Dual feedback linearization control of three-phase pulse-width-modulation voltage-source rectifier based on direct power control

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.

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

The fully rated converter of type 4 wind turbines is capable of providing dynamic voltage control during grid faults by injecting controlled reactive currents. This paper describes three different dynamic voltage control options during unbalanced grid faults: 1) the positive sequence voltage control with only a positive sequence reactive current injection and suppression of the negative sequence current; 2) the positive sequence voltage control with limitation of the positive sequence reactive current injection and suppression of the negative sequence current; and 3) the positive and negative sequence voltage control with both a positive and a negative sequence reactive current injection. These different control options are compared in simulations of a wind power plant connected to a meshed power system, including synchronous generators. It is shown that both the positive sequence voltage control with limitation and the positive and negative sequence voltage control can overcome the voltage rise and voltage distortion that can occur with pure positive sequence voltage control without limitation. Both of these options have a distinct fault response, where the positive and negative sequence voltage control results in a fault response that resembles the fault response of a synchronous generator with higher fault current contributions in the faulted phases.

Optimized hierarchical power oscillations control for distributed generation under unbalanced conditions

A dynamic model for a faulted surface-mount permanent magnet synchronous motor (SPMSM) is derived using a deformed flux model. In reflecting the internal turn short into the dynamics, the variations in inductance and back EMF term were considered. Then, the faulted model was transformed into the two synchronous <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -models: one for the positive sequence and the other for the negative sequence. Also, a torque equation, which shows the relation between the ripple and the negative sequence current, is derived. The negative sequence current should be suppressed to eliminate the torque ripple. The dual current controller is utilized for this purpose: in the dual controller, the positive and negative sequences are controlled separately in their own synchronous frames. Notch filters are utilized in each synchronous frame to extract positive or negative sequence component. Experiments were performed with an SPMSM specially designed to make an internal turn short artificially. The experimental results coincide well with the corresponding simulation results, and exhibit a strength of the dual current controller in suppressing the negative sequence current.

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.

This paper presents a method for estimating the positive and negative sequence grid voltage under unbalanced grid conditions using simple algebraic and trigonometric identities without dedicated grid voltage sensors in three phase grid connected boost rectifier converter system. The estimated voltages are then used to control the positive and negative sequence currents of the boost rectifier. The estimation method is based on the positive and negative sequence active and reactive power delivered by the converter to the grid. In addition to the estimation of the positive and negative sequence grid voltage, the proposed method also generates an estimate of the frequency information of the grid similar to a conventional PLL structure. Detailed theory and results confirm the feasibility of the proposed scheme.

When the grid voltage is imbalance, the current decoupling control strategy in d‐q frame designed for the balanced system still let STATCOM output balanced voltage. In this case, the negative sequence component in the grid voltage may cause the device overcurrent. Here, the positive and negative sequence of grid voltage are separated first and that of the STATCOM output voltage is also controlled independently. However, how to determine and coordinate the positive and negative sequence voltage have not been effectively investigated yet. From the perspective of power balance, this paper gets the quantitative relationship between the phase shift angles of the positive and the negative sequence voltages. It also gets the quantitative relationship between the modulation ratios of the positive and the negative sequence voltages, and hence the effective coordination control strategy of positive and negative voltages of cascaded STATCOM operating under imbalanced grid voltage was developed. In addition, the phase of the positive and negative voltages, which is critical for the proposed coordination control strategy, can be obtained by the advanced open‐loop phase synchronisation scheme. Finally, its detailed implementation was also provided. Simulation results show that the proposed method is correct and effective.

