Guide to designing off-grid and hybrid solar systems

This is a technical guide for those with a basic understanding of solar and off-grid inverters. For less technical information, see the basic guide to selecting a home grid-tie or off-grid solar battery system. Solar and battery storage systems should always be installed by a licensed electrical professional.

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Steps to Designing An off-grid Solar or Battery System

Before purchasing any equipment required for a solar battery (hybrid) or off-grid power system, it is very important to understand the basics of designing and sizing energy storage systems. As explained below, the first part of the process is developing a load profile or using a load calculator to estimate the amount of energy required to be generated and stored daily. If you cannot develop a load table, a professional solar installer or system designer should be consulted.

Step 1 - Estimate the loads

The most important part of designing any off-grid solar or battery system is calculating how much energy is required per day in kWh. For grid-connected sites, detailed load profile data can be obtained using meters to measure the loads directly. For off-grid or stand-alone power systems, always start by using an off-grid load calculator (load table) for summer and winter requirements. The load table will also help calculate surge loads, power factors, and the maximum demand required to size an appropriate off-grid inverter.

Step 2 - Battery sizing

Battery capacity is measured in either Ah or Wh. Lead-acid (deep-cycle) batteries are sized in Ah, while lithium battery capacity is measured in kWh. All loss factors need to be considered to ensure the battery size is adequate to meet the loads, including maximum allowable depth of discharge (DoD) and round-trip efficiency. Also, consider battery type and chemistry, battery voltage range, minimum days of autonomy (continuous days without sunshine), and maximum battery charge rate (C rating), as explained in detail later.

Step 3 - Solar array sizing

A correctly sized solar array is required to charge the battery while also supplying the loads. To ensure the solar array is large enough, consider local conditions, including average solar irradiance throughout the year (peak sun hours), shading issues, panel orientation and tilt angle, cable losses, and temperature derating (loss factors). Our Photonik solar design tool can help estimate solar generation throughout the year, depending on the panel orientation and location.

Step 4 - Inverter selection

After steps 1 to 3 have been established, you can then select a suitable solar inverter or MPPT Solar Charge Controller to match the solar array depending on the panel and string length, which will determine the string voltage. Use a string voltage calculator to calculate the maximum and minimum string voltages. Next, the primary hybrid or off-grid inverter can be selected to meet the continuous and surge loads, taking into account temperature derating and other loss factors explained in more detail below.

18 February 2022

25951

Hello everyone, I am Rose. Today I will introduce inverter to you. The inverter is a converter that converts DC power (battery, storage battery) into constant frequency and constant voltage or frequency modulation and voltage regulation AC power (usually 220V, 50Hz sine wave).

Ⅰ. What are inverters?

The inverter is a device that converts  DC  electricity (battery, storage battery) into  AC  power with a fixed frequency and voltage or with frequency modulation and voltage management (usually 220V, 50Hz sine wave). It is made up of semiconductor power devices as well as drive and control circuits for inverters,  The creation of new high-power semiconductor devices and drive control circuits has been aided by the advancement of microelectronic and power electronics technologies. Insulating gates are now often used in inverters,  Polar transistors, power field-effect transistors, MOS controller thyristors, and intelligent power modules are examples of advanced and easy-to-control high-power technologies. The control circuit has also evolved from an analog integrated circuit to being controlled by a single-chip microprocessor or a digital signal processor, allowing the inverter to progress in the direction of systematization, full control, energy efficiency, and multi-function. Air conditioners, home theaters, electric grinding wheels, electric tools, sewing machines, DVDs,  VCDs, laptops,  TVs, washing machines, range hoods, refrigerators, video recorders, massagers, fans, lighting, and other electronic devices use it.

Ⅱ. The structure of inverters

The input circuit, main power transformer circuit, output circuit, auxiliary circuit, control circuit, and protection circuit  make up the inverter structure as the picture shows:

Structure of inverters

The input circuit is responsible for providing the  DC input voltage; the main inverter circuit completes the inverter program through the action of the semiconductor switching device; the output circuit mainly compensates and corrects the frequency, phase, voltage and current amplitude of the output  AC power of the main inverter circuit to It reaches a certain standard; the control circuit provides pulse signals for the main inverter circuit to control the opening and closing of semiconductor devices; the auxiliary circuit converts the DC voltage of the input circuit into a DC voltage suitable for the operation of the control circuit, and also includes a series of the detection circuit.

Ⅲ. How does inverter work?

The inverter's working concept is similar to that of a switching power supply. It uses an oscillating chip or a specialized circuit to control the output of the oscillating signal, which is amplified to force the  FET  to flip continually. After adjustment, the AC characteristics of can produce a sine wave AC similar to the power grid.

