PDF Power Quality Disturbance Classification Using Frequency Domain Features

Introduction

Power Quality

The term "Power Quality" simply focuses on the measurable characteristics of the power, which is provided in a power system. The term 'power quality' is used to describe electrical power that drives an electrical load and the load's ability to function properly.

Objectives of Power Quality

Power Quality Disturbance

Variations in the peak or RMS voltage are both important for different types of equipment. Random or repeated variations in RMS voltage between 90 and 110% of rated voltage can cause a phenomenon known as "flicker" in lighting equipment.

Discussion on Power Quality Disturbances

Voltage Sag

• Voltage Sag Characteristics
• Voltage Sag Magnitude—Monitoring
• Symptoms of dips, sags and surges
• Causes of voltage sags
• Voltage Sag Magnitude Calculation
• Propagation of Voltage Sags
• Critical Distance
• Voltage Sag Duration
• Methods of dealing with dips, sags and surges
• Equipment Voltage Tolerance
• Mitigation of Voltage Sags

The voltage drop during a fall is because there is a short circuit in the system. The voltage starts to return to its original value at the moment, the short circuit fault is rectified by the protection. A power quality monitor will typically calculate the rms value once per cycle. The main problem is that the so-called post-fault case will affect the case duration.

TABLE -2.1.1 Typical Fault Levels at Different Voltage Levels [29]

Voltage Swell

• Causes and Effects of Voltage Swells

Just like voltage issues, voltage surges are usually associated with system fault conditions - but are much less common. This is especially true for ungrounded or floating delta systems, where the sudden change in ground reference results in a voltage increase on the ungrounded phases. In the event of a voltage surge due to a single line-to-ground fault (SLG) on the system, the result is a temporary voltage rise on the uncaught phases, which lasts for the duration of the fault.

A sudden interruption of the current can cause a high voltage according to the formula: V = L(di/dt), where L is the inductance of the line and di/dt is the change in current flow. In addition, feeding a large capacitor bank can also cause a voltage rise, although it more often causes an oscillatory transition. It may cause component failure of equipment power supplies, although the effect may be a gradual, accumulative effect.

This can cause control problems and hardware failures in the equipment due to overheating which can eventually lead to shutdown.

Harmonic Distortion

• Sources of Harmonics
• Effects of Harmonics
• Harmonic Mitigation
• Harmonic Standard

Loads that produce harmonics can be classified into three main categories. loads with magnetic saturation of iron cores. In most cases, the reduction or elimination of harmonics at their source is only effective during the design or expansion phase of a new facility. Active filters are only now becoming commercially viable products for high power applications and operate in the following way to supply frequency current to the load.

For high power applications or for applications where power factor correction capacitors already exist, it is usually more cost effective to use passive filtering. Even with the potential cost of new capacitors, passive filtering still seems to offer the most cost-effective solution to the harmonic problem at the moment. In conclusion, power system harmonics have been carefully studied for many years and have received a significant increase in research and development activity as a direct result of the proliferation of high power semiconductors.

However, as with all power quality problems, accurate study on the "front-end" will usually reveal potential problems at the design stage, and a lower-cost solution can be implemented before problems arise.

Flicker

• Causes of Flicker
• Mitigation Techniques

A heavy electrical load causes significant changes in current over a short period of time, resulting in vibration. The sudden increase in load increases the current in the distribution line which, in turn, increases the voltage drop across the line. Large industrial load located at the end of a weak distribution network can also be a source of vibration.

The use of static capacitors, power electronic-based switching devices and improvement of system capacity are few of them. Flicker effect can also be minimized by changing the design of motors and using advanced solid state technologies. Static VAR compensators can be used to eliminate the flicker and correct the power factor.

Electronically based thyristor switched capacitors are used to provide the reactive power to the power system in a very short period of time which can minimize the effect of rapid load variations.

Transients

• Impulsive transient
• Oscillatory transient
• Effects of Transient Activity

An impulse transient is a non-energizing steady-state change in frequency of voltage, current, or both, having a unidirectional polarity (primarily positive or negative) that occurs suddenly. Due to the high frequencies involved, the circuit components can quickly change the shape of the impulse transitions and can have significantly different characteristics when viewed from different parts of the power system. A transient with a primary frequency component between 5 and 500 kHz with a duration measured in tens of microseconds or several cycles of the principal frequency is called an intermediate frequency transient.

Back-to-back capacitor excitation results in oscillating transient currents in the tens of kilohertz. One of the most common indicators of transient activity is the premature appearance of black "rings" at the ends of the tubes. Sufficiently large transients will cause sputtering of the anodes. When these sputters deposit on the inside of the tube, the result is the black "tips".

Electrical transformers are forced to operate inefficiently due to the hysteresis losses caused by transients and can run hotter than normal.

