Artificial intelligence-based approaches to control the neutral current compensator and implementation of power quality conditioner for power quality enhancement

Electricity is by all means one of the most basic needs in today‘s world. As a matter of fact, many social and economic activities depend on the quality of electrical power and efficiency. Providing reliable and efficient power supply to the consumers has become one of the difficult tasks of the power engineers. Due to the advancement in power electronic converters and information technologies, a large number of computer products and nonlinear loads have been connected to power distribution systems. The properties of nonlinear loads affect the performance of power supply and reduce the life and efficiency of the modern-day equipment. The other linear loads which are connected to the Point of Common Coupling (PCC) also are affected and degrade the overall performance of power system. Electric power is distributed through a Three-Phase Four-Wire (3P4W) system in many industrial and commercial sectors. These 3P4W distribution systems can be realized by providing the neutral conductor along with three power lines from the generating station or by utilizing a delta-star transformer in the distribution system. Most of the loads in commercial buildings are connected to any one of the phases of 3P4W distribution system. The loads connected to the distribution systems may not be equal, and they lead to unbalanced load conditions. Due to this unbalance load, some part of current flows in the neutral conductor. If the loads connected to the 2 distribution systems are non-linear, then harmonic component of current flows along with the fundamental component in the neutral conductor. These effects of the neutral current lead to an over burden and over sizing of the neutral conductor and affect the performance of the distribution transformer. As a result, load currents and load voltages are also distorted and the overall performance of the power system gets degraded. To prevent these effects, a lot of methods have been proposed in the past decades. It is not an easy task to prevent these effects completely, but they can be reduced to some extent. In order to protect the equipment from damages, some international standards like IEEE 519-1992, IEC 61000-3-2, IEEE 1159 are proposed. These standards set the limits for the variation of voltage and current distortion, below which the equipment can function without any malfunction. This safety margin of standards provides the good power quality and thereby safeguards the equipment from malfunction. The limits given by the standard IEEE 519-1992 for current harmonic distortion and voltage harmonic distortion are depicted in Table 1.1 and Table 1.2 respectively. Thus, for ensuring good power quality requires, good initial design, effective correction equipment and co-operation with the supplier of its customers. ensure that an electric utility can protect its electrical equipment from overheating and loss of life from excessive harmonic current and harmonic voltage. The current distortion limits are 5% and voltage distortion limits are 3% for individual harmonics set by international standards. ISC is the short 3 circuit current at PCC, IL is the fundamental component of maximum demand load current at PCC and TDD is the Total Demand Distortion.

The development of technology in all its areas is progressing at a faster rate. The various effects of harmonics and diode bridge rectifier are discussed in this chapter. The series active filter which is connected in series with the neutral conductor called as neutral current compensator is designed to enhance the power quality. The switching pulses to the inverter of NCC are obtained by implementing hysteresis controller which uses PWM techniques. The performances of the distribution system without compensation and with RUV based hysteresis controller are simulated using MATLAB/Simulink and the results are analysed.

Electric Power Quality (EPQ) is a set of electrical boundaries that allows a piece of equipment to function in its intended manner without any significant loss of performance or life expectancy. Good power quality can be specified as follows:  The supply voltage should be within guaranteed tolerance i.e.10% in voltage variations and ± 2% of frequency deviation.  The waveform should be pure sine wave within allowable limits for distortion.  Voltage should be balanced in all the three phases.  The earthing system should serve its purpose properly.

Any periodic signal (voltage or current) which changes its magnitude and polarity with respect to time is said to be a waveform (Dugan 2004). The analysis of waveform using Fourier series is given by, power supply which is not aninteger multiple of the fundamental frequency is called interharmonics. It creates new problems such as sub-synchronous oscillations, voltage fluctuations, etc. The difference between the harmonicand interharmonic wave form is depicted in Figure.

The PCC is apointinan electrical system where multiple customers or multiple electrical loads may be connected. According to IEEE-519, this should be a point which is accessible to both the utility and the customer for direct measurement.

A switching disturbance of the normal power voltage wave form that lasts for aperiod less than 0.5 cycles is said to be not ching. It is initially in opposite polarity and is thus subtracted from the normal waveform in terms of the peak value of the distortion voltage. This includes complete loss of voltage upto 0.5 cycles.

Voltage swell is an increase in rms value of electric supply voltage or current from 1.1 to 1.9 puofitsrated value and its durations will be from 0.5 cycles to 1 min. supply voltage or current between 0.1 to 0.9 puofitsrated value and its duration will be from 0.5 cycles to 1 min.

The impression of unsteadiness of visual sensation induced by alight stimulus whose luminance or spectral distribution fluctuates with respect to time is known as flicker. The heavy loads can greatly change the load currents in an electrical distribution system. Voltage flicker occurs when heavy loads are periodically turned ON and OFF in a weak distribution system.

Transients may generate in the system itself or may come from the other system. It is nothing but the sudden rise of an electrical signal. The phenomena with the duration of less than one cycle (of the power-system frequency of 50 Hz) are generally referred to as transients. There are mainly two types of transients i.e., Impulsivetransient and Oscillatorytransient.

Animpulsivetransientis as udden change in the steady state condition of voltage, current or both which is unidirectional in polarity (primarily either positive or negative). Impulsive transients are normally characterized by their rise and decay times.

Oscillatory transients show a damped oscillation with a frequency ranging from a few hundred Hertzto several mega hertz. These types of transients are the naturaltransients in electric power systems. Atypical example of an oscillatory transientis created by energizing a capacitor bank.

