Analysis, Calculation and Reduction of Shaft Voltage in Induction Generators

Abstract. This paper presents analysis of shaft voltage in different configurations of a Doubly Fed Induction Generator (DFIG) and an Induction Generator (IG) with a back-to-back inverter in wind turbine applications. Detailed high frequency model of the proposed systems have been developed based on existing capacitive couplings in IG & DFIG structures and common mode voltage sources. In this research work, several arrangements of DFIG based wind energy conversion systems are investigated on shaft voltage calculation and its mitigation techniques. Placements of an LC line filter in different locations and its effects on shaft voltage elimination are studied with mathematical analysis and simulations

Hierarchical Energy Management System for Home-Energy-Hubs Considering Plug-in Electric Vehicles

The escalating demand on Electric Vehicles (EVs) has enhanced the necessity of adequate charging infrastructure, especially in residential areas. This paper proposes a smart charging approach for off-board Electric Vehicles (EVs) chargers in Home-Energy-Hub (HEH) applications along with DC sources such as Photovoltaic (PV) and Battery Storage (BS). The proposed method facilitates smart charging and discharging of EVs to obtain both Vehicle-to-X (V2X) and X-to-Vehicle (X2V) operations focusing on the domestic applications integrated with renewable and storage elements. Furthermore, the optimal State-of-Charge (SOC) profiles for BS and EV in the HEHs system is defined by the extended Bellman-Ford-Moor Algorithm (BFMA). This modified BFMA utilizes the forecasted data such as solar irradiation, electricity tariff, and power consumption to gain economic benefits in HEHs with respect to user and EV requirements. Moreover, the plugging time, duration and initial/final SOC are fluctuating at each connection due to the stochastic nature of EV conditions and user settings. This study presents a laboratory implementation of two-level Hierarchical Energy Management System (HEMS) for HEHs with plug-in electric vehicles. In fact, the primary level includes power converters controller, while the proposed algorithm is implemented in the secondary level. Finally, the simulation and experimental results confirm the effectiveness of the proposed analysis regarding the interaction of HEHs and power grid with EVs behavior

CASCADE TOPOLOGIES FOR THE ASYMMETRIC MULTILEVEL INVERTER BY NEW MODULE TO ACHIEVE MAXIMUM NUMBER OF LEVELS

ABSTRACT:  Multilevel inverters have been introduced as useful devices to connect between DC-AC systems. They are high quality output and cost benefit systems with a wide range of applications. Asymmetric multilevel inverters are a type of multilevel inverters with unequal DC link to create more voltage levels through fewer components. This paper presents new topologies of cascade multilevel inverter by a new module with a reduced component. Base module produces 13 levels by two types of unequal DC sources and 10 switches. Modular can be used to produce more and higher voltages levels. The designing of proposed multilevel inverter makes some preferable index with better quality than similar modular multilevel inverters, such as less semiconductors and DC sources, low switching frequency, creating of negative levels without any additional circuit, and module in cascade connections. Also, two cascade topologies are presented in the modular connections of the proposed module to achieve high and significant number of levels. Nearest level control (NLC) method as a switching technique is used in step changing levels for topologies to get more quality and lower harmonics. The presented module and cascade topologies are simulated by MATLAB/Simulink and are implemented by the experimental prototype in laboratory to validate the performance of proposed topologies in which simulated and experimental results show a good performance with the high quality output

Influence of field-dependent critical current on harmonic AC loss analysis in HTS coils for superconducting transformers supplying non-linear loads : harmonic analysis of HTS transformers

