Power Transformer and feeder
Protection
‫مركز التدريب والتطوير المتكامل أم حراز‬
ENG: ELNAZEER DAFFALLAH A. ELJALEEL M. KHARE
PROTECTION ENGINEER , SEDC
MSC, BSC, IOSH, TOT
Contents
 Introduction
 Definition
 Why
of power transformer
use transformer
 Philosophy
and economic considerations of protection.
 Why
Transformer Protection?
 Why
Do Transformers Fail?
Introduction
The electrical transformer deserves credit as
one of the most important inventions of the
industrial age, which along with steam power,
running water, gas lighting, includes the
harnessing of electricity. In fact, the latter would
not be accomplished without the transformer.
Introduction
 Power Transformer is a vital link in a power
transmission system and distribution.
 A Power Transformer is an expensive part of a power
network.
 The impact of a transformer outage due to fault is
more serious than a transmission line outage.
Definition as per IEC
A static piece of apparatus with two or more
windings
which,
by
electromagnetic
induction, transforms a system of alternating
voltage and current into another system of
voltage and current usually of different values
and at the same frequency for the purpose of
transmitting electrical power.
Why use transformer?
To reduce of transmission
losses.
 For increasing the low
voltage level to high voltage
level.
Why use transformer?
 The voltage level of a power is
increased, the current of the
power is reduced which causes
reduction in ohm or I2R losses
in the system.
 Low level power must be
stepped
up
for
efficient
electrical power transmission.
Power Transformer
Term power transformer is used to refer to those
transformers used in the generator and the
distribution circuits, and these are usually rated at
500 KVA and above.
 Power systems typically consist of a large number
of generation locations, distribution points, and
interconnections within the system or with nearby
systems, such as a neighboring utility.
Power Transformer
Power transformers must be used at each of
these points where there is a transition
between voltage levels.
 The complexity of the system leads to a
variety of transmission and distribution
voltages.
Philosophy and economic considerations of
protection
 Separate the faulted equipment from the
remainder of the system so that the system can
continue to function
 Limit damage to the faulted equipment.
 Minimize the possibility of fire.
 Minimize hazards to personnel.
 Minimize the risk of damage to adjacent high voltage apparatus
Transformer Protection
 The type of protection for the transformers varies
depending on the application and the importance of the
transformer.
 Transformers are protected primarily against faults and
overloads.
 The type of protection used should minimize the time of
disconnection for faults within the transformer and to
reduce the risk of catastrophic failure.
Why Transformer Protection?
Why Transformer Protection?
To Protect the Transformer from severe
damages.
Time required to rectify the Transformer in
case of damages is more and it is very
difficult.
Operation of a power network when the power
transformer is out of service is always difficult.
Why Transformer Protection?
 A Power Transformer
fault therefore often is a
more severe disturbance
for the network than an
overhead line fault which
usually can be repaired
rather quickly.
Transformer components:
Transformer components:
Why Do Transformers Fail?
The electrical windings and the magnetic core in a
transformer are subject to a number of different
forces during operation:
• Expansion and contraction due to thermal cycling
• Vibration
• Local heating due to magnetic flux
• Impact forces due to through-fault current
• Excessive heating due to overloading or inadequate
cooling
What Fail in Transformers?
1- Windings
 Insulation deterioration from:





Moisture
Overheating
Vibration
Voltage surges
Mechanical Stress from
through-faults
What Fail in Transformers?
2- LTCs




Malfunction of mechanical switching mechanism
High resistance contacts
Overheating
Contamination of insulating oil
What Fail in Transformers?
3- Bushings




- General aging
- Contamination
- Cracking
- Internal moisture
What Fail in Transformers?
4- Core Problems
 - Core insulation
failure
 - Open ground strap
 - Shorted laminations
 - Core overheating
 Shell construction is
lighter than core
construction.
• 3-leg shell core
causes zero
sequence coupling.
What Fail in Transformers?
5- Miscellaneous
 - CT Issues
 - Oil leakage
 - Oil contamination
• Metal particles
Moisture
IEEE 37. 91
Transformer Faults
Internal faults
1. Winding Failure
2. Winding inter-turn faults.
3. Core insulation failure,
shorted laminations
4. Over fluxing.
External faults
1. Overloads
2. Overvoltage
3. Over heating
4. External system short
circuits.
