Notes on Short-Circuit Calculation

Contents

1. Synchronous Generator Reactance ................................................................................................ 2

2. Primary Resistance between Generator Star Point and Ground .................................................... 5

3. Short Circuit Contribution of Variable Speed Drives ...................................................................... 5

4. Transformer Impedance ................................................................................................................. 6

5. Fault Current Variables Definition .................................................................................................. 8

6. Conditions for Calculating Maximum/ Minimum Short-Circuit Currents ..................................... 12

7. Circuit Breaker Characteristics ...................................................................................................... 22

8. Fault Current Variables of Interest ............................................................................................... 28

1. Synchronous Generator Reactance

- Saturated reactance values are used for short-circuit calculation as they are lower than

unsaturated reactance.

Fault Current Components (idc, etc.)

2. Primary Resistance between Generator Star Point and Ground

If RSec is known, RPrim can be calculated. RPrim = RSec × N2 N = ratio of generator neutral grounding transformer

3. Short Circuit Contribution of Variable Speed Drives

In practice, the impact of standard PWM drives during a line short circuit condition can be ignored. For dc drives, Current Source Drives and line regenerative PWM drives, the maximum contributing current should be only slightly more than the overload rating of the drive. DC drives and Current Source Drives contain current limit functions which will interrupt excessive currents in milliseconds. In all cases, motors operated on adjustable speed drives, will contribute less than would be experienced with the motor connected directly on line. With the standard PWM drive, there is no contribution to short circuit current when a short circuit condition exists on the incoming ac line.

N2?

4. Transformer Impedance

5. Fault Current Variables Definition

6. Conditions for Calculating Maximum/ Minimum Short-Circuit Currents

7. Circuit Breaker Characteristics

Switch-disconnectors

ABB MV SWGR – UniGear ZS1

ABB Gas Insulated MV CB – HD4

ABB Vacuum MV CB

8. Fault Current Variables of Interest

Ik” : Initial short-circuit current Ik : Steady-state short-circuit current ip : Peak current Ith : Thermal short-circuit current (1.0 s) Ib : Breaking current (0.05 s & 1.0 s) iDC : DC component of short-circuit current (0.05 s & 1.0 s) Iasy : Asymmetrical breaking current (0.05 s & 1.0 s – same setting field as for iDC fault duration) I(0) : Zero-sequence current 3*I(0) : Three times zero-sequence current (transformer neutral earthing size)