Grid Tie Inverter Working Principle


Solar systems are also backed by inverters for converting the direct current generated by solar panels to alternating current. Solar systems need a solar inverter to work efficiently in connection with or without the grid. Today we will learn about the grid tie inverter, its price, and ways to connect it to mains. But before that we will begin with the grid tie inverter working principle.

What is Grid Tie Inverter and what is their function?

It is an electronic component used to harness solar energy by solar panel systems. A GTI or grid-tied inverter is connected to solar panels for converting direct current (DC) generated by solar panels into alternating current (AC). A grid system works without batteries and grid-tied inverters can be used for solar panels, wind turbines, and hydroelectric plants.

Grid-tied inverters can suitably convert current for power grid frequency from 60Hz-50 Hz commonly used for local electrical generators. A GTI takes a variable unregulated voltage from a solar panel array to invert it to AC synchronized with the mains. But when the grid is down a GTI should automatically stop the electric supply to power lines.

What is Grid Tie Inverter Working Principle?

Jan 23 Grid Tie Inverter Working Principle

Before learning about the working principle of a grid tie inverter, you need to understand its circuit. Capacitors (C), diodes (D), inductors (L), transformers (T), and MOSFETs (Q), are the components of its circuit. The entire process is divided into three steps of conversion. A grid-tied inverter has to synchronize its frequency, amplitude, and wave with the utility and feed a sine wave current into the load.

Note: Grid Tied Inverter will be overloaded if the output (Volt) is higher than the utility voltage. And if it is lower, GTI may sink instead of sourcing it.

1st stage

At this point, direct current (DC) input is converted into 60 Hz alternating current (AC). Input voltage is initially raised by a boost converter formed with C2 (capacitor), Q1 (MOSFET), L1 (inductor), and D1 (diode). One of the inputs direct current buses should be grounded for a photovoltaic array of more than 50V. Theoretically, either of two buses can be connected to the Earth, usually the negative one.

The output alternating current conductors in utility-interactive configurations should be isolated from direct current in case direct current input has a conduction pass to the ground.

2nd stage

Obtained alternating current is then converted to the required level by a low-frequency step-up transformer. This is a pulse-width modulated DC-to-DC converter stage, basically an H-bridge, with isolating converter. This stage comprises C3 (capacitor), Q2-Q5 (MOSFET), T1 (transformer), L2 (inductor), and D2-D5 (diode). A half-bridge or forward converter can also be used for power levels less than 1000 watts.

A DC link to the output AC inverter is provided, and its value must be higher than the peak of utility AC voltage. For example, for 120VAC the VDC should be >120° √2 = 168V, typically between 180V and 200 V, and for a 240VAC you would require 350-400 VDC. Another important step in grid tie inverter working principle.

Low-frequency (LF) transformers are also used in some commercial inverter models in the output stage instead of a high-frequency one in the DC-DC section. In this method, input is converted to 60 Hz AC and a transformer changes it to required levels, providing isolation simultaneously.

Note: Conductors from photovoltaic arrays in non-isolated design cannot be bonded to Earth.

However, with a step-up T1 (transformer) boost, the converted stage (1st stage) can be omitted. But this may lead to large leakage inductance leading to voltage spikes on rectifiers and FETs and other undesirable effects. This is mostly the case with transformer-less systems.

3rd stage

It includes another conversion for the remaining part of AC into the output of the second stage. In this stage, another full bridge is used to convert DC to AC. IGBT Q6-Q9 (MOSFET), C4 (capacitor), and L3 (inductor). The MOSFET in this stage works as electronic switches operating in PWM (pulse width modulation) mode. Antiparallel freewheeling diodes are required for this topography that provides an alternate path for the current when switches are off. A positive, negative, or zero potential can be applied across L3 (inductor) by using different switches in H-bridge. Then, the output LC filter reduces high-frequency harmonics to produce a sine wave.

  • Additional Inductor: Circuits should allow a limited current flow into the loads and back into the line. A GTE works as a current-controlled source instead a voltage source since the grid acts as a very impedance source. An L-Grid (additional coupling inductor) is used between a GTE and main lines to absorb extra AC voltage, further reducing current harmonics generated by the pulse width modulation (PWM).

Note: With the introduction of extra poles in the current loop, there are chances for system instability.

Also Read: Understanding Off Grid Solar System Working Principle

What Components are Required to Install Grid Tied Inverter?

After learning about grid tie inverter working principle get to know about its components. Being easy to install and maintain grid-tied inverters requires minimum equipment and average maintenance. A few onsite components and wiring needed during the installation of GTI are as follows.

1. Isolators

They are key electronic components used in solar inverters as they enable communication between low-voltage sensitive control circuits (microcontrollers) and high-voltage components (power transistors).

Also See: 8 Best 3000 Watt Inverter

2. Net Meter

It is like a tracking system that monitors power exchange between the home and the main utility grid. It calculates and credits the owners of solar panel systems for the electricity supplied to the grid from their solar power system.

