Course Title: Protection of Power System (2)
Course Code: EE455
Presentation 9
Protection of Power System -2
Sub-station Transformer Protection
Lecturer Schedule: 13th to 14th Weeks
Prof Mahmoud El-Gammal October 9, 2017 2
Transformer: Failures, Causes & Impact
1. Windings Failures
i. Dielectric Windings Faults
Dielectric internal faults occurs in the windings are
categorized as insulation breakdown between winding and
earth, or between different phases or between adjacent turns
(i.e. inter-turn fault).
Causes of dielectric interturn faults
➢ Degradation of the insulation due to thermal, electrical, and
mechanical stresses.
➢ Insulation breakdown due to over-current and over-voltage
which are high above the rated values caused by short-circuit
faults or lightning surge attack in absence of surge arrestors.
Impacts of dielectric interturn faults
➢ Breaking of windings or complete burn-out.
3 Prof Mahmoud El-Gammal October 9, 2017
Transformer: Failures, Causes & Impact
1. Windings Failures
ii. Copper Conductors Windings Faults
Causes of copper windings faults
The windings are usually of copper. Due to the copper line resistance
thermal losses occur. These thermal losses make hotspots in the
winding due to bad or lack of maintenance.
Impacts of copper windings faults
➢ The overtime thermal losses causes wear and tear and the
decrease of the physical strength up to the point of breaking of
the winding.
4 Prof Mahmoud El-Gammal October 9, 2017
Transformer: Failures, Causes & Impact
1. Windings Failures
iii. Mechanical Windings Faults
Nature of mechanical windings faults
Mechanical windings faults are the distortion, loosening or
displacement of the windings.
Causes of mechanical windings faults
The main reasons that cause mechanical windings fault are the
improper repair, bad maintenance, corrosion, manufacturing
deficiencies, vibration and mechanical movement within the
transformer.
Impacts of mechanical windings faults
➢ Mechanical windings faults results in the decrease of the
performance of the transformer and the tearing of the turn-to-
turn ratio.
5 Prof Mahmoud El-Gammal October 9, 2017
Transformer: Failures, Causes & Impact
2. Bushings Failures
Nature of bushings faults
➢ Bushes are insulating devices that insulate a high voltage
electrical conductor to pass through an earth conductor.
➢ In transformers it provides a current path through the tank wall.
➢ Bushing failure usually occurs over time.
6 Prof Mahmoud El-Gammal October 9, 2017
Transformer: Failures, Causes & Impact
2. Bushings Failures
Causes and Impacts of bushings failures
➢ Loosening of conductors is caused by transformer
vibrations which results in overheating. This heat damage
the insulating paper and the oil used.
➢ Sudden high fault voltages causes partial discharge which
damage the bushes and causes its degeneration and
complete breakdown within hours.
➢ Seal breaking of bushes happen due to ingress of water,
aging or excessive dielectric losses. Due to this fault core
failure of the transformer occurs.
➢ Not replacing of old oil over long time or its deficiency due
to leakage causes internal over-flashing.
7 Prof Mahmoud El-Gammal October 9, 2017
Transformer: Failures, Causes & Impact
3. Under Load Tap Changer (ULTC) Failures
Causes and Impacts of ULTC failures
➢ In Run-Through fault the tap changer takes time and after a
delay changes the turn ratio. The main reason for it is the
relay residue flux and the spring fragile over time.
➢ Lack of maintenance causes the shaft connection between
the tap and the motor driver ULTC to be not synchronous (i.e.
error in movement position).
➢ Old capacitors or burned-out capacitor in the motor causes
the tap changer to fail to control its direction movement.
8 Prof Mahmoud El-Gammal October 9, 2017
Transformer: Failures, Causes & Impact
4. Oil Tank Failures
Function of oil tank
The function of the tank in the transformer is to be a container for
the oil used in it. The oil in the tank is used for insulation and
cooling. The tank can also be used as a support for Buchholz relay
of the transformer protection.
Causes and Impacts of tank failures
➢ The fault in the tank occurs due to environmental stress,
corrosion, high humidity and sun radiation resulting in a
leakage or cracks in the tank walls. From these leakages and
cracks oil spill from the tank causing the reduction of oil.
➢ The reduction in oil level results in the reduction of insulation
breakdown strength of the transformer windings.
➢ The reduction of oil causes over-heating which damages
different parts of the transformer.
