Reverse the rectification circuit, connect one end to direct current (DC), and the other end can lead out alternating current (AC). This is an inverter, a device that converts direct current into alternating current.
Most commercial, industrial, and residential loads require AC power, but AC power cannot be stored in batteries, and battery storage is important for backup power. Nowadays, this defect can be overcome by a DC power supply.
The polarity of DC power does not change over time like AC power, so DC power can be stored in batteries and supercapacitors. So we can first convert AC power to DC power, and then store it in the battery. This way, whenever AC power is needed to operate AC appliances, DC power will be converted back to AC power to operate AC appliances.
According to the input source, connection method, output voltage waveform, etc. of the application, inverters are divided into the following 17 main categories.
1. Classify by input source
The input of an inverter can be a voltage source or a current source, so it is divided into voltage source inverters (VSI) and current source inverters (CSI).
Voltage Source Inverter (VSI)
When the input of the inverter is a constant DC voltage source, the inverter is called a voltage source inverter.
The input of the voltage source inverter has a rigid DC voltage source with zero impedance. In fact, the impedance of a DC voltage source can be ignored. Assuming that VSI is powered by an ideal voltage source (extremely low impedance source), the AC output voltage is entirely determined by the state of the switching devices in the inverter and the applied DC power supply.
Current Source Inverter (CSI)
When the input of the inverter is a constant DC current source, the inverter is called a current source inverter.
Rigid current is supplied from a DC power source to CSI, where the DC power source has high impedance. Usually, large inductors or closed-loop control currents are used to provide rigid currents. The resulting current wave is rigid and not affected by the load. The AC output current is completely determined by the switching devices in the inverter and the state of the DC applied power supply.
2. Classify by output phase
According to the output voltage and current phase, inverters are mainly divided into two categories: single-phase inverters and three-phase inverters.
Single phase inverter
A single-phase inverter converts DC input into single-phase output. The output voltage/current of a single-phase inverter has only one phase, and its nominal frequency is the nominal voltage of 50Hz or 60Hz.
The nominal voltage is defined as the voltage level at which the electrical system operates. There are different nominal voltages, namely 120V, 220V, 440V, 690V, 3.3KV, 6.6KV, 11kV, 33kV, 66kV, 132kV, 220kV, 400kV, and 765kV. Low nominal voltage can be directly achieved through the use of internal transformers or inverters with boost and buck circuits, while for high nominal voltage, external boost transformers are used.
Single phase inverters are used for low loads. Single phase losses are higher, and single-phase efficiency is lower than three-phase inverters. Therefore, three-phase inverters are the preferred choice for high loads.
Three phase inverter
A three-phase inverter converts direct current into three-phase power. A three-phase power supply provides three channels of AC power with uniformly separated phase angles. The amplitude and frequency of all three waves generated at the output end are the same, but slightly vary due to the load, and each wave has a phase shift of 120 degrees between each other.
Basically, a single three-phase inverter consists of three single-phase inverters, each with a phase distance of 120 degrees, and each single-phase inverter is connected to one of the three load terminals.
3. Classified by commutation technology
According to commutation technology, it can be divided into two main types: line commutation and forced commutation inverters. In addition, there can be auxiliary commutation inverters and complementary commutation inverters, but as they are not commonly used, we will briefly discuss the two main types here.
Line reversal
In these types of inverters, the line voltage of the AC circuit can be obtained through equipment; When the current in SCR experiences zero characteristics, the device is turned off. This commutation process is called line commutation, and inverters that work based on this principle are called line commutation inverters.
Forced commutation
In this type of commutation, there will be no zero point in the power supply. That's why some external sources are needed to rectify the device. This commutation process is called forced commutation, and inverters based on this process are called forced commutation inverters.
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4. Classified by connection method
According to the connection method of thyristors in the circuit, it can be divided into series inverters, parallel inverters, and bridge inverters, among which bridge inverters are further divided into half bridge, full bridge, and three-phase bridge.
Series inverter
A series inverter consists of a pair of thyristors and RLC (resistance, inductance, and capacitance) circuits. One thyristor is connected in parallel with the RLC circuit, and one thyristor is connected in series between the DC power supply and the RLC circuit. This type of inverter is called a series inverter because the load is directly connected in series with the DC power source with the help of thyristors.
Series inverters are also known as self commutation inverters because the thyristors of this type of inverter are self commutated by the load. Another name for this inverter is' load commutation inverter '. The reason for giving this name is that LCR is a load that provides commutation.
