Introduction - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 3

which are called L-inverters, R-inverters and C-inverters, respectively. C-inverters are able to
offer much better voltage quality than L-inverters and R-inverters with the same hardware. In
Chapter 8, a strategy that is the same as bypassing the harmonic components in the load current
is presented to improve the voltage quality. Another strategy that falls into this category is to
inject the right amount of voltage harmonics into the reference voltage of an inverter so that it
cancels the harmonic voltage dropped on the output impedance, which improves the quality of
the output voltage. This is presented in Chapter 21, in Part III, after presenting the robust droop
control in Chapter 19. As an application example, the power quality issues in traction power
systems, including current harmonics, negative-sequence currents and low power factor, are
addressed in Chapter 9.
Part II is devoted to the provision of an independently-controlled neutral line, which facil-
itates the implementation of other functions in a power electronic system. The topologies to
provide a neutral line are presented in Chapter 10. In Chapter 11, a controller is designed to
maintain a stable neutral point with classical control strategies, from which the parameters of
the neutral leg are determined. In Chapter 12, a controller is designed with the H ∞ control
strategy, taking the voltage shift of the neutral point and the current flowing into the DC-link
capacitors as feedback. In Chapter 13, an H ∞ current controller is designed to minimise the
current flowing into the DC-link capacitor and a PI controller is designed to bring the DC
voltage shift back to the mid-point of the DC link. These two controllers are decoupled in the
frequency domain and, hence, can be arranged in a parallel control structure. The provision of
an independently-controlled neutral line is applied in Chapter 14, as an application example,
to the generation of an independent three-phase power supply from a single-phase source.
Part III is devoted to power flow control. The control strategies can be classified into two
categories: current-controlled strategies to directly control the current exchanged with the grid
and voltage-controlled strategies to control the voltage of the inverter so that the power flow
is indirectly controlled. Current-controlled strategies are easy to implement but the inverters
equipped with current-controlled strategies cannot directly take part in the regulation of power
system frequency and voltage and, hence, they may cause problems for system stability when
the share of power fed into the grid is significant. The PI control, PR control and DB predictive
control presented in Chapters 15-17 belong to this category. The repetitive controller presented
in Chapter 3, in Part I, also belongs to this category. Voltage-controlled strategies have attracted
a lot of attention from academia and industry in recent years because they are able to take part in
the regulation of system frequency and voltage. In Chapter 18, a control strategy is presented
to make inverters mathematically equivalent to conventional synchronous generators. Such
inverters are called synchronverters. As a result, all the technologies developed for synchronous
generators can be applied to inverters, which considerably facilitates the grid connection of
renewable energy and smart grid integration. A highly compact controller is presented to
implement the functions of frequency control, real power control, voltage control and reactive
power control. In Chapter 19, the parallel operation of inverters is discussed. After presenting
the conventional droop control strategies for L-, R- and C-inverters, the inherent limitations of
the conventional droop control are revealed. The accuracy of power sharing greatly depends on
the accuracy and consistency of the components, and the voltage regulation capability is poor.
Then, robust droop control strategies for R-inverters, L-inverters and C-inverters are presented
so that accurate sharing of both real power and reactive power can be achieved even if there
are component mismatches, numerical errors, disturbances and noises, etc. A byproduct is that
the voltage regulation capability is considerably enhanced as well. In order to improve the