter failure or other reasons, start the diesel generator set and directly supply power to the user through the AC power distribution system.
The power setting of the diesel generator needs to be determined by the specific load power and the power supply guarantee rate of the photovoltaic power generation.

If people use the LOLP method to design, a value of LOLP can be set, such as 0.1, which means that the load loss rate is 10%, that is, 90% of the load power supply guarantee rate is assumed by photovoltaic power generation, and 10% is guaranteed by diesel power generation. In this way, the capacity design of the photovoltaic power generation system can reduce the capacity by 10%. In doing so, the technical and economic indicators are generally appropriate.

The function of the rectifier charging equipment is to convert the alternating current generated by the diesel unit into direct current to charge the energy storage battery pack. The main design requirements for the design of the rectifier and charging equipment for Shuanghu Photovoltaic Power Station are as follows:

The rectification and charging equipment uses KGCA series three-phase bridge thyristor voltage regulation and rectifier circuit. It is composed of KC-04 integrated trigger circuit, PI adjustment control circuit, detection and pulse power amplifier. The screen structure is adopted, and all components are installed in the same box. The instrument, indicator lights and control buttons are installed on the panel. There are three working states: “steady current”, “stabilized” and “manual”, and can be charged with constant current or constant voltage.

Read more: What is the application classification and development of photovoltaic power generation system?