Trend for Static Excitation Systems in Large Thermal Generators - User's Perspective

Owing to the maintenance problem like the frequent replacement of worn-out brushes and slip ring due to sparking/fire of static excitation system, led to development of brushless excitation system. Several manufacturers have used brushless type exciter for small turbo as well as hydro generators. A few suppliers also opted for brushless exciter in large ratings of turbo-generator. The paper appraises on NTPC experience on failures of brushless exciters and associated issues like generation loss, OEM dependency and availability of spares. The high initial response and other performance characteristic advantages due to elimination of machine time constants have been summarized.

As the designs of slip ring assembly in Static Excitation have been established during past decades, it is high time to review the selection criteria for large generator excitation system. All parts except brush-gear assembly are stationary in this scheme and hence the ageing rate of components/equipments is lower. An additional bearing support for slip-ring shaft and availability of high quality of brushes has now improved the system reliability. In comparison, the brushless exciter envisages the complete power circuit on the shaft rotating at 3000 rpm and this assembly is subjected to high mechanical stresses continuously. Generally, electrical failures in machines are caused by mechanical stresses, like abrasion of the insulation, loosening of winding due to vibration, de-lamination of exciter winding insulation due to varying conductor temperatures and opening of poorly brazed joints due to forces arising from revolving electro-magnetic field on flexible connection. The large number of rotating parts in this assembly (diodes, fuses, long flexible links on AC side and rigid links on DC side of rectifiers etc.) is prone to dislodging and thereby causing extensive damage to the generator. Utilities can't ignore these aspects because of the heavy cost incurred on account of generation loss and the cost of repairs.

NTPC has a large fleet with more than 50 units of 500MW and more than thirty (30) units of 200MW. A majority of these units have been supplied from one leading Indian manufacturer has brushless type of exciter. Also a few imported units from GE Alstom, Elektrosila and Siemens also have brushless type of exciter. Some of these units operating since 1984 and many have been in operation for more than 25 years. The NTPC fleet has few 500MW and 200MW units with static type of excitation system also Supplied by BHEL, Siemens and Alstom. A large number of gas projects units (below 200MW rating) have also static type of excitation system in NTPC. The company has a total of more than 100 units in operation catering major share of power supply requirement in India.

The equipments in Static type of excitation System available are highly reliable and no major failure has been reported in large fleet comprising of approximately forty units with 100-200MW in thermal and gas projects.