Ognjen Marjanovic

Real-time monitoring of an industrial batch process

This paper describes the development of a real-time monitoring system for a batch process operated by Aroma and Fine Chemicals Limited. The process shares many similarities with other batch processes in that cycle times can vary considerably, instrumentation is limited and inefficient laboratory assays are required to determine the end-point of each batch. The aim of the work conducted in this study was to develop a data driven system to accurately identify the end-point of the batch. This information can then be used to reduce the overall cycle time of the process. Novel approaches based upon multivariate statistical techniques are shown to provide a soft sensor that can estimate the product quality throughout the batch and provide a long-term estimate of the likely cycle time. This system has been implemented on-line and initial results indicate that it offers potential to significantly reduce operating costs.

Damping of inter-area oscillations in mixed AC/DC networks using WAMS based supplementary controller

The paper presents a supplementary VSC-HVDC Power Oscillation Damping (POD) controller based on wide area measurement signals (WAMS). The controller is designed as Multi Input Single Output (MISO) using a Modal Linear Quadratic Gaussian (MLQG) methodology in order to target critical inter-area electromechanical modes. The approach has been tested on a large (16 machine, 68 bus) test network incorporating parallel HVDC/AC transmission and has shown improved damping compared to a traditional Power System Stabilizer (PSS) based controller structure utilizing local signals. The design process has incorporated the effects of wide area signal transmission delays. Variation in these signal delays and the complete loss of signals has been also investigated to establish the robustness of the WAMS based controller and its sensitivity to loss of signals. Extension of the controller to incorporate reactive power modulation has been investigated, as has variation in available active power modulation capacity. The proposed controller performance has been assessed through small and large disturbance analysis.

Fault detection in continuous processes using multivariate statistical methods

The approach to process monitoring known as multivariate statistical process control (MSPC) has developed as a distinct technology, closely related to the field of fault detection and isolation. A body of technical research and industrial applications indicate a unique applicability to complex large-scale processes, but has paid relatively little attention to generic live process issues. In this paper, the impact of various classes of generic abnormality in the operation of continuous process plants on MSPC monitoring is investigated. It is shown how the effectiveness of the MSPC approach may be understood in terms of model and signal-based fault detection methods, and how the multivariate tools may be configured to maximize their effectiveness. A brief review of MSPC for the process industries is also presented, indicating the current state of the art.

Analysis of Active Power Control for VSC–HVDC

This paper presents a comprehensive analysis of the limitations and the key dynamics of closed-loop active power control systems for voltage-source converter (VSC) HVDC, regarding stability, performance, and robustness. Detailed dynamic models are derived and the controllability and robustness issues for VSC active power control are identified. Limitations imposed by ac system strength, converter operating point, and current control design on the stability and performance of the two leading active power control principles are addressed, using frequency-response analysis and time-domain simulations. The dynamic interactions between the active power control design and the dc voltage droop control are examined. The simulations are performed using average-value VSC models and a high-fidelity modular multilevel converter model. Impacts of the active power control design on dynamic behaviors of multiterminal dc (MTDC) systems are investigated using a four-terminal model. This paper provides a systematic study on the key stability and performance issues associated with the active power control. Furthermore, the methodology offers a framework for the analysis of more complex active power and dc voltage droop controllers for future MTDC systems.

Probabilistic Evaluation of Damping Controller in Networks With Multiple VSC-HVDC Lines

The paper presents a robust probabilistic methodology for assessing the performance of power system controllers. In this study, the proposed probabilistic technique is applied to a novel power oscillation damping (POD) controller structure implemented through active power modulation of multiple voltage source converter based high voltage direct current (VSC-HVDC) links within a large heavily meshed network exhibiting multiple inter-area modes. The modal linear quadratic Gaussian (MLQG) controller, as an example of advanced POD, is implemented in both a centralized and a decentralized form for comparison. Following the development and demonstration of the effectiveness of the controller designs, the robust probabilistic performance evaluation is carried out incorporating outage contingencies for generators, lines, and VSC-HVDC lines. The results obtained demonstrate that both controller structures are largely robust to wide ranging operating conditions. For those conditions where controller performance is unsatisfactory, various mitigation techniques are discussed, including the use of classification tools to guide operational limits. A contingency specific controller design is shown to significantly improve performance when robustness cannot be achieved.

