Zigzag Transformer Research Articles

Research on Hybrid Microgrid Based on Simultaneous AC and DC Distribution Network and Its Power Router

Under the dual pressure of environmental pollution and energy crisis, the global energy consumption structure reform deepens unceasingly and the concept of energy internet has developed rapidly. The widespread volatility, randomness, and uncertainty of distributed new energy generation impose new requirements on distribution systems. The zigzag transformer is used as the coupling and isolating device for simultaneous AC–DC transmission. The basic principle and structure of simultaneous AC–DC power distribution network is analyzed. The topology structure of the simultaneous AC–DC hybrid microgrid and basic operating principle of the microgrid under different operating modes are proposed for the distributed power grid technology. Combined with power electronic technology, a modular multi-interface structure of power routers applied to AC–DC hybrid microgrid and its control strategy are proposed to realize the power routing control of microgrid and ensure reliable operation control of the microgrid. By building the model of simultaneous AC–DC hybrid microgrid and its power router, the rationality and effectiveness of the power router for microgrid routing control are verified.

A Modular Multilevel Converter with a Zigzag Transformer for Bipolar MVDC Distribution Systems

This letter presents a modular multilevel converter (MMC) for bipolar medium-voltage direct-current (MVDC)) distribution systems. By employing a zigzag transformer as a grid interface transformer with a dedicated operation method, the operation of two dc poles of an MMC can be fully decoupled. Consequently, the MMC can provide full bipolar-operation capability. No additional component is required, and no penalty of increased power rating is imposed on the grid interface transformer. The scheme based on the proposed topology is competitive and cost-effective for emerging MVDC distribution systems. Feasibility of the proposed scheme is verified by both simulations and down-scale experiments.

Power Quality Improvement in Utility Interactive Based AC–DC Converter Using Harmonic Current Injection Technique

This paper highlights the power quality issues and explains the remedial measures taken by means of hybrid front-end third harmonic current injection rectifiers. Here, a zig-zag transformer is used as the current injection device so that the advantages pertaining to the zig-zag transformer are effectively utilized. The third harmonic current injection device along with three-level boost converter at the output stage will increase the dc-link voltage. With less boost inductance, generally half of the conventional boost converter inductance is sufficient to implement the proposed converter structure resulting in reduced ripple current and also the device rating is reduced by half of the output voltage. Moreover, the power quality is well improved using third harmonic current modulated front-end structure, which is well appropriate for medium/higher power applications. The experimental prototype of hybrid front-end converter is developed in the laboratory to validate the MATLAB simulation results.

Chebyshev Functional Expansion Based Artificial Neural Network Controller for Shunt Compensation

Three-phase four-wire (TPFW) distribution systems are prone to various power quality (PQ) issues, such as voltage fluctuations, poor power factor, unbalanced load conditions, and the presence of harmonics in current. Mitigation of these PQ problems using appropriate shunt compensator requires advanced control algorithms for control of three-phase voltage source converters (VSC) in a distribution system. In this paper, Chebyshev functional expansion based artificial neural network ( ChANN ) algorithm for shunt compensation using distribution static compensator (DSTATCOM) is proposed. The parameters of ChANN are trained in real time. Implementation results with linear and nonlinear loads are demonstrated on a prototype hardware designed and developed using dSPACE 1104, current and voltage sensors for the realization of DSTATCOM for TPFW system. A zigzag transformer is used along with conventional three-phase, three-wire (TPTW) DSTATCOM to reduce its overall rating. Suitable comparisons with conventional control techniques are also mentioned.

