Classification of inverters and evaluation of their main technical performance

There are many types of inverters, which can be classified in different ways.

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 50-60Hz; the frequency of medium frequency inverter is generally 400Hz to more than 10KHz; the frequency of high frequency inverter is generally more than 10KHz 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 of self-shutdown, and the components lose control after being turned on, so it is called "semi-controlled". Ordinary thyristors belong to this category; the latter has the ability of self-shutdown, that is, the on and 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-commutated inverter and self-commutated inverter.

Main technical performance and evaluation of inverters

1. Technical performance

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 the reverse sequence component to the 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, and its deviation should be within ±1% under normal working conditions.

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 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.

II. Evaluation

In order to correctly select the inverter for the photovoltaic power generation system, the technical performance of the inverter should be evaluated. According to the impact of the inverter on the operating characteristics of the main off-grid photovoltaic power generation system and the requirements of the photovoltaic power generation system for the inverter performance, the evaluation contents are as follows:

1. Rated output capacity

Characterizes the inverter's ability to supply power to the load. An inverter with a high rated output capacity can carry more power loads. However, when the inverter's load is not purely resistive, that is, when the output power is less than 1, the inverter's load capacity will be less than the given rated output capacity value.

2. Output voltage stability

Characterizes the voltage regulation capability of the inverter output voltage. Most inverter products give the deviation of the inverter output voltage within the allowable fluctuation range of the input DC voltage, usually called the voltage regulation rate. High-performance inverters should also give the deviation of the inverter output voltage when the load changes from 0% to 100%, usually called the load regulation rate. The voltage regulation rate of a good-performance inverter should be ≤±3%, and the load regulation rate should be ≤±6%.

3. Overall efficiency

Characterizes the size of the inverter's own power loss, usually expressed in %. Inverters with larger capacity should also give full-load efficiency values ​​and low-load efficiency values. The efficiency of inverters below kW level should be 80% to 85%, and the efficiency of 10kW inverters should be 85% to 90%. The efficiency of the inverter has an important impact on increasing the effective power generation and reducing the power generation cost of the photovoltaic power generation system.

4. Protection function

Overvoltage, overcurrent and short circuit protection are the most basic measures to ensure the safe operation of the inverter. The perfect sine wave inverter also has undervoltage protection, phase loss protection and temperature limit alarm.

5. Starting performance

The inverter should ensure reliable starting under rated load. High-performance inverters can achieve multiple consecutive full-load starts without damaging power devices. Small inverters sometimes use soft starting or current limiting starting for their own safety.

For high-power photovoltaic power generation systems and grid-connected photovoltaic power generation system inverters, technical performance such as waveform distortion and noise level are also very important.

When selecting an inverter for an off-grid photovoltaic power generation system, in addition to the above five basic evaluation items, you should also pay attention to the following points:

(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 "load simultaneity factor".

(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 over-current 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 over-current or short-circuit due to load failure, human error, external interference, etc.

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 it to fail due to component failure. In addition to the manufacturer's need to have 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.