Transformer Protection Scheme
Mechanical
Electrical
1. Buchholz relay
2. Sudden pressure
3. Pressure relief
4. Temperature
protection
1. Bias Differential Protection (87)
2. Over Fluxing (24)
3. Over Voltage (59)
4. Under Voltage (27)
5. Neutral Unbalance (64R)
6. Restricted Earth Fault (64REF)
7. Back up O/C & E/F (50/51/67)
Transformer Protection Scheme
Mechanical Protection
There are two methods of detecting transformer faults other than by
electric measurements:
- Accumulation of gases due to slow decomposition of the
transformer insulation or oil. These relays can detect heating due to
high-resistance joints or due to high eddy currents between
laminations.
- Increases in tank oil or gas pressure caused by internal
transformer faults.
Transformer Protection Scheme
Mechanical Protection
1- Buchholz relay
Transformer Protection Scheme
Mechanical Protection
1- Buchholz relay / Gas accumulator relay
Applicable to conservator tanks
equipped
Operates for small faults by
accumulating the gas over a period of
Time.
Typically used for alarming only
Operates or for large faults that force the
oil through the relay at a high velocity
• Used to trip
• Able to detect a small volume of gas and
accordingly can detect arcs of low
energy
Detects
• High-resistance joints
• High eddy currents between laminations
• Low- and high-energy arcing
• Accelerated aging caused by overloading
Gas analysis
a) Hydrogen is generated by corona or partial discharges. The
presence of other key gases can indicate the source of the discharge.
b) Ethylene (C2H4) is the key gas associated with the thermal
degradation of oil. Trace generation of associated gases (ethane and
methane) may start at 150 C. Significant generation of ethylene
begins around 300 C.
c) Carbon monoxide and carbon dioxide are generated when cellulose
insulation is overheated.
d) Acetylene (C2H2) is produced in significant quantities by arcing in
the oil.
Transformer Protection Scheme
Mechanical Protection
2- Sudden pressure Rely
Sudden-pressure relays respond
to the pressure waves in the
transformer oil caused by the gas
evolution associated with arcing.
Transformer Protection Scheme
Mechanical Protection
2- Sudden pressure Relay
When high current passes through a
shorted turn, a great deal of heat is generated
Detect large and small faults
This heat, along with the accompanying arcing,
breaks down the oil into combustible gases
Gas generation increases pressure within the tank
A sudden increase in gas pressure can be detected
by a sudden-pressure relay located either in the gas
space or under the oil
The sudden-pressure can operate before relays
sensing electrical quantities, thus limiting damage to
the transformer
Transformer Protection Scheme
Mechanical Protection
2- Sudden pressure Relay
Drawback of using sudden-pressure relays is tendency to
operate on high current through-faults
• The sudden high current experienced from ma close-in throughfault causes windings of the transformer to move.
This movement causes a pressure wave that is transmitted
through the oi
Transformer Protection Scheme
Mechanical Protection
2- Sudden pressure Relay
Countermeasures:
Overcurrent relay supervision
• Any high-current condition detected by the instantaneous overcurrent relay
blocks the sudden-pressure relay
• This method limits the sudden-pressure relay to low-current incipient fault
detection.
Place sudden-pressure relays on opposite corners of the
transformer tank.
• Any pressure wave due to through-faults will not be detected by both suddenpressure relays.
• The contacts of the sudden-pressure relay are connected in series so both
must operate before tripping
Sudden Pressure Relay Supervision Scheme
Transformer Protection Scheme
Mechanical Protection
3- Pressure Relief
This is used to evacuate any over pressure inside the transformer to avoid
explosion of the transformer tank.
The pressure relief device limits the tank overpressure and reduces the risk
of tank rupture and uncontrolled oil spill, which might also cause a fire.
Transformer Protection Scheme
Mechanical Protection
3- Pressure Relief
Transformer Protection Scheme
Mechanical Protection
4- Temperature Protection
Transformers may overheat due to the following
reasons:






High ambient temperatures
Failure of cooling system
External fault not cleared promptly
Overload
Abnormal system conditions such as low frequency, high
voltage, nonsinusoidal load current, or phase-voltage
unbalance
Transformer Protection Scheme
Mechanical Protection
4- Temperature Protection
Transformer Overheating
Undesirable results of overheating
Overheating shortens the life of the transformer insulation in proportion to
the duration of the high temperature and in proportion to the degree of the
high temperature.
Severe over temperature may result in an immediate insulation failure (fault)
Overheating can generate gases that could result in an electrical failure
(fault) Severe over temperature may result in the transformer coolant heated
above its flash temperature, with a resultant fire (fault and a bang!).