Multifunctional Three-Phase Four-Leg PV-SVSI With Dynamic Capacity Distribution Method

The unequal single-phase load distribution in three-phase (3P) four-wire (4W) low-voltage (LV) networks can cause significant neutral current and neutral to ground voltage rise problems at both customer and distribution transformer terminals. High neutral current can overload the neutral conductors and can cause electrical safety concerns to the users. To mitigate the high neutral current problem in an unbalanced residential LV network, a multifunctional 3P four-leg (4L) rooftop photovoltaic (PV) smart voltage source inverter (SVSI) is designed with improved active neutral current compensation along with active power export and point of common coupling (PCC) voltage regulation. A novel dynamic capacity distribution (DCD) method is proposed using the available SVSI capacity after active and reactive power operations to achieve higher capacity neutral compensation at the PCC. The performance of the designed 3P-4L PV-SVSI with the DCD method is compared with a traditional 4L SVSI with fixed unbalanced compensation capacity and a passive unbalance compensator, such as a zig-zag transformer, in PSCAD/EMTDC software. Several case studies, such as balanced and unbalanced load changing effects, are presented with actual residential loads connected to an Australian 3P-4W LV network. A Semikron Semiteach modified inverter and a real-time TMSF28335 DSP microcontroller are also used to provide experimental verification on the improvement of the proposed neutral current compensation with the DCD method. Detailed simulations and experimental studies are presented to verify the robustness and efficacy of the proposed control strategy with the designed 3P-4L PV-SVSI.

A 20-Pulse Asymmetric Multiphase Staggering Autoconfigured Transformer For Power Quality Improvement

This letter presents a 20-pulse multiphase staggering autoconfigured transformer (MSAT) for power quality improvement in medium and high power applications. The proposed autoconfigured transformer is a combination of delta and zig-zag transformers. Contrary to the prior art multipulse autoconnected transformers, magnetic rating of the proposed configuration is only 40% of the load rating in addition to improved power quality indices. Moreover, the creation of a neutral point ascribed to inbuilt zig-zag windings supplements as one of the remarkable advantages of the proposed MSAT. Therefore, with this suggested configuration, the performance of zig-zag transformer is accomplished within the autoconfigured transformer. A detailed comparison of the proposed MSAT configuration with that of other multipulse autoconnected transformers in terms of magnetic rating and power quality indices are presented. Further, the feasibility and operability of the proposed MSAT are verified and validated through experimental tests on the laboratory prototype. The test results so obtained complies with the IEEE-519 and IEC 61000-3-2 standards on power quality requirements.

Simple and efficient control of DSTATCOM in three‐phase four‐wire polluted grid system using MCCF‐SOGI based controller

Load current may become highly distorted in a weak power system network due to the presence of non-linear and dynamic loads. Harmonics get penetrated into the supply system too; thereby making it polluted and rich in harmonic content and causing a detrimental effect on the performance of other connected loads. In such cases, the conventional control algorithms fail to work effectively to mitigate power quality (PQ) issues. This study addresses solutions to PQ issues which get aggravated in the presence of distorted supply. This study proposes a multiple complex coefficient filter (MCCF)-second-order generalised integrator (SOGI)-based controller designed to effectively compensate for PQ problems associated with weak grid. A MCCF filter is designed to extract the fundamental components of grid voltage used further for grid synchronisation. The SOGI is designed to extract the fundamental active current magnitude of the load current and reduce PQ issues with the help of Distributed Static Compensator (DSTATCOM). Neutral current compensation is achieved with an appropriately designed zigzag transformer. Extensive simulation and hardware results realising dSPACE 1104 as Digital Signal Processor are considered in this study to show that the performance of the proposed control algorithm under PQ issues with distorted grid.

Harmonic injection scheme for harmonic reduction of three‐phase controlled converters

High harmonics in supply currents of three-phase controlled/uncontrolled converters are creating many problems to the power system and customers at point of common coupling. Third harmonic current injection technique is an excellent option for harmonic reduction of these converters. Minimum total harmonic distortion (THD) for any firing angle of controlled converter is function in phase-angle and amplitude of harmonic injection current that can be controlled by single-phase controlled converter and boost converter, respectively. A novel scheme with three bidirectional switches and single-phase controlled converter to circulate the injection current to supply currents has been introduced. This novel scheme is compared to the state-of-the-art system using zigzag transformer. A novel mathematical analysis showing the optimum values for components on the harmonic injection path at minimum THD and the corresponding efficiency for the proposed scheme and state-of-the-art scheme is introduced. Two lab prototypes for these two schemes have been implemented, discussed, and compared to show the benefits of using the new proposed scheme. The results show the superiority of the new proposed scheme.

