Converting alternating current AC into direct current DC is called rectification, and the circuit that completes rectification is called rectification circuit; while the conversion of direct current DC into alternating current AC is called inverter, and the circuit that completes the inverter function is called inverter circuit; The device is called an inverter. Since most electrical appliances are designed according to AC circuits, various DC/AC inverters are already common components in photovoltaic power generation systems. The early DC/AC inverter was realized by an AC motor with a DC generator, and the modern inverter technology has been based on power electronics technology, semiconductor material and device technology, modern control technology, and pulse width modulation (PWM) technology. , industrial electronic technology and other disciplines above the comprehensive technology.
The inverter matching with solar photovoltaic power generation not only needs 50Hz, 220V AC to adapt to most electrical loads, but by choosing different inverters, you can achieve free output current (voltage) selection waveform, frequency and frequency. The amplitude can achieve various goals such as energy saving, material saving, high efficiency, safety and environmental protection. The main uses are:
①Non-power frequency AC load, such as electromagnetic induction heating, long-wave communication, power ultrasonic application, EDM, negative ion generator, etc.; AC motor frequency conversion speed regulation, including SCR frequency conversion speed regulation, GTR frequency conversion speed regulation and IGBT frequency conversion speed regulation, etc. ;
③The application of high-frequency internal modulation, such as UPS uninterruptible power supply, emergency light power supply, high-frequency electronic rectifier, fast charger, rectifier furnace machine, high-voltage DC power supply and laser power supply, etc.;
④Special inverter power supply, such as grid-connected wind power generation, solar power generation system, motor braking regenerative energy feedback active inverter system, etc.
A typical DC/AC inverter is mainly composed of two parts: semiconductor power integrated devices and inverter circuits.
- Semiconductor power integrated devices
The development of high-power semiconductor switching devices is the basis for the rapid development of inverters. It has been a leap from ordinary thyristors (SCR, commonly known as thyristors) to turn-off transistors (GTO) and high-power transistors (GTR), but the power field New devices such as effect transistors (VMOSFETs) and insulated gate transistors (IGBTs) increase the choice of inverter components, while more new types of electrostatic induction transistors (SIT), electrostatic induction thyristors (SITH), MOS control transistors (MGT) The emergence of high-power devices such as MOS controlled thyristor (MCT) and intelligent power module (IPM) has made the power electronic switching devices available for inverters to be high-frequency, energy-saving, fully controlled, integrated and multi-functional. functional development. Figure 1 shows the classification of power electronic switching devices for inverters.
- Inverter circuit
The core of the inverter system is the inverter switch circuit, referred to as the inverter circuit, which completes the inverter function by turning on and off the semiconductor switching device. However, in addition to the main inverter circuit, a complete inverter circuit must have It consists of control circuit, input circuit, output circuit, auxiliary circuit and protection circuit.
(1) Input circuit Provide the DC voltage to the main inverter circuit to ensure its normal operation.
(2) Output circuit Correct, compensate and adjust the quality of AC power output by the main inverter circuit (including waveform, frequency, electricity, voltage, current amplitude and phase, etc.) to meet user requirements.
(3) Control circuit Provide a series of control pulses to the main inverter circuit to control the on and off of the inverter switch tube, and cooperate with the main inverter circuit to complete the inverter function. In the inverter circuit, the control circuit is as important as the main inverter circuit.
(4) Auxiliary circuit The input voltage is converted into a DC voltage suitable for the work of the control circuit, including a variety of detection circuits.
(5) Protection circuit Input overvoltage and undervoltage protection; output overvoltage and undervoltage protection; overload protection; overcurrent and short circuit protection; overheating protection, etc.
(6) Main inverter circuit The conversion circuit composed of semiconductor switching devices is divided into two categories: isolated and non-isolated. For example, frequency converters, energy feedback, etc. are all non-isolated; UPS, communication basic switching power supplies, etc. are isolated inverter circuits, and the isolated inverter circuit should also include an inverter. Whether it is an isolated or non-isolated main inverter circuit, it is basically a combination of two different topological forms of the boost circuit Buck and the buck circuit Boost. These combinations constitute single-ended (both forward and flyback), push-pull, half-bridge and full-bridge types in the main circuit of the isolated inverter. These circuits can form either single-phase inverters or three-phase inverters.
