Technical Tips | A comprehensive overview of photovoltaic inverters

Inverter, also known as inverter power supply, is a current conversion device that converts DC power into AC power. Photovoltaic inverter is an inverter used in solar photovoltaic power generation system and is an important component in photovoltaic system. The efficiency of inverter affects the efficiency of photovoltaic power generation system, so the selection of inverter is very important. With the continuous development of technology, photovoltaic inverter will also develop towards smaller size, higher efficiency and better performance indicators.

Working principle: The core of the inverter device is the inverter switch circuit, referred to as the inverter circuit. This circuit completes the inverter function by turning on and off the power electronic switch.

Features:

(1) Requires higher efficiency.

As the current price of solar cells is relatively high, in order to maximize the use of solar cells and improve system efficiency, we must find ways to improve the efficiency of the inverter.

(2) Requires high reliability.

At present, photovoltaic power station systems are mainly used in remote areas, and many power stations are unattended and unmaintained. This requires the inverter to have a reasonable circuit structure, strict component screening, and various protection functions, such as: input DC reverse polarity protection, AC output short circuit protection, overheating, overload protection, etc.

(3) The input voltage is required to have a wide adaptability range.

Since the terminal voltage of solar cells varies with load and sunlight intensity, especially when the battery ages, the terminal voltage varies greatly. For example, the terminal voltage of a 12V battery may vary between 10V and 16V. This requires the inverter to work normally within a larger DC input voltage range.

There are many ways to classify inverters. For example, according to the number of phases of the inverter output AC voltage, it can be divided into single-phase inverters and three-phase inverters; according to the different types of semiconductor devices used in the inverter, it can be divided into transistor inverters, thyristor inverters and turn-off thyristor inverters. According to the different principles of the inverter circuit, it can also be divided into self-excited oscillation inverters, step wave superposition inverters and pulse width modulation inverters. According to whether it is used in a grid-connected system or an off-grid system, it can be divided into grid-connected inverters and off-grid inverters. In order to facilitate photovoltaic users to choose inverters, here we only classify the inverters according to the different applicable occasions.

Central Inverter

Centralized inverter technology is that several parallel photovoltaic strings are connected to the DC input of the same centralized inverter. Generally, three-phase IGBT power modules are used for high power, and field effect transistors are used for low power. At the same time, DSP conversion controllers are used to improve the quality of the produced electric energy, making it very close to sine wave current. It is generally used in large photovoltaic power stations (>10kW). The biggest feature is that the system has high power and low cost, but because the output voltage and current of different photovoltaic strings are often not completely matched (especially when the photovoltaic strings are partially blocked due to cloudy, shade, stains, etc.), the use of centralized inverters will lead to reduced efficiency of the inverter process and a decrease in electric energy. At the same time, the power generation reliability of the entire photovoltaic system is affected by the poor working condition of a photovoltaic unit group. The latest research direction is to use space vector modulation control and develop new inverter topology connections to obtain high efficiency under partial load conditions.

String inverter

The string inverter is based on the modular concept. Each photovoltaic string (1-5kw) passes through an inverter, has maximum power peak tracking on the DC side, and is parallel-connected to the grid on the AC side. It has become the most popular inverter in the international market.

Many large-scale photovoltaic power plants use string inverters. The advantage is that they are not affected by module differences and shadowing between strings, and reduce the mismatch between the optimal working point of photovoltaic modules and the inverter, thereby increasing power generation. These technical advantages not only reduce system costs, but also increase system reliability. At the same time, the concept of "master-slave" is introduced between strings, so that when the power of a single string cannot make a single inverter work, the system can connect several groups of photovoltaic strings together and let one or more of them work, thereby producing more electricity.

The latest concept is that several inverters form a "team" to replace the "master-slave" concept, which makes the system reliability a step further. At present, transformerless string inverters have become dominant.

