Filtering and Quality of Electrical Energy PDF

Introduction The fundamental objective of power grids is to provide customers with electrical energy with perfect continuity, in the form of sinusoidal voltage, with pre-established amplitude and frequency values. However, this objective seems ideal and is never easy to achieve, because...

Introduction The fundamental objective of power grids is to provide customers with electrical energy with perfect continuity, in the form of sinusoidal voltage, with pre-established amplitude and frequency values. However, this objective seems ideal and is never easy to achieve, because the electricity grid today is called upon to operate in an increasingly aggressive environment, and consequently, it must face many types of disturbances that can be of internal origin such as the evolution and complexity of the network and the loads connected to it, or external events related to climate change phenomena. Thus, to ensure the objective of quality energy, it is imperative to understand the characteristics, origins, and effects of the various disturbances in order to seek the appropriate remedies. We will start this course with a brief introduction to the basics of power quality. We will then expose the main disturbances affecting the quality of the electrical wave, in particular the harmonies for which we will be particularly interested. We will also talk about their origins, their effects and the standards in force. We will then discuss different solutions considered to overcome the problems related to harmonic disturbances. Definitions: Electrical Power Quality (EEQ) refers to the compliance of the characteristics of the electricity supplied with the standards and specifications required to ensure the stable and efficient operation of electrical equipment. It is measured by various parameters that evaluate the purity, stability, and continuity of the electrical voltage supplied with a near-sinusoidal waveform of the voltages and currents of the power distribution bus at nominal amplitude and frequency. The quality of electrical energy is closely related to the quality of the voltage wave, which is characterized by the following parameters: Perfectly sinusoidal waveform; no distortions, peaks, troughs and amplitude within tolerable limits. Perfect balance and symmetry of the phases in amplitude and phase; Frequency stability. 1. Aspects of electrical power quality: Key aspects of electrical power quality include: Voltage rating: The voltage supplied should be close to the specified rating (e.g., 220 volts AC). Voltage stability: Voltage variation should be kept within acceptable limits to ensure stable operation of electrical equipment. Page 1  Harmonic distortion: The ideal electrical signals are sinusoids, but harmonic distortions can occur due to the presence of non-sinusoidal waves. A high level of harmonic distortion can affect the operation of sensitive equipment. Power factor: The power factor measures the efficiency of converting electrical energy into useful work. A low power factor can lead to inefficient use of energy. Continuity of service: This is the constant availability of electricity without unwanted interruptions. It is affected by fortuitous interruptions, long or short, and scheduled interruptions. It is measured by the number of power cuts at a given point: cut-off/unit of time (example: 5 cuts/month). Frequency regulation: The frequency of electricity must be kept close to the nominal value (e.g., 50 Hz) to ensure the proper functioning of the equipment. Overvoltage and undervoltage: The presence of sudden voltage variations can damage electrical equipment. Grounding quality: Proper grounding is essential to ensure the safety and quality of the electrical grid. Monitoring and controlling these parameters are essential to ensure standard-compliant electrical power quality and to maintain reliable operation of electrical systems. 2. Voltage Wave Quality Ability to supply continuous and satisfactory power to appliances that use electricity. In practice, distributed electrical energy is in the form of a set of voltages constituting a three-phase alternating system, which has four main characteristics: Amplitude Frequency Waveform Symmetry 2. Different disturbances of electrical energy 1. Frequency In the ideal case, the three voltages are alternating and sinusoidal with a constant frequency of 50 or 60 Hz depending on the country. Frequency variations can be caused by significant production losses, the islanding of a generator on its auxiliaries or its transition to a separate network, or a fault whose resulting voltage drop leads to a reduction in load. Frequency quality is usually specified in electrical standards and is closely monitored in power grids to ensure system reliability and stability. Automatic frequency regulation devices are used to maintain frequency at acceptable levels, and synchronization protocols are implemented to ensure consistency between different parts of the power grid. The frequency quality of the electrical voltage encompasses several key aspects: Frequency stability: The frequency should remain close to the nominal value, for example, 50 Hz in many parts of the world. Significant variations in frequency may cause synchronization problems in electrical systems and affect the operation of equipment. Page 2  Frequency fluctuation Frequency fluctuations are characterized by variations in the nominal value of the fundamental frequency of the network (50 or 60 Hz), resulting from variations in the speed of the alternators, following an imbalance between loads and powers brought into play by the power plants, or following the start or stop of a large load. They manifest themselves in disturbances in the speeds and torques of synchronous machines, and sometimes in the complete shutdown of the systems. Frequency regulation: The ability of the power system to maintain frequency at a constant level, even in the face of significant load changes. Synchronization: Frequency synchronization between different parts of the power grid is crucial to ensure coordinated and stable operation. Rejection of frequency harmonics: The absence of unwanted frequency harmonics, which are multiple frequency components of the fundamental frequency, is important to ensure a clean and stable power supply. Transient response: The ability of the system to quickly restore frequency after temporary disturbances, such as large load variations. 