3. Protection Unit

Grid-tied inverters have anti-islanding protection along with other inbuilt protection functions. But adding protection devices like charge controllers, safety switches, and cabling adds an extra layer to the entire system.

4. Solar Panels

These are the most essential components because inverters function by converting DC power generated by solar panels into AC power that is suitable for running appliances.

5. Wiring

Proper wiring with connection to solar panels, main electric panel, and net meter is required. The length, width, and quality of wire should be optimum for the purpose they are used. After this, let’s see how to connect grid tie inverter to mains.

Also See: How to Wire a 5000 Watt Inverter?

How to Connect Grid Tie Inverter to Mains?

JAN 23 Grid Tie Inverter Working PrincipleBefore you connect the grid tie inverter to mains there are a few things to understand.

1. Frequency

Mains have a precise frequency maintained at 50 Hz, and it does vary slightly, but mostly it is between 49.9 and 50.1 Hz.

2. Root Mean Square

It is the RMS of the voltage of 50 Hz signal and the actual signal goes up to 325V. It is essential to know because while connecting an inverter such high temperatures are produced to get current flows into the plugs. Not only is the grid tie inverter working principle but connecting it to mains requires in-depth knowledge about these factors.

3. Voltage Drops

Plugs in the house receive variable voltages. This happens because of the voltage drop across the resistance in connectors, fuses, transformers, and wires, along with other circuit components. As you turn off/on large loads nearby, you will notice sudden fluctuations and changes. Voltage in mains is much more variable, for example, in certain locations, it is supposed to be 230V with a +10%/-6% tolerance.

Also Read: Do I Need A Fuse Between Battery And Inverter

4. Sinusoidal

The signal supplied should be a nice and clear sine wave but at certain points in the sine wave cycle, some non-linear appliances absorb their power from some points. This introduces distortion which is why the signal is not a perfect sine wave. Computers, fluorescent lights, hoovers, and other appliances are of non-linear loads. For connecting an inverter to mains sine wave must be maintained.

And the answer to how to connect the grid tie inverter to mains is that you’ll have to know that the frequency, amplitude, and phase of the power source or inverter should be synchronized. Also, it should feed a sinusoidal current to the load. Otherwise, it might not detect whether the output is high or low and this can be problematic. There should be another coupling inductor L-grid between the system and the grid to absorb extra alternating current voltage. A distance of around 500 millimeters should be maintained between the inverter and the top-bottom-side wall and between two inverters also.

Here I have mentioned the steps for the connection. Grid tie inverter referred to for these steps is Luminous Grid Tie Inverter but most inverters have similar steps.

Step 1: Begin by turning off the main supply now. Switch off the Alternating Current MCB (miniature circuit breaker) in the AC distribution box.

Step 2: Then connect the grid output wire from the inverter to the AC distribution box. Remember the grid tie inverter working principle while making the connection.

Step 3: Connect the output wire from the AC distribution box to the AC MCB box

Step 4: Now the connection between the AC MCB box and net meter also to different appliances in the house will form automatically.

Note: It is recommended to hire technicians and electricians for such work.

Also See: How Does a Hybrid Inverter Work?

What is Grid Tie Inverter Price?

JAN 23 Grid Tie Inverter Working PrincipleA grid tie inverter price depends on its wattage and phases, along with the type of grid tie inverter you choose. Generally, you may have to spend around $911 or more for a grid tie inverter. But mostly inverters are provided as a part of solar power systems and can account for about 20% of the cost of the entire system. Knowledge about the price after learning the grid tie inverter working principle is useful though.

Also Read: Can You Connect Inverters in Series?

What are Types of Grid Tied Inverters?

After understanding grid tie inverters, take a look at the types to choose from.

1. Central inverters: Similar to string inverters in working but with a high-capacity range. It is resistant to environmental damage and suitable for large solar farms.

2. Micrometer: These are the latest technologically advanced devices that function by converting DC to AC directly from the back of solar panels. There are no strings attached thus panels are not shaded. Micrometers offer individual monitoring systems and can be extended as per requirements. These are best for space-restricted solutions but are expensive.

3. Power optimizers: These are also attached to the back of solar panels and work like micrometers. They offer a system-level performance along with individual panel monitoring. Power optimizers are less costly than micrometers.

4. String inverters: They are the most common types of grid-tied inverters. String inverters are wired together in series and the function of each string is to convert Direct Current (DC) to alternating Current (AC). These inverters are reliable and affordable, mostly suitable for commercial and small households.

So, today you learned about the grid tie inverter working principle, which I guess was quite interesting. Considering the components used for grid-tied inverters, their price can be determined, but it can differ from place, manufacturer, and brand, along with other factors. How to connect grid tie inverter to mains? The answer to this is simple, just go through the steps mentioned above but still, we do not recommend self-installation.

Recommended: Can Hybrid Inverter Work Without Battery?

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