Alternate Methods to Short Circuit Breaker Coordination

Transcription

1 Alternate Methods to Short Circuit Breaker Coordination Nguyen Perez, PE Senior Electrical Engineer 2600 Douglas Road, Suite 301 Coral Gables, Florida (p)

2 Importance in Healthcare Facilities selective trip

3 Typical Analysis of Breaker Curves Graphical Representation Breaker Curve Areas of Importance Analysis Below 0.1 Seconds Analysis Above 0.1 Seconds

4 Typical Riser for Short Circuit and Selective Coordination Analysis

5 Basic Curves in Log-Log Graph Long time/short time region Instantaneous region

6 Basic Curves in Log-Log Graph Above 0.1 Second Analysis Below 0.1 Second Analysis

7 Two Classifications of Breakers LVPCB (low voltage power circuit breakers) Tested in accordance with ANSI MCCB (molded case circuit breakers) and ICCB (insulated case circuit breakers) Tested in accordance with UL-489 LVPCB MCCB

8 LVPCB Low voltage ac power circuit breakerdrawout type (shown partially disassembled to show internal features) Usually seen in the largest sizes of protective devices Continuous rating in the 1200A range and above Use as main in switchgear in lieu of switchboard Physical sizes are larger dimension than ICCB or MCCB Draw-out construction

9 MCCB Cutaway view of a typical MCCB Usually seen in smaller frame sizes Wide range of amperage covering sizes from 100A to 6000A Breakers are enclosed in a molded case insulated material does not require maintenance Group mounted in switchboard

10 ICCB Physical size is between MCCB and LVPCB Group mounted construction in switchboard

11 Switchboard vs. Switchgear Switchboard Front connected Group mounted breakers Tested for 3 cycles Switchgear Rear connected Breakers mounted in individual compartments Tested for 30 cycles

12 Switchboard vs. Switchgear A switchboard bus is a component of a prefabricated switchboard assembly manufactured according to standards. According to UL and NEMA PB , switchboards are tested for three cycles at a power factor of 50%, 30%, or 20% for short-circuit ratings of 10,000A, 10,001A 20,000A, or 20,001A 200,000A RMS symmetrical current, respectively. As a result, LVPCBs, ICCBs, and MCCBs must interrupt a fault within three cycles at maximum short-circuit current to protect the bus. It may necessitate using an instantaneous unit on the main circuit breaker to protect the bus at the sacrifice of obtaining selectivity...

13 Testing LVPCB Tested in accordance to ANSI C37.50 ICCB and MCCB Tested in accordance with UL489 UL and ANSI short-circuit test parameters

14 UL Testing Parameters Calibration (200%, 135%) Overload Temperature Rise Endurance Re-calibration Short Circuits 200% Trip Out The UL 489 test standard calls for 4-0 rated copper wire on the side and an optional 10 of rated wire on the load side for a multiple circuit breaker The circuit breaker is tested in an enclosure Dielectric

15 Different Trip Units Thermal Magnetic Trip Unit Curves Fixed instantaneous Adjustable instantaneous Electronic Trip Unit Curves LI LSI LSIG

16 Curves Portions Overload Region Long time pickup Long time delay Instantaneous Region Short time pickup Short time delay Instantaneous pickup

17 Thermal Magnetic Breaker with No Adjustability Overload region Instantaneous region (typically 8 times the continuous rating)

18 Thermal Magnetic Breaker with Some Adjustability Instantaneous is adjusted 5 to 10 times the value of the plug

19 (ETU) Adjustable Settings Totally Adjustable Long Time Pick Up Long Time Delay Short Time Pick Up Short Time Delay Instantaneous Pick Up/Override

20 Different Types of Molded Case Breakers Non-current limiting Semi-current limiting Current limiting Must operate to limit available current and energy to a value less than the energy available during the first one-half cycle of unrestricted current flow Total clearing time equals opening time (time to separate the contacts) plus arcing time (time to extinguish the arc) Symmetrical Available Fault Current

21 How is Breaker Coordination Typically Analyzed? The selective coordination of two or more lowvoltage circuit breakers (molded case or insulated case types) in series with one another can be evaluated using their timecurrent curves. When the curves are plotted together on a single graph (log-log type), trip characteristics of the circuit breakers are evaluated and checked to avoid overlaps between them.

