Power Equipment; Protection and
underground MV cables laying
Eng. Nouh Ahmed Moawwad
Gopa international GMBH
Abu Dhabi, 2010
Power Equipment; Protection and underground MV cables laying
Contents
1.
Introduction ...................................................................................................................................................................... 4
2.
Equipment in a substation ............................................................................................................................................... 4
3.
2.1.
Power transformers and related tests .................................................................................................................... 4

Turns ratio ................................................................................................................................................................ 5

Load losses test ......................................................................................................................................................... 5

No load losses test ..................................................................................................................................................... 5

Percentage impedance (Z) ....................................................................................................................................... 6

Temperature rise test ............................................................................................................................................... 6

Insulation resistance test .......................................................................................................................................... 7

Winding resistance test ............................................................................................................................................ 8

Core balance test ...................................................................................................................................................... 8

Vector group test ...................................................................................................................................................... 9

Separate source voltage withstand test ................................................................................................................. 10

Induced overvoltage test ........................................................................................................................................ 10

Oil breakdown voltage test .................................................................................................................................... 11

Capacitance and dissipation factor tan delta (tan δ) test .................................................................................... 11

Transformer mechanical protections ................................................................................................................... 12
2.2.
Switchgear ............................................................................................................................................................... 13
1.
Busbars .................................................................................................................................................................... 14
2.
Circuit breakers (CB) ............................................................................................................................................ 15
3.
Current Transformers (CT) .................................................................................................................................. 16
4.
Voltage Transformers (VT) ................................................................................................................................... 19
5.
Metering devices ..................................................................................................................................................... 20
6.
Auxiliary wiring...................................................................................................................................................... 20
2.3.
DC systems .............................................................................................................................................................. 21

DC system configurations and components ......................................................................................................... 21

Battery types ........................................................................................................................................................... 22

Testing of DC system.............................................................................................................................................. 22
2.4.
Earthing system: ..................................................................................................................................................... 23

Earthing system tests ............................................................................................................................................. 23
2.5.
Numerical protections relays ................................................................................................................................. 27
2.6.
Power and Fiber Optic Cables .............................................................................................................................. 30
2.6.1.
Power cables.................................................................................................................................................... 31
2.6.2.
Fiber optic cables ............................................................................................................................................ 37
Underground MV Cables Works ................................................................................................................................. 41
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Power Equipment; Protection and underground MV cables laying

Excavation ................................................................................................................................................................... 41

Bedding sand............................................................................................................................................................... 41

Rollers queuing inside trench .................................................................................................................................... 42

Drum preparation ...................................................................................................................................................... 42

Laying the cable and PVC pipes ............................................................................................................................... 43

Fine sand over the cable............................................................................................................................................. 44

Interlocked concrete cover tiles................................................................................................................................. 44

Warning tape .............................................................................................................................................................. 45

Final backfilling .......................................................................................................................................................... 45

Cable joint marker ..................................................................................................................................................... 46

