Neutral Inductor Research Articles

A Generalized Approach for Determining the Current Ripple RMS in Four-Leg Inverters with the Neutral Inductor

This manuscript proposes a novel approach for determining phase and neutral-current-ripple RMS in grid-connected four-leg inverters with the neutral inductor. The harmonic pollution is determined for any arbitrary pulse width modulation (PWM) technique and a generic value of the neutral inductor. Thanks to the proposed approach, it is possible to describe the neutral inductor in a parametric way with respect to phase inductors and obtain a wide range of results, ranging from a direct neutral connection (no neutral inductor) to a conventional three-phase inverter (no fourth wire) for any value of modulation index and common mode injection. The results permit one to compare different design choices in multiple scenarios effectively. The findings were validated by numerical simulations and experimental tests employing the most popular PWM techniques, such as space vector PWM (SVPWM) and discontinuous PWM (DPWM).

Design Methodology for Three-Phase Four-Wire T-Type Inverter With Neutral Inductor

Three-level (3L) converters have been widely used in industry for decades. Compared to the three-phase-three-wire (3P3W) 3L inverter, the three-phase-four-wire (3P4W) one is able to supply the unbalanced loads but has to afford much larger filter inductors because the neutral wire provides a path for high-switching zero-sequence currents. To save filter inductances, a neutral inductor is proposed to insert in the neutral wire. Meanwhile, a complete design methodology is put forward to design the filter inductors and the neutral inductor. With low-frequency zero-sequence currents flowing through the neutral wire, the three-phase load voltages might become unbalanced and/or distorted. To improve the voltage quality, a resonant controller, with the resonant frequency at fundamental output frequency (fo), is presented to add into the zero-sequence voltage loop for balancing load voltages; concurrently, the other resonant controller, with the resonant frequency at 3fo, is presented to insert in the zero-sequence voltage loop or neutral current loop for mitigating voltage distortion. Finally, all of the proposed works are verified on a 3P4W T-type inverter.

Assessment of Model Predictive Voltage Control for Autonomous Four-Leg Inverter

The Finite control set model predictive control (FCS-MPC) is recently introduced to control inverters without the modulation stage. The absence of the modulation stage gives an unpredictable performance of the control system. In this paper, the performance of FCS-MPC is assessed by comparing with PID control which is based on Scalar Pulse Width Modulation (PWM). The two control techniques are applied for load voltage regulation of the autonomous four-leg voltage source inverter (FLVSI). Practically, the predictive control requires a large number of calculations, resulting in high computation time and delay. In this paper, a new finite control set model predictive voltage control (MPVC) algorithm is proposed to predict the load voltages for 15 switching states instead of 16 switching states for reducing the computation time required for the control algorithm. Moreover, the algorithm is optimized by removing the repeated computations and the delay is compensated using the two-step prediction horizon principle. An accurate discrete-time state-space model of the autonomous FLVSI with output LC-filter is used for predicting the load voltages considering the neutral inductance and damping resistance of the LC filter. A simple PID control scheme with decoupled feedforward voltage and current loops is used in the DQ0 reference frame, while MPVC operates in the ABC reference frame. The simulation and experimental results are used to show the full assessment of the MPVC. The prominent outcomes show the ability of the proposed MPVC algorithm to provide high power quality under unbalanced and non-linear load conditions with high stability and robustness.

Optimized Design of the Neutral Inductor and Filter Inductors in Three-Phase Four-Wire Inverter With Split DC-Link Capacitors

The three-phase four-wire inverter with split dc-link capacitors can supply unbalanced loads. For the purpose of reducing the filter inductors, a neutral inductor could be introduced into the neutral line. This paper analyzes the operation principle of the three-phase four-wire inverter with split dc-link capacitors when a neutral inductor is introduced. It is illustrated that the neutral inductor can reduce the zero-sequence switching harmonics in the voltages between the phase-leg midpoints and the output neutral point, thus the filter inductors can be reduced. The optimized design of the neutral inductor and filter inductors is proposed with the considerations of the inductor current ripple, the ability of supplying unbalanced loads, and the total energy stored in the inductors. The equivalent circuits of the three-phase four-wire inverter with split dc-link capacitors and neutral inductor is derived in the α-β-0 frame, and the zero-axis voltage regulator is modified to suppress the third-order harmonic in the zero-sequence current caused by the deadtime of the drive signals for the power switches. Finally, the experimental results from a 9-kW prototype are provided to prove the effectiveness of the proposed optimized design of the neutral inductor and filter inductors and the control strategy of the inverter.

Reducing the Inductors of Rectifiers Having Two Outputs to Improve Power Density

In this paper, a recently reported single-phase rectifier with two outputs (RECTO) is improved to reduce the neutral inductor and the grid inductor, by moving the neutral inductor away from the path of the grid current. The neutral inductor does not carry the grid current any more so the current stress of the neutral inductor can be significantly reduced, and the size of the inductor becomes much smaller. In theory, the current stress can be reduced by at least three times and the size of the inductor can be reduced by nine times. At the same time, the grid inductor can be reduced to achieve the same level of grid-current switching ripples because of the changed operation modes and modulation strategy. Together, the reduced neutral and grid inductors help improve the system power density. It is worth noting that the voltage and current stresses of the switches and the other features of the RECTO, e.g., two dc outputs and unity power factor, are not affected. Comparative experimental results are presented to demonstrate the reduction. If the two load currents are the same then the neutral inductor is only required to handle the switching ripples. This improved RECTO is particularly suitable for applications with two balanced loads without increasing the cost much.

