Zero-sequence Voltage Injection Research Articles

Extension of Negative-Sequence Current Compensation Range Based on Negative- and Zero-Sequence Voltage Injection for Hybrid Cascaded STATCOM

Negative sequence voltage (NSV) injection is an effective cluster balancing control for CHB STATCOM, which has a higher capability in eliminating unbalanced active powers than zero sequence voltage (ZSV) injection. Unfortunately, NSV is employed for negative sequence current control under unbalanced current compensation applications, and only ZSV injection is used for cluster balancing, thereby seriously limiting negative sequence current compensation range (NCCR). To extend NCCR, the converter with a common DC link, called power exchange unit (PEU), is inserted into CHB to form a hybrid cascaded structure, which can freely exchange the unbalanced active powers via the common DC link. However, only ZSV injection method fails to take the advantage of this hybrid cascaded converter (HCC) to extend NCCR. In this paper, the NSV injection with a simple algorithm in dq frame is proposed for the HCC STATCOM with negative sequence current compensation. Owing to the PEU, the NSV of CHB for cluster balancing is counteracted by superposing an opposite NSV into PEU, thus transferring unbalanced active power from CHB to PEU and not affecting the negative sequence current control. Due to the limit of PEU capacity, the ZSV injection is also employed, and thus a novel cluster balancing control based on NSV and ZSV injection is proposed, based on which the NCCR is greatly extended. Finally, the proposed control is verified by the experimental results on a 400V / 7.5kvar HCC STATCOM.

Common-Mode Voltage Reduction for Back-to-Back Two-Level Converters Based on Zero-Sequence Voltage Injection

High frequency and high amplitude common-mode voltage (CMV) can induce a number of problems on the reliable operations of back-to-back two-level converters, such as shaft voltages and bearing currents. To restrain the CMV for back-to-back two-level converters, a simple and effective CMV reduction method based on zero-sequence voltage (ZSV) injection is proposed in this article. By injecting the optimal amount of ZSV on either the rectifier-side or the inverter-side, the corresponding pulse overlapping areas that result in high CMV amplitudes can be eliminated with the proposed method, then the amplitude of CMV can be reduced by half, and the pulse edges of CMV can be reduced by one-third. Simulation and experimental results are presented to verify the effectiveness of the proposed method in CMV reduction, while some comparisons are also conducted to demonstrate the superiority of the proposed method.

Switched-Capacitor-Based Hybrid Clamped Converter for Wide Power Factor Applications

The clamped multilevel converter generally adopts the zero-sequence voltage injection method to realize the voltage balancing of the DC link capacitors. However, the relationship between the reference modulation voltage and the duty ratio of phase neutral point current is often a triangle or trapezoidal function. When the power factor is low, the required optimal zero-sequence voltage will be out of bounds, which will lead the capacitor voltage ripples to become larger or even divergent. Therefore, a novel switched-capacitor-based hybrid clamped(SCB-HC) converter is proposed in this article. The new redundant switching states are obtained by introducing the boost characteristics of the switched-capacitor(SC) circuit. And a hybrid phase-shifted pulse width modulation(PS-PWM) is developed to extend its functional relationship. This ensures the peak current that flow out of the neutral points is fewer, then the capacitor voltage drift is reduced significantly. Moreover, the proposed hybrid PS-PWM method not only ensures that SC circuit switches series and parallel patter once during a carrier period, but also achieves the voltage balancing of SC and DC link midpoint capacitor naturally under ideal conditions. On top of that, the capacitor voltage balancing method is designed to ensure independent and decoupled control of all capacitor voltages. Simulations and experiments show that the proposed topology, modulation method and voltage control method are practical and feasible for wide power factor applications.

Development and testing midpoint voltage balance algorithms of three-level inverters

Nowadays, the use of inverters is increasing in more and more applications. Environmental awareness is becoming a trend, so more and more people install solar energy systems on their houses, which also require the use of inverters. Higher power levels have led to the emergence of three-level inverters which has opened up new challenges, such as the issue of midpoint voltage balancing. This paper discusses voltage balancing and voltage ripple reduction techniques for three-level inverters with Neutral-Point Clamped (NPC) topology. The balancing is based on injecting zero-sequence voltage component in carrierbased modulation. The viability of the modulation techniques are verified by computer simulation and then tested on hardware.