Bentonite injection into cables concrete ducts (for cable cooling) ......................................................................... 46
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Power Equipment; Protection and underground MV cables laying
1. Introduction
This technical manual will give a brief about the equipment inside a substation and testing &
commissioning of this equipment. Testing of the equipment is the final stage in a substation
installation and carried out to check the quality & healthiness of the equipment in the
substation; hence it must be carried out by highly qualified personnel to avoid any hazards
and risks for human and equipment.
Also cable installation works are illustrated briefly in this manual.
2. Equipment in a substation
The major equipment that can be found in a substation is:
1.
2.
3.
4.
5.
6.
Power transformers
Switchgear
DC system
Earthing system
Numerical Protection relays
Electrical and Fiber Optic Cables
Before the commencement of the testing extreme safety precautions must be taken into
account to prevent human injury as well as equipment damage, these precautions shall include
but not limited to the following:
 Qualified personnel who will carry out the tests.
 Surrounding the testing area with warning tape.
 Warning signs.
 Safety officers must be attending during the testing.
2.1. Power transformers and related tests
Any power Transformer name plate includes the following data
as a minimum:
 Rate voltage e.g. 33/11kV,…etc
 Rated power in MVA.
 Cooling type ONAN (oil natural air natural) ; ONAF
(oil natural air forced) ; OFAF…..etc
 Percentage impedance %Z
 Vector group (usually Dyn11 in the distribution)
oil transformer
 AC withstand voltage
 Lightning impulse withstand voltage
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Power Equipment; Protection and underground MV cables laying
Transformer testing will include the following:
 Turns ratio test.
 Load losses test.
 No load losses test.
 Percentage impedance %Z
 Temperature rise test
 DC Insulation resistance (Megger) test.
 Winding resistance test.
 Core balance test.
 Vector group test.
 Separate source voltage withstand test.
 Induced overvoltage test.
 Oil breakdown voltage test.
 Capacitance and dissipation factor
 Turns ratio
Turns ratio=N1/N2 = Vp/Vs.
To check the ratio we apply 400V to the primary side and by measuring the generated
voltage at secondary side, the ratio can be calculated as follows: Turns ratio=
400/measured voltage.
 Load losses test
Load losses are the power lost in the transformer windings due to winding DC resistance;
load losses are also known as copper losses. In this test the load losses are measured at full
load (at rated current). During the Load losses test the secondary is shortened and primary
voltage adjusted to achieve a full load current flow and the absorbed current and losses are
then measured.
Applied Voltage
(V)
Current (A)
Losses (KW)
%Z
 No load losses test
No load loss is also known as iron losses; No load losses occur when there is no load
connected to the transformer and these losses arise from many causes, mainly two reasons:
 Eddy currents:
Ferromagnetic materials are good conductors, and a core made from such a material
constitutes a single short-circuited turn throughout its entire length. Currents
therefore circulate within the core. To minimize the eddy currents the core is made
of laminations electrically insulated from each other, rather than a solid block.
Ieddy α (supply square frequency / square area of the core material)
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Power Equipment; Protection and underground MV cables laying
 Hysteresis losses:
Each time the magnetic field is reversed, a small amount of energy is lost due
to hysteresis within the core.
By applying the rated voltage to the secondary side with the primary winding open
and by measuring the applied voltage and no-load current, no-load losses can be
measured.
 Percentage impedance (Z)
With the secondary winding shortened the voltage required to be applied to the primary
side of a transformer so that full rated current in the secondary side will circulate is called
the impedance voltage (Vimp.).
Percentage impedance Z=Vimp. / Vn
Percentage impedance is very important parameter for a transformer since it affects the
parallel operation of transformers and determines the transformer short circuit level.
In parallel operation assume we have two identical transformers except in Z, so the
transformer of higher Z will carry lower current.
In case of short circuit the transformer of higher Z can withstand higher short circuit
currents.
General points:
 Z is proportional to the transformer rated power and it must not exceed a specific
limit.
 Theoretically, the test voltage can be implemented to the secondary but since the
secondary current is very high and there’s no tester can provide such high current; so
the test is applicable only at high voltage side.
Test procedure:
The secondary winding is shortened by busbars or very short cables (because length of
cables is added to Z); and slowly increase the voltage applied to the primary till 10 or 20%
of nominal current passes then measure the test voltage applied and Z can be calculated as
follows:
Z= (Vt/Vn)/ (It/In) where
Vt= winding test voltage; Vn= winding nominal voltage;
Note: Z is measured during load loss test; in this case It=In; then Z= (Vt/Vn)
 Temperature rise test
This test is to make sure that the transformer during full load will not overheat and exceed
the operational limits of temperature and it’s carried out only in factory as a type test. Heat
sensors are located at both the top and bottom of the transformer tank to measure the
temperature of the oil; then the total losses (load & no-load losses) are applied to the
transformer.
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Power Equipment; Protection and underground MV cables laying
Temperature rise test is carried out in the same manner with load losses except with one
difference which is the voltage applied to the transformer is increased till the current passes
equals to the sum of load and no-load currents. And after reaching steady state (minimum
after 1 hour) ambient temperature and the temperature of the oil (top & bottom) are
measured and the reading is recorded. Then the measurements are repeated each 30
minutes and the average is calculated (for oil & ambient); then oil (top) temperature is
calculated from:
∆θ top oil=θ top average-θ ambient average
And average oil temperature rise:
∆θ oil average = ((θ top average +θ bottom average) /2)-θ ambient average
The winding temperature is extrapolated from resistance/temperature curve immediately
after switch off
Then:
∆θ HV winding=θ HV -θ ambient average
∆θ LV winding=θ LV -θ ambient average\

Winding hot-spot temperature (H.S.T) is the maximum temperature occurring in the winding
and it’s located at winding top point.
H.S.T can be calculated as follows:
Winding temperature gradient= Average winding temp. – Average oil temp.
Then, for distribution transformers:
H.S.T = winding temp. Rise + (1.1*winding temp. gradient)
And for medium size power transformers:
H.S.T = winding temp. Rise + (1.3*winding temp. gradient)
Generally in distribution & medium size power transformer the max. Limits are 45C˚; 55C˚
and 68C˚ for oil temperature rise; winding temperature rise and hot-spot respectively.
 Insulation resistance test
DC resistance of the insulation is measured by means of a DC
voltage instrument; that instrument is called Megger. Generally,
DC insulation resistance must be high (in MΩ)
Assume 11/0.4kV transformer: by means of Megger the
insulation level can be checked between as follows:
Megger
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Power Equipment; Protection and underground MV cables laying

