Arc Protection - Schneider Electric
Photo: J.Vogler C11 Arc Protection Network Protection & Automation Guide
Network Protection & Automation Guide Chapter C11 Arc Protection 1. Introduction 401 2. Arc flash phenomenon 402 3. Causes of arc faults in switchgear 403 4. Impacts and consequences of arc faults 404 5. Arc protection related standards 406 6. Arc protection and mitigation methods 407 7. The principle of arc protection based on simultaneous detection of light and overcurrent 408 8. How to avoid nuisance tripping caused by switching arcs 409 9. Arc protection systems 409 10. Elimination of the arc fault 411 11. Typical application examples 412
Arc Protection C11 1. Introduction High power arc faults in switchgear are rare events but their consequences can be extremely severe. They can be characterised as electrical explosions. Personnel may be seriously affected by the radiation, heat, pressure wave, and flying particles associated with the arc flash. These impacts can also devastate the switchgear and cause substantial economic losses either directly by destroying components of the system or indirectly by causing process outages or medical and legal expenses. Traditional overcurrent protection is inefficient in the detection of arcing faults because it is too slow. Faster protection methods are needed in order to significantly reduce the released energy. Energy Damage & hazard Extensive damage Figure C11.2a: Arcing time 47ms - Minor damage Limited damage Minimal damage Arcing time Figure C11.1: Damage level dependent on the arcing time This chapter describes the state-of-the-art arc protection technology which is based on minimisation of arcing time by very fast arc detection and elimination. By using simultaneous detection of fault arc light and overcurrent it is possible to reliably detect the arc within 2ms and phase-to-phase faults within 1ms. The principle of this protection method and the associated sensors, devices and systems are explained and clarified by typical application examples. Figure C11.1 illustrates that limitation of the arcing time is an efficient way to mitigate damages on arc faults. Figure C11.2a and C11.2b show the results of a 50kA 3 phase internal arc test with short and long arcing time. 401 Figure C11.2b: Arcing time 500 ms - Copper busbars have evaporated Schneider Electric - Network Protection & Automation Guide
Arc Protection 2. Arc flash phenomenon An electric arc is a luminous discharge short circuit through dielectric gas or fluid. Arcs can be categorised as useful arcs (welding), breaking arcs (in switching devices), and fault arcs. Fault arcs can further be divided into low power arcs and high power arcs. Examples of low power arc faults are series faults in power systems (e.g. loose connections) and arcs in photovoltaic low voltage systems. These arcs release very limited amount of energy compared with high power arc faults. This chapter is focused on high power arcs in gases where the fault arc is formed between two or more conductive parts of a power system, most commonly between two or more phases or between phase and earth. The emphasis is in high power fault arcs in air insulated switchgear. An arc fault in switchgear is a short circuit through ionised gas (normally air) between live parts or between one live part and earth. It is very different from a bolted fault with a solid connection. Air-insulated switchgear is commonly used, since dry air in normal temperature is a reasonably good insulator. However, when the temperature of the air is very high, more than 2000 K, thermal ionisation makes air conductive. The ionised air and the ionised material from the electrodes form a conductive plasma channel between the electrodes. The plasma consists mainly of nitrogen and oxygen molecules, atoms and ions of N and O, electrode material and electrons. The plasma is very hot, temperatures as high as 20000 K have been reported, and it radiates light. The light comes from hot particles and from electrons returning from high energy states to lower states. In a high power arc fault very high amounts of electrical energy turns into radiation and thermal energy extremely rapidly. Due to the light and the high temperature with the associated pressure wave and possible flying particles, arc faults can be characterised as electrical explosions (see Figure C11.3). Schneider Electric - Network Protection & Automation Guide Figure C11.3: [Photo: J.Vogler]: Electrical explosions as result of an arc fault 402 C11
