Long-term (2 years) drying shrinkage evaluation of alkali-activated slag mortar: Experiments and partial factor analysis

Alkali-activated slag with many excellent properties was regarded as a novel low carbon building material, has received more and more attention. This research aims to study the impacts of alkali solution (alkali content and modulus), cement and gypsum contents on compressive strength, weight loss and drying shrinkage for alkali-activated slag mortar. Gypsum used as expanding source could compensate the drying shrinkage caused by silica and alkali components in the first three months, but it has a negative impact on the strength. The mainly results can be concluded that the alkali-activated slag blended with a cement content up to 20 wt% could effectively reduce the shrinkage and weight loss and increase the strength. Furthermore, the alkali content was below 3 wt%, the specimens possess relatively lower drying shrinkage. Based on the results of the test and analysis, the partial factors of combined activation on compressive strength and drying shrinkage of alkali-activated slag mortar were put forward. In the meantime, the relationships between compressive strength and combined activation factor are liner at 28, 90 and 365 days. Compressive strength and drying shrinkage could be estimated according to the combined activation and partial factor analysis. This work could provide a reasonable method for preparing the alkali-activated slag mortar and predict the shrinkage at different periods

Control and operation of modular multilevel converters

The increasing trend of electrical power demands in megacities, industries and transportation requires medium- or high-voltage power electronics devices to serve as the enabling components for high power conversion, due to design flexibility, better dynamic performance, energy efficiency and environment friendliness. Multilevel power converters, such as Neutral Point Clamped (NPC) Converters, Cascaded H-Bridge (CHB) Converters and Modular Multilevel Converters (MMCs), are popular commercialized candidates for such high power applications. Among them, the MMC possesses great performance in terms of functionality, hardware implementation, configuration complexity, output power quality, efficiency and reliability, and it shows considerable research potential in the academic world as well. This thesis aims to investigate the control and operation strategies that related to the efficiency, nonlinearity, modularity and reliability of MMCs. At first, the configuration and operation principles of a three phase MMC is introduced. A comprehensive steady state analysis and the equivalent circuit model that are applicable to an MMC connected to either a DC or an AC bus are then presented in this thesis. In addition to better understanding of the MMC operation, the analysis and modeling of the MMC also suggest the conditions that ensure the stability operation of the MMC system. According to the aforementioned analysis and modeling, the control system of the MMC is introduced. The commonly used cascaded control strategy, including output and inner dynamics control loops, has been discussed. The existing modulation methods for MMCs have been summarized, and the phase-shifted pulse width modulation (PS-PWM) are detailed and adopted in this thesis. The circulating current harmonics in each phase are expected to be suppressed in order to improve the performance and reduce the losses of MMCs. The characteristics of the circulating current harmonics are analyzed and it is revealed that the circulating current are dominated by even order harmonics. An even-harmonic repetitive control scheme is proposed to eliminate the even harmonics in the differential current. The proposed repetitive controller has excellent harmonic elimination, with benefits of less memory occupation, less delay period, doubled low frequency gain, faster convergence rate and dynamic response and wider bandwidth at specific frequencies. The full design details of the even-harmonic repetitive control for an MMC system has been presented. Moreover, the MMC is a multi-input multi-output bilinear system. A nonlinear control strategy is preferred for better control performance. The feedback linearization based current control for the MMC is proposed in this thesis to solve the bilinear control problem. The state function model of the MMC is linearized with the help of the feedback linearization technique. The output and inner differential current control loops are decoupled as two independent simple integrators. Classical linear control laws can be easily applied in the two current control loops. The control performance in steady state and during step changes is improved and the controller design process is facilitated. The centralized control system will reduce the modularity of the MMC in terms of software implementation and is not practical in MMC systems with a large number of sub-modules. A distributed control strategy for MMCs is proposed in this thesis. The proposed control strategy is able to achieve all control objectives in conventional control strategies, with significantly reduced data exchanging among central and local controllers through a communication network. % The effectiveness and stability of the control system have been experimentally confirmed. The fault diagnosis and fault tolerant control for an MMC with redundant sub-modules significantly increases the system reliability. A real-time measurement based semiconductor device open-circuit fault diagnosis method has been proposed. This fault diagnosis is implemented in each local controller in the distributed control architecture with local information. It is capable of accurately identifying multiple faults within 3.5ms without false alarm. Based on the performance analysis of an MMC with bypassed faulty sub-modules, a fault tolerant control is accordingly proposed. The fault tolerant control guarantees the performance of the MMC to be the same as in normal operation. The fault tolerant control can be activated with 5ms after the fault occurrence. The MMC is able to seamlessly ride through the switching device open-circuit fault. All the theoretical findings are verified in PLECS simulations. The effectiveness and practicalities of the control algorithms are confirmed on a scale down single-phase MMC prototype.Doctor of Philosophy (EEE

Seamless fault-tolerant operation of a modular multilevel converter with switch open-circuit fault diagnosis in a distributed control architecture

