Inverters

About Inverters

A solar inverter, or PV inverter, converts the variable direct current (DC) output of a photovoltaic (PV) solar panel into a utility frequency alternating current (AC) that can be fed into a commercial electrical grid or used by a local, off-grid electrical network. It is a critical component in a photovoltaic system, allowing the use of ordinary commercial appliances. Solar inverters have special functions adapted for use with photovoltaic arrays, including maximum power point tracking and anti-islanding protection.ization (NEDO).

These Inverters take the DC power, usually from solar panels, and invert it to AC power that matches the local AC grid parameters e.g. frequency 50 or 60 Hz using a built in oscillator, voltage matched exactly and phase when installed in a 3 phase situation so it can be utilised by the property it serves. A high-quality modern GTI has a fixed unity power factor, which means its output voltage and current are perfectly lined up, and its phase angle is within 1 degree of the AC power grid. The inverter has an on-board computer which will sense the current AC grid waveform, and output a voltage to correspond with this grid. However, supplying reactive power to the grid might be necessary to keep the voltage in the local grid inside allowed limitations. Otherwise, in a grid segment with considerable power from renewable sources voltage levels might rise too much at times of high production, i.e. around noon.

A particular safety feature of Grid-tie inverters is that they are designed to quickly disconnect from the grid if the utility grid goes down as in a power cut or utility servicing. Likewise should the grid also change its electrical parameters like frequency or voltage outside those pre- set parameters it will shut down. This characteristic, known as “island protection” preventimg this unwanted export of energy. Normally this is a National requirement that ensures that in the event of a blackout, the grid tie inverter will shut down to prevent the energy from harming any line workers who are sent to fix the power grid.

When installed at premises, the grid tie inverter enables the owner to use an alternative power generation system like solar or wind power to reduce their dependence on the grid. If the alternative power being produced is insufficient, the deficit will be sourced from the electricity grid.

Classification

Solar inverters may be classified into three broad types:

1. Stand-alone inverters, 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 AC source, when available. Normally these do not interface in any way with the utility grid, and as such, are not required to have anti-islanding protection.

2. Grid-tie inverters, which match phase with a utility-supplied sine wave. Grid-tie inverters are designed to shut down automatically upon loss of utility supply, for safety reasons. They do not provide backup power during utility outages.

Grid-tie inverters that are available on the market today use a number of different technologies. The inverters may use the newer high-frequency transformers, conventional low-frequency transformers, or without transformer. Instead of converting direct current directly to 120 or 240 volts AC, high-frequency transformers employ a computerized multi-step process that involves converting the power to high-frequency AC and then back to DC and then to the final AC output voltage Transformerless inverters are lighter and more efficient than their counterparts with transformers.

Most grid-tie inverters on the market include a maximum power point tracker ( MPPT) on the input side that enables the inverter to extract a maximum amount of power from its intended power source. Since MPPT algorithms differ for solar panels and wind turbines, specially made inverters for each of these power sources are available.

When numerous solar panels are linked together in series to make an array voltages can be anything up to 1000 Volts, It could easily be forgotten that it is live, even on a cloudy day.

3. Battery backup inverters, are special inverters which are designed to draw energy from a battery, manage the battery charge via an onboard charger, and export excess energy to the utility grid. These inverters are capable of supplying AC energy to selected loads during a utility outage, and are required to have anti-islanding protection.

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