22 Symmetrical RMS Curve

23 Asymmetry During Fault Conditions Breakers instantaneous region is sensitive to Peak Current

24 Obtaining Peak Current Relationship between X/R ratio, Power Factor and Peak Current Is = Symmetrical current RMS (obtain via short circuit calculations) IP = Peak Current IA = Asymmetrical Current RMS Mm = Maximum Asymmetrical ½ Cycle Mp = Maximum Instantaneous Peak Current factor Asymmetry Factor During 1st (1/2) cycle Power Factor (PF) Asymmetrical Current IA = Is x Mm Peak Current Worst Case Scenario IP = IA+Is Tables of Mp, PF, X/R, Mm are already published in the industry!

25 Obtaining Peak Current Asymmetrical Factors Relationship between X/R ratio, Power Factor and Peak Current IP = Is x Mp (Column 3) With this tool we simply calculate

26 Peak Current Method Instantaneous region is sensitive to Peak Current (IP) Breakers tested with PF of: 250A x 10 x 0.8 x = 2828A 2,000A x 10 x 0.8 x 2.3 = 36,800A UL and ANSI short-circuit test parameters

27 To Obtain Peak Minimum Instantaneous Trip Level of a Breaker Example: 250A Breaker Tested with PF of 100% 250A x 10 x 0.8 x = 2828A 2000A Breaker Tested with PF of 15% 2,000A x 10 x 0.8 x 2.3 = 36,800A

28 Compare Breaker Upstream and Downstream Peak minimum instantaneous trip level of upstream breaker is compared to the peak let-through current of the downstream circuit breaker. Tables of peak currents are published in the industry.

29 Sample Tables for 208V and 480V

30 Closer Look at a Specific Combination Breaker trip curves might indicate overlap which can be interpreted as lack of coordination 4,000A available fault

31 Compare Breakers Upstream and Downstream If the peak let-through current of the downstream circuit breaker is greater than the peak minimum instantaneous trip level of the upstream circuit breaker, the selective coordination level is the minimum instantaneous trip level of the upstream circuit breaker (in RMS) 250A with override set at 21,000A 21,600 x 0.9 x = 44,886A

32 Peak Let-Through Curve Method Point D 44,886 A Point C Point B Point A As in the peak current method; we need to indentify the peak minimum instantaneous trip level of a breaker upstream located in Point D on the graph When comparing Point D with Point C we can see that the breakers are selectively coordinated up to the smallest rating of the two breakers, in this case 35 Ka

33 How Will the Curves Look from 0.1s and Above? Not coordinated with an overlap around 582A

34 More Accurate Information is Observed Below 0.1s Downstream breaker will have opportunity to clear faults in 0.05s Upstream breaker will remain closed for 0.01s

35 Selectivity Tables After pairs of breakers are tested, using a protocol similar to UL testing, the values are published in tables. The intersecting column and row of the tested combination will indicate the level of selective coordination.

36 Adjusting Factor Selectivity tables are developed with typical X/R ratio of 4.9 or 20% PF For a system X/R ratio larger than the test X/R ratio, the available short circuit current equivalent RMS symmetrical rating for comparison with the values in the tables, must be adjusted by a multiplying factor

37 Summary Breaker curves do not always indicate accurate information Breakers could be more selective than curves have demonstrated Alternate methods which can be implemented during design to enhance selective coordination do exist

38 Thank You! Questions?

39 Bibliography IEEE Std IEEE Std IEEE Std Ed Larsen, A New Approach to Low Voltage Circuit Breaker Short-Circuit Selective Coordination IEEE Industrial and Commercial Power System Conference, May 2008 Marcelo Valdes, Cindy Cline, Steve Hansen, Tom Papallo, Selectivity Analysis in Low-Voltage Power Distribution Systems With Fuses and Circuit Breakers IEEE Industrial Applications, 2008 Cooper-Bussmann, EDP-1 bulletin A Simple Approach to Short- Circuit Calculations, 2004