Dynamic Modeling and Control for SPMSMs With Internal Turn Short Fault

Post-fault modeling for dual three-phase permanent magnet synchronous motors (DTP-PMSMs) under the open-phase fault is presented in this article. By using the positive and negative sequence components for the dual three-phase system, the stator currents under the open-phase fault can be treated as a combination of the positive and negative sequence currents. By artfully constructing the negative sequence components in terms of the different initial angles and amplitudes, the single-phase open-circuit fault and certain dual-phase open-circuit fault can be represented. Based on the conventional and modified vector space decomposition (VSD), only the positive sequence components are projected into the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha \beta $ </tex-math></inline-formula> subspace, namely, the torque-producing subspace, and the negative sequence components are transformed into the harmonic subspace. With the conventional proportional–integral (PI) current regulators, the dc components of the positive and negative sequence currents can be readily controlled under the open-phase fault. Experiments based on a DTP-PMSM prototype verify the effectiveness of the proposed open-phase fault modeling using VSD and negative sequence components.

The object of this paper is to point out that the existing methods of measuring power when applied to unbalanced three-phase systems are not equitable for symmetrical polyphase machinery. On the other hand, unsymmetrical loads on polyphase systems are not sufficiently penalized for the trouble which they create in the system. It is first of all shown that a symmetrical generator cannot deliver power except through the balanced components of current. The unbalanced currents are capable of resolution into two balanced systems of currents, one of which is of the same phase sequence as the generator e. m. f., and the other component is of reversed or negative phase sequence. The generator cannot deliver power through the medium of this latter component of the currents, because the instantaneous product of the generated voltage and these currents in the three phases is always zero. However, the volt-ampere product per phase is of great significance, because it is a measure of the effect of the current unbalance on the system. The generator therefore delivers power only through the medium of the positive phase sequence currents. Any power that appears in the system through the negative phase sequence currents is positive phase sequence power which has been supplied by the generator and degraded through unbalanced loads and fed back to the system in the form of negative phase sequence power. This power is always additional loss in all rotating machines on the system, and with the present method of charging, the consumer having symmetrical machines is charged with this additional power, which serves him no useful purpose but reduces the output of his machine and decreases his load power factor. In the paper it is proposed that the positive phase sequence power output only be measured, and the power charges be made on the basis of this measurement. It is further proposed that the unbalanced k-va., which is the product of the positive sequence voltage and the negative sequence current be measured either by means of a negative sequence ammeter, indicating or recording, or a k-va. meter, and a charge made for the amount of unbalance. The user of symmetrical polyphase rotating machinery should then be given a lower rate, based on the estimated cost of unbalance, and the consumer having unbalanced loads should be charged directly for the amount of unbalance he creates, or else should have his positive phase sequence power rate increased, based upon the estimated cost of unbalance. It is pointed out that the unbalanced kv-a. is a factor of the same order of importance as reactive kv-a. and in any system subject to unbalanced conditions this factor should be considered and the unbalanced factor, as well as the power factor, should be measured. The unbalanced factor is the ratio of the negative phase sequence kv-a. and the positive phase sequence kv-a., the former being obtained by taking the product of the positive phase sequence voltage and the negative phase sequence current. Devices for measuring these quantities are being developed and the outfit for making these measurements will be no more complicated than the present existing measurement devices. In fact the tendency is towards greater simplicity. In presenting this subject the author has no intention of suggesting how rates should be made, but merely wishes to point out what factors enter into the question of equitable rates when the polyphase system is subject to unbalance.

Dual feedback linearization control based on direct power control (DPC) for three-phase pulse-width-modulation (PWM) voltage-source rectifier (VSR), which connects to grid voltage during network unbalance, is proposed. And there is a non-linear PI controller as the system's outer loop controller, which can accelerate the response of DC voltage in the case of load changing. The positive sequence and negative sequence instantaneous power models of the PWM rectifier are respectively established. The positive sequence active power and reactive power are entirely produced by positive sequence voltages and positive sequence currents. And the negative sequence active power and reactive power are entirely produced by negative sequence voltages and negative sequence currents. The dual power control scheme with feedback linearization control is presented. The positive sequence instantaneous power and the negative sequence instantaneous power achieve decoupling control. By changing reference, the proposed strategy can achieve different control objectives such as: eliminating active and reactive power oscillation without sinusoidal line current, eliminating active power oscillation with sinusoidal line current, eliminating reactive power oscillation with sinusoidal line current or eliminating negative sequence current. Test results show the good performance of the proposed strategy.