Arc Protection C11 3. Causes of arc faults in switchgear Internal arc faults in switchgear are caused by something that leads to a failure in the insulation. Many arc faults are caused by direct human errors, such as forgotten tools, forgotten earthing connections or errors while working on equipment. Loose connection, vibration, insufficient mechanical dimensioning and overvoltage can also lead to an arc fault. Animals, contamination, dirt, moisture, ageing of insulation, Locations where internal arc faults are most likely to occur (1) Connection compartments Disconnectors Switches Earthing switches Bolted connections and contacts corrosion and maloperation of switching devices are also possible causes of arcs. Many arc faults start as single phase faults and then rapidly escalate into three-phase faults. IEC Standard 62271-200 provides a good overview of possible locations and causes of internal arc faults as shown in Table C11.1. Possible causes of internal arc faults (2) Inadequate design Selection of adequate dimensions Use of appropriate materials Faulty installation Avoidance of crossed cables connections Checking of workmanship on site Correct torque Failure of solid or liquid insulation (defective or missing) Checking of workmanship and/or dielectric test on site Regular checking of liquid levels, where applicable Maloperation Interlocks Delayed reopening Independent manual operation Making capacity for switches and earthing switches Instructions to personnel Corrosion Use of corrosion inhibiting coating and/or greases Use of plating Encapsulation, where possible Supplemental heating to prevent condensation Faulty assembly Checking of workmanship by suitable means Correct torque Adequate locking means During racking-in or racking-out of withdrawable parts E.g. due to dielectric change of state in combination with damage or distortion of the plugging contacts and/or shutters Checking of workmanship at site Ferro-resonance Avoidance of these electrical influences by suitable design of the circuit Short circuit on low-voltage side for VTs Avoid short circuit by proper means for example, protection cover, low-voltage fuses Insufficient maintenance Regular programmed maintenance Instructions to personnel Error by personnel Limitation of access by compartmentalisation Insulation of embedded live parts Instructions to personnel Ageing under electric stresses Partial discharge routine tests Pollution, moisture ingress of dust, vermin, etc Measures to ensure that the specified service conditions are achieved Use of gas filled compartments Overvoltages Surge protection Adequate insulation co-ordination Dielectric tests on site Instrument transformers Circuit-breakers All locations Examples of possible preventive measures (3) Table C11.1: Locations, causes and examples of measures to decrease the probability of internal arc faults [IEC Standard 62271-200] 403 Schneider Electric - Network Protection & Automation Guide
Arc Protection 4. Impacts and consequences of arc faults Figure C11.4 illustrates the impacts and consequences of arc faults. Psychological trauma Vaporised copper (volume 67.000 times solid copper) Pressure wave Toxic gases Intense light, burning and blinding radiation High temperature up to 20.000 K Shrapnel and molten metal Hazardous voltage Heated air pressure and sound wave (Air expands 1.600 fold) Fire Figure C11.4: Consequences of arc faults In a high power arc fault incident huge amounts of electrical energy are converted into radiation and thermal energy. The radiation includes visible light. The very intense light of the arc flash can cause eye damage and it also plays a part in the burning impact of the arc (see Figure C11.5a to C11.5c). Figure C11.5a, 5b, 5c: Light arcing flash Schneider Electric - Network Protection & Automation Guide 404 C11
Arc Protection C11 4. Impacts and consequences of arc faults Most of the burning effect comes from the thermal energy which in fact has many impacts. The high temperature heats up the air, and it vaporises the metal of the busbars. The hot plasma and the convection of the hot gases can cause serious arc burns to personnel. Serious damage to equipment is also possible. If the arcing time is high the busbars can be totally destroyed, and the arc can burn holes to the switchgear housing. Since arcs can ignite fire, additional damage to switchgear or substations is possible. Personnel's clothes may catch fire and increase the risk of burns. Additional safety hazard comes from toxic gases, due to evaporation of metal parts. Humans nearby are also vulnerable to hazardous voltage caused by the fault, and an arc fault may cause subsequent psychological trauma. When the temperature of the air rises the volume of the air increases. Furthermore, when metals evaporate, they expand dramatically (see Figure C11.6a and C11.6b). For example when copper evaporates its volume is 67000 times as large as in the solid form. This instantaneous expansion creates another major impact of the arc, the pressure wave. The pressure wave, often called the arc blast, is dangerous to humans. In addition to ear damage and lung collapse, it can throw humans against walls or to the floor causing bone fractures or more serious injuries. Additional injuries can be caused by flying particles and molten metal from the damaged busbar and assembly. Figure C11.6b: Damage after an arc fault Arc blast gives arc fault incidents an explosive nature. Since the peak value of the pressure, measured at the compartment walls, is normally reached 4-15ms after the ignition of the fault (peak pressure buildup time is relative to volume), mitigation of the pressure impact requires either efficient arc containment, redirection of the arc blast or high speed mitigation of the arc. The above mentioned consequences, serious safety hazard and significant damage to equipment are not the only risks caused by arc faults. Injury or even death of humans may lead to substantial medical and legal expenses. When it comes to equipment, indirect costs are possible as well. Prolonged power and process outages due to the destroyed equipment are possible and in process industries particularly the interruption costs can be very high. Figure C11.6a: Damage after an arc fault 405 Schneider Electric - Network Protection & Automation Guide