DC voltage balance based on zero sequence voltage is commonly used for star-connected cascaded H-Bridge (SCHB) STATCOM to compensate negative sequence current. However, zero sequence voltage injection brings about side-effect of increasing STATCOM output voltage and thus the Negative sequence Current Compensation Range (NCCR) is significantly limited to avoid overmodulation. To extend the NCCR, improving the dc voltage utilization by injecting third harmonic component is a typical method. Compared with third harmonic injection (THI), square wave features lower peak-to-average ratio. Unfortunately, the capability of square wave injection (SWI) for the NCCR extension has not been well explored. In this article, to further extend NCCR, not only the SWI is employed for SCHB STATCOM, but also it is compared with THI in term of NCCR by establishing the relationship between the maximum STATCOM voltage and negative sequence current. The comparison results reveal that the THI and SWI both have their own strengths and weaknesses in NCCR, which highly depends on the amplitudes and phases of positive and negative sequence currents. Furthermore, the criteria associated with the amplitudes and phases of the positive and negative sequence currents are derived to determine the harmonic injection method with larger NCCR by using symbolic-graphic combination method and MATLAB tools. As a result, a new harmonic injection method is proposed by selecting the THI or SWI with larger NCCR flexibly according to the amplitudes and phases of positive and negative sequence currents. Finally, the proposed method is verified by experimental results on a 400 V/7.5 kVar SCHB STATCOM prototype.

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.

The aim of this paper is to compare two different control structures. The Simple Direct Power Control (DPC) and the Direct Power Control with Space Vector Modulation (DPC- SVM) for two level converter applications. The first strategy (DPC) has been developed to control the instantaneous active and reactive power directly by selecting the optimum switching state of the converter. Applied to the Pulse Width Modulation (PWM) converter its main feature is to improve the total power factor and efficiency, even harmonics components existence. In the second structure, the active and reactive powers are used as (PWM) control vari- ables instead of the three-phase line currents usually used in other techniques. It is shown that DPC-SVM exhibits several properties; good dynamic response, constant switching fre- quency, and in particular it provides a sinusoidal line currents. Simulation and experimental results has shown that both control structures achieve good performances. DOI: http://dx.doi.org/10.11591/ijece.v4i3.6074

Fault ride-through capability is an essential requirement for grid-forming inverters (GFMIs) to ensure reliable, secure, and stable power delivery under grid disturbances. During asymmetrical fault conditions, due to the voltage-source behavior of GFMIs, uncontrolled negative sequence currents are injected, which depend directly on fault level and grid impedance. This phenomenon exacerbates power oscillations that significantly affect the DC-side voltage oscillations. Contemporary grid codes mandate the injection of negative sequence currents to support the grid during unbalanced conditions. To address these challenges, this paper proposes a negative sequence voltage control strategy for GFMIs, aiming to regulate negative sequence currents and control power oscillations. The proposed control is implemented in the stationary reference frame, utilizing finite control set model predictive control (FCS-MPC) as the inner control loop to achieve fast and accurate current tracking. Simulations carried out with different short circuit ratios (SCRs) to analyze the impact of grid strength on inverter behavior during asymmetrical faults. Performance evaluation under varying negative sequence voltage references and SCRs demonstrates that the proposed control strategy is effective.

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.

When a grid voltage is unbalanced, the unbalanced grid voltages act as an external disturbance for the grid-connected PWM inverter systems, resulting in an unbalanced grid current. Firstly, the mathematical model of the grid-connected converter, in view of unbalanced grid voltages, is proposed and analyzed in detail. In order to achieve the decomposition of positive and negative sequence voltage, this paper adopts the software phase-locked loop to decouple based on double synchronous reference frame. This method has good dynamic performance. In order to suppress negative sequence current on the AC side, the positive and negative sequence components of the grid current are controlled in the double synchronous reference frame respectively. Finally, Simulation results verify the correctness of the control strategy.

A doubly-fed induction generator grid side converter is studied under voltage disturbances. During voltage dips, conventional current control techniques involving the decoupling of d – q axis in the synchronous reference frame (SRF) exhibits oscillations in the stiff dc link voltage, as well as in the active and reactive power outputs of the voltage source converter (VSC). To mitigate the oscillations, an advanced control technique called the sequence domain control (SDC) is evaluated. This approach implements individual controllers in the positive and negative sequence domains and demonstrates stabilization of the dc link voltage to a greater extent during a disturbance, but is more sluggish than the conventional control. An innovative control technique called direct power control (DPC) is also investigated. This control achieves active and reactive power stability with simple active and reactive power control variables replacing the traditional current control loops. The DPC technique is verified using a prototype in the laboratory. A modified DPC algorithm combining the benefits of DPC and SDC to eliminate the current harmonics created by DPC during system disturbances is introduced. The benefits of the proposed controllers are compared using a simulation of voltage dip on a VSC based on the IEC-61400-21 standard.