Fault conditions

High-voltage contactor-type motor controls depend on power fuses for short-circuit protection. The fuses are coordinated with the overload relays to protect the motor circuit over the full range of fault conditions from overload conditions to solid maximum-current short circmts. 

Fault condition particularly when the LT distribution is fed through a large transformer and the outgoing feeders tre protected by a current limiting device, HRC fuses or breakers. In the event of a fault, on a large... 

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... 

It would also add to the line impedance to contain the severity of the fault conditions. 

Note To avoid unnecessary wear and lear of Ihe machine, ihe blades arc braked when ihe machine is not in operation. In fact the main protection to the iiiachinc is through the brakes only. During an overloading or system fault condition the blades are braked and the machine ceases to generate. The control panel monitors closely all the operating... 

Control and relay panel (which may be microprocessor based), to record, display, monitor and control the generated power, voltage and frequency and also detect fault conditions. 

These are conditions in which overheating of the machine may not trace back to its own thermal curves as in the first case. The temperature rise may now be adiabatic (linear) and not exponential and hence rapid. Now a normal thermal protection device may not be able to respond as in the previous case. Some conditions causing overheating may not necessarily be fault conditions. Nevertheless, they may require fast tripping, and hence are classilled in this category for more clarity. Such conditions may be one or more of the following ... 

A fault condition, such as a short-circuit between phases. 

Frequent starts It is not a fault condition, but rapid heating of motor s stator and rotor due to frequent starts will be no less severe than a fault condition, hence considered in this category. 

These call for a closer proteciion, w hich is possible through a single point niolor protection relay (MPR). Since a single MPR provides protections against unfavourable operating as well as fault conditions, we discuss this relay separately in Section 12.5. 

Coordination of fuses with a transformer Consider a distribution HV/LV transformer. If the fuses are provided on both HV and LV sides, the fuses on the HV side must protect a fault within the transformer while the fuses on the LV side must clear overcurrent and fault conditions on the LV side. Thus, for a fault on the LV side, only the LV side fuses must operate and not the HV side, similar to the requiretnenls discussed above. 

It trips quickly on a fault condition and relieves the motor from the prolonged stresses of the fault, which may cause excessive thermal and electrodynajiiic stresses. 

Thermal capacity to perform the required switching duties and sustain the fault conditions, at least up to the cut-off time of the short-circuit protective device, say, the HRC fuses. 

Below, we analyse the likely fault levels of a system under different circuits and fault conditions for an easy understanding of the subject. It is a prerequisite to decide the level of fault, to select and design the right type of equipment, devices and components and the protective scheme for a particular network. 

The philosophy to assume the impedance of the source of supply (generator or a transformer) as the impedance of the faulty circuit may be far from reality and may give a very high fault current. In actual operation, the fault intensity may be far less, as every device and component connected in the circuit will tend to add to the effective impedance of the faulty circuit and limit the magnitude of the fault current. Figure 13.15 also subscribes to this theory. But it is customary to design the systems for the worst fault conditions which, in all likelihood, may not arise, and decide the protective scheme and the current settings of the protective relays for the minimum possible fault current. 

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. 

Switchgear and controlgear assemblies 13/355 Table 13.9 Fault currents (r.m.s.) in a generator circuit under different fault conditions... 

Before creating a fault condition, to obtain the required /sc the impedance of the test circuit is adjusted so that the required fault current is obtained in all the phases on creating a short-circuit. To provide the required thermal effect (/sc /). the duration of test, /, is then adjusted accor-dingly. The relevant standards therefore stipulate that the test current may be higher or lower than required and can be compensated by adjusting its duration, i. 

A low voliage of 5Vc. at which the transformer is required to maintain its accuracy limit, is of great significance. A protection iran.sformer is required to operate under a fault condition, during which the primary voltage may dip to a value as low as 5% of the rated voltage. 

The rated voltage factor and the corresponding rated time Not applicable Depends upon the system grounding. Refer to Table 15.4 Depends upon the system fault conditions and generally as 1.9 Table 15.4... 

Where three CTs for unrestricted or four CTs for restricted ground fault or combined O/C and G/F protections are employed in the protective circuit, the VA burden of the relay is shared by all the CTs in parallel and a normal VA CT may generally suffice. Such is the case in most of the protective schemes discussed in Sections 21.6 and 15.6.6(1), except for those employing only one CT to detect a ground fault condition, such as for a generator protection with a solidly grounded neutral (Figure 21.12). 

These are auxiliary CTs, and are sometimes necessary to alter the value of the secondary of the main CTs. They help to reduce the saturation level and hence the overloading of the main CTs, particularly during an overload or a fault condition. They are used especially where the instruments to which they are connected are sensitive to overloads. They have to be of wound primary type. So that the main CTs are not overburdened they have a VA load that is as low as possible. Figure 15.20 illustrates the application of such CTs and their selection is made on the following basis ... 

Figure 15.25 A through-fault condition outside the protected zone in a differential scheme...

Figure 15.26 Power circuit and control scheme during a very severe external fault condition...

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