A power survey device called an inverter is required for stand-alone solar systems that use AC loads. The size of the set DC voltage is a significant consideration when choosing an inverter. The inverter's output can be classified into two types: DC output and AC output. The inverter is known as a converter for DC output because it converts DC voltage to DC voltage so that it can provide the voltage required for the functioning of DC loads of various voltages.

kind of inverter

In addition to output power and voltage, waveform and frequency should be addressed for AC output. Pay attention to the inverter's DC voltage requirements and the variation in the surge voltage it can withstand at the input end.

The gate drive circuit of the power switch tube can be controlled by a logic circuit or a dedicated control chip, a general-purpose single-chip microcomputer or a DSP chip, etc. The voltage regulation capability of the inverter output is possible. Using the bridge inverter as an example, the rated voltage of the AC bus output by the inverter should be 10%–20% lower than the rated voltage of the DC bus (the purpose is to make It has certain stability).

The inverter is then controlled by  PWM  to provide a margin whose amplitude may be varied from 10% to 20%, and the modification from low to high is not limited; simply reduce the  PWM on-duty ratio. As a result, the input DC voltage fluctuation range of the inverter is -15 percent to 20%. It is not limited as long as the device's withstand voltage allows. Only the modest output pulse width needs to be adjusted (equivalent to chopping).

When the battery or solar cell output voltage is low, the inverter must be supplied with a booster. In the switching power supply mode, the circuit booster can raise the voltage, or the DC charge pump method can be utilized to enhance the voltage. The inverter boosts the voltage with the output transformer, so the inverter voltage matches the voltage of the battery or solar cell array, and the inverter outputs a lower AC voltage, which is then boosted by the power frequency transformer and fed into the distribution line.

It should be remembered that a portion of the energy will be wasted regardless of whether the transformer or the electronic circuit is boosted. The optimal inverter working mode is when the DC input voltage matches the transmission line voltage and the DC power only goes through one layer of inverter links to minimise conversion link loss. Generally speaking, the inverter's efficiency is greater than 90%. The thermal energy of the power tube and transformer is converted from the energy lost in the inverter link.

This heat is detrimental to the inverter's operation and poses a risk to the device's safety. To remove the heat from the device, a radiator, a fan, or other means must be used. Conduction loss and switching loss are usually two elements of inversion loss.

The MOSFET has a high switching frequency and a significant on-resistance, and the inverter it is used in operates at a frequency of tens to hundreds of kilohertz; the IGBT, on the other hand, has a tiny conduction voltage drop, a moderate switching loss, and a switching frequency. The frequency ranges from several thousand to tens of kilohertz, with the average being fewer than ten kilohertz.

The switch isn't exactly great. The current rises during the turn-on operation, while the tube terminal voltage falls. The turn-on loss occurs when the voltage and current are crossed, and the turn-off loss occurs when the voltage and current are crossed in the other direction. The primary goal of inverter loss reduction is to reduce switching loss. The novel resonant switching inverter reduces switching loss by turning on or off at the zero-crossing point of voltage or current.

The single-phase bridge inverter circuit, as shown in Figure 1-2, will be used to demonstrate the basic operating concept of the inverter circuit (a). The switches S1 to S4 are made up of power electronic devices and auxiliary circuits and are situated on the four arms of the bridge circuit.

Working principles of inverter

The left positive and right negative-positive voltage Uo is obtained on the load when S1 and S4 are closed and S2 and S3 are disconnected, and its waveform is depicted in Figure 1-2. (b). The DC power is converted to AC power, the switching frequency is altered, and the frequency of the output AC power is altered in this way.

When a resistive load is used, the waveforms and phases of the load current io and voltage uo are the same; when a resistive and inductive load is used, the phase of the fundamental wave of the current io lags the shock wave of uo, and the waveforms of the two are also different, as shown in Figure 1- 2(b). If S1 and S4 are both closed before time ti, and uo and io are both positive, and they are disconnected at time t1, and S2 and S3 are both closed at the same time, the polarity of uo becomes negative instantaneously.

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The current direction cannot be altered instantly due to the presence of inductance in the load; instead, it maintains the original direction. The current travels from the power supply load to the positive pole of the power supply via S2 and S3. The load current steadily diminishes as the energy stored in the load is transferred back to the DC power supply. It's modest, and after it reaches zero at time t2, it steadily increases in the other direction. When S2 and S3 are open and S1 and S4 are closed, the scenario is the same. The analysis above is for ideal switches S1–S4, but the actual circuit's working process is more complicated.

