Flexible Nonsinusoidal Zero Sequence Voltage Injection Method to Extend Negative Sequence Current Compensation Range for Star-Connected CHB STATCOM

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.

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 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

This paper presents a dynamic phasor modeling approach to the small signal transient analysis of induction machines with stator inter-turn faults. An important advantage of dynamic phasor modeling is that it results in time-invariant models of unbalanced three-phase systems. The linearized dynamic phasor model facilitates the analytical studies of transients of unbalanced systems using eigenvalues. The linearized q - d model of a machine with stator inter-turn fault has a system matrix having time periodic entries, and is thus unsuitable for eigenvalue analysis. The dynamic phasor model helps in this case. In this paper, the dynamic phasor model of induction machine with stator inter-turn fault is developed, and this is further used for eigenvalue analysis. The eigenvalue comparison of healthy and faulted machines is presented. This analysis is verified by transient simulations of the dynamic phasor model and experiments. The eigenvalue analysis and the results of experiments and simulations show that the effect of stator inter-turn fault in transients of positive and negative sequence currents is very small. Also the stator inter-turn fault produces coupling between transients of positive and negative sequence currents.

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 proposes a point of common coupling (PCC) voltage compensation algorithm using a current limitation strategy for use in distributed generation (DG). The proposed strategy maintains the PCC voltage by prioritizing currents when an output current reference is larger than the current capacity of the power condition system (PCS) of the DG. With this strategy, the DG outputs the active current, reactive current, and the negative sequence current. The DG uses the reactive current for maintaining the PCC voltage within a normal range; the negative sequence current is used for reducing the PCC voltage unbalance. The proposed method was verified using PSIM simulation and experimental results.

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 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 novel inter-cluster direct current (DC) capacitor voltage balancing control scheme for the single-star configured modular multilevel cascaded converter (MMCC)-based static synchronous compensator (STATCOM) under unbalanced grid voltage. The negative-sequence component of grid voltage at the point of common connection (PCC) causes unbalanced active power flow in the phase limbs of converter. This leads to the imbalance of DC voltages of the submodule capacitors across the MMCC phases, and consequently, the malfunction of converter. The proposed solution is to inject both negative-sequence current (NSC) and zero-sequence voltage (ZSV) into the phase limbs of MMCC. A quantification factor Q <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</inf> is used to achieve the sharing of intercluster active power between the NSC and ZSV injection methods. Accurate determination of the quantification factor has been presented. In addition to maintaining the DC voltages of sub-module capacitor across the MMCC phases balanced, it also prevents the overcurrent and overvoltage of converter by injecting NSC and ZSV with the right proportion. The control scheme is validated on a 3.54 kV 1.2 MVA power system using MMCC-based STAT-COM with 3-level bridge cells as sub-modules. The results show that the proposed scheme provides superior effectiveness in eliminating the voltage imbalance of DC capacitor in the phase limb while maintaining low voltage and current ratings.

Modular multilevel converter (MMC) is drawing more and more attention in medium-voltage microgrid applications. Large-scale switching-off loads and grid faults may lead to the voltage swell in microgrid, where MMC should have the ride-through ability. In this paper, a simple operation approach with the fundamental and irregular zero-sequence voltage (ZSV) injection is proposed for MMC under grid voltage swell condition. First, the fundamental ZSV injection is applied to adjust the modulation margin of three phases where the amplitude values of modulation waves in each phase can be the same. Then, if the amplitudes with fundamental ZSV injection are still in over-modulation status, the new irregular ZSV injection is added to further enlarge the operation region. In addition, the limitation of the irregular ZSV injection is analyzed. Simulation and experimental results of MMC under different grid voltage swell conditions verify the effectiveness of the proposed operation approach.

Increased integration of wind power makes higher demands on the stability and protection of Power System. Wind farms are connected to power grid through transmission line. When single-phase fault occurs on the line, positive and negative sequence currents are not equal and are much smaller than zero sequence current. Phase selector based on the theory of current- difference sudden-change may mistake single-phase fault for phase-to-phase fault because positive and negative branch coefficients are not equal. Considering a case of mal-operation of directional pilot protection, the paper studies the impedance characteristics of asynchronous wind generator, analyzes the causes of weak feed and different branch coefficients, and explains their contribution to fault phase selector's mal-operation. The results are validated by simulation and an improvement for fault phase selection with wind farm integration is proposed.

This paper proposes an extended variable structure control strategy for a PV-Battery Virtual Synchronous Generator (VSG) to enhance Low Voltage Ride-Through (LVRT) capability under asymmetrical fault conditions in low-voltage distribution systems. The method introduces dual-sequence current control to actively inject both positive and negative sequence currents during faults. Two realistic local load types-constant impedance (CZ) and constant power (CP)-are analyzed to assess their impact on generation-load match, voltage support, and fault ridethrough performance. Eight different asymmetrical sag conditions are simulated, and the Voltage Unbalance Factor (VUF) is used to evaluate system response. Results show that CZ loads enable more effective VUF suppression due to their passive nature, while CP loads impose stricter current constraints that limit control flexibility. The proposed strategy is particularly suited for Grid-interactive Efficient Buildings (GEBs), where preserving voltage symmetry and complying with current injection requirements are critical during grid disturbances.

This paper compares the performance of four different power electronic converter topologies, which have been proposed for STATCOM applications. Two of the topologies are Modular Multilevel Cascaded Converters (MMCC), whilst the remaining circuits utilize magnetic elements and an open-winding transformer configuration to combine individual power modules. It is assumed that the STATCOM has to work under unbalanced conditions, so that it delivers both positive and negative sequence currents. Simulation studies for the four topologies have been carried out using the simulation tool Saber.

Thermal stresses induced by high root-mean-square (rms) currents in dc-link capacitors significantly affect their lifetimes and compromise the overall reliability of converter systems. This article proposes a novel zero-sequence voltage (ZSV) injection method to reduce the rms current in dc-link capacitors for back-to-back three-level converters. The proposed method adjusts the overlapping intervals of dc bus currents by injecting a differential ZSV on either the rectifier or inverter side, while a common ZSV is injected on both sides to control the neutral point voltage. Simulation and experimental results demonstrate that the proposed method reduces the capacitor rms current by up to 40%–50%, with minimal effect on the total harmonic distortion of the output currents. The proposed method improves system reliability by reducing power losses in the capacitors and extending their operational lifetimes, making it highly suitable for applications that demand both high efficiency and reliability.