Efficient current injection device for harmonic reduction of three‐phase controlled converters

Third harmonic current injection technique is one of the best options for harmonic reduction of three‐phase controlled/uncontrolled converters. Injecting third harmonics current from dc‐bus to the line current reduces its harmonic contents. Minimum THD for any firing angle of controlled converter is function in phase‐angle and amplitude of harmonic injection current that can be controlled by single‐phase controlled and boost converters, respectively. This scheme is used with three bidirectional switches as harmonic injection device to circulate the injection current to supply currents. This scheme is compared with the state‐of‐the‐art system using zigzag transformer. A novel mathematical analysis for the proposed scheme and state‐of‐the‐art scheme is introduced. The mathematical analysis introduces the optimum values for components on the harmonic injection path at minimum THD and the corresponding efficiency of each scheme. The two schemes under study have been simulated using PSIM program. Two lab prototypes for these two schemes have been implemented. Mathematical, simulation, and experimental results for these two schemes have been introduced, compared, and discussed. The results show the superiority of the proposed scheme.

Reduced Active Switch Front-End Multipulse Rectifier With Medium-Frequency Transformer Isolation

This paper presents a reduced switch count multipulse rectifier with medium-frequency (MF) transformer isolation. The proposed topology consists of a three-phase push–pull based ac to dc rectifier with a MF ac link employing two active switches. A three-phase, five-limb, multiwinding MF transformer is employed for isolation. The secondary side of the transformer is connected in a zig-zag configuration and is fed to two six-pulse diode rectifiers, achieving 12-pulse rectifier operation. The primary advantage of the proposed system is reduction in size/weight/volume compared to the conventional 60 Hz magnetic transformer isolation rectifier system. Operating the transformer at 600 Hz is shown to result in three times reduction in size. Furthermore, the proposed system employs only two active semiconductor switching devices operating under a simple pulse width modulation scheme. Also, the zig-zag transformer connection helps to balance leakage inductance on the secondary side. Detailed analysis, simulation, and experimental results on a 208Vl-l, 3.15 kW laboratory prototype are presented to validate the performance of the proposed approach.

Dual-timescale control for power electronic zigzag transformer

Power electronic zigzag transformer is an attractive solution for the flexible interconnection of smart distribution networks. It is constituted by slow-response and low-precision thyristor converters and fast-response and high-accuracy voltage source converters. This paper models its primary circuit and addresses its basic operation mechanism. Then a dual-timescale control scheme is investigated to realize the coordinated regulation of both types of converter. A simulation case is established in PSCAD containing interconnected mid-voltage distribution networks. Simulations with poor- and well-matched control timescales are both carried out. And accordingly, the power flow controllability under these conditions is compared. When the shorter control timescale is no more than tenth of the longer one, the power electronic zigzag transformer will operate with satisfying performances.

A Line Impedance Conditioner for Saturation Mitigation of Zigzag Transformer in Hybrid AC/DC Transmission System Considering Line Unbalances

Hybrid ac/dc transmission extends the power transfer capacity of existing long ac lines closer to their thermal limit, by superposing the dc current onto three-phase ac lines through a zigzag transformer. However, this transformer could suffer saturation under unbalanced line impedance conditions. This paper introduces the concept of hybrid line impedance conditioner (HLIC) as a cost-effective approach to compensate for the line unbalance and therefore avoid saturation. The topology and operation principle are presented. The two-level control strategy is described, which enables autonomous adaptive regulation without the need of system-level control. Design and implementation are also analyzed, including dc-link capacitance as one of the key line conditioner components, HLIC installation, and protection under fault conditions. The cost study on this HLIC-based hybrid system is also performed to reveal the benefits of the solution. Simulation results and experimental results based on a down-scaled prototype are provided to verify the feasibility of the proposed approach.