Connect one terminal of the Megger to the Primary side and other terminal to secondary
side apply 2.5KV for 10 minutes and record the reading after 1 and 10 minutes and
calculate the polarization index (PI) where PI = R10/R1. PI must be not less than 1.1
 Repeat the same between Primary side and earth (secondary earthed) with 5kV.
 Repeat the same between secondary side and earth (Primary earthed) with 1kV
Applied voltage (kV)
Insulation resistance at 60
sec.
Insulation resistance at
600 sec.
PI
HV to earth
5
LV to earth
2.5
HV to LV
2.5
 Winding resistance test
For a transformer the ideal winding has zero DC resistance; but actually this doesn’t
happen and there is a value for the DC resistance .The DC resistance must not exceed a
specific value because the higher DC value means higher heat in the winding and higher
power loss. The tester injects DC current to U-V phases of the primary side then the
generated voltage is measured, the same shall be repeated between V-W and W-U. Then
the same way is repeated for the low voltage side.
R=V/I
For primary side:
Phase
1U-1V
1V-1W
1W-1U
2U-2V
2V-1W
2W-2U
R
For secondary side:
Phase
R
For Dyn-11 transformer:
Delta side: Average resistance per phase= 1.5 measured value
Star side: Average resistance per phase= 0.5 measured value
 Core balance test
Magnetic balance test is done on transformer to see whether the transformer core is
magnetically balanced or not. The test is simply implemented by Applying 400 V 3Ф to
HV side (secondary side open) then disconnecting the phase R and measure VST; VRS; VRT
Then check that VST =VRS+VRT and repeating the same for the other phases.
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Power Equipment; Protection and underground MV cables laying
R
R
S
400v 3Ф
S
HV
LV
T
T
n
Core Balance Test circuit
 Vector group test
Usually in power networks Dyn11 transformers are used;
Meaning of vector group symbol:
 First letter denotes the connection of the high voltage winding
 Second letter denotes the connection of the low voltage winding
 The number means that for a phase the low voltage lags the high voltage with an angle
equals the number multiplied by 30˚.
Example: Dyn11 means the transformer connection of high voltage side is delta and low
voltage side is star and phase shift between primary and secondary is 11x30=330˚ which
means that the secondary lags the primary by 330˚ or the secondary leads the primary by
30˚.Verification of the vector group is very important for the parallel operation of
transformers and it’s carried out by shortening one phase of the primary with same phase at
the secondary side (R to r) and applying 400V to the primary side and recording some
measurements to satisfy the vector group conditions.
U-u
Vn
W
w
v
V
Vector group for Dyn11 transformer
For Dyn11 the following relations must be satisfied:
W-w<W-v
V-v=V-w
UVn + VVn=UV
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Power Equipment; Protection and underground MV cables laying
 Separate source voltage withstand test
The test is implemented using AC high voltage at rated frequency to check the insulation
level.
Apply the test voltage between primary winding and earth with secondary winding earthed
for 1 minute, and then repeat the test with secondary with primary side earthed. The AC
voltage applied depends on the rated voltage of the transformer e.g. 33/11kV transformer
we use 70KV (for1 minute) between primary winding and earth but for secondary winding
28kV is used.
The current measured in the test usually few milliamps.
The pass criterion is to withstand.
Assume 11/0.415kV transformer:
HV to LV & earth
Applied voltage (kV)
28
LV to HV & earth
3
Current (mA)
Result
 Induced overvoltage test
This test is to check the healthiness of the insulation between inter-terns and it will
withstand the temporary overvoltage to which the transformer may be subjected during its
lifetime. It’s implemented by applying double voltage at specific frequency and time to the
secondary winding according to the following duration/frequency relation:
Test duration (sec.) = 120* Fs/Ft
Where: Fs is the supply frequency and Ft is the test frequency
Induced overvoltage test circuit
For 11/0.415kV transformer:
Voltage Frequency
(V)
(HZ)
830
200
The pass criterion is to withstand.
Time
(Sec)
30
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Result
Power Equipment; Protection and underground MV cables laying
 Oil breakdown voltage test
Mineral oils are used for oil immersed transformer to provide high insulation between windings
of transformer as well as the cooling required for heat dissipation generated during the operation
of the transformer. The test is implemented using Oil breakdown voltage tester by taking a sample
of the oil required to be tested and putting it inside the tester which automatically increases the
applied voltage till oil insulation collapses and repeating that step 6 times with 2 minutes
separation between each 2 measurements and find the average to check the healthiness of the oil
insulating characteristics.
 Capacitance and dissipation factor tan delta (tan δ) test
As we know the primary and secondary windings are insulated from each other as well as the
insulation between each winding and the transformer tank; and since we have two conductors
insulated from each other, this forms ideal capacitor.
In ideal capacitor (good insulation) the current is pure capacitive and no resistive current passes
through the insulator.
Dissipation factor or power factor is defined as the ratio between the active (resistive) current and
the capacitive current. As shown in the current phasor
where IR = resistive current & IC = capacitive current & IT = total current
IT