9 Prof Mahmoud El-Gammal October 9, 2017
Transformer Protection
Protection used
Fault Type ANSI
Protective relay
number
Primary winding Phase-phase fault Differential; 87T;
Primary winding Phase-earth fault Overcurrent 50/51
Secondary winding Phase-phase fault Differential 87T
Differential; 87T;
Secondary winding Phase-earth fault
Restricted Earth Fault 87RGF
Interturn Fault Differential; 87T;
Core Fault Thermostat/Buchholz 26/63
Differential; 87T;
Tank Fault Thermostat/Buchholz; 26/63;
Tank-Earth 50G/51G
Over fluxing Over fluxing 24T
RTD temperature
Overheating
monitoring
49T
10 Prof Mahmoud El-Gammal October 9, 2017
Transformer Protection
1) Protection against Windings Faults and Core Insulation Failure
i. Differential relay (ANSI 87T)
Transformer differential relay (87T) provides high sensitivity to
detect low magnitude of fault currents against:
i. Primary winding Phase-to-phase fault.
ii. Primary winding Phase-to-earth fault.
iii. Secondary winding Phase-to-phase fault.
iv. Secondary winding Phase-to-earth fault.
v. Inter-turn winding fault.
vi. Core insulation failure, shorted laminations.
11 Prof Mahmoud El-Gammal October 9, 2017
Transformer Protection
1) Internal three-phase low-magnitude winding faults
i. Differential relay (ANSI 87T)
The major operating challenges to transformer differential
protection are:
Maintaining sensitivity to detect low magnitude internal
faults.
Maintaining security during CT saturation for external
faults while.
➢ CT saturation reduces the secondary output current
from the CT, and causes a false differential current to
appear to the relay.
12 Prof Mahmoud El-Gammal October 9, 2017
Transformer Protection
Transformer Phase Shift between Output and Input Voltages
and Currents
➢ Star connection phase and line voltages
Vab Van Vbn V0 V 120
3 V 30
Phase shift
between line &
phase voltages
➢ Line current equals phase current
13 Prof Mahmoud El-Gammal October 9, 2017
Transformer Protection
Transformer Phase Shift between Output and Input Voltages
and Currents
➢ Delta connection phase and line currents
Ia Iab Ica 3 Iab 30 o
Phase shift
Ia 3 Iab ( 30 o ) between line &
phase currents
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Transformer Protection
Transformer Types of Current Transformer Connections
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Transformer Differential Protection
Compensation for transformer winding configurations
i. Traditional compensation method
CTs are star CTs are delta
connection in connection in
delta-side winding star-side winding
Zero phase shift
between input &
output currents
16 Prof Mahmoud El-Gammal October 9, 2017
Most commonly used wye-delta vector group
and required type of main CT delta connection
on the wye sides of the protected transformer
Required delta CT connection type
IEC Positive-
on wye side of the protected
ANSI sequence No-
vector power transformer and internal
designation Load Voltage
Vector Group setting in numerical
group phasor
differential relay
YNd1 YDAC DAC / Yy0
Dyn1 DABY DAB / Yy0
YNd11 YDAB DAB / Yy0
Dyn11 DACY DAC / Yy0
YNd5 YD150 DAB / Yy6
Dyn5 DY150 DAC / Yy6
Transformer Differential Protection
Compensation for transformer winding configurations
i. Traditional compensation method
When traditional electromechanical
relays are used, the CTs must be
properly connected to have Zero
phase shift between input & output
currents.
➢ The vector group provides the
amount of 30o phase shift that will
occur when the current goes from
the -primary to Y-secondary side
of the transformer
➢ In order to compensate for the 30 degree phase shift, the current
transformers in the Wye side of the transformers must be
connected in delta.
18 Prof Mahmoud El-Gammal October 9, 2017
Transformer Differential Protection
Difficulties and errors in the transformer protection
1. Magnetizing inrush current
The effect of the inrush current on the differential relay is
false tripping the transformer without of any existing type
of faults. From the principle of operation of the differential
relay, the relay compares the line currents coming from
both sides of the power transformer as explained above.
However, the inrush current is flowing only in the lines of
YG-primary side of the power transformer; while circulating
inside phases of -secondary windings. So that, the
differential current will have a significant value due to the
existence of current in only one side.
