Methods to Reduce Arc-Flash Hazards

Transcription

1 Methods to Reduce Arc-Flash Hazards Exercise: Implemetig Istataeous Settigs for a Maiteace Mode Scheme Below is a oe-lie diagram of a substatio with a mai ad two feeders. Because there is virtually o differece betwee the fault curret for a fault o the bus side of a feeder ad for a fault o the load side, there is o opportuity to set a istataeous trip settig o the mai breaker ad maitai selectivity. Selectivity requires that the mai breaker coordiate with the feeder breaker time-overcurret elemets. This results i all bus faults ormally beig cleared with a delayed elemet. Whe the feeders must coordiate with dowlie protective relays, the delay ca be prologed eve more as time-overcurret elemets stack up. A bus fault would be expected to have the highest fault availability o this radial system, ad sigificat exposure for persoel exists i this area. The situatio ca be improved with a istataeous settig that is eabled as eeded whe persoel are i the hazard area. The added feature is easy to implemet ad ca be doe so with o added equipmet costs. The tradeoff for this feature, however, is cocedig proper coordiatio ad possible overtrippig for faults o a feeder whe the maiteace mode is eabled. The same logic ca be used o the mai or feeder breakers. Utility 1,00 MVA X/R = /1.47 kv 1/16/0 MVA Z = 4.5% Relay M1 M1 Relay F1 Relay F F1 F I this exercise, you will use the example system show above to determie the icidet eergy for arcig faults o the bus ad for close-i faults o the feeders. This is the icidet eergy at Breakers F1 ad F that a worker could be exposed to durig a arc-flash evet. For compariso, you will calculate the icidet eergy for both ope-air ad switchgear (box) cofiguratios. You will also see how implemetig a istataeous settig durig maiteace reduces the arc-flash icidet eergy by a factor of 8.3. The calculatios i this exercise are based o the IEEE 1584 empirical method. They meet the Occupatioal Safety ad Health Admiistratio (OSHA) calculatio method recommedatios for arcs i the ope air or i a eclosure (switchgear) at 1.47 kv. Refer to the relay settigs below for the steps i this exercise. Feeder Relays F1 ad F Mai Relay M1 CTR = 10 CTR = 00 51PIP = 5 51PIP = 5 51PIC = U3 51PIC = U3 51P1TD = 5 51PITD = 6 Istataeous Settigs for a Maiteace Mode Scheme Page

2 Step 1: Determie Bolted Three-Phase Fault Curret at Bus The first step i calculatig the icidet eergy is determiig the maximum available bolted three-phase fault curret. The available utility fault curret for this example system is give as 1,00 MVA, ad the X/R ratio is 15. Use the followig formula to covert this iformatio to the trasformer impedace base: kvu MVAt 1 Zsource = 100 ta X/R kvt MVAu Z source = utility impedace i percet based o trasformer base. kv u = utility voltage base. kv t = trasformer voltage base. MVA u = utility fault MVA. MVA t = trasformer MVA base. X/R = utility X/R ratio. The resultig calculatios for this example system are as follows: ( ) (1) Zsource = 100 ta , 00 Zsource = = j (3) Calculate the total impedace to the bus by addig the utility source impedace ad trasformer impedace together, as show below: Ztotal = Zsource + ZXFMR = j j4.5 (4) Ztotal = j5.5 = (i percet) (5) Calculate the trasformer base curret, ad the divide it by the total system impedace (Z total) to get the bolted three-phase fault curret, as follows: 1,000,000 I Base (XFMR) = = 556 A 3 1,470 I Fault 556 = = 10.1 ka (7) () (6) Istataeous Settigs for a Maiteace Mode Scheme Page

