Neutral Voltage Research Articles

Enhanced Boost Factor for Three-Level Quasi-Switched Boost T-Type Inverter

A new modulation strategy has been introduced in this paper in order to enhance the boost factor for the three-level quasi-switched boost T-type inverter (3L-qSBT2I). Under this approach, the component rating of power devices is significantly decreased. Moreover, the use of a larger boost factor produces a smaller shoot-through current. This benefit leads to reducing the conduction loss significantly. Furthermore, the neutral voltage unbalance is also considered. The duty cycle of two active switches of a quasi-switched boost (qSB) network is redetermined based on actual capacitor voltages to recovery balance condition. Noted that the boost factor will not be affected by the proposed capacitor voltage balance strategy. The proposed method is taken into account to be compared with other previous studies. The operation principle and overall control strategy for this configuration are also detailed. The simulation and experiment are implemented with the help of PSIM software and laboratory prototype to demonstrate the accuracy of this strategy.

Voltage compensation in multi-grounded distribution network with a three-phase five-wire DSTATCOM.☆

This paper presents a three-phase static synchronous compensator with five legs to be connected to a multi-grounded neutral distribution network. Several strategies are proposed in the literature to compensate for the imbalances in multi-grounded networks. Typically, three-phase static synchronous compensators with four legs can compensate for positive-, negative- and zero-sequence network voltages. However, they cannot simultaneously compensate also for the neutral voltage of multi-grounded network. Thus, the fifth-leg of the proposed compensator is controlled as a single-phase converter in order to compensate the neutral-voltage at the point of common coupling. Linearised mathematical models, in the synchronous reference frame, are developed and used to design the controllers used to compensate the network’s neutral voltage and to regulate the current synthesized by the fifth-leg of the compensator. A multi-grounded network, derived from the IEEE NEV test feeder, is modelled in the PSCAD/EMTDC to investigate the performance of the proposed compensator. The results of the digital simulations demonstrate the effectiveness of the neutral voltage compensation strategy and validate the control loops designed for the three-phase five-wire static synchronous compensator.

Optimized Filter Design for Common-Mode Current Reduction in Four-Wire Inverter-Fed Motors

Two optimized filter topologies for four-wire electric drive systems (EDSs) are proposed with smaller size, lower power loss, and larger common-mode (CM) current reduction than the usual filter topologies. It has been previously shown that using a four-wire topology reduces the CM current in a wide frequency range, when compared with the three-wire one. However, there still exist low- and high-frequency resonances in the transfer function between neutral and inverter output voltage, resulting in the increase of CM current. By means of the transfer function analysis, two types of optimized filter topology specified for the four-wire EDS are proposed. Unlike conventional CM filter (CMMF), the design of these two new filters also requires a serious consideration of the characteristics of motor and cable impedance. Based on the design procedure, these two filters are built with excellent performance of CM current reduction in the whole frequency range. Besides, compared with conventional CMMF used for three-wire EDS, they also show benefits, such as lower power loss and less implementation space. All theoretical analyses are validated by the experimental measurements and can benefit further studies.

Evaluation of the Detectability of Open Phase Faults in Multiphase Induction Machine Based on Instantaneous Power, Instantaneous Space Vector and Neutral Voltage

This article deals with the effect of the open phases in a Multi-Phase Induction Machine (MPIM). Such systems are known for their reliability even in failure conditions of this type. This is the reason for their presence in a large spectrum of industrial applications. Generally, failures are identified based on signal analysis of electrical measurements. The electrical measurements are given by current, voltage, and instantaneous power. The results obtained through those measurements can be verified through the experimental test bench. The most common fault for multiphase systems is the open phase. In this work, a Six-Phase Induction Machine (SPIM) is considered. A behavior analysis of a SPIM in faulty operating mode is presented. Results of several open phase configurations are illustrated through a comparison study.