19 Prof Mahmoud El-Gammal October 9, 2017
Transformer Differential Protection
Difficulties and errors in the transformer protection
2. False trip due to CTs characteristics
➢ Primary and secondary CTs mismatch characteristics
create a small false differential current; which can be
enough to operate the differential relay.
➢ When saturation happens to one or all CTs at different
levels, false differential current appears in the differential
relay.
20 Prof Mahmoud El-Gammal October 9, 2017
Transformer Differential Protection
Difficulties and errors in the transformer protection
3. False trip due to Tap changers
➢ On-Load Tap-Changer (OLTC) is installed on the main sub-
station power transformers to control automatically the
heavy fluctuations in transformer output voltage.
➢ The transformation ratio of the CTs can be matched with
only one point of the tap-changing range.
➢ Therefore, if the OLTC is changed, unbalance current
flows in the differential relay operating coil causing false
trip signal.
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Transformer Differential Protection
Compensation for transformer winding configurations
ii. Numerical-based compensation method
When Numerical relays are used,
it is possible to connect the CTs in wye and let the relay
calculate the delta compensation with both 3 tap and 30o
phase shift.
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Transformer Differential Protection
Compensation for transformer winding configurations
ii. Numerical-based compensation method
The use of wye connected CTs
has the following advantages:
➢ Wye-connected CTs are easier
to wire and troubleshoot.
➢ Wye-connected CTs see three
times less lead burden for a
three-phase fault than delta-
connected CTs, making CT
saturation less likely.
➢ Residual overcurrent protection can be used, whereas delta-
connected CTs prevent this protection from being used on that
input to the relay.
23 Prof Mahmoud El-Gammal October 9, 2017
Transformer Differential Protection
Compensation for transformer winding configurations
ii. Numerical-based compensation method
With wye compensation, the
currents are only tap adjusted.
With DABY compensation, the
currents are first tap adjusted
and then combined
mathematically to compensate
for 30o phase shift.
24 Prof Mahmoud El-Gammal October 9, 2017
Transformer Differential Protection
Compensation for transformer winding configurations
ii. Numerical-based compensation method
MVA 10 3 C
TAP
3 kVLL CT ratio
C = 1 for wye-connected CTs
C= 3 for delta-connected CTs
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Transformer Differential Protection
Compensation for transformer winding configurations
ii. Numerical-based compensation method
With DABY compensation, the currents are first tap adjusted and
then combined mathematically to compensate for 30o phase shift
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Transformer Differential Protection
Advantages of using Numerical-based differential 87T relay
Numerical-based differential relays meet the challenge for
high sensitivity to internal faults and security against false
tripping caused by CT saturation during external faults in the
following ways:
The restraint current is based on the maximum measured
winding current, as opposed to the traditional magnitude
sum of the currents. This ensures ideal restraint for the
actual fault condition, balancing sensitivity and security.
The differential element uses a dual slope-dual breakpoint
characteristic. The differential element can be set to
account for both DC and AC saturation of the CTs, ensuring
security, while maintaining sensitivity.
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Transformer Differential Protection
Differential relay during transformer saturation & inrush
inrush
Idiff = i1 + i2 Idiff = i1 + i2
I 1 = I2 I1 >> I2
i 2 < i1 i1 >> i2
Idiff > 0 Idiff > 0
saturated
Differential relay during Differential relay during
transformer saturation transformer inrush
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Transformer Differential Protection
i. External Fault
▪ Measuring each current
contribution as “restraint”.
▪ Relay restraint currents:
IRes1 = 30 A & IRes2 = 25 A
➢ IRes = 25 A (smallest of IRes1 & IRes2)
➢ Relay operating current: Iop = 5 A
➢ Ratio of Iop / IRes = 5/25 = 20%
➢ Biased differential relay is a “50% restrained”
➢ Conclusion: 20% is less than 50%, so no trip
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Transformer Differential Protection
ii. Internal Fault
▪ Measuring each current
contribution as “restraint”.
