08-08-2012, 11:49 AM
Integrated Current Control, Energy Control and Energy Balancing of Modular Multilevel Converters
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INTRODUCTION
Modular Multilevel Converters (MMCs) are the novel way
to meet increasing requirements in power electronics and energy
systems. This innovative technology has the capability to
replace standard Voltage Source Converter (VSC) technology
at even lower costs and higher reliability, e.g. in high-voltage
direct current (HVDC) transmission systems. Compared to
conventional VSC technology, MMCs offer advantages such
as higher voltage levels, modular construction, longer maintenance
intervals and improved safety. For power transmission,
stability and robustness of the control and especially the
behavior during grid failures are important economic issues.
CONTROL ASPECTS OF MMCS
Current Control
In general, MMCs have more currents to be controlled than
standard VSCs. For the MMC technology five independent
currents can be derived from the equivalent circuit diagram
in Fig. 2. Besides the clamp currents, like Park-transformed
AC line currents iLp , iLq and the DC current id, so-called
circular currents, which do not affect any clamp current or
behavior, can be defined [1], [2], [4], [5]. Circular currents
can be used for energy balancing between PMs. An adequate
control system has to control all currents without exception.
Bilinear State-Space Model
1) Bilinear Systems: The properties of Bilinear Systems
(BLS), characterized by multiplication of state variables with
control input variables, are detailed in [7]. Plenty of technical
systems can be modeled as BLSs since often energy
states, mass flows or similar are described by a state variable
multiplied with a control input. This fact make BLSs suitable
to model energy systems and power electronics with energy
states.
CONTROL STRUCTURE
An advantageous control structure is given in Fig. 5, in
which an extended estimator/observer determines the current
states and guarantees exact steady-state tracking and disturbance
rejection by subtracting the estimated input disturbance
ˆ, see [6], [8]. Estimated energy values ˆ w of the PMs can be
obtained exemplary by the use of a least squares (LS) estimator
as one strategy to find the energy states of the converter
system. Reference input r, control inputs u and system outputs
y remain as usual denotations of system variables.
CONCLUSION
In this paper a novel strategy integrating current control,
energy control and energy balancing of MMCs has been
proposed. The approach is highly structured and formalized
which is crucial in practical applications and therefore offers
a promising alternative to today’s cascaded structures.