One-cycle control of three-phase active power filter with vector operation

Active power filters (APFs) provides an effective measure to eliminate the power line harmonic/reactive currents generated by nonlinear loads or by distributed energy sources that are connected to the grid. Active power filters are typically connected in parallel to the harmonic/reactive current sources and cancel the harmonic/reactive components in the line current so that the current flow into and from the grid is sinusoidal and in phase with the grid voltage. Since the APFs process only the harmonic/reactive power, their power-handling capability can be much higher than that of the cascade power-factor-correction methods. In this paper, the one-cycle control method is extended to control three-phase APFs. The proposed control approach employs one integrator with reset along with several logic and linear components to control a voltage-source converter to achieve three-phase unity power factor for the current to and from the power grid. No multipliers or sensors for the load current and the APF inductor current are required. Furthermore, there is no need to calculate the reference for controlling APF inductor current so that complicated digital computation is eliminated. The operation switching frequency is constant that is desirable for industrial applications. The proposed control approach features great simplicity, excellent harmonic/reactive current cancellation, and solid stability. It is a cost-effective solution for power quality control for electronic equipment, buildings, industrial facilities, ships, airplanes, distributed power generation stations, etc. All findings are supported by experimental results.     

基于瞬时无功理论的谐波检测方法研究

谐波检测方法在有源电力滤波器中起着关键作用,分析了基于瞬时无功理论的ipiq谐波检测方法的基本原理,并把他推广到任意次谐波的检测。仿真结果表明这种检测方法具有很好的稳态精度和动态性能,即使在电网频率变化的情况下,采用频率跟踪技术后ipiq法也可以快速准确地检测出电网总体谐波或任意次谐波分量。     

An analytical method for calculating harmonic currents of athree-phase diode-bridge rectifier with DC filter

Commonly, three-phase diode bridge rectifiers with an LC filter at the DC side are often used to convert AC input into a DC voltage. It is well known that they generate large amounts of harmonic currents. This paper proposes an analytical method for calculating the harmonic currents for both the continuous and discontinuous current conductions. The equations for the harmonic currents are derived, taking into account the effects of the DC and AC side impedances. All the calculations are conducted only by algebraic calculation with high accuracy. The validity of the proposed method is demonstrated by comparison to the results of time simulation     

Influence of the Electrical Vehicle Charging on the Grid Harmonic

This paper described the impact of the electrical vehicle (EV) charging on the grid harmonic. In view of the randomness of the EV charging process, the harmonic admittance matrix method and superposition method were used to build the single and multiple EVs charging simulation model. By using Matlab as a simulation tool, we analyzed harmonic currents of single and multiple EVs chargers. The results show that the harmonic ratio is beyond the scope of the national harmonic standard. Finally a parallel hybrid active filter (PHAPF) was introduced for governance of harmonic. The experimental results show that net side harmonic currents are significantly reduced by using the PHAPF and meet the national standard GB/Z17625.6-2003 regulations limit.     

The development of a tunable CW magnetron in the K-band region

The development of a low field, K-band, tunable cw magnetron is described. The major characteristics of the device are presented. The research leading towards the development of the magnetron is outlined. This research program had two major objectives: 1) the study of cathode back-bombardment in cw magnetrons, 2) the study of low field operation. The cathode back-bombardment was found to be closely related to two voltage regions at which large anode currents were drawn without any connection to magnetron operation. The close study of these effects made it possible to eliminate them in cw magnetrons through a particular choice of magnetron parameters. The research program culminated in the development of a low field, K-band, tunable cw magnetron, utilizing capacitive Crown of Thorns tuning. At a fixed operating point, an output power exceeding 10 watts at an efficiency exceeding 5 per cent can be obtained over a tuning range of 7 per cent. The tuning range can be doubled by lowering the output power requirements and making use of two operating points.     