3 Step : Determie Arc-Fault Currets The arc-fault curret is typically lower tha the bolted three-phase fault curret due to the arc impedace. The followig calculatios are used to determie the arcig curret: I bf = maximum bus fault curret i ka. I a = maximum arcig curret i ka. LogI = LogI (8) a a LogI a bf I = 10 (9) Use these equatios with I bf equal to 10.1 ka to calculate the arcig curret (I a) for this example system as follows: LogI = Log10.1 = (10) a a = = (11) I ka Step 3: Determie Mai Relay M1 Operatig Time for Bus or Close-I Feeder Faults The IEEE U3 curve formula is as follows: Tp = operatig time i secods. TD = time-dial settigs. M = applied multiples of pickup curret (1) Tp = TD M 1 The U3 curve computatios for the Relay M1 operatig time are as follows: Fault Curret i Primary Amperes 9,800 M = = = 9.8 (13) Relay Secodary i Per Uit CTR Tp( Relay M1) = = 0.81 secods (14) Add a breaker time of secods (5 cycles) to the relay operatig time to get the total operatig time as show below: Relay M1+ Breaker M1 = 0.81 secods secods = secods (15) Step 4: Idetify System Voltages ad Workig Distace Based o IEEE 1584 Table 4, for arcs at 1.47 kv, the gap betwee coductors is 153 mm (0.5 ft) for switchgear or ope-air cofiguratios. Based o IEEE 1584 Table 3, the workig distace is 910 mm (3 ft). Istataeous Settigs for a Maiteace Mode Scheme Page

4 Step 5: Determie Icidet Eergy The empirically derived method preseted i IEEE 1584 provides two equatios to calculate the icidet arc-flash eergy. The first calculates the ormalized icidet eergy. The secod calculates the icidet eergy with specific parameters. The ormalized icidet eergy equatio assumes a typical workig distace of 610 mm ( ft) ad a arc duratio of 0. secods. The ormalized icidet eergy equatio for this example is as follows: LogE = K + K LogI G (16) 1 a LogE E = ormalized icidet eergy i joules per square cetimeter (J/cm ). E = 10 (17) K 1 = for a switchgear cofiguratio ad 0.79 for a ope-air cofiguratio. K = 0.0 for a resistace-grouded system ad for a grouded system. I a = maximum arcig curret i ka. G = gap betwee coductors (153 mm [0.5 ft]). Use this equatio to calculate the ormalized icidet eergy for ope-air cofiguratios i the example system as follows: LogE = Log (18) LogE = (19) E = 10 =. J/cm (0) Next, vary the parameters to calculate the icidet eergy for specific cofiguratios for this example system. For 5 kv ope-air cofiguratios, use a workig distace of 910 mm (3 ft), ad the calculate the icidet eergy for the operatig time of secods. IEEE 1584 provides the followig equatio for calculatig icidet eergy with specific parameters: x t 610 E = Cf E x 0. D (1) E = icidet eergy i J/cm. E = ormalized icidet eergy i J/cm. C f = 1.0 for voltages above 1.0 kv. t = arcig time i secods. D = distace from the possible arc poit (910 mm [3 ft]). x = distace expoet (0.973 for 15 kv switchgear cofiguratios ad.000 for ope-air cofiguratios). The resultig equatio for this example system with a ope-air cofiguratio is as follows: E = = 18.6 J/cm () Istataeous Settigs for a Maiteace Mode Scheme Page