Multivector-Based Model Predictive Current Control With Zero-Sequence Current Suppression for Three-Phase Series-End Winding Permanent Magnet Synchronous Motor Drives

To suppress the zero-sequence current and enhance the steady-state performance of the three-phase series-end winding permanent magnet synchronous motor (TPSW-PMSM) drives, a multi-vector-based model predictive current control scheme with zero-sequence current suppression is developed in this paper. The concept of the desired voltage is utilized to preselect the candidate voltage vectors and offer the guidance of the geometric division of the sector. In one sampling period, multi-voltage vectors are employed, and the combinations include one active voltage vector and one null voltage vector, and two active voltage vectors, which improves the steady-state performance. Meanwhile, the duty cycle of each voltage vector is derived based on an intuitive geometric expression of the cost function. Further, the relationship between the zero-sequence voltage injection and the switching sequence is revealed, and it is employed to suppress the zero-sequence current actively. In addition, overmodulation operation is also taken into consideration in the proposed scheme. Finally, the experimental study validates the effectiveness of the proposed scheme.

Submodule Capacitor Voltage Ripple Reduction of Full-Bridge Submodule-Based MMC (FBSM-MMC) with Non-Sinusoidal Voltage Injection

The full-bridge submodule (FBSM)-based modular multilevel converter (FBSM-MMC) offers promising performance due to its ability to output negative FBSM voltage. This paper studies this ability of the FBSM-MMC under zero-sequence voltage injection (ZSVI) and second-order harmonic current injection (SHCI). The focus of the research is to redistribute the FBSM powers by injecting an additional power degree of freedom, resulting in a smaller FBSM capacitor voltage ripple, while keeping the maximum AC output voltage for a given fundamental frequency component of the arm voltage reference. Accordingly, a control strategy was developed, based on non-sinusoidal ZSVI, and SHCI is proposed for further improving the performance of the FBSM-MMC. The proposed non-sinusoidal ZSVI contains a higher sinusoidal third-order harmonic component than that of pure sinusoidal third-order voltage injection (THVI) when operating under the same maximum AC output voltage. By implementing this solution, a smaller amplitude of the injected second-order harmonic current can be achieved, producing a lower power loss in the FBSM-MMC. Considering the proposed solution, the relationship of the arm powers and FBSM capacitor voltages are also discussed. Finally, the simulation results and experimental results are presented to verify the effectiveness of the theoretical analysis of the proposed method.

An improvement of stability and dynamic response in hybrid AC-DC microgrids using optimal power control management

Microgrid topologies enable more effective use of renewable resources as well as the autonomous process. Microgrids are effective frameworks for managing dispersed assets such as renewable structures with small-scale distributed generator supplies. Smart grids are adaptable electricity networks that have arisen as a result of the introduction of new technologies and features. It is suggested that a three-terminal AC/DC hybrid microgrid with DC two and AC one terminal be used. A two dual active bridge (DAB) with cascaded H-bridge (CHB) converter with AC grid link-based interactions that connect isolated two DC buses are included in the suggested structure. To minimise the energy generation stages and power devices, the DAB converters are intrinsically linked to the CHB of DC rails based on the structure-specific requirements. To present the unnecessary grid currents and DC rail voltage issues exist by this revised structure topology by simply two power alteration stages, a better solution is suggested that employs zero-sequence voltage (ZSV) injection in the CHB converters. The impacts of managed settings on structure stability and dynamic responsiveness are investigated in order to reduce conflicts among ZSV injection by structure voltage/current regulation. This paper compares the performance of a PI and a PR for DC voltage regulation. A comparison of simulation results for both control approaches is provided. The major difficulty is to normalise the system model in order to identify the appropriate controller settings for robust control. The findings are confirmed and compared to the conventional regulator PI. The suggested 3-Φ AC current and DC rail voltage regulating approach's universal efficacy is proved by MATLAB/Simulink analytical model from the three and five-terminal hybrid AC/DC power structures.

An optimization method for reducing the voltage fluctuation of the floating‐capacitor in ANPC five‐level inverter

AbstractThe excessive fluctuation of the floating capacitance's (FCs) voltage affects the output performance and output power of the 3P‐5L‐ANPC converter. In order to minimize the fluctuation of the FCs voltage, an optimization control method based on zero‐sequence voltage (ZSV) injection is proposed. The method takes reducing the maximum amplitude of the voltage fluctuation of the three‐phase floating capacitor as the control target. Firstly, the mathematical equation between the modulation voltage and the voltage fluctuation of the FCs are established, and then the maximum fluctuation amplitude of the FCs voltage is obtained by using the established mathematical equations. Secondly, the range of ZSV injection is determined according to the conditions such as the control of neutral point voltage and the limitation of high‐phase voltage leaps. Finally, the equations of the maximum amplitude in the voltage fluctuation of the three‐phase FCs are established, and the ZSV with the minimum voltage amplitude fluctuation of the FCs is obtained within the effective range of the ZSV injection. Compared with the traditional shifted pulse width modulation (SPWM)/space vector pulse width modulation (SVPWM) method, the proposed method can effectively reduce the fluctuation of the FC voltage by 50%. Simulation and experimental results are presented to verify the effectiveness of the proposed method.

