Negative-sequence Compensation Research Articles

RBE Utilization Improvement and Negative Sequence Compensation for Multi-unit Energy Router in Railway Hub

RBE Utilization Improvement and Negative Sequence Compensation for Multi-unit Energy Router in Railway Hub

Negative Sequence Compensation Method for High-Speed Railway With Integrated Photovoltaic Generation System

The serious negative sequence (NS) issues and energy shortage problems are aroused by the single-phase 25-kV traction power supply system (TPSS) of high-speed railways. To achieve NS compensation and alleviation of energy demand, a TPSS with an integrated PV generation system is proposed. According to the structural characteristics of the TPSS and the PV system, a special topology and an NS compensation method are presented. Moreover, taking the NS control as the primary goal, three operation modes are classified by considering the load conditions of the TPSS and the output state of the PV system. After that, a coordinated control system with a central controller and multiple local controllers is proposed. The central controller can realize the flexible selection of operation modes and calculate the reference power. Meanwhile, the local controllers achieve the accurate tracking of the reference current and the stability of the DC link voltage by switching the control modes. Finally, the feasibility and effectiveness of the proposed system and the NS compensation method are verified by simulations.

Improved Coordinated Control Strategy for Reliability Enhancement of Parallel PFCs With LCC Restriction

Power flow controllers (PFCs), which adopt high voltage and large power converters, are the significant device of cophase traction power supply systems (TPSSs); therefore, it is important to enhance the reliability of PFCs. Due to the limited capacity of power semiconductor switches, parallel connection of PFC units has been encouraged, and system reliability can be enhanced by configuring standby PFC units. However, while the increase in the number of redundant modules is accompanied by higher investment, the reliability improvement also has a saturation effect. In addition, among the control techniques of parallel PFCs, one of the commonly employed methods is current-sharing control, but the thermal stress of converters is increased at low loads. Therefore, in this article, a PFCs’ cost model is built based on life cycle cost (LCC) theory, and the number of basic, redundant PFC units is determined with the constraint of the minimum average annual cost. Furthermore, an improved coordinated control strategy that can alleviate thermal stress is proposed. Under a high load, all the PFC units are operated simultaneously, while, under a low load, the units are operated by turns. Moreover, to ensure consistent reliability and maximize system reliability, the rotation working time of each unit and the load demarcation point are optimized considering negative sequence compensation constraints. Case studies show that the given control strategy can extend the life span up to 26.30% and achieve a 19.11% cost reduction.

A voltage support control strategy based on three-port flexible multi-state switch in distribution networks

Voltage sags in power system may lead to serious problems such as the off-grid of distributed generation and electrical equipment failures. As a novel type of power electronic equipment, a flexible multi-state switch (FMSS) is capable to support the voltage during the grid faults. In this paper, a voltage control strategy to support the voltage in a distribution network is proposed by introducing three-port FMSS. The positive–negative-sequence compensation (PNSC) scheme is adopted to control the active and reactive current. This control scheme eliminates active power oscillations at the port of voltage sags and reduces coupling oscillations of other ports. Based on the characteristics of the voltage support under PNSC scheme, two voltage support strategies are proposed. A proportional-integral controller is introduced to provide the reactive power references, which eliminates the errors when estimating the grid voltage and impedance. A current limiting scheme is adopted to keep the port current in a safe range by adjusting the active and reactive power references. The voltage support strategies in two different voltage sags are simulated, and results show the feasibility and effectiveness of the proposed control strategies.

