Zero Sequence Circulating Current Suppression Research Articles

Active Vector Modification for Circulating Current Suppression of Paralleled Inverters Using Two-Step Sector Indexing-Based Discontinuous SVPWM

Due to the discrepancy of paralleled inverters, the zero-sequence circulating current (ZSCC) is a critical issue in a paralleled inverter fed permanent magnet synchronous machine (PMSM) drive system. The conventional solution to this issue is to adjust the duration of two zero vectors, however, will not be appropriate under the discontinuous space vector pulse-width modulation (DSVPWM) control using only one zero vector. For overcoming the above issues, an active vector modification based ZSCC suppression and a two-step sector indexing based DSVPWM are proposed in this paper. It uses both reference voltage and current vectors to jointly determine the drive signals in a two-step manner, which avoids the circulating current jump due to the coexistence of different zero vectors and can also reduce the switching loss by 50% in theory. Meanwhile, the low-frequency ZSCC can be online suppressed by a proposed real-time modification of durations of active vectors, which is finally verified together with the proposed two-step sector indexing method on a PMSM fed by two paralleled inverters.

Improved Finite-Time Control for Circulating Current Suppression of Multiparalleled Rectifiers

Multi-paralleled three-level rectifiers (MP3LRs) have the advantages of higher-power rating and higher reliability. However, the zero-sequence circulating current (ZSCC) will occur when MP3LRs are connected to a common dc bus, which will distort the grid current, and threaten the safe and reliable operation of MP3LRs. Significantly, the ZSCC suppression of the MP3LRs is rarely studied. To overcome the limitation, an improved ZSCC suppression method for MP3LRs is proposed in this paper. Firstly, a ZSCC model of the MP3LRs system is established. Secondly, according to the ZSCC model, the ZSCC controller based on “Finite-time control + Feedforward” is designed, and the strict theoretical proof of system stability is carried out. Thirdly, this method is realized by using the output of ZSCC controller to modify the three-phase switching time of multiple rectifiers (2nd to Nth rectifiers). Consequently, the method proposed in this paper can suppress the ZSCC of MP3LRs, completely. Compared with the conventional ZSCC suppression methods, the proposed method has better ZSCC suppression performance and stronger anti-interference capability. Meanwhile, it does not increase any hardware cost. Finally, simulation and experimental results verify the effectiveness of the proposed method.

Nonlinear Modeling and Improved Suppression of Zero-Sequence Circulating Current for Parallel Three-Level Inverters

The parallel operation of three-level inverters can increase the current capacity and power rating of the system. However, the problem of zero-sequence circulating current (ZSCC) inevitably emerges due to the common-mode voltage difference (CMVD) between the parallel three-level inverters (PTLIs). Especially, the inverter nonlinear characteristics (INCs) will worsen the ZSCC problem and weaken the suppression effect of ZSCC. In this paper, a nonlinear equivalent model of ZSCC is established considering the INCs. Then, the simplified graphical analysis method is proposed to implement the frequency analysis of ZSCC. Furthermore, an improved ZSCC suppression strategy for the PTLIs is proposed. The INCs and original CMVD are regarded as the total disturbance and a nonlinear disturbance observer (NDOB) is designed to estimate it. Based on the NDOB, a feedback-feedforward control method is adopted to suppress ZSCC. According to the amplitude-frequency characteristics of the closed-loop control system, the controller parameters design and performance analysis are performed. The control structure is simple, the parameter design is convenient, and the suppression effect of ZSCC is evidently improved. Finally, the simulation and experimental results verify the validity of proposed strategy.

PROPORTIONAL INTEGRAL QUASI RESONANT CONTROLLER FOR ZERO-SEQUENCE CIRCULATING CURRENT AND RIPPLES SUPPRESSION IN PARALLEL THREE-PHASE PWM RECTIFIERS

Parallel three-phase pulse with modulation (PWM) rectifiers with common sc and ac buses are widely used in power systems due to their many advantages such as flexibility, sinusoidal grid currents, lower switching frequency, and good reliability. However, this topology suffers from zero-sequence circulating current (ZSCC) generated by numerous reasons including filters inductors unbalanced, unequal dead time, and losses of synchronism between the control of each rectifier, which will distort the ac-side currents and increase power losses. This paper proposes both an adjusted space vector pulse width modulated (ASVPWM) method and proportional integral quasi resonant controller (PIQRC) method not only to force the ZSCC to be zero but also to reduce its ripples, which results in low frequency harmonic components in the ac side currents. This twofold objective can be achieved by adjusting the zero-vector duty ratios of ASVPWM to suppress the ZSCC and by using PIQRC to mitigate its predominant harmonics. Finally, the superiority and efficiency of the proposed control method in terms of ZSCC suppression and current ripple reduction are verified through comparative analysis with the conventional ZSCC-PI controller.