Active Power Backflow Suppression Strategy of Cascaded PV Solid-State Transformer Under Interphase Short-Circuit Fault Condition

Currently, the grid-tied guidelines all require large and medium-sized photovoltaic (PV) power stations to stay connected to the grid during low-voltage ride through (LVRT). However, three-phase cascaded PV solid-state transformer (SST) has the inherent active power backflow problem during asymmetric voltage sags, causing that H-bridge dc-bus voltages are out of control and that the converter will be disconnected from the grid because of overvoltage fault. The existing control strategy is capable of avoiding active power backflow in theory by injecting appropriate zero-sequence voltage, but the overmodulation range is larger during interphase short-circuit fault, weakening active power backflow suppression effect. For this issue, this article proposes a combined control method of positive–negative-sequence maximum–minimum harmonic zero-sequence voltage injection strategy and adaptive zero-sequence voltage compensation strategy, which can shrink effectively active power backflow range of three-phase cascaded PV SST, increasing its adaptability to different output powers and different grid drop depths, and then improving LVRT capability of the PV power generation system. Finally, a low-voltage and low-power experimental prototype is made to verify the effectiveness and feasibility of the proposed control strategy.

Zero-Sequence Current Suppression Method for Fault-Tolerant OW-PMSM Drive With Asymmetric Zero-Sequence Voltage Injection

For the open winding permanent magnet synchronous motor (OW-PMSM) with leg open-circuit faults, a novel zero-sequence current (ZSC) suppression method with asymmetric zero-sequence voltage (ZSV) injection is proposed in this article. To realize the fault-tolerant control (FTC) in leg open-circuit fault conditions, the leg-sharing method is developed, where the output terminal of the faulty leg is connected to that of the remaining equivalent leg. However, the ZSC increases in the FTC mode, which causes severe torque ripple and system performance degradation. If the traditional ZSC suppression method is still adopted, the suppression performance is decreased. Besides, an undesired voltage vector is introduced, resulting in current distortion. To solve these problems, a novel ZSC suppression strategy is put forward. By asymmetrically adjusting the voltage output of the remaining legs, the ZSC of OW-PMSM in FTC mode can be effectively suppressed. Moreover, the proposed strategy is suitable for both the single-leg and double-leg FTC modes, where the ZSC suppression performance is almost the same as that of the traditional method in normal operation mode. Furthermore, experiments are performed on an OW-PMSM to verify the effectiveness of the proposed ZSC suppression scheme.

A Coordination Control With Extra Active Power Exchange Way to Extend Negative Sequence Current Compensation Range for STATCOM Based on Hybrid Cascaded Converter

The star-connected Cascaded H-Bridge (CHB) converter is one of the most attractive multilevel topologies for medium/high-voltage STATCOM. However, due to the fact that CHB has separated dc-links, three-phase unbalanced active powers cannot exchange automatically, which, thus, requires extra zero sequence voltage (ZSV) to eliminate them. Especially when negative sequence current is compensated, the negative sequence current compensation range (NCCR) is seriously limited. To extend the NCCR, this article combines the CHB and the converter with a common dc-link, which is utilized to exchange and eliminate the unbalanced active power generated from CHB. Based on this new active power exchange way and conventional ZSV injection, a novel cluster voltage balancing control method is proposed to eliminate the unbalanced cluster active powers. Moreover, based on the proposed control, the NCCR is identified, which is larger than that based on the conventional control. Finally, the proposed control and the derived NCCR are verified by simulation and experimental results on a 400 V/7.5 kvar Hybrid Cascaded Converter STATCOM.

A Hybrid Modulation Method With the Maximum Controllable Range of the Neutral-Point Current for Three-Level NPC

In order to analyze the maximum neutral-point (NP) voltage controllability of the three-level NPC under carrier-based pulsewidth modulation, this article deduces the accurate relationship between the three degrees of freedom under carrier-based pulsewidth modulation (zero-sequence voltage, zero-level disassembly phase, and the extent of zero-level disassembled) and the NP current. Meanwhile, a hybrid method is proposed to obtain the suitable values of the three degrees of freedom that can achieve the maximum controllable range of the NP current directly. Therefore, the proposed hybrid method avoids the huge computational burden caused by the cyclic traversal method. Compared with the traditional method of injecting zero-sequence voltage or disassembling the zero level, the proposed method can realize the maximum controllability of the NP voltage under the full range of the power factor and the modulation index with a lower switching frequency. The proposed method is verified by experiments.

