Negative Sequence Current Compensation 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.

Active and Reactive Power Allocation Strategy to Maximize the Range of Negative-Sequence Current Compensation Based on Hybrid Cascaded STATCOM

Active and Reactive Power Allocation Strategy to Maximize the Range of Negative-Sequence Current Compensation Based on Hybrid Cascaded STATCOM

Negative-Sequence Current Compensation-Based Coordinated Control Strategy for Dual-Cage-Rotor Brushless Doubly Fed Induction Generator Under Unbalanced Grid Conditions

This article proposes a negative sequence current compensation based coordinated control strategy for dual-cage rotor brushless doubly fed induction generator (DCR-BDFIG), where the machine side converter (MSC) and grid side converter (GSC) cooperates to mitigate the imbalance in the control winding (CW) current, total output power and total grid current. Based on the machine dynamic model withstanding unbalanced grid voltage, the negative sequence current references, including four targets for MSC and three targets for GSC, are calculated and sent to the inner-current-loop controllers, where the GSC current controller is designed based on super–twisting sliding mode control (ST–SMC) with the stability analyzed. This proposed control strategy omits the outer-loop controller in the negative sequence frame, which has a more simple control structure and becomes more easy-implemented. Finally, the whole proposed coordinated control system is designed and implemented in both simulation and experiments under unbalanced grid conditions, verifying the effectiveness of the proposed control strategy.

An Improved Negative-Sequence Current Compensation Method for Star-Connected CHB STATCOM

Negative-sequence current is required to be intentionally injected to support the power grid or compensate the imbalanced load in asymmetrical faults utilizing STATCOM. However, the extreme compensation range of negative-sequence current injection becomes complicated when grid voltage becomes asymmetrical. This article presents a novel off-line strategy for negative-sequence current compensation under two typical kinds of asymmetrical grid faults — single-phase voltage drop and double-phase voltage drop, with the following two benefits simultaneously: maximizing negative current compensation capacity; avoiding iterations and reducing computation burdens. In this paper, the mechanism of DC voltage balancing among three phases in STATCOM is analyzed and a new identification method of negative-sequence current compensation range is explored out. To simplify the calculation, this paper takes the ratio of negative-sequence reactive ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Iq-</i> ) and active ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Id-</i> ) current as a variable ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</i> ). In this way, the relationship between the extreme compensation range, asymmetrical voltage sag depth of power grid <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</i> is drawn with MATLAB. Different from the previous iterative method, the off-line identification method is applied to avoid huge iterations. By fitting the curve, the precise ranges of negative-sequence current compensation under different asymmetrical grid faults are approximated as simple polynomials, thus significantly reducing the computation burdens. Simulation and experimental results verify the practicability of the proposed compensation method.

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.

Modulated Model Predictive Control With Branch and Band Scheme for Unbalanced Load Compensation by MMCC-STATCOM

This article presents a novel modulated model-predictive control (MMPC) scheme for modular multilevel cascaded converter-based static synchronous compensator (STATCOMs) (MMCC-STATCOMs) to compensate unbalanced load current and regulate reactive power flow. By adding a common-mode voltage (CMV) to the phase-voltages of the star-connected MMCC current model, the method allows natural injection of a nonsinusoidal voltage to the neutral point of the converter, hence achieving interphase cluster voltage balance. Moreover, the imposed CMV is shown to extend the operating ranges of MMCC-STATCOMs when used for negative sequence current compensation. The proposed MMPC method incorporates a modified branch and bound algorithm to optimize the per-phase switch duty ratios. It is shown to be computationally more efficient compared to model-predictive control schemes using the optimal voltage level method combined with voltage sorting schemes. Experimental results with different weighting factors confirm the effectiveness of this control scheme and compared favorably with the conventional scheme of injecting only a sinusoidal zero-sequence voltage.

