Individual Channel Design Research Articles

Small-signal stability analysis of doubly-fed pumped storage plants based on impedance modeling and individual channel design

The small-signal stability (SSS) of the doubly-fed variable-speed pumped storage plant (VSPSP) is analysed based on the impedance modelling and Individual Channel Design (ICD) framework. There exists couplings between the d and q frame in the impedance model of doubly-fed induction machine (DFIM), which have a significant influence on the stability. Considering that the couplings are hard to reduce in different domains, this paper apply a more transparent ICD framework instead of the widely used Generalized Nyquist Criterion (GNC) to analyse the SSS of VSPSP. Firstly, the impedance model of DFIM in d-q domain is derived through transfer matrix. Then two single input single output (SISO) channel transmittances, which represent the dynamics of d-q frame respectively, are reformulated based on ICD. Therefore, the classical Nyquist/Bode analysis is utilized in the individual channels. It is proved that the ICD can preserve the loop interactions in the individual channels although the cross-coupling is strong. The influences of electrical and control parameters as well as operating mode on the SSS of DFIM based VSPSP are analysed, and the structural robustness of the channels are evaluated. The results show that the system is more prone to instability in pump mode.

Individual Channel Design-Based Precise Analysis and Design for Three-Phase Grid-Tied Inverter With LCL-Filter Under Unbalanced Grid Impedance

Three-phase grid-tied inverter with LCL filter is usually designed to operate under symmetric grid impedance. However, in actual operations, the equivalent three-phase grid impedance tends to be unbalanced, which turns the three-phase grid-tied inverter with LCL filter into a highly coupled multiple-input-multiple-out system. Traditionally, the impact of the cross-coupling on the stability is directly overlooked, which may lead to imprecise stability analysis. To overcome this issue, this article proposes an analysis and design method for three-phase grid-tied inverter with LCL filter under the unbalanced grid impedance based on the individual channel analysis and design. First, the effect of unbalanced grid impedance on the structural robustness is comprehensively evaluated. Then, the control system is simplified with no loss of structural information. Thus, the stability can be precisely analyzed and, simultaneously, the controller parameters can be easily tuned by applying Bode/Nyquist plots. Simulation and experimental results are provided to demonstrate the validity and effectiveness of the proposed method.

Analysis of Low-Frequency Stability in Grid-Tied DFIGs by Nonminimum Phase Zero Identification

For power converter-interfaced wind-generation systems, low-frequency instability issues arising from the interactions among converter outer power flow control (PFC) loops have got increased attention recently. This paper explores the possible origin of such instability in grid-tied doubly fed induction generator systems. To comprehensively analyze this multiloop interacted system, a reduced transfer matrix model is first constructed allowing for the PFC dynamics. Then the individual channel analysis and design approach is employed to decompose such closed-loop multivariable system into components such as local subsystem and loop interactions. Subsequently, frequency-response behavior analyses of these components are individually conducted at various operating points to study their relative contribution to overall stability. Results indicate that local active power control loop will induce a right-half-plane zero (RHPZ) for certain high-power operating conditions, and the resulting inverse response leads system unstable. Meanwhile, the critical operating point at which the RHPZ arises is analytically derived, and demonstrated to be close to the actual dynamic stability boundary.

Analysis of the damping characteristics of two power electronics-based devices using ‘individual channel analysis and design’

Abstract A comparison of the capabilities of two quite distinct power electronics-based ‘flexible AC transmission systems’ devices is presented. In particular, the damping of low frequency electromechanical oscillations is investigated aiming at improving the performance of power systems. The comparison is made using frequency domain methods under the ‘individual channel analysis and design’ framework. A synchronous generator feeding into a system with large inertia is used for such a purpose. Two system configurations including compensation are analysed: (a) in series in the form of a thyristor-controlled series compensator, and (b) in shunt through a static VAr compensator featuring a damping controller. Analyses are carried out to elucidate the dynamic behaviour of the synchronous generator in the presence of the power electronics-based controllers and for the case when no controller is present. Performance and robustness assessments are given particular emphasis. The crux of the matter is the comparison between the abilities of the static VAr compensator and the thyristor-controlled series compensator to eliminate the problematic switch-back characteristic intrinsic to synchronous generator operation by using the physical insight afforded by ‘individual channel analysis and design’.