ИССЛЕДОВАНИЕ ПРОБЛЕМ НАСТРОЙКИ ДУГОГАСЯЩИХ РЕАКТОРОВ

The paper considers methods of configuration of arc-suppression coils in cable, air and mixed distribution networks of 6-35 kV. A special attention the authors have dedicated to influence of external factors (natural unbalance of network and interference) on configuration quality, as well as the maintenance of value of neutral point displacement voltage in acceptable limits. As a result, the authors have analysed the existing methods and suggested mew ones for elimination of influence of the specified factors.

Improved DTC Technique for THL-NPC VSI fed Five Phase Induction Motor Drive Based on VVs Assessment Over a Wide Speed Range.

A three-level neutral point clamped voltage source inverter fed five-phase induction motor (FPIM) drive is assessed by analyzing common-mode voltage (CMV). The presence of CMV leads to motor bearing fault and phase winding insulation failure due to additional voltage stress. To nullify the effect of CMV, this article proposes a modified direct torque control (DTC) based on the virtual vector (VV) concept. The proposed DTC utilizes the appropriate nonvertex voltage vectors to form VVs that neutralize the average volt-second in the $xy$ plane, maintain dc-link capacitor voltage balancing, and limits the switching voltage stress. The theoretical analysis is carried out to investigate the VVs effect on change in flux and torque response. Based on this assessment, a modified lookup table is designed for a wide range of speed operations. For the low-speed operation, i.e., below 25% of rated speed, the small VVs provide better drive performance instead of zero VV. The proposed DTC method is verified at steady-state and dynamic conditions through experimental investigations. Comparative results are provided for the assessment of the proposed DTC method in comparison to existed methods. It is concluded from the findings that the proposed DTC method is a promising approach to improving FPIM performance at high- and low-speed operations with zero CMV output.

Визначення оптимального значення опору резистора заземлення нейтралі в мережах 20 кВ

At present, there is an objective need for a solution in the power system of Ukraine problems of improving the principles of construction of network topology by voltage classes and types of performance on based on scientifically sound technical solutions using modern methods and technologies. The article is devoted to the study of the optimal value of the resistance of the neutral grounding resistor in 20 kV networks. Distribution electric networks of Ukraine with a voltage of 6 (10) kV are included in the final link systems of providing consumers with electric energy. They are in direct interaction both with the consumer, and regional and main electric networks. Therefore, the state and the operation of 6 (10) kV electrical distribution networks has a significant impact on performance reliability, quality and efficiency of the integrated power system of Ukraine. Transition from voltage level 6 (10) kV on 20 kV is an actual and priority step to increase the efficiency of operation distribution network and a significant reduction in electricity losses. The paper presents the types of resistors and options for their installation in the neutral voltage network 20 kV. The relationship between the resistance of the neutral grounding resistor and the single-phase current is established short circuit, which directly affects the types of relay protections, as well as their number. Simulation of the 20 kV network operation mode was performed, based on the results of which the optimal one was chosen the value of the resistance of the neutral grounding resistor in the presented network, which is achieved sufficient sensitivity of relay protection devices at the lowest short-circuit current. This one was chosen for the study neutral grounding method, because resistive grounding has the following advantages: no arcs high-frequency overvoltages and multiple damages in the network; no need to disconnect the first single-phase earth fault; reducing the likelihood of impressions of staff and outsiders at single-phase short circuit; almost complete exclusion of the possibility of transition of single-phase circuit to ground in multiphase; simple implementation of sensitive and selective relay protection single-phase ground fault.

Design of STATCOM for reactive power control using multilevel inverter

STATCOM a power electronic based device is widely used for reactive power compensation. In the present work, a STATCOM is realized by employing a 3-level neutral point clamped inverter which is controlled by space vector pulse width modulation technique. Switching sequence for STATCOM is designed by minimizing the deviation in neutral voltage. Subsequently, the STATCOM is connected to the load side of a power network and by varying source voltage, reactive power control is accomplished resulting in voltage control of load side and improving the power quality of the network. The present work will be useful to the utilities dealing with reactive power and voltage control of AC transmission networks.

