An inverter is a device that converts direct current from a photovoltaic power generation system into alternating current. The output power of inverters usually comes in different capacities, such as 5 kW, 50 kW, 500 kW, etc. The quantity of photovoltaic modules is determined based on the capacity of the inverter. For example, a 1 kW inverter can connect 6-8 photovoltaic panels of around 100 watts, and this combination can achieve maximum energy utilization. Similarly, for a 350kW photovoltaic system, typically 5-7 inverters are required; Assuming we use each inverter with a rated power of 50kW, we would need at least 7 inverters to achieve a power generation of 350kW. We also need to consider the relationship between the number of inverters and operating efficiency, as well as the difficulty level of operators and technicians for these devices. Too few inverters can easily lead to single point failures, while too many inverters can also cause maintenance difficulties and high costs.
Photovoltaic inverters can be classified into the following types according to different classification methods:
Off grid inverter: also known as an independent inverter. It can supply power to the load independently without being affected by the power grid or other sources. Used in independent systems, the photovoltaic array charges the battery, and the inverter uses the DC voltage of the battery as the energy source.
Grid connected inverter: Grid connected inverters can provide AC power from storage devices to AC loads and provide additional power to the grid. The output voltage of the inverter can be fed back to a commercial AC power source, so the output sine wave needs to be in the same phase, frequency, and voltage as the power source.
Dual peak inverter: also known as backup battery inverter. A special inverter that uses a battery as its power source, coupled with a battery charger to charge the battery. If there is too much power, it will be recharged to the AC power source. Dual peak inverters can operate as both grid connected and off grid inverters.
Voltage Source Inverter (VSI): The input of the inverter is a constant DC voltage source. Suitable for occasions that require stable voltage output, such as motor drive.
Current Source Inverter (CSI): The input of the inverter is a constant DC current source. Suitable for situations with large load changes, it can provide stable current output.
Single phase inverter: converts DC input into single-phase output. The output voltage and current of a single-phase inverter have only one phase, with a nominal frequency of 50Hz or 60Hz. Commonly used in low load scenarios such as household appliances and small devices.
Three phase inverter: converts direct current into three-phase power supply. A three-phase power supply provides three intersecting and evenly separated alternating currents. All three waves generated at the output end have the same amplitude and frequency. Suitable for high load applications such as industrial equipment and large motors.
Line commutated inverter: The line voltage of the AC circuit of the inverter can be obtained through the device. When the current in the SCR experiences zero characteristics, the device is turned off. Suitable for situations that require precise control of current.
Forced commutation inverter: In this type of commutation, there will be no zero point in the power supply. Therefore, some external resources are needed to rectify the device. Suitable for situations that require external control.
Series inverter: The load is directly connected in series with the DC power supply with the help of thyristors. Suitable for situations where direct series connection of loads is required.
Parallel inverter: The conduction and turn off of thyristors are controlled by commutation capacitors connected between them. In working condition, the capacitor is connected in parallel with the load through a transformer. Suitable for situations where parallel loads are required.
Half bridge inverter: requires two electronic switches to operate. Switches can be MOSFETs, IJBTs, BJTs, or thyristors. Suitable for small and medium power applications.
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. Suitable for high-power applications.
Three phase bridge inverter: Industrial and other heavy loads require three-phase power supply.
Square wave inverter: The simplest inverter that converts direct current into alternating current, but the output waveform is not a pure sine wave, only has a square wave at the output end.
Quasi sine wave inverter: a modified sine wave inverter with stepped sine waves.
Pure sine wave inverter: Pure sine wave inverter converts DC to almost pure sine AC, and the output waveform is much smoother than square wave and quasi sine wave inverters.
Two level inverter: A two-level inverter has two output levels, with alternating positive and negative output voltages at the fundamental frequency.
Multi level inverter: A multi-level inverter converts a DC signal into a multi-level stepped waveform, and the output waveform of the multi-level inverter is not directly positive and negative alternating, but multi-level alternating.
If classified according to the connection method between photovoltaic modules and inverters, they can be divided into centralized inverters and string inverters.