A mode-matched force-rebalance control for a MEMS vibratory gyroscope

The force-to-rebalance (FTR) closed-loop control is widely used in MEMS vibratory gyroscopes. However, most of these applications may operate in split modes, as mode matching is usually conducted open loop. There is a lack of discussion explicitly addressing the significance of mode matching in the FTR operation mode. This paper investigates the influence of mode mistuning on the FTR closed-loop control of a MEMS Coriolis vibratory gyroscope (CVG), and proposes a novel tuning method using real time control forces to achieve a mode-matched FTR control. The analysis and design of the FTR is based on the time averaged equations of motion, where the sense mode vibration is decomposed into the quadrature and in-phase channels with cross coupling determined by the frequency mismatch between the drive and sense modes of vibration. The control design is treated as a 2 × 2 multivariable control problem using the individual channel design (ICD) framework. Independent control design for each of the two channels allows the bandwidth of the quadrature loop to be significantly less than the in-phase loop. The characteristics of mode mistuning can be extracted from the real time feedback forces. Using this information, the desirable mode-matched uncoupled FTR can be implemented. The FTR closed-loop control eliminates the influences of frequency mismatch on the zero rate output and linearity of the scale factor. It therefore relaxes the degree to which the modes need to be tuned. It is shown in this study that matching the modes in the FTR control scheme improves noise performance and measurement accuracy over the non-tuned case. Experimental results of real time FTR control and Allan deviation tests are provided to verify the analysis.

Induction Motor Control: Multivariable Analysis and Effective Decentralized Control of Stator Currents for High-Performance Applications

The adequate control of stator currents is a fundamental requirement for several high-performance induction motor (IM) control schemes. In this context, classical linear controllers remain widely employed due to their simplicity and success in industrial applications. However, the models and methods commonly used for control design lack valuable information, which is fundamental to guarantee robustness and high performance. Following this line, the design and existence of linear fixed controllers is examined using individual channel analysis and design. The studies presented here aim to establish guidelines for the design of simple (time invariant, low order, stable, minimum phase, and decentralized) yet robust and high-performance linear controllers. Such characteristics ease the implementation task and are well suited for engineering applications, making the resulting controllers a good alternative for the stator current control required for high-performance IM schemes such as field-oriented, passivity-based, and intelligent control. Illustrative examples are presented to demonstrate the analysis and controller design of an IM, with results validated in a real-time experimental platform. It is shown that it is possible to completely decouple the stator current subsystem without the use of additional decoupling elements.

Loop Decoupling Design Based on Identification Model of DTG

To solve the coupling problems cause by flexible structure of Dynamic Tuned Gyroscope (DTG), solutions was proposed through the combination of loop control and decoupling within Individual Channel Design (ICD) framework during the DTG lock loop design. Firstly, theoretical model of DTG and its actual coupling property were presented. Then, the diagonal controller was designed for the diagonal elements and counter-diagonal elements of DTG transfer function matrix under the framework of ICD. The stability and coupling relationship of individual channel were also analyzed for both two cases. Finally, a validation experiment was carried out for a certain type of DTG when it works in tuned state and mistuned state with 20% perturbation of tuned speed. The experimental results show that the diagonal controller for both two cases can stabilize the loop. But in mistuned state, the coupling degree of diagonal elements case grows up to 15%.This shows that the controller robustness for counter-diagonal elements is greater.

An Application of the Individual Channel Analysis and Design Approach to Control of a Two-input Two-output Coupled-tanks System

Frequency-domain methods have provided an established approach to the analysis and design of single-loop feedback control systems in many application areas for many years. Individual Channel Analysis and Design (ICAD) is a more recent development that allows neo-classical frequency-domain analysis and design methods to be applied to multi-input multi-output control problems. This paper provides a case study illustrating the use of the ICAD methodology for an application involving liquid-level control for a system based on two coupled tanks. The complete nonlinear dynamic model of the plant is presented for a case involving two input flows of liquid and two output variables, which are the depths of liquid in the two tanks. Linear continuous proportional plus integral controllers are designed on the basis of linearised plant models to meet a given set of performance specifications for this two-input two-output multivariable control system and a computer simulation of the nonlinear model and the controllers is then used to demonstrate that the overall closed-loop performance meets the given requirements. The resulting system has been implemented in hardware and the paper includes experimental results which demonstrate good agreement with simulation predictions. The performance is satisfactory in terms of steady-state behaviour, transient responses, interaction between the controlled variables, disturbance rejection and robustness to changes within the plant. Further simulation results, some of which involve investigations that could not be carried out in a readily repeatable fashion by experimental testing, give support to the conclusion that this neo-classical ICAD framework can provide additional insight within the analysis and design processes for multi-input multi-output feedback control systems.

Control of Handling Stability in Four-Wheel Steering Vehicles Based on Individual Channel Design

This paper presents a new steering control structure for vehicles equipped with four-wheel steering system. A linear model of the lateral dynamics is used in this paper. This control structure is based on a simplified linear model of the lateral dynamics of such vehicles and aims to decouple the control of sideslip from the control of yaw rate. The control design is based on a linear multivariable plant and the front and rear steering angles, According to the Individual Channel Design paradigm. The proposed control structure has been applied to design sideslip and yaw rate controllers using a more accurate model of the lateral dynamics of four-wheel steering vehicles. Simulations are used to illustrate the performance and robustness of the designed controllers.

