TRENDS IN PRACTISE AS AFFECTING COORDINATION 177 



more remote units in the sequence. The development of relaying 

 practise has included various methods of securing selectivity independ- 

 ently of time. This has accomplished large increases in the over-all 

 speed of operation, at the same time improving selectivity. Coinci- 

 dent with these improvements there has also been a substantial gain 

 through greater precision in design and workmanship and improved 

 application of relays and related devices. These trends definitely aid 

 coordination by reducing duration of transients, eliminating faulty re- 

 lay operation, and steadily reducing the radius of influence of system 

 abnormals. 



With the growth in power systems and major interconnections, the 

 use of bus or feeder current limiting reactors or other means of limiting 

 the concentration of fault current flow is being given increasing applica- 

 tion. Such practise acts to restrict the magnitude of inductive tran- 

 sients. In distribution systems the growing use of feeder reactors has 

 a similar effect in matters of coordination. 



For well known reasons, among which are the avoidance of transient 

 over-voltages resulting from arcing grounds and the economies made 

 possible in apparatus insulation, it is predominant practise in America 

 to ground the neutrals of transmission systems at important trans- 

 forming centers, sometimes through resistors or reactors but usually 

 solidly. In view of the prevalence of the latter method, a large pro- 

 portion of higher voltage transformers now in service have been con- 

 structed with insulation between the neutral ends of the grounded 

 windings and the core and tank, designed to support only the neutral 

 potentials produced by fault currents regulating through the unavoid- 

 able impedance of grounding connections. The economies resulting 

 from this method of construction become greater as rated operating 

 voltages rise. The use of solidly grounded neutrals tends to make 

 coordination more difficult in view of the possibilities for increased 

 flow of earth currents. 



On some large power networks with relatively great possible concen- 

 trations of short-circuit power and solidly grounded neutrals tenden- 

 cies towards instability of operation have appeared. In some instances 

 also oil circuit breaker characteristics, particularly as regards the older 

 breakers in service, have become a source of concern. For these rea- 

 sons, in these situations, increasing study and consideration are being 

 given to the use of current limiting devices in the neutral where the 

 characteristics of the apparatus and limitations of relaying will permit 

 of such operation. 



In some European countries, particularly in Germany, where ground- 

 ing for the purpose of power system voltage stabilization is excluded 



