Technical Concepts - Fault Analysis Methods

Transcription

1 RATIO X/R Technical Concepts - Fault Analysis Methods Technical Concepts - Fault Analysis Methods (Continued) "What is the difference between a traditional fault calculation and an ANSI-based fault calculation" is a common question. While a thorough review of the ANSI C37 standards is required for complete understanding, the following summary highlights the primary conceptual differences. Sample AC/DC or DC only factors are shown below. The applicable curves change depending on whether you are calculating fault currents to compare with Total Rated Breakers (pre-1962) or Symmetrical Rated Breakers. They also vary for generator contributions depending on whether they are considered local or remote. The primary differences include: 1) How AC and DC decay are handled for interrupting calculations. 2) How the X/R ratios are calculated. 3) How the X/R ratios are used in the calculations. 4) Whether the calculated currents are adjusted to account for differences in application versus breaker testing conditions. A traditional fault calculation reports the initial symmetrical fault current and initial asymmetrical fault current (the difference being the DC offset) as shown in the following figure. It is also possible to report the asymmetrical fault current at any time using the DC decay, which is based on the system X/R. The DC component is calculated by equation from the system impedance and decays more slowly at higher X/R values. Notice that for traditional fault calculations, the only difference between the Asymmetrical current and the Symmetrical current is the DC offset and decay. This assumes that there is no AC decay from rotating equipment. The initial symmetrical and momentary asymmetrical values are not affected by the AC decay, however interrupting values will be affected depending on the amount of motor and local generator contribution. DC Offset Total Asymmetrical Current AC Component DC Component UTILITY Rather than calculate the DC decay by equation, the ANSI standard adjusts the motor and local generator contributions in an effort to account for both the AC and DC decay. The following adjustment factors are used in the ANSI C and C37.5 standards for the interrupting calculations. MULTIPLYING FACTORS FOR E/X AMPERES LOCAL GENERATORS MOTORS Synchronous Induction Type of Contribution X"d Multiplier Utility 1.0 (no adjustment) Synchronous Motors 1.5 Induction Motors >1000 HP <= 1800 RPM 1.5 > 250 HP <= 3600 RPM >1000 HP <= 1800 RPM <250 HP <= 3600 RPM 3.0 <50 HP Excluded from Calculation Further AC/DC or DC only adjustments are made using multiplying factors published in the standard. These factors are multiplied times the calculated fault current and serve to account for variations in X/R ratios and to compare total asymmetrical values to equivalent symmetrical values. Similarly, a low voltage factor is applied to calculated fault currents used to select low voltage breakers when the calculated system X/R is larger than the X/R used to test the circuit breaker. Another subtle difference between traditional fault calculation methods and ANSI C37 fault calculations is that the ANSI method calculates X/R ratios from separate X and R network calculations, whereas the traditional methods calculate the X/R ratio using complex impedance values. Since the decrement value adjustments are based on X/R ratios, the differences between complex network calculations and separate R and X calculations can affect the calculated fault currents. This simplified description of the differences between traditional fault calculations methods and the ANSI C37 calculation methods is intended to provide a general overview and is not intended as an application guide. There are several details left out of this discussion for simplicity. Refer to the appropriate ANSI standards and NEMA publications for detailed application instructions. The A_FAULT module rigorously follows the ANSI C37 standards. The DAPPER Comprehensive fault module follows the traditional complex calculation method.

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47