TRANSFORMER PROTECTION

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
• 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
HV2
HV1
Time
LV

Current
IF(LV) IF(HV)
1.2IF(LV)
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 - I2 OPERATE
I1 BIAS BIAS I2
I1 - I2
OPERATE RESTRAIN

I1 + I 2 Mean Thro
2 Current

Bias = Differential (or Spill)

Current Mean Through 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 a2 P1 P2
S1 S2

P1 S1 P2
REF
S2 P1
P2
S1
S2
To differential relay
 Integral Vectorial and Ratio
Compensation
Power transformer

Ratio
correction

Vectorial
correction
Differential
Virtual interposing CT element Virtual interposing CT
In Zone Earthing Transformer
a1 a2
P1 P2
A1 A2 P2 P1

S2 S1

S2 S1 T2 T1 P1 P2
 Three Winding Transformer
63MVA 25MVA
132KV 11KV
300/5 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 No Load
Current Current at
Twice Normal
Flux
Parallel Transformers
T1 N A B C

T2
 Inter-Turn Fault

CT
E
Load
Shorted
turn

Nominal turns ratio - 11,000 / 240

Fault turns ratio - 11,000 / 1
Current ratio - 1 / 11,000

Requires Buchholz relay

Buchholz Relay Installation
3 x internal pipe
Conservator
diameter (minimum)
5 x internal pipe
diameter (minimum)

Oil conservator
3 minimum

Transformer
 Buchholz Relay
Petcock
Alarm bucket Counter balance
weight

Mercury switch
Oil level

To oil From transformer

conservator

Trip bucket
Aperture adjuster

Drain plug Deflector plate

 Overfluxing Basic Theory

2 m
V = kf
Causes m
Low frequency
High voltage Ie

Geomagnetic disturbances
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
On
Top oil of I load Fan
power control
transformer Off

On
Pump
control
Off
Heater Temp. indication
Local
Thermal Temperature Remote
replica sensing resistor
 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
100 characteristic

Single
10 characteristic:
1 2 3 4 5 6 = 5 mins
Current (multiple of thermal setting)
ZA
DIGITAL RELAYS FOR TRANSFORMER
THERMAL WINDING PROTECTION