Fundamental of Short Circuit Currents

 

Medium Voltage Switchgear | Source: Schneider Electric

A short circuit is one of the major incidents affecting electrical systems.

Short-circuit current at a given point in the system is expressed as the RMS value Isc (in kA) of its AC component. The maximum instantaneous value that short-circuit current can reach is the peak value Ip of the first half cycle. 

The peak value can be much higher than √2 * Isc, because of the damped DC component that can be superimposed on the AC component.This random DC component depends on the instantaneous value of the voltage at the start of the shortcircuit and on the system characteristics.

The consequences of the short circuit are: 

• a short circuit disturbs the system environment around the fault point by causing a sudden drop in voltage,
• it requires a part of the system (often a large part) to be disconnected through the operation of the protection devices, 
• all equipment and connections (cables, lines) subjected to a short circuit undergo strong mechanical stress (electrodynamic forces) which can cause breaks, and thermal stress which can melt conductors and destroy insulation, 
• at the fault point, there is often a high power electrical arc, causing very heavy damage that can quickly spread all around.

Related Article: Sources of Short Circuit Currents

Type of Short Circuit

• Symmetrical Three Phase Fault
• Unbalanced Line to Line Fault
• Unbalanced Single Line to Ground  

Short Circuit at Generator Terminals 

It is more complicated to calculate short circuit current at a synchronous generator's terminals than at the terminals of a transformer connected to the system. This is because the internal impedance of the machine cannot be considered constant after the start of the fault. 

It increases progressively and the current becomes weaker, passing through three characteristic stages:

• Subtransient: (approx. 0.01 to 0.1 sec). Short-circuit current (rms value of the AC component) is high: 5 to 10 times permanent rated current. 
• Transient: (between 0.1 and 1 sec). Shortcircuit current drops to between 2 and 6 times rated current. 
• Continuous: Short-circuit current drops to between 0.5 and 2 times rated current.

Related Article: Fundamentals of Generator Protection

Short Circuit Currents at Generator Terminals

It can be concluded that short-circuits at generator terminals are difficult to assess, and that their low, decreasing value makes protection setting difficult.

Equipment Behavior During Short Circuit

There are 2 types of system equipment, the type that intervenes and the type that does not intervene at the time of a fault.

Passive equipment

This category comprises all equipment which, due to its function, must have the capacity to transport both normal current and short-circuit current without damage. This equipment includes cables, lines, busbars, disconnecting switches, switches, transformers, series reactances and capacitors, instrument transformers. 

For this equipment, the capacity to withstand a short-circuit without damage is defined in terms of: 

• electrodynamic withstand (expressed in peak kA), characterizing mechanical resistance to electrodynamic stress. 
• thermal withstand (expressed in RMS kA for 1 to 5 seconds) characterizing maximum admitted overheating. 

Active equipment

This category comprises the equipment designed to clear short circuit currents: circuit breakers and fuses. This property is expressed by the breaking capacity and if required, by the making capacity upon the occurrence of a fault. 

Breaking Capacity 

This basic characteristic of a switching device is the maximum current (in rms kA) it is capable of breaking in the specific conditions defined by the standards, it generally refers to the rms value of the AC component of the short circuit current; sometimes, for certain switchgear, the rms value of the sum of the 2 components is specified: AC and DC; it is then "unbalanced current". 

The breaking capacity requires other data such as:

• voltage, X/R ratio of broken circuit
• system natural frequency
• number of breaks at maximum current, for example the cycle: B - M/B - M/B (B = breaking; M = making)
• status of the device after the test. The breaking capacity appears to be a fairly complicated characteristic to define: it, therefore, comes as no surprise that the same device can be assigned different breaking capacities depending on the standard by which it is defined.

Making capacity upon the occurrence of a short-circuit

In general, this characteristic is implicitly defined by the breaking capacity: a device should have the capacity to "make" upon the occurrence of a short-circuit that it has the capacity to break. Sometimes making capacity needs to be higher, for example for AC generator circuit breakers. 

The making capacity is defined at peak kA since the 1st asymmetric peak is the most restrictive one from an electrodynamic point of view.

Short-circuit current presumed to be "broken" 

Some devices have the capacity to limit the current they are going to break. Their breaking capacity is defined as the maximum current presumed to be broken that would develop in the case of a full short circuit at the upstream terminals of the device. 

Reference: 

• Protection Guide and Control
• Publisher: Merlin Gerin | Download