Fundamental Analysis of the Electromechanical Oscillation Damping Control Loop of the Static VAr Compensator Using Individual Channel Analysis and Design

This paper presents the first individual channel analysis and design (ICAD) of a high-order synchronous generator and a static VAr compensator (SVC) featuring a damping control loop. Particular emphasis is given to the closed-loop performance and robustness assessments. Fundamental analyses are carried out using ICAD and its frequency-domain approach to explain the dynamic behavior of the generator affected by an SVC with damping capabilities. A coordinated SVC voltage and damping control is achieved in a straightforward fashion owing to the transparent manner in which ICAD treats the complex interactions taking place between the synchronous generator and the SVC with a damping control loop. Using the ICAD framework, an indepth comparison is made between the competing abilities to provide system damping of the SVC and the power system stabilizer.

Individual Channel Analysis of the Thyristor-Controlled Series Compensator Performance

The TCSC is the electronically-controlled counterpart of the conventional series bank of capacitors. A mature member of the FACTS technology, the TCSC has the ability to regulate power flows along the compensated line and to rapidly modulate its effective impedance. In this paper its performance is evaluated using Individual Channel Analysis and Design. Fundamental analysis is carried out to explain the generator dynamic behavior as affected by the TCSC. Moreover, a control system design for the system is presented, with particular emphasis in the closed-loop performance and stability and structural robustness assessment. It is formally shown that the incorporation of a TCSC operating in its capacitive range improves the dynamical performance of the synchronous machine by decreasing the electrical distance and therefore considerably reducing the awkward switchback characteristic exhibited by synchronous generators. It is also formally proven in the paper that the inductive operation should be avoided as it impairs system operation. In general, the TCSC inclusion brings on fragility into the global system, making it non-minimum phase and introducing adverse dynamics in the speed channel of the synchronous machine. Moreover, it is shown that the minimum-phase condition may also be present in cases featuring high capacitive compensation levels.

Fundamental Analysis of the Static VAr Compensator Performance Using Individual Channel Analysis and Design

In this paper the performance of a synchronous generator – SVC system is evaluated using Individual Channel Analysis and Design (ICAD), a control-oriented framework suitable for small-signal stability assessments. The SVC is already a mature piece of technology, which has become very popular for providing fast-acting reactive power support. The great benefits of ICAD in control system design tasks are elucidated. Fundamental analysis is carried out explaining the generator dynamic behavior as affected by the SVC. A multivariable control system design for the system is presented, with particular emphasis in the closed-loop performance and stability and structural robustness assessment. It is formally shown in the paper that although the addition of the SVC with no damping control loop does not improve the dynamic of the system, its inclusion is very effective in enhancing voltage stability. Moreover, ICAD analysis shows that with the use of the SVC the dynamical structure of the system is preserved and no considerable coupling or adverse dynamics are added to the plant.

Synchronous Generators Modeling and Control Using the Framework of Individual Channel Analysis and Design: Part 1

In this paper a comprehensive dynamical assessment of a high order synchronous generator plant is carried out using the Individual Channel Analysis and Design (ICAD) framework –a multivariable control engineering tool that allows robustness and system performance evaluations. The great benefits of ICAD are elucidated and contrasted to those provided by the long-time honored block diagram representations. Several models used for the small signal stability analysis of synchronous generators are evaluated under the framework of ICAD. The study, which builds on pioneering work, reveals the great advantages of carrying out control system analysis and design with higher order generator models. Moreover, careful analysis of the ICAD's Multivariable Structure Function (MSF) helps to explain, formally, why some operating conditions of the control system are more critical than others. Furthermore, correct interpretations of MSFs are amenable to robust and stable control system designs. Two kinds of studies are considered in the paper; one assesses operation under various power factor conditions and the other under a varying tie-line reactance. The control system design and stability and structural robustness assessment of the system are presented in the second part of this paper. Moreover, results obtained under the ICAD framework are compared with those arising from conventional controllers.

Individual Channel Design for Synchronous Generators

In this paper a novel control strategy for a real synchronous generator is presented. It is based on linear, diagonal, low order, minimum-phase, fixed, and stable controllers. The controller was obtained via individual channel design (ICD), a framework that allows analysis and synthesis of multivariable control systems by means of classical techniques based on the Bode and Nyquist plots. The generator is part of a one machine – infinite bus system. A linearised model around an operating point in a rotating coordinate frame is used. The theoretical principles behind this control strategy are summarised for completeness. The results using this novel control strategy are obtained throughout simulations in MATLAB®.

