Mike Krzywosz. Resilient Communications Energy for our Connected World 1

Transcription

1 Mike Krzywosz Resilient Communications Energy for our Connected World 1

2 General Arc Flash Principles 3 categories of electrical hazards: electrocution, arc flash, arc blast. Arc Flash event category creates heat and can burn human skin. DC Arc Flash calculation methods exist to determine the heat energy levels that can cause skin burns first, second, or 3 rd degree burns. Unit is calorie/cm 2 (burn energy per area). Called incident energy. (Also want to determine arc flash boundary distance). The heat part is burn energy. Example: 1.2cal/cm 2 is lighter burn for 1 seconds for 2 nd degree burn. PPE = Personal Protective Equipment, required clothing ( Arc Rated ) and equipment to prevent injury from arc flash and electrocution. Safety from electrical hazards is main goal: if working on live equipment, want to identify risk and determine PPE Level to be worn, to protect workers. Difference between 48VDC telecom and 400VDC power need an arc flash safety assessment. 2

3 DC Arc Flash Hazard Analysis NFPA 70E 2012 is Basis ( Bible for Arc Flash (AF)) NFPA 70E 2012 Edition provides three DC AF methods: 1. TABLE LOOK UP METHOD. Provides PPE Level if voltage and short circuit current are known. 2. DOAN CALCULATION METHOD. Simplified calculation method. Determines AF incident energy (cal/cm 2 ). 3. AMMERMAN CALCULATION METHOD. More complex calculation method. Determines AF IE. Today, DC Arc Flash calculation methods need more refinement still being developed. Arc Flash Hazard threshold at DC voltage of >100V and Current >1kA THESE 3 METHODS YIELD DIFFERENT RESULTS! Get experts and AHJ 3

4 DC Arc Flash Hazard Analysis PPE & HRC Level and Incident Energy (applies to AC or DC) Hazard / Risk Category and PPE Calculated Incident Energy (Calories/cm 2 ) Minimum Arc Flash Rated PPE Required (arc rating value) 0 0 < IE 1.2 Non arc rated clothing, safety glasses, & rubber gloves < IE 4 4 (2nd degree burn at 1.2) 2 4 < IE 8 8 Requires a face shield, safety glasses, and a balaclava be worn. Can cause 3rd degree burns on skin. 3 8 < IE < IE Between 25 and 40, you must wear the Moon suit IE 40 NFPA 70E does not go beyond 4 (not officially rated), >4 but could exist by energy calculation method. Work deenergized! 4

5 General Outline of 380VDC 120kW Power System Datacenter Example 5

6 400VDC 120kW Power System Equipment Example 6

7 DC Arc Flash Method Comparisons Arc Flash NFPA 70E Look up Table 130.7(C)(15)(b) EXAMPLE: Single 5kA Battery and 380VDC 120kW System Tasks Performed on Energized Equipment Hazard / Risk Category and PPE LEVEL Storage Batteries, DC Switchboards, and other DC supply sources. 2 second arc duration, 18 working distance >100V to <250Vdc Where arcing current is 1kA and <4kA 1 Where arcing current is 4kA and <7kA 2 >250V to <600Vdc Where arcing current is 1kA and <1.5kA 1 Where arcing current is 1.5kA and <3kA 2 Where arcing current is 3kA and <7kA 3 Where arcing current is 7kA and <10kA 4 7

8 DC Arc Flash Method Comparisons NFPA 70E Annex D8.1.1 DOAN Calculation Method Assumes maximum power in DC arc occurs when arcing current is half of BF current. Arcing time used is either 2 second jump back rule, or clearing time of upstream OCPD. Working distance is typically 18 (45.72cm). Goal is to calculate burn Incident Energy, IE. NOTE: For exposure where the arc is in a box ( arc in thebox ) or enclosure, it would be prudent to use a multiplying factor of 3 for the resulting incident energy value. I arc = 0.5 x I bf IE m = 0.01 x V sys x I arc x T arc / D 2 where I arc = arcing current <Amps> I bf = system bolted fault current <Amps> IE m = estimated DC arc flash incident energy at the maximum power point <cal/cm 2 > V sys = system voltage <V> T arc = arcing time <sec> D = working distance <cm> 8