Automatic network capacitive balancing technique for resonant grounded power distribution systems

This study presents an automatic network balancing technique to limit the capacitive unbalance in resonant grounded power distribution systems (RGPDSs). The aim of this capacitive balancing technique is to minimise the unbalance current through the line to ground (which is the current through the neutral of the system) in order to automatically limit the neutral voltage. The proposed technique is designed by combining the weighted-sum technique and genetic algorithm (GA), where distributed switched capacitor banks (SCBs) are used for balancing RGPDSs. The proposed technique is employed to optimise available SCBs for limiting the network unbalance at the substation under a pre-defined threshold considering all possible network configurations. The unbalances at different locations of the network are also minimised to limit the system unbalance within the threshold due to minor changes in network parameters. Since the lifetime of capacitor banks relies on the switching, the proposed technique is designed in such a way that the system balance is achieved with the minimum switching. The performance of the proposed technique is evaluated through simulation studies in MATLAB/SimpowerSystems environment. Simulation results show that the proposed technique works well and capable to maintain the capacitive balance of the system with changes in network configurations.

Assessing Multiple Operationally-Stable Power Flow Solutions in Unbalanced Impedance-Grounded Systems: Initialization Procedure and Analysis

The power flow is usually formulated by nonlinear equations and may present multiple solutions. However, most of these solutions do not represent a practical situation but are mathematical findings. Remarkably, in unbalanced multiphase systems with impedance-grounded loads, a phenomenon can occur where two or more solutions may especially show practical significance. These solutions are called operationally-stable solutions (solutions which for a given loading level the nodal voltages, currents, and losses are feasible) and may be obtained in Distribution Systems (DS). In this paper, some relevant aspects regarding the multiple solutions in unbalanced grounded systems are explored: (i) the development of a simple analytical model considering the feeder and loads to analyze the multiple solutions generically; (ii) a direct procedure for initializing neutral voltages to find possible operationally-stable solutions or reduce the number of iterations to achieve convergence; (iii) a study on multiple solutions using the continuation power flow to analyze their behavior and verify their impacts on DS. The proposed assumptions have been confirmed with numerical experiments over IEEE 123, Simple NEV, and IEEE 8500 test feeders.

Simultaneous Switching Loss Reduction and Neutral-Point Voltage Balance Scheme for Single-Phase Three-Level T-Type Inverter

Single-phase three-level T-type inverter (3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I) has revealed superior performances in many applications. However, it inevitably faces the challenge of high switching loss and imbalanced neutral-point voltage with conventional carrier based pulsewidth modulation (CBPWM). To address these issues, an improved CBPWM strategy with maximum offset voltage injection is proposed to reduce the switching loss and balance the neutral voltage simultaneously. Neutral-point voltage balance is maintained by selecting P-type or N-type switching sequence, which depends on the sign of offset voltage. Also, switching loss is mitigated by reducing the number of total switching commutation. First, the restriction of injected offset voltage is investigated and when the offset voltage is maximum, the switching commutation is reduced. Then, to further reduce the number of switching commutation, a new set of carrier signals are proposed to realize zero switching commutation between P-type and N-type switching sequences. After the analysis, the proposed modulation method also has better common-mode voltage restriction and faster response of neutral-point voltage balance. Simulation and experiment have been conducted to verify the theoretical analysis.

PWM Investigation of a Field-Oriented Controlled Five-Phase PMSM Under Two-Phase Open Faults

The decoupled model of Five-Phase Permanent Magnet Synchronous Motors (5Ph PMSMs) under a two-phase open circuit has been developed recently, yet the corresponding PWM has not been fully exploited. The Pulse Width Modulation (PWM), which is straightforward for a balanced system, can be problematic to fit a 5Ph PMSM under faulty conditions. During the open faults, the neutral voltage is drifting over the mid-point of DC bus, thus, the PWM typically designed for a balanced system always fails. In this article, an affine transformation is first presented to offset the drifting neutral effect, and only then PWMs can be effective. A quasi-sinusoidal PWM, a Space Vector PWM, and a min-max Carrier-based PWM are presented and compared experimentally. Each method has its own merits and demerits, and their effectiveness or rather the proposed affine transformation is validated by the experimental results.