Parallel inverter
A parallel inverter consists of two thyristors, a capacitor, a center tap transformer, and an inductor. Thyristors are used to provide a path for current flow, while inductors are used to keep the current source constant. The conduction and turn off of these thyristors are controlled by the commutation capacitors connected between them.
It is called a parallel inverter because in operation, the capacitor is connected in parallel with the load through a transformer.
Half bridge inverter
A half bridge inverter requires two electronic switches to operate. Switches can be MOSFETs, IJBTs, BJTs, or thyristors. A half bridge with thyristor and BJT switches requires two additional diodes, except for pure resistive loads, while MOSFETs have built-in diodes. In short, two switches are sufficient to meet pure resistive loads, while other loads (inductors and capacitors) require two additional diodes. These diodes are called feedback diodes or freewheeling diodes.
The working principle of a half bridge inverter is the same for all switches, but here we are discussing a half bridge with thyristor switches. There are two complementary thyristors, which means conducting one thyristor at a time. For resistive loads, the circuit operates in two modes. The switching frequency will determine the output frequency. When the output frequency is 50HZ, each thyristor conducts once for 20ms.
Full bridge inverter
A single-phase full bridge inverter has four controlled switches used to control the direction of current flow in the load. This bridge has 4 feedback diodes that can feed back the energy stored in the load to the power supply. These feedback diodes only function when all thyristors are turned off and the load is not a purely resistive load.
For any load, only 2 thyristors are working at a time. Thyristors T1 and T2 will conduct in one cycle, while T3 and T4 will conduct in another cycle. In other words, when T1 and T2 are in the ON state, T3 and T4 are in the OFF state, while when T3 and T4 are in the ON state, the other two are in the OFF state. Opening two or more thyristors at once can cause a short circuit, generate excessive heat, and immediately burn out the circuit.
Three phase bridge inverter
Industrial and other heavy loads require three-phase power supply. In order to operate these heavy loads from storage devices or other DC power sources, a three-phase inverter is required. A three-phase bridge inverter can be used for this purpose.
A three-phase bridge inverter is another type of bridge inverter, consisting of 6 controlled switches and 6 diodes, as shown in the figure.
5. Classified by operating mode
According to the operating mode, inverters are divided into three main categories:
Independent inverter
The independent inverter is directly connected to the load and will not be interrupted by other power sources. Independent inverter or "off grid mode inverter", the inverter supplies power to the load independently without being affected by the grid or other power sources.
These inverters are called off grid mode inverters because they are not affected by the utility grid. These inverters cannot be connected to the utility grid because they do not have synchronization capability, where synchronization is the process of matching the phase and nominal frequency (50/60hz) of two AC power sources.
Grid connected inverter
Grid connected or grid connected inverters (GTI) have two main functions. One function of grid connected inverters is to provide AC power from storage devices (DC power sources) to AC loads, while another function of grid connected inverters is to provide additional power to the grid.
Grid connected inverters, also known as utility interactive inverters, grid interconnection inverters, or grid feedback inverters, synchronize the frequency and phase of the current to adapt to the utility grid. By increasing the voltage level of the inverter, power is transmitted from the DC power source to the utility grid.
Dual peak inverter
The dual peak inverter can operate as both a grid connected inverter and an independent inverter. These inverters can inject additional energy from renewable energy sources and storage devices into the grid, and retrieve electricity from the grid when the energy generated by renewable energy is insufficient. In other words, these inverters can operate as independent inverters and grid connected inverters according to the requirements of the load. Dual peak inverters are multifunctional, including the functions of independent inverters and grid connected inverters.
The function of a dual peak inverter will vary with the load. If there is a problem with the power grid or when the power of renewable energy is sufficient to meet the load, its function will be changed to an independent inverter (it becomes an independent inverter). In this case, the transfer switch will disconnect the inverter from the grid.
Once renewable energy begins to generate additional energy, the operating mode will shift from independent mode to grid connected mode. The inverter synchronizes its phase and frequency with the inverter and begins to inject additional energy into the grid.
6. Classify by output waveform
The ideal inverter refers to an inverter that converts DC signals into pure sinusoidal AC outputs. The problem with actual inverters is that their output signals are not purely sinusoidal. According to the output waveform, inverters are divided into three categories:
Square wave inverter
These are the simplest inverters for converting direct current to alternating current, but the output waveform is not the required pure sine wave. These inverters have square waves at the output end. In other words, these inverters convert DC input into AC in the form of square waves. Meanwhile, square wave inverters are also cheaper.
