Environmental Engineering Reference
In-Depth Information
compliant but not in agreement with the model, the model must be altered; if
it is not compliant with the code, there are likely to be significant commercial
Fault level issues
All embedded generation presents potential fault level issues. Fault level is a
measure of the energy dissipated at a point by the worst possible fault. In practice
the fault level - the product of per unit fault current at the point of interest and
nominal per unit voltage (1.0 pu) and specified in MVA - is used to
assess the capability of equipment to make, break and pass the severe currents
experienced under short circuit conditions
establish suitable settings for protective relays and devices
form the basis for many 'rule of thumb' calculations related to voltage
depression during load or network switching, capacitor block sizing or har-
monic distortion levels
If the fault level is excessive, switchgear may not be able to deal safely with
the currents involved. If it is too low, network protection may not operate as
designed, switching operations may result in large step voltages and harmonic
distortion may be severe. DC drives and FACTS devices may experience com-
mutation failures.
Manufacturers of wind turbine equipment have generally now undertaken tests
to demonstrate the contribution which a wind turbine makes to fault current
immediately upon application of the fault and how that changes over time until the
fault is cleared by protection. It is said that this is reliable information for three-
phase faults but that there is less certainty for single-phase to ground faults. That
could present an issue for utilities which operate effectively earthed systems, where
fault current experienced by switchgear for single-phase to earth faults may be
higher than in each phase of a three-phase fault.
4.7.1 Equipment capability
Three quantities are needed to determine equipment capability:
the maximum current which might flow
the maximum time for which it will be allowed to flow
the power factor of the current relative to the voltage at that point
Switchgear is usually specified as having symmetrical and asymmetrical load
making and load breaking duties as well as a continuous load current rating. The make
duty is to cope with a situation where a circuit breaker may be closed when a fault is
already present. It must not suffer damage and explode, endangering the person
closing it. The break duty is to ensure that the breaker can interrupt the fault safely.
Equipment experiences two physical effects when fault currents flow. The
dominant factor in most equipment tends to be thermal. This is proportional to
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