Modular multilevel converter losses model for HVdc applications

Multi-terminal high voltage dc (HVdc) grids can eventually became a feasible solution to transport energy to remote and/ or distant areas and its exploitation depend, among other things, on the performance of the converter terminals. Therefore, to optimize the power transmission strategy along such a grid, it is necessary to recognize the efficiency of all the converters in all points of operation, namely with the different load conditions. In this vision, the aim of this work is to provide the methodology to model the modular multilevel converter (MMC) efficiency by means of a mathematical expression that can describe, over a broad range of active and reactive power flow combinations, the power losses generated by the semiconductors. According to the presented methodology, a polynomial-based model with a reduced number of coefficients is deducted, in such a way that can be directly used for optimal power flow (OPF) studies. The accuracy of the proposed model is characterized by an absolute relative error, at the worst scenario, approximately equal to 3%.Postprint (author's final draft

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

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

Investigation of FACTS devices to improve power quality in distribution networks

Flexible AC transmission system (FACTS) technologies are power electronic solutions that improve power transmission through enhanced power transfer volume and stability, and resolve quality and reliability issues in distribution networks carrying sensitive equipment and non-linear loads. The use of FACTS in distribution systems is still in its infancy. Voltages and power ratings in distribution networks are at a level where realistic FACTS devices can be deployed. Efficient power converters and therefore loss minimisation are crucial prerequisites for deployment of FACTS devices. This thesis investigates high power semiconductor device losses in detail. Analytical closed form equations are developed for conduction loss in power devices as a function of device ratings and operating conditions. These formulae have been shown to predict losses very accurately, in line with manufacturer data. The developed formulae enable circuit designers to quickly estimate circuit losses and determine the sensitivity of those losses to device voltage and current ratings, and thus select the optimal semiconductor device for a specific application. It is shown that in the case of majority carrier devices (such as power MOSFETs), the conduction power loss (at rated current) increases linearly in relation to the varying rated current (at constant blocking voltage), but is a square root of the variable blocking voltage when rated current is fixed. For minority carrier devices (such as a pin diode or IGBT), a similar relationship is observed for varying current, however where the blocking voltage is altered, power losses are derived as a square root with an offset (from the origin). Finally, this thesis conducts a power loss-oriented evaluation of cascade type multilevel converters suited to reactive power compensation in 11kV and 33kV systems. The cascade cell converter is constructed from a series arrangement of cell modules. Two prospective structures of cascade type converters were compared as a case study: the traditional type which uses equal-sized cells in its chain, and a second with a ternary relationship between its dc-link voltages. Modelling (at 81 and 27 levels) was carried out under steady state conditions, with simplified models based on the switching function and using standard circuit simulators. A detailed survey of non punch through (NPT) and punch through (PT) IGBTs was completed for the purpose of designing the two cascaded converters. Results show that conduction losses are dominant in both types of converters in NPT and PT IGBTs for 11kV and 33kV systems. The equal-sized converter is only likely to be useful in one case (27-levels in the 33kV system). The ternary-sequence converter produces lower losses in all other cases, and this is especially noticeable for the 81-level converter operating in an 11kV network

Cost-Effective Model Predictive Control Techniques for Modular Multilevel Converters

In this thesis, model predictive control (MPC) techniques are investigated with their applications to modular multilevel converters (MMCs). Since normally a large number of submodule (SM) capacitor voltages and gate signals need to be handled in an MMC, the MPC schemes studied in this thesis are employed for determining only the voltage levels of converter arms, while gate signals are subsequently generated by the conventional sorting method. Emphasis is given to inner-loop current control in terms of phase current and circulating current, aiming at performance enhancement and computation reduction. A variable rounding level control (VRLC) approach is developed in this thesis, which is based on a modification of the conventional nearest level control (NLC) scheme: instead of the conventional nearest integer function, a proper rounding function is selected for each arm of the MMC employing the MPC method. As a result, the simplicity of the NLC is maintained while the current regulating ability is improved. The VRLC technique can also be generalized from an MPC perspective. Different current controllers can be considered to generate the arm voltage references as input of the VRLC block, thus refining the control sets of the MPC. Based on the decoupled current models, the accumulated effect of SM capacitor voltage ripples is investigated, revealing that the VRLC strategy may not achieve a proper performance if the accumulated ripple is nontrivial compared to the voltage per level. Two indexes are also proposed for quantifying the current controllability of the VRLC. Benefiting from this analysis, A SM-grouping solution is put forward to apply such MPC techniques to an MMC with a large number of SMs, leading to an equivalent operation of an MMC with much reduced number of SMs, which significantly increases the current regulating capability with reduced complexity. As an example, the SM-grouping VRLC proposal is analyzed and its system design principles are described. This thesis also develops another MPC technique which directly optimizes the cost function using quadratic programming technique. Both a rigorous and a simplified procedure are provided to solve the optimization problem. Compared with the conventional finite control set (FCS)-MPC method which evaluates all voltage level combinations, the proposed scheme presents apparent advantage in terms of calculation cost while achieving similar performance

