Iec tr 61000 2 8 2002

Compatibilité électromagnétique CEM –Partie 2-8: Environnement – Creux de tension et coupure s brèves sur les réseaux d'électricité publics incluant des résultats de mesures statistiques

Source des creux de tension

La source principale des creux de tension observés sur le réseau public est le court-circuit électrique se produisant à un point quelconque dans le réseau d'alimentation électrique.

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r.m.s value of the voltage on an electricity supply system specified for the purpose of defining the end of a voltage dip

NOTE Typically, the value used for the end threshold has been the same as the start threshold or has exceeded it by 0,01 of the reference voltage.

The measurement of voltage dips and short interruptions is crucial in electricity supply systems A voltage dip is defined as a situation where the root mean square (r.m.s.) value of the voltage across all phases falls below a specified threshold, categorizing it as a short interruption.

2.7 residual voltage (of voltage dip) minimum value of r.m.s voltage recorded during a voltage dip or short interruption

NOTE The residual voltage may be expressed as a value in volts or as a percentage or per unit value relative to the reference voltage.

2.8 depth (of voltage dip) difference between the reference voltage and the residual voltage

NOTE 1 The depth may be expressed as a value in volts or as a percentage or per unit value relative to the reference voltage.

The term 'depth' is often used descriptively to refer to the voltage dimension of a voltage dip, without specifying whether it indicates residual voltage or depth It is important to clarify this meaning based on the context in which it is applied.

The duration of a voltage dip refers to the time interval between when the voltage at a specific point in an electricity supply system drops below a defined start threshold and when it subsequently rises back to the end threshold.

In polyphase events, the approach to defining the start and end of a voltage dip can differ It is anticipated that future practices will establish that a dip commences when the voltage of at least one phase drops below the designated dip start threshold and concludes when the voltage across all phases rises to or exceeds the dip end threshold.

(voltage dip) sliding reference voltage

The measurement of voltage dips and short interruptions involves continuously calculating the root mean square (r.m.s.) value of the voltage at a specific point in an electricity supply system over a defined interval This calculation serves to represent the voltage value immediately before a dip, which is essential for establishing a reference voltage.

NOTE The specified interval is much longer than the duration of a voltage dip.

3 Description of voltage dips and short interruptions

The primary source of voltage dips observed on the public network is the electrical short circuit occurring at any point on the electricity supply system.

Short circuits lead to a significant increase in current, which in turn causes substantial voltage drops across the impedances of the electrical supply network The occurrence of short circuit faults is inevitable in electrical networks, arising from various causes Fundamentally, these faults involve a failure in the dielectric insulation between two structures that are intended to be isolated from each other and are typically maintained at different potentials.

Beaucoup de courts-circuits sont provoqués par des surtensions, qui sollicitent l'isolation au- delà de son aptitude La foudre atmosphérique est une cause notable de telles surtensions.

Alternatively, insulation can be weakened, damaged, or destroyed due to various climatic effects such as wind, snow, ice, and salt fog Additionally, it may suffer from impacts or contact with animals, vehicles, and excavators, as well as deterioration caused by aging.

A typical electrical grid transmits energy from various sources, such as power plants, to multiple loads, including motors, resistive elements for lighting and heating, and power supply modules for electronic devices The entire network, comprising generators, loads, and their connections, functions as a unique, integrated, and dynamic system Any change in voltage, current, or impedance at one point instantly affects every other point within the system.

Most networks operate as three-phase systems Short circuits can occur between phases, between a phase and neutral, or between a phase and ground One, two, or all three phases may be involved in these incidents.

A l’endroit du court-circuit, la tension s'effondre effectivement à zéro Simultanément, à presque chaque autre point du réseau la tension est réduite de même ou, plus généralement, à un degré moindre.

Power circuits are equipped with protective devices that disconnect the energy source in the event of a short circuit Once this disconnection occurs, the voltage is immediately restored to its previous level at all points except those that are disconnected Some faults are self-extinguishing, meaning the short circuit disappears and the voltage is restored before the disconnection takes place.

La réduction soudaine de tension, suivie du rétablissement de tension, comme décrit précé- demment, est le phénomène connu sous le nom de creux de tension.

The activation of large loads, transformer magnetization, the startup of large motors, and significant fluctuations associated with certain loads can all cause substantial current variations similar to a short-circuit current Although the effect is typically less severe at the point of occurrence, the resulting voltage changes at specific locations may be indistinguishable from those caused by a short circuit, thus being classified as a voltage dip However, in public network management, supply conditions impose permissible limits on these voltage fluctuations.

Durée du creux de tension

À moins qu'un défaut auto-extincteur ne soit impliqué, la durée des creux de tension est régie par la vitesse du fonctionnement des dispositifs protecteurs.

