Current Proportional–Integral Control - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 240

Figure 15.1 Block diagram of a current-controlled VSI in the synchronously rotating reference
( dq ) frame

which determines the real power and reactive power exchanged with the grid, respectively.
The inverter in Figure 15.1 is assumed to be powered by a constant DC power source and,
hence, no controller is needed to regulate the DC-link voltage. Otherwise, a controller can be
introduced to regulate the DC-link voltage and to generate I d accordingly.
A particular feature of the control structure shown in Figure 15.1 is that the grid voltage is
feed-forwarded, after being transformed into the dq -components U d and U q , and added to the
output of the current controllers after the abc

dq transformation. This allows the voltage
of the inverter to be the same as the grid voltage when the circuit breaker is not turned on. It
also improves the dynamics of the controller during grid voltage fluctuations, leading to a fast
response for the control system (Timbus et al . 2009). In order to improve the performance of
the PI controller in this structure, cross-coupling terms (

→

± ω L ) are used (Blaabjerg et al . 2006;

Timbus et al . 2006c).
The PI controller implemented in the dq frame is traditionally considered to be a good
solution for regulating sinusoidal currents in balanced three-phase systems. However, it is
not good at correcting unbalanced disturbance currents unless the positive- and negative-
sequence currents are dealt with separately. Moreover, the compensation capability of low-
order harmonics is poor. The PI controller in the synchronously rotating reference frame is more
complex than the PR controller in the stationary frame to be discussed in Chapter 16 because
it requires two coordinate transformations with the knowledge of the phase information of
the grid voltage.
The scheme has the particular advantage of independent control of the real and reactive
components of the grid currents, which can be directly translated into real and reactive power
(Twining and Holmes 2003). As a result, it is possible to set the real power and reactive power
sent to the grid directly.