Battery Energy Support to Cascaded H-Bridge Converter-Fed Large-Scale PV System During Unbalanced Power Generation

The cascaded H-bridge (CHB) multilevel converter is an attractive solution for integrating photovoltaic (PV) generators with the ac grid. However, the power generated in the PV panels may be unequal due to partial shading, temperature change, and dust layer accumulation leading to unequal and unbalanced power between the phases and possible violation of power quality constraints. Fundamental frequency zero-sequence voltage (FFZSV) injection can be applied, but this method can produce overmodulation in conditions with high PV power mismatch. A new FFZSV is proposed in this article assuming a CHB with a battery energy support (BES) to compensate for the effects of unbalanced PV power generation. A barycentric coordinate representation helps to determine the capacity of the BES required. A coordinated control between the PV system and BES is also proposed in the article. Additionally, a novel approach to deliberate FFZSV injection is discussed in this article by which the maximum charge and discharge rate for the BES can be obtained along with balanced current injection into the grid. The effectiveness of the proposed technique is verified in numerical simulations and with experimental results.

Research on STATCOM control strategy of Cascaded H-bridge considering unbalanced conditions

STATCOM, a star cascade H bridge, usually uses zero-sequence voltage injection to achieve inter-phase DC voltage balance. Different from negative sequence current injection, the phase-to-phase DC voltage balance control based on zero sequence voltage injection does not generate zero sequence current in three-phase and three-wire systems, which is beneficial to the power quality of the system. In the traditional control algorithm, the zero sequence voltage is calculated by the unbalanced active power and current. The control algorithm contains a large number of complex operations and matrix operations, which need to consume a lot of controller resources and is not conducive to system design and expansion. Moreover, in order to improve the dynamic performance under voltage imbalance, the positive and negative sequence separation is introduced into the feedforward control to increase the complexity of the control algorithm. In order to simplify the control algorithm, a PI regulator is proposed to linearly adjust the DC component of zero sequence voltage in DQ coordinate system. At the same time, through theoretical analysis, it is found that the zero-sequence voltage in feedforward control is equal to the zero-sequence voltage in power grid, so the positive and negative sequence separation can be removed, further simplifying the algorithm.

Symmetric Cascaded H-Bridge Multilevel Inverter With Enhanced Multi-Phase Fault Tolerant Capability

In this article, a new method is introduced for operation of three-phase symmetric cascaded H-bridge (CHB) multilevel inverter after failure of power switches. When a switch fails, the faulty leg of the H-bridge is bypassed and the remaining leg is kept in use. Even with multiple faults in different H-bridge cells, the CHB is optimally reconfigured to continue its operation with its maximum healthy capacity. Based on the remaining operational part of the converter, a fault tolerant control scheme with zero-sequence voltage injection is proposed to synthesize a balanced line-to-line voltage. Using this method, the postfault line-to-line voltage of CHB is maximized with the minimum common mode voltage (CMV). Reduction of CMV is of importance in motor drive application. Also, fundamental component of CMV affects the real power of CHB phase legs. Through reducing the fundamental CMV, unidirectional power transfer from the dc side to the ac load is achieved in a wide operation region. The performance of the presented fault tolerant technique is verified in MATLAB/Simulink environment and validated on a three-phase five-level hardware prototype.

Novel Control Scheme for Five-Level Hybrid Flying-Capacitor Inverters Without DC-Link Balancing Circuits

In this article, a novel control scheme for five-level hybrid flying-capacitor inverters is proposed, which can balance the neutral-point voltages across split dc-link capacitors and simultaneously regulate the flying-capacitor voltages.Each capacitor voltage is maintained by regulating the currents that flow through the dc-link neutral points and the flying capacitors. The key components affecting these currents are analyzed to construct a control scheme based on the injection of zero-sequence voltage to the modulation voltage reference.This technique eliminates the need for the auxiliary balancing circuit, which was previously used in the conventional operating schemes to regulate the dc-link capacitor voltages and, thus, reduces the device count and converter volume. The effectiveness of the proposed method for maintaining the capacitor voltages within the allowable range has been verified under various operating conditions through the simulation and experimental results.

