Differential protection schemes

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

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

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. 

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 ... 

Figure 15.22 A circulating current scheme to provide a phase and a ground fault differential protection...

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... 

Differential proteetion (Relay Code 87) To detect a stator phase-lo-phase fault by a three-pole differential protection relay, current setting 10 0%. For scheme diagrams, refer to Section 15.6.6(1). 

The dominant protection scheme for generators and motors is the differential relay. Access to all enti y points of the protected zone is usually readily available, no coordination with the protection of other... 

Differential relaying is the universal bus and transformer protection scheme. The inrush current associated with power transformers rec uires a special differential relay utilizing filters to provide harmonic restraint to differentiate between energizing current and fault current. 

As demonstrated by MacMillan and coworkers, a-oxygenated aldehydes are very good reaction partners in the aldehyde-aldehyde crossed-aldol reaction. The products are tetroses, and one further aldol step affords a range of hexoses, i.e. differentially protected monosaccharides, in a two-step synthesis (Scheme 20) [203],... 

Recently McDonald et al. reported the synthesis of (1 —> 5)-linked D-mannoseptanosyl di- and trisaccharides.80 Once again they utilized differentially protected phenyl 1-thio-a-D-mannoseptanoside as donor and methyl a-D-mannoseptanoside as acceptor to synthesize the di- and tri-a-septanosides exclusively. The strategy used for their synthesis mirrored that depicted in Scheme 26. The septanoside structures and their properties in a glycosidase assay are presented in Section IV. 

Following a similar strategy, Miller [92] has documented the ring opening of a-azido (3-lactams with a-amino acid esters, Scheme 24. Treatment of a-azido (3-lactams 68/69 with glycine methyl ester thus afforded dipeptides 70/71 with differentially protected amino groups. From dipeptide 70, a synthesis of the rhodo-peptin B5 analogue 72 was further demonstrated. 

In this development, both amino moieties are differentially protected and thus, incorporation of these amino acids into peptide chains either at the a- or p-position is possible. This procedure has also been applied to the synthesis of piperazine-2-carboxylic acids and derived peptides [135], Scheme 51. For example, the bicyclic a-hydroxy (S-lactam 161, upon ring expansion and subsequent coupling of the resulting NCA 162 with a-amino esters, affords 163 in good yield. 

This method enables the catalytic asymmetric synthesis of differentially protected 3-aminoaspartate, a nitrogen analogue of dialkyl tartrate, the utility of which was demonstrated by the product syn-80 being converted into a precursor 81 of strepto-lidine lactam (Scheme 5.42). 

Extensive optimization studies identified highly electron-deficient 2,4-dinitrobenzyl-substituted aziridines as the most reactive substrates, chromium as the metal of choice, and indanol-derived Schiff bases as the most effective ligands. In this ring-opening process, catalyst 61 provided the highest selectivities. Using these optimized conditions, a variety of aziridines were selectively opened in a very efficient manner (Scheme 17.21).51 This reaction can provide an easy access to C2-symmetric 1,2-diamines, a valuable class of chiral auxiliaries, and even to less accessible non-C2-symmetric 1,2-diamines because of the differentially protected amines of the ring-opened products. 

Because of the relative fragility of acyclic acetals, the principal challenge in their use lies in defining the mildest conditions for their hydrolytic removal All of our examples are taken from syntheses in which the liberated carbonyl is prone to further acid-catalysed reactions. However, we begin with an example of selective hydrolysis of only one of three differentially protected carbonyls spanning a mere 6-carbon chain [Scheme 2.47],l(M The acyclic acetal was easily removed in the presence of a dioxolane and a dithiane. 

Conversion of a 1,2- or 1,3-diol to the p-methoxyphenyl acetal (see below) followed by regioselective reductive cleavage of the acetal using diisobutylalane can be used for the differential protection of a 1,2- or 1,3-dioL. In the example shown in Scheme 4.198, two regioisomeric cleavage products were obtained (8 1) with the major p-methoxybenzyl ether being isolated in 72% yield.144 A related transformation has been applied to 3,4-dimethoxyphenyl acetals.365... 

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