▪ Relay restraint currents:
IRes1 = 30 A & IRes2 = 1 A
➢ IRes = 1 A (smallest of IRes1 & IRes2)
➢ Relay operating current: Iop = 31 A
➢ Ratio of Iop / IRes = 31/1 = 310%
➢ Biased differential relay is a “50% restrained”
➢ Conclusion: 310% is much bigger than 50%, so trip
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Transformer Differential Protection
Settings calculations
Step-1:
Calculate the rated load of primary
and secondary sides from:
rated MVA 10 3 25 10 3
INp 131.2 A
3 kVNp 3 110
rated MVA 10 3 25 10 3
INs 687.3 A
3 kVNs 3 21
Step-2: Calculate tap settings
The current mismatch from the power transformer ratio and CT
ratios is solved by using tap settings so that the currents are
compared on a per-unit basis.
rated CT Ip 300
Tap Setting p 2.287 A
INp 131.2
rated CT I s 1000
Tap Setting s 1.455 A
INs 687.3
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Transformer Differential Protection
Settings calculations
Step-3: Select the turn point-1 current
Set high enough to avoid operation
because of steady-state CT errors
and transformer magnetizing current.
The typical TP1 range is 0.3 to 0.5×IN.
ITP1 is fixed @ 50% IN
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Transformer Differential Protection
Settings calculations
Step-4: Calculate the low percentage
bias setting (slope k1 or starting ratio)
Should the tap changer be
uncompensated, the starting ratio
setting is calculated as sum of
▪ 5% (HV CT error)
▪ 5% (LV CT error)
▪ 9×1.67% (Tap changer error)
▪ 4% (typical relay operation error)
▪ 5% (typical margin)
➢ Starting ratio setting (k1) = 34%
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Transformer Differential Protection
Settings calculations
Step-4 cont. :
Calculate the low percentage bias
setting (slope k1 or starting ratio)
Should the tap changer be
automatically compensated (by means
of Tap position input), the starting
ratio setting is calculated as sum of :
▪ 5% (HV CT error)
▪ 5% (LV CT error)
▪ 4% (relay operation error)
▪ 5% (typical margin)
➢ Starting ratio setting (k1) = 19%
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Transformer Differential Protection
Settings calculations
Step-5: Calculate the basic setting
Basic setting defines the minimum
sensitivity of the differential
protection.
Basic setting allows for the no-load
current of the power transformer.
Calculate basic setting beginning
from TP1 as:
Basic setting = 0.5saturation ratio
+ %core losses
Typically core losses is taken as
10% if actual value is unknown
Basic Setting 0.5 34 10 27%
or
Ipu , min 27 100 1 A 0.27 A 270 mA
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Transformer Differential Protection
Settings calculations
Step-6: Select the turn point-2 current
In a power transformer protection
application the second Turn-point2 is
normally chosen in the range 1.5 ... 2.
With the setting 1.5, the protection is
somewhat more stable against out-
of-zone faults, whereas the setting
2.0 provides somewhat more
sensitive protection for in-zone
faults.
Select ITP2 = 200%IN
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Transformer Differential Protection
Settings calculations
Step-7: Calculate the high percentage
bias setting (slope k2)
Set Slope 2 in the range of 50 to 70
percent to avoid problems with CT
saturation for high fault currents.
Select K2 = 70%
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Transformer Protection
2) Phase and Ground Over-Current protection
Causes of over current in transformer:
Over current may occur due to over loading of the
transformer. IEC recommends that transformer capability
loadings are
➢ 150%FLC for transformer ratings less than 100MVA
➢ 130%FLC for transformer ratings higher than 100MVA
If any internal fault occurs like short circuit or ground faults
there will be over current flow through the transformer
windings from the source.
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Transformer Protection
2) Phase and Ground Over-Current protection
ii. High set Over-Current relay (ANSI 50/51)
Values of 51 (Low and High set
OC) settings are adjusted
according to relay co-ordination
with other down-stream
protective devices
39 Prof Mahmoud El-Gammal October 9, 2017
Transformer Protection
2) Phase and Ground Over-Current protection
ii. High set Over-Current relay (ANSI 50/51)
Overcurrent of long duration (i.e.
overloads) is generally detected
by IDMT delayed (ANSI 51)
overcurrent relay.
An instantaneous overcurrent
protection (ANSI 50) device
located at the transformer primary
ensures Instantaneous release in
(15 ~ 50)ms against violent short
circuit currents.
➢ instantaneous pickup of
numerical 50-OCR is set at
1.3×Iinrush @ 15-50 ms
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Transformer Protection
2) Phase and Ground Over-Current protection
ii. High set Over-Current relay (ANSI 50/51)
Short-time delayed 51 (Low and
High set OC) relay located at the
transformer primary as back-up
protection.