Arc Protection 5. Arc protection related standards In some countries arc protection based on simultaneous detection of light and overcurrent is a ‘de facto’ standard which means that practically all new industrial switchgear and primary substations of the utilities are equipped with the technology. However, currently in 2015, there are no international standards directly standardising methodology or equipment for arc protection. Currently available standards concerning arc fault issues are the following: a. IEC 62271-200, High-voltage switchgear and controlgear - Part 200: AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV, known as the international switchgear standard b. IEC 60364 Low-voltage electrical installations c. IEEE Std 1584 -2002, IEEE Guide for Performing ArcFlash Hazard Calculations d. NFPA 70E , Standard for Electrical Safety in the Workplace, 2015 Edition, NFPA (National Fire Protection Association) IEC 62271-200 (Edition 2.0, 2011) is a MV switchgear standard and it "specifies requirements for prefabricated metal-enclosed switchgear and controlgear for alternating current of rated voltages above 1 kV and up to and including 52 kV for indoor and outdoor installation and for service frequencies up to and including 60 Hz. Enclosures may include fixed and removable components and may be filled with fluid (liquid or gas) to provide insulation." Arc faults are briefly discussed in the standard. The standard aims at preventing the occurrence of internal arc faults. It gives a good list of locations where faults are most likely to occur, and explains causes of failure and possible measures to decrease the probability of faults. Additionally IEC 62271-200 gives examples of supplementary measures - in practice arc protection technologies - to provide protection to persons: a. rapid fault clearance times initiated by detectors sensitive to light, pressure or heat or by a differential busbar protection b. application of suitable fuses in combination with switching devices to limit the let-through current and fault duration c. fast elimination of arc by diverting it to metallic short circuit by means of fast-sensing and fast-closing devices d. remote operation instead of operation in front of the switchgear and controlgear e. pressure-relief device f. transfer of a withdrawable part to or from the service position only when the front door is closed IEC 62271-200 recognises two important ratings of the arc fault currents: a) three-phase arc fault current and b) single Schneider Electric - Network Protection & Automation Guide phase-to earth arc fault current. IEEE Std 1584 -2002, IEEE Guide for Performing ArcFlash Hazard Calculations, is a safety oriented guide. It provides techniques to apply in determining the arc-flash hazard distance and the incident energy to which employees could be exposed during their work on or near electrical equipment. Its applications cover an empirically derived model including enclosed equipment and open lines for voltages from 208 V to 15 kV, and a theoretically derived model applicable for any voltage. The standard also provides a good list of arc fault related definitions. One of the most central definitions is the concept of incident energy: The amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. Incident energy is measured in joules per centimetre squared (J/cm²). The incident energy concept is used for developing strategies to minimise burn injuries. The guide is based upon testing and analysis of the hazards presented by incident energy. It provides a detailed step-bystep process for arc flash analysis. This analysis ends up with determining the incident energy level and the flash-protection boundary (The distance from live parts that are uninsulated or exposed within which a person could receive a second degree burn) based on incident energy of 5.0 J/cm². One should note that the analysis only covers the thermal impact of the arc fault, not the pressure related impact for example. The standard is well known but mostly utilised in North America. Although incident energy levels are seldom calculated in Europe, incident energy calculations are a useful tool when comparing the effectiveness of different arc protection methods. Because the incident energy level depends on four key parameters: the arcing current, the voltage, the working distance and the arcing time, it is relatively easy to see that normally the most practical factors in the mitigation of the thermal impacts of arc faults are the arcing time and the arcing current. IEEE Std 1584 -2002 includes reporting of the tests carried out