Transformer Protection Scheme
Other means of thermal protection
Top-oil temperature
Fuses and overcurrent relays
Thermal relays for tank temperature
Overexcitation protection
Transformer Protection Scheme
Mechanical Protection
Fire Protection
It can occur because of deterioration of insulation
in the transformer.
This produces arcing which in turn overheats the
insulating oil and causes the tanks to rupture;
further arcing then will start a fire.
Fires are also initiated by lightning and
occasionally by dirty insulators on the outside of the
tanks.
Transformer Protection Scheme
Mechanical Protection
Fire Protection
Transformer Protection Scheme
Mechanical Protection
Lightning Protection
Lightning overvoltage surges originate from
atmospheric discharges and they can reach their peak
within a few microseconds and subsequently decay
very rapidly.
The charge from the surge produces both short
duration high current impulse and long duration
continuing current impulse which affects the
transformer insulation system.
Transformer Protection Scheme
Mechanical Protection
Lightning Protection
Transformer Protection Scheme
Mechanical Protection
Celica Gel Breather
Silica gel breathers is used on the conservator of
oil filled transformers.
The purpose of these silica gel breathers is to
absorb the moisture in the air sucked in by the
transformer during the breathing process.
Transformer Protection Scheme
Mechanical Protection
Celica Gel Breather
During the breathing process, the incoming air
may consist of moisture and dirt which should be
removed in order to prevent any damage.
Hence the air is made to pass through the silica
gel breather, which will absorb the moisture in the air
and ensures that only dry air enters in to the
transformer.
Transformer Protection Scheme
Mechanical Protection
Celica Gel Breather
Silica gel in the breather will be
blue when installed and they turn
to pink colour when they absorb
moisture which indicates the
crystals should be replaced.
These breathers also have an oil
cup fitted with, so that the dust
particles get settled in the cup.
Transformer Protection Scheme
Mechanical Protection
Oil Level Gauge
Transformers with oil conservator (expansion
tank) often have an oil level monitor.
Usually, the monitor has two contacts for alarm.
One contact is for maximum oil level alarm and
the other contact is for minimum oil level alarm.
Transformer Protection Scheme
Mechanical Protection
Oil Level Gauge
When oil level is low from
fixed minimum oil level then
minimum oil level alarm is
ringing.
When oil level is high from
fixed maximum oil level then
maximum oil level alarm is
ringing.
Transformer Protection Scheme
Mechanical Protection
Oil Level Gauge
Transformer Protection Scheme
Mechanical Protection
Transformer Protection Scheme
Fault clearing
fault-clearing devices to be used should consider factors such as
a)Installation and maintenance cost
b) Fault-clearing time ver elatito fire hazard and
repair or replacement costs of the transformer
c)System stability and reliability
d)System operating limitations
e)Device interrupting capability
Transformer Protection Scheme
Fault clearing
1- Relay tripping circuits
2- Circuit breakers
3- Fuses
Transformer Protection Scheme
IEEE Devices used in Transformer Protection
24: Overexcitation (V/Hz)
26: Thermal Device
46: Negative Sequence Overcurrent
49: Thermal Overload
50: Instantaneous Phase Overcurrent
50G: Instantaneous Ground Overcurrent
50N: Instantaneous Residual Overcurrent
50BF: Breaker Failure
Transformer Protection Scheme
IEEE Devices used in Transformer Protection
51G: Ground Inverse Time
Overcurrent
51N: Residual Inverse Time
Overcurrent
63: Sudden Pressure Relay
(Buccholtz Relay)
64G: Transformer Tank Ground
Overcurrent
81U: Underfrequency
87H: Unrestrained Phase Differential
87T: Transformer Phase Differential
with Restraints
87GD: Ground Differential (also
known as “restricted earth fault”)
Transformer Protection Functions
Internal Faults:
Through Faults:
87T Phase Differential with
50/51 Phase Overcurrent
Restraints
50G/51G Ground
87H Unrestrained Phase
Overcurrent
Differential
50N/51N
Instantaneous
87GD Three Ground
Residual Overcurrent
Differential elements
46 Negative Sequence
(Restricted Earth Fault)
Overcurrent
64G Tank Ground
Overcurrent
Transformer Protection Functions
Abnormal Operating
Conditions:
27 Undervoltage
24 Overexcitation (V/Hz)
49 Thermal Overload
81U Underfrequency
50BF Breaker Failure
Asset Management
Functions:
TF Through Fault
Monitoring
BM Breaker Monitoring
TCM Trip Circuit Monitoring
Transformer Protection Functions
High Side Overcurrent