➢ Values of 51 (Low and High set
OC) settings are adjusted
according to relay co-ordination
with other down-stream
protective devices.
The over current relays cannot distinguish between external short
circuit, over load and internal faults of the transformer, so backup
protection over current and earth fault protection connected to in-
feed side of the transformer will operate.
41 Prof Mahmoud El-Gammal October 9, 2017
Transformer Protection
2) Phase and Ground Over-Current protection
ii. High set Over-Current relay (ANSI 50/51)
Short-time delayed 51G ground
fault relay located at the
grounded-transformer neutral as
back-up protection for ground
fault currents.
➢ Earth fault current setting
20% of maximum earth fault
with time delay according to
discrimination.
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Transformer Protection
ii. High set Over-Current relay (ANSI 50/51)
Transformer operating region
located at and to the left of
the FLA and inrush points.
Transformer damage region
located to the right and
above the damage curves.
Protective device operating
region located between the
transformer operating and
damage regions.
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Transformer Protection
ii. High set Over-Current relay (ANSI 50/51)
The primary OCR (50)
instantaneous setting should
override the inrush current.
The primary OCR (51) IDMT
curve should be below the
primary-side thermal damage
curve.
The primary OCR (51) IDMT
curve should be below the
ANSI point.
I ANSI 937 A @ 4.5 s
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Transformer Protection
3. Sensitive Ground Fault Protection to limit Transformer Damage
iii. Restricted Ground Fault relay (ANSI 87RGF)
Differential and overcurrent
protection do not provide
adequate protection for wye-
connected windings with
grounded neutrals.
➢ Faults close to the neutral
produces lesser fault current
as shown by the current
distribution curve.
The restricted ground fault
function can be used to provide
differential protection for such
ground faults, down to faults at
5% of the transformer winding.
45 Prof Mahmoud El-Gammal October 9, 2017
iii. Restricted Ground Fault relay (ANSI 87RGF)
Basics
An external fault in the star side
will result in current flowing in the
line CT of the affected phase and at
the same time a balancing current
flows in the neutral current
transformer, hence the resultant
current in the relay is therefore
zero. So this 87RGF relay will not
be actuated for external earth fault.
But during internal fault the neutral
current transformer only carries
the unbalance fault current and
operation of 87RGF relay takes
place.
46 Prof Mahmoud El-Gammal October 9, 2017
iii. Restricted Ground Fault relay (ANSI 87RGF)
Basics
REF is sensitive (more so than
biased differential protection) and
fast acting to clear the internal
earth fault.
Fault detection is confined to the
zone between the two CTs hence
the name 'Restricted Earth Fault'.
The protection scheme is
comparatively cheaper than
differential protection scheme.
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iii. Restricted Ground Fault relay (ANSI 87RGF)
Scheme Arrangements
REF: 3 Wire + Earth, REF: 4 Wire + Earth,
4 CTs 5 CTs
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Transformer Protection
4. Over Flux Protection
In transformer the core flux density B al V/f ratio.
V/f ratio is generally maintained constant but if due to any reason
voltage is increased above nominal voltage or the frequency is
reduced below nominal frequency transformer may experience
overfluxing and may also reach under saturation.
Overfluxing may be due to
➢ sudden load-rejection overvoltages,
➢ the excitation of generator at low frequency, or
➢ when transmission line is lightly loaded and proper shunt VAR-
compensation is not provide.
When a transformer core is overexcited, the core is operating in a
non-linear magnetic region, and creates harmonic components in
the exciting current. A significant amount of current at the 5th
harmonic is characteristic of overexcitation
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Transformer Protection
4. Over Flux Protection
The capability of a transformer to withstand higher V/f values
i.e. over fluxing effect, is limited to a few minutes as furnished
below in the Table:
V/f ratio in % of (V/f)rated 1.1 1.2 1.25 1.3 1.4
Duration of withstand limit continuou
2.0 1.0 0.5 0.0
(in minutes) s
From the table above it may be seen that when over fluxing
due to system hazards reaches such that the factor %V/f
ratio attains a values 1.4, the transformer shall be tripped
out of service instantaneously otherwise there may be a
permanent damage
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Transformer Protection
4. Over Flux Protection
iv. Over-fluxing (V/Hz) relay (ANSI 24)
Characteristics
➢ It is usual to provide a
definite time-delayed alarm
setting for overfluxing %V/f
ratio exceeds 1.1.
