Autonomous voltage inverters, types, device and principle of operation, how to choose

В Recently, due to the constant increase in the cost of electricity and the increasing popularity of “green” technologies for generating electricity, the use of solar panels has become increasingly popular. Moreover, this and other similar means are allocated to a separate group called alternative energy.

In the middle latitudes of our country, solar panels can function normally only in the spring and summer. Nevertheless, the feasibility study demonstrates a certain profitability of its installation.

What is a standalone inverter

An inverter in power supply engineering is a device that provides a transition from direct voltage to alternating voltage.

As one of the functional modules, it is included in the list of mandatory solar battery blocks and allows you to get standard single-phase or three-phase mains voltage from direct current.

Depending on the design features, the applied switching circuit and the list of tasks to be solved, the inverter can have a different design, which is schematically reflected in the classification of the figure.

The device can be considered as an uninterruptible power supply with extended functionality.

At the same time, it differs from conventional entry-level UPSs primarily in the following main features:

• contains several equal inputs for connecting various sources of electrical energy to them;
• independently controls the sources of electricity generation, providing the voltage and frequency of the power network normalized by the standards in the entire range of permitted loads;
• provides complete decoupling of external electrical input from the house network, for which the functions of the source of electrical energy, regardless of the operating mode, are always taken over by the inverter.

The latter feature determined the generally accepted designation of this device as an autonomous inverter.

Where is it used and how is it turned on

With regard to solar energy, an autonomous inverter, as a device that primarily performs the functions of choosing one of the possible sources of power supply, is installed between the output of the solar battery and the input shield.

In order to ensure maximum operational flexibility of the in-house wiring, the connection of the external input can also be physically carried out on the house input shield, which is separately highlighted in Figure 2.

At the same time, this input is supplied with all the necessary accessories and circuit breakers for protection against short circuits, excessively high leakage currents, and similar ones.

The strength of this approach lies in the fact that, if necessary, without problems, by simply re-switching just a few outputs, you can switch to a typical power supply scheme in which there are no alternative sources.

Устройство

From a functional point of view, an autonomous inverter is an uninterruptible power supply, supplemented by a multi-input power switch, an output voltage conditioner and equipped with a control unit.

The algorithm for the operation of the control unit in some cases can vary within a fairly wide range.

The block diagram of this device, which indicates the individual blocks and shows the features of their interaction, is shown in Figure 3.

It is believed that the matching by the type of current (DC – AC) and the voltage values ​​of a particular input and common output is carried out in the switch circuit.

External input, solar and storage batteries, as well as a gas generator in this case are considered as mutually complementary sources of energy and cannot function in parallel.

Older models of inverters have the ability to independently determine this sequence by the user or project developer through appropriate programming.

This allows you to fully take into account the local features of the electrical facilities implemented at a particular residential property.

With appropriate programming in the mode of receiving energy from an external input or a gas generator, it is additionally possible to charge the battery to the level of full or another also selectable capacity.

Difference from grid and hybrid inverter

Potentially, all the described functions can also be performed by the so-called. a hybrid inverter, which from this point of view can be considered as the most technically advanced representative of the technology of the variety in question.

Its main difference is the ability to return excess generated electricity back to the grid.

The practical application of hybrid inverters is hampered mainly not by the technical problems of implementing this technique, but by the lack of an appropriate legal framework.

The current regulatory documents do not provide for the very possibility of independent generation of electricity by a private individual and its sale to an energy sales company.

Taking into account the hierarchy shown in Figure 1, the grid inverter is considered to be a lower class device against the background of a hybrid, which implements the following simple two-mode operation algorithm:

• during the day, if there is sufficient power supplied by the solar battery, the house network is disconnected from the electrical input and is fully provided with electrical energy from an alternative source;
• in the morning, in the evening and at night, as well as in cloudy weather, when the solar panel is not able to ensure the normal functioning of household consumers, the inverter is turned off and, due to the bypass switch, the household’s power supply is completely supplied from the electricity supply company network.

Types according to the method of switching current

The output voltage driver 3 or 220 V, which is necessarily present in any inverter, separately highlighted in the diagram of Figure 380, is implemented only according to the pulse circuit.

All this makes it possible to increase the overall efficiency of the device to values ​​above 90%, Figure 4.

In fact, the main losses occur at the moment of their transition from one state to another, which determines the presence of additional high requirements for the key elements of the device and their speed.