A Control Strategy for Voltage Unbalance Mitigation in an Islanded Microgrid Considering Demand Side Management Capability

Inverter-based distributed generators (DGs) have been customarily used for voltage unbalance (VU) mitigation in microgrids. The sole dependency on DGs for VU mitigation may not be justified, particularly in islanded microgrids. Demand side management can be a potential candidate for VU mitigation in microgrids. Therefore, this paper presents a new VU mitigation scheme for an islanded microgrid by coordinating photovoltaic (PV) grid-tied inverters, and thermostatically controlled loads (TCLs). A negative sequence compensation loop working in parallel with a positive sequence compensation loop is designed for PV inverters for VU mitigation. Also, a voltage dependent model of TCLs, unlike the conventional one, along with the control strategy for VU mitigation is presented. The proposed control of TCLs not only minimizes VU, and hence, increases the dynamic reactive power reserve of the inverter based DGs, but also effectively maintains the customers’ thermal comfort. The proposed VU mitigation scheme is studied with comprehensive simulations in PSCAD/EMTDC, and is verified using a real-time digital simulator, OPAL-RT. Simulations and real-time results of the proposed scheme demonstrated the regulation of VU factor within the permissible range defined by IEC and CIGRE Working Group 36.07.

Multiport Railway Power Conditioner and Its Management Control Strategy With Renewable Energy Access

To achieve the mutual benefits between railway systems and renewable energy systems, a multiport railway power conditioner (MP-RPC) with the renewable energy access is proposed to realize the on-site power consumption and negative-sequence compensation. Its power-flow management strategy with five classic operation modes is proposed and analyzed for MP-RPC. The key part of MP-RPC is three-port isolated dc/dc converters, which can be used for the isolation and power transmission. Through the small-signal modeling and analysis of the three-port converter, the port power control strategy with feed-forward decoupling is applied. The coupling system is decoupled into two independent control loops based on the feed-forward decoupling matrix, which can effectively improve the dynamic performance of system. Finally, the traction power platform and simulated prototype are built in the Laboratory, and the topology and control methods of MP-RPC are verified effectively.

Ancillary Services via VSIs in Microgrids With Maximum DC-Bus Voltage Utilization

Grid-interactive inverters allow distributed generation units to provide various ancillary services in microgrids. As the linear modulation region of inverters is restricted by the dc-bus voltage, providing ancillary services may drive the fundamental positive-sequence inverter voltage to the overmodulation region. Therefore, to operate in the linear modulation region the maximum active- and reactive-power of the inverter may have to be reduced unless pulsewidth modulation (PWM) references are adjusted. In this paper, an atypical PWM method is proposed for maximizing dc-bus utilization of grid-interactive two-level voltage source inverters (2L-VSI) providing ancillary services. Two important ancillary services, i.e., negative-sequence compensation and harmonic compensation are considered in this paper. The proposed method increases the maximum active- and reactive-power that a 2L-VSI can deliver when providing ancillary services by injecting a common-mode component in the references computed based on instantaneous reference magnitudes. The validity of the proposed technique is verified through simulation, as well as experimental data obtained using a 208-V three phase grid-connected 2L-VSI.

Comparison of DSCC and SDBC Modular Multilevel Converters for STATCOM Application During Negative Sequence Compensation

Among the various topologies proposed in this literature, the modular multilevel converter (MMC) is considered the next generation of converters for medium- and high-voltage applications. This paper presents a comparison between two variants of MMCs for STATCOM applications: The double-star chopper cell-MMC and the single-delta bridge cell-MMC. These converter topologies are compared and benchmarked during positive and negative sequence compensation. The comparisons are supported by analytical results regarding minimum effective dc-link voltage, number of cells, current rating, energy storage requirements, and operation during unbalanced conditions. The dynamic behavior, power losses, and cost of both solutions are evaluated through simulation model of a 15 mega volt-ampere (MVA) MMC STATCOM.

