Short Circuit

SHORT-CIRCUIT ANALYSIS IEC STANDARD

Purpose of Short Circuit Studies We wish to: • Verify protective device close and latch capability. • Verify protective device interrupting capability. • Protect equipment from large mechanical forces (max. fault kA). • Protect equipment from thermal stress (I2t). • Select ratings/settings for relay coordination.

Types of Short Circuit Faults

Types of Short Circuit Faults

Fault Current • IL-G can range in utility systems from a few % to possibly 115% (if X0 < X1) of I3-phase (85% of all faults). • In industrial systems a situation where IL-G >I3-phase is rare. Typically IL-G ≈ 0.87 x I3-phase • In industrial systems, 3-phase fault conditions are generally only considered as it results in maximum current.

Short Circuit Phenomenon

di v(t)  Ri  L  Vm  Sin(t   ) (1) dt Solving equation 1 yields the following expression R - t Vm Vm i(t)   sin(t   -  )   sin( -  ) e L Z Z     Steady State

Transient (DC Offset)

AC (symmetrical) with no AC decay

DC Current

Machine Reactance ( λ = L I )

AC Decay Current

Definitions IEC (60909) I’’k • Initial Symmetrical Short Circuit Current. This is the fault level value.

Ip • Peak Short Circuit Current

Ib • Symmetrical Short Circuit Breaking Current

Ik • Steady State Short Circuit Current

Isc Contributing Elements •Generator •Power Grid •Synchronous Motors •Induction Machines •Lumped Loads (with some % motor load) •Inverters, PV Array, WTG •I0 from Yg-Delta Connected Transformer

Types of Short Circuits When selected

Maximum Voltage Factor is used. • Ratio between pre-fault voltage and nominal voltage.

Minimum Impedance is used. • All negative tolerances are applied and minimum resistance temperature is used.

Types of Short Circuits When selected

Minimum Voltage Factor is used. • Ratio between pre-fault voltage and nominal voltage.

Maximum Impedance is used. • All positive tolerances are applied and maximum resistance temperature is used.

Types of Short Circuits When selected

Required to account for: • Variations due to time and place • Transformer taps • Static loads and capacitances • Generator and motor subtransient behaviour

Zero Sequence Model

When this option is checked: • Branch susceptances and static loads including capacitors will be considered. • Recommended by IEC for systems with: • Isolated neutral • Resonant earthed neutrals • Earthed neutrals with earth fault factor > 1.4

IEC SC 60909 Calculation

Transient Fault Current Calculation (IEC 61363)

Transient Fault Current Calculation (IEC 61363)

Transient Fault Current Calculation (IEC 61363)

Transient Fault Current Calculation (IEC 61363)

Transient Fault Current Calculation (IEC 61363)

Unbalanced Fault Display and Reports

Short-circuit Device duty Analyzer •Comparison of multiple reports with different configurations with fast 64-bit analyzer •Critical and Marginal alerts •Worst case duty evaluation •Sorting and filtering individual columns based on study •Export results based on evaluation from analyzer.

Short-circuit Device duty Analyzer

Short-circuit Device duty Analyzer

Steps for Device Sizing 1. Identify all possible operating modes (revisions, configurations and study options) 2.Set up scenarios to include all cases. 3.Determine worst case fault current for each device. 4.Select device rating based on worst case fault current and company design guidelines. 5.Re-run device evaluation.

Unbalanced Fault Display and Reports

Unbalanced Fault Display and Reports