Interruption

• Short-duration Interruption
• Effects of Interruption
• Sustained Interruption
• Interruption - Prevention and Protection
• Synopsis

Interrupts are further divided into: instantaneous (1/2 to 30 cycles), Momentary (30 cycles to 3 seconds) and transient (3 seconds to 1 minute). Most outages result from reclosing of circuit breakers or reclosers attempting to clear non-permanent faults by first opening and then reclosing after a short time delay. The equipment is usually in the distribution system, but in some locations momentary interruptions also occur due to faults in the sub-transmission system.

On the other hand, a time-delayed reclosing of the protective device may cause a momentary or temporary interruption. In addition, these are specific power system phenomena and have no relation to the use of the term outage. Sustained outages are usually caused by permanent faults due to storms, trees hitting lines or poles, utility or customer equipment outages in the power system, or faulty coordination of protective devices.

In other words, if other customers on the same circuit are also affected, then there is a high probability that the disturbance is due to interruption and not voltage sag.

Notching

Existing Methods

• Support vector machines:
• FkNN classifier:
• Feature selection by GA
• Wavelet Transform

Minimizing the first term of (1) reduces the complexity of the SVM, and minimizing the second term reduces the number of training errors. As the value of m approaches infinity, the classification result is the same as that of the clear k-NN algorithm result. They operate on string structures called chromosomes, usually a concatenated list of binary digits that represent the encoding of the control parameters of a given problem.

The feature vector is formed by applying some of the statistical measures such as energy, entropy, standard deviation and so on, the resulting WPT coefficients. The classification process using all 96 feature vectors may result in less accuracy, as some of the features may have an overlapping nature between two classes, leading to misclassification. Reproduction involves the formation of a new population, usually with the same total number of chromosomes, by selecting from members of the current population following a specific scheme.

The WT multi-resolution analysis is based on decomposing the original signal into different signals at different resolution levels.

Proposed Method

Comparison in between the disturbance signals & normal signal

• Difference in between sag & normal wave
• Difference in between Swell & normal signal
• Difference in between Harmonics & Normal wave
• Difference in between Flicker & Normal signal
• Difference in between Transient & Normal signal
• Difference in between Interruption & Normal signal
• Difference in between Sag with Harmonics & Normal signal
• Difference in between Swell with Harmonics & Normal signal

From the figures we see that the magnitude of the sag wave is smaller than the supply voltage wave for 2.5 cycles. In the figure above we see the curve of the harmonic signal with a normal sine wave.

Solution for extraction of disturbance signals

• Proposed Feature extraction by DCT-domain

Advantages

Experimental Results

Proposed Process

For higher value of DCT coefficients, the accuracy of finding our subjected signal is higher. We consider 100 signals for our feature extraction. So, the size of combined feature matrix becomes 100 × 1602. Using these 50 values, we have the position of our target signals' DCT values ​​in the 1 × 1602 average matrix.

Then, using the positions of the mean matrix, we get the positions of the DCT values ​​in our priority signals. The exact exposed signal was obtained from the DCT value of each signal.

Result

From the table, we see that our proposed method also has higher accuracy in extracting the disturbance signal. A value reported by many power quality monitoring instruments that represents the ratio between the peak value of the measured waveform and the effective value of the waveform. The frequencies involved are created by non-linear loads, or loads where the current waveform does not match the supply voltage waveform.

A condition in which the power system resonates near one of the main harmonics being harmonics produced by non-linear elements in the system, thus exacerbating harmonic distortion. Unwanted electrical signals that produce undesired effects on the circuits of the control systems in which they occur. The ratio of the root mean square (rms) of the harmonic content to the rms of the fundamental quantity, expressed as a percentage of the fundamental.

A steady-state deviation from an ideal power frequency sine wave, characterized primarily by the spectral content of the deviation.

Comparison of classification accuracy with other methods

Conclusion

The advantage of this method is that it can realize short-term detection of power quality disturbances under strong noise, and the detection accuracy is higher than that of DB1 wave analysis, and good accuracy can be achieved in finding the original signal from most distortions. 28] ‘Optimal feature selection for classification of power quality disturbances using a wavelet packet-based fuzzy k-nearest neighbor algorithm’. Demir, “Automatic classification of power quality events and disturbances using wavelet transform and support vector machines.”

Wang, “Digital System for Power Quality Disturbance Detection and Classification,” in Asia-Pacific Power and Energy Engineering Conference,. A connecting connection, whether intentional or unintentional, by which an electrical circuit or equipment is connected to earth, or to a conductive body of relatively large size serving in place of earth. A frequency component of a periodic quantity that is not an integer of the frequency at which the utility system is designed to operate.

An electrical load that draws current intermittently or whose impedance changes throughout the cycle of the input AC voltage waveform.