There are two main obstacles in front of the development of high temperature superconducting (HTS) technology for electric power network applications; tape price and cooling cost. In order to reduce cooling cost, it is vital to evaluate AC transport current loss of the tapes precisely and then reduce it by some design innovative approaches. In addition, AC transport current loss in HTS material is a critical design variable for large-scale power network applications such as HTS transformers, superconducting fault current limiters, and power cables, since they are continuously carrying the network/load current during their operating life. In existing power networks, harmonic production sources are commonly used and thus, currents are distorted. Therefore, the effect of nonsinusoidal current on the critical apparatus in the network such as transformer must be studied. In this paper, AC transport current loss of a single-turn 2G YBCO HTS coil was modeled and numerically calculated under nonsinusoidal transport current using finite element method. Furthermore, influence of dependency of critical current density to magnetic field on the AC transport current loss of HTS coil when carries distorted currents was considered. It was observed that nonsinusoidal current causes excessive losses in HTS coil. On the other hand, a case study on an HTS transformer supplying non-linear load was considered to study the loss increment as well as heat load change. It was observed that current harmonics increases the AC loss, and heat load of transformer and decreases the efficiency, consequently

Analysis, calculation and reduction of shaft voltage in induction generators

This paper presents analysis of shaft voltage in different configurations of a Doubly Fed Induction Generator (DFIG) and an Induction Generator (IG) with a back-to-back inverter in wind turbine applications. Detailed high frequency model of the proposed systems have been developed based on existing capacitive couplings in IG & DFIG structures and common mode voltage sources. In this research work, several arrangements of DFIG based wind energy conversion systems are investigated on shaft voltage calculation and its mitigation techniques. Placements of an LC line filter in different locations and its effects on shaft voltage elimination are studied with mathematical analysis and simulations

Effects of Switching Time on Output Voltages of a Multilevel Inverter Used in High Frequency Applications

Employing multilevel inverters is a proper solution to reduce harmonic content of output voltage and electromagnetic interference (EMI) in high power electronic applications. In this paper, a new pulse width modulation method for multilevel inverters is proposed in which power devices' on-off switching times have been considered. This method can be surveyed in order to analyze the effect of switching time on harmonic contents of output voltage in high frequency applications when a switching time is not negligible compare to a switching cycle. FFT calculation and analysis of output voltage waveforms and harmonic contents with regard to switching time variation are presented in this paper for a single phase (3, 5)-level inverters used in high voltage and high frequency converters. Mathematical analysis and MATLAB simulation results have been carried out to validate the proposed method

Different approaches to reduce shaft voltage in AC generators

This paper presents several shaft voltage reduction techniques for doubly-fed induction generators in wind turbine applications. These techniques includes: pulse width modulated voltage without zero vectors, multi-level inverters with proper PWM strategy, better generator design to minimize effective capacitive couplings in shaft voltage, active common-mode filter, reducing dc-link voltage and increasing modulation index. These methods have been verified with mathematical analysis and simulations

Analysis of shaft voltage in a doubly-fed induction generator

Fast switching transients and common mode voltage generated by pulse width modulated voltage in high frequency applications may cause many unwanted problems such as shaft voltage and resultant bearing currents. The main objective of this research work is to analyse shaft voltage generation in a doubly-fed induction generator (DFIG) with a back to back converter. A detailed high frequency model of the proposed system has been developed based on capacitive couplings between different objects of the machine. The proposed model can be used for shaft voltage calculations and finding parameters which have key effect on shaft voltage and resultant bearing currents. A discussion about the presented technique for shaft voltage elimination in existing literature is also presented based on mathematical analysis and simulations

Remediation strategies of shaft and common mode voltages in adjustable speed drive systems

