The issue of power quality is now recognised as an essential feature of a successful electric power system. This is mainly due to the rapid increase of loads, which generate noise and, at the same time, are sensitive to the noise present in the supply system. As a result, power quality monitoring has become an important issue in modern power systems.
This paper presents a technique for classifying electrical power quality disturbance events. The technique is based on a novel Self-Adapting Artificial Neural Network (SAANN), which has the unique capability of adapting to new disturbance features.
In the proposed technique, distinctive feature vectors from disturbance events captured are extracted using Fast Fourier Transform (FFT) and Discrete Wavelet Transform (DWT). The feature vectors are then fed to two SAANN based classifiers, which classify the captured events into different categories of power quality disturbances. The proposed technique is tested using a number of disturbance events and results are presented.
A disturbance classification technique employing new SAANNs has been developed and is described in this paper. Unlike other neural networks, the SAANN always learn and adapt itself to new environments. This makes the classification accuracy of the SAANN dependent upon the frequency of occurrence of a certain event. However, this problem can be overcomed if the SAANN is trained using known disturbance signals selected from each disturbance type before installing the classifier on a real system.
The advantage of the SAANN is that training data need not be real disturbance signals, as they only provide a basis to start with. Once the classifier is connected to a real system, it adapts itself to the real data. The features associated with power quality disturbances vary from time to time as new types of disturbance sources may be added to the power system.
The proposed technique has the capability of adapting to new disturbance features by its self-growing technique. In addition, it learns the statistical properties of known disturbances by self tuning the weight vectors with every input disturbance. This makes the proposed SAANN technique ideally suited for power quality disturbance classification.
2.6. A NEW APPROACH TO MONITORING ELECTRIC POWER QUALITY
The paper presents an adaptive neural network approach for the estimation of harmonic distortions and power quality in power networks. The neural estimator is based on the use of linear adaptive neural elements called adalines the learning parameter of the proposed algorithm is suitably adjusted to provide fast convergence and noise rejection for tracking distorted signals in the power networks. Several numerical tests have been conducted for the adaptive estimation of harmonic components, total harmonic distortions, power quality of simulated waveforms in power networks supplying converter loads and switched capacitors. Laboratory test results are also presented in support of the performance of the new algorithm
The single-layer adaptive neural network presents a very realistic and promising approach for fast estimation of power network signal parameters corrupted by noise, decaying dc components and harmonics. The non-linear adaptation of the weight vector for the adaline is performed using a difference error equation and provides an accurate estimation of amplitude and phase of the 3-phase voltage and current phasors corrupted by harmonics and noise.
The learning parameter a is also adapted for providing fast convergence and noise rejection. Numerical simulation tests using the EMTDC software package clearly demonstrate the capability of the algorithm in quantifying power quality from noisy data. Real-time laboratory tests conï¬rm the validity of the new approach for computing harmonic distortions and power quality on-line.
2.7. A CONTROL STRATEGY BASED ON UTT AND ISCT FOR 3P4W UPQC
This paper presents a novel control strategy of a three phase four-wire Unified Power Quality (UPQC) for an improvement in power quality. The UPQC is realized by integration of series and shunt active power filters (APFs) sharing a common dc bus capacitor. The shunt APF is realized using a thee-phase, four leg voltage source inverter (VSI) and the series APF is realized using a three-phase, three leg VSI. A control technique based on unit vector template technique (UTT) is used to get the reference signals for series APF, while instantaneous sequence component theory (ISCT) is used for the control of Shunt APF.
The performance of the implemented control algorithm is evaluated in terms of power-factor correction, load balancing, neutral source current mitigation and mitigation of voltage and current harmonics, voltage sag and swell in a three-phase four-wire distribution system for different combination of linear and non-linear loads. In this proposed control scheme of UPQC, the current/voltage control is applied over the fundamental supply currents/voltages instead of fast changing APFs currents/voltages, there , by reducing the computational delay and the required sensors. MATLAB/ Simulink based simulations are obtained, which support the functionality of the UPQC. MATLAB/Simulink based simulations are obtained, which support the functionality of the UPQC.
The proposed control scheme based on UTT and ISCT for the three-phase four-wire UPQC has been validated through simulation results, using MATLAB software along with simulink and sim-power system toolbox. The performance of the UPQC has been observed to be satisfactory for various power quality improvements like load balancing; source neutral current mitigation, power-factor correction, voltage and current harmonic mitigation, mitigation of voltage sag and swell.
The source current THD is improved from 15.10 % to 4.36 %, while the load voltage THD is improved form 5.38 % to 2.47 %. In addition to this the performance of UPQC has been found satisfactory during transient conditions.