PWM Common Mode Reference Generation for Maximizing the Linear Modulation Region of CHB Converters in Islanded Microgrids

Microgrid-interactive converters are increasingly expected to provide auxiliary services, particularly during islanded microgrid operation. Unbalanced, non-sinusoidal pulse width modulation (PWM) references may be required when converters provide auxiliary services, such as negative-sequence compensation. Since PWM references cannot exceed dc-bus voltage defined limits without causing overmodulation, the maximum positive-sequence voltage producible by microgrid-interactive converters may be diminished unless reference waveforms are adjusted. In this paper, a technique for maximizing the linear modulation region by injecting a common mode component into PWM references is proposed for microgrid-interactive cascaded H-bridge (CHB) converters. The technique presented can be implemented in real-time without lookup tables, is controller independent, is compatible with any carrier-based PWM method, enabling its use for symmetric and asymmetric CHB, and enables continued operation after internal fault events when utilized with fault tolerant PWM schemes. Experimental data verify the common mode injection technique.

DSCC-MMC STATCOM Main Circuit Parameters Design Considering Positive and Negative Sequence Compensation

The double-star chopper cell modular multilevel converter (DSCC-MMC) has been employed in several applications as HVDC, energy storage, renewable energy, electrical drives and STATCOMs. Generally, the DSCC-MMC main circuit parameter design presented in literature considers balanced currents flowing through the converter. Nevertheless, in STATCOM application, the converter can compensate negative sequence components and unbalanced currents flow through the DSCC-MMC, resulting in different stresses in the converter phases. Therefore, this work presents a detailed design methodology of the DSCC-MMC main circuit parameters, considering both positive and negative sequence current compensations. The dc-link voltage, number of submodules, power semiconductor thermal stresses, submodule capacitance and arm inductances are designed. Expressions for the energy storage requirements are derived when negative sequence is compensated. A case study considering a 15-MVA STATCOM is presented, and simulation results validate the proposed design methodology. Finally, the converter power losses and thermal stresses in the power semiconductors are evaluated.

Compensation of Magnetizing Current for Enhanced Operation of DFIG Under Grid Unbalance

In the context of a doubly fed induction generator (DFIG) connected to the utility grid under unbalanced voltage conditions, the controller design needs to ensure additional challenges such as restricting the total harmonic distortion (THD) in grid current, minimizing the pulsations in generated power, torque, dc-link voltage, etc., apart from facilitating the generator power control. In this study, a negative-sequence compensation scheme for the magnetizing current in a DFIG is investigated. The reference currents are modified to include the negative-sequence compensating component for the magnetizing current. A rotor-side converter is employed to compensate the pulsations in magnetizing current thereby minimizing the ripple in power and torque. A grid-side converter is used to maintain unity power factor and a constant dc-link voltage. It is revealed that controlling the magnetizing current as a single control target enables simultaneous reduction of pulsations in torque, power, and dc-link voltage and also minimizes unbalance in currents. The effectiveness of the scheme is validated through detailed analysis and numerical simulations in PSCAD/EMTDC for a practical 250 kW DFIG wind generation system and further, through experimental results for a 2.3 kW DFIG test setup. Typical experimental results for the laboratory test setup show an improved performance.

Unbalanced PCC voltage regulation with positive‐ and negative‐sequence compensation tactics for MMC‐DSTATCOM

Unbalanced voltage fluctuation at the point of common coupling (PCC) is a challenging power quality problem in power distribution system. This study presents an unbalanced PCC voltage regulation scheme with positive- and negative-sequence control for modular multilevel converter (MMC) in medium-voltage distribution static synchronous compensator (DSTATCOM) application. The proposed positive-sequence control is utilised to regulate the positive-sequence component of PCC voltage to rated value, while the negative-sequence control aims to cancel out the negative-sequence component of PCC voltage completely. Compensation commands whether in positive- or negative-sequence control are generated from unbalanced PCC voltage without utilisation of unbalanced load currents. DC circulating currents transferring active power among three phases of MMC are also analysed to illustrate power balance issue resulting from negative-sequence control. Then comparison of negative-sequence compensation capacity between MMC and full-bridge cascaded converter with delta-connection is illustrated. Experiment results acquired from a downscaled 85 V/1.5 kVA apparatus validate the effectiveness of the proposed control scheme.