Tan (δ) = IR/IC
IR
(δ)
IC
Test Procedure
Step 1:
Isolate the equipment, ground all incoming and outgoing cables and disconnect all incoming and
outgoing cables from the transformer bushing terminals.
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Power Equipment; Protection and underground MV cables laying
Step 2:
Isolate the neutral bushing connection (if applicable) from the transformer grounding bar.
Step 3:
Short-circuit all high voltage bushing terminals together.
Step 4:
Short-circuit all low voltage bushing terminals and the neutral bushing terminal together.
Step 5:
Connect the capacitance and dissipation factor test set.( Refer to Figure above)
Step 6:
Record the capacitance and dissipation factor values once the null meter is balance for both phasing
position. Record values for the five test-variable selector switch position.
Usually minimum 10kV is used for the test and the max. Value for tan delta for a power transformer at
20˚C does not exceed 0.5%. (The smaller tan delta, the better insulation)
 Transformer mechanical protections
Usually power transformers are equipped with mechanical protections to disconnect the
primary side of the transformer in case of over currents; these protections are as follows:
1. Buchholz relay.
2. Winding temperature sensor.
3. Oil temperature sensor.
4. Pressure relief device.
5. Oil level float.
6. Transformer dehydrator or breather.
 Buchholz relay:
Buchholz relay or gas relay is a safety device mounted on some oilfilled power transformers equipped with an external overhead oil
reservoir called a conservator. The Buchholz Relay is used as a
protective device sensitive to the effects of dielectric failure inside the
transformer.
Buchholz relay
The function of the relay is to detect an abnormal condition within the tank and send an alarm or
trip signal. Under normal conditions the relay is completely full of oil.
In Fault conditions within a transformer, gases are produced and collected in a chamber inside the
relay. It includes 2 stages (alarm and trip).
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Power Equipment; Protection and underground MV cables laying
 Winding temperature sensor:
An RTD (resistive temperature detector) sensor e.g.
Pt100 or a thermocouple is inserted inside the low
voltage winding (higher current, hence higher heat) to
measure the temperature and transmits the signal to a
temperature gauge with 2 stages (alarm and trip).
Pt100
 Oil temperature sensor: Same like winding temperature sensor.
Note: alarm and trip stages of the oil temperature gauge are set to lower values than that of
the winding because the winding is the source of heat.
 Pressure relief device:
This is the last line of protection which works only due to high pressure inside the
transformer and includes a trip stage only.
 Oil leakage detector: by means of a float any leakage of the transformer oil can be
detected.
 Transformer dehydrator or breather:
It’s a glass tube filled with Silicagel material to absorb moisture from
the air entering the transformer due to oil level change (expansion and
contraction) resulted from temperature variation because the
temperature increases drastically in operation.
Dehydrator
2.2. Switchgear
Switchgear is the equipment which controls the power flow to the load. It incorporates busbars;
circuit breakers; voltage and current transformer; protections…etc.
All above mentioned equipment should be tested before energization to check the healthiness of
each part inside the switchgear.
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Power Equipment; Protection and underground MV cables laying
Types of switchgear:
Switchgear can be classified based on
The insulating medium between phases to:
 Air insulated switchgear (AIS)
 Gas insulated switchgear (GIS)
In GIS SF6 gas is used for insulation. Since SF6 has higher dielectric strength
than air. And thus GIS is used for higher voltages and requires much less space
than AIS but at higher cost.
AIS switchgear
GIS switchgear
Generally, the equipment in switchgear is as follows:
1. Busbars
A busbar is a thick strip of copper or aluminum that
transfers power within a switchboard, distribution
board, substation or other electrical apparatus. Busbars
are used to carry very large currents, or to distribute
current to multiple devices within switchgear or
equipment.
MV busbars tests will include insulation resistance and
high voltage tests.
Busbars
1: DC resistance test by Megger (before and after HV test):
5KV DC is applied between phase to phase and phase to earth (other 2 phases are
earthed).
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Power Equipment; Protection and underground MV cables laying
2: High Voltage test:
By applying the HV between phase to phase and phase to earth (other 2
Phases are earthed). (70kV is applied to 33kV busbar; 50kV for 22kV busbar; 28kV for
11kV busbar and 3kV for LV busbar)
Note: (At site 80% of the factory acceptance test is applied e.g.
56kV for 33kV and 22.5kV for 11kV).
2. Circuit breakers (CB)
Circuit breaker is used to control the power flow from supply to the
load. Circuit breakers are classified based on the insulating medium
of the interrupter. Types of circuit breakers are vacuum; air and SF6.
Tests will include insulation resistance, HV, timing and contact
resistance.
Circuit Breaker
1: insulation resistance test:
This test is carried out to check the insulation level between the phases and between phases
and the earth; also to check the insulation level of the insulating medium.
The test is carried out in open position to check the insulation level of the insulating
medium, then in close position between the phases and between phases and the earth.
2: Timing test:
Opening and closing time of a CB are measured during this test; it’s worthy to note that
opening time is more important because in case of a current fault the CB must trip to
prevent damage of the load.
Usually a healthy CB opening and closing times are between 40 to70 milliseconds.
To measure the CB closing and opening a CB timing tester is used as shown below; the
tester generates open and close commands to the CB and by means of feedback from the
CB poles it measures the time between the command signal and feedback signal.
CB
YC/YO
Tester
YC = closing coil
Timing test
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YO= opening coil
Power Equipment; Protection and underground MV cables laying
3: Contact resistance test:
During the CB is closed the current flows to the load so the contact points of the poles have
a specific resistance (ideal case zero); this resistance causes heat and deterioration of the
CB life time. Contact resistance must be minimum value usually in μ Ω.
The test is implemented as follows:
Close the CB and inject DC current into R phase of the CB by the Feedback voltage the
contact resistance can be measured. R=V/I
I
C.B (closed)
V
I
Tester
Contact resistance test
4: high voltage test:
Same method like DC insulation resistance except that the applied voltage is AC with a
value depends on the CB rated voltage e.g. 70kV is applied to 33kV busbar; 50kV for 22kV
busbar and 28kV for 11kV busbar.
Note: (At site 80% of the factory acceptance test is applied e.g. 56kV for 33kV and 22.5kV
for 11kV).
3. Current Transformers (CT)
Current transformer is an instrument by which the high
currents can be stepped down to be easy to be measured.
Current transformer has a standard secondary current 1
and 5; so any CT transformation ratio is X/1 or X/5.
CT accuracy: it comprises number-letter-number