In electrical networks, protection devices include fuses and circuit breakers controlled by various types of relays Protection relays are typically designed with an inverse time characteristic, meaning that the lower the short-circuit current, the longer the fault-clearing time Fuses exhibit similar characteristics The time characteristics and settings of fuses and relays are meticulously organized and coordinated to ensure that a short circuit detected by multiple devices is cleared at the most appropriate location.

Short circuits lead to a significant surge in current, resulting in substantial voltage drops across the supply system's impedances These faults are inevitable in electrical systems and can arise from various factors, primarily due to a failure in the insulation between two structures that are meant to remain electrically isolated and operate at different potentials.

Many short circuits are caused by overvoltages, which stress the insulation beyond its capacity.

Atmospheric lightning is a significant source of overvoltages, while other weather conditions such as wind, snow, ice, and salt spray can also compromise insulation Additionally, factors like animal contact, vehicle impacts, and the wear and tear from aging can further weaken or damage insulation.

The electricity supply system is a dynamic and integrated network that transmits energy from various generating stations to multiple loads, including motors, lighting, heating elements, and electronic devices Any alteration in voltage, current, or impedance at one location within the system immediately affects all other points, highlighting the interconnected nature of the entire system.

Most supply systems are three-phase systems The short circuit can occur between phases, phase and neutral, or phase and earth Any number of phases can be involved.

During a short circuit, the voltage at the fault location drops to zero, while the voltage at nearly all other points in the system also decreases, typically to a lesser degree.

Supply systems utilize protective devices to isolate short circuits from the energy source Once this disconnection occurs, the voltage at all points, except those that are disconnected, quickly recovers to nearly its original value.

Some faults are self-clearing: the short circuit disappears and the voltage recovers before disconnection can take place.

The sudden reduction of voltage, followed by voltage recovery, as just described, is the phenomenon known as voltage dip (also known as voltage sag).

Large loads, transformer energization, and the starting of large motors can cause significant current changes akin to short circuit currents While the immediate effects are typically less severe, the voltage fluctuations observed at specific locations can resemble those caused by short circuits, leading to their classification as voltage dips Public network management imposes limits on permissible voltage fluctuations to ensure stable supply conditions.

Unless a self-clearing fault is involved, the duration of voltage dips is governed by the speed of operation of the protective devices.

Protective devices, primarily fuses and circuit breakers, are managed by various types of relays These protection relays typically feature an inverse time characteristic, meaning that a lower short circuit current results in a longer fault clearance time Fuses exhibit similar behavior The time characteristics and settings of both fuses and relays are meticulously calibrated and coordinated to ensure that when a short circuit is detected by multiple devices, it is cleared at the most suitable point.

Many short circuits are cleared within the time range of 100 ms to 500 ms Faster clearing times are often achieved for short circuits on main transmission lines, while the clearing of short circuits in distribution networks can be significantly slower.

Quand une fluctuation de courant autre qu'un court-circuit est la source du creux de tension, la durée est régie par l'événement qui en est la cause.

Certain loads draw a significant inrush current when voltage is restored after a dip, which delays the voltage recovery and extends the duration of the voltage sag A similar effect occurs when transformers enter saturation during the voltage restoration process.

Amplitude du creux de tension

L’amplitude du creux de tension est régie par la position du point d'observation par rapport à l'emplacement du court-circuit et à la ou aux sources d’alimentation.

Le réseau peut être représenté par un circuit équivalent simple reliant le point d'observation à une source unique équivalente et au site du défaut (voir la Figure 1) La tension entière

The voltage drop at the observation point is influenced by the relative magnitudes of the two impedances connecting it to the source and the short circuit This distribution of voltage, which can range from 0% to 100%, is determined by the impedance between the source and the short circuit.

Creux de tension aux points d'observation O 1 et O 2 pour un court-circuit en F et source équivalente unique en S

(exprimés en termes de tension résiduelle unitaire).

Figure 1 – Circuit équivalent pour un creux de tension

Generally, the closer the observation point is to the short circuit, the closer the voltage at that point is to the fault voltage In other words, the voltage dip approaches the maximum possible depth (zero residual voltage) near the short circuit Conversely, if an observation point is near a generating source or stored energy sources, such as rotating machines, the effect is to bring the observation point closer to the equivalent single source.

Figure 1 illustrates how this mitigates the severity of the observed voltage dip However, if the duration of the dip is extended, an increase in current occurs due to the deceleration of the motors, which can exacerbate the severity of the dip.

The visibility of a voltage dip caused by a short circuit at a specific observation point depends on its location within the network A short circuit in a transmission network can lead to significant voltage dips that may impact a large area, even hundreds of kilometers away In contrast, a short circuit in a distribution network affects a much smaller region Observation points within the same network may experience substantial dips, but the severity of these dips will be considerably reduced on neighboring circuits, and at greater distances, the dips will be barely noticeable.

Short circuits are typically cleared within 100 ms to 500 ms, with quicker clearance times observed on major transmission lines In contrast, the clearance of short circuits on distribution circuits tends to be significantly slower.