The usual strategy for current regulation of distributed generation sources in an unbalanced three phase system is to separately control the positive and negative sequence currents using independent positive and negative synchronously rotating frame current regulators. This approach requires the measured phase currents to be separated into positive and negative sequence components before transformation into their respective synchronous rotating frames, which is computationally expensive, potentially introduces errors and compromises dynamic and transient performance (particularly for weak grids). This paper presents a simpler approach for controlling unbalanced three phase currents that does not require separation of the measured currents into their individual sequence components, and thus avoids these problems. The approach is combined with classical synchronous frame PI regulators with maximized gains, to achieve excellent unbalanced current control with a minimum of complexity. Simulation and experimental results are provided to verify the strategy.

This paper presents the implementation of the extended vector control on a new type of brushless doubly-fed generator when subjected to unsymmetrical voltage dips. This new type of generator has partially rated power electronic converter and exciter mounted on the shaft and rotating with it. The unbalanced voltages introduce positive and negative sequence components in the generator. The negative sequence components gives rise to torque oscillations and extra heating in the generator and power electronics which could affect the life time of the shaft, gearbox and electronic equipment. Two separate synchronous reference frames are defined and standard PI controllers are implemented in the reference frames in order to control the positive and negative sequence currents independently. Through these means the effect of the negative sequence currents is mitigated and it is shown that the oscillations in the behavior of this new type of generator are considerably reduced.

This paper presents a noninvasive and online method for detection of stator winding faults in three-phase induction motors from observation of negative sequence supply current. A power decomposition technique (PDT) has been used to derive positive and negative sequence components of measured voltages and currents. A fault detection algorithm has been developed to characterise the effects of supply imbalance and nonlinear motor effects (motor iron saturation, winding imbalance, rotor static eccentricity), which also generate negative sequence current. The effects of motor heating and variation in negative sequence resistance with slip change are minimised by using only motor negative sequence reactance to calculate supply negative sequence current. Change in the motor negative sequence impedance under supply imbalance due to load variation (mainly because of closed rotor slots effect) has been included. Experimental results on several motors show that negative sequence impedance can vary between 10% to 50%. Semiempirical formulas based on theoretical and experimental results have been proposed to eliminate the effects of supply imbalance, load, and voltage variation. Compensation for this negative sequence current before making the fault decision enables a high fault sensitivity to be achieved.

Single-phase conversion to three-phase power conversion is a special case of an unbalanced three-phase power compensator. An unbalanced three-phase current can be split into its symmetrical components in classical power system theory. A single-phase supply or load is an extreme case of an unbalanced system. In the case of a three-wire system, the current components are made up of a positive current sequence and balanced negative phase sequence current. In this paper it is shown how the negative sequence current can be eliminated without the use of active power. The opposite negative sequence is injected into the three lines by a three-phase inverter. The inverter has a space vector modulated PWM switching function. A digital signal processor (DSP) produces the control signal for the inverter semiconductor switches. The inverter is a full bridge IGBT (insulated gate bipolar transistor) inverter running in a space vector modulation scheme (SVM). The space vector, which represents the switching states of the six switches of the inverter, consists of the sum of two circular trajectories. One trajectory runs clockwise and the other anticlockwise. The result is an elliptical trajectory. The invertor voltage space vector is calculated on line from the single-phase current and therefore requires a fast processor. A DSP is used to achieve this.

This paper presents an analysis and control design of a doubly-fed induction generator (DFIG)-based wind generation system operating under unbalanced network conditions. A DFIG system model in the positive and negative synchronous reference frames is presented. Variations of stator active and reactive powers and generator torque are fully defined in the presence of negative sequence voltage and current. Alternative DFIG control targets during network unbalance, such as reducing stator current unbalance, torque, and power pulsations minimization, are identified. A rotor current control strategy based on positive and negative (dq) reference frames is used to provide precise control of the rotor positive and negative sequence currents. Simulation results using EMTDC/PSCAD are presented for a 2-MW DFIG wind generation system. It shows that conventional vector control of DFIG without considering network unbalance results in excessive oscillations on the stator active/reactive power, electromagnetic torque, and stator/rotor currents even with a small stator voltage unbalance. In contrast, with the proposed control strategy, enhanced system control and operation such as minimizing oscillations in either active power, or electromagnetic torque, or stator or rotor currents can be achieved

According to symmetric component theory, negative phase sequence in induction motors is caused due to unbalanced windings or due to the application of unbalanced supply voltages on the stator terminals. A magnetic field is created by this negative phase sequence component and this field rotates in the opposite direction causing a decrease in the motor output torque. In this article, a novel negative phase sequence detector for relaying for motor/generator protection and motor condition monitoring is proposed without using any negative sequence filter. In the proposed scheme, a phase sensitive rectification has been used for extraction resistive and reactive components of negative phase sequence current. A Matlab Simulink model has been developed to simulate the performance of phase sensitive rectification (PSR) and necessary computation of rms value of negative sequence component.

