607 research outputs found
Generalized Voltage-based State-Space Modelling of Modular Multilevel Converters with Constant Equilibrium in Steady-State
This paper demonstrates that the sum and difference of the upper and lower
arm voltages are suitable variables for deriving a generalized state-space
model of an MMC which settles at a constant equilibrium in steady-state
operation, while including the internal voltage and current dynamics. The
presented modelling approach allows for separating the multiple frequency
components appearing within the MMC as a first step of the model derivation, to
avoid variables containing multiple frequency components in steady-state. On
this basis, it is shown that Park transformations at three different
frequencies (, and ) can be applied for deriving
a model formulation where all state-variables will settle at constant values in
steady-state, corresponding to an equilibrium point of the model. The resulting
model is accurately capturing the internal current and voltage dynamics of a
three-phase MMC, independently from how the control system is implemented. The
main advantage of this model formulation is that it can be linearised, allowing
for eigenvalue-based analysis of the MMC dynamics. Furthermore, the model can
be utilized for control system design by multi-variable methods requiring any
stable equilibrium to be defined by a fixed operating point. Time-domain
simulations in comparison to an established average model of the MMC, as well
as results from a detailed simulation model of an MMC with 400 sub-modules per
arm, are presented as verification of the validity and accuracy of the
developed model
Dual Channel Control with DC Fault Ride Through for MMC-based, Isolated DC/DC Converter
This study is sponsored by the Engineering and Physical Sciences Research Council (EPSRC) grant no EP/K006428/1, 2013.D. Jovcic and H. Zhang are with the School of Engineering, University of Aberdeen, AB24 3UE, U.K. ([email protected], [email protected]).Peer reviewedPostprin
Global Tracking Passivity--based PI Control of Bilinear Systems and its Application to the Boost and Modular Multilevel Converters
This paper deals with the problem of trajectory tracking of a class of
bilinear systems with time--varying measurable disturbance. A set of matrices
{A,B_i} has been identified, via a linear matrix inequality, for which it is
possible to ensure global tracking of (admissible, differentiable) trajectories
with a simple linear time--varying PI controller. Instrumental to establish the
result is the construction of an output signal with respect to which the
incremental model is passive. The result is applied to the boost and the
modular multilevel converter for which experimental results are given.Comment: 9 pages, 10 figure
Modeling and enhanced control of hybrid full bridge–half bridge MMCs for HVDC grid studies
Modular multilevel converters (MMCs) are expected to play an important role in future
high voltage direct current (HVDC) grids. Moreover, advanced MMC topologies may include various
submodule (SM) types. In this sense, the modeling of MMCs is paramount for HVDC grid studies.
Detailed models of MMCs are cumbersome for electromagnetic transient (EMT) programs due to
the high number of components and large simulation times. For this reason, simplified models that
reduce the computation times while reproducing the dynamics of the MMCs are needed. However, up
to now, the models already developed do not consider hybrid MMCs, which consist of different types
of SMs. In this paper, a procedure to simulate MMCs having different SM topologies is proposed.
First, the structure of hybrid MMCs and the modeling method is presented. Next, an enhanced
procedure to compute the number of SMs to be inserted that takes into account the different behavior
of full-bridge SMs (FB-SMs) and half-bridge submodules (HB-SMs) is proposed in order to improve
the steady-state and dynamic response of hybrid MMCs. Finally, the MMC model and its control are
validated by means of detailed PSCAD simulations for both steady-state and transients conditions
(AC and DC faults)
Management and Protection of High-Voltage Direct Current Systems Based on Modular Multilevel Converters
The electrical grid is undergoing large changes due to the massive integration of renewable energy systems and the electrification of transport and heating sectors. These new resources are typically non-dispatchable and dependent on external factors (e.g., weather, user patterns). These two aspects make the generation and demand less predictable, facilitating a larger power variability. As a consequence, rejecting disturbances and respecting power quality constraints gets more challenging, as small power imbalances can create large frequency deviations with faster transients.
In order to deal with these challenges, the energy system needs an upgraded infrastructure and improved control system. In this regard, high-voltage direct current (HVdc) systems can increase the controllability of the power system, facilitating the integration of large renewable energy systems.
This thesis contributes to the advancement of the state of the art in HVdc systems, addressing the modeling, control and protection of HVdc systems, adopting modular multilevel converter (MMC) technology, with focus in providing services to ac systems.
HVdc system control and protection studies need for an accurate HVdc terminal modeling in largely different time frames. Thus, as a first step, this thesis presents a guideline for the necessary level of deepness of the power electronics modeling with respect to the power system problem under study.
Starting from a proper modeling for power system studies, this thesis proposes an HVdc frequency regulation approach, which adapts the power consumption of voltage-dependent loads by means of controlled reactive power injections, that control the voltage in the grid. This solution enables a fast and accurate load power control, able to minimize the frequency swing in asynchronous or embedded HVdc applications.
One key challenge of HVdc systems is a proper protection system and particularly dc circuit breaker (CB) design, which necessitates fault current analysis for a large number of grid scenarios and parameters. This thesis applies the knowledge developed in the modeling and control of HVdc systems, to develop a fast and accurate fault current estimation method for MMC-based HVdc system. This method, including the HVdc control, achieved to accurately estimate the fault current peak value and slope with very small computational effort compared to the conventional approach using EMT-simulations.
This work is concluded introducing a new protection methodology, that involves the fault blocking capability of MMCs with mixed submodule (SM) structure, without the need for an additional CB. The main focus is the adaption of the MMC topology with reduced number of bipolar SM to achieve similar fault clearing performance as with dc CB and tolerable SM over-voltage
Modular multilevel converter based HVDC transmission system for offshore wind farms
This doctoral thesis falls within the scope of electronic power converters oriented to high voltage transmission applications, in particular the power generated in remote offshore wind farms by means of HVDC subsea cables.
This research is focused on the Modular Multilevel Converter (MMC) with two level submodules but also with multilevel topology submodules such as 3L-FC (three level flying capacitors) and 3L-NPC (three level neutral point capacitors). The main contribution of this thesis is the developed PWM based modulation strategy which allows the balancing of the total amount of submodules capacitors. It is applicable to the aforementioned submodule topologies under different working conditions as evidenced by experimental results
Small-Signal Dynamic DQ Model of Modular Multilevel Converter for System Studies
This work was funded by the European Research Council under the Ideas program in FP7 under Grant 259328, 2010. Paper no. TPWRD-00046-2015.Peer reviewedPostprin
MMC impedance modelling and interaction of converters in close proximity
This paper develops a small-signal impedance model of modular multilevel converters (MMCs) using harmonic state-space (HSS) method and studies the stability in a multiple converter scenario. In order to simplify analysis on the coupling characteristics between different frequencies in MMCs, the proposed model is developed in the positive-negative-zero (PN0) sequence-frame, where the zero-sequence current in three-phase three-wire system is directly set to zero without introducing complicated method. A simple 2 by 2 admittance matrix in PN0-frame is extracted from the MMC small-signal model for ease of system stability analysis. Using the developed impedance model, the multi-infeed interaction factor (MIIF) measure is adopted to analyze the most significant interactions for multi-infeed converter systems to be prioritized. Different outer-loop controllers are adopted and compared in the analysis to illustrate the effect of different control modes on converter impedance and system stability. Analytical studies and time-domain simulation results are provided to validate the proposed model and stability analysis
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