Enhanced harmonic generation in magnetrons

An X-band rising sun harmonic-generator magnetron has been developed. In order to obtain enhanced second harmonic output power, the magnetron was designed so that the resonant frequency of the second-order π mode was twice the frequency of the first-order π mode. Second harmonic efficiency, the ratio of second harmonic output power to input power, was strongly magnetic field dependent. Large values of second harmonic efficiency and output power occurred for operation in the "valley of efficiency" of the rising sun anode, i.e., for magnetic fields near B=13,000/λ1gauss where λ1is the wavelength of the first-order π mode. Maximum second harmonic efficiency and output power at λ2=1.69 cm were 13 per cent and 46 kw, respectively. For operation at these magnetic fields the ratio of generated second harmonic to fundamental power was of the order unity. This large power ratio can be understood in terms of a phase shift between the fundamental voltage and the current which drives it, together with a modified rotating wave hypothesis where a waving motion, caused by the Γ=0 Hartree component of the azimuthal electric field, is superimposed on the conventional rotation of the space-charge spokes.     

Online grid measurement and ENS detection for PV inverter running on highly inductive grid

Photovoltaic (PV) and other sources of renewable energy are being used increasingly in grid-connected systems, for which stronger power quality requirements are being issued. Continuous grid monitoring should be considered so as to provide safe connections and disconnections from the grid. This letter gives an overview of the methods used for online grid measurement with PV inverters. Emphasis is placed on a method based on the injection of a noncharacteristic harmonic in the grid. Since this injection is regarded as a disturbance for the grid, different issues, i.e., the influence on total harmonic distortion (THD), the accuracy of line impedance measurement and the ENS (German abbreviation of Main Monitoring units with allocated Switching Devices) detection are studied. Laboratory results conducted on an existing PV inverter are presented to demonstrate the behavior of the PV inverter under different grid conditions. Some of the injection parameters are tuned in order to get an accurate measurement of line impedance.     

Third Harmonic Injection Increasing the Efficiency of High-Power HF Amplifiers

The paper deals with the influence of the third harmonic injection into the grid and anode qirclits of a tuned high power amplifier, in order to improve the efficiency. The linear triode model is used, enabling avaluation of all power balance important parameters in a closed form. The results obtained are compared with the performances of the same amplifiers, without injection.     

两种电铁谐波及无功电流检测方法的研究与仿真

针对电气化铁道供电系统对谐波及无功电流检测的要求,介绍了两种检测方法,并分别给出了检测原理图。前者是基于鉴相原理的瞬时谐波电流检测法,后者是基于瞬时无功功率理论的谐波分离法,对该方法基于不同截止频率时的LPF进行了分析比较。理论分析和仿真结果证明两种方法都能实时地检测出电铁供电系统中的瞬时谐波及无功电流,前者算法和实现更为简单。稳态运行的仿真波形证实了文中所提方法的有效性。     

Control of distributed generation systems - Part II: Load sharing control

This work is concerned with the control strategy for the parallel operation of distributed generation systems (DGS) in a standalone ac power supply. The proposed control method uses only low-bandwidth data communication signals between each generation system in addition to the locally measurable feedback signals. This is achieved by combining two control methods: droop control method and average power control method. The average power method with slow update rate is used in order to overcome the sensitivity about voltage and current measurement errors. In addition, a harmonic droop scheme for sharing harmonic content of the load currents is proposed based on the voltages and currents control algorithm. Experimental and simulation studies using two parallel three-phase pulsewidth modulation (PWM) inverters are presented to show the effectiveness of the proposed control.     

An analysis of a super wide–band FM line discriminator

This paper presents a dynamic analysis of an extremely wide-band FM line discriminator consisting of a pair of transmission lines and coupling resistors. The relationship between the harmonic distortion of the output waveform of the discriminator and the permissible frequency deviation of the input signal is analyzed in detail by the method of Fourier transform. It is shown that the coupling resistor constant γ (see Fig. 1) should be set at unity from the viewpoint of the sensitivity of the discriminator and the constant input impedance of the discriminator over the frequency deviation of the input signal. Theoretically, an FM signal having 100 per cent frequency deviation could be detected by this discriminator with 2.67 per cent harmonic distortion while the input impedance of the discriminator remains constant (equal to characteristic impedance of the transmission line Z0) during the frequency deviation. Experimental results are presented to verify the theoretical results obtained. The characteristic curve of the discriminator is linear for 40 per cent frequency deviation from the carrier frequency 85.5 Mc, and its input impedance remains within ±14 per cent of the idea value Z0.     