➢ Inverse time-delayed or an
instantaneous trip setting
when duration and severity
of the condition crosses safe
operation limit.
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Transformer Protection
5. Transformer Tank Protection
v. Transformer tank grounded relay (ANSI 64T)
Principle of this protection consists in
connection of the transformer frame
with the earthing system through a CT.
An overcurrent relay is connected to
the CT output terminals.
This protection will act in case of all
faults, when voltage will be on the
transformer frame (e.g. a bush
breakdown) and current will flow from
frame to ground through CT. Therefore
the transformer tank has to be
insulated from the ground.
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Transformer Protection
5. Transformer Tank Protection
v. Transformer tank grounded relay (ANSI 64T)
The setting value of the tank
earth-fault protection is given:
Rg
Ipu IFmax
Ri R g
Ri insulation resistance of the transformer tank
resistance between the grounded point of
Rg the transformer tank and the grounding of
the transformer winding neutral point
maximal value of fault current through
IFmax
ground
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Transformer Protection
6. Transformer Over-Load Protection
vi. Thermal over-load relay (ANSI 49RMS)
This protection function is used to protect HV/LV
transformers against overloads, based on measurement of
the current drawn.
The current measured is an RMS 3-phase current which
takes into account harmonics up to number 13.
The 49RMS relay function is based on a thermal model witch
calculates the temperature rise from current measurements
according to functional standard IEC 60255-149 and take
into account the I2R losses due to the specific operational
current and the simultaneous cooling effect of the coolant.
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Transformer Protection
6. Transformer Over-Load Protection
vi. Thermal over-load relay (ANSI 49RMS)
The time-current characteristics according to functional standard
IEC 60255-149 is:
2 2 t Operating time
I Ip
t T ln T Thermal time constant of the transformer
I2 k I 2
B I Overload current
IB Thermal overload current setting
k constant
Specified load current before the overload
Ip
occurs
The tripping time during an overload condition takes the prior level
of load current into consideration. An alarm can be set to operate
before the tripping condition is reached.
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Transformer Protection
6. Transformer Over-Load Protection
vi. Thermal over-load relay (ANSI 49RMS)
A 49 thermal protection function can be implemented to make
decisions based on top-oil temperature or winding hottest-spot
temperature limits.
The 49 function includes 12 stages of settings that could be
used to issue
➢ alarms when the transformer is 80-90% of maximum
current load or
➢ tripping decisions when the transformer has reached
maximum allowable temperature.
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Transformer Protection
vii. Buchholz relay (ANSI 63)
Buchholz relay ONLY applies to a
transformer with a Conservator.
Buchholz relay is a type of oil and gas
actuated protection relay universally
used on all oil immersed transformers
having rating more than 500 kVA.
Buchholz relay is used for the
protection of internal transformer
short circuit faults such as
➢ inter turn faults,
➢ incipient winding faults, and
➢ core faults that may occur due to the
impulse breakdown of the insulating
oil.
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Transformer Protection
vii. Buchholz relay (ANSI 63)
Minor fault occurs inside the
transformer produces heat which
decomposes oil and gas bubbles
moves towards the conservator
through the pipe line. The
displacements of oil gas tilts the
hinged float at the top of the chamber
thereby the mercury switch closes the
contacts of the alarm circuit.
During the occurrence of severe faults
the amount of volume of gas evolves
will be large and the float at the
bottom of the chamber is tilted and
the trip circuit is closed to operate
the circuit breaker and isolates the
transformer.
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Transformer Protection
MV/LV & Low power rating
(2) Fuse or 2-setting overcurrent
(3) Earth fault
(4) Buchholz or DGPT
(6) Neutral earth protection
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Transformer Protection
MV/LV & High power rating
(1) Thermal overload
(2) Fuse or 2-setting overcurrent
(3) Earth fault
(4) Buchholz or DGPT
(5) Tank earth leakage
(6) Neutral earth protection
60 Prof Mahmoud El-Gammal October 9, 2017
Transformer Protection
MV/MV & High power rating
(1) Thermal overload
(2) Fuse or 2-setting overcurrent
(3) Earth fault
(4) Buchholz or DGPT
(5) Tank earth leakage
(6) Neutral earth protection
(7) Transformer differential
(8) Restricted earth fault protection
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