with current-limiting fuses. The published figures confirm the risk related to current-limiting fuses: high incident energy levels occur when the fault current is low, and the fuse is not in its intended range of operation. NFPA 70E, Standard for Electrical Safety in the Workplace by National Fire Protection Association addresses electrical safety-related work practices, safety-related maintenance requirements and other administrative controls for the practical safeguarding of employees. It has some links to arc protection and it provides some commonly used arc fault related definitions, such as: a. Arc Flash Boundary: When an arc flash hazard exists, the distance from a prospective arc source at which a person could receive a second degree burn if an electrical arc flash were to occur. (A second degree burn is possible 406 C11
Arc Protection C11 5. Arc protection related standards by an exposure of unprotected skin to an electric arc flash above the incident energy level of 5 J/cm² b. Arc-Resistant Switchgear: Equipment designed to withstand the effects of an internal arcing fault and that directs the internally released energy away from the employee NFPA 70E includes an informative annex giving guidance on selection of arc-rated clothing and other PPE (Personal Protective Equipment) when it is not practical to eliminate exposure to incident energy. 6. Arc protection and mitigation methods Naturally the primary goal is to prevent arc faults by, for example, careful design, education of personnel and adequate maintenance of equipment. In some cases early detection of developing faults is possible by special monitoring equipment. However, it is very difficult to totally eliminate arc faults in distribution systems. As shown previously, the incident energy depends on the voltage, working distance, arc current and arc duration. From an arc protection point of view it is normally not possible to effect the applied voltage. The working distance is only related to humans working on the equipment, and it is often difficult to increase this distance. In practice there are two major approaches for decreasing the released energy: limitation of the arc current or reduction of the arc time. Method Incident energy calculations, based on testing, show that the released energy is proportional to the arc time. When traditional overcurrent protection is applied, the arc time is normally some hundreds of milliseconds. This leads to extensive damage and a serious safety hazard. This is why several different arc fault mitigation approaches have been introduced, that are much more efficient than overcurrent protection. The following Table C11.2 presents evaluation of some well known arc mitigation methods. Benefits Drawbacks Arc-resistant switchgear Equipment designed to withstand the effects of an internal arc fault and that directs the internally released energy away from the employee Good protection for personnel at least when the doors are closed Provides protection against the pressure impact Gives very good protection if used in combination with fast protection If used as the only arc mitigation approach, provides no protection to the equipment in the enclosure Maintenance switch A switch that when turned on (during maintenance at a substation) makes circuit breakers operate without any intentional delay Rather good protection of personnel Effective only during maintenance Zone-selective interlocking Rather simple, relatively low costs Not very fast Current-limiting fuses Very fast operation and good protection if the fault current is in the operation range of the fuse Limits both current and arc time When fault current is low (as it can be for various reasons) the arc time and the incident energy are high Current-limiting reactors Limit fault current Increase cost and losses Limited effect Busbar differential protection Fast protection Complicated settings Requires careful CT selection Does not operate in cable terminal faults Table C11.2: Comparison of mitigation methods 407 Schneider Electric - Network Protection & Automation Guide
Arc Protection 7. The principle of arc protection based on simultaneous detection of light and overcurrent The leading method in arc protection is based on simultaneous very fast detection of light and overcurrent as shown in Figure C11.7. This approach can be divided into two parts: Arc detection and Arc mitigation. The arc can be detected within 1 milliseconds which is outstanding performance compared with conventional protection technology. The arc time varies according to the elimination technology. When applying conventional circuit breakers the arc time is some tens of milliseconds. If a short-circuit device is applied, the arcing time is less than half cycle. Light Overcurrent & Figure C11.8: Point type of optical sensor Trip signal Figure C11.7: Modern arc protection logic Normally the overcurrent condition increases costs only a little since existing current transformers can be used. However, there are applications where "light only" based arc detection can be applied. For example, if the probability of intense external light can be practically closed out, measurement of current would be very difficult, or where low cost is essential the "light only" condition can be justified. 