A feature of pulse circuits is that, unlike analog ones, the output voltage is not pure, but the so-called. approximate sinusoid.

The quality of the formation of this sinusoid and the degree of its closeness to normal is largely determined by the speed and resistance in the open and closed states of key semiconductor devices.

These may be:

• powerful transistors, incl. IGBT type;
• thyristors of various structures.

Key components based on high-power transistors are used mainly in low-power inverters.

In inverters of medium and high power, semiconductor switches are implemented on single- and dual-operation thyristors, which, due to internal positive feedback, can significantly reduce the duration of the transient process from open to off state and vice versa.

In terms of their parameters, these devices can be considered fairly close analogues, but the two-operation version of the thyristor, due to the transfer of part of the control functions directly to the semiconductor structure, makes it possible to obtain simpler circuit solutions with fewer components and, accordingly, is more reliable.

The principle of operation of a two-stroke type generator

The second key (after the switch) component of the inverter is the alternating voltage generator.

In the circuitry of autonomous inverters, the construction of such a generator according to a push-pull (balanced) circuit has become most widespread.

Regardless of the type of key element used in it (transistor thyristors), a balanced circuit is used to obtain an alternating output voltage, the main components of which are shown in Figure 5.

The scheme functions as follows. The constant voltage created by the “AND” source (depending on the operating mode, its functions can be performed by a solar battery, a rectifier of a gas generator or a mains input, a battery) is applied to the key elements “K”, which are connected in parallel, and to the midpoint of the primary winding of the output transformer.

The upper element “K” is responsible for the formation of a positive voltage half-wave, and the lower one is negative.

The output transformer provides a power alternating voltage that is symmetrical with respect to zero, and also allows you to obtain the galvanic isolation of individual blocks of the power network required by electrical safety rules.

The control scheme can implement various strategies, incl. use well-established and convenient from a circuitry point of view PWM modulation.

The principle of operation of the resonance-type inverter generator

The resonant scheme for constructing an autonomous inverter generator is inferior in popularity to the push-pull, incl. due to the difficulties of ensuring normal operation at idle.

From a circuitry point of view, it compares favorably with its push-pull counterpart by the possibility of being implemented on only one active element (because of the rather low efficiency at powers above 200 – 300 W it becomes inefficient).

The idea of ​​a resonant circuit is that an alternating voltage is created by an oscillatory circuit, i.e. with the right selection of parameters and, first of all, the choice of denominations L and C, its shape will be close to sinusoidal.

Depending on the type of connection of the oscillatory circuit and the load, such generators are divided into serial, parallel and partially parallel.

Additionally, there are closed and open circuits, the difference between which is only whether a direct current flows through the inductance.

If it is present, they speak of closed circuits, and in the absence of open resonant generators.

One of the possible schemes of the simplest resonant inverters is shown in Figure 6.

Application of Capacitors and Flywheel Diodes in Autonomous Resonant Inverter Circuits

A certain increase in the conversion efficiency is achieved by introducing various additional elements into the circuit of resonant inverters. Most often, capacitors and the so-called are used. reverse diodes.

Capacitor C1 in Figure 6 is connected in parallel with the load if it has significant inductance. The purpose of this element is to maximize the parameter cosφ.

Any of the reverse diodes is locked in the open state of the key element and opens when it goes into the locked state, which allows you to “reset” the energy of the reactive elements L and C back to the “AND” source.

Criteria for choosing stand-alone inverters

When choosing a stand-alone inverter, pay attention to several main characteristics. We highlight the main parameters and their features.

Number of phases

When choosing the number of phases, consider the following points:

1. If three-phase voltage (380 V) is suitable for your house, the stand-alone inverter must also be three-phase.
2. In a situation where only single-phase voltage (220 V) is connected to the machine, the equipment must be appropriate.

Rated / peak output power

It is optimal that the rated power of an autonomous inverter is equal to the sum of the loads (consumers in the house). For reliability, it is better to buy equipment with a margin and taking into account inrush currents.

To calculate, multiply the starting current by the voltage in the house and compare with the peak power parameter (the first figure should be lower).

If the developer did not indicate the peak power parameter of an autonomous inverter, this means that the nominal parameter is actually peak.

U shape out

This is a key parameter on which the quality of the receivers depends.

There are three types here:

1. Pure sine.
2. Quasi sine wave.
3. Rectangular sine wave.

This is because an inductive load is very sensitive to the shape of the voltage. If the output of the device is a square sine wave, the main equipment will not work and may break.

