Design and control of single-phase dynamic voltage restorer

Dynamic voltage restorer (DVR) is a custom power device used in electrical distribution system for power quality improvement. It ensures regulated voltage supply to the sensitive loads, even in case of voltage sag and swell disturbances in the distribution network. It is a series connected device and compensates voltage sag and swell by injecting a voltage with the help of a series transformer. The injection of an appropriate voltage component in the event of a voltage disturbance requires a certain amount of real and reactive power. Conventionally, DVR consists of an energy storage device, which supplies the required power over the limited duration of the sags. Large magnitude and long duration of sags lead to heavy financial investment in energy storage unit. To overcome this limitation, a single-phase back-to-back converter-based DVR is implemented in this work, which eliminates energy storage requirement. The integration of series and shunt converter makes the DVR capable of bidirectional flow of energy. Therefore, the key advantage of this topology is its capability to compensate for long-term voltage sag and swell. Modelling of the DVR and its controller design is included in this paper. The effectiveness of control schemes, protection schemes and starting sequence of operation of DVR is verified through detailed simulation studies. A scaled down laboratory prototype of DVR is developed. The viability of these schemes is confirmed by the experimental results generated from the laboratory prototype. Various challenges faced during the prototype development and corresponding solutions are also discussed in this paper.     

Effects of unsymmetrical voltage sags on squirrel-cage induction motors

The consequences of unsymmetrical voltage sags on the three-phase squirrel-cage induction motor behaviour are analysed; namely torque and current peaks, and speed loss. These effects depend on several variables such as sag duration, depth and sag type. The study takes into account that the voltage recovery instant can take only discrete values since fault clearing occurs in the natural current zeroes. The torque and current peaks have a periodical dependence on the sag duration, and a linear dependence on depth. The study of the influence of sag type on speed loss shows that sag types C and D have the same behaviour, and the same is true for sag types E, F and G.     

Design and analysis of dynamic voltage restorer for deep voltage sag and harmonic compensation

A dynamic voltage restorer (DVR) to compensate deep voltage sags and harmonics is proposed. The DVR consists of shunt and series converters connected back-to-back through a DC-to-DC step up converter. The presence of the DC-to-DC step converter permits the DVR to compensate deep voltage sags for long duration. The series converter is connected to the supply side whereas the shunt converter is connected to the load side. With this configuration, there is no need for large DC capacitors. A design procedure for the components of the DVR is presented under a voltage sag condition. The control system of the proposed DVR is based on hysteresis voltage control. Besides voltage sag compensation, the capability of compensating load voltage harmonics has been added to the DVR to increase the power quality benefits to the load with almost negligible effect on the sag compensation capability. The proposed DVR is modelled and simulated using SIMULINK/MATLAB environment. Time domain simulations are used to verify the operation of the DVR with linear and non-linear loads.     

Detection of voltage sag sources based on instantaneous voltage and current vectors and orthogonal Clarke?s transformation

Different methods for voltage sag source detection are discussed. They are based either on disturbance energy, voltage-current characteristic or active (real) current component. It is shown that, in the cases of asymmetrical voltage sags, both current-based methods known from literature do not work well. These two methods are therefore generalised using a vector-space approach. Furthermore, three new methods are introduced using orthogonal Clarke's transformation, which can be used to detect the sources of those voltage sags provoked by earth faults. All the discussed methods for voltage sag source detection have been tested by applying extensive simulations, laboratory tests and field testing. The results obtained show the very high effectiveness of the proposed methods, which are superior to the methods known from literature, especially for detecting sources of asymmetrical voltage sags.     

Advanced voltage sag characterisation ii: point on wave

Two important but often neglected characteristics of voltage sags and short interruptions are: point on wave of sag/interruption initiation and point on wave of sag/interruption ending. A critical review of existing approaches is presented and an appropriate definition of point-on-wave values proposed. An identification of point-on-wave values in monitoring applications is discussed and problems related to identification/assessment of point-on-wave values highlighted. Examples of equipment sensitivity to these phenomena are analysed. The results related to identified ranges and relationships of point-on-wave values are reviewed, using examples of sags and interruptions recorded in power supply systems. Adequate procedure for testing single-phase and three-phase equipment to sags and interruptions are discussed with respect to point-on-wave values.     