2. Symmetry A three-phase voltage system (currents) is said to be balanced if the amplitudes of the three voltages/phase currents are equal and are separated from each other by an electrical angle of 120° in phase. 3. Waveform The waveform of the three voltages forming a three-phase system must be as close as possible to a sinusoid. In the event of waveform disturbances, the voltage is no longer sinusoidal and can generally be considered as a fundamental wave at 50Hz associated with waves of a frequency multiple integers of 50 Hz called harmonics or is not an integer multiple of 50 Hz (interharmonics). Voltages can also contain permanent but non-periodic signals, which are then called noises. 4. Amplitude The voltage amplitude is an important factor for the quality of electricity. It is generally the first contractual commitment of the energy distributor. Usually, the amplitude of the voltage should be maintained within a range of ±10% around the nominal value. In the ideal case, all three voltages have the same amplitude, which is a constant. However, several disruptive phenomena can affect the amplitude of voltages. Depending on the variation in amplitude, two main families of disturbances can be distinguished: Voltage dips, power outages and surges. These disturbances are characterized by significant variations in amplitude. They are mainly caused by short circuits and can have significant consequences for electrical equipment. Voltage variations. These disturbances are characterized by variations in the amplitude of the voltage of less than 10% of its nominal value. They are usually due to fluctuating loads or changes Page 3  A. Disturbances (Definitions and Origins) 1 Disturbances: Disturbances are all phenomena internal or external to the network, having the power to temporarily or permanently modify the voltage or current wave. Classification These disturbances can be classified according to two criteria: 1.1.1 Depending on the duration of persistence: a) Periodic disturbances (which last over time), such as: ✓ Harmonic distortions ✓ Voltage drops due to the flow of reactive power into the grid ✓ Imbalances b) Aperiodic disturbances (all fugitive phenomena that are often very difficult to predict), such as: ✓ Voltage dips ✓ Transient overvoltages 1.1.2 According to their manifestations (consequences on electrical quantities): ✓ Disturbances on the amplitude of the voltage-Flicker ✓ Imbalance of three-phase systems 2- Voltage Disturbances 2.1 Voltage dips and outages A voltage sag is a sudden decrease in the amplitude of the voltage to a value between 10 and 90% of its nominal value. A voltage dip is characterized by a depth ∆V and a duration ∆T taken conventionally greater than 10 ms and usually less than 3 min. The short break is a special case of voltage dip with a depth greater than 90%. Brief cuts, lasting between 1 s and 3 minutes. long outages, lasting 3 minutes or more. Figure 2.1 Voltage Dips  Origin The main causes of voltage dips are disturbances due to the operation of the networks, such as the energization of large transformers, short circuits, the activation of capacitors, the simple starting of large motors. etc. Belongings Voltage dips have the effect of disturbing torques in rotating machinery, tripping contactors, untimely failures... etc. 2.2 Power surges Overvoltage is any voltage applied to equipment whose peak value exceeds the limits of a range (One +10%). Surges have three natures: 1) Temporary power surge rate: It is a sudden increase in the RMS value of the voltage on one or more phases (more than 110% of the nominal voltage) for a duration of 3 sec to 1 min, they are at the same frequency as that of the network. Origin: ▪ A lack of insulation between phase and ground, ▪ Ferro-resonance caused by an inductive circuit and a capacitor, ▪ Overcompensation of reactive energy, 2) Surge of operation, Shunting overvoltages result from a modification of the structure of the electrical network, commissioning of a capacitor, a vacuum line. 3) Atmospheric overvoltage Overhead distribution networks are the most affected by overvoltages and overcurrents of atmospheric origin (lightning). Origin: ▪ Direct lightning strikes (on a line or on a structure). ▪ The indirect effects of a lightning strike (induced surges and increases in the earth's potential). Consequences : - Dielectric breakdown. - Long cut-off - Disruption of control, command and communication circuits. - Uncontrolled stopping or starting of electrical machines. 2.3 Voltage variation 2.3.1 Slow voltage variations Definition : An increase or decrease in voltage caused by a slow change in the total load of the distribution system or part of it. Origin : Raising or decreasing the electrical load (variation) on the electrical network. Typical values: ✓ 95% of the 10-minute RMS averaged values should be within the defined range of rated voltage One +/- 10%. ✓ 100% of the 10-minute averaged RMS values must be within the +10% range and 15% (compared to One). +10% =On e -10% -15% Figure 2.2Example for typical 230V voltage values Consequences: These variations are often acceptable for equipment. 