A Method for Calculating the Asymmetry in the Shunt Parameters of Power Lines in Compensated Distribution Networks

In compensated distributed networks (CDN), minimizing the asymmetry in the shunt parameters of the power lines is a key factor for avoiding excessive neutral voltages which is necessary not only for safety but also for improving the sensitivity of detecting ground faults, particularly when bush-fire mitigation is a major concern. This paper offers a comprehensive analysis and modeling of the asymmetry of the power lines in CDN. Unlike the existing analyses which assume the asymmetry can be lumped in one phase, the proposed analysis is based on more practical assumption by considering the asymmetry in all of the phases of the power lines. The proposed analysis is used to develop an equivalent circuit for asymmetrical CDN which then used to develop a new method for calculating the per-phase asymmetries in the shunt parameters. The proposed method has been validated based on simulations of real networks.

An Improved Space Vector Modulation With Capacitor Voltage Control for T-Type Five-Level Nested Neutral Point Piloted Converter

In this article, a novel control method is proposed to control capacitor voltages for the T-type five-level nested neutral point piloted (5L-NNPP) converter. Due to the diversity of switching states for NNPP topology, there is no guarantee that the redundant vectors have the opposite effect on the dc bus neutral voltage. Meanwhile, the voltage balance controls between the dc bus neutral voltage and three-phase flying capacitor voltage are coupled and interacted with each other in the 5L-NNPP converter. This has severe voltage balance problems with the conventional space vector modulation (SVM), which limits its wide applications, especially under low output frequency. To solve the voltage balance problems, an improved SVM and voltage balance control (VBC) function are proposed to balance dc bus capacitor voltages and flying capacitor voltages, which realizes decoupling of voltage control. The switching states, switching principle, and dwell time calculation are developed, and the flowchart of VBC function is also elaborated on. The performance of the proposed improved SVM is investigated and compared with conventional SVM. The effectiveness and validity of the improved SVM with a VBC function is verified by simulation and experimental results of the 5L-NNPP converter.

Distributed Power Quality Conditioning System for Three-Phase Four-Wire Low Voltage Networks

This paper presents distributed power quality conditioning system to compensate current and voltage distortions in three-phase four-wire networks caused by unbalanced non-linear single-phase loads. The proposed conditioning system consists of several hybrid filter units installed in various nodes for compensation of excessive neutral currents and voltage distortions in a selected area of the distribution network. The system is open and it can be easily modified by installation of new filter units. A novel hybrid filter design procedure based on filter frequency characteristics optimization in the parameters of passive and active parts is presented. A digital system of control signal computation for active filters based on the use of modern methods of spectral analysis is considered. The proposed digital control system enables selective compensation of fundamental and harmonic components. The mathematical model of the proposed power quality conditioning system is developed in the MatLab software. The simulated results show that the presented conditioning system reduces the level of neutral conductor currents and the voltage unbalance as well ensures harmonic compensation in three-phase four-wire networks.

Multiobjective Optimization Technique for Mitigating Unbalance and Improving Voltage Considering Higher Penetration of Electric Vehicles and Distributed Generation

The increasing penetration of distributed generations (DGs) and electric vehicles (EVs) offers not only several opportunities but also introduces many challenges for the distribution system operators (DSOs) regarding power quality. This article investigates the network performances due to uncoordinated DG and EV distribution. It also considers power quality-related performances such as the neutral current, energy loss, voltage imbalance, and bus voltage as a multiobjective optimization problem. The differential evolution optimization algorithm is employed to solve the multiobjective optimization problem to coordinate EV and DG in a distribution grid. This article proposed a method to coordinate EV and DG distribution. The proposed method allows DSOs to jointly optimize the phase sequence and optimal dispatch of DGs to improve the network's performance. If the network requires further improvement, the EV charging or discharging rate is coordinated for a particular location. The efficacy of the proposed method is tested in an Australian low-voltage distribution grid considering the amount of imbalance due to higher penetration of DG and EV. It is observed that the proposed method reduces voltage unbalance factor by up to 98.24%, neutral current up to 94%, and energy loss by 59.45%, and improve bus voltage by 10.42%.