An effective utilization scheme for regenerative braking energy based on power regulation with a genetic algorithm

To ensure that regenerative braking energy is fully utilized by traction trains in the whole railway power supply systems, an effective utilization scheme of the regenerative braking energy based on power regulation with a genetic algorithm (GA) is proposed in this paper. This scheme consists of railway power conditioners (RPCs), energy transfer converters (ETCs) and a central controller. A RPC connects the two power arms of each TS, and an ETC connects the two power arms of each section post (SP). The RPCs and ETCs are controlled to transfer the regenerative braking power distributed by the central controller, which is calculated by the GA. The detailed calculation steps are described. Moreover, the principle and control strategies of the negative sequence current compensation by the RPCs are given based on the optimization results of the GA. Finally, optimization calculations and hardware in loop (HIL) experiments were performed. The results show that the utilization rate of regenerative braking energy is about 93.3% and the three-phase current imbalance is reduced by about 98.6% in the case system with the proposed scheme. This result verifies the effectiveness and feasibility of the proposed scheme.

A Negative Sequence Current Phasor Compensation Technique for the Accurate Detection of Stator Shorted Turn Faults in Induction Motors

Stator faults are the most critical faults in induction motors as they develop quickly hence requiring fast online diagnostic methods. A number of online condition monitoring techniques are proposed in the literature to respond to such faults, including the signature analysis of the stator current, vibration monitoring, flux leakage monitoring, negative sequence components of voltage and current and sequence components monitoring based on the identification of asymmetrical behavior in a machine. Negative sequence components of voltage and current and sequence components monitoring are commonly considered for the identification of asymmetrical behavior of induction motors. Negative sequence current analysis is a sensitive technique for the detection of shorted turns as it directly measures the asymmetry produced by the fault. However, the technique is sensitive to other asymmetrical faults and disturbances, which can also produce negative sequence currents. These disturbances include sensor errors, inherent asymmetry and voltage unbalance. This paper provides a comprehensive investigation of the disturbances using a motor model along with experimental measurements under varying load conditions. Then, a new phasor compensation technique is explained to eliminate such disturbances effectively. This technique enables the accurate detection of even relatively small shorted turn faults, even at an early stage. The technique is tested experimentally, and a set of practical results are given to validate the methods developed.

Collaborative compensation strategy of electrified railways and DFIG-based wind farm for suppressing voltage unbalance

Single-phase traction loads of electrified railways have aroused the worst voltage unbalance (VU) problem in a weak remote grid, where possesses a number of wind farms (WFs). To utilize the compensator capacity effectively, this paper focuses on VU compensation effort sharing for traction power supply system (TPSS) and WF. Firstly, to achieve the stable VU compensation of point of common coupling (PCC), a VU propagation model is proposed to reveal the VU contribution of TPSS and WF, which provides a compensation target. Then, in terms of the VU active compensation ability of double fed induction generotor (DFIG), a negative sequence current (NSC) optimal compensation model is established under constraints of stable operation limits. Furthermore, a collaborative control strategy with the objective of VU limit at PCC is presented to provide reference current for TPSS and WF. Finally, simulation results are carried out to verify the effectiveness of the proposed strategy, and the compensator capacity of TPSS can be reduced by 18% when the active compensation ability of WF is taken into account.

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

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.

Negative Sequence Current and Reactive Power Comprehensive Compensation for Freight Railway Considering the Impact of DFIGs

To solve the power quality problems caused by freight railways in weak grids with high penetration of wind generation, this paper proposes a comprehensive control method for negative sequence current (NSC) suppression and reactive power compensation. Firstly, a co-phase traction power supply system (CTPSS) is introduced, and its compensation principle is analyzed. Secondly, to bring a doubly fed induction generator (DFIG) into full play in suppressing voltage unbalance (VU) in the grid, a VU compensation model of stator is developed. Moreover, a comprehensive compensation method is presented to achieve the dynamic compensation of VU and reactive power. Finally, the effectiveness of the proposed approach is demonstrated using a simulation, which can fully solve the power quality issues and effectively reduce the capacity of the CTPSS.