敏感用户高品质交-直流级联供电方案研究

工业过程中大量使用的变频器(variable frequency drive,VFD)在采用传统交流供电时易受到电压暂降影响,传统治理方式存在检测延时、控制复杂等问题。为此,该文从敏感负荷变频器在电网扰动工况下的供电需求量化出发,围绕供电拓扑设计、DC/DC变换器的控制策略及储能单元容量配置等方面,构建一种实用化工业用交-直流级联供电方案。该文利用电压暂降过程中直流供电回路与敏感负荷间的压差自感应控制,实现交-直流供电回路无扰动切换,提高电压暂降治理响应的快速性;采用两相交错并联DC/DC变换器接入集中式储能,减少储能系统纹波系数,并给出两相均流控制方法。最后,通过Matlab/Simulink中的建模仿真与样机的实验测试,验证所提方案能够在交流系统发生不同严重程度电压暂降过程中为各类敏感负荷提供稳定的连续供电,为解决工业敏感用户高品质供电问题提供可实用的新思路。     

Proposal of DC shield reactor type superconducting fault current limiter

Saturated DC reactor type superconducting fault current limiter (SFCL) had been proposed two years ago. It was classified to rectifier type SFCL. The changing inductance value with the operating mode has superior characteristics to reduce voltage sag during step increase of the load current. But it has the disadvantage of its weight. In this paper, rectifier type SFCL with shielded reactor has been proposed. The reactor which has superconducting ring or tube inside its winding is substituted to the DC link of the rectifier. The configuration looks like an air core transformer with secondary short winding. When the current through the bulk shield-ring reaches to a certain level, the flux penetrates to the shield body and finite impedance appears in the primary winding. In other words, when the surface flux density exceeds its critical flux density, the flux penetrates into the bulk superconductor, and increases equivalent inductance. The equivalent transient resistance of the shield was represented as a function of exponential of the time. Using this equivalent transient resistance, the transient impedance was expressed. The transient wave analysis using EMTDC (electro-magnetic transients in DC systems) has been described. Simulated waveforms are shown considering the source inductance, the leakage inductance, the coupling coefficient and the forward voltage drop of the semiconductor. And voltage sag was also investigated with 50% step load increase.Preliminary design was also performed. The coil size and number of turns are designed to obtain adequate inductance for the current limitation, and the central magnetic field of the coils are calculated. There is optimal aspect ratio to minimize the magnetic field with restriction in outer diameter of the coil.     

A Study on Measurement Error during Alternating Current Induced Voltage Tests on Large Transformers

1IntroductionIn a resistance, inductance, and capacitance series circuit, the voltage across the capacitance canbe greater than the source voltage due to the compensative effect of the inductance. This phenome-non, referred to as the capacitive rise, has been fully studied and favorably employed in electricaland electronic systems, but has not been paid much attention in high voltage tests.To guarantee safe and reliable operation, transformers of220 kVand above are recommended in IEC(Interna…     

Transient response analysis of a Rosen-type piezoelectric transformer and its applications

This paper investigates the characteristics of voltage transient response, such as the maximum voltage, direct current (DC) time constant, alternating current (AC) time constant and oscillation frequency, and their applications for a Rosen-type piezoelectric transformer (PT). The transient response is induced immediately after an AC voltage connected to the PT is switched off. For the applications, the maximum voltage is used to elucidate how to cause an electrical shock for users of the PT under open-circuit operation. The DC time constant and the AC time constant are used to estimate the equivalent resistance of the mechanical loss and the dielectric resistance of the dielectric loss, respectively. Also, the AC time constant is used to estimate the quality factor of the PT. Additionally, the oscillation frequency is used as an antiresonant frequency of the PT. In order to verify the above characteristics and applications, both an equivalent circuit with initial conditions and a drive system with a control switch interposed between the PT and its AC voltage source are adopted to derive the transient response and measure its characteristics. Effects of the load resistance of the PT and the switching-off time of the voltage source on the transient response are measured and discussed.     