When a current fluctuation other than a short circuit is the source of the voltage dip, the duration is governed by that of the causative event.

Some loads draw a large inrush of current as the voltage recovers at the end of a voltage dip.

Delaying voltage recovery and prolonging voltage dips can occur when transformers enter saturation during the recovery process.

The magnitude of the voltage dip is governed by the position of the observation point in relation to the site of the short circuit and the source(s) of supply.

The system can be represented by a simple equivalent circuit connecting the observation point to a single equivalent source and to the site of the fault (see Figure 1.) The entire voltage

The voltage drop at the observation point is influenced by the impedance between the source and the short circuit, with 100% of the voltage dissipated across this impedance The extent of the voltage dip can vary from 0% to 100%, depending on the relative magnitudes of the impedances connecting the observation point to both the source and the short circuit.

Voltage dips at observation points O 1 and O 2 for short circuit at F and single equivalent source at S (expressed in terms of residual voltage per unit.)

Figure 1 – Equivalent circuit for voltage dips

The proximity of the observation point to a short circuit site significantly influences the voltage levels, with the voltage at the observation point nearing zero as it approaches the fault location Conversely, when the observation point is closer to a generation source or stored energy, such as rotating machines, the voltage dip is less severe, as it resembles the effect of a single equivalent source However, if the voltage dip persists for an extended period, decelerating motors may draw increased current, potentially exacerbating the dip's severity.

The impact of a short circuit on voltage levels varies based on the locations within the supply system A short circuit occurring in the transmission system can cause significant voltage dips that are detectable over extensive areas, even hundreds of kilometers away Conversely, short circuits in distribution circuits have a limited observable effect While observation points on the same distribution circuit may experience pronounced voltage dips, neighboring circuits will see a marked reduction in dip severity, and at greater distances, the voltage dip becomes nearly imperceptible.

At a specific observation point within or near a private facility, it is indeed possible for a short circuit or similar event to occur within that facility The voltage dip observed may equal or exceed a dip caused by a short circuit in the public transmission or distribution network.

The magnitude of the observed voltage dip is influenced by the specific phases involved, both from the short circuit and the observation point, as well as the winding connections (such as star-delta or star-star) of all transformers between these two locations.

3.3.1 Importance du raccordement des transformateurs et des charges

The amplitude of the voltage dip observed from a specific event varies depending on whether the observation point and the event are on the same side or on opposite sides of the network or customer transformer The sequence of phases during a short circuit or any other event, the measurement system's configuration, and the connection methods of the primary and secondary windings of the transformer are all crucial factors in this context.

For a transformer connected in Dyn or Dy configuration, a ground fault on a single conductor can result in a voltage dip of 0 V (residual voltage) on one phase at the primary side However, on the secondary side, the voltage between phases and neutral on two phases can reach up to 58% of the original voltage.

In industrial installations, loads that are sensitive to voltage dips, such as converters, power controls, motors, and control equipment, are often connected between phases As a result, these loads experience phase-to-phase voltage dips rather than phase-to-neutral dips It is essential to consider this distinction when noting whether measurements are taken phase-to-neutral, between phases, or both.

Table 1 summarizes the voltage dips observed on the secondary side of various step-down transformers, caused by a phase-to-ground fault on a single primary conductor, resulting in a 100% voltage drop on phase 1 The power supply network is assumed to be a directly grounded neutral system.

Tableau 1 – Tensions secondaires d'un transformateur avec un défaut d'une phase à la terre au primaire

Tension phase-neutre Tension phase-phase

Coupures brèves

A circuit breaker or fuse operates by disconnecting a section of the network from the power source In a radial network, this action halts the supply to all downstream areas Conversely, in a mesh network, multiple disconnections are required to eliminate the fault Consequently, electricity users connected to the disconnected section experience a power outage.

In aerial networks, automatic re-energization sequences are commonly applied to circuit breakers that interrupt fault currents These sequences aim to restore the network to normal conditions as quickly as possible in the event of a transient fault, such as an arc caused by a surge that does not cause serious or permanent damage to the involved components If the initial re-energization attempt is unsuccessful, additional attempts may be made at preset intervals Should the fault persist after the pre-established open-close operation sequence is completed, the circuit breaker remains open and will not close until necessary repairs are made at the fault location It is important to note that each re-energization attempt while the fault is still present causes an additional voltage dip, with the observed depth depending on the observation point's location.

In addition to the actual isolation of the fault, various maneuvers are often carried out, either automatically or manually, to minimize the extent of the network affected and the number of users interrupted as a result of the initial fault removal action.

A single fault can lead to a complex series of maneuvers, perceived by users as interruptions of varying durations Depending on the network structure, the specific case, and the individual positions of users relative to the fault locations and involved switches, some users may experience very brief outages, while others might have to wait for repairs to restore service.

Des interruptions ayant une durée jusqu’à 1 min (ou, dans le cas de certaines ré-alimen- tations, jusqu’à 3 min) sont classifiées conventionnellement comme coupures brèves.