Modularity and high reliability from redundancy are the two attractive advantages of modular multilevel converters (MMCs). This paper elaborates a switch open-circuit fault diagnosis and a fault-tolerant operation scheme for MMCs with distributed control. The proposed fault diagnosis and fault-tolerant control method can significantly improve the reliability of the MMC while maintaining the modularity of its software implementation. By distributing fault diagnosis into submodules, its local controller is capable of identifying the switching devices in open-circuit fault without extra hardware circuitry. Based on the real-time measurements of submodule terminal voltage and arm current, single, or multiple faulty switches can be identified within 3.5 ms without triggering faulty alarms. Furthermore, a new fault-tolerant operation is proposed to maintain the output current, internal dynamics, and switching harmonics unchanged after the faulty submodule is bypassed. This is achieved by resetting the period and phase registers in the local controller according to the information of bypassed submodules. The control loops of the MMC are not influenced by the proposed fault diagnosis and fault-tolerant operation, making the operation transition seamless and reliable. Experimental results show that fault identification and system reconfiguration can be completed within 5 ms, and the MMC can seamlessly and smooth ride through the switch open-circuit faults without severe malfunction and catastrophic damages.NRF (Natl Research Foundation, S’pore)Accepted versio

Reliability- and cost-based redundancy design for modular multilevel converter

The modular multilevel converter (MMC) is very attractive in a high-voltage dc (HVdc) transmission system due to its high modularity and scalability. It is also a promising candidate for medium-voltage applications. An MMC has many power devices that lead to a relatively low reliability based only on a direct failure rate calculation. The reliability of an MMC can be improved by redundant designs. Lots of redundant designs and strategies have been proposed in recent years, while their reliability improvements are hardly quantified in existing literature. In practice, reliability improvement with constraint costs is a challenge. In view of these, this paper proposes a reliability and cost assessment scheme for MMC redundant designs and strategies. Reliability and cost are quantified for comparison purposes. A medium-voltage application is used to demonstrate the reliability and cost assessment scheme. The reliability analysis is based on a combinatorial model and the Poisson process model. The cost analysis focuses on capital cost and power loss cost. The proposed reliability and cost analysis scheme provides a guideline for MMC redundant design and redundant strategy selection.National Research Foundation (NRF)Accepted versio

Model-predictive current control of modular multilevel converters with phase-shifted pulsewidth modulation

Model-predictive current control (MPCC) is a promising candidate for modular multilevel converter (MMC) control due to its advantages of direct modeling and fast dynamic response. The conventional MPCC, which obtains the optimal control input by evaluating a cost function for all the possible switching states, may make the MPCC impractical due to the exponentially increasing computation burden with the increasing number of submodules (SMs). On the other hand, the MPCC experiences high load current and circulating current tracking errors, since only one switching state is selected and applied during one control period. To address these issues, this paper proposes an MPCC with phase-shifted pulsewidth modulation (PS-PWM) for improving the steady-state control performance. The arm voltages are considered as a whole to implement the proposed MPCC. The optimal duty cycle is obtained based on the load and circulating current tracking error minimization and applied using the PS-PWM. As a result, the computation burden is unrelated to the number of SMs by avoiding the exhaustive evaluation process for all the possible switching states. A better steady-state performance with smaller tracking errors is achieved with the similar switching frequency, and the tedious tuning process of the weighting factor is eliminated. Experimental results are presented to demonstrate the effectiveness of the proposed MPCC

Influence of calcination temperature on the structure and hydration of MgO

The performance of magnesium oxide-based cements and expansion agent closely relates to the reactivity of MgO. This paper systematically investigates the structure and hydration properties of MgO calcined from amorphous magnesite at a wide range of temperatures from 400 to 1150 ℃. XRD, SEM, BET and PSD tests were carried out to determine the structure and particle properties of MgO. The hydration reactivity of MgO was indexed with isothermal calorimeter, DTG, XRD and TEM test. Increasing temperature augments the crystallinity and size of MgO nanograins from about 23.85 nm to 86.37 nm, decreases the specific surface area but makes the particles agglomerate into smaller size. The increasing temperature prolongs the induction period and retards the hydration rate of main peak. The total cumulative heat from complete hydration of MgO is with a similar value of about 800 J/g. The hydration reactivity of MgO can be accurately indexed by its heat release. MgO calcined at 700 ℃ has high reactivity and it dissolves quickly leading to the precipitation of needle Mg(OH)2 initially. The needle brucite has poor crystallinity but it transforms into flake and clavate shape. MgO calcined at 1150 ℃ has very low dissolving rate. The dissolution starts from the (1 1 0) truncation at <1 0 0> edges, and then the cuts at (1 1 1) occur to create an octahedron shape. Some clavate brucite initially precipitates instead of needle brucite during the hydration of low reactive MgO

Feedback linearization-based current control strategy for modular multilevel converters

Modular multilevel converters (MMCs) are multi-input multi-output (MIMO) nonlinear systems. The control systems for MMCs are required to simultaneously achieve multiple control objectives, e.g., output current regulation, submodule capacitor voltage control, and circulating ripple currents suppression. Existing cascaded control strategies for MMCs achieve those control objectives with relatively complex controllers, and the controller parameter design is normally difficult for such nonlinear systems with highly coupled states. In view of this, a feedback linearization-based current control strategy is proposed for an MMC system in this paper. The nonlinear state function model of the MMC is presented and transformed to a linearized and decoupled form with the help of the input-output feedback linearization technique. Based on the linearized system, simple linear controllers are employed to regulate the output and inner differential currents of the MMC, which significantly reduces the difficulty in controller design. The stability of the proposed control strategy is analyzed. The experimental verification results show that, compared to the conventional cascaded control strategies for MMCs, the proposed feedback linearization control strategy is able to achieve improved steady-state and dynamic performances. The robustness of the proposed control strategy against parametric uncertainties is experimentally investigated.NRF (Natl Research Foundation, S’pore