- PWM pulse width modulation technology
To effectively realize the power conversion in the inverter technology, it is often necessary to consider the semiconductor switching device, the main circuit and the control circuit as a whole. In the early stage, the RLC resonant circuit composed of the basic load resistance-capacitance-inductor was used for power conversion during the resonance process, which was called PFM conversion technology. PFM conversion has the advantages of low switching loss, low electromagnetic interference, high frequency and high efficiency, but it has disadvantages such as high voltage and current ratings of the switching tube, and difficulty in output filtering. With the development of semiconductor switching devices, a new and simple power conversion technology, namely pulse width modulation, called PWM conversion technology, is used for power conversion, which can effectively suppress harmonics and have a good dynamic response.
3.1 The basic principle of PWM conversion technology
The PWM conversion circuit can have a resistive load or an inductive load, but its working conditions are different with these two loads. Their basic circuit forms are shown in Figure 2. RL, and L represent resistive load and inductive load respectively; Q is the switch tube, D is the freewheeling diode; Vd is the DC input power supply voltage; VG is the voltage of the drive control signal of the inverter main switch tube; VT is the inverter main switch The terminal voltage of the tube.
The circuit of PWM conversion is relatively simple, the technology is mature, and the application is very common, and its working waveform is shown in Figure 3.
With resistive load
When the inductive load is used, the current changes. Within TON, there is . When L is very large, it can be considered that IT is basically unchanged, IT=Im.
The basic working principle of PWM conversion is to fix the operating frequency of the inverter and adjust the time ratio δ of the switching tube conduction, so as to adjust the size of the output (voltage and current supplied to the load) or stabilize the output.
From Figure 4, the duty cycle δ can be written as:
It should be pointed out that the above 8 is the on-duty ratio of the switch. For the power conversion circuit of single-ended operation, although δ≤0.5, it is equal to the duty cycle of power transmission; while for the power conversion circuit of double-ended operation, of course, δ≤0.5, but it is equal to half of the duty cycle of power transmission , the maximum duty cycle of power transfer can be approximately equal to 1.
3.2 Features and applications of PWM conversion
PWM conversion can be used in almost all inverter main circuit topologies, such as single-ended, push-pull, half-bridge, full-bridge, buck, boost, buck-boost, isolated, non-isolated type, single-phase inverter, three-phase inverter, passive inverter, source inverter, square wave output inverter, sine output inverter, GTO inverter, GTR inverter, VMOSFET inverter, IGBT inverter, etc. PWM control can be used.
- Sine wave PWM technology
Sine wave PWM technology, also known as SPWM technology, is more common in use. Single-phase SPWM is relatively simple and generally has two forms, as shown in Figure 4.
(1) Unipolar SPWM waveform After the unipolar sine wave is formed into unipolar, the pulse train is obtained by comparing it with the unipolar symmetrical triangular wave, and then the inverted signal is used to invert the phase, as shown in Figure 4.
(2) Bipolar SPWM waveform The bipolar SPWM waveform can be obtained by directly comparing the sine wave with the bipolar symmetrical triangular wave, as shown in Figure 4(b).
(3) Three-phase SPWM waveform The three-phase SPWM waveform also has two kinds of unipolar and bipolar, and the method of generation is similar to that of single-phase SPWM. One form of the three-phase SPWM waveform is shown in Figure 5. In the figure, eT is a triangular wave signal, while eRa, eRb, and eRc are three-phase sinusoidal modulation signals; EAO, EBO, and ECO are SPWM waveforms of three-phase A, B, and C, respectively. Obviously, they are all bipolar.
(4) Purpose of SPWM Single-phase sine wave SPWM is mainly used in single-phase DC/AC inverter and UPS. Three-phase sine wave SPWM is mainly used in three-phase inverters, three-phase UPS and three-phase active inverter systems, such as grid-connected photovoltaic power generation systems, grid-connected wind power generation systems, etc.
With the development of large-scale integrated circuit manufacturing technology, the early PWM technology of discrete component assembly has been eliminated, and replaced by perfect overvoltage, overcurrent, undervoltage protection circuits and integration of program control, remote control and synchronous operation functions SPMW chip. An intelligent micro-miniature inverter mainly composed of SPWM chips can be directly installed on the back of photovoltaic modules, which can directly convert the DC power emitted by the photovoltaic modules into standard sine wave AC power and connect it to the grid. It provides convenience for solar photovoltaic building integration and reducing the loss of photovoltaic module interconnection and assembly.
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