Current-Stress Reduction for the Neutral Inductor of $\theta$ -Converters

Following the recently proposed θ-converter that reduces voltage ripples, leakage currents, and total capacitance required in a single-phase converter, this paper further improves the θ-converter by reducing the current stress of the neutral inductor, by relocating it to the dc side without affecting the other functions of the θ-converter. Based on the average circuit model of the proposed converter, it is found that the current flowing through the neutral inductor can be reduced by at least three times, and hence, the size of the inductor can be reduced by at least nine times. How to operate the converter to achieve the reduction of the current stress is discussed in detail. Moreover, the selection criteria on the neutral inductor and other passive components used in the proposed converter are discussed in order to minimize their usage. Intensive experimental results are presented to demonstrate that the proposed converter can indeed reduce the current-stress while the other functions of the θ-converter are not affected.

Neutral Point Voltage Ripple Suppression for a Three-Phase Four-Wire Inverter With an Independently Controlled Neutral Module

An independently controlled neutral module (ICNM) can be adopted in a three-phase four-wire inverter to form the neutral line and provide the path for zero-sequence current when the loads are unbalanced. The original control objective of the ICNM is to stabilize the neutral point voltage. Since there is a −180° phase jump at the resonant frequency of the neutral inductor and split dc-link capacitors in the system loop gain, system instability may occur. In order to ensure system stability, the inductor current feedback scheme (ICFS) can be incorporated, which is equivalent to introducing a resistor in series with the inductor branch. However, it will lead to increased zero-sequence current flowing through the dc-link capacitors, thus increasing the neutral point voltage ripple. In this paper, three improved control schemes, namely, notch-filter-incorporated inductor current feedback scheme (NF-ICFS), proportional resonant cascaded regulator plus inductor current feedback scheme (PR+ICFS), and neutral current feed-forward plus inductor current feedback scheme (NCFF+ICFS), are proposed, which not only ensure system stability, but also maintain a small neutral point voltage ripple. A 10-kW prototype of the three-phase four-wire inverter with ICNM is built and tested in the laboratory, and the experimental results verify the effectiveness of the proposed control schemes.

The events for prevention of resonant overvoltages in main power electrical networks

science , pr. Peremogy, 56, с. Kyiv, 03680, Ukraine.The study of the origin of conditions and possible consequences of abnormal modes power electrical networks with extreme values parameters, such as overvoltage, traditionally performed using mathematical models with linear and symmetrical elements. This is due to great experience in the development and use of this class of models on practice, since the basis for decision-making in the design and operation of electrical networks is simulation of results primarily in normal modes. At the same time, a more detailed analysis shows that the presence in modern electrical networks sources of distortion, mainly unbalance, influences the current values of the parameters modes, including the probability of transition of these values through thresholds.Trends of modern electricity power grid point to the growing role of extra high voltage (EHV) transmission lines as those that constitute the system and interconnect systems. It should be noted that the study of overvoltage for this class lines must be done very carefully, particularly with regard to the impact of sources of distortion. This is due to the lack of practical operational reserve insulation, designed for extreme values, as a such reserve for EHV is expensive. Thus, analysis of the possibility of overvoltage in EHV lines should be done not only for normal, but also for abnormal asymmetrical modes.In main electric power networks are used, above all, structural measures to reduce and suppress the resonant overvoltage associated with changes in workload schemes and the transition from an abnormal to normal mode. However, special devices or setting modes by the criterion of reducing the overvoltage can be used when needed, which have been established for other purposes. At the same time the resonant overvoltage caused by current resonance, resulting in influence on the kind of overvoltage is possible by changing the parameters of elements connected in parallel to overhead line. These include, in particular: the neutral inductance and shunt reactors. In this case, the criterion of the occurrence of resonance overvoltage is the length of the line at which the condition of complete current resonance is performed.Reference 5, figures 1.

Design and Performance of an Adaptive Low-DC-Voltage-Controlled LC-Hybrid Active Power Filter With a Neutral Inductor in Three-Phase Four-Wire Power Systems

This paper proposes an adaptive low-dc-link-voltage-controlled LC coupling hybrid active power filter ( LC-HAPF) with a neutral inductor, which can compensate both dynamic reactive power and current harmonics in three-phase four-wire distribution power systems. Due to its adaptive low-dc-link-voltage characteristic, it can obtain the least switching loss and switching noise and the best compensating performances, compared with the conventional fixed and newly adaptive dc-voltage-controlled LC-HAPFs. The design procedures of the dc-link voltage controller are discussed, so that the proportional and integral gains can be designed accordingly. Moreover, the general design procedures for the adaptive dc-voltage-controlled LC-HAPF with a neutral inductor are also given. The validity and effectiveness of the adaptive dc-link voltage-controlled LC -HAPF with a neutral inductor are confirmed by experimental results obtained from a 220-V 10-kVA laboratory prototype compared with the conventional fixed and adaptive dc-link voltage-controlled LC-HAPFs without a neutral inductor.

