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Grid-tied inverters - How they Work

The Grid

This refers to the electricity supply network, which gets its power input from power stations around the country, and which supplies it to consumers, very often at 220-240 VAC 50 Hz. These power stations are usually very large, each providing power in the order of megawatts at an overall efficiency of 35 - 38%. Generally, individual consumers can use a maximum of 14 - 15 kW corresponding to a 60A supply at 230-240 VAC.

The Tie

Where a small generator is to be connected to The Grid, the grid tied inverter output must be synchronised to the mains frequency, and organised so that the power it will attempt to supply to the grid is well-controlled. Generally, sine wave inverters are easy to implement using modern existing electronic technology, but herein lies a problem:

Ideally, the grid will carry a pure sine wave of voltage, but in practice, this is rarely the case, and distortions caused by the presence of harmonic voltages will cause the waveform to be distorted. This distortion, being produced at a very low impedance, is impossible to overcome, at least for individual systems, and therefore the small producer generally simply has to live with it. This means that the grid-tied inverter will generally use the mains voltage envelope itself as a template for the current waveform it will put out, in order to avoid the generation of large local harmonic currents which will result if the inverter tries to "correct" the grid waveform by outputting a pure sine wave. Click here for a block diagram

The Inverter

The grid-tied inverter differs from the stand-alone variety in that the control circuit has to be able to operate in the presence of the existing grid voltage and force the grid to accept power instead of providing it. Because the grid is essentially a very low impedance voltage source, the inverter must be able to act as a current source, only allowing the desired amount of current to be sent into the grid. This process requires close control of the inverter output voltage. Generally, the inverter will control its bulk DC input voltage and use this to determine the output power level. The power level signal is then used to determine the output power, and the inverter output will be adjusted upward until this amount of power is delivered to the grid.

To meet standards such as G83 in the UK and VDE0126 in Germany, grid-tied inverters have to have an over/undervoltage protection, and be able to detect the existence of over and under frequency conditions on the line. They also have to avoid injecting direct-current (DC) into the grid and need to produce a balanced output such that there is no current leakage to ground.

There are a number of ISO and IEC safety-related standards which may apply such as:

  1. IEC 60146-2 Semiconductor converters - Part 2: Self-commutated semiconductor converters including direct d.c. converters
  2. IEC/TS 61287-2 Power convertors installed on board railway rolling stock - Part 2: Additional technical information
  3. IEC 61377 Railway applications - Rolling stock - Part 1: Combined testing of inverter-fed alternating current motors and their control system
  4. IEC 61727 Photovoltaic (PV) systems - Characteristics of the utility interface
  5. IEC 62093
all of which may be found on the IEC website

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