Causes des creux de tension et des coupures brèves

As previously established, the primary cause of voltage dips, which can occasionally lead to brief outages or are associated with them, is the significant current surge involved in a short circuit within an electrical network, along with occasional large-scale load fluctuations.

L'écoulement du courant à travers les impédances des composants de réseau a comme conséquence des chutes de tension, qui diminuent pour un instant la tension fournie aux utilisateurs de l'électricité.

Dielectric breakdown in short circuits occurs due to stress from overvoltage or the weakening, damage, or failure of insulation in various ways The causes of defects leading to these outcomes are numerous.

– événements atmosphériques: foudre et tempête de vent, neige, glace, dépôt de sel ou polluants atmosphériques sur des isolateurs, débris transportés par le vent;

– interférence et dommages mécaniques: collision par véhicule, élément de construction, excavateur, animaux et oiseaux, branches d’arbres, vandalisme et actes de malveillance;

– panne de réseau dans une usine: détérioration due au vieillissement, corrosion, putréfaction, vices latents de fabrication ou de construction;

– accidents ou erreurs de fonctionnement et de maintenance;

– Événements naturels d'importance: inondations, éboulements, tremblements de terre, avalanches.

A circuit breaker or fuse functions by disconnecting a portion of the system from its energy source, leading to a complete supply interruption for all downstream components in a radial circuit In contrast, a meshed network requires multiple disconnections to effectively clear a fault Consequently, electricity users within the affected segment experience a disruption in their power supply.

Automatic reclosing sequences are commonly utilized in overhead networks to manage circuit breakers that interrupt fault currents These sequences aim to quickly restore the circuit to normal operation, particularly when the fault is identified as transient or self-clearing.

In the event of a flashover caused by over-voltage, the components typically do not suffer serious or permanent damage If the initial attempt to reclose the circuit breaker fails, additional attempts may be made at predetermined intervals Should the fault persist after completing the scheduled open-reclose operations, the circuit breaker will remain open until necessary repairs are conducted at the fault location It is important to note that each reclosure while the fault is still present leads to an additional voltage dip, the severity of which varies based on the observation point's location.

To minimize the impact of a fault on the network and reduce the number of affected users, additional switching is typically performed after isolating the fault This process can be executed either automatically or manually.

A single fault in the network can trigger a series of complex switching operations, leading to interruptions of varying durations for users The impact of these interruptions depends on the network's structure and the users' locations in relation to the fault and switches Consequently, some users may face only brief interruptions, while others might have to wait for repairs to restore their service.

Interruptions having a duration up to 1 min (or, in the case of some reclosing schemes, up to 3 min) are classified conventionally as short interruptions.

3.5 Causes of voltage dips and short interruptions

Voltage dips, often linked to short interruptions, primarily occur due to significant current surges from short circuits in electrical systems or large-scale load fluctuations These current flows through network component impedances lead to temporary voltage drops, reducing the voltage supplied to users.

Dielectric breakdown in short circuits occurs due to overvoltage stress or compromised insulation, which may be weakened, damaged, or bridged Various factors contribute to these faults, leading to such outcomes.

– atmospheric events: lightning and wind storms, snow, ice, deposition of salt or atmospheric pollutants on insulators, wind-borne debris;

– mechanical interference and damage: contact by vehicles, construction equipment, excavation equipment, animals and birds, growing trees, vandalism and malicious damage;

– breakdown of network plant: deterioration with age, corrosion, rot, latent manufacturing or construction faults;

– accidents or errors in operation and maintenance;

– major natural events: floods, landslides, earthquakes, avalanches.

Certain defects caused by these factors are unavoidable across all networks Some types of networks are more susceptible to either most of these causes or a significant portion of them.

En particulier, des réseaux aériens sont exposés à la plupart de ces causes.

Voltage dips caused by load fluctuations are linked to the startup of large motors, especially in remote locations powered by long lines, as well as motors with significant load variations, such as arc furnaces and welding equipment However, in public network management, supply conditions typically impose limits on such fluctuations.

Exemple de défaut dans un réseau MT

La Figure 2 montre les creux de tension et les coupures brèves résultant d'un défaut sur un feeder MT Trois cas sont représentés:

– un défaut fugitif qui s'avère être supprimé après la première manœuvre de réenclenchement;

– un défaut semi-permanent qui persiste après la première manœuvre de réenclenchement mais s'avère être supprimé à la deuxième manœuvre de réenclenchement (après un certain délai);

– un défaut permanent qui persiste toujours après que toute la séquence de réenclenche- ment a été effectuée.

In each instance, voltage dips and interruptions are illustrated as experienced by two clients: one on the same conductor as the fault but upstream, and the other on a different conductor from the same busbar The times displayed are for illustration purposes, with actual times varying based on the settings applied to a specific network.

Faults in networks are unavoidable due to various causes, with certain types of networks being more susceptible to these issues Overhead networks, in particular, face a higher risk from a wider range of potential causes.