2.3.2 Rapid Voltage Variations-Flicker Voltage fluctuations are rapid and repetitive variations in the RMS value of the voltage. These variations have a moderate amplitude (usually ±10%), but can occur several times per second, and can be cyclical or random. This voltage fluctuation is observed through the phenomenon of flickering, which reflects an impression of instability of visual tension due to a light stimulus whose brightness or spectral distribution fluctuates over time. Figure 2.3 Voltage Fluctuation Origin: Voltage fluctuations are caused primarily by loads that vary rapidly and continuously in their current demand. The most well-known examples are welding machines, arc furnaces, frequent start motors, laser printers, microwaves,... etc. 3- Imbalance A three-phase voltage system (currents) is said to be balanced if the amplitudes of the three voltages/phase currents are equal and are separated from each other by an electrical angle of 120° in phase. There is an imbalance in the system of three-phase voltages or currents, when asymmetries of amplitudes and/or phases are recorded in a steady state. Figure 2.4 Imbalance of the three-phase electrical system Origins : Imbalances are mainly caused by asymmetries of the impedances of the network lines, asymmetry of the impedances of the loads, and also by single-phase or two-phase short circuits. Examples: ✓ Poor distribution of LV single-phase connections. ✓ Poor phase balancing for three-phase connections. ✓ Poor distribution of public lighting. ✓ Fusion fusible BT. ✓ Wrong connection (bad contact). 3.1 Three-phase voltage unbalance rate The three-phase voltage unbalance rate measures the disparity between the voltages of the three phases of a three-phase electrical system. In an ideal and balanced three-phase system, the voltages of the three phases are equal in amplitude and out of phase with each other in an equivalent way. However, imbalances in amplitude, phase, or both can occur, and Unbalance rate defined by NEMA (National Equipment Manufacturers Association) NEMA defines the voltage three-phase system unbalance by :  A= Maximum deviation of the RMS values of compound voltages With respect to the Average Value of the RMS Values of the Compound Voltages Average Value of the RMS × 100 Values of the Compound Voltages This rate of imbalance in a three-phase electrical network can be obtained as follows: 1. Measure the three compound RMS voltages (phase-phase). 2. Calculate the average voltage. 3. Determine the voltage that has the greatest deviation from the calculated average voltage. 4. Calculate the ratio between this difference and the average voltage. If the ratio is greater than 2%, a three-phase voltage imbalance problem is considered to exist. Example 1. Measured voltages between phases: Vab = 600 V; V bc = 630 V; VAC = 570 V 2. Vm = (600 + 630 + 570) / 3 = 600 V 3. ΔV = 630 V – 600 V = 30 V 4. A = (30 V ÷ 600 V) × 100 = 5% (very high imbalance) 4- Frequency variations This type of disturbance is extremely rare, it can be observed when the sum of the power produced in the electricity grid is different from that consumed. Figure 2.5 Frequency variations 5- Harmonics and inter-harmonics Harmonics are permanent disturbances affecting the waveform of the network voltage. These disturbances result from the superposition, on the fundamental wave (50Hz or 60Hz), of waves that are also sinusoidal but of frequencies that are multiples integer of that of the fundamental. The superposition of these waves on the fundamental wave results in a deformation of the latter. This distortion is called 'Harmonic Distortion'. Figure 2.6 Synthesis of a signal from harmonics  Origin: Harmonics come mainly from nonlinear loads from integrating equipment such as basic power electronics elements (diodes, thyristors,... etc.), nonlinear resistors, saturated transformers... etc. whose characteristic is to absorb a current that does not have the same shape as the voltage that supplies them. This current is rich in harmonic components whose spectrum will depend on the nature of the charge. These harmonic currents flowing through the impedances of the network create harmonic voltages that can d isrupt the operation of other users connected to the same source. Interharmonics are also sinusoidal waves, but of frequencies not multiples of the fundamental frequency. They are also responsible for harmonic distortions and their main sources are static frequency converters, induction furnaces, arc furnaces, etc. etc. 4.1 Harmonic distortions: Harmonic distortion is any steady-state deviation of the ideal sinusoidal shape of the voltage or current wave. Belongings: The consequences of harmonics can be instantaneous or long-term. Instant effects: ▪ Functional synchronization or switching disorders, ▪ Untimely disjunctions. ▪ Measurement errors on energy meters, ▪ or even the destruction of equipment (capacitors, circuit breakers). ▪ Vibrations and noises of electromagnetic devices. ▪ Noise in telecommunications networks and damage to image (television) and sound quality Long-term effects: The most important long-term effect is of a thermal nature, which results in heating leading to premature fatigue of the equipment and lines and leads to decommissioning of the equipment. ▪ Heating of cables and equipment (transformers, motors,... etc: ▪ Heating of capacitors caused by hysteresis losses in the dielectric. ▪ Heating of rotating machinery (caused by static and rotor losses), and transformers.  B. Effects of Disturbances Regardless of the disturbance, the effects can be classified in two different ways: Instantaneous effects: untimely operation of contactors or protective devices, malfunction or stoppage of a machine. Delayed effects: energy losses, accelerated ageing of the material due to heating and Efforts Electrodynamic Additional Generated by the perturbation. 