Analysis and Comparison of Modular Railway Power Conditioner for High-Speed Railway Traction System

With the rapid development of modern electrified railway, negative-sequence current minimization is one of the most important considerations in the high-speed railway traction system. In the past, many multiple or multilevel topologies with high compensation capacity have been introduced for railway power conditioner (RPC). This paper presents a simplified quantitative comparison of five previous modular RPC topologies for negative sequence compensation in V/V and SCOTT traction systems, aiming for an optimal selection of the compensators. Performance criteria such as transformer requirement, voltage stress and current stress of a power switch, numbers of the power switches and capacitor are derived by analytical methods. Moreover, the numerical comparison of operating controllers is completed for modular RPCs. In addition, power losses of five modular RPCs are obtained by theoretical analysis, IPOSIM calculation as well as PSIM simulation. These calculations are validated via simulations results in PSIM. The main conclusion is that presented modular RPCs can be divided into general purpose RPC and special purpose RPC in terms of the behavior and efficiency. It is helpful to choose the appropriate topology for specific applications.

A Nine Switch Converter-Based Fault Ride Through Topology for Wind Turbine Applications

This paper proposes a new fault ride-through (FRT) configuration for a fixed-speed induction generator-based–wind turbine (FSIG–WT). It utilizes a nine-switch converter (NSC) to enable shunt and series compensation in response to the system dynamics and grid faults. The steady-state control strategy and transient-management scheme are developed to ensure proper performance in steady-state, dynamic operation as well as enhanced FRT capability of the FSIG–WT. In addition to that, the developed control strategy tackles the balanced and unbalanced operation of the electric grid with an enhanced positive- and negative-sequence compensation control scheme. The recently reported grid synchronization scheme, namely, the frequency-locked loop (FLL) is implemented to achieve fast phase detection and transient response. Consequently, the performance of the FLL has been compared with the multiple reference–frame–phase-locked loop (MRF–PLL) to demonstrate its superior performance for enhancing transient response. The proposed topology, FLL, and control strategies are verified through a comprehensive simulation study to demonstrate the enhancement of the transient response and FRT capability of FSIG-WT in adherence to grid-code requirements.

Industrial frequency single-phase AC traction power supply system for urban rail transit and its key technologies

To avoid stray current and maintain the benefit of no phase-split in the DC traction power supply system, an AC traction power supply system was proposed for the urban public transport such as metro and light rail transit. The proposed system consists of a main substation (MSS) and cable traction network (CTN). The MSS includes a single-phase main traction transformer and a negative-sequence compensation device, while the CTN includes double-core cables, traction transformers, overhead catenary system, rails, etc. Several key techniques for the proposed system were put forward and discussed, which can be summarized as (1) the power supply principle, equivalent circuit and transmission ability of the CTN, the cable-catenary matching technique, and the selection of catenary voltage level; (2) the segmentation technology and status identification method for traction power supply network, distributed and centralized protection schemes, etc.; (3) a power supply scheme for single-line MSS and a power supply scheme of MSS shared by two or more lines. The proposed industrial frequency single-phase AC traction power supply system shows an excellent technical performance, good economy, and high reliability, hence provides a new alternative for metro and urban rail transit power supply systems.

Direct Voltage Control With Slip Angle Estimation to Extend the Range of Supported Asymmetric Loads for Stand-Alone DFIG

The control system for stand-alone doubly fed induction generator (DFIG) is commonly based on the “sensorless” direct voltage control. This paper adopts the direct voltage control and employs negative-sequence compensation through rotor-side converter to support asymmetric “unbalanced” loads. This paper demonstrates the limitation of the conventional direct voltage control to obtain the slip angle, and the consequent limitation of the negative-sequence compensation control to support full range of asymmetric loads. To overcome these limitations, this paper proposes a new estimator of the angle of the rotor current in the synchronous reference frame. The proposed estimator requires one DFIG parameter only which is the stator inductance, which can be measured allowing using real machine parameter. This paper proposes a simple method to integrate the proposed estimator into the direct voltage control, which enables it to obtain the slip angle, and it enables the negative-sequence compensation to support full range of asymmetric loads. Experimental results for a 1.1-kW prototype DFIG are provided to validate the proposed method and to demonstrate its advantages over the conventional direct voltage control.