Total Harmonic Distortion (THD) is an important figure of meritused to quantify the level of harmonics in voltage or current waveforms. Thetotal harmonic currentdistortionis defined as the squarerootof the sum ofthe squares of the current harmonic components to its fundamental currentcomponents.

THDI =

THDI = total current harmonic distortion, In = Order of currentharmonics (3rd,5th,andso The total harmonic voltage distortionis defined as the square rootof sum of the squares of the voltage harmonic components to its fundamental voltage components. THDv = THDV = total voltage harmonic distortion, Vn= Order of voltage harmonics (3rd,5th,etc.), V1=fundamental voltage component (50Hz).

Electrical power system harmonic problems are mainly due to the substantial increase of nonlinear loads such as the use of power electronic circuits and devices. These kinds of equipment createload-generated harmonics through out the system. Ingeneral, the sources of harmonics are due to  Power Electronic Converters.  Devices which include semi-conductor elements.  Lightening equipment working by gas discharge principle.  Personal Computers.  Electronic ballasts.  Uninterrupted power supplies.  Switching power supplies.  Welding machines and arcfurnaces used in industries.  Control circuits used by switching devices.  Frequency converters.  Static VAR compensators.

Power quality monitoring is the process of gathering, analyzing andinterpreting raw measurement data into useful information. The process ofgathering data is usually carried out by continuous measurement of voltageand current over an extended period. Hence in the last decade many utilitycompanies have implemented extensive power quality monitoring programs. Several common objectives of power quality monitoring are, a. Monitoring to characterize the system perform b. Monitoring to characterize the specific problems c. Monitoring as part of an enhanced power quality service d. Monitoring as part of predictive or just-in-time maintenance.

Most of the house holding equipment‘s and other sensitive components uses diode bridge rectifier for the rectification processes whichare the main causes of the harmonics. The single-phase diode rectifier has become a popular power source for these appliances because of its reduced cost and relatively low sensitivity to supply voltage variations. The thesismainly concentrates on these devices because all the circuits considered for the simulation consists of diode bridge rectifier whichis taken asthe source of the harmonics for the analysis. The effect of diode bridge rectifier with smoothening capacitor is very harmful to the load current as shown infigure 2.6.The harmonic spectrum implies that the load current is affecteddue to 3rd order harmonics of 72%, 5th of 60%, 7th of 40%, 9th of 22.6% and soon. To illustrate the cumulative effect of this type of diode bridge rectifierload, the following case study is analyzed

The analysis on the effect of harmonic sowing to diodebride rectifier as a nonlinear load is simulated using MATLAB. The system parameters are given in Table and MATLAB model of system is shown in Figure 3 The system voltage is taken as 240 V/120 V, 50 Hz. In order to create an unbalance system, the different values of resistances are connected in each phase of the 3P4W distribution system.

To realize the 3P4W distribution system, three single phase diodebridge rectifiers with different values of load resistance is connected to thesystem. The capacitance in the rectifier side isused to remove th eDCripples. Result-1 Result-2

The areas of power quality issues and remedial measures have received considerable attention from the power utilities, power consumers andpower equipment manufacturers over the last decade. The large-scale use of bulk powerthyristor converters and industrial electronic equipment resulted in wave form pollution at all levels in the power systems from mid-eightiesonwards. The problem became more serious with the proliferation of nonlinear loads (rectifiers, arc furnaces, variable speed drives, UPS, computer load, printers and domestic electronic equipment) in the industrial, commercial and residential sectors in the past decade. These loads draw nonsinusoidal currents from the supply and lead to voltage distortion and related system problems. With the widespread use of power electronics at all levels, the polluting loads spread out system wide. Power quality improvement measures concentrated on a few bulk power points and they turned out to be insufficient in mitigating system wide problems. The paper concentrated on two important effects of power quality.The first one is neutral current harmonics which flow due to nonlinear andunbalanced system of the loads in the 3P4W distribution system. The effects of neutral current were realized by modeling a 120V, 50Hz distribution system using MATLAB simulink and the mitigation of these neutral current harmonics has already been discussed in the literature. The seexisting methods have draw backs like circuit complexity, lack of lower order harmonic reduction and they use complex zigzag transformer connections. To overcome these difficulties, neutral current compensator was designed using rotating unit vector-based hysteresis controller, back propagated artificial neural network controller, rotating unit vector with fuzzy logic hysteresis controller and solar PV based fuzzy logichysteresis controllers were proposed. RUV controller was used to extract the harmonic components present in the neutral conductor. The second one is the load voltage harmonics, load current harmonics and unbalanced load which occurs due to non-linearity and unbalanced load present in the distribution system. These effects of load voltage and load current harmonics affect the overall performance of the coupling also get degraded. The various filtering techniques like passive filters, seriesactive filters, shunt active filters, hybrid filters and FACTS controllers were implemented and discussed in the literature as well. Although the existing methods provide the distortionlevels with in the good standards achieving balanced load currents from the unbalanced system was a difficult task which leads to the flow of neutral current. Also, the existing controllers were not able to maintain the constant voltage across the inverter input dc-link capacitor voltage. The proposed method uses unified powerquality conditioner, which compensated both voltage and current harmonic distortion in the distribution system and also proved to be capable of balancing a nun balanced system. The proposed method used synchronous reference frame controller for extracting reference signal for voltage and current compensation along with modified phase locked loop. Fuzzy hysteresis voltage and current controllers were used to generate the switching signals for UPQC inverter circuits. The implementation of fuzzy logic controller in the proposed method was used to optimize the upper and lower bands of the hysteresis controller.

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Associate Professor & HOD CSE, VIT, Bargarh