Voltage dips caused by load fluctuations often occur during the startup of large motors, particularly in remote areas served by long transmission lines This issue is also prevalent with equipment that experiences significant load variations, such as arc furnaces and welding machines In public network management, however, there are typically limits imposed on these fluctuations as a condition for supply.

3.6 Example of fault on MV network

Figure 2 illustrates the voltage dips and short interruptions resulting from a fault on an MV feeder Three cases are shown:

– a transient fault which is found to have cleared at the first reclose operation;

– a semi-permanent fault which still remains at the first reclose operation, but is found to have cleared at the second (delayed) reclose operation;

– a permanent fault which still remains after the full reclose sequence has been completed.

The voltage dips and interruptions are illustrated from the perspective of two customers: one located upstream on the same feeder as the fault, and the other on a different feeder connected to the same busbar The times presented are for demonstration purposes, as actual timings vary based on the specific settings used for each network.

Client alimenté par le départ 1 t t t t t t

Cas d'un défaut semi-permanent

Apparition d'un défaut t 0 - t 1 Temps de détection du défaut + délai t 1 - t 2 Ouverture du départ du défaut t 2

Le défaut a disparu (dans le cas d'un défaut fugitif)

Client alimenté par le départ 2

Figure 2 – Creux de tension et coupures brèves résultant d'un défaut en réseau MT

Case of a semi-permanent fault

16,5 s to 31,5 s Case of a permanent fault t 0

Appearance of the fault t 0 - t 1 Detection time of the fault + timing t 1 - t 2 Opening of the out-going feeder at fault t 2

The fault has disappeared (case of a transient fault)

Figure 2 – Voltage dips and short interruptions resulting from fault on MV network

4 Effets des creux de tension et des coupures brèves

Effets généraux

This document, part of the IEC 61000 series, addresses the effects related to electromagnetic compatibility (EMC), specifically the potential degradation of equipment performance EMC phenomena such as voltage dips and short interruptions can lead to connected devices not operating as intended.

The fundamental relationship between the network and the connected equipment is that the network acts as an energy source from which the equipment draws the necessary energy to perform its intended function The amount of energy extracted and its usage largely depend on the design and operation of the equipment, including the incorporated switching and control devices, and is only limited by the network's capacity to supply energy at the equipment's connection point.

The ability of the network to deliver energy decreases as voltage drops Voltage dips and brief outages result in a reduction or temporary interruption of energy supply to equipment This leads to performance degradation that varies depending on the type of equipment involved, potentially culminating in a complete operational shutdown.

One option often implemented in equipment design or installation is to incorporate a protective device that interrupts the power supply when the voltage drops below a predetermined threshold This device prevents damage and other undesirable effects caused by reduced voltage Such protection can effectively turn a voltage dip into a prolonged outage for the affected equipment.

The long interruption is not caused by a voltage dip; rather, it is the intended outcome of a protective device designed to respond in this manner to a reduction in voltage.

The severity of the effects of voltage sags and brief outages depends not only on their direct impact on the affected equipment but also on the criticality of the functions that the equipment performs Modern manufacturing methods often involve complex continuous processes that rely on multiple devices working in unison A failure or shutdown of any device due to a voltage sag or brief outage can necessitate halting the entire process, leading to production losses and potential serious damage or congestion of equipment This can be one of the most severe and costly consequences of voltage sags and brief outages However, such failures or significant losses are influenced by the design of the process and are considered indirect or secondary effects of the voltage sag or brief outage.

The considerations of electromagnetic compatibility (EMC) focus on the direct effects on the performance of devices powered by the electrical grid Common effects are specifically described for various types of equipment in the following paragraphs, although the list is not exhaustive.

NOTE Un déphasage subit peut accompagner le creux de tension et peut avoir un effet significatif sur certains équipements Ce phénomène n'est pas développé dans ce rapport.

4 Effects of voltage dips and short interruptions

This document, part of the IEC 61000 series, addresses the effects related to electromagnetic compatibility (EMC), specifically the potential degradation of equipment performance Voltage dips and short interruptions are identified as EMC phenomena that can lead to unintended operation of equipment connected to the supply network.

The supply system serves as a vital energy source for connected equipment, providing the necessary energy for its intended functions The energy drawn and its application largely depend on the design and operation of the utilization equipment.

(including the switching and control features incorporated in it), limited only by the capacity of the network to deliver energy at the point of connection of the equipment.

As voltage decreases, the network's energy delivery capacity diminishes, resulting in voltage dips and short interruptions that temporarily reduce or halt energy flow to equipment This can lead to varying degrees of performance degradation, potentially culminating in a complete operational shutdown, depending on the type of equipment affected.

Incorporating a protective device in equipment design or installation can interrupt the power supply when voltage drops below a specified threshold, preventing damage from reduced voltage conditions This protection may transform a brief voltage dip into a prolonged interruption, which is an intentional outcome of the protective feature designed to respond to low voltage situations.