1 Effects of voltage dips and outages The main consequences of voltage dips and outages on the main equipment used in the industrial, tertiary and domestic sectors: 1.1 Asynchronous motor ❖ During a voltage dip, the torque of an asynchronous motor (proportional to the square of the voltage) decreases abruptly and causes a slowdown, the motor can stop (stall) if the motor torque becomes less than the resisting torque. ❖ After a cut, the return of the voltage generates a reacceleration current draw close to the starting current and the duration of which depends on the duration of the cut. ❖ When the installation has many motors, their simultaneous reacceleration can cause a voltage drop in the upstream impedances of the network which lengthens the duration of the trough and can make reacceleration difficult (long restarts with overheating) or even impossible (motor torque lower than the resisting torque). ❖ These overcurrents and the resulting voltage drops have consequences for the motor (additional heating and electrodynamic forces in the coils that can cause insulation breaks and jolts on the torque with abnormal mechanical stresses on the couplings and gearboxes, resulting in premature wear or even breakage) but also on other equipment such as contactors (wear or even welding of the contacts). ❖ Overcurrents can lead to the tripping of the general protection of the installation, thus causing the process to stop. 1.2 Synchronous motor ❖ The effects are more or less identical to the case of asynchronous motors. However, synchronous motors can withstand larger voltage dips (around 50%) without losing the possibility of over- excitation and the proportionality of their torque with the voltage due to their generally higher inertia. ❖ In the event of a stall, the engine stops, and the whole starting process must be resumed, which is quite complex. 1.3 Actuators  ❖ Control devices (contactors, circuit breakers equipped with low-voltage coils) supplied directly from the network are sensitive to voltage dips whose depth exceeds 25% of One. Indeed, for a conventional contactor, there is a minimum voltage value to be respected (known as the deposition voltage) below which the poles separate and then transform a voltage dip (of a few tens of milliseconds) or a short break into a long break (of several hours). 1.4 Computer equipment ❖ Computer equipment (computers, measuring devices) now occupies a predominant place in the monitoring and control of installations, management and production. These devices are all sensitive to voltage dips that are deeper than 10% of Un. The Information Technology Industry Council (ITI) curve indicates, in a time-amplitude plane, the typical tolerance of computer equipment to voltage dips, cuts, and overvoltages. Operating outside these limits leads to data loss, incorrect commands, shutdown or failure of devices. Fig. 4 : Typical tolerance defined by the ITI curve 1.5 Variable Speed Machines The problems posed by voltage dips applied to variable speed drives are: ❖ Inability to supply sufficient voltage to the motor (loss of torque, retardation), ❖ Inability to operate control circuits supplied directly from the grid, ❖ Overcurrent on return of the voltage (recharging of the filter capacitor of the dimmers), ❖ Overcurrent and current imbalance in the event of voltage dips on a single phase, ❖ Loss of control of DC drives in inverter operation (energy recovery braking). Variable speed drives usually fail for a voltage drop of more than 15%. 1.6 Lighting ❖ Voltage dips cause incandescent lamps and fluorescent tubes to age prematurely. ❖ Voltage dips with a depth of 50% or more and a duration of about 50 ms cause discharge lamps to go out. 2- Effects of Harmonics The harmonic currents associated with the different impedances of the network will give rise, according to Ohm's law, to harmonic voltages, which will be added, or deduced, from the fundamental voltage generated by the network. The resulting voltage is no longer sinusoidal and moreover this voltage is common to all the other receivers in the network. The pollution then present on the distribution network is harmful to the proper functioning of all the receivers connected to this same network. The standard sets the harmonic voltage values that must not be exceeded to ensure the correct operation of the receivers. In the context of the supply of electricity, the harmonic voltage levels must not exceed the values specified in the following table. These values represent individual rates calculated in relation to the fundamental at 50 Hz, bearing in mind that the overall voltage harmonic rate must not exceed 8% in a low- voltage distribution installation. The individual harmonic rate values are given in the following table. Their consequences are related to the increase in peak values (dielectric breakdown) and RMS values (additional heating) and the frequency spectrum (vibration and mechanical fatigue) of voltages and currents. Their effects always have an economic impact due to the additional cost linked to: ❖ A deterioration in the energy efficiency of the installation (energy losses), ❖ Oversizing of equipment, ❖ A loss of productivity (accelerated aging of equipment, untimely tripping). Above a voltage harmonic distortion rate of 8%, malfunctions are likely. Between 5 and 8%, malfunctions are possible. 2.1 Instant or short-term effects: ✓ Untimely activation of the protections: ✓ Perturbations Induced some Systems at Current Low (remote control, telecommunication, computer screen, television). ✓ Abnormal acoustic vibrations and noises (LV switchboards, motors, transformers). ✓ Destruction by thermal overload of capacitors. If the natural frequency of the capacitor- upstream network assembly is close to a harmonic rank, there is resonance and amplification of the corresponding harmonic. ✓ Loss of accuracy of measuring devices. 2.2 Long-term effects: A current overload causes additional heating and therefore premature aging of the equipment: ✓ Heating of sources: transformers, alternators (by increasing Joule losses, iron losses), ✓ Mechanical fatigue (pulsating torques in asynchronous machines), ✓ Heating of the receivers: of the phase conductors and the neutral by increasing joule and dielectric losses. ✓ Capacitors are particularly sensitive to harmonics because their impedance decreases proportionally to the harmonic rank. ✓ Destruction of equipment (capacitors, circuit breakers, etc.). ✓ An overload and additional heating of the neutral conductor can be the consequence of the presence of harmonic currents 3 and multiples of 3 present in the phase conductors that  are added in the neutral. 