IEEE C57.116-1989 pdf free.IEEE Guide for Transformers Directly Connected to Generators.
IEEE C57.116 describes selection and application considerations for the unit transformer and unit auxiliaries transformer. Consideration is given to connections that include direct connection and connections through generator breakers and load break switches. The considerations referred to in this guide apply to hydroelectric and thermal electric generating stations. Various transformer connections and possible operating problems under normal and abnormal conditions are treated.
Phasing procedures, basic impulse insulation level selection, and loading practices are not covered.
The unit-connected generating system came into usage as generator size and generator bus fauk current levels began to exceed the capability of breakers that were available w isolate the generator from the power system. Generally, the generator auxiliaries load is fed from a transformer that is connected directly to (he generator bus. An alternate source of power is normally supplied from the transmission system by a second transformer during unit startup. shutdown. and other contingencies. Such a system is shown in Fig 1. Typically, an isolated phase bus is used for connection between the generator and its UT and UAT; thus, the system is unit connected.
Studies indicate that through faults for the UT are normally not severe in terms of either thermal or mechanical duty. However, the sLresses in the UAT can be severe under certain conditions, such as low-side faults, paralleling out of phase with the SST. and generator load rejection.
On units without generator breakers, faults at location (3) in Fig 1 will he fed predominantly by two sources, namely. through the UT from the system and from the generator. Normally the differential relay protection system, which will detect faults at location (3), will initiate a unit trip that will disconnect the unit from the system and remove the generator excitation. The loss of load may result in a higher voltage on the generator that will continue to feed the fault. This higher voltage due to load rejection will increase the generator’s current contribution to a level higher than before load rejection. However, the system contribution will be absent. The generator’s contribution will continue at a level and for a period that depends on the generator’s fault-current decrement characteristics. The fault-current decrement characteristics are a function of the generator short-circuit parameters and its excitation system. As a result of the generators fault-current decrement characteristics, faults at location (3) in Fig 1 may exceed the thermal and mechanical capabilities of the UAT if it is designed solely in accordance with ANSI/IEEE C57. 12.00-1987.IEEE C57.116-1989 pdf download.