Back up to differential,
sudden pressure
Coordinated with line
protection off the bus
• Do not want to trip for
low-side external faults
Transformer Protection Functions
Low Side Overcurrent
Provides protection
against uncleared faults
downstream of the transformer
May consist of phase and
ground elements
Coordinated with downline
protection off the bus
Simulation Session Using Etap Software
Transformer Protection Functions
Negative Sequence Overcurrent
Negative sequence overcurrent
provides protection against
Unbalanced loads
Open conductors
Phase-to-phase faults
Ground faults
Does not protect against 3-phase
faults
Transformer Protection Functions
Negative Sequence Overcurrent
Can be connected in the primary
supply to protect for secondary phase-toground or phase-to-phase faults
Helpful on delta-wye grounded transformers
where only 58% of the secondary p.u. phaseto-ground fault current appears in any one
primary phase conductor
Transformer Protection Functions
Through Fault
Provides protection against
cumulative through fault damage
Typically alarm function
Transformer Protection Functions
Through Fault
A transformer is like a motor that does not spin
There are still forces acting in it
That is why we care about limiting through-faults
Transformer Protection Functions
Through Fault
Through Fault Monitoring
Protection against heavy prolonged through faults
Transformer Category
-IEEE Std. C57.109-1985 Curves
Transformer Protection Functions
Through Fault
Through Fault Damage Mechanisms
Thermal Limits for prolonged
through faults typically 1-5X
rated
Time limit of many seconds
Mechanical Limits for shorter
duration through faults typically
greater than 5X rated
Time limit of few seconds
NOTE: Occurrence limits on each
Transformer Class Graph
Transformer Protection Functions
Through Fault
Through Fault Function Settings (TF)
Should have a current threshold to discriminate between mechanical and
thermal damage areas
• May ignore through faults in the thermal damage zone that fails to meet
recording criteria
Should have a minimum through fault event time delay to ignore short
transient through faults
Should have a through fault operations counter
• Any through fault that meets recording criteria increments counter
Should have a preset for application on existing assets with through fault
history
Should have cumulative I2t setting
• How total damage is tracked
Should use inrush restraint to not record inrush periods
• Inrush does not place the mechanical forces to the transformer as does a through fault
Transformer Protection Functions
Overexcitation
Responds to overfluxing; excessive
V/Hz
• 120V/60Hz = 2 = 1pu
Constant operational limits
O ANSI C37.106 & C57.12
• 1.05 loaded, 1.10 unloaded
O Inverse time curves typically available for
values over the constant allowable level
Transformer Protection Functions
Overexcitation
Overfluxing is a voltage and frequency based issue
Overfluxing protection needs to be voltage and frequency
based (V/Hz)
Although 5th harmonic is generated during an overfluxing
event, there is no correlation between levels of 5th harmonic
and severity of overfluxing
Apparatus (transformers and generators) is rated with V/Hz
withstand curves and limits – not 5th harmonic withstand limits
Transformer Protection Functions
Overexcitation vs. Overvoltage
Overvoltage protection reacts to dielectric limits.
• Exceed those limits and risk punching a hole in the insulation
• Time is not negotiable
Overexcitation protection reacts to overfluxing
• Overfluxing causes heating
• The voltage excursion may be less than the prohibited dielectric
limits (overvoltage limit)
• Time is not negotiable
• The excess current cause excess heating which will cumulatively
damage the asset, and if left long enough, will cause a catastrophic
failure
Transformer Protection Functions
Causes of Overexcitation
Generating Plants
o Excitation system runaway
o Sudden loss of load
o Operational issues (reduced frequency)
Static starts
Pumped hydro starting
Rotor warming
Transmission Systems
o Voltage and Reactive Support Control Failures
Capacitor banks ON when they should be OFF
Shunt reactors OFF when they should be ON
Near-end breaker failures resulting in voltage rise on line
• Ferranti Effect
Runaway LTCs
Load Loss on Long Lines (Capacitive Charging Voltage Rise)
Transformer Protection Functions
Differential Relay Protection
This scheme is employed for the protection of
transformers
against internal short circuits. It provides the best overall
protection for internal faults.
It compares the current entering the transformer with
the current leaving the element.