3 Effects of Power Surges Their consequences are very diverse depending on the time of application, repetitiveness, amplitude, etc.: ❖ Dielectric breakdown, cause of destruction of sensitive equipment (electronic components, etc.), ❖ Degradation of equipment by aging (non-destructive but repeated overvoltages), ❖ Long outage caused by the destruction of equipment (loss of invoicing for distributors, loss of production for manufacturers), ❖ Disturbances in low-current control and communication circuits. ❖ Electrodynamic and thermal (fire) stresses caused by: ✓ Lightning essentially : Overhead networks are the most affected by lightning, but installations supplied by underground networks can experience high voltage stresses in the event of lightning strikes near the site. ✓ Surge of operation which is repetitive and whose probability of occurrence is much higher than that of lightning and of longer duration. They can lead to damage as significant as lightning. 4 Effects of Voltage Variations and Fluctuations: Since the fluctuations have an amplitude of no more than ± 10%, most devices are not disturbed. The main effect of voltage fluctuations is : ❖ Fluctuation in the brightness of the lamps (flicker): Physiological discomfort (visual and nervous fatigue) depends on the amplitude of the fluctuations, the frequency of repetition of the variations, the spectral composition and the duration of the disturbance. ❖ Variations in the RMS voltage value can have undesirable effects on the torque and speed of rotating machinery, 5 Effects of Imbalance: ❖ Malfunctions ❖ Vibrations, noises ❖ Poor operation of a single-phase device powered by a very low voltage. ❖ Destruction of a single-phase device powered by too high a voltage. ❖ Concerning three-phase power electronics devices, mainly rectifier bridges, operation in the presence of imbalance leads to the appearance of harmonics of multiple rank 3. ❖ The consequence of the reverse components on rotating machinery is the creation of a field rotating in the opposite direction to the normal direction of rotation, resulting in parasitic braking torque and additional losses that cause the cables and the machine to heat up  Origins of Power Quality Deterioration 1- Concept of linear and nonlinear loads 1.1 Origin of deforming loads Resistors, inductors and capacitances, when combined, form so-called "linear" loads. That is to say, under sinusoidal voltage, they consume currents of sinusoidal shapes identical to the voltage. Figure 3.1 Signals relating to a linear load. Today, with the contribution of electronics integrated into many electrical devices, charges produce distorted currents whose appearance is no longer sinusoidal. These currents are then composed of harmonics, multiples of the frequency of the fundamental (50 Hz for example). Figure 3.2 Signals relating to a nonlinear load. The distinction between linear and nonlinear loads is made: or by recognizing the type of load when the integrated technology is known. or by measurement in order to verify the characteristics of the current called by the load. Some examples of equipment responsible for signal distortion : ✓ Power electronics converters. ✓ Welding machines, arc furnaces. ✓ Switched-mode power supplies in the tertiary and industrial sectors as well as in household appliances. ✓ All equipment with semiconductor devices. 1.2 A so-called linear load application A so-called linear load corresponds to any category of load found through conventional receivers such as electric convectors, incandescent lamps or any receiver with resistive, inductive or capacitive elements such as electric motors. Also, the linear charge, when subjected to a sinusoidal voltage, absorbs a current of the same pace. There is thus, at all times, proportionality between voltage and current. Figure 3.3 Proportionality between voltage and current for a so-called linear load. The power absorbed corresponding to these two electrical quantities, voltage and current, is the simple product of these components for a so-called linear load associating simple purely resistive elements. A phase shift exists when we observe a shift between two signals on the same circuit in relation to the other in time. Figure 3.4 Phase difference between voltage and current. 1.2.1 Power Factor: The phase difference observed then refers to the power factor involved in the calculation of the power absorbed by the receiver in addition to the components: voltage and current. The power factor k of a circuit is the ratio of the active power P to the apparent power S. This is always less than or equal to 1. 1.2.2 Cosφ: The phase shift between the voltage and the current if they are sinusoidal is denoted .  is taken positive for inductive phenomena. This phase shift gives rise to the existence of active, reactive and apparent powers. Since only active power is required, reactive power should be combated in order to reduce apparent power. 1.2.3 Active power The active power consumed by a linear receiver in the sinusoidal regime is given by the following relationships: In single-phase: P = U × I × cos φ In three-phase: P = √3 × U × I × cos φ 1.2.4 The apparent power: The apparent power being calculated as follows: Where P is the active power and Q is the reactive power. 1.3 A so-called nonlinear load application The so-called nonlinear charge corresponds to a type of charge composed of semiconductor elements, essential components of electronic devices. The nonlinear load subjected to a sinusoidal voltage absorbs a current called deformed. There is therefore more proportionality between voltage and current. Figure 3.5 Non-linearity between voltage and current for a deforming load 2- Nonlinear Load and Deformed Power 2.1 Deforming Power D By developing this notion of power factor, we see the appearance of a new term D in the following expression, materializing the deforming power:  ▪ This deforming power D reflects the effects of the harmonic distortion on the installation under consideration. The degradation of the power factor value is therefore increased, on this type of installation, compared to an installation with only linear loads. This is due to the presence of harmonics from nonlinear loads. A distorted signal is the sum of the sinusoidal signals, amplitudes, frequencies, and multiples of the frequency of the fundamental signal.  