Three‐dimensional Modeling and Performance Optimization of Proton Conducting Solid Oxide Electrolysis Cell▴

AbstractIn this study, a three‐dimensional model is developed to study a novel proton conducting solid oxide electrolysis cell (H‐SOEC) with porous current collector to alleviate the H2O‐starvation problem in oxygen electrode. The comparison between conventional straight‐channel structure and the new porous structure is conducted in detail. It is found that the porous current collector can increase the flow uniformity and facilitate the diffusion of H2O, which in turn improves the average performance of the H‐SOEC. The electrolysis efficiency can be increased by 6.4% compared to straight‐channel structure. It is also found that the thermal neutral voltage is decreased when the porous current collector is adopted. Besides, the evaluation of syngas production for CO2‐supplied H‐SOEC is performed. Compared to H2 production, the efficiency of syngas production can be increased by up to 13.5%.

Fast Commutation Error Compensation for BLDC Motors Based on Virtual Neutral Voltage

Precise commutation signal is the prerequisite for the efficient operation of brushless direct current (BLDC) motors. However, the low-pass filter, nonideal current, circuit, and software retarding can bring about the commutation errors that will deteriorate the performance of BLDC motors, for example, resulting in larger current ripple and noise. To eliminate the commutation errors quickly and improve the performance of the motor, a novel compensation method is proposed in this letter. The compensation time is calculated by a counter. And the start-stop signals of the counter are obtained by the virtual neutral voltage. The proposed method requires none voltage or current sensor and has a faster compensation speed and higher precision than the traditional compensation method based on the integration of virtual neutral voltage. The effectiveness of the proposed method is verified by comparable exp eriments using an 80-W BLDC motor.

REAL TIME ASSESSMENT OF 500-KVA, 11/0.415 KV DISTRIBUTION TRANSFORMER SITUATED AT BELLS UNIVERSITY OF TECHNOLOGY, OTA, OGUN STATE USING FLUKE 435 SERIES II

Distribution transformer being major equipment in utility companies deserves to be routinely scrutinized for its performance with a view to ensure continuous supply of energy to end-users as well as to sustain improved revenue collection by the utility company. Presented in this paper is the real time assessment of 500-kVA, 11/0.415 kV distribution transformer situated at Bells University of Technology, Ota, Ogun State using power quality and energy analyzer equipment. The parameters measured on real time include phase to neutral voltage, phase to phase voltage, root mean square and peak current, frequency, active power, reactive and apparent power and line power factor. The assessment revealed wide variation of system voltages far from the standard, however, the line power factor and as well as frequency of operation was observed to be within the standard. Based on this analysis, several findings and appropriate recommendations were suggested to improve the supply of energy in the study area. It is hope that the findings of this research will be of immense benefit to distribution engineers at the Department of works in Bells University of Technology, Ota for field compliance.

Solid Oxide Electrolyser Cell Testing Up to the Above 30,000 h Time Range

A running steam electrolysis test with a 10Sc1CeSZ electrolyte supported cell with Ni/GDC hydrogen and LSCF oxygen electrodes has passed 34,000 h with -0.6 Acm-2 current density. The high specific ionic electrolyte conductivity allowed a moderate initial cell temperature (~780°C) for operation with a cell voltage close to the thermal neutral voltage (U th ~1.29 V), i.e. with highest energy-conversion efficiency. Part of the wide margin for temperature increase was used for compensation of the voltage degradation, done after 20 and 31 kh, respectively. The resulting compensation rate of 0.9°C/kh extrapolates to a degradation compensated lifetime beyond current system requirements (>50,000 h). Compensation was traced with voltage-current curves and with impedance. Voltage degradation for constant temperature amounted, after several kh initial stabilisation, to (4 - 5) mV/kh (0.3 - 0.4 %/kh). Degradation was mainly ohmic, without clearly detectable non-ohmic electrode contributions. Non-ohmic terms therefore added to the compensation of ohmic degradation via temperature increase.