A novel current injection device for harmonic reduction of three-phase controlled converters in renewable energy utility interfacing

Three-phase controlled/uncontrolled converters have been used in renewable energy applications such as photovoltaic and wind energy as rectifiers and inverters. The main drawback of these converters is the high THD (Total Harmonic Distortion) in their line currents. Many techniques have been used to remedy this problem. Third harmonic current injection is one of the best solutions for the harmonic reduction in line currents of these converters. Injecting third harmonic current from the dc-bus to the line currents reduces its harmonic contents. Minimum THD for any firing angle of the controlled converter is a function of the phase-angle and amplitude of harmonic injection current that can be controlled by a single-phase controlled converter and a boost converter, respectively. This scheme is used with a zigzag transformer to circulate the harmonic injection current to the line currents. A novel mathematical analysis for the proposed scheme and state of the art scheme is introduced. The mathematical analysis introduces the optimum values for components on the harmonic injection path at minimum THD and the corresponding efficiency. The scheme under study has been simulated using the PSIM program. A lab prototype for this scheme has been implemented. Mathematical, simulation, and experimental results for this scheme have been introduced, compared, and discussed. The results show the superiority of the proposed scheme.

Implementation of the Predictive Current Control of Multi-Pulse Flexible Topology Thyristor Rectifier Under Unbalanced Source Voltages

Predictive current control strategy has captured much concern recently in power converters. As for the multi-pulse flexible-topology thyristor rectifiers (m-PFTTR), the symmetry of the input line-to-line voltages has great influence upon the implementation of the control strategy. Existing literatures have mainly focused on the implementation under ideal condition, i.e., the input line-to-line voltages, being critical to the predictive current controller, were supposed to be symmetrical. In fact, the unbalance of the input line-to-line voltages usually exists in the windings of the zigzag transformer at front end and the A/D converter of the digital signal processor (DSP). This paper presents a new method to dynamically regulate the input line-to-line voltages to ensure the symmetry of each other. It is realized through the instantaneous correction of the bias values in the course of sampling, and the correction is embodied in the modified control algorithm. The paper includes analyses of the m-PFTTR as well as the control algorithm, simulation and experiments. Simulation and experimental results demonstrate the effectiveness of the presented method.

Power quality improvement for grid interconnected solar PV system using neural network control algorithm

This paper deals with neural network control algorithm-based grid connected to solar photo voltaic (PV) system consisting of DC-DC converter, solar PV with maximum power point tracking (MPPT) controller, three-leg voltage source converter (VSC), ripple filter at PCC, interfacing inductor and three phase grid connected to three phase linear/nonlinear loads. The reference solar-grid current for three-leg VSC are estimated using neural network control algorithm. The neural network based on least mean square (LMS) control algorithm is also known as adaptive linear element to estimate reference fundamental grid currents. A three phase non-isolated zigzag transformer is connected to solar grid PCC for neutral current compensation. The proposed solar PV grid connected system maintains UPF at the grid, reactive power compensation for ZVR operation along with load balancing, neutral current compensation and harmonic compensation. In the proposed solar PV system, MPPT is obtained using DC-DC boost converter and DC bus voltage is controlled by using DC bus proportional integral (PI). The neural network control algorithm based solar PV system is modelled in MATLAB R2013a along with SIMULINK.

Transformer-Less Unified Power-Flow Controller Using the Cascade Multilevel Inverter

The conventional unified power-flow controller (UPFC) that consists of two back-to-back inverters requires bulky and often complicated zigzag transformers for isolation and reaching high voltage. This paper proposes a completely transformer-less UPFC based on an innovative configuration of two cascade multilevel inverters (CMIs). The unique configuration and control of the two CMIs as a power-flow controller make it possible to independently control active and reactive power flows over a line. The new UPFC offers several advantages over the traditional technology, such as transformer-less, light weight, high efficiency, high reliability, low cost, and fast dynamic response. The transformer-less UPFC is thereby very suited for fast and distributed power flow control, such as wind and solar power transmission. Experimental results based on 13.8-kV/2-MV·A transformer-less UPFC prototype are shown to validate the theoretical analysis and operating principle.

