Grid-tied inverter – learn the basics of it

What is a grid-tied inverter?

Grid-connected inverters are generally divided into photovoltaic power generation grid-connected inverters, wind power grid-connected inverters, power equipment power generation grid-connected inverters and other power generation equipment grid-connected inverters. The biggest feature of grid-tied inverter is high system power and low cost.

Grid-tied inverters are generally used in large-scale PV power plant systems, where many parallel PV strings are connected to the DC input of the same centralized inverter. Typically, three-phase IGBT power modules are used for high power and field-effect transistors are used for smaller power. At the same time, the DSP conversion controller is used to improve the quality of the power generation so that it is very close to the sine wave current.

How the grid-tied inverter works

When the public grid is powered off, the grid side is equivalent to a short-circuit state. At this point, the inverter running on the grid is automatically protected due to overload. When the microprocessor detects an overload, in addition to blocking the SPWM signal, it also disconnects the circuit breaker connected to the grid. At this point, if the solar array has an energy output, the inverter will operate in a separate operating state.

When operated alone, the control is relatively simple, i.e., the negative feedback state of the AC voltage. The microprocessor senses the inverter output voltage and compares it to the reference voltage (typically 220V), and then controls the PWM output duty cycle to achieve inverting and stabilization.

Of course, the premise of independent operation is that the solar array can provide enough power at the time. If the load is too large or the sunshine conditions are poor, and the inverter cannot output enough power, the terminal voltage of the solar array will drop, thus reducing the output AC voltage and entering a low-voltage protection state. When power is restored to the grid, it automatically switches to a feedback state.

The main features of the grid-tied inverter:

1. efficiency

   High Since the current price of solar cells is relatively high, it is necessary to find ways to improve the efficiency of the inverter in order to maximize the use of solar cells and improve the efficiency of the system.

2. High reliability

   At present, photovoltaic power station systems are mainly used in remote areas, and many power stations are unattended and unmaintained. This requires the inverter to have a reasonable circuit structure, strict component selection, and requires the inverter to have a variety of protection functions.

3. Wide range of input voltage

   Because the terminal voltage of a solar cell varies with the load and the intensity of sunlight. Especially when the battery ages, its terminal voltage varies greatly. For example, for a 12V battery, its terminal voltage may vary between 10V and 16V. This requires the inverter to operate normally over a wide range of DC input voltages.

What is the difference between an off-grid inverter and a grid-tied inverter?

An off-grid inverter is a system that is decoupled from the public grid. The direct current generated by the solar panels is first stored in the battery, which is then delivered to an off-grid inverter for reversing. The alternating current generated can be supplied directly to the load. After use, it can also be returned to battery storage. It is more suitable for desert areas, plateau areas, forest areas and other areas without grid coverage, which can ensure that electricity is needed at any time.

Grid-tied inverters are typically used in systems in large power stations where many parallel PV strings are connected to the DC input of the same centralized inverter. The biggest difference between a grid-tied inverter is that it doesn't use any battery storage. The direct current generated by the solar panels can be directly switched to alternating current through an inverter and delivered directly to the public power system. However, there is a corresponding threshold, that is, the relevant regulations and policies of the local power grid need to be met, otherwise the grid connection cannot be provided.

Key points for choosing a grid-tied inverter

1. It has real-time monitoring function

If the photovoltaic power generation system is integrated into the public power grid, the output status of the photovoltaic power generation system (start, operation, dormancy, stop, fault state) and the public power grid status need to be monitored in real time, and the public power grid should be judged based on the detection results. Requires on/off control operation or other operation. This ensures the safe and reliable operation of the photovoltaic power generation system and the public power grid.

2.Has Maximum Power Tracking Function (MPPT)

Since the output curve of the PV array is nonlinear, in order to ensure that the PV array outputs power at maximum power, the grid-connected PV inverter (hereinafter referred to as the "inverter") needs to adopt the maximum power tracking control technology (MPPT).

3. Inverter output efficiency

At full load, the inverter output efficiency needs to exceed 95%. When the input power of the inverter is 10% of its rated power, the output efficiency of the inverter needs to exceed 90%.

4. The output waveform of the inverter must meet the requirements of grid connection

The output voltage waveform, amplitude and phase of the inverter should be consistent with the public network, so as to achieve a smooth power supply to the public network without interference.

5. The ability to protect the islands

When the public power grid is interrupted due to external electrical faults, misoperation, natural factors, etc., in order to prevent damage to the safety of electrical equipment and grid maintenance personnel, the inverter must have island protection capabilities.

6. It has perfect protection function

The inverter should have functions such as input under-voltage, over-current protection, over-current protection, output short-circuit protection, input reverse polarity protection, lightning protection, etc.

7. Inverter input DC voltage range

Since the terminal voltage of the PV array varies greatly with the change of load and sunshine intensity, the inverter needs to work normally within a large DC input voltage range, and the maximum input DC voltage of the inverter should be greater than the maximum voltage of the PV array.

8. The output voltage and frequency of the inverter

The rated output voltage of the inverter should be equal to the rated voltage of the public network, and the rated frequency of the inverter should be equal to the frequency of the public network.