Comparison of Three Major Types of Inverters
| Project | Centralized inverter | String inverter | Micro inverter |
| Centralized large-scale power station | Applicable | Applicable | Inapplicable |
| Distributed large-scale industrial and commercial rooftop power station | Applicable | Applicable | Inapplicable |
| Distributed small and medium-sized industrial and commercial rooftop power stations | Inapplicable | Applicable | Applicable |
| Distributed rooftop power station for household use | Inapplicable | Applicable | Applicable |
| Maximum power tracking corresponds to the number of components | A large number of strings | 1-4 sets of strings | Single string |
| Maximum power tracking voltage range | Narrow | Wide | Wide |
| System power generation efficiency | General | High | Highest |
| Installation land occupation | Need an independent computer room | No need | No need |
| Outdoor installation | Not allowed | Allowed | Allowed |
| Maintainability | General | Easy maintenance | Difficult to maintain |
| Compare projects | 80KW string inverter | 500KW centralized inverter |
| Combiner box | No need for combiner box, DC input is subdivided into each string | Need 12 combiner boxes for centralized convergence |
| DC wiring | Simple wiring on the DC side, distributed on-site grid connection; Short DC cable, low cost | The wiring on the DC side is relatively complex and has a long distance. If necessary, multiple levels of busbars need to be installed, which results in relatively high costs |
| AC wiring | The connection distance of the AC side cable is long, and each inverter requires an AC circuit breaker, which can be connected to the grid locally or through AC convergence | The distance from the AC side to the transformer is very short, the line loss is small, and the AC wiring is simple and cost-effective |
| Output voltage | Output three-phase AC 400V, can be connected to DC low voltage grid without the need for isolation transformers | Output three-phase AC 315V, grid connection requires the addition of a 400V isolation transformer |
| Protection level | Protection level IP65, can be installed outdoors, and can be installed nearby around the components. | The protection level is IP20, installed indoors or constructed outdoors |
| Cooling method | Intelligent air cooling | Forced air cooling requires high flow air ducts |
| Working voltage range | Wide range MPPT voltage, 200-850V, capable of generating electricity even in low light weather such as rainy days | The MPPT range is 500-820V, and the power generation range is relatively narrow |
| Efficiency | The highest efficiency is 99%, and the comprehensive efficiency is 98.65% | The maximum efficiency of a transformer without insulation is 98.0%, with a comprehensive efficiency of 97.5%. The maximum efficiency of a transformer with insulation is 97.0%, with a comprehensive efficiency of 96.5% |
| Power Quality | Single unit THD<3%, total THD of 20 units together exceeds 5%. No isolation transformer, high DC component | Single THD<3%, parallel connection of two units is about 3%, and there is no DC component when an isolation transformer is added |
| Power grid regulation | Featuring low voltage ride through, power factor adjustment, and recording of power grid faults | There is a low voltage ride through function, and the power grid can adjust the power factor. The active and reactive power functions are relatively weak |
| MPPT channels | 1 inverter with 6 MPPT channels, 1MW project with 72 MPPT channels, has advantages in multiple angles | Two MPPT channels generate high power in flat and unobstructed areas |
| Safety | Without DC circuit breakers and AC circuit breakers, the safety is slightly inferior | There are DC circuit breakers and AC circuit breakers that can be disconnected according to different fault conditions, with good safety |
| Number | System capacity | Inverter selection | Description |
| 1 | Below 400KW | String inverter | For systems below 400kw, the cost difference between string inverters and centralized inverters is not significant, but string inverters generate 5% to 10% more electricity |
| 2 | 400KW to 2MW | String inverter | For systems between 400KW and 2MW, the cost of string inverters is 5% higher than centralized inverters, but the power generation of string inverters is 5% to 10% higher. The total revenue of the string inverter system is good |
| 3 | 2MW to 6MW | Centralized inverters are used for ground power stations with uniform sunlight, and string type inverters are used for roofs | Select according to the actual installation site |
| 4 | Above 6MW | Centralized inverter | Centralized inverters can meet the requirements of the power grid |
Centralized inverter: It is used to convert the direct current of multiple photovoltaic strings into a total, and is suitable for large photovoltaic power plants, such as large factories, desert power plants, ground power plants, etc. Its characteristics include: high power, with a single capacity generally above 500KW, suitable for large photovoltaic power stations. High power quality, low harmonic content, high power quality, complete protection functions, and high safety. Convenient management, with a small number of inverters, easy to manage, few components, and good stability. The main disadvantages are: the MPPT voltage range is narrow, and the component configuration is not flexible; Need a dedicated computer room, installation is not flexible; The self power consumption and ventilation and heat dissipation in the computer room consume a lot of electricity.
String inverter: It is used to invert the DC power of each photovoltaic string separately, and is suitable for small and medium-sized photovoltaic systems, small and medium-sized rooftop photovoltaic power generation systems, and small ground power stations. Its characteristics include: low power, with individual power generally below 100KW, but with technological progress, currently more mature power can reach 350kW. Flexible component configuration, wide MPPT voltage range, more flexible component configuration, suitable for various lighting conditions. High power generation efficiency, unaffected by module differences and shading between strings, maximizing power generation. Small size, small footprint, no need for dedicated computer room, flexible installation. Easy maintenance: low self power consumption, minimal impact of faults, and convenient maintenance. The relationship between inverter voltage and component quantity configuration should start from the principle of the inverter and be discussed as follows:
A string inverter needs to boost and stabilize the DC voltage to a certain value (this is called DC bus voltage) before it can be converted to AC power. 230V AC output, with a DC bus voltage of around 360V; 400V AC output, with a DC bus voltage of around 600V; 500V AC output, with a DC bus voltage of around 750V; 540V AC output, with a DC bus voltage of around 800V. But the series voltage of the components is generally not so high and needs to be adjusted by the circuit. Inverters generally use PWM to adjust, and there is a term called duty cycle, which is equal to the series voltage of the components/DC bus voltage. Duty cycle is closely related to efficiency. The larger the duty cycle, the smaller the voltage difference, and the higher the efficiency. By mastering this secret, there is no need to calculate complex formulas when matching components with inverters. Try to match the string voltage around the rated operating voltage of the inverter for the highest efficiency, and it will not exceed the maximum voltage at extreme low temperatures. During operation, it will also be within the full load MPPT voltage range, which is absolutely simple and practical. Taking a single crystal 450W module as an example, with a working voltage of 41V, a single-phase 220V inverter, an input rated voltage of 360V, and the best configuration with 9 modules; A three-phase 400V output inverter with a rated input voltage of 600V, preferably equipped with 15 components.