Ⅳ. The features of inverters

1. High conversion efficiency and fast startup;

2. Good safety performance: the product has 5 protection functions of short circuit, overload, over/under voltage, and over temperature;

3. Good physical properties: The product adopts an all-aluminum shell, which has good heat dissipation performance, hard oxidation treatment on the surface, good friction resistance, and can resist extrusion or impact from a certain external force;

4. With load adaptability and stability.

Ⅴ. Precautions for using inverter

1. The DC voltage should be consistent

Each inverter has access to DC voltage values such as 12V, 24V, and so on, and it is needed that the battery voltage chosen matches the inverter's DC input voltage. A 12V inverter, for example, must choose a 12V battery.

2. The inverter's output power must be more than the power consumed by the electrical appliances, with a wider margin left for electrical appliances that consume a lot of power at startups, such as refrigerators and air conditioners.

3. The positive and negative poles must be connected correctly

The DC voltage connected to the inverter is marked with positive and negative poles. Red is positive (+), black is negative (-), the battery is also marked with positive and negative, red is positive (+), black is negative (-), must be connected to positive (red to red), negative Negative (black to black). The diameter of the connecting wire must be thick enough, and the length of the connecting wire should be minimized.

4. It should be kept in a well-ventilated and dry location, away from flammable and explosive materials, and no other items should be placed or covered on the machine. The temperature outside is not higher than 40 degrees Celsius.

5. It is not possible to charge an invert at the same time. That is, while the inverter, the charging connector cannot be inserted into the electrical circuit of the inverter output.

6. The time between two restarts must be at least 5 seconds (cut off the input power).

7. To maintain the machine clean and neat, wipe it off with a dry cloth or an anti-static cloth.

8. Before connecting the machine's input and output, make sure the machine's enclosure is properly grounded.

9. It is highly banned for users to open the chassis for operation and use in order to avoid accidents.

10. If you believe that the machine is defective, do not run or use it; instead, switch off the input and output as soon as possible, and have it checked and maintained by certified maintenance workers or maintenance units.

11. When connecting the battery, make sure you don't have any other metal things on your hands to prevent the battery from shorting out and burning the human body.

12. The installation environment should meet the following parameters, which are based on safety and performance considerations:

1) Keep dry: avoid soaking in water or rain.

2) Shade and cool: the temperature ranges from 0 to 40 degrees Celsius.

3) Ventilation: keep no foreign objects within 5 cm of the shell, and ventilate the other end faces.

13. Methods of installation and use

1) Turn the switch off, then insert the cigar head into the car's cigarette lighter socket to make sure it's in place and making excellent contact;

2) Verify that the combined power of all connected electrical appliances in the two sockets is less than the nominal power of G-ICE by inserting the electrical appliance's 220V plug directly into the 220V socket on one end of the converter and ensuring that the sum of the power of all connected electrical appliances in the two sockets is less than the nominal power of G-ICE.

3) Turn on the converter switch; the green indicator light will illuminate, indicating that the converter is operating normally; 4) The red indicator light will illuminate, indicating that the converter has been shut off due to overvoltage/Undervoltage/overload/overtemperature;

5) In many circumstances, the converter alerts or shuts down during routine use due to the restricted output of the car cigarette lighter plug. As long as the car is started or the power consumption is lowered, it can return to normal.

14. Precautions

1) At startup, TVs, monitors, motors, and other electronic devices achieve their maximum power. Although the converter can resist peak power double the nominal power, some electrical appliances that meet the specifications may have peak power that exceeds the converter's max output power. The current is switched off and the overload safeguard is triggered. Multiple electrical appliances may be driven at the same time, which is possible. The electrical switch should be turned off at this point, the converter switch should be turned on, and then the electrical switches should be switched on one by one, starting with the electrical appliance with the highest peak value.

2) The battery voltage begins to decline during use. The alarm will ring when the voltage at the DC input end of the converter drops to 10.4-11V. The computer, as well as any other sensitive electrical appliances, should be turned off at this time. The device will automatically turn off when the voltage hits 9.7-10.3V, preventing the battery from being overcharged. The red indicator illuminates once the power protection is turned off.

3) The vehicle should be started promptly, and the battery should be charged to avoid a power outage, which will affect the car's start and battery life.

4) Although the converter has no overvoltage protection, if the input voltage exceeds 16V, the converter may be damaged; 5) The surface temperature of the shell will rise to 60 °C after continuous use; pay attention to smooth airflow and stay away from objects that are easily affected by high temperatures.

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