The impact of voltage dips and short interruptions is influenced by the criticality of the equipment's function In modern manufacturing, where complex processes rely on multiple interconnected devices, a failure due to a voltage dip can halt the entire operation, leading to significant product loss and potential damage to equipment This consequential damage is often one of the most severe and costly outcomes of such disturbances, highlighting the importance of process design in mitigating indirect effects.

EMC considerations directly impact the performance of appliances connected to the electricity network This article highlights specific effects on various types of equipment, although the list provided is not comprehensive.

NOTE A sudden phase shift can accompany the voltage dip and can have a significant effect on some equipment.

This phenomenon is not discussed further in this report.

Effets sur des dispositifs particuliers

4.2.1 Équipement de technologie de l’information (IT) et de contrôle de procédé

Generally, the main functional units of these devices require direct current power supplies, which are provided by power supply modules that convert the alternating current from the public power grid Typically, the minimum voltage reached during a voltage dip is significant for these power supply modules.

Figure 3 illustrates the well-known ITIC curve for minimum immunity to voltage sags, including voltages above the normal range Equipment users must assess whether the impact of more severe sags than those depicted on the curve necessitates additional measures to ensure satisfactory performance Depending on the equipment's application, failures can have safety implications or other consequences, with traffic signal outages being one of many possible examples.

1 às 10 às 100 às 1 ms 10 ms 100 ms 1 s 10 s 100 s

Tens ion en pour un de l a t en si on a ss ignée

Figure 3 – Courbe ITIC (CBEMA) pour un équipement raccordé à un réseau 120 V 60 Hz

Relays and contactors can activate when the voltage drops below approximately 80% of the nominal level for more than one cycle The implications of this can differ based on the application, but they can be significantly severe in terms of financial impact or safety concerns.

The operating point of an asynchronous motor is determined by the balance between the motor's characteristic torque-speed curve, which is influenced by the square of the voltage, and the mechanical load During a voltage dip, the motor's torque begins to decrease, leading to a reduction in speed, while there may be an increase in current until a new operating point is established.

4.2 Effects on some particular devices

4.2.1 IT and process control equipment

Power supply modules convert a.c power from the public supply into d.c power for the principal functional units of the equipment The minimum voltage during a voltage dip is crucial for these modules, as illustrated by the ITIC curve, which outlines minimum immunity objectives for dips and includes voltages above the normal range Users must assess whether severe dips beyond the curve's parameters necessitate additional measures to ensure satisfactory performance, as failures can have significant safety implications, such as in traffic signaling systems.

1 às 10 às 100 às 1 ms 10 ms 100 ms 1 s 10 s 100 s

P er uni t of rat ed vo ltage

Figure 3 – ITIC (CBEMA) curve for equipment connected to 120 V 60 Hz systems

AC relays and contactors may disengage if the voltage falls below approximately 80% of the nominal level for more than one cycle The impact of this voltage drop can differ based on the application, potentially leading to significant safety risks or financial losses.

The operation of an asynchronous motor is determined by the interplay between its torque-speed characteristic, which is influenced by the square of the voltage, and the mechanical load When a voltage dip occurs, the motor's torque decreases, leading to a reduction in speed, while the current may increase until a new operational point is established.

Induction motors with a maximum torque exceeding 2.2 times their rated value exhibit low sensitivity to voltage dips, provided the residual voltage remains above 70% of the nominal voltage During such dips, the current increases by approximately 25% to 35%, while the power drawn from the grid remains relatively stable or slightly decreases If the torque defined by the load is fairly constant, the speed only decreases by a small percentage, corresponding to the increased slip due to reduced flux in the motor The primary effects are thermal, with a time constant greater than the duration of the longest dips The overcurrent resulting from voltage recovery is typically limited and, for directly connected motors, does not exceed the usual starting current.

Deeper troughs have effects similar to brief cuts on engine performance Two distinct behaviors are observed, depending on the mechanical time constant, which is the ratio of total inertia to the engine's nominal torque.

When the mechanical time constant is significantly larger than the duration of the dip, the speed only decreases slightly The time constant of the flow typically ranges in the hundreds of milliseconds, which allows for the possibility that during recovery, the electromotive force (e.m.f.) may be out of phase with the supply voltage Consequently, the resulting transient current peak can exceed the normal starting current.

When the mechanical time constant is small relative to the duration of the dip, the speed decreases significantly, causing the motor to nearly stop The current spike resulting from the voltage recovery corresponds to the normal starting current.

NOTE Il faut considérer la possibilité de déclencher les relais ou les contacteurs de protection du moteur – voir 4.2.2.

The recovery of voltage following a dip can be a critical phase, especially when multiple motors are connected to the same bus In such cases, the high current spike during voltage recovery may lead to a secondary voltage drop, delaying the restoration of voltage and postponing the acceleration of the motors to their normal speed In some instances, it may be impossible to resume acceleration, necessitating disconnection.

