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Advanced understanding and future development trend of grid-connected inverter(2)

Jan 06, 2021

Advanced understanding and future development trend of grid-connected inverter(2)

Main technical indicators of photovoltaic inverters

1. Stability of output voltage

In the photovoltaic system, the electric energy generated by the solar cell is stored by the battery, and then converted into 220V or 380V alternating current through the inverter. However, the battery is affected by its own charging and discharging, and its output voltage varies widely. For example, the nominal 12V battery can vary from 10.8 to 14.4V (exceeding this range may cause damage to the battery). For a qualified inverter, when the input terminal voltage changes within this range, the change in its steady-state output voltage should not exceed Plusmn; 5% of the rated value. At the same time, when the load changes suddenly, its output voltage deviation should not It exceeds ±10% of the rated value.

2. Waveform distortion of output voltage

For sine wave inverters, the maximum allowable waveform distortion (or harmonic content) should be specified. Usually expressed by the total waveform distortion of the output voltage, its value should not exceed 5% (single-phase output allows l0%). Since the high-order harmonic current output by the inverter will produce additional losses such as eddy currents on the inductive load, if the inverter waveform distortion is too large, it will cause serious heating of the load components, which is not conducive to the safety of electrical equipment and seriously affects the system Operating efficiency.

3. Rated output frequency

For loads that include motors, such as washing machines, refrigerators, etc., because the motor's best frequency operating point is 50Hz, too high or too low a frequency will cause the equipment to heat up and reduce the operating efficiency and service life of the system. The output frequency should be a relatively stable value, usually 50Hz, and its deviation should be within Plusmn;l% under normal working conditions.

4. Load power factor

Characterize the inverter's ability to carry inductive load or capacitive load. The load power factor of the sine wave inverter is 0.7 to 0.9, and the rated value is 0.9. In the case of a certain load power, if the power factor of the inverter is low, the capacity of the inverter required will increase. On the one hand, it will increase the cost. At the same time, the apparent power of the AC circuit of the photovoltaic system will increase. As the current increases, losses will inevitably increase, and system efficiency will also decrease.

5. Inverter efficiency

The efficiency of the inverter refers to the ratio of its output power to the input power under specified working conditions, expressed as a percentage. In general, the nominal efficiency of a photovoltaic inverter refers to a purely resistive load, at 80% load s efficiency. As the overall cost of the photovoltaic system is relatively high, the efficiency of the photovoltaic inverter should be maximized, the system cost should be reduced, and the cost performance of the photovoltaic system should be improved. At present, the nominal efficiency of mainstream inverters is between 80% and 95%, and the efficiency of low-power inverters is required to be no less than 85%. In the actual design process of the photovoltaic system, not only the high-efficiency inverter should be selected, but also the reasonable configuration of the system should be adopted to make the photovoltaic system load work near the best efficiency point as much as possible.

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, and the unit is expressed in VA or kVA. The rated capacity of the inverter is when the output power factor is 1 (that is, pure resistive load), the rated output voltage is the product of the rated output current.

7. Protective measures

An inverter with good performance should also have complete protection functions or measures to deal with various abnormal situations in actual use, so that the inverter itself and other components of the system are protected from damage.

(1) Enter the undervoltage protection account:

When the input voltage is lower than 85% of the rated voltage, the inverter should be protected and displayed.

(2) Enter the overvoltage insurance account:

When the input voltage is higher than 130% of the rated voltage, the inverter should be protected and displayed.

(3) Overcurrent protection:

The overcurrent protection of the inverter should be able to ensure timely action when the load is short-circuited or the current exceeds the allowable value to protect it from surge current damage. When the working current exceeds 150% of the rated, the inverter should be able to automatically protect.

(4) Output short-circuit insurance customers

The inverter short-circuit protection action time should not exceed 0.5s.

(5) Input reverse connection protection:

When the positive and negative input terminals are connected reversely, the inverter should have a protective function and display.

(6) Lightning protection:

The inverter shall have lightning protection.

(7) Over-temperature protection, etc.

In addition, for inverters without voltage stabilization measures, the inverter should also have output over-voltage protection measures to protect the load from over-voltage damage.

Working principle of grid-connected inverter

8. Starting characteristics

It characterizes the ability of the inverter to start with load and its dynamic performance. The inverter should be guaranteed to start reliably under rated load.

9. Noise

Components such as transformers, filter inductors, electromagnetic switches, and fans in power electronic equipment all generate noise. When the inverter is in normal operation, its noise should not exceed 80dB, and the noise of a small inverter should not exceed 65dB.

Selection skills

The selection of the inverter must first consider having sufficient rated capacity to meet the electrical power requirements of the equipment under the maximum load. For an inverter with a single device as a 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 is 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 the impact of capacitive and inductive loads.

For general inductive loads, such as motors, refrigerators, air conditioners, washing machines, high-power water pumps, etc., when starting, the instantaneous power may be 5-6 times its rated power. At this time, the inverter will endure a great instant surge. For this type of system, the rated capacity of the inverter should have sufficient margin to ensure that the load can be started reliably, and the high-performance inverter can be started at full load multiple times without damaging the power devices. For its own safety, small inverters sometimes need to use soft start or current limiting start.

Installation precautions and maintenance

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

2. When selecting the installation site, it should be ensured that there is no interference from any other power electronic equipment in the surrounding area.

3. Before making electrical connections, be sure to use opaque materials to cover the photovoltaic panels or disconnect the DC side circuit breaker. Exposure to sunlight, photovoltaic arrays will generate dangerous voltages.

4. All installation operations must be completed by professional and technical personnel only.

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

development trend

For solar inverters, improving the conversion efficiency of power is an eternal topic, but when the efficiency of the system is getting higher and higher, almost close to 100%, further efficiency improvement will be accompanied by lower cost performance, so how to maintain A very high efficiency, while maintaining good price competitiveness will be an important topic at present.

Compared with efforts to improve the efficiency of inverters, how to improve the efficiency of the entire inverter system is gradually becoming another important issue for solar systems. In a solar array, when a partial shadow of 2~3% of the area appears, for an inverter with an MPPT function, when the output power of the system is bad, there will even be a power drop of about 20%! In order to better adapt to situations like this, it is very effective to use one-to-one MPPT or multiple MPPT control functions for single or partial solar modules.

Because the inverter system is in the state of grid-connected operation, the leakage of the system to the ground will cause serious safety problems; in addition, in order to improve the efficiency of the system, most of the solar arrays are connected in series to form a high DC output voltage; Due to the occurrence of abnormal conditions between the electrodes, it is easy to produce a DC arc. Due to the high DC voltage, it is very difficult to extinguish the arc and it is very easy to cause a fire. With the widespread adoption of solar inverter systems, system safety issues will also be an important part of inverter technology.

In addition, the power system is ushering in the rapid development and popularization of smart grid technology. A large number of solar and other new energy power systems are connected to the grid, which poses new technical challenges to the stability of the smart grid system. Designing an inverter system that can be more quickly, accurately, and intelligently compatible with smart grids will become a necessary condition for solar inverter systems in the future.

In general, the development of inverter technology is developed with the development of power electronics technology, microelectronic technology and modern control theory. As time goes by, inverter technology is moving towards higher frequency, higher power, higher efficiency, and smaller volume.