TRANSFORMER PROTECTION
Extent of
damage
Fault level
Duration of
fault
Winding failures
Voltage
regulating load
changers
Transformer
faults
Transformer
bushing failure
Transformer core
problem
Miscellaneous
failures
Insulation
breakdown
Time
Ageing of
insulation
Temperature
Condition
leading to faults
Over heating
due to excitation
Oil
contamination%
& oil leakage
Reduced cooling
Improve cooling
system of
possible
Reduced load
FAULT OF TRANSFORMER














Earth fault on H.V external connection
Phases to phase fault on H.V external connection
Internal earth fault on H.V windings
Internal phase to phase fault on H.V windings.
Short circuit between turns L.V windings.
Earth fault on L.V external winding
Phase to phase fault on L.V external connection.
Internal earth fault on L.V windings
Internal phase to phase fault on L.V winding
Short circuit b/w turn L.V windings
Earth fault on tertiary windings
Short circuit b/W turns tertiary windings
Sustained system phase to phase fault
Sustained system earth fault
Differential relay
L.V side three phase
over current and
earth fault relay
132KV/11KV
POWER
TRANSFORMER
H.V side three
phases over current
and earth fault relay
D.C trip circuit
supervision relay
Trip and lock out
relay
Percentage biased
Transformer
differential relay
High REF impedance
Circulating current
differential protection
HV side over current
and earth fault relay
LV side over current
and earth fault relay
Over excitation relay
220/132/11KV AUTO
TRANSFORMER
Thermal over load
relay
Overall percentage
biased differential
relay
L.H&H.V Connection
circulating protection
Tertiary over current
protection
D.C trip circuit
supervision relay
Percentage biased
Transformer
differential relay
High REF impedance
Circulating current
differential
protection
HV side over current
and earth fault
relay
LV side over current
and earth fault
relay
Over excitation
relay
500/220KV AUTO
TRANSFORMER
Thermal over load
relay
Overall percentage
biased differential
relay
L.H&H.V Connection
circulating
protection
Tertiary over
current protection
D.C trip circuit
supervision relay
Over voltage relay
Transformer
buchhloz relay
Tap changer
buchhloz relay
MECHANICAL
PROTECTION:
Winding
Temperature
0IL
temperature
Pressure
relieve valve
• Factors:
• The shape, magnitude and duration of the inrush
current depend on the factors:
• Size of power transformer
• Source Impendence
• The magnetic properties of the core i.e.
saturation density
• The remanence of core
• Resistance in power system from source to
transformer.
• The moment when transformer is switch on.
Effect of magnetising current

