Current Transient fault

Cuncnt imiisieiits A similar situation will arise wheti a switching ON operation of the rectifier unit occurs hen it is a thyristor rectifier. Under load conditions, the stored magnetic energy in the incoming supply system, which can be the feeding transformer and the line reactances similar to a fault condition discussed earlier, may cause a current transient which can be expressed by... 

This can be defined by the most severe external fault at which the schetne will remain inoperative. It should also remain inoperative in healthy conditions. That is it should be immune to the momentary voltage or current transients and normal harmonic contents in the circulating current. Series LC-filter circuits are generally provided with the relay coil to suppress the harmonics and to detect the fault current more precisely. 

The instant (sub-transient) fault current, /jjgf, through a generator in a symmetrical three-phase system, irrespective of the condition of neutral as defined in Table 13.9 will be... 

The following discussion and worked example for an LNG plant show how to carry out simple but reasonably accurate estimates of the sub-transient fault current and its decay in the first few cycles. Following is a discussion on how to assess the fault breaking current. 

The method adopted below is based upon the principles set out in IEC60363 and 1EC60909, both of which describe how to calculate sub-transient and transient fault currents, and are well suited to oil industry power systems. The method will use the per-unit system of parameters and variables. Choose the base MVA to be Nbase-... 

Many power system networks can be reduced to a simple series-connected circuit containing a resistance R and an inductance L, for the purpose of calculating the transient fault current. Furthermore a... 

The total of these currents is the peak asymmetrical sub-transient fault current which is 61,375 amps. This is a conservative summation because it assumes that the three peaks occur at the same time. The fault making duty of the main switchboard must be greater than this value of current,... 

Significant contribution of sub-transient fault current from induction motor consumers. 

Current standards on railway safety as EN 50128 or IEEE 1483 name them or require some countermeasures which do not necessarily cover transient failures completely. ISO 26262-109 names transient faults and describes, how they should be accounted for in reliability and safety calculations. But the standards do not give a systematic approach against transient failures or provide a deeper discussion on the causes and nature of transient faults. 

It is important for this type of safety based on self-tests, to have an application that is as robust as possible regarding transient faults that are not covered by periodic self-tests. This is the case for the majority of current automotive applications. For example, data stored in memory are refreshed before their impact becomes critical. 

I In the event of a fault in the d.c. link it will add to the circuit impedance and limit the rate of rise of fault current, since under a transient condition... 

The magnitudes of fault currents under different conditions of fault are analysed in Table 13.9. Figure 14.5 has been redrawn in Figure 13.20 for a generator circuit illustrating the sub-transient, transient and steady-state currents on an actual fault. The curve depicts the most severe fault condition which occurs when the circuit voltage is the minimum, i.e. at Vq, causing the maximum asymmetry atid the associated d.c. component. 

Since the ground fault currents in generators can be higher (Section 20.10.1) than the sub-transient state current, special care need be taken while grounding a generator to limit the ground fault current. Section 20.10.1 covers this aspect also. 

In certain cases, where a long delay may be necessary for the protective scheme to operate, it may be desirable to use the maximum steady-state short-circuit current V2 /j, for a more appropriate setting, rather than the maximum transient current i2 /, as by then the fault current will also fall to a near steady-state value, /sKr.m s.i-... 

A fault current on a power system is normally asymmetrical as discussed next, and is composed of a symmetrical a.c. component /sar.m.s.) nd an asymmetrical sub-transient d.c. component (Figure 14.5). The forces arising out of /jc aie referred to as electromagnetic and... 

The fault ctirrents also develop electrodynamic foi ces, Fii, as in equation (28.4) due to the sub-transient d.c. component. These forces play an important role in the meehanical design of the interrupting device, the load-bearing and mounting structuies for the interrupter and the bus system, and the hardware used in a switchgear assembly. All such mechanical parts, supports and hardw-are should be adequate to withstand such forces when they arise, A procedure to arrive at the ideal size of the current-carrying components, mounting structure, type of supports and hardware cte, is discussed in detail in Example 28.12. 

Protection against electrodynamic forces. These are caused by transient currents, such as on faults, and mainly affect the overhangs or the parts of the windings that fall outside the stator slots. These parts are specially braced and strengthened at the time of manufacture. 

This is to achieve a higher level of fault current to obtain a quicker tripping on fault. It is obtained when the system has a ground fault factor not exceeding 1.4 (Vg < O.SVf), as noted above. A solidly grounded system will provide effective grounding. This system will reduce the transient oscillations and allow a current sufficient to select a ground fault protection. It is normally applicable to an LT system. 

I-3.6.2 Electronic control, monitoring, and hydrogen gas measurement equipment shall be properly grounded and isolated from piping to help prevent overpressure/ accidental shutoff situations caused by equipment failure due to lightning strikes and electrical transients and to prevent safety hazards caused by fault currents. Electrical isolation equipment for corrosion control purposes should not be installed in buildings unless specifically designed to be used in combustible atmospheres. 

Transient current capability to operate over current devices to clear equipment or cable faults... 

Fig. 9.4a Transient of a three kilowatt SOFC system undergoing a load fault. Data points are shown at 1-minute intervals. Following the fault, a small amount of current remains on the stack to service balance-of-plant loads.

Calculation of fault current - rms symmetrical values Calculate the Sub-Transient symmetrical RMS Fault Current Contributions 11.6.1 Calculate the sub-transient peak fault current contributions... 

Tables 5.1-5.4 show the approximate resistance and reactance values in per-unit for two-pole and four-pole low voltage induction motors that are generally of the Design D type. Tables 5.5-5.8 show the approximate resistance and reactance values in per-unit for two-pole and four-pole high voltage induction motors that are of the reduced starting current type. In the absence of exact data from a manufacturer these data can be used for system studies such as starting motors, transient stability and fault current contribution. The data from a manufacturer should be used for calculations and system studies that are to be carried out during the detailed design phase of a project.

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. 

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