(Second of a Series)
THE NEED FOR FAULT CALCULATIONS IN DESIGN
Fault Calculations must precede any effort to procure system protection devices. This activity is supposed to be one major part of the design process, but is oftentimes skipped or omitted. Several provisions of the National Electrical Code, the Philippine Electrical Code & ANSI/IEEE Publications relate to proper system protection. Safe and reliable operation of the industrial plant based on these provisions mandate that electrical systems must be protected adequately & effectively.
While over-current protective devices (OCPD’s) are provided for overload protection for system components such as switchgears, busses, wires & cables, motor controllers, etc; it is also necessary as discussed earlier to place protection for more damaging events such as faults. To obtain a reliable operation and to assure that system components are protected from damage during abnormal events, it is necessary to first calculate the fault duties at various points in the electrical system while still on the drawing boards and adequate protective devices must subsequently be in place to anticipate these faults.
For all possible conditions, it is the responsibility of the system designer to design electric power systems with adequate control of short circuits as one major consideration. It is also the plant engineer’s responsibility to see to it that the protective relaying devices are armed to pre-determined settings either by himself or by consultants. As can be recalled, uncontrolled short circuits can cause service outages with accompanying production downtime and associated inconvenience, interruption of essential facilities, extensive equipment damage, personnel injury or fatality and a possible full-blown fire.
It is therefore important to de-mystify the stigma of faults and its counter-measures. Again, the system designer is responsible for the selection of the right equipment; and would generally have the task of calculating system short circuits. But alas… - procedures & techniques for these calculations are not generally available in one dissertation but are scattered among many publications & technical papers.
Fault Calculations also result to at least three very significant outputs which will become the bases of the following:
1) First: Proper selection of protective equipment ratings as circuit breakers or fuses that suit to system requirements;
2) Second: Realistic arming up of protective relays to trigger operation of circuit breakers once faults do occur;
3) Thirdly, Proper coordination of operation of these protective devices to effect selective interruptions of the only required breakers to trip faulted circuits without the hassle of rendering the other portions of the system powerless.
TYPES OF POWER SYSTEM SHORT CIRCUITS
Contributing sources of Short Circuit Currents into an industrial plant in focus include: Utility Generation thru the Industry Substation, Local Generation, In-Plant Synchronous Motors and Induction Motors. Capacitor discharge currents are normally neglected due to their short time duration.
In several forms or ways, short circuits can occur on a three-phase system. Again, for any type of faults, the protective devices must interrupt the faulted circuits while at the same time be able to withstand these faults.
a) THREE-PHASE BOLTED SHORT CIRCUITS
Three-phase bolted short-circuit describes the condition where the three conductors are physically held as if they were bolted together. In this condition, the impedance between these conductors or terminals is zero. This establishes a “worst case” condition, which results in maximum thermal and mechanical stress in the system. While ‘bolted short circuit condition’ seldom occurs (only in cases of errors in connecting buses or cables), it generally results in maximum short-circuit values and for this reason that the “basic short circuit calculation” in power systems is employed.
Subsequently, it is from these maximum values that the selection of fault duty ratings of circuit breakers, other protective devices and switchgear withstand ratings shall be based, busways included.
b) LINE-TO-LINE BOLTED SHORT CIRCUIT
From the three-phase fault calculation, other types of fault conditions can be obtained. The levels of line-to-line bolted short circuit currents in most three-phase power systems are approximately 87% of three-phase bolted short circuit currents, but this calculation is seldom required because it is not the maximum value, especially for establishing circuit breaker ratings. But then these values are needed as basis in relay settings or other purposes.
c) LINE-TO-GROUND BOLTED SHORT CIRCUITS
In solidly grounded systems, single line-to-ground bolted short circuit current in general, can be almost equal to the three-phase bolted short circuit current. Most of the time, SLG fault currents are lower than the 3Φ short circuit current due to the impedance of the ground return circuit and due to the non-zero-sequence current contribution from the motors which are usually ungrounded. Line-to-ground fault calculations are seldom necessary in solidly grounded low voltage industrial or commercial systems, but are needed in medium voltage systems for relay setting purposes.
In resistance-grounded medium voltage systems (common in 4.16 to 23kV) the neutral grounding resistor is generally selected to limit ground fault current to a value ranging between a few hundreds & a few thousand amps. Magnitudes of Line-to-Ground fault currents on these systems are controlled primarily by the resistor itself and a line-to-ground fault calculation using symmetrical components is generally required to size up the resistor.
d) ARCING SHORT CIRCUITS
In the real world, faults in many power systems tend to be arcing in nature. Statistics say that Single-Line-to-Ground Arcing Faults are the most frequent faults experienced in any power system.
Arcing faults are much lower level short circuit currents than the bolted ones at the same fault point. These lower levels of currents are due to the impedance of the arc ‘inserted’ into the circuit. Normally, arcing fault currents fall in the range from 40% to 50% of the bolted values.
(to be continued...)
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