Negative Sequence Current Optimizing Control Based on Railway Static Power Conditioner in V/v Traction Power Supply System

In order to bring railway static power conditioner (RPC) into full play in suppressing negative sequence current in the V/v traction power supply system, the reason of negative sequence current and compensation mechanism were analyzed, and the mathematical model with minimum negative sequence current under the constraint of voltage fluctuation, power factor, device capacity, transformer winding capacity, and energy conservation was set up. For solving the small-scale multidimensional nonlinear and constrained optimization problem, an intelligent algorithm based on sequential quadratic programming (SQP) method is proposed through a comparative analysis of existing optimization algorithms and the traditional analytical method. The proposed algorithm is capable to complete an optimizing computation process in several milliseconds with the precision of 0.1 A, and its computational efficiency and precision can meet the needs for real-time control of RPC. A self-adaption real-time optimization computing platform was built in combination with detectable analogue quantities of traction power supply system and RPC, including catenary network voltages, feeder currents, and compensation currents. Simulation and engineering experimentation results are provided to illustrate that the model and its computation are effective and feasible.

Control strategy for microgrid under three-phase unbalance condition

Microgrid (MG) is generally developed at utility terminal which contains lots of unbalanced loads and distributed generations (DGs). The interaction between MG and the unbalance loads or DGs will degrades the control performance of interfaced inverter in MG and dramatically leads to MG voltage unbalance. In this paper, a negative-sequence compensation based three-phase voltage unified correction strategy is proposed. While MG operates in islanded mode, a positive virtual impedance control is used to eliminate the negative voltage resulted from the negative-sequence current, and then a positive-sequence voltage control loop and negative-sequence control loop are used to improve the inverter control performance. While MG operates in grid-tied mode, the inverter operates as a negative-sequence current source to compensate the negative-sequence currents of loads to guarantee the point of common coupling (PCC) voltage balance. By using the proposed strategy, the voltage control performance of inverter can be improved; the MG power quality can be enhanced significantly. Simulation and experimental results verify the effectiveness of the proposed method.

Voltage ripple analysis of simplified active power compensator for negative sequence and reactive power compensation

As for the compensation of unbalanced load, a simplified active power quality compensator (SAPC) was discussed, which consists of a three-phase two-leg inverter and a voltage-balance circuit. According to the principle of power balance, the analytical expression of 2nd ripple in dc-link voltage is derived, and it can be used by the control system of SAPC to avoid 3rd harmonic output. According to the structure characteristic of SAPC, the odd ripples in dc-link voltage is analysed mathematically, and the analytical expression of odd ripples is derived in this study, which can give a clear picture about the link between the output current of SAPC and the ripples essentially. Then an integrated control method is proposed for SAPC, which can associate the merits of feedback control and feedforward control, and it can effectively improve the response performance. Finally, simulation and experimental results have verified the analysis and control method correctly.

A Negative Sequence Compensation Method Based on a Two-Phase Three-Wire Converter for a High-Speed Railway Traction Power Supply System

This paper presents a negative sequence compensation system based on a novel two-phase three-wire converter to eliminate a negative sequence current for the high-speed railway traction power system with a three-phase V/V traction transformer. In this compensation system, the proposed two-phase three-wire converter is fed by two single-phase power sources formed with two step-down transformers connecting to the two feeder sections. The proposed converter contains three switch legs, one of which is connected to the common ground wire of two single-phase voltages. Hence, a switch leg is saved compared with two conventional single-phase converters, and the ratings of the power switches are not increased. In order to enhance the dynamic and steady-state performances of the compensation system, a compound control method composed of hysteresis control and dividing frequency control is presented. Simulation and experimental results are presented to demonstrate that the proposed compensator and its control strategy are very effective.