 First number is the CT accuracy in percent as
long as the CT burden is not exceeded.
Current transformer
 The letter indicates to the CT type (measurement or protection)
 The last number indicates the accuracy limit factor.
Example: 5P10 CT means that it’s a protection CT and it’s accurate within 5% as
long as the CT secondary current does not exceed 10 times the rated current.
Types of CT’s: CT can be classified into measurement and protection depending on the
purpose the CT will be used for.
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Power Equipment; Protection and underground MV cables laying
 Measurement CT is very sensitive (accuracy around 0.5%) in the range below the
nominal current and it used for tariff meters which payment will be based on.
 Protection CT is not with the same sensitivity like measurement CT (accuracy 5% and
10%) and it’s able to measure high currents up to 20 times of the nominal current without
saturation.
Protection CT is dedicated to measure over and short circuit
currents to disconnect the load.
CT tests will include ratio, winding resistance, saturation curve,
insulation resistance and polarity check.
It’s worthy to note that the CT secondary winding must always be
kept shortened; otherwise the CT will get damaged due to the
huge voltage generated on the secondary winding.
1. Turns Ratio test:
Using primary injection test set by injecting AC current through
the primary side and measuring the resulting secondary current the
ratio can be calculated as follows:
Ratio= IP/IS where IP= the injected current and IS = the secondary
current.
Primary current injection tester
CT core no.
PRY. Amps
Sec. amps
2. Winding resistance test:
CT winding resistance is required to be minimum value to minimize the voltage (CT
burden) generated on the secondary winding.
By injecting DC current into the CT and by the feedback voltage the winding resistance
can be measured. R=V/I
CT core no.
Resistance(Ω)
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Power Equipment; Protection and underground MV cables laying
3. Saturation curve test:
Saturation region of a CT is the region where the
relation between the primary and secondary
currents is no longer linear. For measurement of
saturation voltage apply the AC voltage across the
secondary winding and measure the resulting
current. The point where 10% voltage increase
results in 50% increase is called the Knee point.
CT Saturation curve
4. Polarity check:
To check the correctness of the relative polarity between primary side and secondary, use a
12V battery; switch and analogue ammeter. Then apply impulse voltage across the primary
side by turning the switch on / off and note the ammeter deflection direction at secondary
winding
Switch
P1
P2
CT
S1
S2
Micro ampere
Ammeter
CT polarity check
5. Insulation resistance test:
For a CT used in MV switchgear the insulation test shall be carried out as follows:
 Apply 5kV DC between primary terminal and the earth for 1 minute.
 Apply 1kV DC between secondary winding and the earth for 1 minute.
 Apply 2.5kV DC between primary terminal and secondary winding for 1 minute.
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Power Equipment; Protection and underground MV cables laying
Applied voltage (kV)
Insulation resistance at 60 sec.
Insulation resistance at 600 sec.
PRY. to
earth
5
PRY to SEC
SEC. to earth
2.5
1
4. Voltage Transformers (VT)
Voltage transformer is an instrument by which the
high voltage can be stepped down to be easy to be
measured.
Voltage transformers are used for measurement and
protection; usual step down ratio is X/110V where
X is the busbar voltage.
Voltage transformer
VT Tests will include
 Turns ratio
 Winding resistance
 Insulation resistance
 Polarity check.
1- Turns ratio:
Using 400V voltage source apply the voltage to the primary side and measuring the
resulting secondary voltage the ratio can be calculated as follows:
Ratio= VP/VS where VP= primary voltage and VS = secondary voltage
2- Winding resistance:
VT winding resistance is measured to make sure that there is no damage in the VT.
It’s measured simply by injecting DC current into the CT and by the feedback voltage the
winding resistance can be measured. R=V/I
For primary side:
Phase
R
1U-1V
1V-1W
1W-1U
Phase
R
2U-2V
2V-1W
2W-2U
For secondary side:
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Power Equipment; Protection and underground MV cables laying
3- Insulation resistance:
VT insulation test shall be carried out as follows:
 Apply 5kV DC between primary and earth for 1 minute.
 Apply 2.5kV DC between primary and secondary for 1 minute.
 Apply 1kV DC between secondary and the earth for 1 minute.
Applied voltage (kV)
Insulation resistance at 60 sec.
Insulation resistance at 600 sec.
PRY. to
earth
5
PRY to SEC
SEC. to earth
2.5
1
4- Polarity check.
To check the correctness of the relative polarity between primary side and secondary;
use12V battery; switch and analogue ammeter then apply impulse voltage across the
primary side by turning the switch on / off and note the ammeter deflection direction at
secondary winding
5. Metering devices
In the switchgear metering devices are used to measure elec. Quantities like current,
voltage; power …etc
 Ammeters are connected to the secondary side of the CT. to calibrate the ammeter we
apply secondary injection to the ammeter (1Ampere) and check the ammeter reading.
Phase
Injected current
R
Y
B
Ammeter reading
Error
 Voltmeters are connected to the secondary side of the VT. To calibrate the voltmeter we
apply 110V to the voltmeter and check the voltmeter reading.
Phase
Applied Voltage
Voltmeter reading
Error
R
Y
B
6. Auxiliary wiring
Insulation level has to be checked between the wiring and earth by applying 2kV/1 min.
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Power Equipment; Protection and underground MV cables laying
2.3. DC systems
DC system in a substation represents the nerves in human body; the function of the DC systems is
to provide the DC supply for all auxiliary and substation DC dependant equipment, through the
DC distribution boards. This equipment comprises the protection relays, control systems and the
communication systems in addition to the emergency lighting.
 DC system configurations and components
1. Batteries bank connected in series.
2. AC to DC converter (charger)
3. DC distribution board
 AC/DC converter is sized so that it can cater for 100% of the load
 Battery is sized so that it can cater for 100% of the load plus 25% for ageing.
 DC system in a substation can comprise single or dual charger with one or two
battery banks.
 The output voltage of the DC system can be 110V or 48V depending on the load
requirements.
 AC supply of the DC system can be three or single phase.
 In normal operation of the DC system, the load is fed by the AC supply (through
AC/DC converter) and in case of supply absence, the load is fed by the battery
bank.
 DC system is equipped with over/under voltage protection relays as well as earth
fault monitor relay to check that positive & negative poles of the DC system are
strictly insulated from the earth and in case of any insulation failure it detects the
leakage current and hence launches earth fault alarm.
Earth fault monitor relay has three terminals, those terminals are connected to
earth; positive and negative poles.
AC supply
AC/DC
A
Battery bank
DC distribution board
DC system – single charger
Page 21
Power Equipment; Protection and underground MV cables laying
Single Battery Bank
Dual charger with single battery bank