Impact of DC Breaker Systems on Multiterminal VSC-HVDC Stability

The use of VSC-HVDC grids for offshore wind farm integration will require the use of dc breaker systems and presently they require dc reactors to limit the rate of rise of fault current. The introduction of large dc reactors throughout a VSC-HVDC system can have a significant impact on its stable operation and will require additional control. This paper analyzes this problem and proposes a PSS-like control (DCPSS) to aid dc grid stability and cope with this effect. A generalized analytical model for studies on dc voltage control is presented. Key stability and transient performance issues caused by the use of the dc reactors in a multiterminal system are investigated by analyzing poles, zeros, and frequency responses of open-loop and closed-loop models. Design and location identification methods for the DCPSS are provided. An excellent damping enhancement is achieved by this controller. The analytical studies and time-domain simulations in this paper are performed based on two VSC-HVDC models.

Damping of electromechanical oscillations by VSC-HVDC active power modulation with supplementary wams based modal LQG controller

This paper presents a Multi Input Single Output (MISO) Modal Linear Quadratic Gaussian (MLQG) Power Oscillation Damping (POD) controller for Voltage Source Converter based HVDC systems. Applying this technique to a small (four machine) test network incorporating parallel HVDC/AC transmission has lead to improved damping of electromechanical modes compared with a traditional Power System Stabilizer (PSS) based controller structure. Furthermore, the MLQG controller has demonstrated greater robustness to varying operating conditions. The performance of the proposed control structure is assessed through small and large disturbance analysis.

Minimising conservatism in infinite-horizon LQR control

This paper studies the formulation of the constrained infinite horizon linear quadratic regulator control law (CIHLQR). Results from recent studies in this area are extended to show that conditions used in the standard formulation of the CIHLQR law are not necessary, but merely sufficient. Through the use of a novel proof it is shown that for a general SISO system with input constraints and certain conditions imposed, saturated LQR provides the same control sequence as CIHLQR. It is further shown that saturated LQR is equivalent to the CIHLQR in the case of first-order systems, subject to both state and control constraints. Finally, the region of constrained stabilisability is characterised for the case of open-loop unstable first-order systems.

Multivariate Statistical Analysis of Raman Images of a Pharmaceutical Tablet

This paper describes the application of principal component analysis (PCA) and independent component analysis (ICA) to identify the reference spectra of a pharmaceutical tablets constituent compounds from Raman spectroscopic data. The analysis shows, first with a simulated data set and then with data collected from a pharmaceutical tablet, that both PCA and ICA are able to identify most of the features present in the reference spectra of the constituent compounds. However, the results suggest that the ICA method may be more appropriate when attempting to identify unknown reference spectra from a sample. The resulting PCA and ICA models are subsequently used to estimate the relative concentrations of the constituent compounds and to produce spatial distribution images of the analyzed tablet. These images provide a visual representation of the spatial distribution of the constituent compounds throughout the tablet. Images associated with the ICA scores are found to be more informative and not as affected by measurement noise as the PCA based score images. The paper concludes with a discussion of the future work that needs to be undertaken for ICA to gain wider acceptance in the applied spectroscopy community.

Excitation control and voltage regulation of switched reluctance generators above base speed operation

Switched reluctance (SR) generators are a candidate technology in vehicle electric power generation applications that require operation under harsh conditions such as high temperature and high speed. This paper presents the excitation control and DC link voltage regulator design for a SR generator system. Excitation control is designed with consideration of the machine electrical dynamics in single pulse mode of operation at different speeds, and dynamically locates the conduction period to a predefined region of the inductance profile for higher efficiency. This excitation controller is compared with a fixed turn-on angle and variable turn-off angle based excitation controller. The discrete nature of the SR generator excitation and per-stroke average DC link dynamics control issue is treated in discrete-time domain. The DC link voltage regulation is formulated as a multirate proportional integral (PI) control scheme. The proposed controller is validated and analyzed by means of a finite element (FE) model based dynamic simulation of a three phase SR generator.