An adaptive detecting method for harmonic and reactive currents

The conventional detecting methods for power system harmonic and reactive currents have various limitations. Based on adaptive interference canceling theory, a new adaptive detecting method for harmonic and reactive currents is presented, the corresponding detection system is developed, and experiments are performed which verify the feasibility of the method proposed. The new real-time detecting method is useful for dynamic compensation equipment such as active power filters, static VAr compensators, etc     

Short term braking capability during power interruptions for integrated matrix converter-motor drives

The new trend in adjustable speed drives (ASD) is to integrate the inverter and the motor into a single unit in order to reduce the production cost, the commissioning time, and the physical size of the equipment. This last issue becomes more important, making the matrix converter topology more attractive. Sinusoidal input currents and bidirectional power flow are other advantages of the matrix converter but it is less immune to power grid disturbances compared to a standard ASD. In hoisting applications, short-term braking capability during a power outage is needed until the mechanical brake engages or to perform more effective a combined braking. This paper proposes a new method to provide short-term braking capability during a power outage for matrix converters. A braking chopper is needed in the clamp circuit, which allows for a drastically reduction of the capacitor size. The power flow in the clamp circuit may be reduced by increasing the harmonic content in the motor currents, which causes higher motor losses. Experiments prove the validity of the proposed method.     

Low-harmonic, three-phase rectifier that applies current injection and a passive resistance emulator

A new three-phase diode bridge rectifier that provides low harmonic distortion of the input currents applying current injection technique is proposed in this paper. The rectifier applies a novel passive resistance emulator consisting of four diodes and a transformer with the volt-ampere rating of 3.57% of the rectifier-rated power. Optimization of the transformer turns ratio is performed in order to minimize the input current total harmonic distortion (THD). It is shown that with the optimal turns ratio the input current THD equals 3.72%. Dependence of the input current THD on the load current is analyzed, and it is shown that at low load currents the rectifier operates in the discontinuous conduction mode with the THD of 7.79%. The analytical results are experimentally verified on a 2 kW rectifier, indicating that the input current THD lower than 8% is provided within a wide range of the load current variations applying simple circuitry.     

Anode sputtering in high-voltage mercury arc valves

A method is described whereby the anode sputtering rate during recovery of a mercury arc valve can be estimated from its transient inverse current and voltage waveforms. Inverse currents have been measured in an experimental valve, from which corresponding anode sputtering rates are deduced for a variety of cathode coolant temperatures and commutation conditions. These measurements substantiate proposed methods of reducing anode sputtering in valves. The accuracy of the method has been confirmed by examining sputtered deposits found in an experimental valve after several hundred hours of rectifier operation. Major applications of this method are in the development and testing of valves for high-voltage direct current (hvdc) transmission. These include rapid assessment of the effect of design modifications on anode sputtering, and increasing the anode sputtering rate by a known factor through manipulation of the test conditions (e.g., cathode coolant temperature and dI/dt), thereby considerably shortening the time required for sputtering life tests. The latter application is particularly valuable in the case of single-gap valves where the effects of anode sputtering during recovery are likely to be the ultimate limitation on serviceable life. The saving in testing time is especially important because of the high operating cost of test plant at relevant levels, in our case 50 kV, 1000-ampere dc bridge rating.     