Double Battery Bank
Dual charger with dual battery bank
Battery types
Batteries used in the DC systems are rechargeable and can
be categorized into:
 Nickel Cadmium battery:
Nickel Cadmium battery has a long life of
approximately 20 years with reliable efficiency, the cell
voltage is 1.4V and the total voltage of the battery bank
used can be 110V or 48V (depending on the number of
series cells).
Battery bank


Lead-Acid battery:
Lead-Acid battery has life reaches around 5 years with reliable efficiency, the cell voltage
is 12V and the total voltage of the battery bank used can be 110V or 48V.
Testing of DC system
Capacity test
 Insulation test
 Polarity test
 Voltage level
 Capacity test: the power of a DC system battery bank is measured in Ampere. Hour (Ah).In
this test the DC system is connected to the supply to charge the batteries with the load
disconnected and the current & time are recorded till the battery reaches full charge and the

Page 22
Power Equipment; Protection and underground MV cables laying
same is done during discharging test; then compare to the manufacturer data given for the
DC system.
 Insulation resistance: positive and negative poles must be fully insulated from the earth; to
verify the insulation we use Megger to check the insulation between these poles and
the earth.
 Polarity test: the load must be connected in a correct manner to the DC distribution board to
avoid any problems for the system; this is simply verified by multi-meter.
 Voltage level: DC output voltage of the charger and the voltage level of each battery module
are to be checked by voltmeter.
2.4. Earthing system:
Earthing system provides protection for both human and equipment inside a substation as it
ensures that all exposed conductive surfaces are at the same electrical potential as the surface of
the Earth, to avoid the risk of electrical shock if a person touches a device in which an
insulation fault has occurred. It ensures that in the case of an insulation fault (a "short circuit"),
a very high current flows, which will trigger an overcurrent protection device (fuse, circuit
breaker) that disconnects the power supply.
All metallic parts in a substation must be connected to the earthing system via suitable cables or
busbars.
In a substation, earthing resistance must not exceed 1Ω.

Earthing system tests
Soil resistivity test
Earth resistance test.
Step and touch voltage test

Soil resistivity test
The first step in an earthing system design is the measurement of the soil resistivity
The measuring technique to be followed for soil resistance measurement shall be the one developed by Dr. F.
Wenner, which is sometimes known as the ‘four electrodes method’ because four electrodes (spikes) are
inserted into the ground and connected to the terminals of the earth tester.
The four spikes are equally spaced in a straight line and driven into the ground such that the depth
of insertion is <1/3 of the distance “a” between the spikes.
The spikes are connected to the earth resistance tester by multi-core cables.
Current is passed through the ground via the two outer spikes, which are connected to the current
terminals of the tester. The voltage appearing between the inner spikes connected to the potential
terminals of the tester as a result of the current flowing is measured.
The formula for calculating the specific apparent soil resistance at a certain depth of b is:
4  a RM
 
1
( 2 a)


 2

2
  a   4  b  
a
a2  b2
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Power Equipment; Protection and underground MV cables laying
RM =
the measured soil resistance in 
a
=
the distance between the spikes in m
b
=
the depth of spikes

=
the apparent soil resistivity at a depth of ‘b’ in  x m.
If measured soil resistance values are very high and leading to difficulties in designing the
earth system within the specified limits, ground (soil resistivity) improvement materials shall
be applied.
Where
Voltage rods
Current rods
Soil resistivity test
Page 24
Power Equipment; Protection and underground MV cables laying
Soil resistance meter
Test circuit
Page 25
Power Equipment; Protection and underground MV cables laying

Earthing resistance test:
In this test the earthing system resistance is measured; to simulate the potential distribution that
occurs during short circuit conditions in Substation a heavy current is injected to the earthing
system. The test current shall be selected as high as
possible, to generate voltage high enough to be
measured. This will be minimum 100A.