This letter proposes a new algorithm for the fast extraction of symmetrical components from unbalanced three-phase signals. Being different from the conventional methods for the decomposition of positive and negative sequence components commonly realized at the cost of introducing significant delay, the proposed algorithm assumes the non-nominal dq-transformation to first produce the special d-axis and q-axis components, which will become the pure high-frequency ac signals, and then uses the typical half-cycle delay method to remove the unwanted negative or positive sequence component. Finally, the proper amplitude and phase angle coefficients can be easily calculated to compensate the remaining high-frequency d-axis and q-axis components before transforming them to the expected d-axis and q-axis values of corresponding symmetrical components. The performance of the proposed algorithm is evaluated through simulation and experiment results.

In this paper, the voltage sag information extraction is taken as the research object, and a simulation model based on voltage sag caused by different transmission lines is established. On account of the problems that the detection techniques of fundamental harmonic of a positive and negative sequence for common grid voltage of power network have poor ability of suppressing harmonic interference and slow detection speed, a method of instantaneous symmetrical component based on adaptive notch filter is proposed. Three adaptive notch filters are used to extract the fundamental components and their 90° phase-shift components of the three-phase unbalanced voltage signal, and the instantaneous symmetrical components are used to separate the fundamental positive and negative sequence voltages of these components. The improved method are simulated. The results of simulation demonstrate that the proposed and improved algorithm can quickly and accurately detect the magnitude of voltage positive and negative sequence components and track the change of grid frequency.

This paper introduces a novel method of islanding detection for distributed generation networks employing negative sequence impedance measurement. The negative sequence impedance islanding detection method (NSIID) is an improvement on previous impedance measurement techniques for islanding detection because it has a small non-detection zone and large threshold window, which previous techniques have had difficulty achieving. Experiments were conducted on a distributed generator fed commercial building by measuring naturally occurring and injected negative sequence components. Results indicate that naturally occurring unbalanced load conditions are readily measurable and correlate to the system Thevenin impedance. The experimental measurements also confirmed the accuracy of the negative sequence components exhibited in simulations, which were employed to analyze islanded conditions. This work further demonstrated that injecting negative sequence current into the distribution network increases the measurability of negative sequence currents, improving the accuracy of our proposed islanding detection technique.

Doubly fed induction generator (DFIG) still shares a large part in today's wind power market. It provides the benefits of variable speed operation cost-effectively, and can control its active and reactive power independently. Crowbar protection is often adopted to protect the rotor-side voltage source converter (VSC) from transient overcurrent during grid voltage dip. But under unbalanced grid voltage condition, the severe problems are not the transient overcurrent, but the electric torque pulsation and dc voltage ripple in the back-to-back VSCs. This paper develops dynamic models in MATLAB/Simulink, validates it through experiments, investigates the behavior of DFIG during unbalanced grid voltage condition, and proposes new controllers in separated positive and negative sequence. Methods to separate positive and negative sequence components in real time are also developed, and their responses to unsymmetrical voltage dip are compared. Simulation results prove that the separated positive and negative sequence controllers limit the torque pulsation and dc voltage ripple effectively.

At present, the asymmetric faults that take place in a power grid result in the grid current imbalance, and feed into the grid power inducing oscillation, which seriously affects the stability of power grid. For the asymmetric fault, the second-order generalized integrator(SOGI) is used to separate the positive and negative sequence components. The negative sequence voltage feedforward control method with notch filter is proposed. It can suppress the influence of the double frequency alternating components generated by the line side negative sequence components, and improve the low voltage ride through performance of the photovoltaic power generation system. Then, the key parameters of the inverter are identified by chaotic particle swarm optimization(CPSO) algorithm. The simulation model in Matlab/Simulink verifies the feasibility and effectiveness of the proposed method.

This paper presents a comprehensive study on a novel voltage injection based offline parameter identification method for surface mounted permanent magnet synchronous motors (SPMSMs). It gives solutions to obtain stator resistance, d- and q-axes inductances, and permanent magnet (PM) flux linkage that are totally independent of current and speed controllers, and it is able to track variations in q-axis inductance caused by magnetic saturation. With the proposed voltage amplitude selection strategies, a closed-loop-like current and speed control is achieved throughout the identification process. It provides a marked difference compared with the existing methods that are based on open-loop voltage injection and renders a more simplified and industry-friendly solution compared with methods that rely on controllers. Inverter nonlinearity effect compensation is not required because its voltage error is removed by enabling the motor to function at a designed routine. The proposed method is validated through two SPMSMs with different power rates. It shows that the required parameters can be accurately identified and the proportional-integral current controller auto-tuning is achieved only with very limited motor data such as rated current and number of pole pairs.