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101 renewable - direct grid [grid-tie] power inverters

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Last Updated
7th of January, 2020

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Power Grid Inverter Schematic

Inverters are having as main function to transform a DC input into an AC output. This process goes through a chain of sequences where the DC input is chopped in a square wave and later modified into a sine wave and synchronized with the grid through build in phase coupling functions. The process of transforming the DC input into an AC output  by inverter is a  PWM - Pulse with Modulation - transformation. The table bellow illustrates the wave types an inverter is generating at different moments.

THD = The total harmonic distortion


Grid tie inverter voltage output

Modified sine wave inverters:~

square wave

The output of a modified square wave or quasi-square is similar to a square wave except that the output wave goes through zero volts before switching positive or negative. This process is simple and it comes at a low cost  at $0.10USD/Watt or less. In the same time this type of output is compatible with the impute value of most electronic devices we would like to energize using modified sine wave inverters, with exception of sensitive or specialized equipment like laser printers, fluorescent lighting, and audio equipment. Most AC motors will run using this type of power source albeit a reduction in efficiency of approximately 20%.

Pure sine wave inverters:~

A pure sine wave inverter produces a nearly perfect sine wave output (less than 3% total harmonic distortion) that is essentially same as utility-grid supplied sin wave power. Thus it becomes compatible with all AC electronic devices. This is the type of wave used in the grid-tie inverter. Its design is more complex and will cost more per unit power. The type of electrical inverter is a high-power electronic oscillator. It is named Inverter because early mechanical AC to DC converters was made to work in reverse and was named "inverted" when they were used to convert DC to AC.

Solar inverters may be classified into three broad types:~

Stand-alone inverters:~
or off-grid, are used in isolated systems where the inverter draws its DC energy from batteries charged by photovoltaic arrays. Many stand-alone inverters also incorporate integral battery chargers to replenish the battery from an backup AC source, when available. Normally these type of inverters do not interface with the utility-grid, and they do not require anti-islanding protection.

Grid-tie inverters:~

They match phase with a utility-grid sine wave and they use phase coupling functions.  Grid-tie inverters are designed to disconnect automatically upon loss of utility-grid power, for safety reasons. They are equipped with anti-islanding protection.

Backup inverters:~

are off-grid or grid-tie type of inverters special designed to draw energy from a battery bank or an external power generator, recharge the battery bank via an onboard charger. Those inverters are capable to supply AC power to selected loads - critical loads - during a utility-grid outage and they are required to have anti-islanding protection.

Some of the main feature incorporated into a Power Invertors:~

Maximum power point tracking (M.P.P.T.):~

Maximum power point tracking is a technique that solar inverters use to get the maximum possible power output from an existing PV array. Solar cells have a complex relationship between solar irradiation, temperature and total resistance that will produce a non-linear output known as I-V curve. It is the purpose of the MPPT system to sample the output of the solar panel and apply a resistance (load) to obtain maximum power for any given environmental conditions. Essentially, this defines the current that the inverter should draw from the PV array in order to get the maximum possible power output (since power equals voltage times current).

Anti-islanding protection:~

In the event of a power failure on the grid, it is generally required that any grid-tie inverters attached to the grid turn off in a short period of time. This prevents the inverters from continuing to feed power into small sections of the grid, known as "islands". Powered islands present a risk to workers who may expect the area to be unpowered, but equally important is the issue that without a grid signal to synchronize to, the power output of the inverters may drift from the tolerances required by customer equipment connected to it.

Galvanic Isolation between the AC and DC sections of the solar power plant and TL Inverters:~

It is about No Isolated and Isolated Grid-tied inverters, the majority of the inverters sold on the market are incorporating high-frequency components as IGBTs or thyristors and some of the manufacturer are supplying the chopping bridges without a transformer. It is a transformerless AC power generation and we call them TL Inverters (Transformerless Inverters). That it means it is not a physical separation between the DC side (strings and dc power optimizers) and the AC side (grid tied connected). There are concerns about having transformerless electrical systems feed into the public utility grid because of the lack of galvanic isolation between the DC and AC circuits could allow the passage of dangerous DC faults to be transmitted to the AC side. There for an additional transformer on the AC side will do just that, galvanic separate the DC from AC. This solution comes at an economical cost: copper and additional weight. Since 2010, TL Inverters have been UL certified and are offering the level of isolation between the DC side and the AC side requested by the utility power line. 

Inverter intercommunication and monitoring:~

Some models include communications “link,” which allows interconnecting multiple inverters to provide synchronized higher wattage and voltage outputs. In this situation, we will have a master inverter which monitors the well-being of a chain of similar inverters.

High-Efficiency conversion:~

One of the major component to maximizing the power output of a solar installation is to utilize high-efficiency conversion.  Efficiency is the measure of power out of the inverter as a percentage of the power into the inverter.  Thus, high-efficiency PV inverters use less of power in the conversion process and supply more of the power for use.

HF transformer switchover:~ 

Transformers used by inverters are high-frequency (HF) type of transformer. The automatic transformer switchover facility produces three efficiency peaks. The result: a constant level of efficiency across the entire input voltage range, resulting in higher yields. Other advantages of HF transformer technology are the compact, lightweight design, a high level of efficiency and safety as a result of the electrical isolation.

Integrated WLAN interface:~ 

Simple, user-friendly system monitoring is very important. With the Inverter Data Manager, inverter manufacturer offers a WLAN interface incorporated into the inverter itself. The inverter is connected to the internet and it grants the perfect overview of how the PV system is operating.

Secure Power Supply (S.P.S.):~

One of the many unique features of the TL-US residential series is its innovative Secure Power Supply.  Secure Power Supply (SPS) feature enables the inverters to supply up to 12 A at 120VAC to an outlet not connected to the distribution panel, where is energizing some critical loads during outages or is used for a dedicated in house power ring for as long the sun shines. A grid-tie inverter equipped with an SPS outlet has a very limitate off-grid functionality.

Article 690.11 Arc-Fault Circuit Protection (DC Side). 

PV systems with DC source and/or output circuits on or penetrating a building operating at a PV system maximum system voltage of 80 volts or greater shall be protected by a listed (DC) arc-fault circuit interrupter, PV type, or other system components listed to provide equivalent protection. The PV arc-fault protection means shall comply with the following requirements:

(1) The system shall detect and interrupt arcing faults resulting from a failure in the intended continuity of a conductor, connection, module, or  other system components in the direct-current PV source and output circuits.
(2) The system shall disable or disconnect one of the following:
   a. Inverters or charge controllers connected to the fault circuit when the fault is detected
   b. The system components within the arcing circuit.
(3) The system shall require that the disabled or disconnected equipment be manually restarted.
(4) The system shall have an annunciation that must be manually disabled.

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