2019 Innovations in Power and Advanced Computing Technologies (i-PACT) 1 Fault minimization by phaser correction of transformer Vidhatri Gujela Dept. of Electrical & Electronics Engineering HMFA Memorial Institute of Engineering & Technology Allahabad, India vidhatrigujela@gmail.com Om Prakash Gujela Dept. of Electrical & Electronics Engineering Bakhtiyarpur College of Engineering Patna, India opgujela@gmail.com Abstract— This paper presents the research done on minimization of fault by phaser correction of station service transformer. During the research, it was observed that due to mismatching of phaser of the station service transformer and unit auxiliary transformer there was a resultant voltage, which should ideally be zero. The reason was found as phaser difference between the two transformers. Vector group of station service transformer had been changed from Dy11 to Dy1. After this correction the resultant voltage were measured and it was found that the problem has been minimized. Keywords— Dy connection; SST; UAT; Vector group; Bus- bar. I. INTRODUCTION Every generating plant(Hydro) of larger capacity requires auxiliary system which is directly or indirectly related to operation of the plant. The auxiliaries may be grouped as unit auxiliary and station service. When the machine is generating the unit auxiliaries which are directly linked with the unit i.e. cooling system, high pressure lub system etc. draw power from the unit itself. There are systems like air compressor (low & high pressure), drainage & dewatering system, illumination etc which are essential whether unit is running or not. Further when unit is put to start , the unit requires its own auxiliaries to run, but since the unit has not started generating the power to them needs to be fed from some other sources, which is station service, of course when the unit gets synchronized, the load is shifted to unit as unit auxiliary as depicted above. The same consequence occurs during stopping operation also i.e. load gets shifted from unit to station service. Hence, the role of station service is essential for running of the power plant. A. Unit Auxiliary Board(UAB): 415 volt AC power is supplied to UAB of each unit through its own Unit Auxiliary Transformer (UAT) which is directly connected to generator terminals through bus duct. The power supply to the unit auxiliaries like primary and secondary cooling water pumps, OPU of inlet valve, OPU of governor, oil circulating pumps of the step up transformer, CGL system, are distributed from UABs. All the UABs have 3 nos. 2500 A ACBs, one is for incoming from Station Service Board (SSB) and other is for incoming from the UAT, the remaining is spare feeder. At the time of starting the 415 volt AC power is supplied to the unit auxiliaryes through the SSB and as soon the machine is synchronized, the supply changes over to UAT. This changes over takes place in AUTO, however provision for manual operation is also avalable. The incoming breaker of UAB-1 & UAB-2 is connected through SSB-1 and of UAB-3 is connected through SSB-2. MCBs are provided to supply power to different outgoing circuits from the UABs. UAB bus is directly extended to adjacent unit MCC panel where the motor starters are loacated. B. Station Service Board (SSB): There are two number of SSBs which is supplied by two 2000 KVA Station Service Transformes (SST). SSB supplies power to different power house auxiliaries: illumination system, HP & LP compressors, drainage & dewatering pumps, battery chargers etc. Power will always be extended upto UABs. The 415 volt AC power is supplied to SSBs by 2 nos 2000 KVA station service transformer, which is fed from the 11 KV switch gear panel located at Gas Insulated Sub- station(GIS). The supply from SSTs is switched on by 3200A main incomer circuit breakers. There is a provision to connect SSB-1 bus coupler from SSB-2 through a 2500 A Circuit Breaker(CB) or vice versa. Different outgoing feeders are installed with different rating CBs as per requirement for outgoing supply from the SSBs. Both the SSBs feed power to Portyard Service Board (PSB) and Transformer Service Board (TSB). C. Problem Observed: In hydro power station the frequent tripping of secondary and primary motors may observed from the starting of the commissioning of the project. This happen during the time of changing of power supply from the Station Service Transformer to Unit Auxiliary Transformer. Figure 1. Single line diagram of 11KV System Fig. 1 depicts single line diagram of the unit auxiliary board. When command is given for start of unit, the auxiliary supply coming from the station service through the breaker no S1 is on. After the unit synchronized with the grid the breaker S1 is off and the breaker S2 is on to provide power from unit auxiliary transformer. But at the time of changeover of power 978-1-5386-8190-9/19/$31.00 © 2019 IEEE