The Zero-Sequence Circulating Current Suppression Method for Parallel Three-Level Rectifiers System Under the Open-Circuit Fault of Neutral-Point Switches

In parallel T-type three-level rectifiers system (PT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 3LRs), when the open-circuit fault (OCF) of neutral-point (NP) switches occurs, the existing zero-sequence circulating current (ZSCC) suppression methods are not capable of suppressing the ZSCC any more since the ZSCC model of PT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 3LRs is changed. Significantly, the ZSCC suppression issue of PT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 3LRs under the OCF of NP switches has not been researched in previous literatures. To overcome this limitation, a novel ZSCC suppression method for PT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 3LRs with the OCF of NP switches is proposed. First of all, according to the principle of area equivalence, the fault-tolerant control (FTC) is accomplished by reconstructing the switching sequence. Second, a novel ZSCC model with FTC is established. Then, according to the novel ZSCC model, the ZSCC control method based on “PI + feedforward” is proposed. This method is realized by modifying the turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> time of three-phase of fault rectifier in PT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 3LRs. Consequently, the method proposed in this article effectively restrains the ZSCC of PT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 3LRs with the OCF of NP switches. Finally, the simulation and experiment results verified the correctness of proposed method.

Fault-Tolerant Sequential MPC for Vertical Switch Open-Circuit Fault and ZSCC Suppression for Parallel T-Type Converters

A fault-tolerant control (FTC) is proposed for enhancing the reliability of power electronic systems. The software-based FTC of parallel-connected three-level T-type converters (3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Cs) for suppressing zero-sequence circulating current (ZSCC) induced by a vertical switch open-circuit fault is investigated. A simplified sequential model predictive control (SSMPC)-based FTC technique is developed. Parallel-3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Cs and their prediction models are introduced, followed by an SSMPC for the no-fault condition. The model inaccuracies under vertical switch faults are elaborated. For the faulty 3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> C, two fault-tolerant SSMPC (FT-SSMPC) methods are provided by creating a sequential predictive controller that considers ZSCC suppression and grid current tracking. The control policies for the ZSCC are changed from the standard feedback-free to feedback-based cost function (CF) optimization using the unimproved FT-SSMPC. Furthermore, an improved FT-SSMPC is proposed using a phase-deficient CF, which increases the accuracy of the mathematical relation of the fault condition. After fault diagnosis, the proposed method achieves neutral-point voltage balance and excellent point-of-common-coupling current, effectively suppresses the ZSCC, and increases the control reliability. Finally, experiments demonstrate the effectiveness of the proposed FT-SSMPC under various conditions.

The Tolerance Control Method Based on Zero-Sequence Circulating Current Suppression for Parallel T-Type Three-Level Rectifiers

Zero-sequence circulating current (ZSCC) suppression and fault-tolerant control (FTC) are important guarantees for the safe and reliable operation of parallel T-type three-level rectifiers <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$({\mathrm {PT}}^{2}3{\mathrm {LRs}})$ </tex-math></inline-formula> . However, none of the existing literature can solve the FTC under the ZSCC suppression of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {PT}}^{2}3{\mathrm {LRs}}$ </tex-math></inline-formula> when the open switch fault (OSF) occurs in neutral-point (NP) switches. In order to solve these issues, a novel FTC method based on ZSCC suppression is proposed in this article. First, an improved finite-time controller is constructed to suppress the ZSCC by adjusting the difference of zero-sequence duty ratio between two rectifiers, completely. Then, based on the principle of area equivalence, the switching sequence is reconstructed by changing the state switching mode of the fault phase in real-time. As a result, the FTC under the OSF of NP switches is realized without affecting the suppression of ZSCC. Compared with the existing methods, the proposed method not only realizes the suppression of the ZSCC of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {PT}}^{2}3{\mathrm {LRs}}$ </tex-math></inline-formula> but also completes the FTC under the OSF of NP switches, which guarantees the ZSCC suppression ability under the OSF of NP switches. Consequently, the power quality of the ac side of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {PT}}^{2}3{\mathrm {LRs}}$ </tex-math></inline-formula> is improved significantly during the normal operation and the operation under the OSF of NP switches. Thus, the safe and reliable operation of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {PT}}^{2}3{\mathrm {LRs}}$ </tex-math></inline-formula> is guaranteed effectively. Finally, the feasibility and correctness of the proposed method are verified by experiment results.