DC side and AC side cascaded multilevel inverter topologies: A comparative study due to variation in design features

This paper presents a comparative study between DC side and AC side cascaded topologies for the hybrid modular multilevel converter (MMC) which are becoming popular in recent years. A multilevel converter with half or full bridge sub modules connected across DC link is another alternative for high-voltage applications as it has the same number of sub modules and footprint as AC side cascaded topology with the same DC link voltage and AC side voltage. The compared AC side cascaded structure offers a two-level converter as the high voltage stage and cascaded H-bridge (which is full bridge) sub modules with electrically isolated DC sources or capacitors for the low voltage stages which has number of features suitable for HVDC application. The comparison aspects are investigated against 6 different converter sub module number and configuration options for losses, harmonic profile of the output voltage, and the DC fault current characteristics (before blocking the IGBT gate signals during the DC fault) with the same input DC voltage and the same load for both (DC and AC side) topologies. The major results and findings of this investigation are presented, compared and discussed

Power losses in HVDC converter stations.

Masters Degree. University of Kwa-Zulu Natal, Durban.In transmission systems, particularly when the power is transported over a transmission line of distance 500 km and above, a considerable amount of power is lost during power system operations, which consist of all the components that are used in generation and transmission of power. Therefore, it is imperative to estimate the power losses due to some power equipment on the electrical network during transmission systems. More so, it is of importance to comprehend the pros and cons of both LCC-based and VSC-based transmission systems and subsequently carry out detailed research on power losses of both systems using the calculation methods listed in standards. It is the purpose of this research work to determine and calculate the overall losses of various equipment of high-voltage direct current (HVDC) converter stations under operating and standby modes using standards IEC 61803, IEEE 1158, IEC 62751-1-2 and the component datasheet parameters (Phase Control Thyristor Type DCR3030V42 and Dynex IGBT module DIM1200ASM45-TS000). The loss calculations in this research work are precisely applicable to all parts of the converter station and cover standby, partial-load, and full-load losses using the standardized calculation methods stipulated in the aforementioned standards. Furthermore, Switching losses, as well as conduction losses are included in the calculation using a simplified analytical model, based on the standards IEC 62751-1-2 and power semiconductor (Dynex) datasheet information. Therefore, an analytical method was adopted to estimate the power losses of VSC-based HVDC system of two-level, three-level and modular multilevel VSC configurations. Finally, the various HVDC technologies (circuit simulations) models were implemented in the Matlab-Simulink environment. The Matlab models were used to estimate the power losses of these technologies converter losses for various operating conditions. The simulation technique has been devised to provide an independent crosscheck on the results obtained using idealized mathematical representations (analytical technique). Subsequent to these circuit implementations, some results were obtained and consequently validated with other commercial power loss simulation tools or electronic software, such as Semisel and Melcosim. The use of different contrasting techniques to provide equivalent characteristics losses calculations provide a good method of validating the feasibility of the HVDC technology loss study, giving confidence in the results for the converter losses that have been obtained. This research work is based on an existing method of loss evaluation, but strictly followed the IEEE loss calculation methods stipulated in standards. The major contribution of this research work was the new approach adopted in the power loss evaluation of various HVDC technologies such as the LCC-based and VSC-based topologies of the converter stations using the idealized mathematical representations stipulated in standards IEC 61803, IEEE 1158, IEC 62751-1-2 and the component datasheet parameter, which signifies the novel output of this research work