9 DC Arc Flash Method Comparisons NFPA 70E Annex D8.1.2 Detailed AMMERMAN calculation Method R Arc = ( G) / I 0.88 Arc Calculation method includes GAP distance. Box Size Panelboard (Small) LV Switchgear (Medium) MV Switchgear (Large) Dimensions of Box, W x H x D (inches) 12 x 14 x x 20 x x 30 x 30 Formula Factors Arc in the Box K A (mm) G = Gap <mm> This is the actual spacing between L-L (plus to minus) Assumed for start of iteration: I DC = 0.50 x I bf I Arc = V dc / (R DC + R Arc ) Where: R DC = R Bat + R Conductors Solve for both I Arc and R Arc iteratively until both converge. Power ARC = I 2 Arc x R Arc IE Arc = Power x T Arc, or IE Arc = ( I 2 Arc x R Arc ) x T Arc Open Air Arc: IE Arc = E Arc / (4 π D 2 ) Where D= working distance <mm> So IE Arc = ( I 2 Arc x R Arc ) x T Arc / (4 π D 2 ), <J/mm 2 > or 23.9 x J/mm 2 = <cal/cm 2 > Arc in the Box: IE Box = K x IE Arc / (A 2 +D 2 ) Where K and A are experimentally derived constants, and are dependent on box geometry. Use appropriate box concentration factor from table at left. 9

10 400VDC System Based Upon VRLA Battery Design (REF ONLY) Similar to 48VDC Telecom Power Model VRLA = Valve Regulated Lead Acid, Sealed 7 Strings x 48VDC = 336VDC Nominal (28 quantity 12V Jars in series = 336VDC, 168 cells) Using 540Watt per cell UPS battery. NOTE: about 135Amp hr jars at 10hr Rate. Rectifier Output Range: 336VDC to 400VDC Battery Floated to 380VDC Multiple VRLA Battery Cabinets in parallel provide back up power. Typically, 1 to 4 cabinets, in 15 minute back up increments 10

11 System Analysis Basis & VRLA Battery Info Battery jar bolted fault current: 5kA per String Short Circuit Current can be lower (depending on circuit impedance) Arc Fault current is lower still Battery Bolted Fault current = Current at 0V fault Battery Manufacturers don t do real 0V tests, but high rate discharge tests and extrapolate to 0V per various methods. IEC 946, or IEC

12 VRLA Battery, 540Wpc 12V Jar Examples Battery Supplier VRLA Top Post Vendor P/N Battery Short Circuit Current (Amps) Vendor Method to Determine Short Circuit Current Watt per cell Enersys 12HX540FR 4775A IEEE Wpc (10x 1 min discharge) C&D UPS12-540MR 537Wpc 5000A IEC (Replaced by -21) GNB /Exide S12V550NGF 3158A IEC Wpc (Slope method) 12

13 One Line Arc Fault Diagram 13

14 Fault Location Analysis Fault Location #1 Example (Battery Cabinet and 400A C. Breaker) Fault on unprotected side of battery breaker. Breaker trip curve does not help here. DC Circuit Breaker Trip Curve 400A Example 14

15 Arc Flash Incident Energy Calculation DOAN METHOD RESULTS Fault Location # #1 Inside Bat Cab #2 DC Power Sys Bus #3 Downstream of T5 400A output breaker #4 Downstream of XT1 Load breaker NOTE: PPE for Electrocution still REQUIRED! REFERENCE ONLY Arc in Free Air Resultant Incident Energy (Cal/cm 2 ) Box Concentration Factor using 3x Prudent NFPA FACTOR FINAL Arc in the Box Arc Flash Incident Energy (Cal/cm 2 ) PPE LEVEL Required for Arc Flash Hazard Same as above Same as above

16 Arc Flash Incident Energy Calculation AMMERMAN METHOD RESULTS Fault Location # #1 Inside Bat Cab #2 DC Power Sys Bus #3 Downstream of T5 400A output breaker #4 Downstream of XT1 Load breaker REFERENCE ONLY Arc in Free Air Resultant Incident Energy (Cal/cm 2 ) Box Concentration Factor for Large Box FINAL Arc in the Box Arc Flash Incident Energy (Cal/cm 2 ) PPE LEVEL Required for Arc Flash Hazard Same as above Same as above

17 Study Summary 3 ways to find DC Arc Flash Hazard Level 2 ways to calculate AF Incident Energy Doan Method and Ammerman Method The 2 calculation methods do not agree More studies needed in DC arc flash for standardization Get right personnel and experts involved System: 120kW Power System with Total of 20kA Battery Fault Current Fault Location #1 (Inside Bat Cabinet) Arc in the Box (cal/cm 2 ) Fault Location #2 (on 400VDC Sys bus) Arc in the Box (cal/cm 2 ) NPFA 7 Look up Table Method HRC Level 3 >HRC Level 4 DoanMethod 26.4(Level4) 0.86(Level0) Ammerman Method 6.8 (Level 2) 0.22 (Level 0) 17