The simplest structure of these inverters can be an H-bridge inverter. As shown in the figure, using SPDT (single push double throw) switches before the transformer can achieve a simpler version. This transformer will also help achieve any desired output voltage level.
The operation of a given model is extremely simple. Simply turning on and off the switch will simultaneously change the current at the output terminal. In other words, switching single pole double throw at the desired frequency will generate AC square waves at the output of a typical inverter (i.e. center tap transformer). The harmonic distortion of a typical sine wave is about 45%, which can be further reduced by using filters to filter out some harmonics.
Quasi sine wave inverter
Quasi sine wave inverter, also known as modified sine wave inverter with stepped sine waves. In other words, the output signals of these inverters gradually increase in positive polarity. After reaching the positive peak, the output signal gradually decreases until it reaches the negative peak, as shown in the figure.
The structure of a quasi sine wave inverter is much simpler than a pure sine wave inverter, but more complex than a pure square wave inverter.
Although the final output waveform of these inverters is not a pure sine wave, the harmonic distortion of the output is still reduced to 24%. Filtering will further reduce distortion, but the amount of distortion is still significant. For this reason, these inverters are not the preferred choice for driving various loads, including electronic circuits.
Quasi sine waves may permanently damage electronic devices with timers in the circuit. If connected to a quasi sine wave inverter, all electrical appliances with motors will not work as efficiently as those connected to a pure sine wave inverter. In addition, rapid waveform transitions may cause noise. Due to these issues, the application of quasi sine wave inverters is limited.
Pure sine wave inverter
A pure sine inverter converts DC to almost pure sine AC. The output waveform of a pure sine wave inverter is still not an ideal sine wave, but it is much smoother than square wave and quasi sine wave inverters.
The output waveform of a pure sine wave inverter has extremely low harmonics. Harmonics are sine waves with odd multiples of the fundamental frequency of different amplitudes. Harmonics are very unpopular because they can cause serious problems with various electrical appliances. By using various PWM techniques and then passing the output signal through a low-pass filter, these harmonics can be further reduced.
The construction and operation of pure sine wave inverters are much more complex than square wave and modified square wave inverters.
These inverters are superior to the first two inverters because most electrical equipment requires pure sine waves to operate better. As mentioned earlier, square wave or quasi sine wave inverters can damage electrical appliances, especially those equipped with motors. Therefore, for practical use, a pure sine inverter is used.
7. Classified by the number of output levels
The output level of any inverter can be at least two or more. According to the number of output levels, inverters are divided into two categories: two-level inverters and multi-level inverters.
Two level inverter
A two-level inverter has two output levels. The output voltage alternates between positive and negative, and alternates at the fundamental frequency (50Hz or 60Hz).
Some so-called 'two-level inverters' have three levels in their output waveform. The reason for classifying three-level inverters into this category is that one of the levels is zero voltage. Actually, zero is the third level, but it is still classified as a two-stage inverter.
A two-level inverter circuit consists of a source and some switches that control current or voltage. Due to the limitations of switch losses and device ratings, the high-frequency operation of two-level inverters in high-voltage applications is restricted. However, the rated value of the switch can be increased through series and parallel combinations. The group of switches that provide a positive half cycle in a two-level inverter is called a positive group switch, while the other group of switches that provide a negative half cycle is called a negative group switch.
Due to the following reasons, a two-level inverter is not preferred. Inverters require the minimum number of switches and power sources to operate and convert power in small voltage steps. A smaller voltage step will provide high-quality waveforms. In addition, it can also reduce voltage (dv/dt) stress and electromagnetic compatibility issues on the load. Therefore, multi-level inverters are the more practical first choice.
Multi level inverter (MLI)
A multi-level inverter converts DC signals into multi-level stepped waveforms. The output waveform of a multi-level inverter is not directly positive and negative alternating, but multi-level alternating. Due to the fact that the smoothness of the waveform is directly proportional to the number of voltage levels. Therefore, multi-level inverters will produce smoother waveforms. As mentioned earlier, this characteristic makes it suitable for practical applications.
Conclusion:
This article introduces 17 main types of inverters, but in fact, there are many other classifications of inverters. For example, multi-level inverters can also be divided into flying capacitor inverters (FCMI), diode clamped inverters (DCMI), and cascaded H-bridge inverters.
From a practical application perspective, three-phase inverters are suitable for high load applications, pure sine inverters can better protect electrical appliances, and multi-level inverters are more practical choices.