A quasi-sine wave is a kind of compromise between a pure and a rectangular sine wave. Most of the models of autonomous inverters on the market with this characteristic are of high quality. But you need to be careful, because there are also unreliable options.

Equipment protection

A good stand-alone inverter model should have a complete set of various protective characteristics.

Let’s highlight the main types of protection:

• from overheating;
• battery protection;
• from the CC;
• from output overload.

If the model has a fan to force the temperature down, check with the consultant if it functions in all situations or turns on only when the load rises above a certain value.

In the best models, the fan turns off at minimum load. As a result, a stand-alone inverter produces less noise, which is important when installing it in a residential building.

Performance

By the parameter of efficiency, you can understand how much energy the device consumes without use. The best representatives have an efficiency in the range from 90 to 95%. If this parameter is less than 90%, 1/10 of the energy will be wasted, which is unacceptable for solar stations.

Own consumption

The indicator displays how much power the equipment consumes without a load connected to it. It is optimal if this parameter is not more than 1% of the rated power.

Availability of sleep / standby mode

The essence of the option is to turn off the device if it is not used for a long time and there is no load.

In this case, its own power drops to three to six watts. At the same time, the autonomous inverter is in constant current tracking mode in order to turn on at full power at any time.

If no shutdown is provided, it is possible that the autonomous inverter will remain in standby mode when a low-power load is connected, for example, charging.

In conclusion, we note that do not take too cheap devices, because their quality may be far from ideal. It is better to choose models taking into account the manufacturer, characteristics and other parameters.

Popular Models

To simplify the choice, we will consider several models of autonomous inverters, highlight their nuances and parameters.

Inverter / UPS SILA EP20-300

The SILA EP20-300 model is a universal equipment that combines the options of an uninterruptible power supply, a charger and a converter.

The possibility of automatic transfer of modes of receipt of U from the battery or from the network is provided. Ensures continuous operation of connected equipment.

The inverter is used as a UPS (in the presence of a battery) or in combination with a solar power plant (an external charge controller is required). The country of origin is Taiwan.

The main parameters:

1. Protection against short circuit, high and low voltage.
2. Fast switching – 6 ms.
3. Autostart after protection work.
4. LCD display.
5. Automatic charging (3-step): constant current / voltage, support charge.
6. Pure sine output.

Features:

1. Large range U at the output – from 140 to 280 V.
2. Current regulation – from 5 to 10 A (set by the program).
3. Stabilization U at the output.
4. Rated / maximum power – 300 / 900 W.
5. Battery voltage – 12 V.
6. Efficiency – 90%.
7. Personal consumption – 24 watts.
8. Warranty – 2 years.

The SILA EP20-300 is capable of operating at 0 to 90% humidity and over a wide temperature range. The dimensions of the autonomous inverter are only 31,5×14,5×21 cm, and the weight is 7,5 kg. Similar models are available for sale at a power of 600 and 1000 watts.

Inverter / UPS Must EP3000 Pro 1K

A model that combines the options of charger, voltage converter and UPS.

There is an option of automatic transfer of operating modes – from the battery and from the network. It is actively used as a UPS (requires a battery) and a solar power station (requires an external controller). Country of origin – Taiwan.

Pros:

1. The presence of a transformer.
2. Ability to work at 3-fold overload.
3. Fast switching time – up to 10 ms.
4. LCD screen.
5. Choice of charging current.
6. Auto-charge mode (3-step).
7. Autostart after the protection is triggered.
8. Stabilization U at the output.
9. Protection against short circuit and overload.

Main parameters:

1. Power – 1000 W (maximum – 3000 VA).
2. Battery U – 12 V.
3. U at the output – 220-240 watts.
4. Own consumption current – 5 A
5. Battery charge current – 35 A.
6. Output voltage – from 155 to 280 V.
7. Warranty – two years.

The Must EP3000 Pro 1K UPS model operates at humidity from 0 to 90%, has dimensions of 42,6×20,6×17,8 cm and weighs 16,5 kg. On the market there are models of higher power – from 2 to 6 kW.

SolarWorks VM 3000-24 Plus (PF 1.0)

The model is used as the main and backup source of supply. It combines the options of a charge controller, an inverter, as well as a charger for recharging the battery from 220 V.

The inverter collects energy from solar panels, the grid or solar modules, and then outputs it to the grid. As a result, consumers receive uninterrupted power. Suitable for offices and homes. Often used to protect server, PC, heating boilers.