基于谐波测量的改进余弦窗函数

电网电压监测设备在监测运行设备的电流电压波形时通常采用谐波分析法,针对传统FFT谐波分析法在电网频率波动时谐波测量会产生栅栏效应和频谱泄漏的问题,在研究余弦窗特性的基础上提出了一种改进3项余弦窗。通过频谱衰减曲线与传统余弦窗对比,采用基于数据样本的非线性拟合方法,拟合出了改进窗插值算法的谐波幅值相位修正系数,使改进窗插值算法能实际应用于谐波测量。通过谐波测量仿真对比,验证了加改进余弦窗的谐波测量准确度更高。     

A Unified Artificial Neural Network Architecture for Active Power Filters

In this paper, an efficient and reliable neural active power filter (APF) to estimate and compensate for harmonic distortions from an AC line is proposed. The proposed filter is completely based on Adaline neural networks which are organized in different independent blocks. We introduce a neural method based on Adalines for the online extraction of the voltage components to recover a balanced and equilibrated voltage system, and three different methods for harmonic filtering. These three methods efficiently separate the fundamental harmonic from the distortion harmonics of the measured currents. According to either the Instantaneous Power Theory or to the Fourier series analysis of the currents, each of these methods are based on a specific decomposition. The original decomposition of the currents or of the powers then allows defining the architecture and the inputs of Adaline neural networks. Different learning schemes are then used to control the inverter to inject elaborated reference currents in the power system. Results obtained by simulation and their real-time validation in experiments are presented to compare the compensation methods. By their learning capabilities, artificial neural networks are able to take into account time-varying parameters, and thus appreciably improve the performance of traditional compensating methods. The effectiveness of the algorithms is demonstrated in their application to harmonics compensation in power systems     

基于STM32的电网谐波检测仪的研究

电力系统在运行过程中,会产生很多谐波,严重影响到电能的质量,为了方便实时监测谐波以及加强谐波管理,谐波检测仪就显得尤为重要。以传统单片机设计的谐波检测仪的电参数测量方法精度不够高,越来越难以满足目前电网检测的需要,而以DSP设计的检测仪成本高、难推广。针对这种现状,设计了一种多功能、低成本、高精度的电网谐波检测仪,它是基于最新Cortex-M3内核的STM32,并结合了三相多功能高精度计量芯片ADE7878,具有很强的实时性、精确度,为电力系统谐波的检测和抑制提供了重要的依据。     

Coupled Circular Cylindrical Rods Between Parallel Ground Planes

The normalized self and mutual capacitances of periodic, circular cylindrical rods located between parallel ground planes are presented graphically. The capacitances were determined by solving the appropriate integral equation by numerical methods. Charts of self and mutual capacitance are given for rod diameter-to-ground plane spacing ratios varying from 0.05 to 0.8 and for very small to very large spacings between rods. Accuracy of the data is believed to be generally better than 2 per cent for the normalized mutual capacitance and generally better than 1 per cent for the normalized self capacitance. An approximate design method is also presented that permits using the data to synthesize filters (such as interdigital and comb-line filters) that require rods of nonequal diameters and spacings. An example of the design method is given, and a filter is constructed from the resulting data. The filter response was measured and found to agree closely with that called for by the theory.     

A Flexible Selective Harmonic Mitigation Technique to Meet Grid Codes in Three-Level PWM Converters

Due to the development of new grid codes, power converters' output signal harmonic control is currently becoming extremely important in medium- and high-power applications. By taking this new scenario into account, a new method to generate switching three-level pulsewidth-modulation (PWM) patterns to meet specific grid codes is presented. The proposed method, which is named selective harmonic mitigation PWM , generates switching three-level PWM patterns with high quality from the point of view of harmonic content, avoiding the elimination of some specific harmonics and studying all harmonics and the total harmonic distortion as a global problem by using a general-purpose random-search heuristic algorithm. This fact leads to a drastic reduction or even avoidance of the bulky and costly grid connection tuned filters of power systems. Any harmonic shaping can be considered due to the flexibility of the method. Power devices switching constraints are considered to obtain directly applicable results. As a practical example, limits from one actual grid code have been used to get the experimental results by means of a 150-kVA three-level diode-clamped converter test bench. Comparisons between the proposed technique, optimized PWM and Selective Harmonic Elimination methods have been carried out. The results obtained with this new method greatly improve previous ones.