Photovoltaic water pump controller

Controller

The output characteristic curve of the photovoltaic array has a strong linear relationship with the solar irradiance, ambient temperature, cloudy, sunny, rainy, foggy and other meteorological conditions. Its output changes with the sunshine, which is the DC power. The photovoltaic water pump, which is the load of the photovoltaic array, has a driving motor that is sometimes a DC motor, sometimes an AC motor or even other new motors. They also have nonlinear properties. In this case, in order to make the photovoltaic pump system work in a relatively ideal working condition, and in any sunshine, the maximum potential of the photovoltaic array output power must be exerted. This requires an adapter to achieve a harmonious, efficient and stable working state between the photovoltaic load. The content of the adapter is mainly the maximum power point tracker, inverter and some protection facilities.

Maximum Power Point Tracker (MPPT)

From the volt-ampere characteristic curve of the photovoltaic array, it can be seen that the maximum power point position of the photovoltaic array under different solar irradiances is not fixed, and when the ambient temperature changes, the maximum power "point position corresponding to the same irradiance will also change. In order to achieve maximum power "point tracking to obtain the most energy under the current sunshine, MPPT is usually made into two forms, which are introduced below. Constant voltage maximum power point tracker (CVT MPPT)

Carefully observe the black dots in the figure that represent the maximum power output, which are the positions of the maximum power points. They are all located near the straight line of Umax=const, especially when the solar radiation is strong, they are closer to Umax=const. At the same time, considering that the solar cell has the following temperature characteristics: when the temperature rises, its open circuit voltage UOC will decrease and the short circuit current Isc will increase slightly under the same solar radiation conditions. Considering that the ambient temperature is generally higher when the solar radiation is high, and the ambient temperature is generally lower when the solar radiation is low, combined with the temperature characteristics of the solar cell, they are just conducive to making the trajectory of the maximum power point in a day closer to a vertical line Umax=const. That is to say, in engineering, people are allowed to treat the trajectory of the maximum power point as a vertical line Umax=const approximately, which constitutes the theoretical basis of the TcvT MPPT.

·True MPPT

The CT-type MPPT has its shortcomings, mainly because the open circuit voltage Uoc and the maximum power point voltage U of the photovoltaic array are greatly affected by temperature. Once Um is set, there will be a large deviation in winter and summer, which will cause a considerable loss of energy. Therefore, with the continuous improvement of the performance/price of microcomputer chips and their real-time performance, many systems have begun to adopt the "true MPPT" technology.

In the "true MPPT" technology, people adopt the concept of self-optimization, measure the output power of the photovoltaic array in real time, and automatically find the maximum power point after comparison. Continuously search, continuously adjust, and continuously search again..., and so on and so forth, the system has been in micro-adjustment. This "real MPPT" can automatically adapt to the large temperature difference between winter and summer without manual intervention, which is very helpful to improve the system's efficiency throughout the year.

Variable frequency inverter

The output of the photovoltaic array after passing through the maximum power point tracker is a DC voltage. If the drive motor used by the water pump is a DC motor, of course, it can be directly connected when the voltage values of the two are adapted. The motor will drive the water pump to rotate and pump water, such as the quiet products of Solarjack Company in the United States in the early years. Since the cost of DC motors is generally high, it is also necessary to regularly maintain or replace its brushes. In recent years, due to the progress of new speed control theory and power electronic devices and technologies, AC speed control technology has made great progress, and its efficiency has gradually caught up with DC motors, and its The convenience and firmness of brushed DC motors far exceed those of DC motors, so the brushed DC motor drive method is gradually being eliminated, and is replaced mainly by high-efficiency three-phase asynchronous motors and DC brushless motors, and occasionally permanent magnet synchronous motors or reluctance motors. The latter types of motors must be driven by dedicated frequency conversion devices or corresponding power electronic drive circuits. : Here, the basic principle of the drive is explained by taking the drive of a three-phase asynchronous motor as an example.

AC drives are often divided into two categories: square wave drive (including step wave drive) and sinusoidal wave drive. Generally speaking, photovoltaic water pump systems with lower power (below 300W) mostly use square wave drive, and when the power is higher, sinusoidal wave drive is often used to limit its harmonic loss. Regardless of the type of drive used, its basic circuit structure is the same. It can be divided into the following four parts, namely:

(1) Switching power supply part: its function is to provide power for the controller. The controller often requires a control power supply such as ±5V or +12V, and the output voltage of the solar cell array cannot be used directly for this purpose in most cases. Therefore, a DC/DC conversion device is required to convert the DC voltage of the array into the required DC voltage, which is the switching power supply.

(2) Main circuit and its drive circuit

The main components of the three-phase inverter circuit as the main circuit are power electronic devices, which constitute a full-bridge inverter circuit. The large-capacity electrolytic capacitor is directly connected across the DC side as an energy storage element. When the inverter circuit is turned off, the solar cell array charges the capacitor. When the inverter circuit is turned on, the capacitor and the solar cell array together supply power to the load.

The design and production of the drive circuit should be carried out carefully. When using power MOSFETs, the gate drive circuit should have good performance reliably.

3) Control circuit

At present, the control circuits of many photovoltaic water pumps have adopted advanced single-chip microcomputer technology. After the development process of MsC-51 series and MCS96 series, the more satisfactory 8XCI96 series has been launched recently, including the 80C196MC series specially used for motor speed regulation. In addition to many common features of the 196 series, it is also particularly suitable for motor drive. Through assembly language programming, the following functions are mainly completed in this system. 

·Complete the protection functions required by the system, such as overcurrent, undervoltage, low speed, and dry protection, and display the fault status;

·Detect the current and voltage on the DC side of the main circuit, calculate the output power of the solar cell array, and complete the tracking of the maximum power point of the array output during the variable frequency speed regulation process;

·Send SPWM signals according to magnetic flux tracking or other corresponding variable frequency speed regulation principles.

(4) Protection circuit

For the sake of safe operation of the system, it is necessary to set up many protection links such as overcurrent, overvoltage, overload, underload, undervoltage, water drying up, automatic restart under various conditions after shutdown, etc., which should be set in the circuit according to the selected control devices and control circuits. Since photovoltaic water pumps work automatically at sunrise and sunset in most cases, very reliable protection measures must be adopted.