AC motors are largely used in a wide range of modern systems, from household appliances to automated industry applications such as: ventilations systems, fans, pumps, conveyors and machine tool drives. Inverters are widely used in industrial and commercial applications due to the growing need for speed control in ASD systems. Fast switching transients and the common mode voltage, in interaction with parasitic capacitive couplings, may cause many unwanted problems in the ASD applications. These include shaft voltage and leakage currents. One of the inherent characteristics of Pulse Width Modulation (PWM) techniques is the generation of the common mode voltage, which is defined as the voltage between the electrical neutral of the inverter output and the ground. Shaft voltage can cause bearing currents when it exceeds the amount of breakdown voltage level of the thin lubricant film between the inner and outer rings of the bearing. This phenomenon is the main reason for early bearing failures. A rapid development in power switches technology has lead to a drastic decrement of switching rise and fall times. Because there is considerable capacitance between the stator windings and the frame, there can be a significant capacitive current (ground current escaping to earth through stray capacitors inside a motor) if the common mode voltage has high frequency components. This current leads to noises and Electromagnetic Interferences (EMI) issues in motor drive systems. These problems have been dealt with using a variety of methods which have been reported in the literature. However, cost and maintenance issues have prevented these methods from being widely accepted. Extra cost or rating of the inverter switches is usually the price to pay for such approaches. Thus, the determination of cost-effective techniques for shaft and common mode voltage reduction in ASD systems, with the focus on the first step of the design process, is the targeted scope of this thesis. An introduction to this research – including a description of the research problem, the literature review and an account of the research progress linking the research papers – is presented in Chapter 1. Electrical power generation from renewable energy sources, such as wind energy systems, has become a crucial issue because of environmental problems and a predicted future shortage of traditional energy sources. Thus, Chapter 2 focuses on the shaft voltage analysis of stator-fed induction generators (IG) and Doubly Fed Induction Generators DFIGs in wind turbine applications. This shaft voltage analysis includes: topologies, high frequency modelling, calculation and mitigation techniques. A back-to-back AC-DC-AC converter is investigated in terms of shaft voltage generation in a DFIG. Different topologies of LC filter placement are analysed in an effort to eliminate the shaft voltage. Different capacitive couplings exist in the motor/generator structure and any change in design parameters affects the capacitive couplings. Thus, an appropriate design for AC motors should lead to the smallest possible shaft voltage. Calculation of the shaft voltage based on different capacitive couplings, and an investigation of the effects of different design parameters are discussed in Chapter 3. This is achieved through 2-D and 3-D finite element simulation and experimental analysis. End-winding parameters of the motor are also effective factors in the calculation of the shaft voltage and have not been taken into account in previous reported studies. Calculation of the end-winding capacitances is rather complex because of the diversity of end winding shapes and the complexity of their geometry. A comprehensive analysis of these capacitances has been carried out with 3-D finite element simulations and experimental studies to determine their effective design parameters. These are documented in Chapter 4. Results of this analysis show that, by choosing appropriate design parameters, it is possible to decrease the shaft voltage and resultant bearing current in the primary stage of generator/motor design without using any additional active and passive filter-based techniques. The common mode voltage is defined by a switching pattern and, by using the appropriate pattern; the common mode voltage level can be controlled. Therefore, any PWM pattern which eliminates or minimizes the common mode voltage will be an effective shaft voltage reduction technique. Thus, common mode voltage reduction of a three-phase AC motor supplied with a single-phase diode rectifier is the focus of Chapter 5. The proposed strategy is mainly based on proper utilization of the zero vectors. Multilevel inverters are also used in ASD systems which have more voltage levels and switching states, and can provide more possibilities to reduce common mode voltage. A description of common mode voltage of multilevel inverters is investigated in Chapter 6. Chapter 7 investigates the elimination techniques of the shaft voltage in a DFIG based on the methods presented in the literature by the use of simulation results. However, it could be shown that every solution to reduce the shaft voltage in DFIG systems has its own characteristics, and these have to be taken into account in determining the most effective strategy. Calculation of the capacitive coupling and electric fields between the outer and inner races and the balls at different motor speeds in symmetrical and asymmetrical shaft and balls positions is discussed in Chapter 8. The analysis is carried out using finite element simulations to determine the conditions which will increase the probability of high rates of bearing failure due to current discharges through the balls and races

End-winding effect on shaft voltage in AC generators

This paper presents effects of end-winding on shaft voltage in AC generators. A variety of design parameters have been considered to calculate the parasitic capacitive couplings in the machine structure with Finite Elements simulations and mathematical calculations. End-winding capacitances have also been calculated to have a precise estimation of shaft voltage and its relationship with design parameters in AC generators