NNTPS 2X500 MW TRANSFORMER
DETAILS
• Generating Transformer(GT)-2 Nos.
 3 Single Phase Transformer
 200 MVA (each)
 21 / 400/√3 kV (each)
 Vector group: YNd11
• Unit Transformer(UT)- 4 Nos.
 Three Phase Transformer
 40 MVA
 21/11.5 Kv
 Vector group : Dyn1
• Unit Auxiliary Transformer(UAT)- 4 Nos.
 Three Phase Transformer
 16 MVA
 11/3.3 kV
 Vector group : Dyn11
• Station Transformer(ST)-2 Nos.
 Three Phase Transformer (with 2 -
LV windings)
 80 MVA
 220/11 kV
 Vector group: YNyn0yn0d11
• Inter Connecting Transformer(ICT)-2
Nos.
 3 Single Phase Transformer
 167 MVA(each)
 400/ 220/√3 kV
 Vector group: Yna0d11
TRANSFORMER
PROTECTION
• Differential Protection
• Restricted Earth Fault (REF) Protection
• Over flux Protection
• HV Backup Protection
• LV Backup Protection
• Over load function
• Buchholz Protection
• Pressure Relief Valve(PRV)
• High Winding Temperature
• Low Oil Level
RELAYS AND THEIR PROTECTION
FUNCTION
• MICOM P643
Differential
Earth Fault
• MICOM P141
IDMT Overcurrent
DT Overcurrent
Restricted Earth Fault
TRANSFORMER PROTECTION
DIFFERENTIAL
PROTECTION
Fig: Typical Connection Drawing for
Differential Relay
DIFFERENTIAL PROTECTION
• It compares the currents at both sides of
•
•
•
•
transformer
If there is no fault/ through fault,
incoming and outgoing currents are
equal and there the difference current
will be zero.
For internal faults, there is difference
current and relay operates.
For higher stability for through fault
currents, percentage differential
protection is used.
Ratio Correction factor and vector
compensation are provided to suitable
for different CT ratios and to take care
of phase shifting on secondary of PTR
DIFFERENTIAL PROTECTION
• Difference Current = I1-I2
• Restraining Current = (I1+ I2
)/2
• Slope 1 is provided to take of
spill currents due to tap
changer
• Slope 2 is provided for
stability under through fault
condition by considering CT
saturation
• Tripping will be issued based
on the difference current with
respect to restraining current.
DIFFERENTIAL PROTECTION
TYPICAL SETTINGS:
• I>Diff = 0.10 A to 0.30 A
• I>>Diff = 10 A
• Slope = 30% to 60%
• 2nd harmonic setting =
15%
• 5th harmonic setting =
25%
RESTRICTED EARTH FAULT (REF)
PROTECTION
• Faults nearer to neutral of the
•
•
•
•
power transformers will not
covered in the differential
protection
To take care of the faults in the
winding nearer to neutral, REF
protection will be provided.
It is provided on 400kV power
transformers.
It compares the neutral current with
the vector sum of three phase
currents.
If the difference current is more
than preset value, trip signal will be
issued.
Fig: REF Connection diagram
RESTRICTED EARTH FAULT (REF)
PROTECTION
• High Impedance Principle is used.
• Through fault stability is arrived by
Stabilizing Resistor
• Stabilizing Resistor Value is
calculated by formula
• RST= [Ifx (RCT+ 2RL)]/Current
Setting
• RST= Stabilizing Resistance
• RCT= CT Secondary Resistance
• RL = Lead Resistance
TYPICAL SETTINGS:
• Current Setting = 0.02 A
• RST= 500 -2500 Ohms
Fig: REF Connection diagram
OVERFLUX PROTECTION
• Flux generated in the transformer is given by
• E = 4.44 f ϕ t
• ϕ = K.(V/f)
• The Over-fluxing condition in a transformer can occur during system over voltage
and/or under frequency conditions (V/F)
• This will cause an increase in the iron loss and magnetizing current
• Under over-excitation, the core bolts which normally carry little flux may be
subjected to large component of flux
• Under such condition, the bolts may be rapidly heated to a temperature which
destroys their own insulation and will damage the coil insulation if the condition
continues
OVERFLUX PROTECTION
OPERATING PRINCIPLE:
• It continuously measure the ratio of voltage to frequency.
• If the value is higher than preset value, warning or tripping will be issued depending on the operating time.
• Two Stages of protection will be provided. One for alarm and one for tripping
• Definite Time Setting is used for Alarm
• Inverse Characteristic Curve is used for tripping.
TYPICAL SETTINGS:
For Alarm
• V/f setting = 110% of rated V/f
• Time Delay = 5 sec
For Tripping
• V/f setting = 112% of rated V/f
• Time Delay = Set to trip in 60 sec at 120% of rated V/f
HV & LV BACKUP PROTECTION
• Used as backup protection for
transformer differential and
REF protection.
• It also act as backup
protection for feeders
• Typically, provided with 3 SI
IDMT O/C and and 1 SI IDMT
E/F relay
• Highset Instantaneous
element is provided for clear
the internal fault in very less
time.
HV & LV BACKUP PROTECTION
• Time of operation shall be
typically from 600 ms to 800 ms
• Time of operation for SI IDMT
relay is given by
• ATO = 0.14 x TMS/(PSM)0.02–
1)
• Where PSM = fault current/(PS x
CTR)
• PS = Plug Setting
• CTR = CT Ratio
OVER LOAD PROTECTION
• Continuous over loading may lead to the damage of power
transformer.
• Over load function is provided to detect the over load condition and
issue an alarm signal.
• It is typically an over current relay with definite time setting.
Typical Setting:
• Current Setting= 110%
• Time Delay = 5 sec
BUCHHOLZ PROTECTION
• Provided for sensing slow
development of faults inside the
transformer
• Whenever a fault in transformer
develops slowly, heat is
produced locally, which begins
to decompose solid of liquid
insulated materials and produce
inflammable gas.
• The gas produced get collected
inside the relay which acts
mercury switch.
• It is placed between the main
tank and conservator tank.
BUCHHOLZ PROTECTION
PRV PROTECTION
• Internal faults developed
inside the transformer
produces high pressures
inside the transformer.
• During this conditions, the
pressure inside the PTR shall
be release immediately to
avoid bulging of main tank
and for clear the faults
instantaneously.
• For this purpose, Pressure
Relief Value (PRV) is
provided on the main tank.
PRESSURE RELIEF VALVE
WINDING AND OIL TEMPERATURE
INDICATOR
•
•
•
Higher load current causes increase in winding temperature
It causes failure of insulation material and damages it.
Winding Temperature indicator will measure the temperature of oil and issues alarm signal and trip
signal at pre-determined settings.
• With increase in winding temperature, surrounding oil temperature will also increase.
• It causes decomposition of oil and if allows continuously the insulation properties of the oil
deteriorate quickly.
• Oil Temperature indicator will measure the temperature of oil and issues alarm signal and trip signal at
pre-determined settings.
Typical Setting:
• OT Alarm = 85 C
• OT Trip= 95 C
• WT Alarm = 90 C Fan Start= 60 C(Winding Temp)
• WT Trip= 100 C Fan Stop= 55 C(Winding Temp)
OTI AND WTI INDICATORS
OIL LEVEL INDICATOR
• Entire winding of the power transformer shall be immersed in
the oil for proper cooling under low and heavy load conditions
• If oil level fall below the core and winding causes increase in its
temperature and causes failure of insulation winding
• Oil level indicator will measure the level of oil and issues
warning/ tripping signal at pre-determined value.
OIL LEVEL INDICATOR