If they are equal there is no fault inside the zone of
protection
If they are not equal it means that a fault occurs
between the two ends.
Transformer Protection Functions
Differential Relay Protection
What goes into a “unit” comes out of a “unit”
Kirchoff’s Law: The sum of the currents entering and leaving a
junction is zero
Straight forward concept, but not that simple in practice with
transformers
A host of issues challenges security and reliability of
transformer differential protection
Transformer Protection Functions
Differential Relay Protection
It can detect the any faults occurred in the
zone of protection of transformer (CT zone).
Transformer Protection Functions
Differential Relay Protection
Unique Issues Applying to
Transformer Differential Protection
CT ratio caused current mismatch
Transformation ratio caused current mismatch (fixed taps)
LTC induced current mismatch
Delta-wye transformation of currents
- Vector group and current derivation issues
Zero-sequence current elimination for external ground
faults on wye windings
Inrush phenomena and its resultant current mismatch
Transformer Protection Functions
Differential Relay Protection
Unique Issues Applying to
Transformer Differential Protection
Harmonic content available during inrush period due to pointon-wave switching
Especially with newer transformers with step-lap core construction
Overexcitation phenomena and its resultant current mismatch
Internal ground fault sensitivity concerns
Switch onto fault concerns
CT saturation, remanance and tolerance
Transformer Protection Functions
Transformer Protection Functions
Transformer Protection Functions
Transformer Protection Functions
Transformer Protection Functions
Transformer Protection Functions
Transformer Protection Functions
Classical Differential Compensation
CT ratios must be selected to account for:
- Transformer ratios
- If delta or wye connected CTs are applied
- Delta increases ratio by 1.73
Delta CTs must be used to filter zero-sequence
current on wye transformer windings
Transformer Protection Functions
Transformer Protection Functions
Compensation in Digital Relays
Transformer ratio
CT ratio
Phase angle shift and √3 factor due to delta/wye
connection
Zero-sequence current filtering for wye windings so
the differential quantities do not occur from external
ground faults
Transformer Protection Functions
Phase Angle Compensation in Numerical Relays
Phase angle shift due to transformer connection in
electromechanical and static relays is accomplished
using appropriate connection of the CTs
The phase angle shift in Numerical Relays can be
compensated in software for any transformer with zero
or 30° increments
All CTs may be connected in WYE which allows the same CTs
to be used for both metering and backup
overcurrent functions
Some numerical relays will allow for delta CTs to
accommodate legacy upgrade applications
Benefits of Wye CTs
Phase segregated line
currents
- Individual line current
oscillography
- Currents may be easily used
for overcurrent protection and
metering
- Easier to commission and
troubleshoot
- Zero sequence elimination
performed by calculation
NOTE:
For protection upgrade
applications where one wants to
keep the existing
wiring, the relay must:
• Accept either delta or wye CTs
• For delta CTs, recalculate the phase
currents for overcurrent functions
Inrush Detection and Restraint
Characterized by current into one winding of transformer, and
not out of the other winding(s)
• This causes a differential element to pickup
Use inrush restraint to block differential element during inrush
period
• Initial inrush occurs during transformer energizing as the core magnetizes
• Sympathy inrush occurs from adjacent transformer(s)
energizing, fault removal, allowing the transformer to undergo a low level
inrush
• Recovery Inrush occurs after an out-of-zone fault is cleared and the fault
induced depressed voltage suddenly rises to rated.
Transformer Protection Functions
Restricted Earth Faults Relay Protection
Restricted Earth Fault (REF) means an earth fault from a
restricted/localized zone of a circuit.
The term "REF protection method " means not to sense any
earth faults outside this restricted zone. This type of protection is
prevalent in Dyn group of transformers (Delta Primary and Star
Secondary).
Transformer Protection Functions
Restricted Earth Faults Relay Protection
Differential protection has excellent operation in most fault cases, but in
the situations that a single phase to ground fault that current increases
slightly and causes differential protection not to detect the fault.
Restricted earth fault (REF) relay can be used as a complementary of
differential protection.
Differential relay will operate for earth faults inside the zone only if the
earth fault current is more than the bias setting in the relay. The normal
bias setting in a differential relay is 20%.
So, complete earth fault protection is not possible with differential relay.
That is why you need a restricted earth fault relay with sensitive settings
like 5%.
Summary
Summary
References
IEEE C37.91, Guide for Power Transformer Protection
 WSU Hands-On Relay School 2018, Wayne Hartmann
Network And Protection Guid
Thank You