• Introduction
• DG set
• Operating parameters
• Theory of operation
• Guidelines on the selection of a DG set
• Types of loads
• Starting of a DG set
• Protection of a DG set
• Parallel operation
• Procedure of parallel operation
• Fault levels and grounding of generators
• Recommended protection for a synchronizing scheme
• Load sharing by two or more generators
• Total automation through PLCs

APPENDIX Selection of power cables PVC, paper insulated and XLPE and ampacity

Introduction
It is common practice to provide a standby emergency source of supply at all important installations such as large factories, railways, airports, hospitals and other essential services. This is usually achieved with the use of a captive diesel generator (DG) set (Figure 16.1). Here we briefly discuss these machines, their characteristics and selection for a required application. We also consider schemes that are commonly used to start a DG set and run it individually or in parallel with an existing source of supply, which may be another DG set or an infinite bus.

Ground fault level

Solid grounding
Generators have a different grounding practice from others due to their zero phase sequence reactance, which is much less than its positive or negative phase sequence reactances (Section 13.4.1(5) and Table 13.6). See the above example where x0 is only 4.1% compared to  as 16.2%. As a result, the ground fault current in a generator circuit is much greater than its three-phase symmetrical fault current. This current rises further when they are individually grounded and more than one unit are running in parallel at a time. It is worth mentioning that when two or more generators are running in parallel and all of them are grounded, they form a closed circuit to cause circulating current. This may occur even in a healthy system due to unbalance, not because of single-phase loads but unequal generator phase currents due to eddy currents. These are important aspects and must be considered while deciding on the grounding method of a solidly grounded generator.
   
Appendix 16.1
Introduction
To provide a reference for those working on power projects or at sites, we provide some important data on different types of LV and HV power cables in this appendix. The cables described here are in use for all kinds of power distribution applications. Of these, XLPE cables are also used for power transmission applications. To help a user select the most appropriate types of cables, we also provide a brief comparative chart of the various types of cables being manufactured. Tables giving the technical particulars of such cables in all voltage ratings have also been provided.
The selection process of power cables is almost the same as that of a bus system discussed in Section 28.3. For simplicity we consider only the basic data for selection which would suffice the majority of applications. For accurate calculations a similar approach will be essential as for the bus systems (Chapter 28). For site conditions and laying arrangements which may influence the basic rating of a cable, corresponding derating factors have also been provided. The information covered here will be useful to users to meet their cable requirements, although the data may vary marginally for different manufacturers. For more data on cables, not covered here, reference may be made to the cable manu-facturers.
The choice of any of the cables mentioned in Table A16.1 will depend upon the site conditions, fault level and the voltage rating of the system. A brief comparison of all these insulating systems is given in Table A16.2.