Oleh Kiselychnyk

Analysis of Triggered Self-Excitation in Induction Generators and Experimental Validation

In self-excited induction machines, a power generating mode of operation can often be attained only by precharging at least one of the capacitors connected to the windings. The paper shows how a carefully derived state-space model with nonlinear magnetic characteristics enables the assessment of all possible operating regimes including their stability properties. In particular, the analysis reveals the possible existence of an unstable operating regime, which creates a barrier that must be overcome through precharged capacitors. In such case, the analytical results of the paper yield a simple formula that predicts the voltage needed to trigger self-excitation. Close to the boundary, voltages can be generated for extended periods of time before growing to a stable operating regime, or collapsing to zero. Experimental results validate the results of the paper on the transient properties of self-excited induction generators.

The Complex Hurwitz Test for the Analysis of Spontaneous Self-Excitation in Induction Generators

Spontaneous self-excitation in induction generators is a fascinating phenomenon triggered by the instability of a zero equilibrium state. Prediction of this condition for various values of free parameters requires many computations of the eigenvalues of a 6 × 6 matrix over a large space. The technical note uses a novel approach to stability using a transformation of the state-space system and an extension of the Hurwitz test to polynomials with complex coefficients. The analytic formulas that are obtained give the values of the minimum load resistance, the range of capacitor values, and the range of speeds for which spontaneous self-excitation appears. The technical note concludes with an illustration of the results on an example.

Comparison of two magnetic saturation models of induction machines

The paper considers two models of induction machines accounting for magnetic saturation. The first is a systematically-designed model based on fundamental principles, while the second is a simplified model that neglects certain terms in the first model. The paper shows that both models predict the same responses in the linear region, as well as in the nonlinear region but only for steady-state operation. To investigate the possible superiority of one model over the other, the paper considers the measurement of transient responses where the models predict different behaviors. Experimental conditions are planned by computing the eigenvalues of the linearized systems and selecting conditions with maximal differences in settling time. Surprisingly, however, the experimental data shows relatively little differences between the predictions of the models and fails to favor one model over the other.

Linearized State-Space Model of a Self-Excited Induction Generator Suitable for the Design of Voltage Controllers

The complexity and strong nonlinearity of the model of a self-excited induction generator hinder the systematic design of a voltage regulation system. Using a special reference frame aligned with the stator voltage vector, the paper succeeds in developing a control-oriented linearized model that relates small deviations of the capacitance, load admittance, and angular velocity, to corresponding deviations of the voltage amplitude. Transfer functions are also computed based on the linear model. A stability analysis predicts rapidly decaying oscillatory transients combined with a primary component with slower exponential decay. Simulated transient responses of the full and linearized models demonstrate the validity of the approximation and are in good agreement with experiments.

Model of a self-excited induction generator for the design of capacitor-controlled voltage regulators

The systematic design of voltage regulation systems for self-excited induction generators requires the development of a control-oriented model. The paper considers the situation where the peak magnitude of the stator voltages is regulated through adjustable capacitors connected to the windings. A transfer function model is difficult to obtain, due to the strong nonlinearity of the self-excitation phenomenon, and to unconventional features of the problem. Nevertheless, the paper succeeds in computing a transfer function relating small deviations of the capacitance to small deviations of the voltage magnitude using a clever choice of reference frame. The linearized system is found to be stable for all operating points under consideration, and the eigenvalues of the system predict rapidly-decaying oscillatory transients combined with a slower exponentially decaying component. Results of simulations of the full nonlinear model and of the linearized system demonstrate the validity of the approximation for small deviations. Experimental results also show a good match between measured data and the identified model.

