power transformer protection ... presentation

7/28/2019 Power Transformer Protection ... Presentation

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Power Transformer Protection

Er.Sandeep Yadav , Xen , Protection Division ,

HVPNL,Gurgaon

A Seminar on Protection SystemOrganized by CBIP


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Over-current ProtectionDuring a fault , the secondary current(s) of the C.T. increases and if the C.T. sec. current increase beyond the setting of the relay , the relay will operates within a

time (as per the fault current)

Important


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Generally IDMTL(Inverse Minimum Definite Time Lag) relays are used in HVPNL for protection of primary & secondary side as well as feeder protection of thepower transformer. An IDMTL Over current/Earth fault relay has a Aluminum Disc and will take a time to trip after occurrence a fault. The tripping time of the relay is inversely proportional to the fault current.If fault current is high , operating time is less and if fault current is less , relay will take a long time to operate.The main drawback of IDMTL relay is to take some minimum operation time even for extreme high fault currents, say as high as 8-10 times the rating of transformer.If relay take even a small time to isolate (trip) such heavy fault currents , the power transformer may be in trouble during heavy faults.So , we need a method by which the IDML relay allow to operate for fault currents below say 5x (5 times the capacity of transformer) and if fault current goes beyond 5x then plain over-current relay (non-IDMTL) will trip the transformer instantaneously.This Non-IDMTL Over-current relay is called High-set or Instantaneous Relay.


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Highset relay is always connected in series with the IDMTL relay so that it can see the same current as seen by IDMTL relay as show in picture below.Highset relays are never connected in incomers i.e. 132KV I/c,66KV I/c,33KV I/c & 11KV I/c and always provided on outgoing circuits i.e. 11KV Outgoing feeders ,33KV Outgoings feeders etc so that for heavy outgoing feeder fault , the incomer should not trip and disturb other healthy feeders connected to it.


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In LV side of power transformer , all outgoing feeders are equipped with highset relay to trip the feeder instantaneously during a heavy fault. As already indicated , no highset relay is provided on the LV Incomer panel/VCB of the power transformer. As the LV bus is fed by LV Incomer so if there is some heavy/serious fault in the LV bus i.e. 11KV bus for a 66/11 or 132/11KV TF , 11KV Incomer panels relay will take some time to trip being IDMTL relay and as bus fault is very nearer to T/F , such delay in tripping cant be tolerated. So there is a provision of providing the Highset on HV side of TF to trip the transformer for LV bus faults.


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In a Power Transformer , Restricted Earth Fault (R.E.F.) protection is provided on both H.V. and L.V. side if transformer is of Yy0 type. If primary is Delta connected (if T/F is Delta-Star type) , no need of R.E.F. on Delta (HV) side. All the Earth Faults on outgoing feeders are looked by their relays. Earth fault in bus zonewill be seen by E/f relay on LV incomer.Supposean earth fault in a 11KV Incomer cablepuncture of 11KV or 33KV L.A. of power transformerDamage of TF CT on HV side or LV side with a flash with groundDamage of LV cable box (LV incomer Indoor/Outdoor)Damage of HV or LV winding of transformer after shortening with core/earthBird-age on transformer of HV/LV jack buses etcSuch faults are very nearer to transformer and transformer should be isolated from fault to avoid any mis-happening. We need a relay which will automatically sense thezone between HV C.T.s to HV winding/core (for HV R.E.F.) and between LV Winding/core to LV incomer C.Ts (for LV R.E.F.).This relay needs to sense and isolate the earth faults in the above said zones so called Restricted Earth Fault Relay.As we need immediate tripping , so these are non IDMTL relays i.e. dont have a rotating disk e.g. CAG14 (Alstom) etc.So following may be cloncluded.A H.V. R.E.F. relay will sense any earth fault in H.V. C.Ts , HV Jack Bus, H.V. L.As , TF H.V. Bushing/core and winding.SimilarlyA L.V. R.E.F. relay will sense any earth fault in L.V.Core/Winding/Bushing of the transformer , LV Jack bus (11KV Incomer cables in case of 132/11 or 66/11KV T/F), LV L.As & L.V. C.T.s ( 33KV I/c C.Ts in case of 132/33KV T/f & 11KV Incomer C.Ts in case of 132/11KV T/F.


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Path of Current during Earth Fault on any outgoing feederFor an Earth fault , the fault current have to return to neutral bushing of the source transformer through NCT as shown above whatever be the distance of locatio

n of Earth fault from source transformer


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REF relay during Normal Loading conditionDuring normal condition , the phase currents in all phases will be approx. same and so Ict=0 (as vector sum of three phases currents 120deg apart will be zero.T

he current in NCT secondary will also be zero due to no Earth fault & no current in NCT primary i.e. in normal condition , there wont be any current throug

h neutral circuit of Power transformer.

So in normal situation , no current will flow through REF relay and it will not operate.


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REF relay during External Earth Fault ConditionDuring External Earth Fault condition, the fault current will flow through the NCT as well as through the REF core of the faulty phase CT.As direction of both

the currents are in opposition so nullify both the currents in relay and REF relay wont operate.

SO, THE REF RELAY WONT OPERATE FOR A OUT-OF-ZONE EARTH FAULT


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REF relay during In-zone Earth Fault ConditionDuring In-zone Earth Fault condition, the fault current will only flow through the NCT & no t from the REF core of Incomer CT being fault before Incomer pa

nel.The current from NCT goes to REF relay and trips it being no counter current from REF cores of Incomers CTs to oppose it.

SO, THE REF RELAY SHOULD OPERATE FOR A IN-ZONE EARTH FAULT


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Differential ProtectionDifferential Protection is most sensitive & fast acting protection of the power transformer. It sense the difference in the currents received from the HV side & L

V side CTs differential cores. It covers the area from the differential core of HV CTs to the differential core of LVCTs including Las , HV/LV jack buses , TF

winding , cores & bushings etc


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A Buchholz relay is a gas and oil operated device installed in the pipe work between the top of the transformer main tank and the conservator. A second relay is sometimes used for the tap changer selector chamber. The function of the relay is to detect an abnormal condition within the tank and send analarm or trip signal. Under normal conditions the relay is completely full of oil. Operation occurs when floats are displaced by an accumulation of gas, or a flap is moved by a surge of oil. Almost all large oil-filled transformers are equipped with a Buchholz relay, first developed by Max Buchholz in 1921.


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PRESSURE RELIEF DEVICEThe pressure relief device (PRD) is designed to open and close automatically when the pressure reaches the operating pressure of the PRD.The pressure relief device will open and remain open until the pressure falls to the reseal pressure. The pressure relief device reseals at a positive pressure.


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Magnetic Oil Level Gauges ( Indicator )are most commonly fitted on conservator of Power/ Distribution Transformers.These Gauges give visual indication of oil level and provide Alarm facility at desired level. Float movement due to rise a

nd fall of oil level gives rotary movement to indicating pointer through bevel gears and magnets.


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THANK YOU !!

https://www.facebook.com/sanrwr

+919310404372

[email protected]@hvpn.gov.in

Er.Sandeep YadavExecutive Engineer,

Protection Division,HVPNL

Gurgaon (Haryana)Certified Energy Auditor from Bureau of Energy Efficiency

Govt. of INDIA
https://www.facebook.com/sanrwrmailto:[email protected]:[email protected]://www.facebook.com/sanrwr

power transformer protection ... presentation

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