1. Problems with active inverter systems
The active inverter system is an important part of the solar energy utilization system, and its main function is to convert the direct current energy generated by solar photovoltaic power into electrical energy and input it into the AC power grid. The current working principle of active solar inverters is to convert the DC voltage emitted by silicon photovoltaic cells into AC voltage and input it into the power grid through a PWM system. The active solar inverter operating in this way has the following drawbacks:
(1) Electromagnetic pollution is severe. No matter which PWM system works with, due to the use of pulse width modulation, it will generate a large number of harmonics, causing serious high-frequency pollution to the power grid. This useless electromagnetic power increases the losses of the power grid and electrical equipment, while also causing serious electromagnetic pollution to the environment.
(2) The price is expensive. Active solar inverters account for approximately 30% of the total project cost in solar power generation systems. The active inverter system operating in this way is relatively expensive due to the use of expensive high-power and high-voltage MOS or IGBT components, as well as the complexity of the technology.
(3) There are potential security risks. The main components of PWM working mode are power semiconductor devices. Once a fault occurs, it can cause a short circuit in the power grid and equipment damage.
2. Design of Rotating Active Solar Inverters
In the presence of excitation, if a DC voltage is applied to the brush, the motor will rotate. However, in the rotor circuit coil, it is a complete single-phase AC signal, with each wire turn connected to the brush through a rectifier at the highest or lowest voltage. If the electric brush is not considered, simply facing out the two symmetrical points of the armature with a sliding ring can obtain a pure AC voltage. If the voltage at this time is close to the voltage of the power grid, and the speed is synchronized with the speed of the power grid, the AC power of the motor will be connected to the power grid, and the motor will be dragged into synchronization by the power grid. If the excitation is further strengthened at this time, the energy from the DC power supply can be fed back to the grid.
The example is single-phase operation, and the design idea is to change the winding of the motor to a three-phase winding, which can form a three-phase rotating active inverter circuit.
2.1 Composition of Rotating Active Solar Inverters
The active solar inverter designed in this article is divided into two parts: the circuit part and the mechanical part.
1) Circuit section
The schematic diagram of the circuit is shown in Figure 1.
In Figure 1, the upper part of the dashed line is installed on the fixed part of the rotating solar inverter, and the lower part of the dashed line is installed on the rotating part of the rotating solar inverter. The solar cell 8 is connected to the DC winding 1 of the rotating solar inverter through the three-phase distribution switch 7, forming a rotating magnetic field in the stator winding of the rotating solar inverter, which drives the rotor of the rotating solar inverter to rotate.
The step-up (or step-down) winding of the autotransformer structure is connected to the public power grid. After the excitation voltage is applied to the excitation coil, the magnetic field lines are coupled to the excitation isolation coil 5 through the air gap. The DC voltage rectified by rectifier bridge 4 is applied to excitation winding 3. The rotor adopts a convex pole composite excitation method, with the magnetic poles divided into a permanent magnet part and an excitation part. The magnetic flux of the permanent magnet part is used to maintain the normal power generation voltage, and the excitation part is used to regulate the load.
The stator winding adopts an autotransformer type structure, mainly to improve the efficiency of the rotating solar inverter. Therefore, it is best to design a voltage ratio around 1.
The rotor adopts a salient pole composite excitation method, mainly to reduce excitation power and improve efficiency.
2) Mechanical part
The mechanical schematic diagram of the rotating solar inverter is shown in Figure 2. The mechanical part adopts an integral sealing structure, and the stator is equipped with three-phase distributed windings. The rotor is composed of hollow cast aluminum, which is equipped with a magnetic yoke and permanent magnet magnetic steel. The excitation stator and its winding are assembled on a cast aluminum rotor, and the excitation rotor and its winding shaft 17 are connected to the end cover 9 outside. The three-phase rectifier bridge is fixed on the cast aluminum rotor. The three-phase distribution code disk 15 is fixed on the shaft 17, and the three-phase distribution switch is fixed on the left end cover 3.
2.2 Design of three-phase guide switch
The function of the designed three-phase guide switch is to sequentially connect the DC voltage to three different windings, forming a rotating magnetic field in the rotating solar inverter. This causes the rotor of the rotating solar inverter to rotate under the action of the rotating magnetic field, thereby inducing three-phase AC voltage in the three-phase winding. The three-phase guide switch is composed of a three-phase distribution encoder and a three-phase distribution switch. Figure 3 shows the case where the number of poles is equal to 1.
Design two distribution switches on each winding: one connected to the positive pole of the solar cell, called a forward switch; A negative switch is connected to the negative pole of a solar cell. The three-phase distribution code disk is composed of two symmetrical 150 ° sectors, with the red one only working on the positive switch and the green one only working on the negative switch.