Protection schemes

Some of the terpolymers containing high levels of AGE give superior sour gasoline and ozone resistance, particularly dynamic ozone resistance. Since the unsaturation is not in the polymer backbone, it can be, and apparentiy is, sacrificed under sour gasoline or ozone aging. This protection scheme is limited with the peroxide and sulfur cure systems as they involve the aHyl functionaUty of the polymer. The protection is maximized when a dinucleophilic curative, such as trithiocyanurate, is used. 

Some overload-protection schemes measure motor-winding temperature directly various methods are used. Small single-phase motors are available with built-in overload protection, A thermostat built into the motor senses motor-winding temperature directly. 

FIG. 29-8 Typical high-voltage ac motor starter illiistrating several protective schemes fuses, overload relays, ground-fault relays, and differential relays with the associated current transformer that act as fault-current sensors. In practice, the differential protection current transformers are located at the motor, hut the relays are part of the starter. 

InHBr, AcOH, 25°, 30min. The5-Cbzgroup is removed slowly under these conditions, but the N-Cbz group is completely cleaved, thus providing some selectivity in the protection scheme for cysteine. 

Due to a high lime constant of the dampening circuit T= LJR (R being the resistance of the circuit) it will also delay occurrence of the fault by which time the circuit s protective scheme may initiate operation. 

For meticulous protection, therefore, it is advisable to use a motor protection relay for large LT and all HT motors. The following motor details and working conditions are essential to know before making i proper selection of a protective scheme ... 

Having discussed the effect of the above parameters on the motor s performance, we will now illustrate by way of an example a general case to broadly suggest a procedure that can be followed to select the protective scheme for a motor. For more detailed selection of the motor protection relay, reference may be made to the relay manufacturer. 

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. 

This is a simple calculation to determine the maximum symmetrical fault level of a system, to select the type of equipment, devices and bus system etc. But to decide on a realistic protective scheme, the asymmetrical value of the fault current must be estimated by including all the likely impedances of the circuit. 

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-... 

We also provide a brief reference to a protective scheme, usually adopted in a large power-generating station as in Section 16.8.2. 

The control and protection scheme diagram for all the above requirements is given in Figure 13.51. It is presumed that all the drives are provided with direct on-line switching. 

The choice of class 3P or 6P will depend upon Ihe application and the protection scheme of the system. The following may be considered as a rule of thumb when making this choice. 

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). 

Both requirements of measuring and protection cannot be met through one transformer generally. Thus two sets of transformers are required for a power circuit associated with a protection scheme, one for measurement and the other for protection. 

Consider a protective scheme having a total VA burden of 15 and requiring an accuracy limit factor (ALF) of 20 ... 

For high set protective schemes, where to operate the protective relays, the primary fault currents are likely to be extremely high, as in the above case. Here it is advisable to consider a higher primary current than the rated for the protection CTs and thus indirectly reduce the ALF and the product of VA X ALF. In some cases, by doing so, even one set of CTs may meet the protective scheme requirement. 

Consider a system being fed through a transformer of 1500 kVA, 11/0.433 kV, having a rated LV current of 2000 A. The protection CT ratio on the LV side for the high set relay may be considered as 4000/5 A (depending upon the setting of the relay) rather than a conventional 2000/5 A, thus reducing the ALF of the previous example from 20 to 10. Now only one set of 15 P10 CTs will suffice, to feed the total protective scheme and have a VA x ALF of not more than 150. 

These are protection CTs lor special applications such as biased differential protection, restricted ground fault protection and distance protection schemes, where it is not possible to easily identify the elass of accuracy, the accuracy limit factor and the rated burden of the CTs and where a full primary fault current is required to be transformed to the secondary without saturation, to accurately monitor the level of fault and/or unbalance. The type of application tind the relay being used determine the knee point voltage. The knee point voltage and the excitation current of the CTs now form the basic design parameters for such CTs. They are classified as class PS CTs and can be identified by the following characteristics ... 

We discuss below a high-impedance differential protection scheme to provide a detailed procedure to select PS Class CTs. 

Figure 15.24 Equivalent control circuit diagram for a differential ground fault protection scheme of Figure 15.22...

If there are N number of CTs connected in parallel, the magnetizing current will flow through all of them. In a CiF protection scheme all the three CTs of all the feeders being protected together will fall in parallel, while in case of a combined GF and phase fault protection scheme, only one third of these CTs will fall in parallel. The CT in the faulty circuit must be able to draw enough current to feed the magnetizing losses of all the CTs falling in parallel and the relay pickup current, The sensitivity of the differential scheme can therefore be expressed more appropriately as... 

Since it determines the sensitivity level of the protection scheme, it must be kept as low as possible to detect even a small fault. To achieve a high degree of sensitivity it is therefore essential... 

The most severe fault is the capacity of the machine or the system being protected to feed the fault, and is determined by its fault level as indicated in Tables 13.7 and 13.10. To consider a higher fault level than this, such as of the main power supply, is of little relevance as it would fall outside the detection zone of the CTs and would serve no useful purpose except to further improve the stability level of the protective scheme. 

Figure 15.32 Phase and ground fault differential protection scheme for a transformer and feeder bus protection...

This is a complex subject and requires detailed engineering and application of the various protection schemes for... 

Figure 16.14 A typical protection scheme in a large generating station...

Where this is not necessary, it can be assessed by the equipment s exposure to such TOVs and surges and thus determine the appropriate level of BIL. lEC 60071-2 provides guidelines for the most appropriate surge protection scheme and is discussed in Section 18.6. 

Duration This will depend upon the ground fault protection scheme adopted and may be considered as follows ... 

Important parameters for selecting a ground fault protection scheme 20/669... 

Below we briefly discuss the criteria and theory of selecting a grounding system to achieve a desired level of fault current to suit a predetermined ground fault protection scheme, i.e. type of grounding and grounding impedance to suit the system voltage, type of installation, and location of installation. 

When some extra impedance R, Xq, Xi or a combination of these is introduced into the ground circuit it will become possible to alter the magnitude and the characteristic of the ground circuit current, /g, to suit an already designed ground fault protection scheme as discussed below. 

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