The operation of a synchronous motor is characterized by torque and speed on the output side, while voltage and active power define the input side Key variables such as flux, reactive power, and the rotor's angular displacement are interconnected with voltage and torque A voltage dip can be tolerated as long as stable operating conditions are reestablished, typically applicable for dips with a residual voltage of 75% or 80% (direct component) Additionally, the excitation circuit may be impacted and should be taken into account.

Severe conditions hinder the establishment of new stable operating conditions and lead to a loss of synchronism by increasing the rotor's angular displacement to the stability limit The attainment of this limit angle depends on the duration of the voltage dip, the level to which the voltage is reduced, and the mechanical time constant A comprehensive analysis is complex and must consider the damper windings, which can generate asynchronous torque.

Power drive systems (PDS) can be highly sensitive to even minor voltage sags, making the effects of these sags and brief interruptions quite complex It is essential to consider both the individual components and the system as a whole Typically, such systems include a power converter/inverter, a motor, a control element, and several auxiliary components.

Induction motors with a maximum torque higher than 2,2 times the rated value are very tolerant of dips presenting a positive phase sequence residual voltage above 70 % of the rated voltage.

Considérations générales

Standards address electromagnetic compatibility by setting coordinated limits on emissions and immunity The goal is to prevent excessive electromagnetic interference from being emitted while ensuring that equipment exposed to such disturbances has adequate immunity, allowing it to function as intended.

In the case of voltage dips and brief interruptions, which are normal responses of an electrical network to short circuits or sudden current surges, the level of disturbance has two dimensions: residual voltage and duration An emission limit should address both dimensions.

Residual tension generally cannot be altered It ranges from zero volts to approximately the normal supply voltage level, depending on the relative positions of the short circuit locations, observation points, and generation sources.

The duration of a voltage dip can be adjusted to some extent, primarily influenced by the speed at which short circuits are cleared A crucial protective measure against short circuits involves selecting the operating delays of switches and relays at various points in the network to ensure that each short circuit is addressed at the most appropriate location Consequently, the elimination time, and thus the duration of voltage dips and brief interruptions, is contingent upon the location of the short circuit If the initiating event is not a short circuit, the duration will depend on the specific event in question.

There is limited scope for emission reduction regarding disturbance levels In specific cases, various methods can influence the frequency of voltage sags and short interruptions by taking actions to minimize a network's exposure to faults Therefore, it is essential to assess the feasibility of providing immunity in equipment exposed to voltage sags and short interruptions.

The control element plays a crucial role in managing the response of other components during voltage dips or short interruptions A reduction in voltage decreases the power available to the motor and the connected equipment, potentially resulting in a loss of control Regenerative converters are particularly sensitive and may need specialized management, especially when voltage dips or short interruptions occur alongside reversed power flow.

The converter has little or no available energy storage capability Generally, the driven equipment has some energy storage capability, which can be used under certain conditions.

High-pressure discharge lamps can be extinguished by voltage dips that drop the voltage below 90% of the nominal value, leading to cooling and pressure loss Consequently, these lamps may take several minutes to restart Additionally, lighting systems with electronic components may also be impacted.

The standard method for ensuring electromagnetic compatibility involves implementing coordinated limits for both emissions and immunity This approach aims to minimize excessive electromagnetic disturbances while ensuring that equipment exposed to such disturbances maintains a sufficient level of immunity, allowing it to function as intended.

Voltage dips and short interruptions are natural responses of electrical systems to short circuits or current surges The disturbance level is characterized by two key dimensions: residual voltage and duration Therefore, any emission limit must address both of these factors.

The residual voltage typically remains unchanged, varying from zero volts to the normal supply voltage level, influenced by the locations of the observation point, short circuit, and generation sources.

The duration of short circuit events can be adjusted, primarily influenced by the speed of clearing these faults Effective short circuit protection requires that the operating times of switches and relays vary at different network points, ensuring optimal fault clearance Consequently, the duration of voltage dips and short interruptions is contingent upon the location of the short circuit, while other events will have their own specific duration factors.

Thus, there is limited scope for emission limitation in terms of the level of the disturbance.

In certain situations, it is possible to minimize the occurrence of dips and brief interruptions in a network by implementing measures that reduce its exposure to faults.

Therefore it is necessary to consider whether it is possible to provide immunity in equipment that is exposed to voltage dips and short interruptions.

Moderate depth and duration voltage dips can be managed by various equipment that possess inherent immunity due to their energy or inertia storage capabilities Additionally, design adjustments may be implemented to enhance this protective feature.

However, for brief outages and severe voltage dips, immunity, in its strict sense, is not a feasible concept The key aspect of the event is that it involves a complete cessation or significant reduction of energy supply for a short period No electrical device can operate as intended without its energy supply.

Consequently, such immunity against disruptions is likely to be extrinsic, involving either the rapid restoration of energy through an alternative source or ensuring that the equipment and its associated processes are designed to adapt predictably to brief power outages or reductions in power supply.

