transformer protection
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POWER TRANSFORMER PROTECTION
By
Bhallamudi SridharDy.Manager, Bangalore SSDate:18-07-2008
Introduction
• The power transformer is the most important links in power system
• A transformer fault will cause a large interruption in power supply and also cause damage to power system stability.
Need of protection
• To limit the damage in case of fault• To limit the damage in case of fault• To isolate the faulty element to protect the
power system.
Types of faults in Transformer
INTERNAL FAULTS
Earth faultsPhase to Phase faults Inter turn faultsCore faults
EXTERNAL FAULTS
Over-load System faults(phase to
phase/ phase to earth) Over voltage Reduced system
frequency.
Classification of transformer protection
• Protection against electrical faults• Protection against abnormal conditions.• Transformer protective devices
Protection against electrical faults
• Unit protection -against internal faults
1.Differential relay
2.Restricted earth fault relay
• Non unit (back up) protection-against external faults
1.Directional over current and earth fault
Differential protection
I1-I2I1 I2
I1 I2
I1 I2
Principle of operation
• During normal condition and external fault condition the protective system is balanced
• This balance is disturbed for internal faults.• The differential current flows through the relay
coil and it operate.
Peculiarity of the Transformer Differential Protection
1. Vector group (e. g. Yd5) 2. Different CT´s, tap changer, magnetising current
vector group adaptation restraint function (stabilising) is necessary
3. Dynamic currents I = f (Irestr.)
Irestr. = |I1| + |I2| inrush current overflux (overexcitation) CT saturation during external faults
blocking via harmonics
saturation detector
trip region
-I2
I1
150°current transformer
tap changer,CT adaptation
(will be eliminated)magnetising current
I
I
ITr, IRestr.
Example of an Inrush CurrentA unit transformer (IN = 396 A) was switched on from the high voltage side
Rush Stabilisation
Recognise inrush condition by evaluating the ratio 2nd harmonic I2HAR to basic wave IDiff.
Time limit for cross-block. Reliable reaction to the inrush condition with cross-block.Trip of a short circuit after the set time delay.
Recognise over-excitation by evaluating the ratio 3rd or 5th harmonic to basic wave
filter window1 cycle L1-block
L2-block
L3-block
Cross-block = No (phase separate blocking)
Cross-block = Yes (blocking of all phases)
Idiff, L2 > trip blocking
O R1
t
t1P 2P 3P
iRUSH = iDiff
15 % setting valueblock
no block
I2HAR
Idiff
Inrush currentin one phase
L1-block
L2-block
L3-block
Idiff, L1 > trip blocking
Idiff, L3 > trip blocking
IDiff > trip blocking for a limited time
&
&
&
Tripping Characteristic
flexible adaptation to various transformers, e.g. with tap changer or different main CTs
high stability against external faults with CT saturation
fast tripping for solid short-circuits within one period
Restricted earth fault protection
• REF is used for protect the earth fault inside the transformer• Protection is provided using bushing core CTs at 400KV,
220KV and neutral.• All the CTs has been connected in parallel• as the net current always zero for normal and through
faults.
• Whenever any earth fault occur in the REF zone of transformer the vector sum of currents come into unbalance and resultant current will flow through REF relay and it will operate
CT ICT
CT
I1 I2
Directional over current & EF fault
• It is provided as a back up protection to prevent the transformer subjected to prevent high mechanical stresses during through faults.
• Inverse definite minimum time relays with directional elements as well as over current instantaneous elements are used.
• The relay are always made look towards the transformer.
• This protection is to prevent the transformer from high system voltage and low frequency condition.
• The flux density of the transformer is directly proportional to the V/f
• Power transformers are designed to with stand Vn/fn =1.1 where Vn is the normal highest r.m.s and fn is the standard frequency.
Protection against abnormal condition-Over fluxing protection
Over fluxing protection
V/f/Vn/fn 1.1 1.2 1.25 1.3 1.4
Duration of withstand limit(min)
continues
2 1 0.5 0
Transformer protective devices- non electrical
• Besides the electrical relay , a power transformer has protected with following safety and monitoring devices.
1. Buchholz relay2. Pressure relief device3. Winding / oil temperature indicators
Buchholz protection
• This is gas generated relay connected in between the main tank and conservator
• This relay is generally used to detect the following faults:
1. Core faults2. Inter turn faults3. Tank faults4. Hot spot
Pressure relief device protection
• To prevent the pressure rise of inside the transformer above 10 PSI during
1. High over load peaks2. Prolonged overloads3. Arcing faults within oil.• Operation of the PRD will cause either alarm
or instantaneous trip of the transformer.
Winding / oil temperature indicators
• The function of temperature indicator is to indicate and control the temperature of the oil in the transformer.
• Transformer temperature increasing due to over loading or internal faults.
• Typical settings of temperature indicators are as follows.
`
• O T I • 85ºC
• W T I• 95ºC
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