An improved data-driven based model predictive control for zero-sequence circulating current suppression in paralleled converters

To address the requirement of improved power capacity, reliability and operation, parallel converters have been widely applied in micro-grid. However, the connections between DC and AC sides of different parallel converters may lead to the surge of zero-sequence circulating current (ZSCC), which would result in the deterioration of system functionality. Regarding the stated problem above, this paper proposes an improved data-driven based model predictive control for parallel converters. Specifically, a model-free adaptive control (MFAC), which is based on the measured input/output data, is incorporated into the finite control-set model predictive control (FCS-MPC) framework with multi-objective constraints, thus allowing for the suppression of ZSCC and the improvement of robustness performance. Furthermore, to avoid the tuning of weighting factors, a novel cost function is constructed based on the multi-objective decision making method. The main contribution of the proposed methodology relies on the fact that no knowledge of any model parameters and weighting factors in whole control process are required, which leads to a significant enhancement in the robustness and reliability of the control system in the presence of parametric uncertainties. Furthermore, by this proposal, the accurate current tracking and ZSCC suppression can be achieved. Finally, the validity of the proposed FCS-MPC scheme is verified by comprehensive case studies.

An Optimized Zero-Sequence Voltage Injection Method for Eliminating Circulating Current and Reducing Common Mode Voltage of Parallel-Connected Three-Level Converters

Parallel three-level inverters have attracted considerable attention due to the advantages to increase the power rating and the flexibility of power conversion systems. However, this type of configuration requires to suppress the zero-sequence circulating current (ZSCC) through either modulation or active control strategies, to avoid overloading problems of a single three-level inverter. For the operation of paralleled three-level T-type inverters, another requirement is to reduce the common mode voltage (CMV) associated with a pulsewidth modulation method. To better solve these problems simultaneously, an optimized zero-sequence voltage injection method is proposed to both reduce CMV and eliminate ZSCC for multiparallel-connected three-level inverters. A generalized CMV reduction strategy is developed, which can reduce the magnitude of CMV to one half of the value using the conventional space vector modulation. The highlight of the method is that ZSCC is eliminated by adding an optimal compensation value to the reference duty signals with lower CMV. This compensation value is calculated simply by ZSCC, duty cycles, and filter parameters. The better ZSCC suppression performance can be obtained in both steady and transient states with the proposed scheme, thus, improving the quality of currents.

Modeling and Suppression of Circulating Currents for Multi-Paralleled Three-Level T-Type Inverters

Paralleled inverters have the merits of high power rating, improved reliability, and convenient maintenance. However, zero-sequence circulating current (ZSCC) will occur and lead to current distortion and system loss. The ZSCC control is more complicated when three-level topology is utilized and paralleled number is increased. Therefore, this paper proposes a simplified ZSCC suppression scheme for multi-paralleled three-level T-type inverters (3LT2Is). A novel equivalent model is developed and the ZSCC controller based on the new model is easier to implement. Thus, the number of paralleled inverters can be increased without complicating the control scheme and the inverters could be switched on or off optionally. Besides, Feed-Forward (FF) strategy is adopted to eliminate the ZSCC spikes caused by symmetrical three-level space vector modulation. The effectiveness of the proposed control scheme is verified by both simulation and experimental results.

Investigation of Zero Sequence Circulating Current Suppression for Parallel Three-Phase Grid-Connected Converters Without Communication

The parallel three-phase grid-connected converters (GCCs) with common dc and ac buses can increase the system power level effectively. However, zero sequence circulating current (ZSCC) will occur inevitably. In this paper, the mechanisms and characteristics of ZSCC in two parallel three-phase GCCs are first investigated based on three typical cases. From the analysis, it was found that not all ZSCC can be suppressed. Generally speaking, ZSCC caused by inconsistent modulation voltages between modules cannot be effectively handled. And ZSCC caused by different injected zero sequence voltages (ZSVs) or asynchronous carriers can be suppressed. Based on the theoretical analysis, identical ZSV injection and carrier synchronization methods without communication are, thus, proposed to suppress ZSCC at the source, which are easily implemented without adding any hardware. Finally, the theoretical analysis and the effectiveness of the proposed methods on ZSCC suppression are well-verified by experiments.

