TRANSFORMER PROTECTION
Jan 17, 2016
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
Ageing of insulation
Time
Temperature
Improve cooling system of possible
Condition leading to faults
Over heating due to excitation
Oil contamination%
& oil leakage
Reduced cooling
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
Over current Relay Applied to a Transformer
51
HV2
Time
51
HV1
HV1
LV
HV2
51
LV
IF(LV)
1.2IF(LV)
IF(HV)
Current
Use of Instantaneous Over current Protection
Source
50
51
LV
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 magnetizing inrush current.
PROTECTED ZONE
HV
R
LV
·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
Differential Current
I1
BIAS
BIAS I2
I1 - I2
OPERATE
OPERATE
I1 - I2
I1 + I2
2
RESTRAIN
Mean Thro
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 Relay
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
`
Combined Differential and Restricted
Earthfault Protection
A2
A1 a1
a2
P1
P2 S1 S2
P1
P2
S1
S2
REF
P2
P1
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
P1 P2
A1
A2
a1 a2 P2
P1
S2
S1
S2
S1 T2
T1
P1
P2
Three Winding Transformer
63MVA
25MVA
300/5 132KV 11KV
1600/5
50MVA 33KV
1000/5
4.59
2.88
5.51
5
10.33
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
Load
Nominal turns ratio Fault turns ratio
Current ratio
- 11,000 / 240 - 11,000 / 1
- 1 / 11,000
Requires Buchholz relay
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
Drain plug
Counter balance weight
Oil level
From transformer
Aperture adjuster
Deflector plate
Overfluxing Basic Theory
Causes
V = kf
2m
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 f K
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
AVR
G
RL
VT
THERMAL OVERLOAD:
· EFFECT OF OVER LOAD ON TRANSFORMER INSULATION LIFE:
Overheating Protection
I load
TD setting
Trip
Alarm
Top oil of power transformer
I load
On
Off
Fan control
On
Pump control
Off
Heater Temp. indication
Thermal
Temperature Local
Remote
replica sensing resistor
Overload Protection
· Overcurrent protection designed for fault condition
· Thermal replica provides better protection for overload
–
Current based
Time
–
– 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
Single 10
1
2
3
4
5
6
characteristic:
= 5 mins
Current (multiple of thermal setting) ZA
DIGITAL RELAYS FOR TRANSFORMER THERMAL WINDING PROTECTION