影响电压暂降问题的多源属性排序方法

对电压暂降的多源属性进行分析,不仅能为供电工程的规划设计、设备选型、运行维护等环节提供全过程技术支撑,还可以有效评估电能质量事件的风险及其严重程度。该文提出电压暂降的多源属性贡献度排序方法。首先,列举电压暂降的多源属性,将其分为内源属性和外源属性;然后,采用设备兼容度指标,计算多源属性下设备的暂降兼容度,并建立基于层次分析法的三层致因体系,针对多源致因属性的权重进行计算;最后,利用蒙特卡洛法进行仿真,结合实际工程数据,分析多源致因属性下发生电压暂降的严重性程度,评估结果验证该方法可行。     

Multilayer piezoelectric ceramic transformer with low temperature sintering

The low-fired high performance piezoelectric ceramics used for multilayer piezoelectric transformer were investigated. Based on the transient liquid phase sintering mechanism, by doping suitable eutectic additives and optimizing processing, the sintering temperature of the quaternary system piezoelectric ceramics with high piezoelectric properties could be lower to about 960–1000°C. The low-temperature sintering multilayer piezoelectric transformer (MPT) has been developed. Some characteristics of MPT were systemically studied. The measurements include the frequency response of input impedance, frequency response of phase difference between input voltage and current, frequency shifting with load, input impedance changing with load, phase difference between input voltage and current shifting with load, and phase difference between input voltage and vibration velocity. The vibration modes and resonance characters of MPT were measured by a Laser Doppler Scanning Vibrometer. Several kinds of MPT with high voltage step-up ratio, high power density, high transfer efficiency and low cost have been industrially produced and commercialized. It reveals a broad application prospect for back-light power of liquid crystal display and piezo-ionizer etc.     

新型智能路灯照明调控系统

路灯照明光源通常是高压钠灯和汞灯,调控设备具有变压与稳压的双重功能,通过控制电路和时控电路,实现最优化的智能控制。智能照明调控装置,通过补偿变压器、多抽头变压器,实现主回路输出电压的增减。智能照明调控设备结合节能产品的优点,并克服存存的缺陷。     

有载自动调容配电变压器的降损节能效益分析

本文分析了调容变压器降损节能原理,对有载自动调容变压器的组成和工作原理进行介绍,根据典型日、月实际运行负荷数据,绘制了日、月负荷曲线;对SZ11-M-T型有载自动调容型变压器与S11普通型配电变压器的在农村电网中的动态功率损耗进行经济效益分析,在我国农村电网中,有载调容配电变压器的应用范围非常广泛。     

A reliable high-powered constant current supply

The use of series-connected saturable reactors operating in the constrained mode has proven to be a reliable and trouble-free means for developing constant current power such as that required by transoceanic cable systems. As the art has progressed towards the use of higher and higher ac power frequencies, however, it has become increasingly difficult to design a single ac power source of substantial power rating without resorting to rather complex and inefficient circuitry. In practice, the saturable reactors are used in conjunction with transformers. This paper demonstrates that by departing from convention in the design of the saturable reactor and output transformer, it is possible, using a single reactor and transformer, to combine the outputs from a multiplicity of ac power sources or converter stages on a load sharing basis so as to obtain a single supply with the required power rating. The particular arrangement of the specially designed saturable reactor and output transformer permits a unique interaction to be established in which the two complement one another; the reactor controlling the summation of source currents to achieve output regulation and the transformer proportioning these currents among the sources to insure proper sharing of load power. Variations in load power are automatically accompanied by a shift in the operating power factor of the individual sources. Also, because the method affords electrical isolation, sources of unequal current, voltage, and power ratings can readily be combined. Furthermore, this technique permits the use of excess (redundant) ac sources so that normal operation can be maintained following the loss of output from any one of the sources.     

Design and implementation of PV-based three-phase four-wire series hybrid active power filter for power quality improvement

This paper proposes a Photovoltaic (PV)-based three-phase four-wire Series Hybrid Active Power Filter (SHAPF), it comprises of a Series Active Power Filter (SAPF) and an LC shunt passive filter. The proposed system eliminates both the current and voltage harmonics and compensates reactive power, neutral current and voltage interruption. A SAPF demands a source of energy for compensating the voltage sag/swell. This system found a new topology for SHAPF which utilizes the PV with DC–DC boost converter as a source of DC power for the series active filter. The compensation current reference evaluation is based on the twin formulation of the vectorial theory of electrical power theorem with Fuzzy Logic Controller (FLC). The PV array/battery managed DC–DC boost converter is employed to step up the voltage to meet the DC bus voltage requirement of the three-leg Voltage Source Inverter (VSI). The foremost benefit of the proposed system is that, it will provide uninterrupted compensation for the whole day. This system utilizes the renewable energy; accordingly saves the energy and provides the uninterruptable power supply to critical/sensitive load, through the PV array/battery bank during both day time and night time. An experimental model was established and results were obtained, which indicated the capability of the proposed control scheme.     