7.1 Fast optical detection of light There is a strong correlation between the power of the arc and the intensity of the observed light. Fault arcs can be detected practically immediately by using light sensitive sensors, such as photodiode (point type of sensor) or optical fibre (loop or point type of sensor). There is not an exact universal sensitivity threshold value which could always differentiate between light emanating from arc faults and the light coming from other sources. Practical experience has shown that sensitivity of approximately 10000 lux (visible light) gives excellent results. Sensors with the sensitivity of 10000 lux are very likely to detect the light in all relevant arc fault situations with metal-enclosed switchgear while at the same time the risk of false activation is low. This is true especially in the cases where the detection of the arc is confirmed by the simultaneous detection of overcurrent. Point type of sensors (Figure C11.8) enable more selective protection than fibre loop sensors, identifying the location of the arc more accurately. Loop sensors (Figure C11.9) are a cost effective solution for applications where protection selectivity is not a critical. Schneider Electric - Network Protection & Automation Guide Figure C11.9: Fibre type of optical sensor 7.2 Fast detection of overcurrent In order to minimise the possible nuisance tripping caused by external light, an overcurrent condition, i.e. detection of overcurrent (secondary sensor) is often required in parallel with the detection of light (primary sensor). The current can be measured with normal (existing) current transformers. In arc protection applications it is however necessary to minimise the operation time and special methods are used to enable the rapid detection of the overcurrent. Very fast (less than 1ms) detection of overcurrent is possible by applying an analogue comparator. The method is illustrated in Figure C11.10. Because many arc faults start as single-phase faults, it is justified to detect phase-to-earth faults as well. If the arc is detected and eliminated before it escalates into high-power three-phase fault, the damage will be lower. The detection of phase-to-earth arc fault is normally based on simultaneous detection of light and zero-sequence overcurrent, but zero sequence voltage can be utilised as well. 3xl 3xI Io U f A/D 3xl Io Pickup value 3xl Io Comp Io I CONF. MEMORY FPGA CPU Figure C11.10: A method for very fast detection of overcurrent, utilising an analogue comparator 408 C11
Arc Protection C11 8. How to avoid nuisance tripping caused by switching arcs In almost all cases, both in medium voltage (MV) and low voltage (LV) systems, the trip condition of simultaneous detection of light and overcurrent has proven to be successful. However, some low voltage circuit breakers (air-magnetic type) emit light and other type of pollution while operating. This problem can be mitigated by using special types of light sensors, less sensitive or designed for limited wavelength ranges or by applying pressure sensors. 9. Arc protection systems 9.1 Stand-alone arc protection systems The simplest arc protection solutions can be based on standalone devices (Figure C11.11). When "light only" detection criteria is applied, all that is required is the optical sensors, and a device that collects the information from the sensors and sends the trip command to the appropriate circuit breaker. Some wind power applications, secondary substations and limited low voltage switchboards are examples of possible application areas for stand-alone devices. An example of applications is a primary substation (HV/MV) where arc faults can be selectively tripped . e.g. in cable terminations of outgoing feeders - a typical location of arc faults. Vamp 321 ON OK F2 F1 I O vamp Cable Vamp 321 L (CB, BB) ON OK F1 I F2 O vamp CB BB CB CB Cable Figure C11.11: Stand alone arc protection system 9.2 Arc protection integrated in numerical protection relays Another cost effective and very widely applied solution is to integrate arc protection into normal protection devices (Figure C11.12). Because most relays already include current measurement, it is relatively easy to add the input for light sensors to achieve the light and overcurrent based trip condition. However, the overcurrent detection must be very fast. Cable Vamp 321 ON Vamp 321 ON OK F1 I OK F2 F1 O I vamp F2 O vamp L (CB, BB) Figure C11.12: Integrated arc protection function in a protection device When the relays are equipped with communication and several light sensor inputs, selective arc protection can be provided. 409 Schneider Electric - Network Protection & Automation Guide
