DC Link Requirements for Grid Tie Inverters

  

  

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A common question we field regarding Grid-Tie inverters goes something like "Can I interface to a 480V grid with a 680V DC Link?".  To answer this question, lets consider a typical Grid Tie Inverter or Active Front End application as illustrated in the figure below.

Generally, our application code uses center-aligned, space vector pulse width modulation (SVM) techniques for maximum DC Link voltage utilization.  Using SVM, the switch-mode AC voltage output of the inverter, before the filter components, is limited by the dead-time of the power stage and any minimum pulse requirements of the IGBT drivers.  For example, 3usec of dead-time when switching at 10kHz, results in a maximum achievable duty-cyle of 97%.   Depending on theswitching frequency, these hardware limitations will set the maximum, peak, line to line output voltage to ~95% to 97% of the DC Link.  Note however, that this is the point at which the duty-cycle is clamped, and the controls become non-linear.  In practice, you will want to allow several percent duty-cycle for control headroom in order to provide the controller linear operating range.  As such, 92-94% is a more practical design value.  

The previous discussion addresses the maximum obtainable voltage at the output of the IGBTs.  Considering a GTI or AFE converter can source current into the grid, one also has to allow for voltage drops across the filter components at maximum output current.  Let's consider a typical 100kW Grid Tie Inverter interfacing to a 480V AC line.  At 10% high line, the maximum peak line voltage is 480*1.1*SQRT(2) = 747Vpk.  Assumming 30Vpk is dropped across the filter components, the required minimum DC Link voltage is given by (747+30)/0.92 = 845V.