Comparison of Two Magnetic Saturation Models of Induction Machines and Experimental Validation

This paper develops a systematic comparison of two nonlinear models of induction machines in magnetic saturation using stator and rotor currents as state variables. One of the models accounts for dynamic cross-saturation effects, whereas the other neglects them. Analytic derivations yield an explicit description of the difference between the models showing that differences can only be observed through transient responses in the saturated region. To refine the comparison, and exclude conditions in the linear magnetic region, the dynamics of self-excited induction generators around stable operating points is analyzed. Unexpected and interesting features of the models are revealed through their linearization in the reference frame aligned with the stator voltage vector, followed by computation of the transfer functions from perturbations to state deviations. The analysis predicts a slower exponential convergence of the simplified model compared to the full model, despite very close responses in the initial period. The comparison is validated via thorough experiments and simulations. This paper provides experimental evidence of the higher accuracy of the full model for transients deep into the saturated region. For realistic operating conditions, the difference is found to be rather small, and often comparable to the steady-state error caused by inaccuracies in the parameters.

A combined MTPA and maximum efficiency control strategy for IPMSM motor drive systems

For electric vehicle applications, it is desirable to achieve maximum torque output of the traction motor for fast acceleration and minimize electrical losses, from energy saving point of view, which is achieved by Maximum Torque Per Ampere (MTPA) and Maximum Efficiency (ME) control methods, respectively. This paper introduces a compromise solution between MTPA and ME approaches obtained for Interior Permanent Magnet Synchronous Motor (IPMSM) drive, smoothly combining them and yielding a desired priority of the maximum torque or efficiency depending on the operating conditions. The proposed approach does not require switching between the methods which can worsen the dynamic performance of the system, and it is easy in implementation. The paper demonstrates an application of the proposed methodology with respect to the referred torque profile. For lower torques, the system is closer to the MTPA control which is useful for the fast dynamics. For higher torques, where power losses are higher, the system is closer to the ME approach providing almost the minimum of electric energy losses, which is useful for preventing battery discharge. The methodology is justified analytically and verified via simulations of steady state and dynamic characteristics of a torque vector control system.

Steady-state and dynamic characteristics of self-excited induction generators with resistive-inductive loads

The paper presents a detailed analysis of self-excited induction generators with resistive-inductive loads. A nonlinear dynamic model is derived that enables a unified analysis of both the steady-state and transient characteristics of the machine. Steady-state operating modes are first determined, and self-excitation boundaries are derived. Computations are straightforward and do not require iterations. The dynamic model is also used to compute the eigenvalues of the linearized system around an operating mode. Experimental data show good agreement with computational results using the dynamic model. It is found that the addition of inductance generally, but not necessarily, enlarges the set of conditions where self-excitation is possible, and increases the stability of the operating modes.

Mathematical Modeling and Control of a Cost Effective AC Voltage Stabilizer

AC voltage regulation is required in both the domestic and industrial sectors to avoid undesired effects from random voltage variations of the power supply. The paper introduces an ac voltage stabilizer/converter (ACVS) that is based on a controllable autotransformer technology. The proposed ACVS offers a specified strategy of voltage regulation, less harmonics, and low cost. The paper explains the operating principle of the ACVS and derives its nonlinear mathematical model. To ensure the desired performance of the ACVS while it is subject to uncertain input voltage and load variations, an optimal control strategy is designed. It is achieved via transforming the ACVS model extending with fictive axis emulation into a rotating reference frame and the linearization of the model via specific orientation of the reference frame and introducing a linear control action. Operation of the ACVS is simulated under different disturbances due to load and grid voltage changes, and compared to voltage stabilization with application of I and PI controllers. Experimental results are presented to demonstrate the voltage regulation technology.

Adaptive torque control of IPMSM motor drives for electric vehicles

During real-time operation of electric vehicles, motor parameter variations can significantly effect and deteriorate control performances of electric traction drives. For an open-loop torque control system, the parameter mismatch between the electric motor and the controller results in large steady-state error of the output torque response. This paper presents an adaptive torque controller based on Model Reference Adaptive Control technique for Interior Permanent Magnet Synchronous Motor drive systems. The proposed adaptive torque control scheme incorporating with a low-pass filter based torque estimator can provide the robustness against motor parameter variations. The simulation results show that the proposed torque control system using SVPWM-based vector control scheme can meet torque tracking requirements, and minimize steady-state error caused by parameter variations.