In-depth summary of the working principle of photovoltaic inverter

Understanding the working principle of the inverter is the core of the entire inverter research and development and production process, and is directly related to the inverter conversion efficiency. For this reason, the photovoltaic circle, manufacturers and users are very concerned about this. There are too many answers on the Internet about the working principle of the inverter. In order to let everyone have a complete understanding of the working principle of the inverter, Omnik has made a detailed summary based on many years of technical experience, hoping that it will be of some help to friends who are concerned.

Understanding the concept of inverter

The process of converting AC power into DC power is called rectification, the circuit that completes the rectification function is called a rectification circuit, and the device that implements the rectification process is called a rectification device or a rectifier. Correspondingly, the process of converting DC power into AC power is called inversion, the circuit that completes the inversion function is called an inverter circuit, and the device that implements the inversion process is called an inverter device or an inverter.

Detailed explanation of inverter classification

1. According to the frequency of the inverter outputting AC power, it can be divided into industrial frequency inverter, medium frequency inverter and high frequency inverter. The frequency of industrial frequency inverter is 50-60 Hz; the frequency of medium frequency inverter is generally 400 Hz to more than 10 kHz; the frequency of high frequency inverter is generally more than 10 kHz to MHz.

2. According to the number of phases of the inverter output, it can be divided into single-phase inverter, three-phase inverter and multi-phase inverter.

3. According to the destination of the power output by the inverter, it can be divided into active inverter and passive inverter. Any inverter that transmits the power output of the inverter to the industrial power grid is called an active inverter; any inverter that transmits the power output of the inverter to a certain power load is called a passive inverter.

4. According to the form of the inverter main circuit, it can be divided into single-ended inverter, push-pull inverter, half-bridge inverter and full-bridge inverter.

5. According to the type of the main switch device of the inverter, it can be divided into thyristor inverter, transistor inverter, field effect inverter and insulated gate bipolar transistor (IGBT) inverter, etc. It can also be summarized into two categories: "semi-controlled" inverter and "full-controlled" inverter. The former does not have the ability to self-turn off, and the components lose control after turning on, so it is called "semi-controlled". Ordinary thyristors belong to this category; the latter has the ability to self-turn off, that is, the turn-on and turn-off of the device can be controlled by the control electrode, so it is called "full-controlled". Power field effect transistors and insulated gate bipolar transistors (IGBT) belong to this category.

6. According to the DC power supply, it can be divided into voltage source inverter (VSI) and current source inverter (CSI). In the former, the DC voltage is nearly constant and the output voltage is an alternating square wave; in the latter, the DC current is nearly constant and the output current is an alternating square wave.

7. According to the waveform of the inverter output voltage or current, it can be divided into sinusoidal wave output inverter and non-sinusoidal wave output inverter.

8. According to the inverter control method, it can be divided into frequency modulation (PFM) inverter and pulse width modulation (PWM) inverter.

9. According to the working mode of the inverter switching circuit, it can be divided into resonant inverter, fixed frequency hard switching inverter and fixed frequency soft switching inverter.

10. According to the inverter commutation method, it can be divided into load commutation inverter and self-commutation inverter.

The basic structure of the inverter

The direct function of the inverter is to convert DC power into AC power. The core of the inverter 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. The on and off of the power electronic switch device requires a certain driving pulse, which may be adjusted by changing a voltage signal. The circuit that generates and adjusts the pulse is usually called a control circuit or control loop. The basic structure of the inverter device, in addition to the above-mentioned inverter circuit and control circuit, also includes a protection circuit, an output circuit, an input circuit, an output circuit, etc.

Working Principle of Inverter

1. Working principle of full-controlled inverter: It is a commonly used single-phase output full-bridge inverter main circuit, and the AC components use IGBT tubes Q11, Q12, Q13, and Q14. The conduction or cutoff of the IGBT tube is controlled by PWM pulse width modulation.

When the inverter circuit is connected to the DC power supply, Q11 and Q14 are turned on first, and Q1 and Q13 are turned off. Then the current is output from the positive pole of the DC power supply, and returns to the negative pole of the power supply through Q11, L or inductor, and the primary coil of the transformer (Figure 1-2) to Q14. When Q11 and Q14 are turned off, Q12 and Q13 are turned on, and the current returns to the negative pole of the power supply from the positive pole of the power supply through Q13, the primary coil of the transformer 2-1 inductor to Q12. At this time, positive and negative alternating square waves have been formed on the primary coil of the transformer. Using high-frequency PWM control, two pairs of IGBT tubes are alternately repeated to generate AC voltage on the transformer. Due to the action of the LC AC filter, a sinusoidal AC voltage is formed at the output end.