2.8. ARBITRARY WAVEFORM GENERATOR:
An arbitrary waveform generator (AWG) is a piece of electronic test equipment used to generate electrical waveforms. These waveforms can be either repetitive or single-shot (once only) in which case some kind of triggering source is required (internal or external). The resulting waveforms can be injected into a device under test and analyzed as they progress through it, confirming the proper operation of the device or pinpointing a fault in it.
Unlike function generators, AWGs can generate any arbitrarily defined waveshape as their output. The waveform is usually defined as a series of "waypoints" (specific voltage targets occurring at specific times along the waveform) and the AWG can either jump to those levels or use any of several methods to interpolate between those levels.
For example, a 50% duty cycle square wave is easily obtained by defining just two points: At t0, set the output voltage to 100% and at t50%, set the output voltage back to 0. Set the AWG to jump (not interpolate) between these values and the result is the desired square wave. By comparison, a triangle wave could be produced from the same data simply by setting the AWG to linearly interpolate between these two points.
Because AWGs synthesize the waveforms using digital signal processing techniques, their maximum frequency is usually limited to no more than a few gigahertz. The output connector from the device is usually a BNC connector and requires a 50 or 75 ohm termination.
AWGs, like most signal generators, may also contain an attenuator, various means of modulating the output waveform, and often contain the ability to automatically and repetitively "sweep" the frequency of the output waveform (by means of a voltage-controlled oscillator) between two operator-determined limits. This capability makes it very easy to evaluate the frequency response of a given electronic circuit.
Some AWGs also operate as conventional function generators. These can include standard waveforms such as sine, square, ramp, triangle, noise and pulse. Some units include additional built-in waveforms such as exponential rise and fall times, sinx/x, and cardiac. Some AWGs allow users to retrieve waveforms from a number of digital and mixed-signal oscilloscopes. Some AWG's may display a graph of the waveform on their screen - a graph mode. Some AWGs have the ability to output a pattern of words on a multiple-bit connector to simulate data transmission, combining the properties of both AWGs and digital pattern generators.
One feature of DDS-based arbitrary waveform generators is that their digital nature allows multiple channels to be operated with precisely controlled phase offsets or ratio-related frequencies. This allows the generation of polyphase sine waves, I-Q constellations, or simulation of signals from geared mechanical systems such as jet engines. Complex channel-channel modulations are also possible. AWGs may also be contained within music synthesizers.
2.9. THREE-PHASE ELECTRIC POWER:
Three-phase electric power is a common method of alternating current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by electrical grids worldwide to transfer power. It is also used to power large motors and other heavy loads. A three-phase system is generally more economical than others because it uses less conductor material to transmit electric power than equivalent single-phase or two-phase systems at the same voltage The three-phase system was invented by Galileo Ferraris and Nikola Tesla in 1887 and 1888.
In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electric current. This delay between phases has the effect of giving constant power transfer over each cycle of the current and also makes it possible to produce a rotating magnetic field in an electric motor.
Three-phase systems may have a neutral wire. A neutral wire allows the three-phase system to use a higher voltage while still supporting lower-voltage single-phase appliances. In high-voltage distribution situations, it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).
Three-phase has properties that make it very desirable in electric power systems:
The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load. This makes it possible to eliminate or reduce the size of the neutral conductor; all the phase conductors carry the same current and so can be the same size, for a balanced load.
Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations. Three-phase systems can produce a magnetic field that rotates in a specified direction, which simplifies the design of electric motors.
Three is the lowest phase order to exhibit all of these properties. Most household loads are single-phase. In North America and a few other places, three-phase power generally does not enter homes. Even in areas where it does, it is typically split out at the main distribution board and the individual loads are fed from a single phase. Sometimes it is used to power electric stoves and electric clothes dryers. The three phases are typically indicated by colours which vary by country.
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The need for performance arbitrary waveform generation is broad and spans over a wide array of applications. The industry-leading AWG7000 Series arbitrary waveform generators (AWG) represent a cutting edge benchmark in performance, sample rate, signal fidelity, and timing resolution. The ability to create, generate, or replicate either ideal, distorted, or "real-life" signals is essential in the design and testing process.
The AWG7000 Series of AWGs, with up to 24 GS/s and 10-bit vertical resolution, delivers the industry's best signal stimulus solution for ever-increasing measurement challenges. This allows for easy generation of very complex signals, including complete control over signal characteristics. The capabilities of the AWG7000 Series are further enhanced by the addition of key features:
The Equation Editor is an ASCII text editor that uses text strings to create waveforms by loading, editing, and compiling equation files. The editor provides control and flexibility to create more complex waveforms using customer-defined parameters.