Modeling and analysis of dual-output piezoelectric transformer operating at the thickness-shear vibration mode.

In our previous study, the multioutput piezoelectric transformer operating at the thickness-shear vibration mode was proposed and experimentally investigated. By designing a new construction of support and lead wire connection, a power density of 52.7 W/cm3 and a total output power of 169.8 W were achieved at a temperature rise less than 20 degrees C. In this work, a theoretical model was developed for the dual-output piezoelectric transformer operating at the thickness-shear vibration mode. The equivalent circuit parameters of the piezoelectric transformer were derived. Based on this, the impedance characteristics, equivalent inductance, capacitance ratio, voltage gain, and efficiency of the piezoelectric transformer were calculated. The theoretical results were verified by experimental data. Furthermore, the effect of the transformer size on the voltage gain, efficiency, output power and power density, and the effect of the load of one output on the voltage gain of another output were analyzed. Some useful guidelines were achieved by these analyses.     

Series compensation method to mitigate harmonics and voltage sags and swells

The use of a series compensator (SC) to improve power quality is investigated. The role of the compensator is not only to mitigate the effects of voltage sag and swell, but also to control the level of harmonic distortions in the network. This is accomplished through extension of the voltage injection scheme of the SC by the incorporation of load current feedback and an external inductive filter. The compensation process is accompanied by an exchange of energy between the SC and the external network. Through a phase adjustment technique, it is shown that the voltage quality across the terminals of the protected load can be controlled. Simulation studies, supported by laboratory experiments, show that the resulting series compensation scheme is effective in improving the supply quality of the power system.     

High power universal piezoelectric transformer.

This study describes a multilayer piezoelectric voltage and power transformer that has one direction poling, operates in a wide-frequency range and delivers both step-up and step-down voltages by inverting the electrical connections. In this design, the input and output electrodes are on the same side of the disk and are isolated from each other by a fixed isolation gap. The electrode pattern is a ring/dot structure such that it uses radial mode for both input and output part that are built-in on the same ceramic disk. A prototype transformer was fabricated of size 15 x 2.78 mm2 having mass of 3.8 gm. In the step-down configuration at the constant output power of 6 W, the transformer characteristics across a 100 ohms load were found to be efficiency = 92%, gain = 0.21 input voltage = 110 Vrms, and temperature rise = 20 degrees C from the room temperature. In the step-up configuration at the constant output power of 5 W, the transformer characteristics across a 5 kohms load were found to be efficiency = 97%, gain = 9.5, input voltage = 16 Vrms, and temperature rise = 8 degrees C from the room temperature. A detailed equivalent circuit analysis of the transformer was done, and the results were found to be in excellent agreement with the experimental results.     

High power universal piezoelectric transformer

This study describes a multilayer piezoelectric voltage and power transformer that has one direction poling, operates in a wide-frequency range and delivers both step-up and step-down voltages by inverting the electrical connections. In this design, the input and output electrodes are on the same side of the disk and are isolated from each other by a fixed isolation gap. The electrode pattern is a ring/dot structure such that it uses radial mode for both input and output part that are built-in on the same ceramic disk. A prototype transformer was fabricated of size 15 x 2.78 mm2 having mass of 3.8 gm. In the step-down configuration at the constant output power of 6 W, the transformer characteristics across a 100 omega load were found to be efficiency = 92%, gain = 0.21 input voltage = 110 V(rms), and temperature rise = 20 degrees C from the room temperature. In the step-up configuration at the constant output power of 5 W, the transformer characteristics across a 5 komega load were found to be efficiency = 97%, gain = 9.5, input voltage = 16 V(rms), and temperature rise = 8 degrees C from the room temperature. A detailed equivalent circuit analysis of the transformer was done, and the results were found to be in excellent agreement with the experimental results.