Negative Sequence Current Compensation for Induction Motor Stator Inter-Turn Short Circuit and Off-Diagonal Term in Sequence Impedance Matrix as Fault Indicator

This work proposes an artificial neural network supported method to establish an automatic detection of stator turn fault in induction motor. The sequence current analysis is done for turn fault condition. Various factors influencing the total measured negative sequence current such as unbalanced voltage and inherent asymmetry have been reviewed. To compensate the voltage unbalance and non-idealities in the machine, utilization of measured negative sequence current, impedance, admittance, or semi empirical formula is developed. The output of a well-trained feed forward back propagation neural network classifies the severity of fault level in stator winding. The method of considering the effects of turn faults on inter-turn fault detection improves sensitivity meanwhile reduces the prospect of misdetection.

A Novel Multistation Cooperative Negative Sequence Current Compensation Method for High-Speed Railway

Nowadays, the negative sequence current (NSC) issue is the most serious power quality problem for the high-speed railway power supply system, and the single substation NSC compensation methods cannot reduce the installed capacity of the compensating devices any further. A novel NSC compensation method, multistation cooperative compensation (MSCC), is proposed in this article. In order to adapt to the MSCC, a new NSC compensation principle, which is different from the traditional compensation principles, is carried out. Then, the cooperative control strategy is analyzed according to the new NSC compensation principle and the topology of the multiple traction substations. The compensating device is based on three-phase, cascaded, H-bridge converter which is suitable for the new NSC compensation principle. The capacity analysis results show that the MSCC can reduce the installed capacity of the compensating device for each single substation by 33.3% and even more under the constraint condition. A lower installed capacity means lower cost and smaller size. This advantage and the effectiveness of the MSCC are verified by both simulations and small-capacity experiments.

Analysis and Control of Modular Multilevel Converter With Split Energy Storage for Railway Traction Power Conditioner

In this paper, a railway power conditioner (RPC) based on a modular multilevel converter (MMC) with a split supercapacitor energy storage system (SCESS) is studied. In this case, the MMC-SCESS-based RPC could not only provide normal negative-sequence current compensation, but also reduce the impact of power fluctuations caused by the locomotive braking or startup on the electric grid. First, this paper analyzes the power flow patterns and deduces the reference circulating current under different operation modes of the MMC-SCESS-based RPC. Then, the control objectives of the MMC-SCESS-based RPC are divided into two categories—the balance control and the current tracking control. The balance control methods are developed for the submodule capacitor voltages and the state of charge of the supercapacitor, which are associated with the operation modes. To ensure the current tracking performance, a model-predictive direct current control method is presented for the MMC and the bidirectional energy converter. Finally, the effectiveness of the proposed control methods is verified by the experimental results of a downscaled prototype.

Control Strategy for Arc-Suppression-Coil-Grounded Star-Connected Power Electronic Transformers

The grounding of low-frequency distribution transformers is a critical concern that ensures the proper operation and protection of the distribution system. The power electronic transformer (PET), as a future alternative low-frequency distribution transformer, also needs grounding. This paper studies PET operation with arc-suppression-coil (ASC) grounding in the Y-connected medium-voltage (MV) stage, focusing on the following two aspects. 1) The MV stage current stress brought by the ASC is analyzed, and a negative-sequence current compensation method is proposed to reduce the MV stage current stress. 2) The ASC impedance adjustment range that the MV stage can provide by zero-sequence voltage injection is analyzed, and a virtual impedance method is proposed to achieve this adjustment. The effectiveness of the proposed ASC-PET as well as its control strategy is verified by both simulation and experiment results.

Unbalanced and Reactive Load Compensation Using MMCC-Based SATCOMs With Third-Harmonic Injection

Modular multilevel-cascaded converter-based STATCOMs are scalable to higher voltages without requiring a step-up transformer with multiple windings and can be realized using a low switching frequency, giving lower harmonic content and, hence, a reduced filtering requirement. This paper presents a new injection technique to extend the operating ranges of modular multilevel-cascaded converters STATCOMs when used for negative sequence and reactive current compensation. A nonsinusoidal voltage or current containing a fundamental and its third-harmonic component is injected to achieve phase cluster voltage balance. This technique reduces the maximum dc-link voltage for the star configuration, and the maximum converter phase circulating current for the delta case, compared to applying only sinusoidal zero-sequence components for mitigating the same degree of load unbalance. The analysis is confirmed experimentally, showing that the third-harmonic injection can allow a significant improvement of STATCOM capability for simultaneous compensation of unbalanced load and reactive current.