5 Covert the arc eergy ito calories per square cetimeter (cal/cm ) usig the followig formula: The result is as follows: 5.0 J/cm = 1. cal/cm (3) 1. E = 18.6 = 4.5 cal/cm (4) 5 Now, calculate the icidet eergy if a switchgear cofiguratio is used, ad compare it with the ope-air cofiguratio results. The calculatios for icidet eergy ormalized for switchgear are as follows: LogE = Log = 0.57 (5) 0.57 E = 10 = 3.73 J/cm (6) Vary the parameters to calculate the icidet eergy for the switchgear cofiguratio example, as show below: E = = 47.6 J/cm (7) 1. E = 47.6 = 11.4 cal/cm (8) 5 Lookig at these results, otice that for the same fault curret ad operatig times, the icidet eergy is much larger for switchgear applicatios (11.4 cal/cm ) tha ope-air applicatios (4.5 cal/cm ). It is good practice to check the icidet eergy at a lower fault curret level (80 to 85 percet) to see if the combiatio of lower fault curret ad loger trip times icreases the icidet eergy. I this example, the icidet eergy is highest at the calculated fault curret of 10.1 ka. Step 6: Implemet Istataeous Settigs for a Maiteace Mode i Relay M1 Arc-flash eergy is proportioal to V I t. Notice from the icidet eergy equatio for specific parameters (repeated below) that reducig time (t) reduces the arc-flash eergy by the same proportio. x t 610 E = Cf E x 0. D (9) Also otice from this equatio that by reducig the arcig time (t), icidet eergy is reduced by that same proportio. To reduce the arcig time, you eed to implemet a faster trippig time i the relay. Implemetig faster trippig time i Relay M1 reduces the icidet eergy whe work is doe o Breakers F1 or F. The pickup time of the overcurret elemet i a moder relay is ot istataeous. The pickup ca be delayed by filterig iside the relay ad by the time it takes for the output cotact of the relay to close. This time is typically 0.05 secods (1.5 cycles). The relay operatig time usig the U3 curve was previously calculated to be 0.81 secods with a breaker time of secods. The istataeous time is 0.05 secods, for a total operatig time of 0.05 secods secods = secods. Istataeous Settigs for a Maiteace Mode Scheme Page

6 The eergy reductio based o the ew operatig time of secods is as follows: = = The result is 8.37 times less eergy whe usig maiteace mode tha whe ot usig it. Now, usig the icidet eergy equatio for switchgear, the eergy reductio result should also be Calculate the ew icidet eergy with a relay istataeous time of 0.05 secods, as show below: (30) Eew = = 5.73 J/cm (31) 1. Eew = 5.73 = 1.37 cal/cm (3) 5 Compare this maiteace mode icidet eergy result with the previous calculatio that used the U3 curve of 11.4 cal/cm : = (33) The result is 8.3 times less eergy whe usig maiteace mode with a istataeous settig, which is essetially the same as the previous 8.37 reductio i eergy. This is a major reductio i icidet eergy. The method of usig istataeous settigs for a maiteace mode is also iexpesive to implemet, makig it a ecoomical solutio to reduce arc-flash icidet eergy. Table 1 shows the icidet eergy chage whe usig maiteace mode. Table 1 Icidet Eergy for Example System Relay M1 Settigs With Breaker M1 Time Ope Air (cal/cm ) Switchgear (cal/cm ) 0.9-secod delay secod delay (maiteace mode) Step 7: Cosider Coditios Whe Selectig Istataeous Curret Pickup Take care whe implemetig the maiteace mode scheme to uderstad how it works ad the type of load ad system that it will be applied o. If there are trasiet coditios (such as motor startig or irush) that cause the curret see by the relay to go above the pickup settig, the scheme would eed to be modified to accommodate those coditios to avoid uisace operatios that ca be extremely disruptive ad costly. Step 8: Cotrol Maiteace Mode Turig It O ad Off Eablig ad disablig the maiteace mode ca be accomplished i several ways. For distributio feeders, the hot-lie tag fuctio (if set up for arc-flash detectio) ca be tured o ad off from the frot pael of the relay. For may arc-flash applicatios, the relay ca be located i the hazard area where persoel have to adhere to more restrictive persoal protective equipmet (PPE) requiremets whe eablig or disablig the maiteace mode. Havig local cotrol oly over the maiteace mode ca result i a higher probability of huma error, leadig to failig to eable the mode whe eeded or Istataeous Settigs for a Maiteace Mode Scheme Page

7 leavig it i the mode whe ot eeded. Whe supervisory cotrol ad data acquisitio (SCADA) ad/or commuicatios are available to the relay via metallic, fiber, or wireless media, the maiteace mode ca be remotely applied ad moitored. Abset the commuicatios, the digital I/O i the relay ca be used to set up automatic eablig based o a door alarm or remote switch so that persoel do ot have to be i the hazard area to chage modes. A output cotact from the relay ca be used to give a clear idicatio of the mode from a distace via a lamp or beaco. Istataeous Settigs for a Maiteace Mode Scheme Page