Faults Busbars

If the short-time lating of the interrupting device is higher than the fatilt level of the system, which is the case with modern interrupting devices, the fault level of the system alone will prevail for the busbars, components and hardwaie. For example, for a system fault level of. SO kA. if the interrupter used is of 65 kA short-time rating, the bus system and all associated components w ill be designed for 50 kA onlv. 

Manufacturers may adopt different practices to mount the main and auxiliary busbars, depending upon the size, rating and fault level of the system. Some of the recommended and more common of these are illustrated in Figure I3.30(a)-(d) and discussed briefly below,... 

To obtain a strong busbar mounting system, suitable to withstand the electrodynamic forces arising out of a system fault, modern practice is to make use of thermosetting plastics, such as DMC (Dough Moulding Compounds)... 

Petroleum jelly is not recommended due to its low tracking lempeialure. The minimum tracking temperature is recommended to be 200°C, the same as for the busbars during a fault. Also refer to Section 28.4.1. 

Only high tensile (HT) fasteners must be used for busbar Jointing and their interconnections or links not only to take care of the fault level but to also maintain the recommended contact pressure over a long period of operation as noted in Table 29.1. An ordinary fastener may not be able to withstand or sustain this torque for long. Similarly, the busbar supports, which are mounted on only two or three fasteners, should also be fitted with these fasteners. 

Assume the temperature of the busbars at the time of fault = 85°C and rectangular flats of electrolytic grade E-91E or its equivalent. Busbars chosen for each phase - four (152.4 mm X 6.35 mm) - which are more than the minimum size required to account for the thermal effects during a short circuit condition... 

F = maximum electrodynamic forces acting on each support, in the event of a fault, as calculated above = 514 kgf / = centre distance between two busbar supports = 40 cm M = sectional modulus of each busbar at section x - x... 

The minimum shearing strength of aluminium is 1650 kg/cm (Table 30.1) which is much larger than the actual force to which the busbars will be subject, in the event of a fault. They are thus more than adequate in cross-section and numbers. Other than bending stress, there is no significant tensile or shearing force acting on the busbars. 

It is possible that during the fault only one ot the insulators is subject to the transitory first peak of the fault, as there may be slight misalignment between the insulators, asymmetry in the busbars, an imperfect bolt fixing and their fastening, or a combination of such factors. To be on the safe side it is advisable to consider each support and its fasteners to be suitable to withstand the forces by themselves. We have assumed a factor of safety of 100% in all the above calculations to account for this. 

Often a failure on a fault may be due not to the inadequate size of busbars, fasteners or insulators but to poor alignment of the insulators or to too large a gap between the busbar and the insulator slots. It may be a consequence of an inappropriate mounting or unequal width of the busbars or insulator slots. In such cases, load sharing w ill be uneven and the weakest section may fail. This can be illustrated as follows ... 

A loose fit of busbars itiside the slots may cause e.xcessive vibrations on a fault and may lead to loosening of the fasteners and shearing of the wedges and/or the edges and the fingers of the insulators. Even the insulator mounting section. v - x may become vulnerable to failure. 

Moulded-in busbars 6 Isolator blades with moving contacts 7 Hinged front door 8 Door interlocked with isolator 9 Flameproof window for instruments, indicator lights and operation counter 10 Flameproof windows for visual isolation 11 Fault reset and/or earth fault test push-buttons under covers 12 Static protection unit ... 

Busbar nominal fault breaking capacity in kA at 1 or 3 seconds. 

Details of special devices such as transducers, automatic voltage regulators, synchronising schemes, fault limiting reactors, reduced voltage motor starters, busbar trunking. 

Switchgear tends to be operated infrequently, whereas motor control centres operate frequently as required by the process that uses the motor. Apart from the incomers and busbar section circuit breakers, the motor control centres are designed with contactors and fuses (or some types of moulded case circuit breakers in low voltage equipment) that will interrupt fault currents within a fraction of a cycle of AC current. Circuit breakers need several cycles of fault current to flow before interruption is complete. Consequently the components within a circuit breaker must withstand the higher forces and heat produced when several complete cycles of fault current flow. 

In the above simple example some of the margin between 60 kA and 80 kA will be taken up by the sub-transient contributions from the motors. It can be noted at this point that if the transformer is subsequently increased in rating by the addition of forced air fans, then the fault current passed by the transformer will be unchanged. It is advisable to specify the rating of the transformer in its forced air-cooled mode of operation, if such cooling is considered likely to be needed in the future. This would ensure that the incoming circuit breakers and busbar normal rated currents would be correctly matched to the transformers. 

Suppose the generator is connected to a nearby switchboard. The generator and busbar section circuit breakers will need to at least withstand the fault current given in (7.1). The equation consists of three essential parts -... 

Hence the circuit breakers and the busbars in the switchboard may have to be derated for the breaking duty. The amount of DC component, or off-set as it is often called, depends upon the point in time set by (po when the fault is applied. The occurrence of 100% off-set is seldom but cannot be ignored. The design and selection of the switchboard should be based on 100% off-set, especially if it is fed by generators and feeds a group of high voltage motors. 