Arc Protection 9. Arc protection systems 9.3 Dedicated arc protection systems Arc protection is most often implemented by a separate system using arc flash detectors connected to dedicated arc protection relays (Figure C11.13). Overcurrent and earth-fault protection is carried out by other relays. A comprehensive, selective arc protection system comprises of optical sensors, current transformers (normally no additional CTs are needed), I/O units collecting data from the sensors and CTs, communication cabling, and a master unit or several master units for final collection of all the sensor data. The master unit(s) are measuring the current and tripping the appropriate A separate system enables large installations with selective protection and multiple protection zones. It provides very high speed protection and can also provide some protection redundancy. CBFP CBFP* Point sensor circuit breakers, if both light and overcurrent are detected. Very high speed communication between the components is an essential feature of the system, transmitting information on detected light, detected overcurrent, addresses (location) of detection, and trip commands. T2 T4 VAMP 321 T, T1, T2, T3, T4 Trip S1, S2, S3 . Sensors * Either CBFP or Direct Zone 1 trip VAM 4CD V T3 T T1 Zone 4 T1 Zone 1 VAM 12L V Zone 2 T3 Zone 2.1 S1 T1 Zone 2.2 Zone 2.3 S2 T2 VAM 12LD Zone e 2.4 4 S3 S1 T2 Zone 3 Zone Z one 2.5 5 Zone 2.6 T3 T3 S2 S3 S T T1 VAM 12LD T VAM 3L V Figure C11.13: Dedicated arc protection system Schneider Electric - Network Protection & Automation Guide 410 C11
Arc Protection C11 10. Elimination of the arc fault 10.1 The importance of the arc elimination time The incident energy is proportional to the arcing time. From a protection point of view the arcing time consists of two components: arc detection time and arc elimination time. By applying arc detection methods described above, minimal arc detection time can be achieved. The arcing time then depends almost entirely on arc elimination time. 10.2 Circuit breakers In most applications arc protection relays send the trip signal to appropriate circuit breakers which then open the circuit and extinguish the fault arc. In MV applications using arc protection relays and CBs the total arcing time is in the order of 60ms, consisting of 1ms detection time and 60ms CB operation time. The operating times of LV CBs are usually shorter than MV CBs' operation times. When total arcing time is in only a few tens of milliseconds, the thermal impacts of faults arcs are efficiently mitigated (see Figure C11.14). Fast communication between the arc protection relay and the short-circuit device is vital. The combination of optical arc detection and a short-circuit device provides extremely fast and efficient protection. The arcing time is only a few milliseconds. The thermal impact of the fault is minimal, and the pressure impact is significantly mitigated. 10.4 Current-limiting fuses The use of current-limiting fuses in arc protection requires good product and system knowledge otherwise the protection level may be much lower than expected. CL fuses can be very efficient in both limiting the current and reducing the arc time. When the fault current is in the current-limiting range, the fuse is able to break the current very rapidly, and also reduce the peak current. The reduction of the peak current is a benefit, because high current causes mechanical forces that are detrimental to transformers feeding the current. Figure C11.15 below illustrates the current-limiting impact of a CL fuse. I B Prospective wave (I rms A) Limited wave Cut-off current (peak value) t Figure C11.15: Current-limitation of a CL fuse Figure C11.14: Low impact on an arc fault with fast arc protection trip 10.3 Short-circuit devices (arc eliminators) Arc eliminating by means of a short-circuit device (crowbar unit, arc quencher or high speed earthing device) is recognised by IEC Standard 62271-200 as an option to provide the highest possible level of protection to personnel in case of an internal arc in MV switchgear. When using a short-circuit device the arc protection systems sends trip commands to both the arc eliminator and the appropriate circuit breakers. The short
405 Schneider Electric - Network Protection & Automation Guide Arc Protection Most of the burning effect comes from the thermal energy which in fact has many impacts. the high tempera ture heats up the air, and it vaporises the metal of the busbars. the hot plasma and the convection of the hot gases can cause serious arc burns to personnel.
For Reference Only ARC 8D ARC 10D ARC 12D ARC 15D Dimensions And Specs 1 Recommended Enclosures ARC 8D 2 ARC 10D 2 ARC 12D 3 ARC 15D 3 . .65 cu.ft. .75 cu.ft. .85 cu.ft. Minimum SPL SQ ARC 10D2 ARC 10D4. 3 TOP NOTE: These woo
High Current Level Arc "Parallel Arc" An Arc Fault That Occurs at 75Amps and higher An Arc Fault That Occurs Line-Line or Line-Neutral Types Of Arcing Faults Low Current Level Arc "Series Arc" An arc fault at low levels down to 5 Amps An arc fault at a break or gap in a single conductor in series with a connected load
Arc Flash Facts Arc Flash Fact Sheet Brady Arc Flash Training Aids Promote awareness of the dangers associated with arc flash accidents and make sure your workers know how to protect themselves! Poster Highlights the common causes of arc flash and provides safe work practices and personal protection equipment requirements
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