Step and touch voltage
"Step voltage” is the voltage between the feet of a
person standing near an energized grounded object. It is
equal to the difference in voltage between two points at
different distances from the "electrode". A person could
be at risk of injury during a fault simply by standing
near the grounding point.
"Touch voltage" is the voltage between the energized
object and the feet of a person in contact with the object.
It is equal to the difference in voltage between the object
(which is at a distance of 0 feet) and a point some distance away. For example, a crane that was
grounded to the system neutral and that contacted an energized line would expose any person in
contact with the crane or its un-insulated load line to a touch potential nearly equal to the full
fault voltage.
Touch voltage
Is measured with digital voltmeter and 1k resistance connected in parallel to the voltmeter to
simulate the human body resistance.
Measuring electrodes that simulate the feet, total surface area of 400 cm2 loaded with minimum
20Kg each, will be used for measurement of touch voltage. Both electrodes will be connected
together to the voltmeter and the other side of voltmeter will be connected to point electrode.
Point electrode will be used as the measurement electrode to simulate the hand. Any coats of
paint on the place of measurement (but not insulation) shall be reliably pierced.
When measuring the touch voltage on an installation component the electrode shall be set up 1m
away from the accessible installation component and on a damp cloth or a film of water if the
base is concrete or dry ground.
Metal Surface
V
R = 1kΩ
R=human resistance
F
F
0.5
F=human feet
1M
Touch voltage test
Page 26
Power Equipment; Protection and underground MV cables laying
Step voltage is measured with digital voltmeter and 1k resistance connected in parallel to the
voltmeter to simulate the human body resistance. Measuring electrodes that simulate the feet,
total surface area of 400 cm2 loaded with minimum 20Kg each, will be used for measurement of
step voltage. Voltmeter will be connected between electrodes that simulate the feet.
Step voltage test
2.5. Numerical protections relays
The numeric protective relay, or digital relay, is
a protective relay that uses a microprocessor to analyze
power system voltages and currents for the purpose of
detection of faults in an electric power system of
detection of faults in electric power systems.
The numeric relay receives the signals from the
secondary windings of current and voltage transformers.
Numerical Protection relays
Theses relays are set to a specific setting value and when the value is exceeded the relay operates
and disconnects the protected load or area.
In the world of electrical protections, there are so many protection functions ,mostly common are:
1. Protection functions
 Over/under voltage protection
Over/under voltage protection requires voltage transformers and senses any increase/decrese in
the voltage level in a three-phase circuit. If this change in the voltage exceeds a pre-determined
value a circuit breaker would trip.

Overcurrent (OC) protection
Overcurrent protection requires a current transformer which simply measures the current in a
circuit. If this current exceeds a pre-determined level, the overcurrent protection realy initiates a
signal to the trip coil in a circuit breaker.
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Power Equipment; Protection and underground MV cables laying
OC & EF relay wiring

Earth fault (EF) protection
Earth fault protection requires current transformers and senses an imbalance in a three-phase
circuit. Normally a three-phase circuit is in balance, so if a single (or multiple) phases are
connected to earth an imbalance in current is detected.
If this imbalance exceeds a pre-determined value a circuit breaker
should operate.

Differential protection
A common method for of protection for high voltage apparatus
such as transformers and power lines is current differential
protection. This type of protection works on the basic theory of
Kirchhoff's current law which states that the sum of the currents
entering a node will equal zero.It requires a set of current
transformers at each end of the power line or each side of the
transformer. The current protection relay then compares
Diff. protection principle
the currents and calculates the difference between the two.
Under normal conditions, the currents in the current transformers secondaries are equal,
i.e. IW1 = IW2 and no current flows through the current relay.
If a fault develops inside of the protected zone, currents IW1 and IW2 are no longer equal, therefore a
current will flow through the current relay.
Actually, there are no current transformers symetrical 100% and hence, slight different will
appear so that some problems arise due to :
 Necessary inrush current.

Change of the transformation ratio through transformer tapping

Tolerances of the current transformers of the system
Page 28
Power Equipment; Protection and underground MV cables laying

Measuring accuracy of the protection.