Appears on one side of transformer only

Seen as fault by differential relay

Normal steady state magnetising current is less than relay
setting

Transient magnetising inrush could cause relay to operate
• The vector group shows the connection of
windings of transformer and numerical index
(hour numbers) for displacement of vector of
two star voltages.
• Capital Letter
DY11
Small
letter ( clock dial reference)
• The first capital letter donates the connection
of high voltage winding of transformer
• The small letter represent the connection of
low voltage secondary winding of transformer
• Yy0d5
•
• The first capital letter Y is referred to H.V or
primary winding, the second letter y is
referred as secondary winding and third letter
is referred as tertiary winding.
• Primary winding is taken as phase referred ‘O’
means that phase angle b/W H.V and M.V
winding is zero. Whereas ‘5’ denotes that
phase angle b/W H.V and tertiary winding is
150 (5x30)
OVER CURRENT PROTECTION
As it names implies, relay will pick up when it
exceeds its present value
TYPES:
The types of over current relay are based on the
relay characteristics over can be classified into three
groups.
• Definite current or instantaneous
• Definite time
• Inverse time
Vécurent Relay Applied to a
Transformer
51
51
51
HV2
HV1
LV
HV1
HV2
Time
LV
IF(LV)
IF(HV)
1.2IF(LV)
Current
Use of Instantaneous Overcurrent
Protection
Source
LV
50
51
Differential Protection
• Overall differential protection may be justified for larger
transformers (generally > 5MVA).
•
Provides fast operation on any winding
• Measuring principle :
• Based on the same circulating current principle as the
restricted earth fault protection
• However, it employs the biasing technique, to maintain stability
for heavy thro’ fault current
• Biasing allows mismatch between CT outputs.
• It is essential for transformers with tap changing facility.
• Another important requirement of transformer differential
protection is immunity to magnetising inrush current.
PROTECTED ZONE
HV
LV
R
• Correct application of differential
protection requires CT ratio and winding
connections to match those of
transformer.
• CT secondary circuit should be a
“replica” of primary system.
• Consider :
• (1) Difference in current magnitude
• (2) Phase shift
• (3) Zero sequence currents
Biased Differential Scheme
Differentia
l
Current
I1
BIAS
OPERATE
BIAS I2
I1 - I2
OPERATE
I1 - I2
RESTRAIN
I1 + I2
2
Bias = Differential (or Spill)
Current Mean Through Current
Mean Thro
Current
Restricted E/F Protection
Low Voltage Windings (1)
A B C N
LV restricted E/F
protection trips
both HV and LV breaker
Recommended setting : 10% rated
Restricted E/F Protection
Low Voltage Windings (2)
A B C N
LV restricted E/F protection trips both HV and LV breaker
Recommended setting : 10% rated
Delta Winding Restricted Earth Fault
Source
Protected zone
REF
 Delta winding cannot supply zero
sequence current to system
 Stability : Consider max LV fault level
 Recommended setting : less than 30% minimum
earth fault level
Protection of Auto-Transformer
by High Impedance Differential
Relays (2)
(b) Phase and Earth Fault Scheme
A
B
C
a
b
c
87 87 87
n
`
Combined Differential and Restricted
Earthfault Protection
A2
A1 a1
P1
P2
S1
S2
a2
REF
P1
S1
P2
S2
P2
P1
S1
S2
To differential relay
Integral Vectorial and Ratio
Compensation
Power transformer
Ratio
correction
Vectorial
correction
Virtual interposing CT
Differential
element
Virtual interposing CT
In Zone Earthing Transformer
P1
P2
a2
a1
A1
A2
S2
S1 T2
T1
P1
P2
P2
P1
S2
S1
Three Winding Transformer
63MVA
132KV
300/5
25MVA
11KV
1600/5
50MVA
33KV
1000/5
4.59
5.51
10.33
2.88
5
2.88
5
All interposing C.T. ratio’s refer to
common MVA base (63MVA
Transformer Magnetising
Characteristic
Twice
Normal
Flux
Normal
Flux
Normal
No
Load
Current
No Load
Current at
Twice Normal
Flux
Parallel Transformers
T1
T2
N A B C
Inter-Turn Fault
E
CT
Shorted
turn
Nominal turns ratio
Fault turns ratio
Current ratio
- 11,000 / 240
- 11,000 / 1
- 1 / 11,000
Requires Buchholz relay
Load
Buchholz Relay Installation
3 x internal pipe
diameter (minimum)
Conservator
5 x internal pipe
diameter (minimum)
Oil conservator
3 minimum
Transformer
Buchholz Relay
Petcock
Alarm bucket
Mercury switch
To oil
conservator
Trip bucket
Counter balance
weight
Oil level
From transformer
Aperture adjuster
Drain plug
Deflector plate
Overfluxing Basic Theory
V = kf
Causes
2m
m
Low frequency
High voltage
Geomagnetic disturbances
Ie
Effects
Tripping of differential element (Transient overfluxing)
Damage to transformers (Prolonged overfluxing)
EFFECTS OF OVER FLUXING:
•
•
•
•
Increase in magnetizing current
Increase in winding temperature
Increase in noise and vibration
Overheating of laminations and metal parts
(cause by stray flux)
V/Hz Overfluxing Protection
V  K
f
Trip and alarm outputs for clearing prolonged overfluxing
Alarm : Definite time characteristic to initiate corrective action
Trip : IDMT or DT characteristic to clear overfluxing condition
Settings
Pick-up 1.5 to 3.0
i.e. 110V x 1.05 = 2.31
50Hz
DT setting range 0.1 to 60 seconds
V/H CHARACTERISTIC:
Over-fluxing Relay
Ex
G
VT
AVR
RL
THERMAL OVERLOAD:
• EFFECT OF OVER LOAD ON TRANSFORMER
INSULATION LIFE:
Overheating Protection
Trip
I load
Alarm
TD
setting
Top oil of
power
transformer
On
Fan
control
I load
Off
On
Pump
control
Off
Temp. indication
Heater
Local
Thermal
replica
Temperature
sensing resistor
Remote
Overload Protection
• Overcurrent protection designed for fault
condition
• Thermal replica provides better protection
for overload
–
–
–
–
–
Time
Current based
Flexible characteristics
Single or dual time constant
Reset facility
Non-volatile
Current
Thermal Overload Oil Filled Transformers
Trip time (s)
10000
Single
characteristic:
 = 120 mins
1000
Dual characteristic
100
10
1
ZA
2
3
4
5
6
Current (multiple of thermal setting)
Single
characteristic:
 = 5 mins
DIGITAL RELAYS FOR TRANSFORMER
THERMAL WINDING PROTECTION