Solutions for improving power quality Harmonics There are three possible ways to eliminate them, or at least reduce their influence: 1- Reduction of the harmonic currents generated 2- Modification of the installation 3- Filtering 1- Reducing Generated Harmonic Currents 1-1 Placing Inductors in the Installation Setting up an inductor in series with the power supply reduces the harmonics of the current (especially those of high rank). The inductor acts as a filter by providing greater impedance at high frequencies, thus limiting the passage of harmonics and helping to mitigate their impact on the electrical system of the power circuit. 1-2 Using Twelve-Phase Rectifiers The principle consists of using a transformer with two secondaries delivering voltages offset by 30° between them, each of these secondaries feeds a Graëtz bridge rectifier which performs a hexaphase rectification. Rectifiers must provide identical direct currents so that the alternating currents they draw from the transformers' secondaries have the same values. Under these conditions, there is a recombination of the harmonic currents, generated by each rectifier at the primary of the transformer. The line current has a shape closer to a sinusoid than the current obtained with a single rectifier. 2- Modification of the installation The modification strategy consists of modifying the characteristics of the loads or sources, in order to reduce the generation of harmonics and their propagation in the network. It is preferable to connect pollutant loads as far upstream as possible to limit the propagation of disturbances throughout the network, thus preserving the quality of the energy for other users. 3- Filtering 3-1 Passive filtering 3-1-1 Working Principle: It consists of achieving a low impedance at the frequencies to be attenuated thanks to the arrangement of passive components (Inductance, Capacitor, Resistance) in order to constitute one or more traps for the harmonics whose propagation is to be avoided, so that the current on the source side remains as close as possible to the sinusoidal shape. This assembly is placed in a branch on the network. Multiple passive filters in parallel may be required to filter multiple components. The sizing of harmonic filters must be careful: a poorly designed passive filter can lead to resonances whose effect is to amplify frequencies that were not annoying before its installation Principle Characteristics Bypass by a passive filter tuned to each No harmonic current limits harmonic frequency to be eliminated Reactive energy compensation assured. Elimination of one or more harmonic ranks (usually: 5, 7, 11). A filter for one or two rows to compensate. Benefits Economical: low cost of the component used. No monitoring to ensure during operation Disadvantages Sensitivity to variations in the impedance of the electrical network and harmonic decontamination is only effective for the current installation (Adding or removing loads may render the solution inoperative). Risk of harmonic amplification in the event of a network modification. Often difficult to implement in existing plants 3-1-2 Calculation of passive filters The calculation of a passive filter consists of sizing its RLC elements. The parameters taken into account in this calculation are often : 1. The nominal voltage V (fundamental) of operation, i.e., the voltage at the busbar where the filter is connected. 2. The reactive power QF that this filter is called upon to compensate. 3. The harmonic rank(s) hn to which this filter is tuned. 4. The quality factor Q of the filter, which determines the sharpness of the passive filter setting, is calculated by: Xn: Characteristic reactance of the filter R : value of the resistor in the circuit The quality factor is between 30 and 100. 3-1-2 Possible passive filter configurations Type Scheme Resonant filter Tuned to a specific frequency, and therefore capable of trapping (eliminating) a single harmonic. High-pass filter Tune to a frequency from which it must trap all the higher harmonics, thus capable of eliminating several harmonics. Dual Resonance Filter Tune to two distinct frequencies to be able to eliminate two harmonics. Type C filters It is also a high-pass filter but with better characteristics (compensation of reactive power, and less losses). 3-1-3 Resonance phenomenon The operation of passive filters (L-C) including inductors, capacitors, and sometimes resistors relies on the "resonance phenomenon" that occurs due to variations in the frequency of inductors and capacitors. The reactance XL of an inductor L is greater the higher the frequency f : The reactance XC of a capacitor C is smaller the higher the frequency: When this filter resonates against a harmonic of hn rank, the corresponding reactances, inductive (XLn) and capacitive (XCn ) are equal, and therefore, the resistance, which is generally low, is the only impedance in the circuit. We then draw the relations: We then deduce the characteristic reactance of the Xn filter as : The resonance of this filter will take place at the hn rank : hn : Harmonic rank f1 : Fundamental frequency (50 Hz for example) fn: Resonant frequency of the filter at the hn rank ω1 : Pulsation of the fundamental frequency (ω1 =2πf1 ) QC : Capacitance of the capacitor SCC: The short-circuit power of busbar This resonance will therefore take place at the rank hn, if the inductive reactance is chosen as : The phenomenon of resonance gives rise to extreme values of voltage and current. Depending on the nature of the scheme (parallel resonance or series resonance), the resulting overcurrents and overvoltages can cause serious damage to the installation (destruction of capacitors for example). 