Features:

1. The output U is pure sine.
2. Three modes of operation: autonomous, parallel (with a network with a backup source).
3. The impressive LCD display displaying the necessary information.
4. Auto restart when power is restored.
5. Setting the battery charge level, 3-stage mode.
6. Protection against overheating, short circuit and overload.
7. Ability to connect to a PC.
8. Changing the U range at the input.
9. Thoughtful design that positively affects battery performance.

Main characteristics:

1. Solarworks brand.
2. The country of origin is China.
3. Controller type – MPRT.
4. The maximum power of solar panels is 1,5 kW.
5. Operating voltage – from 30 to 115 V.
6. The highest charge current from the network / solar panel is 60 A.
7. Rated / peak power – 3 kW / 6 kW.
8. Nom. U battery / charge maintenance – 24 / 27 V.
9. Efficiency – 90-93%.
10. Warranty – 12 months.

The SolarWorks VM 3000-24 Plus model operates at humidity from 5 to 95%, has dimensions of 10x30x44 cm and weight of 9,5 kg.

Victron Energy EasySolar 48/3000/35-50

The model under consideration is equipped with one or two MPPT controllers built into the equipment case and providing battery charging from the mains.

Features:

1. Brand: Victron Energy.
2. Manufacturer – the Netherlands.
3. The number of phases is one.
4. Indication – LEDs.
5. Protection – against short circuit, overload, overheating, overvoltage and low battery voltage.

Features:

1. Rated power – 2,5 kW.
2. U at the input – from 38 to 63 V.
3. U at the output – 230 V.
4. Efficiency – 95%.
5. Warranty – 5 years.
6. Dimensions – 36,2×37,4×21,8 cm.
7. Weight – 21 kg.

ABi-Solar SL MPPT 5048

Model ABi-Solar SL MPPT 5048 is a stand-alone inverter with MPPT controllers or PWM charge controllers.

Depending on the situation, they can perform the options of a charge controller, inverter and mains charger. Can be used as a UPS.

Features:

1. The output is pure sine.
2. Input U parameters – it is possible to configure.
3. Ability to change the charge current and battery priority.
4. Operation from the voltage of the industrial network or generator.
5. Autostart after recovery U.
6. Several types of protection – against short circuit, overheating and overload.
7. Cold start option.

Features:

1. Brand — ABi-Solar.
2. Country of origin – Taiwan.
3. Rated power – 4 kW.
4. Efficiency – 93%.
5. Indication — LCD display.
6. Dimensions – 14×29,5×54 cm.
7. Weight – 13,5 kg.
8. The number of phases is one.
9. Warranty – one year.

SMA Sunny Island 3324

A universal model that is suitable for stand-alone systems of medium and small levels, with a power of two to five kilowatts. Works flawlessly in high temperatures and bad weather.

1. Brand: SMA.
2. The country of origin is Germany.
3. Protection against full discharge of the battery.
4. Thermal protection.
5. The number of phases is three.
6. LCD display with basic information.

1. Rated power – 3,3 kW.
2. Input/Output Voltage – 172,5 to 250V/230V
3. Efficiency – 94,5%.
4. Dimensions – 59x39x24,5 cm.
5. Weight – 39 kg.
6. Warranty – 5 years.

Mars BN3024E

Autonomous inverter iMars BN3024E is a universal model with a high-quality sine wave, relatively high efficiency and convenient control.

The manufacturer has provided a built-in solar controller, the ability to work with generators, solar panels and renewable energy sources.

1. The output is pure sine.
2. Overload capability up to 300%.
3. Stable U output.
4. Three charge stages with an intelligent system.
5. Voltage regulation.
6. Fast switching – up to 10 ms.
7. Control of indicators by means of the LCD display.

1. Brand: iMars.
2. The country of origin is China.
3. Rated power – 3000 watts.
4. The number of phases is one.
5. U at the input – from 155 to 272 V.
6. Battery voltage – 24 V.
7. Efficiency – 88%.
8. Indication – LEDs.
9. Dimensions – 18×26,4×46 cm.
10. Weight – 26 kg.
11. Warranty – one year.

Conclusion

The market for stand-alone inverters is wide enough to choose a model that takes into account the characteristics of a home, office or other facility. The main thing when buying is to look not only at the cost, but also at the characteristics, brand reputation, reliability and warranty period of the device.