Helicopter translational rate command using individual channel analysis and design

Translational rate command (TRC) in helicopter flight involves controlling the earth-referenced forward and side velocities, using the helicopter’s cyclic control. Handling qualities requirements for TRC systems suggest that the design should comprise an inner attitude command-attitude hold (ACAH) loop, with TRC control as an outer loop. This forms a non-square system, and a method to determine the implications of this structure for stability robustness is proposed here. Control synthesis was performed using the method of individual channel analysis and design. This technique is a neo-classical, frequency-domain control analysis and design method for multivariable systems. The control law obtained is very simple, and the handling qualities, assessed by non-linear simulation, are predicted to be at ‘Level 1’ (satisfactory without improvement). The conditions for stability robustness are satisfied at frequencies of significance for the handling qualities.

Helicopter Attitude Command Attitude Hold Using Individual Channel Analysis and Design

A control law was designed for a linearized model of a typical combat rotorcraft trimmed to 30-kn forward e ight. Although based on a single e ight condition, the same controller is found, in simulation, to give level 1 performance fortherangeofspeedsfrom hover to 80 kn, for handling qualities based on small-amplitude motions. Control synthesis was performed using the method of individual channel analysis and design (ICAD). ICAD is a neoclassical, frequency-domain control analysis and design method for multivariable systems. Its most distinctive feature is the use of multivariable structure functions, which make explicit the role of cross coupling in the design process and on the robustness. The control law so obtained is very simple, and the results suggest that modern control methods, based on optimal synthesis, are not a necessity for the helicopter e ight control problem.

An approach to multivariable combustion control design

In this paper, an approach to multivariable combustion control design within the Individual Channel Design (ICD) framework for analysis and control design is presented. ICD is a framework which involves an interplay between customer specification, uncertain plant characteristics, and the multivariable feed-back design itself. Established multivariable methods and process engineering knowledge can be incorporated or evaluated within the ICD framework. The combustion control has been designed and evaluated with a computer simulation of both a linearized model and a nonlinear model of the closed-loop system. The ICD multivariable framework shows in a highly transparent manner, by way of simple graphical frequency response indicators, what the main possibilities and difficulties posed by a combustion process for multivariable control are, and how much trade-off between control specifications is possible. Solutions are also presented for problems such as: integrity of closed-loop control, balance of input-output channels, simple and transparent controller structure, and robustness.

A submarine depth control system design

Submarines operating at deep submergence can be considered to be in a disturbance-free environment. Under these conditions the design of depth-keeping controllers is a straightforward task. At shallow submergence under rough sea conditions and at low speed, accurate depth-keeping controller designs require particular attention. In these circumstances, the submarine is subject to severe disturbances, imposing additional restrictions on the designer. A submarine low-depth multivariable autopilot has been developed by applying classical Bode and Nyquist techniques. It is shown that a successful multivariable depth-keeping autopilot design can be produced using the framework of individual channel design. It is also shown that, the performance of the resulting linear fixed controller obtained, can be extended to a wide range of the submarine's operational envelope. This is achieved by considering the nonlinear effect of the speed on the nominal design. With such a modification, there is no need to implement a ...

Helicopter flight control using individual channel design

Using the multivariable analysis framework known as individual channel design (ICD), it is shown that for a typical strongly cross-coupled single main rotor helicopter at 80 knots forward flight, the standard 4-input 4-output multivariable control problem, for design purposes, decomposes without significant loss of structural (interaction) information into two simpler 2-input 2-output multivariable problems: one for the longitudinal dynamics and one for the lateral dynamics. Following on this analysis of multivariable structure, the control systems design is built up in a systematic fashion and a novel type of feedforward control is used to overcome a severe lack of robustness as well as to decouple the lateral dynamics into two SISO subsystems round crossover frequency. A diagonal (4*4) feedback controller matrix is then used, the elements of which are designed on the basis of the decoupled lateral and longitudinal dynamics. Resulting closed-loop bandwidths of all four channels are within Level 1 handling quality specifications and step responses are satisfactory with acceptably low cross-coupling.

Investigation of the ICD structure of systems defined by state-space models

The individual channel design (ICD) pole-zero structure of systems defined by linear state-space models is investigated. The existence of ‘fictitious’ cancelling poles and zeros and highly structured plant transfer-function matrices leads to a requirement, within ICD, that these systems be handled differently from systems given directly (Leithead and O'Reilly 1992 b) in transfer-function matrix form. Both the pole-zero structures of multiple-channels and of multiple-channels with infinite gain are presented, and it is noted that multiple-channel zeros which coincide with the plant poles are fictitious. A similar statement of pole-zero structure is made for each of the m open-loop Individual Channels Cj. Further, the number Z of RHPZs of Channel Cj is given by Z = N + P − Q where N is the net number of clockwise encirclements of the point (1,0) by the Nyquist plot of the multivariable structure function γj(s), P is the number of RHPPs of γj(s), and Q is the number of RHPPs of the plant. Finally, it is esta...