A Novel STATCOM Based on Flying Capacitor Modular Multilevel Converter

A new static synchronous compensator (STATCOM) based on Flying Capacitor modular multilevel converter (DCM2C) is proposed in this thesis. In this converter topology the capacitor voltage is clamped by using a low power rating diode in each sub-module (SM). The quantity of voltage sensors is significantly reduced and is free from the number of voltage levels. Furthermore the voltage balancing control method becomes very simple and the capacitor voltage balance speed is fast. Based on the structure of MMC the DCM2C-STATCOM has the capability of Var compensation and negative sequence current compensation. The topology characteristics and compensation control method of DCM2C-STATCOM are investigated in this thesis. That the capacitor voltage of the proposed DCM2C-STATCOM can be well balanced and the Var and negative-sequence current compensation are effective.

Reliable stator fault detection based on the induction motor negative sequence current compensation

The objective of this work is the compensation of the negative sequence current of the faulty induction motor in the aim to increase the accuracy and the sensitivity of the incipient stator fault detection under different disturbances. This is because, the negative sequence current generated in a faulty machine is the superposition of the different negative sequences current generated by the fault and others disturbances, which are the inherent asymmetry in the machine, the unbalanced supply voltage and the sensor inaccuracy. Thus, this paper proposes an efficient experimental method that allows the compensation of the negative sequence current of the faulty machine, by subtracting the negative sequence current of the different disturbances from the total negative sequence current of the faulty machine to isolate the negative sequence current due to the fault. Thus, the negative sequence currents of the different disturbances are isolated using an original experimental technique. The efficiency of the proposed method is validated experimentally on a 1.1kW motor under inter-turns short circuit faults and phase to phase faults.

A Novel STATCOM Based on Diode-Clamped Modular Multilevel Converters

A new static synchronous compensator (STATCOM) based on the diode-clamped modular multilevel converter (DCM2 C) is proposed in this paper. In this converter topology, the capacitor voltage is clamped by using a low power rating diode in each submodule. The quantity of voltage sensors is significantly reduced and is free from the number of voltage levels. Furthermore, the voltage balancing control method becomes very simple and the capacitor voltage balance speed is fast. Based on the structure of modular multilevel converter, the DCM2C-STATCOM has the capability of Var compensation and negative-sequence current compensation. The topology characteristics and compensation control method of DCM2C-STATCOM are investigated in this paper. Experimental results obtained from a laboratory prototype validate that the capacitor voltage of the proposed DCM2C-STATCOM can be well balanced and the Var and negative-sequence current compensations are effective.

Simple Zero-Sequence-Voltage-Based Cluster Voltage Balancing Control and the Negative Sequence Current Compensation Regulation Identification for Star-Connected Cascaded H-Bridge STATCOM

In some application circumstances, both positive and negative sequence reactive power compensations are desired from the star-connected cascaded H-bridge static synchronous compensator (CHB STATCOM). However, the imbalanced phase currents/voltages will cause uneven active power flow among three clusters, which makes the cluster voltage balance more challenging. In this scenario, only zero sequence voltage is available to balance the cluster voltages. Although the zero sequence voltage can be calculated in all operating conditions based on the equilibrium of the active power for each phase cluster, the calculation is definitely complicated due to the zero sequence voltage expressed in the form of trigonometric function in the abc or dq 0 frame. In this paper, the zero sequence voltage is represented in a dc form in the dq frame through introducing a virtually orthogonal zero sequence voltage. As a result, a linear relationship between the active powers and the zero sequence voltage is established, based on which, a simple zero sequence voltage calculation method is proposed with a small calculation burden. Furthermore, the accurate compensation region for the negative sequence current injection is identified by establishing the relationship between the maximum converter voltage and the negative sequence currents taking into consideration the initial phase angle of the negative sequence currents, while still keeping the cluster voltage balanced by using the zero sequence voltage control. The effectiveness of the proposed control and the compensation region are verified by the simulation and experimental results on a 400 V/±7.5 kVAr star-connected CHB STATCOM.