3-1-4 Series resonance: The simplified expression of the overall impedance, obtained from the elements L and C in series, is expressed by the following relationship: At resonance, the numerator is e q u a l t o z e r o , T The total impedance ZT tends towards a minimum value as illustrated in the figure opposite (Impedance scan). For this particular frequency fr, the current is only limited by the low resistance of the circuit, which is usually low, so the current value will be high. Resonant Series Filter Calculation Calculating such a filter for a resonance at a given frequency follows the following steps : Ih n 1. Calculates the capacitive reactance : From the necessary reactive power required QC by the harmonic source, the capacitive reactance of the filter is deduced as: 2. Calculate the inductive reactance : To trap the harmonic of rank hn, the inductive reactance of the filter is: 3. Calculation of Resistance : The resistance of the filter is deduced from the quality factor Q of the filter by: The capacity (Var) of the filter can then be deduced by: Since , The impedance of the resonant filter is given at each frequency - of rank h - by : The ZF module is : Neglecting the resistor R (R ≈ 0), the voltage across the capacitor is given by: V1 : The busbar voltage at the fundamental frequency. At the resonant frequency this voltage then becomes : With; VC1 The voltage of the fundamental component across the capacitor Vbus 1 The tension of the fundamental component of the busbar VCn The voltage across the capacitor at the resonant frequency Vbus n The busbar voltage at the resonant frequency Xn The characteristic reactance of the filter Q The quality factor of the filter  Th Example : Filter granted to 11 harmonic, with QC = 2 Mvar, V= 33 kV, Q=60 V2 XC √XCXL 544,5 XC = = 544, 5 Ω, XL = = 0.825 Ω 𝑍𝐹(ℎ) = 0,825 + 𝑗 (4,5ℎ − ) 2 = 4.5 Ω, R = 𝑄𝐶 ℎ𝑛 𝑄 ℎ 3-2 Active Filtering Introduction: The disadvantages of passive filters (non-adaptation to load and lattice variations, resonance phenomenon) and the appearance of new semiconductor components, such as GTO thyristors and IGBT transistors, have led to the design of a new filter structure called active power filters (APFs). Working Principle: Active filters are single/three-phase inverters acting as a source of current or voltage by injecting harmonic components in series or parallel into the electrical network in opposition to those generated by the pollutant load, therefore, the harmonic components of the load are eliminated and the current from the source will acquire an almost sinusoidal shape. Figure 4.7. Basic diagram of a parallel active filter An active filter consists of two main parts : ▪ A power part : Often a voltage inverter connected to the common coupling point (PCC) with a capacitive DC bus, ▪ A control part : which consists of calculating the harmonic current of the nonlinear load in real time and forcing the inverter to inject it in phase opposition to the grid so that the sum iF+iL is always sinusoidal. 𝑖𝐿 = 𝑖1 + 𝑖ℎ 𝑖𝐹 = −𝑖ℎ ⇒ is = iL + iF = i1 i1 : Fundamental component of charging current Benefits of these active filters: The physical volume of the filter is smaller. The filtering capacity is higher. Active filters react in real time (actively) to the harmonics present to eliminate them; (Very good flexibility and adaptability) However, they have some disadvantages : - Their high cost has limited their implementation in industry. - The losses are higher. Active Filter Classification Active filters can be classified according to several criteria, such as : Its configuration in relation to the network. The type of inverter he uses. The number of threads. Regarding the configuration in relation to the network, a distinction is made between the active shunt filter, the serial active filter, and the universal active filter. Shunt Filtering (Parallel): Figure 4.8. Active shunt filtering (Parallel) The inverter is connected to the grid through an inductive filter, and possibly a transformer. On the DC side, the inverter uses a capacitance as a voltage source. The purpose of this configuration is to decouple the disturbances caused by the pollutant load from the upstream electricity grid. The pollutant load represented by a rectifier supplying a load RL absorbs a current iL which contains, in addition to the fundamental, harmonic components which the active filter must eliminate by injecting a harmonic current if in the opposite direction into the network, thus the source current is remains sinusoidal and possibly in phase with the voltage. Serial filtering: Figure 4.8. Serial Active Filtering This configuration is used to ensure a quality voltage wave. The voltage inverter is connected to the grid through a passive LC-type filter and a series transformer. The active filter must inject a certain voltage vc to the disturbed voltage wave vL. This topology can also act as a dynamic voltage restorer to protect the load from potential voltage dips or cuts. Universal filtering Figure 4.9. Universal Active Filtering This configuration is a combination of an active shunt filter and a serial active filter. This structure improves the quality of the tension. The main functions of the serial filter are: ✓ Isolate the harmonics between the source and the pollutant load. ✓ Compensate for reactive power and unbalanced voltages. The role of the shunt filter is: ✓ Absorb harmonic currents. ✓ Compensate for reactive power.  ✓ DC bus voltage regulation between the two active filters. 3-3 Hybrid Filtering The hybrid filter recombines between active and passive filters to reduce the size and cost of filtering. One or more passive filters are tuned for the removal of the most persistent harmonics. The active filter follows the evolution of the load and the high harmonic ranks. Advantages of the hybrid filter: Combines the advantages of passive and active filtering solutions and covers a wide range of power and performance. Filtering over a wide frequency band (elimination of harmonics from ranks 2 to 25). Reactive energy compensation. Great current filtering capability. Good technical and economic solution for "network" filtering. 3-3-1 Filters hybrid shunt Figure 4.10. Hybrid shunt filter In this topology : The passive shunt filter is used to eliminate the lowest rank (the 5th ' for example).  The active shunt filter is designed for other high ranks. The active filter can protect against possible resonance between the passive filter and the network impedance. 