Micro inverter

In traditional PV systems, the DC input of each string inverter is connected in series by about 10 photovoltaic panels. If one of the 10 panels in series does not work well, the entire string will be affected. If the inverter uses the same MPPT for multiple inputs, each input will also be affected, greatly reducing the power generation efficiency. In actual applications, various obstructions such as clouds, trees, chimneys, animals, dust, ice and snow can cause the above factors, which is very common. In the PV system of microinverters, each panel is connected to a microinverter. If one of the panels does not work well, only this panel will be affected. The other photovoltaic panels will operate in the best working state, making the overall system more efficient and generating more power. In actual applications, if a string inverter fails, it will cause several kilowatts of panels to fail to function, while the impact caused by the failure of a microinverter is quite small.

Power Optimizer

The installation of a power optimizer in a solar power generation system can greatly improve the conversion efficiency and simplify the inverter function to reduce costs. To realize a smart solar power generation system, installing a power optimizer can ensure that each solar cell performs at its best and monitor the battery consumption status at any time. The power optimizer is a device between the power generation system and the inverter. Its main task is to replace the original optimal power point tracking function of the inverter. The power optimizer performs extremely fast optimal power point tracking scans in an analog manner by simplifying the circuit and corresponding one power optimizer to a single solar cell, so that each solar cell can achieve optimal power point tracking. In addition, it can monitor the battery status anytime and anywhere by inserting a communication chip, and report problems immediately so that relevant personnel can repair them as soon as possible.

The inverter not only has the function of DC-AC conversion, but also has the function of maximizing the performance of solar cells and the system fault protection function. In summary, there are automatic operation and shutdown function, maximum power tracking control function, anti-single operation function (for grid-connected system), automatic voltage adjustment function (for grid-connected system), DC detection function (for grid-connected system), and DC grounding detection function (for grid-connected system). Here we briefly introduce the automatic operation and shutdown function and the maximum power tracking control function.

Application of inverter in photovoltaic system

(1) Automatic operation and shutdown function

After sunrise in the morning, the intensity of solar radiation gradually increases, and the output of the solar cell also increases accordingly. When the output power required by the inverter is reached, the inverter automatically starts to operate. After entering operation, the inverter will monitor the output of the solar cell module at all times. As long as the output power of the solar cell module is greater than the output power required by the inverter, the inverter will continue to operate until it stops at sunset. The inverter can also operate even on rainy days. When the output of the solar cell module becomes smaller and the inverter output is close to 0, the inverter will enter the standby state.

(2) Maximum power tracking control function

The output of solar cell modules varies with the intensity of solar radiation and the temperature of the solar cell modules themselves (chip temperature). In addition, since the voltage of solar cell modules decreases as the current increases, there is an optimal operating point that can obtain maximum power. The intensity of solar radiation is changing, and obviously the optimal operating point is also changing. In relation to these changes, the operating point of the solar cell module is always kept at the maximum power point, and the system always obtains the maximum power output from the solar cell module. This control is called maximum power tracking control. The biggest feature of the inverter used in the solar power generation system is that it includes the maximum power point tracking (MPPT) function.

1. Output voltage stability

2. Output voltage waveform distortion

3. Rated output frequency

4. Load power factor

5. Inverter efficiency

6. Rated output current (or rated output capacity)

7. Protective measures

8. Starting characteristics

9. Noise

When selecting an inverter, the first thing to consider is whether it has sufficient rated capacity to meet the power requirements of the equipment under maximum load. For an inverter with a single device as the load, the selection of its rated capacity is relatively simple.

When the electrical equipment is a pure resistive load or the power factor is greater than 0.9, the rated capacity of the inverter can be selected to be 1.1 to 1.15 times the capacity of the electrical equipment. At the same time, the inverter should also have the ability to resist capacitive and inductive load impacts.

For general inductive loads, such as motors, refrigerators, air conditioners, washing machines, high-power water pumps, etc., their instantaneous power may be 5 to 6 times their rated power when starting. At this time, the inverter will be subjected to a large instantaneous surge. For such systems, the rated capacity of the inverter should have sufficient margin to ensure that the load can start reliably. High-performance inverters can achieve multiple consecutive full-load starts without damaging power devices. For the sake of their own safety, small inverters sometimes need to use soft start or current limiting start.