In some cases it is also necessary to consider the fault current contributed by motor consumers, particularly if large synchronous motors are fed from the same busbars as the main generators or main transformer infeeds, see Chapter 11. Induction motors contribute fault current during the sub-transient period and so extra allowance must be made when calculating the making duty. 

Moulded case circuit breakers are also available for incoming and busbar section purposes, with ratings up to 6000 A and service voltages between 220 V and 660 V. (At 415 V a 4000 A circuit breaker would satisfy the duty of a 2500 kVA feeder transformer with about 15% spare capacity.) These are also available as 4-pole units. Circuit breakers having ratings of 800 A and above are often provided with several adjustments that widely modify the shape of the complete protection curve, as described in Chapter 12. This enables the curve to coordinate with almost any other protective device or equipment that is immediately upstream or downstream of the circuit breaker. Some circuit breakers with the higher rated currents are also provided with integral earth fault protection facilities. 

When fuses or moulded case circuit breakers are applied to a circuit it is necessary to ensure that their /-squared-t characteristics coordinate properly with the thermal capabilities of the downstream equipment, especially the cables. In order to determine the /-squared-t characteristics of a protective device it is assumed that the current in the device suddenly changes from a normal load value to the fault value in a very short period of time, i.e. similar to a step change in a control system. Hence for each value of current along the x-axis of the device s time-current characteristic the value of the current squared multiplied by the corresponding time can be plotted. For cables and busbars the /-squared-t function equals a constant (k) for each cross-sectional area of conductor, as explained... 

For AC systems the calculation of the short-circuit current is more complicated, particularly when generators and motors are both present in the system. The simplest calculations occur when the source of voltage can be assumed to be of constant magnitude during the fault duration. In AC systems the source impedance will be the addition of the cable impedance, busbar impedance, transformer internal impedance, the appropriate internal impedance of the generator, the appropriate internal impedance of the motors in system and the impedance of the overhead transmission lines. 

The cables and busbars connecting the transformers to the switchboards are very short in comparison with the length of the transmission lines and the transformer reactances and so their impedances may be ignored. Consider the fault being applied to the busbars of the T4 switchboard. The fault circuit for the switching configuration shown is through T2 and T4. 

As a project moves into the detail design phase it acquires more precise data for all aspects of the work. It is then possible to calculate the fault currents more accurately. However, it should be noted that the tolerances on most of the data are seldom better than plus or minus 15%, and so increasing the quantity of data will not necessarily improve the results significantly. During the detail design phase the power system tends to be modified and additional switchboards added. It is then necessary to calcnlate the fault currents at least at the busbars of each switchboard, and this can become a laborious task if hand calcnlations are attempted. 

All these considerations apply to HV motors, particularly if they are fed directly from the main generator switchboard. LV motors can often be grouped together and considered as one large equivalent motor. It is sometimes possible to ignore the contributions from LV motors because their circuits often have a low X-to-R ratio, which causes the motor contribution to decay very fast. Also, the connected cables, busbars and transformers in the circuit will tend to attenuate the motor fault contribution. 

It is interesting to note that in this case there will be no current flowing in the reactor that couples the two outer busbars. However, this reactor cannot be omitted because it serves its purpose when faults occur at the outer switchboards. 

The situation for the secondary circuit is different. The purpose of instantaneous protection is to detect major faults at or near to the secondary terminals and at the downstream switchgear e.g. busbar fault. This protection must also be coordinated with the instantaneous protection settings of downstream circuits e.g. static loads, motors. The settings chosen are much less sensitive to the upstream source characteristics than those of the primary protection, because of the inclusion of the leakage impedance of the transformer in the faulted circuit. 

Faults can occur within the busbar and riser compartments of switchboards. However, modern switchgear is very reliable and snch fanlts are rare. Many pnrchasers of switchgear specify insulated and segregated bnsbars and risers, with all the connectors, clamps, nnts and bolts to be fully shrouded with flame retardant material, see sub-section 7.2.4. 

Where a bus-section circuit breaker is used to divide the busbars (during abnormal operating conditions) each set of busbars is protected as a separate zone. Each zone consists of the incomers, the outgoing circuits and the bus-section circuit breaker(s). An accurate current transformer is connected in each line of each circuit. All the current leaving the zone must be balanced by current from the incomer circuits. A fault in the zone will be detected by the (87) relay. Rapid operation is required... 

It is not normally necessary to provide overcurrent protection in the bus-section circuit because the presence of overcurrent, not caused by an in-zone fault, would be detected by an outgoing circuit relay. For the busbar to be overloaded the outgoing system must also be overloaded. Introducing an overcurrent relay in the bus-section circuit will add comphcation to the coordination of the incoming and outgoing relays, since a time margin is necessary between each relay. In systems where there are large induction motors the coordination can already be awkward to achieve. 

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