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Grid-tie inverter

Grid-tie inverter is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Grid-tie inverter rather than just read about it. In short: A grid-tie inverter converts direct current (DC) into an alternating current (AC) suitable for injecting into an electrical power grid, at the same voltage and frequency of that power grid. Grid-tie inverters are used between local electrical power generators: solar panel, wind turbine, hydro-electric, and the grid.

Grid-tie inverter — main illustration
Grid-tie inverter — illustration

Key takeaways

  • Grid-tie inverter belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Grid-tie inverter to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Grid-tie inverter from memory before moving on to harder problems.

Reference excerpt

A grid-tie inverter converts direct current (DC) into an alternating current (AC) suitable for injecting into an electrical power grid, at the same voltage and frequency of that power grid. Grid-tie inverters are used between local electrical power generators: solar panel, wind turbine, hydro-electric, and the grid. To inject electrical power efficiently and safely into the grid, grid-tie inverters must accurately match the voltage, frequency and phase of the grid sine wave AC waveform.

Payment for injected power Electricity companies, in some countries, pay for electrical power that is injected into the electricity utility grid. Payment is arranged in several ways. With net metering the electricity company pays for the net power injected into the grid, as recorded by a meter on the customer's premises. For example, a customer may consume 400 kilowatt-hours over a month and may return 500 kilowatt-hours to the grid in the same month. In this case the electricity company would pay for the 100 kilowatt hours balance of power fed back into the grid. In the US, net metering policies vary by jurisdiction. Feed-in tariff, based on a contract with a distribution company or other power authority, is where the customer is paid for electrical power injected into the grid. In the United States, grid-interactive power systems are specified in the National Electrical Code (NEC), which also mandates requirements for grid-interactive inverters.

Operation Grid-tie inverters convert DC electrical power into AC power suitable for injecting into the electric utility company grid. The grid tie inverter (GTI) must match the phase of the grid and maintain the output voltage slightly higher than the grid voltage at any instant. A high-quality modern grid-tie inverter has a fixed unity power factor, which means its output voltage and current are perfectly lined up, and its phase angle is within 1° of the AC power grid. The inverter has an internal computer that senses the current AC grid waveform, and outputs a voltage to correspond with the grid. However, supplying reactive power to the grid might be necessary to keep the voltage in the local grid inside allowable limits. Grid-tie inverters are designed to disconnect quickly from the grid if the utility grid goes down. In the United States, there is an NEC requirement that in the event of a blackout, the grid tie inverter shut down to prevent the electricity it generates from harming persons repairing the power grid. Properly configured, a grid tie inverter enables a building to use an alternative power generation system such as solar or wind power without extensive rewiring and without batteries. If the system produces insufficient power, the utility grid makes up the deficit.

Types

Grid-tie inverters include conventional low-frequency types with transformer coupling, newer high-frequency types, also with transformer coupling, and transformerless types. Instead of converting direct current directly into AC suitable for the grid, high-frequency transformers types use a computer process to convert the power to a high-frequency and then back to DC and then to the final AC output voltage suitable for the grid. Transformerless inverters, which are popular in Europe, are lighter, smaller, and more efficient than inverters with transformers. But transformerless inverters have been slow to enter the US market because of concerns that transformerless inverters, which do not have galvanic isolation between the DC side and grid, could inject dangerous DC voltages and currents into the grid under fault conditions. However, since 2005, the NFPA's NEC allows transformerless, or non-galvanically isolated, inverters by removing the requirement that all solar electric systems be negative grounded and specifying new safety requirements. Amendments to VDE 0126-1-1 and IEC 6210 define the design and procedures needed for such systems: primarily, ground current measurement and DC to grid isolation tests.

Datasheets Manufacturers datasheets for their inverters usually include the following data:

… excerpt ends here. Continue reading the full article.

Illustrations

Grid-tie inverter: Inverter for grid-tied solar panel
Inverter for grid-tied solar panel
Grid-tie inverter: Three-phase grid-tie inverter for large solar panel systems
Three-phase grid-tie inverter for large solar panel systems
Grid-tie inverter: Inside an SWEA 250 W transformer-coupled grid-tie inverter
Inside an SWEA 250 W transformer-coupled grid-tie inverter

Worked examples

Example 1 — a first encounter with Grid-tie inverter

Start with the simplest possible case. Write down what Grid-tie inverter claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Grid-tie inverter before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Grid-tie inverter ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Grid-tie inverter

In research
Grid-tie inverter appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Grid-tie inverter in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Grid-tie inverter is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electric power systems components, Inverters, so understanding it makes those chapters shorter.
In everyday life
Look for Grid-tie inverter outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Grid-tie inverter in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Grid-tie inverter means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Grid-tie inverter out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Grid-tie inverter in simple terms?

A grid-tie inverter converts direct current (DC) into an alternating current (AC) suitable for injecting into an electrical power grid, at the same voltage and frequency of that power grid. Grid-tie inverters are used between local electrical power generators: solar panel, wind turbine, hydro-elect…

Why does Grid-tie inverter matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Grid-tie inverter?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Grid-tie inverter.

Tags

  • Electric power systems components
  • Inverters

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