Quelques exemples des mesures réparatrices

5.2.1 Machine rotative avec inertie additionnelle

A straightforward method to navigate voltage dips and brief outages for rotating equipment is to increase their inertia However, this approach is primarily applicable in specialized applications, such as metallurgy, and is often used in conjunction with other methods to mitigate sudden load variations The effectiveness of this configuration relies on the ratio of inertia to actual load, typically allowing for response times in the range of several seconds.

5.2.2 Machine rotative avec volant et moteur ou système d'alimentation de secours

A large mass connected to a motor/generator spins at very high speeds in a vacuum, accumulating energy up to several megawatt-seconds This energy is supplied to the system through a converter, with typical available power reaching several hundred kilowatts.

Moderate voltage dips can be tolerated by certain equipment due to its inherent immunity, which may stem from factors like inertia or energy storage capacity Additionally, design modifications can be implemented to enhance this immunity.

Short interruptions and severe voltage dips present challenges that immunity cannot effectively address These events are characterized by a complete halt or significant reduction in energy supply for a brief period Consequently, no electrical device can function properly without its energy source.

To mitigate disturbances, immunity is often extrinsic, focusing on either quickly restoring energy from an alternative source or ensuring that equipment and processes can adapt effectively to brief power interruptions or reductions.

Remedial measures often utilize energy storage systems to temporarily provide the energy needed to compensate for voltage dips and short interruptions The effectiveness of this equipment in managing such disturbances relies on the balance between the stored energy and the power demands of the process Additionally, it is crucial to consider the necessary reaction time, which can be just a few milliseconds Due to the high costs associated with energy storage, protective measures are typically focused on the most sensitive components of the process.

Other remedial measures, lacking energy storage capabilities, are unable to address supply interruptions; however, they can mitigate voltage dips, maintaining a residual voltage of approximately 50% These measures vary in their capacity to withstand different levels of voltage reduction.

Generally, the duration of the dip is not an important parameter in these methods Omission of the energy source results in costs that are typically lower.

This technical report offers examples to enhance understanding of voltage dips and short interruptions While mitigation strategies are not covered here, they should be evaluated through a combination of economic and technical analysis, as detailed in IEEE 1346-1998 [5].

5.2 Some examples of remedial measures

5.2.1 Rotating machine with additional inertia

To effectively manage voltage dips and brief interruptions in rotating equipment, increasing inertia is a straightforward approach This method is primarily applicable in specialized settings, such as steelworks, where it also helps to mitigate abrupt load variations The effectiveness of this configuration is influenced by the interplay between inertia and the actual load, generally providing support for several seconds.

5.2.2 Rotating machine with flywheel and engine or emergency power system

A high-speed rotating mass combined with a motor/generator operates in a vacuum, storing energy of several megawatt-seconds This energy is delivered to the system through a converter, providing power levels that can reach several hundred kilowatts.

5.2.3 Alimentation non interruptible d'énergie (UPS)

Uninterruptible power supply (UPS) systems are widely used to protect sensitive equipment from voltage fluctuations and power outages Typically, the load is supplied through a converter connected to a power source, such as batteries The storage capacity of these systems varies significantly based on specific requirements and is primarily limited by energy storage costs Real-world applications range from small low-voltage loads to those requiring several hundred kilowatts.

5.2.4 Bobine en supraconducteur emmagasinant de l’énergie (SMES)

SMES can store several megawatt-seconds of energy in a superconducting coil Typically, it can compensate for outages or voltage dips lasting several hundred milliseconds for loads with high power demand, depending on the design.

5.2.5 Compensateur statique de puissance réactive (SVC)

A static reactive power compensator typically consists of capacitors and/or passive filter circuits with an inductive coil in parallel, controlled by thyristors It continuously provides adjustable reactive power.

(équilibrée ou déséquilibrée) au système, permettant de ce fait l'ajustement de la tension.

Typically, SVCs are connected to medium and high voltage networks with a rated power ranging from several megavars (Mvar) to several hundred megavars They primarily facilitate voltage control at major nodes within the distribution network While they can be designed to compensate for voltage dips, their capabilities in this application are somewhat limited Generally, the voltage regulation capabilities of an SVC range from 10% to 20% of the network voltage.

5.2.6 Restaurateur dynamique de tension (DVR)

During voltage dips, dynamic voltage restorers compensate for the missing voltage amplitude, both balanced and unbalanced, using power electronics through a transformer in series with the load For residual voltages below 50%, the voltage can be restored for several milliseconds Applications are available for loads ranging from several tens of kilowatts (LV) to several tens of megawatts (MV).

Tiêu đề	IEC TR 61000-2-8 2002
Trường học	Not specified
Chuyên ngành	Electromagnetic Compatibility
Thể loại	Technical report
Năm xuất bản	2002
Thành phố	Geneva

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Số trang	98
Dung lượng	1,48 MB