When Q11 and Q14 are turned off, in order to release the stored energy, diodes D11 and D12 are connected in parallel at the IGBT to return the energy to the DC power supply.

2. Working principle of semi-controlled inverter: Semi-controlled inverter uses thyristor components. The main circuit of the improved parallel inverter is shown in Figure 4. In the figure, Th1 and Th2 are thyristors working alternately. If Th1 is triggered and turned on first, the current flows through Th1 through the transformer. At the same time, due to the inductive effect of the transformer, the commutation capacitor C is charged to twice the power supply voltage. Then Th2 is triggered and turned on. Because the anode of Th2 is reverse biased, Th1 is cut off and returns to the blocking state. In this way, Th1 and Th2 commutate, and then capacitor C is charged with reverse polarity. In this way, the thyristors are triggered alternately, and the current flows alternately to the primary of the transformer, and AC power is obtained at the secondary of the transformer.

In the circuit, the inductor L can limit the discharge current of the commutation capacitor C, prolong the discharge time, and ensure that the circuit shutdown time is greater than the thyristor shutdown time without the need for a large-capacity capacitor. D1 and D2 are two feedback diodes that can release the energy in the inductor L and send the remaining commutation energy back to the power supply to complete the energy feedback function.

Main technical performance of inverter

1. Rated output voltage

It indicates the rated voltage value that the inverter should be able to output within the specified allowable fluctuation range of the input DC voltage. The stability accuracy of the output rated voltage value is generally specified as follows:

(1) During steady-state operation, the voltage fluctuation range should be limited, for example, its deviation should not exceed ±3% or ±5% of the rated value.

(2) Under dynamic conditions with sudden load changes (rated load 0% → 50% → 100%) or other interference factors, the output voltage deviation should not exceed ±8% or ±10% of the rated value.

2. Output voltage imbalance

Under normal working conditions, the three-phase voltage imbalance (the ratio of reverse sequence component to positive sequence component) output by the inverter should not exceed a specified value, generally expressed in %, such as 5% or 8%.

3. Output voltage waveform distortion

When the inverter output voltage is sinusoidal, the maximum allowable waveform distortion (or harmonic content) should be specified. It is usually expressed as the total waveform distortion of the output voltage, and its value should not exceed 5% (10% is allowed for single-phase output).

4. Rated output frequency

The frequency of the inverter output AC voltage should be a relatively stable value, usually 50Hz. Under normal working conditions, its deviation should be within ±1%.

5. Load power factor

Characterizes the inverter's ability to carry inductive or capacitive loads. Under sine wave conditions, the load power factor is 0.7 to 0.9 (lagging), and the rated value is 0.9.

6. Rated output current (or rated output capacity)

Indicates the rated output current of the inverter within the specified load power factor range. Some inverter products give the rated output capacity, which is expressed in VA or kVA. The rated capacity of the inverter is the product of the rated output voltage and the rated output current when the output power factor is 1 (i.e. pure resistive load).

7. Rated output efficiency

The efficiency of the inverter is the ratio of its output power to its input power under specified working conditions, expressed in %. The efficiency of the inverter at rated output capacity is the full load efficiency, and the efficiency at 10% of the rated output capacity is the low load efficiency. 8. Protection

(1) Overvoltage protection: For inverters without voltage stabilization measures, output overvoltage protection measures should be taken to protect the load from damage caused by output overvoltage.

(2) Overcurrent protection: The inverter's overcurrent protection should be able to ensure timely action when a short circuit occurs in the load or the current exceeds the allowable value, thereby protecting it from damage caused by surge current.

9. Starting characteristics

Characterizes the inverter's ability to start with load and its performance during dynamic operation. The inverter should ensure reliable starting under rated load.

10. Noise

Transformers, filter inductors, electromagnetic switches, fans and other components in power electronic equipment will generate noise. When the inverter is operating normally, its noise should not exceed 80dB, and the noise of a small inverter should not exceed 65dB.

