An inverter is a type of power electronic device used to convert electricity from one form to another, such as direct current to alternating current, at the necessary frequency and voltage.
It can be classified according to the power supply and the related topology in the power supply circuit, mainly divided into VSI (voltage source inverter), also known as three phase inverter and CSI (current source inverter) two types. It's the difference between voltage vs current.
Among them, the VSI inverter has a DC voltage source with a small impedance at the input, while the CSI inverter has a high impedance DC current source. In this article, we will describe the working principle and application characteristics of a simple type of VSI inverter, that is, a three phase inverter.
1. Basic concept of three phase inverter
As we all know, inverters can convert direct current to alternating current, like power inverter 1000w. If it is used to change the DC voltage into a three phase AC power source, it is called a three phase inverter. Typically, these are used in high power and variable frequency drive applications such as HVDC transmission.
In a three phase inverter, electricity can be transmitted over the network with the help of three different currents that are out of phase with each other. In single-phase inverters, electricity can be transmitted through a single phase. For example, if there are three phase connections in the home energy storage, the inverter can be connected to one of the phases.
2. The working principle of a three phase inverter
The working principle of the three phase inverter is that it includes three single-phase inverter switches, each of which can be connected to the load side. For basic control systems, the operation of the three switches can be synchronized so that a single switch operates at every 60 degrees of the basic O/P waveform, creating an O/P waveform that includes six line-to-line steps.
This waveform includes a zero voltage stage between the positive and negative parts of the square wave. Once carrier-based PWM techniques are applied to these waveforms, the basic shape of the waveform can be taken, cancelling the third harmonic including its multiple. In fact, both single-phase and three phase inverters include 180 degrees and 120 degrees.
3. The main applications of three phase inverters
The application of a three phase inverter includes the following aspects:
- Variable frequency drive.
- High power such as high voltage direct current transmission.
- Three phase square wave inverters are used in UPS circuits.
- Low-cost solid-state inverter charger circuit.
4. Single phase inverter vs three phase inverter
Energy is the cornerstone of human social development, and inverter is one of the core technologies for building a new power energy conversion system. Single-phase inverters and three phase inverters have their own characteristics, each with its own advantages and disadvantages.
Photovoltaic power generation systems are generally divided into single-phase grid-connected power generation systems and three phase grid-connected power generation systems.
Generally speaking, systems less than 5KW generally use single-phase grid-connected systems, and systems larger than 5KW generally use three phase grid-connected systems. What is the difference between a single-phase inverter and a three phase inverter?
5. What is the phase in the inverter
We need to first understand what "phase" is. In power systems, "phase" refers to three phase live wires (A, B, C, or L1, L2, L3), and transformers usually output three live wires and one neutral wire (N wire). Note that "live wire" here refers to wires with a large current and the potential to be live, which are usually used for power transmission.
The phase in inverter
The "neutral wire" is the wire connected to the ground and used to return the current. In addition, some transformers also lead out a ground wire (PE), which is to ensure the safe grounding of the equipment and further protect personal safety.
6. What is the wire in the inverter
After talking about "phase", let's talk about "line", there are the following types of common ones:
- Single-phase double wire: one live wire and one zero wire.
- Single-phase three wires: one live wire plus one zero wire and one ground wire.
- Three phase three-wire (TT system): three live wires, mostly lines above 10KV.
- Three phase four-wire (TN-C system): three live wires and one neutral wire, mainly 380V/220V (400V/230V) low-voltage lines. The working neutral line N doubles as the grounding protection line PE, which can be called the protective neutral line, expressed by NPE, and is suitable for electricity places with basically balanced three phase loads.
- Three phase five-wire (TN-S system): three live wires + one zero wire + one ground wire, the working zero line N and the special protection line PE are strictly separated power supply system, used in places with high safety requirements and grounding requirements for electrical equipment.
Under normal circumstances, single-phase grid-connected inverters are incorporated into single-phase two-wire, single-phase three-wire power grid lines. The three phase grid-connected inverter is integrated into the three phase four-wire and three phase five-wire power grid lines.
In addition, there is a medium and high voltage grid-connected three phase inverter, such as 480V/800V grid-connected, which needs to increase the corresponding step-up transformer connection.
7. What are single phase and three phase
Single-phase inverter is to convert direct current inverter into AC output, single-phase inverter is to convert the output AC voltage to single-phase, such as AC 220V or 230V. There are three jacks at the interface of the single-phase inverter, which are marked "N", "L" and "PE".
- L indicates a live wire (marked with the letter 'L' Live Wire) with a red or brown line.
- N indicates the neutral line (marked with the letter 'N' Neutral wire) with a blue or white line.
- PE stands for ground (marked with the letter 'E' Earth) with yellow-green lines.
Three phase inverter is to convert the output AC voltage is three phase, such as AC 380V or 400V, three phase electricity is composed of three AC potentials with the same frequency, equal amplitude, and phase difference of 120°. The 3000w inverter can easily bring home appliances.
Simply summarized: 5-hole interface, 400V voltage level. This is why when installing a three phase inverter, you often hear names such as three phase four-wire, three phase five-wire, etc.
8. How to choose the inverter specifically
Starting from the application scenario of distributed pv system to match the inverter, the appropriate inverter can play the greatest role according to local conditions. How to choose a specific model? Single phase or triple phase?
Depends on the capacity of the grid and system.
- Home roof or courtyard, small installed capacity, generally choose single-phase string inverter.
- When the roof area is large, if the grid has three phases, it is recommended to choose a small three phase inverter.
- Industrial and commercial rooftops, complex mountain and greenhouse projects.
- In the case of irregular orientation, local occlusion, large installed capacity, low voltage or medium voltage and multiple grid-connected voltages, a large three phase multi-channel MPPT string inverter is generally selected.
- For large ground and desert power stations, it is recommended to choose large string or centralized inverters.
9. ConclusionThe above is about the working principle, conduction mode and application characteristics of the three phase inverter. Three phase inverters are generally used to convert DC to AC output, including three arms, typically delayed by an angle of 120° to produce three phase AC power. In addition, its switches have a 50% ratio.