Electric Power System Overcurrents

Short Circuit Current

Normal, or load, current may be defined as the current specifically designed to be drawn by a load under normal, operating conditions. Depending upon the nature of the load, the value of the normal current may vary from a low level to a full-load level. Motors offer a good example. Normal motor current varies from low values (under light loading) to medium values (under medium loading) to maximum values (under maximum loading). 

The maximum load current is called full load current and is included on the motor nameplate as FLA (full load Amperes). Normal current, therefore, may vary from low values to FLA values. Additionally, normal current flows only in the normal circuit path. The normal circuit path includes the phase and neutral conductors. It does not include equipment grounding conductors.

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Overload Current

Overload current is greater in magnitude than full-load current and flows only in the normal circuit path. It is commonly caused by overloaded equipment, single-phasing, or low line voltage, and thus is considered to be an abnormal current. Some overload currents, such as motor starting currents, are only temporary, however, and are treated as normal currents. Motor starting current is a function of the motor design and maybe as much as twenty times full-load current in extreme cases. Motor starting current is called locked-rotor current and is included on the motor nameplate as LRA (Locked-Rotor Amperes). Overload current, then, is greater in magnitude than full-load amperes but less than locked rotor amperes and flows only in the normal circuit path. 

The short circuit current is greater than the locked rotor current and may range upwards of thousands of amperes. The maximum value is limited by the maximum short circuit available on the system at fault point. Short circuit current may be further classified as bolted or arcing. 

• Bolted short-circuit current. Bolted short circuit current results from phase conductors becoming solidly connected together. This may occur from improper connections or metal objects becoming lodged between phases. A large amount of short circuit current will flow into a bolted fault. 
• Arcing short-circuits current. Arcing short circuit current results from phase conductors making less than solid contact. This condition may result from loose connection or insulation failure. When this happens, an arc is necessary to sustain current flow thru the loose connection. Since the arc presents an impedance to the flow of current, smaller amounts of current will flow into an arcing fault than will flow into a bolted fault. 

Ground Fault Current

Ground fault current consists of any current which flows outside the normal circuit path. A ground fault condition then results in current flow in the equipment grounding conductor for low voltage systems. In medium and high-voltage systems, ground-fault current may return to the source thru the earth Ground fault protection of medium voltage and high voltage systems has been successfully protected for years, using ground current relays, Ground fault protection for low voltage systems is a considerable problem because of the presence and nature of low level arcing ground faults. The ground-fault current on low voltage systems may be classified as leakage, bolted, or arcing. 

• Leakage ground-fault current. Leakage ground-fault current is the low magnitude current (milliampere range) associated with portable tools and appliances. It is caused by insulation failure and is a serious shock hazard. Personnel protection is accomplished by using ground-fault circuit interrupters (GFCI) in the form of GFCI receptacles or GFCI-circuit-breakers. 
• Bolted ground-fault current. Bolted ground-fault current results when phase conductors become solidly connected to ground (i.e., the equipment grounding conductor or to a grounded metallic object). Bolted ground-fault current may equal or even exceed three-phase, bolted short-circuit current if the system is solidly grounded. Equipment protection is accomplished by using standard phase and ground overcurrent devices depending upon system voltage levels. 
• Arcing ground-fault current. Arcing ground-fault current results from a less than solid connection between phase conductors and ground. Because an arc is necessary to sustain the current flow through the connection, the magnitude of arcing ground-fault current will be less than that of bolted ground-fault current. Depending upon the arc impedance, arcing ground-fault current may be as low as several amperes (low-level) or as high as 20-38 percent of three-phase, bolted short-circuit current (high level) on a 480V system. 

Considerable research has been conducted in the area of arcing ground-fault current magnitudes on low voltage systems. Some designers use the 38 percent value while others use the 20 percent figure. NEMA PB2.2 applies ground-fault damage curves instead of performing a calculation. Equipment protection is accomplished by using ground-fault protective (GFP) devices. Due to ionization of the air, arcing ground faults may escalate into phase-to-phase or three-phase faults.

Sources of Short Circuit Current

All sources of short-circuit current and the impedances of these sources must be considered when designing coordinated power system protection. 

Synchronous generators. 

When a short-circuit occurs downstream of a synchronous generator, the generator may continue to produce output voltage and current if the field excitation is maintained and the prime mover continues turning the generator at synchronous speed. The flow of short-circuit current from the generator into the fault is limited only by the generator impedance and downstream circuit impedances. The magnitude of generator fault current depends on the armature and field characteristics, the time duration of the fault, and the load on the generator. The ability of a generator to supply current during a fault is a function of the excitation system.

• Some generator excitation systems do not have the ability to sustain short-circuit current. The magnitude of fault current is determined by the generator reactance, and, for such systems, can be essentially zero in 1.0 to 1.5 seconds. 
• Static exciters derive excitation voltage from the generator terminals. Since static exciters do not sustain short-circuit current, protective devices on the system will not operate properly, or at all. Static exciters, therefore, are not recommended. Static exciters with current boost should be specified for applications requiring static excitation. 
• Round-rotor generators with brushless exciters, typically above 10 MVA, can sustain short-circuit current for several seconds. Salient-pole generators less than 10 MVA, also with brushless exciters, will typically sustain short-circuit current at 300 percent of generator full load amperes.

 

Synchronous motors. 

When a short-circuit occurs upstream of a synchronous motor, the system voltage goes to zero, and the motor begins losing speed. As the motor slows down, the inertia of the load is actually turning the motor and causing it to act as a generator. The synchronous motor has a dc field winding, like a generator, and actually delivers short-circuit current into the fault until the motor completely stops. As with a generator, the short-circuit current is limited only by the synchronous motor impedance and the circuit impedance between the motor and the fault. 

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Induction motors. 

With one slight difference, a short-circuit upstream of an induction motor produces the same effect as with a synchronous motor. Since the induction motor has no dc field winding, there is no sustained field current in the rotor to provide flux as is the case with a synchronous machine. Consequently, the short-circuit current decays very quickly. 

Supply transformers. 

Supply transformers are not sources of short-circuit current. Transformers merely deliver short-circuit current from the utility generators to the fault point. In the process, transformers change the voltage and current magnitudes. Transformer impedances will also limit the amount of short-circuit current from the utility generators. Standard tolerance on impedance is plus or minus 7.5 percent for two-winding transformers and plus or minus 10 percent for three-winding transformers. The minus tolerance should be used for short circuit studies and the plus tolerance for load flow and voltage regulation studies

Source: 

• Technical Manual | Coordinated Power Systems Protection | pp. 2-1 to 2-3
• Publisher: US Army
• Download Here