Enhancement of Performance, Availability, and Flexibility of a Battery Energy Storage System Based on a Modular Multilevel Cascaded Converter (MMCC-SSBC)

This paper provides a theoretical and experimental discussion about the performance, availability, and flexibility of a battery energy storage system (BESS) using a modular multilevel cascaded converter based on single-star bridge cells (MMCC-SSBC). The SSBC-based BESS produces three-phase multilevel voltage waveforms, and eliminates both harmonic filters and a complicated zig-zag transformer from the ac side. The circuit modularity of the SSBC enables the usage of multiple individual low-voltage battery modules. Along with control strategy based on zero-sequence-voltage injection, this modularity enhances the availability and flexibility of the BESS. A three-phase laboratory downscaled system rated at 140 V, 10 kW, and 21 kW·h is designed, constructed, and tested to verify the operating principles and performance. The tested BESS produces a current total harmonic distortion (THD) of 3.0% even when it operates with different active-power commands for individual bridge cells. In addition, zero-voltage ride-through capability is verified for the severest single-phase, two-phase, and three-phase voltage sags. Mathematical analysis and experimental verification validate the downscaled system and performance, making the BESS prospective.

Modulation and Control of Transformerless UPFC

In this paper, a modulation and control method for the new transformerless unified power flow controller (UPFC) is presented. As is well known, the conventional UPFC that consists of two back-to-back inverters requires bulky and often complicated zigzag transformers for isolation and reaching high power rating with desired voltage waveforms. To overcome this problem, a completely transformerless UPFC based on an innovative configuration of two cascade multilevel inverters has been proposed. The new UPFC offers several advantages over the traditional technology, such as transformerless, light weight, high efficiency, low cost and fast dynamic response. This paper focuses on the modulation and control for this new transformerless UPFC, including optimized fundamental frequency modulation for low total harmonic distortion and high efficiency, independent active and reactive power control over the transmission line, dc-link voltage balance control, etc. The new UPFC with proposed control method is verified by experiments based on 4160-V test setup. Both the steady-state and dynamic-response results will be shown in this paper.

고조파발생에 따른 영향과 대책 연구

Skyscrapers, large business buildings, and IT consumers use many appliances, and the electrical power stems can cause fires by overheating. This can result in damaged capacitors, lost data, rising ground potential, and communication obstacles from linear or nonlinear high frequency. To make sure of that we investigated 7 spots of a building, among which 6-spots were fair but the other one needed high frequency control. Spots 3, 6, and 7 needed diagnostic workup, and spots 2, 3, and 5 considered 5 high frequency currents. A phase is all of good but the high frequency current is greater than the standard level except for spot 1. As a result, a zigzag transformer or active filter needs to be installed, and the efficiency needs to be upgraded by investigating load unbalance factors and power factors.

Fuel-efficient Induction Generator Based Diesel Generator Set for Stand-alone Supply Systems

This article proposes a fuel-efficient, diesel engine driven, self-excited induction generator based diesel generator set for stand-alone supply systems. The system uses a static compensator along with a battery energy storage system. The static compensator with a battery energy storage system provides the voltage control, load compensation, harmonics elimination, load balancing, and reactive power compensation. The fuel efficiency of the generator is improved by loading the generator continuously between 80% and 100% of its rated capacity. The battery energy storage system takes the power from the generator during light load periods and supplies the power to the load during overloads. In addition, a continuous loading of the generator eliminates the problem of frequency variation of the induction generator under varying loads. The proposed system is designed to feed both three-phase as well as single-phase loads. A zigzag transformer is used for neutral current compensation. A power balance theory based control algorithm has been used to estimate the reference source currents.