A new method for online thyristor conduction monitoring based on thyristor current waveform recording in static excitation system

This paper describes present system of thyristor conduction monitoring, problems facing by maintenance engineer and proposed a method for “On line thyristor monitoring” in static excitation system based on thyristor current waveform recording for better and effective monitoring.

The purpose of the excitation system is to regulate terminal voltage of the synchronous machine, meet excitation power requirements under all normal operating conditions and enable maximum utilization of machine capability. The excitation equipment comprises of regulation cubicle (DVR system), thyristor cubicles, field flashing cubicle, field breaker cubicle and transformer cubicle. One thyristor cubicle consists of six thyristor (full wave Thyristor Bridge) and generally there are four such cubicles (banks) connected in parallel to share the total generator field current in a 200 MW generator excitation system. Since all the four banks connected in parallel the thyristors will carry current depending on there parallel circuit resistance. If one thyristor of any particular bank not conducted then the current of the non-conducting thyristor will shared by the corresponding thyristor located in the other three banks.

Generally, following methods are presently used for thyristor monitoring of static excitation system.

Periodic monitoring of all four-thyristor banks average current from panel ammeter. This method unable to indicate individual thyristor conduction and current sharing.

Here, a new method is proposed for online conduction and current sharing monitoring for static excitation system. For this Software simulation of static excitation done in MATLAB software and thyristor bridges current waveform under different condition studied and same had been demonstrated at one of the 200 MW unit of FSTPS NTPC Ltd. The method is very effective and advantageous for detection of thyristor current sharing and help to balancing the thyristor current in static excitation system. Field recorded waveform before and after current balancing of a bank in 200 MW unit are also presented in this paper. This system of monitoring is very much advantageous for old AVR base excitation system, and also for DVR system without conduction monitoring unit.