February 2011
Intermediate
416 pages
11h 24m
English
12
Grid Current Control
12.1 Introduction
When wind and photovoltaic systems are grid-connected using a voltage source converter as the front-end, the grid current is usually controlled in order to control active and reactive power exchange, as discussed in previous chapters [1]. The position of voltage and current sensors can be different with respect to Figure 12.1, as already discussed in Chapter 11.
Figure 12.1 Block diagram of a typical three-phase distributed inverter (in PV system L2 is usually not present)
Moreover, in this chapter the influence of the Cf capacitor of the filter will be neglected since it only deals with the switching ripple frequencies. In fact, at frequencies lower than half of the resonance frequency the LCL-filter inverter model and the L-filter inverter models are practically the same. The LCL-filter-based inverter (Figure 12.2 shows the frequency characteristic i/υ of the LCL filter) behaves as if the capacitor is not present, i = ig (hence in the following i will be used in all the equations and schemes of the reviewed current control techniques) and the frequency characteristic is equivalent to the frequency characteristic of a filter made by the sum of the inductances downstream of the converter (L1, L2 and Lg).
Figure 12.2 Frequency characteristic (i/v) of the LCL filter
This chapter reviews basic current control techniques and modulation ...