The technical performance of the inverters for high-power photovoltaic power generation systems and grid-connected photovoltaic power generation systems, such as waveform distortion and noise level, is also very important. When selecting inverters for off-grid photovoltaic power generation systems, the following points should also be noted:

(1) It should have sufficient rated output capacity and load capacity. 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 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 as 1.1 to 1.15 times the capacity of the electrical equipment. When the inverter is loaded with multiple devices, the selection of the inverter capacity should consider the possibility of several electrical equipment working at the same time, that is, the "simultaneous load coefficient".

(2) It should have high voltage stability. In off-grid photovoltaic power generation systems, batteries are used as energy storage devices. When a battery with a nominal voltage of 12V is in a floating charge state, the terminal voltage can reach 13.5V, and a short-term overcharge state can reach 15V. When the battery is discharged with load, the terminal voltage can drop to 10.5V or lower. The fluctuation of the battery terminal voltage can reach about 30% of the nominal voltage. This requires the inverter to have good voltage regulation performance to ensure that the photovoltaic power generation system is powered by a stable AC voltage.

(3) High or relatively high efficiency under various loads. High overall efficiency is a significant feature of photovoltaic power inverters that distinguishes them from general-purpose inverters. The actual efficiency of a 10kW-class general-purpose inverter is only 70% to 80%. When it is used in a photovoltaic power generation system, it will cause an energy loss of 20% to 30% of the total power generation. Therefore, in the design of inverters dedicated to photovoltaic power generation systems, special attention should be paid to reducing their own power loss and improving overall efficiency. Therefore, this is an important measure to improve the technical and economic indicators of photovoltaic power generation systems. The requirements for photovoltaic power generation inverters in terms of overall efficiency are: rated load efficiency of inverters below kW ≥ 80% to 85%, low load efficiency ≥ 65% to 75%; rated load efficiency of 10kW-class inverters ≥ 85% to 90%, low load efficiency ≥ 70% to 80%.

(4) It should have good overcurrent protection and short-circuit protection functions. During the normal operation of the photovoltaic power generation system, it is entirely possible that the power supply system will have overcurrent or short circuit due to load failure, personnel misoperation and external interference. The inverter is most sensitive to overcurrent and short circuit in the external circuit and is the weak link in the photovoltaic power generation system. Therefore, when selecting an inverter, it must have good self-protection functions against overcurrent and short circuit.

(5) Easy maintenance. After a high-quality inverter has been in operation for several years, it is normal for the inverter to fail due to component failure. In addition to the manufacturer's need for a good after-sales service system, the manufacturer is also required to have good maintainability in terms of inverter production process, structure and component selection. For example, there are sufficient spare parts for damaged components or they are easy to buy, and the components are interchangeable; in terms of process structure, the components are easy to disassemble and replace. In this way, even if the inverter fails, it can quickly return to normal.

Inverter use, maintenance and repair

use

1. Connect and install the equipment in strict accordance with the requirements of the inverter operation and maintenance manual. During installation, carefully check whether the wire diameter meets the requirements; whether the components and terminals are loose during transportation; whether the insulation is good; and whether the grounding of the system meets the requirements.

2. The inverter should be operated and used strictly in accordance with the regulations in the inverter operation and maintenance manual. In particular: before starting the inverter, pay attention to whether the input voltage is normal; during operation, pay attention to whether the order of power on and off is correct, and whether the indications of each meter and indicator light are normal.

3. Inverters generally have automatic protection for disconnection, overcurrent, overvoltage, overheating, etc., so when these phenomena occur, there is no need to shut down the inverter manually; the protection points of automatic protection are generally set at the factory and do not need to be adjusted.

4. There is high voltage inside the inverter cabinet. Operators are generally not allowed to open the cabinet door, which should be locked at ordinary times.

5. When the room temperature exceeds 30°C, heat dissipation and cooling measures should be taken to prevent equipment failure and extend the service life of the equipment.

Maintenance and repair

1. Check the connections of the inverter regularly to see if they are secure and loose. In particular, check the fan, power module, input terminal, output terminal, and grounding.

2. Once the inverter is shut down due to an alarm, do not restart it immediately. Find out the cause and repair it before restarting it. The inspection should be carried out strictly according to the steps specified in the inverter maintenance manual.

3. Operators must be specially trained and be able to identify the causes of common faults and eliminate them, such as being able to expertly replace fuses, components, and damaged circuit boards. Untrained personnel are not allowed to operate or use the equipment.

4. If an accident that is difficult to eliminate occurs or the cause of the accident is unclear, a detailed record of the accident should be kept and the inverter manufacturer should be notified in a timely manner for resolution.