Improved Modulation Mechanism of Parallel-Operated T-Type Three-Level PWM Rectifiers for Neutral-Point Potential Balancing and Circulating Current Suppression

In high-power applications, parallel-operated T-type three-level pulse width modulation (PWM) rectifiers (T3LPRs) are widely employed to improve the power capacity and system reliability. For parallel-operated T3LPRs, there are two important issues need to be properly addressed: 1) the neutral-point potential (NPP) balancing, and 2) the zero sequence circulating current (ZSCC) between the common ac and dc bus. In this paper, the fluctuation of NPP in a T3LPR and the generative mechanism of ZSCC in parallel-operated T3LPRs are first investigated. The zero sequence voltage (ZSV) difference between the parallel-operated T3LPRs is identified as the main excitation source of the ZSCC problem. Meanwhile, the disconnection of the neutral point and the control effect difference of NPP balancing in each T3LPR also affect the ZSCCs within the parallel-operated T3LPRs. To adjust the ZSV of T3LPRs, the dwell time of the positive small voltage vector in each switching period of the converter, which is usually adjusted to balance the NPP, is also required to be controlled to suppress ZSCC. Therefore, in this paper, to avoid the possible conflict, an improved modulation mechanism is designed to guarantee simultaneous ZSCC suppression and NPP balancing. The neutral points of each T3LPR are connected. Then, by adjusting the dwell time of the positive small voltage vector, the charging time of two dc capacitors is adjusted to balance the NPP, and the ZSV differences between different T3LPRs are adjusted to suppress ZSCCs. To avoid the control effect difference of independent NPP balancing in different T3LPRs, all T3LPRs share the same value marking the difference between two dc capacitor voltages. Thus, a communication with a greatly reduced baud rate requirement is needed for NPP balancing to transfer the difference between the two dc capacitor voltages. Moreover, no extra circuit is needed for ZSCC suppression. These approaches make the proposed improved space vector PWM strategy much more practical and easy to employ. The proposed method is verified with experiments on two 3-kW T3LPR prototypes.

Model predictive control for parallel three‐level T‐type grid‐connected inverters in renewable power generations

As the penetration of renewable power generation units connected to the grid increases, high power quality and high efficiency have been widespread concerned. The parallel 3LT 2 Is are necessary to improve the quality and efficiency of applications. The use of common dc-link without isolating transformers will cause the zero-sequence circulating current (ZSCC) problem. This study proposes a finite control set model predictive current control to suppress the ZSCC, balance the neutral-point (NP) voltage as well as control the currents with fast transient response. The cause of the ZSCC generated in the parallel 3LT 2 I is analysed. The proposed ZSCC suppression technique is achieved through changing the redundant small vectors in the parallel 3LT 2 Is. Moreover, the coupling problem between the ZSCC and NP balance is solved by the amplitude comparison of NP voltage and ZSCC. As a result, better ZSCC suppression and NP voltage performance can be achieved in different output currents and filter inductance condition. The proposed method has been verified experimentally.

Zero-Sequence Voltage Modulation Strategy for Multiparallel Converters Circulating Current Suppression

A zero-sequence circulating current (ZSCC) is typically generated among the multiparallel converters that share the common dc link and ac side without isolated transformers under the space vector modulation (SVM), due to the injected third-order zero-sequence voltage (ZSV). This paper analyzes SVM techniques, studies the effects of the ZSV on the ZSCC control loop in detail, and then proposes an improved SVM scheme to suppress the impact of the ZSV on the ZSCC. The proposed strategy can effectively realize the ZSCC suppression, even though the parallel converter modules have asymmetrical current references and filter inductances. The simulation and experimental results based on the parallel converters clearly verify the effectiveness of the proposed control.

Space Vector Modulation for Circulating Current Suppression Using Deadbeat Control Strategy in Parallel Three-Level Neutral-Clamped Inverters

Parallel three-level T-type inverters (3LT2Is) can effectively increase the power rating. However, when multiple 3LT2Is are connected to a single dc bus, the zero-sequence circulating current (ZSCC) will occur. The ZSCC will distort the output currents and increase the system loss. To overcome this drawback, this paper proposes a novel deadbeat control scheme for the ZSCC suppression. A generalized model for analyzing ZSCC in parallel 3LT2 Is is developed for designing the deadbeat control method. The proposed deadbeat control scheme is realized by real time adjusting the dwell time of small vectors in each space vector modulation. Compared to the conventional PI control method, the influence of different sector is eliminated, thus, improving the output current waveform and reducing the ZSCC. The effectiveness of the proposed method is verified by simulation and experiments.