In addition, the inverter must have a certain overload capacity. When the input voltage and output power are rated and the ambient temperature is 25°C, the inverter's continuous and reliable working time should not be less than 4 hours; when the input voltage is rated and the output power is 125% of the rated value, the inverter's safe working time should not be less than 1 minute; when the input voltage is rated and the output power is 150% of the rated value, the inverter's safe working time should not be less than 10 seconds.

Application examples:

The main load in the photovoltaic system is a 150W refrigerator. When working normally, an AC inverter with a rated capacity of 180W can work reliably. However, since the refrigerator is an inductive load, its power consumption at the moment of starting can reach 5 to 6 times the rated power. Therefore, the output power of the inverter can reach 800W when the load starts. Considering the overload capacity of the inverter, a 500W inverter can work reliably.

When there are multiple loads in the system, the selection of inverter capacity should also consider the possibility of several power loads working at the same time, that is, the "load simultaneity coefficient".

Notes on inverter installation and maintenance:

1. Before installation, you should first check whether the inverter is damaged during transportation.

2. When selecting an installation site, ensure that there is no interference from any other power electronic equipment in the surrounding area.

3. Before making electrical connections, be sure to cover the photovoltaic panels with opaque materials or disconnect the DC circuit breaker. The photovoltaic array will generate dangerous voltage if exposed to sunlight.

4. All installation operations must be completed by professional technicians only.

5. The cables used in the photovoltaic system power generation system must be firmly connected, well insulated and of appropriate specifications.

6. All electrical installations must meet local and national electrical standards.

7. The inverter can be connected to the grid only after obtaining permission from the local power department and all electrical connections have been completed by professional technicians.

8. Before performing any maintenance work, first disconnect the electrical connection between the inverter and the grid, and then disconnect the DC side electrical connection.

9. Wait for at least 5 minutes until the internal components are fully discharged before performing maintenance work.

10. Any fault that affects the safety performance of the inverter must be eliminated immediately before the inverter can be turned on again.

11. Avoid unnecessary circuit board contact.

12. Comply with electrostatic protection regulations and wear an anti-static wrist strap.

13. Pay attention to and comply with the warning signs on the product.

14. Before operation, conduct a preliminary visual inspection of the equipment to see if it is damaged or in other dangerous conditions.

15. Pay attention to the hot surface of the inverter. For example, the heat sink of the power semiconductor will remain at a high temperature for a period of time after the inverter is powered off.

Installation process of photovoltaic inverter:

For solar inverters, improving the conversion efficiency of power is an eternal topic. However, when the efficiency of the system is getting higher and higher, almost approaching 100%, further efficiency improvement will be accompanied by low cost performance. Therefore, how to maintain a very high efficiency and maintain good price competitiveness will be an important issue at present.

Compared with the efforts to improve the efficiency of the inverter, how to improve the efficiency of the entire inverter system is gradually becoming another important issue for solar energy systems. In a solar array, when a local shadow of 2~3% of the area appears, for an inverter with an MPPT function, the system output power at this time may even drop by about 20% when the power is poor! In order to better adapt to such a situation, it is very effective to use a one-to-one MPPT or multiple MPPT control functions for a single or partial solar module.

Since the inverter system is in grid-connected operation, leakage from the system to the ground will cause serious safety problems. In addition, in order to improve the efficiency of the system, most solar arrays are connected in series to form a very high DC output voltage. Therefore, DC arcs are easily generated due to abnormal conditions between electrodes. Due to the high DC voltage, it is very difficult to extinguish the arc, which can easily lead to fire. With the widespread adoption of solar inverter systems, the issue of system safety will also be an important part of inverter technology.

In addition, the power system is experiencing rapid development and popularization of smart grid technology. The grid connection of a large number of new energy power systems such as solar energy has brought new technical challenges to the stability of smart grid systems. Designing an inverter system that can be compatible with smart grids more quickly, accurately and intelligently will become a necessary condition for future solar inverter systems.

In general, the development of inverter technology is developing along with the development of power electronics technology, microelectronics technology and modern control theory. As time goes by, inverter technology is developing towards higher frequency, greater power, higher efficiency and smaller size.