Improved control scheme for simultaneous reduction of common-mode voltage and current harmonic distortion of the Vienna-type rectifier with balanced and unbalanced neutral-point voltages

As a non-generative-boost rectifier, the Vienna-type rectifier has been proposed and applied in several applications, such as wind power generation systems, battery charging system, etc. This paper further proposes an improved control scheme for reducing common-mode voltage (CMV) and input current harmonic distortions under balanced and unbalanced neutral-point (NP) voltages. At first, the operating fundamentals of the Vienna-type rectifier are presented. The zero-sequence voltage and its corresponding ranges are carefully derived to control the dc-link capacitor voltages separately. Considering the specific constraint of this rectifier, the zero-sequence voltage is further revised to reduce the current harmonic distortions around the zero-crossing points. Doing so, the simultaneous reduction of CMV and input current harmonic distortions can be realized, which is applicable to balanced and unbalanced NP voltages. The comparisons of the proposed scheme, the conventional zero-sequence voltage injection scheme, and the CMV reduction method under balanced NP voltage condition are conducted, which demonstrate the salient advantages of the presented scheme.

A Carrier-Based Two-Phase-Clamped DPWM Strategy With Zero-Sequence Voltage Injection for Three-Phase Quasi-Two-Stage Buck-Type Rectifiers

A three-phase buck-type rectifier features a step-down ac–dc conversion function, which is considered as a prominent solution for electric vehicle chargers and telecommunication systems integrated to the grid above 380 V line to line. However, traditional solutions for those applications employ cascaded architectures with an ac–dc boost-type stage and a dc–dc buck-type stage, which may suffer from high switching losses and large dc-link capacitor volume. To relieve this issue, a straightforward carrier-based two-phase-clamped discontinuous pulsewidth modulation (DPWM) strategy with generalized zero-sequence voltage injection is proposed in this article for the commonly employed cascaded circuit. This method can stop the switching actions in the front-end stage during two-third of the grid period, which can yield to the best switching loss reduction. The operations of the front- and back-end converter stages become highly coupled to each other, which reduces the size requirement of the capacitor in the dc link. Therefore, the equivalent circuit behaves as a quasi-two-stage buck-type rectifier allowing an enhancement of the system power density by improving power conversion efficiency and by reducing the volume of passive components and heat sink. The proposed carrier-based two-phase-clamped DPWM strategy is described, analyzed, validated, and compared with different pulsewidth modulation methods on PLECS-based simulation and a 5-kW prototype.

An Optimal Zero-Sequence Voltage Injection Strategy for DVR Under Asymmetric Sag

This article introduces an optimal zero-sequence voltage injection strategy (OZVIS) for the three-phase dynamic voltage restorer (DVR), which can effectively reduce the output voltage magnitude of DVR in case of asymmetric sags in a three-phase, three-wire power system, so as to improve the compensation capacity of the device. The conventional compensation strategy improves the active power or reduces the voltage amplitude of the DVR through phase rotation, but the two functions are difficult to achieve together. In this article, the operation principle of OZVIS is first introduced. Then, the optimality and effectiveness of the strategy are theoretically demonstrated by means of a plane geometric modeling approach. In addition, the reduction extent using OZVIS in different voltage sag types is quantified, which is based on the minimum energy compensation strategy. Furthermore, a digital “plug-in” algorithm implementation for the proposed strategy is elaborated. Finally, the experimental results will be shown to verify the feasibility and effectiveness of OZVIS under an asymmetric situation. Moreover, the adaptability under a symmetric situation will also be illustrated.

Low Switching Frequency SPWM Strategies for Open-Winding Machine With Low Current Harmonics

In this article, two novel sinusoidal pulsewidth modulation (SPWM) strategies with simple implementation are proposed for an open-winding machine fed by isolated dc-bus dual two-level three-phase inverters to reduce switching frequency and current harmonics. The proposed strategies utilize zero sequence voltage injection and unbalanced reference voltage distribution to generate discontinuous PWM with low current harmonics and less switching action. Two different zero sequence voltages are introduced in this article corresponding to two proposed PWM strategies with different switching actions in a PWM cycle. The novelty of this article is low current harmonics and less switching actions in a PWM cycle compared with the conventional SPWM strategy. Moreover, compared with the subhexagonal center PWM strategy, the proposed SPWM strategies have advantages of easy implementation and half switching commutations with the same current harmonic characteristics. The superiority of the proposed SPWM strategies is validated by simulation and experiment results.