Modeling, Analysis, and Mitigation of Load Neutral Point Voltage for Three-Phase Four-Leg Inverter

The three-phase four-leg inverter is very suitable for the high-power uninterruptible power supply application, and the load neutral point voltage (LNPV) should meet the requirement. In this paper, two factors influencing the LNPV were revealed first, which are the switching states and the ratio of the neutral inductance to the phase inductance ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> ). Then, the 16 switching states can be classified into four groups, and the ones in the first and the second group come to the minimum amplitude of LNPV with a specific ratio <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> respectively. Third, three methods to mitigate the LNPV were proposed based on the analysis of LNPV. In the first method, switching states in the first group are just utilized, and the LNPV can be eliminated with the ratio <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> of unity. In the second method, the switching states in the fourth class are avoided, and the LNPV can be reduced. Common mode filter is used in the third method and the LNPV can be mitigated effectively with the ratio <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> of unity. Furthermore, the power losses in the first and the second method were estimated and were compared with the original situation. Finally, the proposed methods were verified by simulation and experimental results.

Minimum inverter capacity design for LC-hybrid active power filters in three-phase four-wire distribution systems

This study presents a minimum inverter capacity design for three-phase four-wire centre-split inductor-capacitor (LC) coupling hybrid active power filters (LC-HAPFs). Based on its equivalent circuit models in d - q -0 coordinate, the coupling part filtering characteristics of the LC-HAPF without or with neutral inductor can be more clearly illustrated and easily understood, compared with the past analysis based on the generic filter structure. According to the current quality data, the minimum dc-link voltage expressions for the LC-HAPF without and with neutral inductor are deduced and compared. Conventionally, the coupling LC is usually tuned at a higher fifth- or seventh-order harmonic frequency to reduce its cost and size compared with third-order case. When triplen harmonic currents exist significantly, the LC-HAPF with a small tuned neutral inductor can further reduce its minimum dc-link voltage requirement. Thus, the initial cost, switching loss and switching noise of the LC-HAPF can be lowered. Representative simulation and experimental results of the three-phase four-wire LC-HAPF with neutral inductor are presented to verify the filtering characteristics analysis and minimum dc-link voltage expressions, to show the effectiveness of reducing its inverter capacity, switching loss and switching noise in current quality compensation compared with the conventional LC-HAPF.

A study on the effects of the neutral inductor on the modeling and performance of a four-wire three-phase/switch/level fixed-frequency rectifier

This paper presents a performance evaluation of three-phase, three-switch, three-level, fixed-frequency pulse-width-modulated (PWM) rectifier, when an inductive connection is added between the DC-side capacitive mid-point and the AC-side source neutral. A new continuous-mode model that takes into account the presence of the added zero-sequence inductor is developed using the state–state averaging technique in the synchronous frame. The stability and the performance of the converter in terms of input current Total Harmonic Distortion (THD), power factor and DC voltage regulation are investigated, even for operation under unbalanced DC loads. The converter characteristics are evaluated for different values of the zero-sequence inductor in an attempt to derive useful optimized criteria for the design of the converter. The proposed study is supported by numerical simulations that are carried out a switching-functions-based model of the converter.

The Capacitive Coupling Between EHV Lines and Nearby Pipelines

This paper addresses the effect of the electrostatic field due to extra high voltage (EHV) overhead transmission lines on pipelines in the vicinity of those power lines. Two measures for this effect are used: the maximum electric field on the pipe surface and the total electric charge on the pipeline per unit length. A mathematical model is presented for assessing these two measures. The results of applying the model to a situation involving a single-circuit, six-bundle, three-phase, 750-kV EHV line with flat conductor configuration are presented and discussed. The first set of results refers to a parallel pipeline of radius 0.5 m. The dependence of both the maximum electric field (at the top of the pipe) and the charge per meter on the distance between the pipeline and the tower center is similar to the distribution of the electric field beneath the tower at the ground surface. It shows maxima of both quantities if the pipe is exactly under one of the outer phases of the power line. At a distance of about 35 m from the tower center, both the electric field and the charge per unit length drop to 50% of their maximum values. The electric field is found to increase almost linearly with the clearance between the pipeline and the ground surface. The charge changes in a more complicated way with the clearance. It decreases if the pipe clearance increases from 0 to 0.2 m, then increases steadily beyond this value. For a given distance from the 750-kV line and for a fixed clearance from the ground, both the electric field and the electric charge per unit length on the pipeline will increase with the pipe radius. This paper will deal also with the impact of the pipeline on the nearby EHV power line and its associated network. The results will show that for a solidly earthed power network, the presence of the pipeline will be accompanied by a slight increase in the neutral current. On the other hand, for a power network with an inductively earthed neutral, there will be a tendency toward a parallel resonance that can occur for particular values of the neutral inductance and pipe radius. This resonance will result in an increase in the system's neutral potential, which will be primarily limited by the network losses.