3-3-2 Hybrid Filter Series Figure 4.11. Hybrid Filter Series The series hybrid filter is composed of a series active filter and a shunt passive filter. The active crossover is used to eliminate the problems caused by the passive crossover (such as resonance and the influence of source impedance), and to improve compensation performance. The role of the passive filter is to deflect harmonic currents through its ability to locally modify the impedance of the network. 3-4 Active Filter Converters: There are two types of converters that can be used as an active filter : A voltage inverter : Uses a DC voltage source represented by a capacitance, Fig (a). This inverter is more used for its better efficiency, size, and price. A current inverter : Uses a current source represented by the inductance Fig (b). Figure 4-12 Applicable Power Converters for Three-Phase Active Filters (a) voltage inverter, (b) current inverter. Harmonic distortion standards Nomes are guidelines, set up by technical expertise organizations, that define the limits of stability and reliability of the electricity grid. These directives concern the producer, distributor and consumer of electrical energy. Objectives of the standards: To limit the levels of harmonic distortions to reduce their effects on the network and loads. Norme IEEE (Institute of Electrical and Electronics Engineers) Sets limits for individual and total voltage distortion rates. Sets the individual and total current distortion rate limits for nonlinear loads at the PCC. These limits depend on the ratio (ISC/IL) between the maximum short-circuit current ISC of the grid and the load current IL 1) Distribution networks  Parallel Active Filter Principle of parallel active filtering: Figure. Active shunt filtering (Parallel) The parallel active filter is designed to provide the if compensation current to cancel out the harmonic components of the ich nonlinear load current, such that the current supplied by the iS array is sinusoidal. The current absorbed by the load is given by : icha : active component of ich ichr : reactive component of ich Harmonic components of the current Ich The current supplied by the source is equal to: The parallel active filter provides the deforming power and reactive power: General description of the Voltage Structure Parallel Active Filter: Active filters are composed of inverters which are static power converters powered by a continuous source of current or voltage that act as harmonic current sources in phase opposition to the load current in order to restore a quasi-sinusoidal source current. structure of the active filter consists of two parts : Power part Control part Power part The power part consists of: 1- a voltage inverter based on power switches, controllable at ignition and blocking (GTO, IGBT,... etc.) with antiparallel diodes. 2- an energy storage circuit, often capacitive. 3- a coupling filter. Control part The control-command part consists of: 1- the method of identifying disturbed currents. 2- DC voltage regulation applied to energy storage elements. 3- the regulation of currents injected into the grid from the voltage inverter. 4- The voltage inverter control. Three-Legs Voltage Inverter Each Leg of the inverter has two bidirectional current switches, controlled at closing and opening. The energy storage on the DC side is done through a Cdc capacitor having at its terminals a voltage denoted vdc, regulated to a positive value. This capacitor acts as a direct voltage source. The control of the two semiconductors of the same arm is done in a complementary way, that is, if the first is open the other is closed. With this principle, the opening and closing of the switches depends on the status of the control signals (S1, S2, S3): The voltages between phases, imposed by the inverter, are then defined by : The output voltages of the inverter, denoted vfk , with k = {1,2,3} , are referenced with respect to the grid neutral and verify the following equations: Since the network voltages are assumed to be balanced and given that the sum of the currents injected by the inverter is zero, we can write: We then deduce the following relationship : The equations of the output voltages of the inverter as a function of the states of the switches are then expressed by : We can express eight possible cases of the output voltage of the active filter Vf (referred to the neutral n of the source), as shown in the table below, where Vf is the vector representation of the voltages supplied by the inverter (vf1, vf2, vf3) in the orthogonal coordinate system (α, β). Figure (.....) represents this vector in the coordinate system (α, β).  Figure (.....): Vector representation of the voltages provided by the three-arm voltage inverter Mathematical Model of the Parallel Active Filter in the Three-Phase Coordinate System The phase voltage equation of the three-phase parallel active filter is given by : Then, the equations of the three phases are given by : Techniques for extracting harmonic currents Identification in the frequency domain: - Discrete Fourier transform (DFT) ,Fast Fourier transform (FFT) - Recursive discrete Fourier transform (RDFT) - Filter of Kalman Inappropriate : The large number of computational iterations and the slow response time compared to the time required for real-time filtering applications. Time domain identification: Advantages over frequency-domain methods : - Speed - Minimal calculation time 1) Method of instantaneous active and reactive powers : Transition from three-phase systems consisting of simple voltages and line currents to a two-phase system (αβ coordinate system) using the Concordia transformation, in order to calculate instantaneous powers. 2) Synchronous frame method : The three-phase currents of the load are expressed in the system (dq) using a phase-locked loop (PLL) in each phase to detect the angular position θ of the Synchronous frame DQ. The latter is synchronized with three-phase voltages and rotates at a constant speed. 3) Notch filter method : The charge current is filtered by a notch filter (Notch filter) which eliminates the fundamental component while allowing the harmonic components to pass through.