Furthermore, in case of a short circuit the transformation error of the CT’s.
To overcome these problems, the conection inside the relay is modified as shown below:
Now, I1-I2 is called as the operating current and (I1+I2)/2 is called the restraining current.
The protection relay will not operate till Iop is more than IRes by a preset percentage not less than
10% and hence the the name precentage differential protection.
Example: Assume a slight difference in the current in normal operation as follows:
I1=10 & I2=9, without the restraining coils a trip will occur which is not required.
But after the insertion of restraining coils then, Iop=10-9=1 A & IRes=(10+9)/2=9.5A
This means no trip will happen.
K= Iop /IRes
Percentage differential protection

Distance protection
Distance protection is the most important
protection used on transmission lines. The
major advantage of the distance protection
over traditional over current protection is
the ability to determine the fault location.
Distance Protection principle
Principle of operation: The relay in the distance protection measures the voltage VR and the
current IR . in normal conditions the result of (VR/IR) is equal to (Zload + Zline) -note that Zline << Zload)and in case of a line short circuit as we know R= ρL/A , then Resitance seen by the relay is
proportional to the distance and then the result is just a part of Zline and that part equals to the
distance from the relay to the fault location.
Page 29
Power Equipment; Protection and underground MV cables laying

Over/under frequency protection
Over/under frequency protection requires voltage transformers and senses any increase/decrese in
the frequency in a three-phase circuit. If this change in the voltage exceeds a pre-determined
value a circuit breaker would trip.

Restricted earth fault (REF) protection
The REF protection method is applied to
transformers or generators and is more
sensitive than the method known as differential
protection.
An REF relay works by measuring the actual
current flowing to earth from the frame of the
unit. If that current exceeds a certain preset
maximum value of milliamps (mA) then the
relay will trip to cut off the power supply to the
unit.
Differential protection can also be used to protect the windings of a transformer by comparing the
current in the power supply's neutral wire with the current in the phase wire. If the currents are
equal then the differential protection relay will not operate. If there is a current imbalance then
the differential protection relay operates.
REF protection is applied on transformers in order to detect ground faults on a given winding
more sensitively than differential protection.
REF protection is applied for high power transformers (usually more than 15MVA) with star
secondary winding.
To check this function a test known as “stability test is carried out”;
In this test as shown in the test circuit the transformer secondary winding is bypassed and the
current is injected between R phase and neutral (before CT’s) and verify zero current in the REF
measurements, the same shall be repeated for each phase.
Note that the 4 CT’s must be identical.
2.6. Power and Fiber Optic Cables
For transmission of electrical energy from the generation stations till the consumer, electrical
cables are used for this purpose; and these cables are widely varying in different types.
Power cables may be installed as permanent wiring within buildings, buried in the ground, run
overhead.
For protection and communications a type of cables so called fiber optic cables are used.
Page 30
Power Equipment; Protection and underground MV cables laying
2.6.1. Power cables
Power cables are used to transfer power from supply to
the load. Generally cables are classified based on the
insulation material, conductor material, voltage level,
cross section and number of cores.
 As for insulation materials the most commonly
used are XLPE (cross linked polyethylene) and
PVC (polyvinyl chloride) cables but there many
other types like PE and EP rubber.
11kV XLPE cables
The table below shows the most important differences between XLPE and PVC cables
characteristic
Max. continuous operating temperature, ˚C
XLPE
90
PVC
70
Max. conductor temperature at short circuit current, ˚C
250
160
Fire resistance
Poor
excellent
Flexibility
Good
excellent
 Cable voltage rating can be categorized into low, medium and high voltage cables.
 Cable can be single or multi-cores.
 Cable core can be either copper or aluminum.
No.
1
2
3
4
5
6
7
8
9
Description
Conductor
Conductor screen
Insulation
Insulation screen
Metallic layer
Inner sheath
Armour
Outer sheath
Conductive layer
material
Copper, water tight
Semi conducting
XLPE, Tree-retardant
Semi conducting
Copper tape
PE-ST7
Aluminum wire
PE-ST7
Semi conducting
33kV single core power cable cross section
Page 31
Power Equipment; Protection and underground MV cables laying
Voltage denoting of a power cable:
Standard rated voltage of a cable is denoted by U0/U (Um).
Where,
U0: is the rated voltage between cable conductor and metallic screen or earth.
U: is the rated voltage between cable conductors (phase to phase voltage)
Um: is the maximum continuously permissible voltage of the cable
Cable selection:
The following factors are important when selecting a cable which is required to
transfer electrical energy from the source to the consumer:
1.
Load to be fed by the cable.
2.
Maximum operating voltage.
3.
Insulation type.
4.
Cable length.
5.
Voltage drop.
6.
Derating factors due to cable installation conditions.

Cable derating factors:
Cable derating factor is the value which the cable carrying capacity is reduced by due to the
conditions of the cable installation, these conditions can briefed as follows:
 Cable installation method:
Cable carrying capacity is affected by the medium the cable installed at. It drastically
varies from air to ground.

Depth of the trench:
The depth which the cable is laid at is inversely affecting the ampacity of the cable e.g. a
240mm2 cable buried at 2m depth; the cable carrying capacity is reduced down to 85%.

Soil thermal resistivity:
The higher value of Soil thermal resistivity the more derating of the cable; as a standard
value it must not exceed 120 C°.cm/Watt.

Spacing between the cables:
Due to current passing through the cable, the cables have mutual affect on each other;
hence a minimum spacing between the cables must be preserved.

Temperature:
The higher temperature of the medium; the more derating of the cable, normally for the
cables laid in the ground; fine sand is used under and over the cable to transfer the heat
from the cable to the out.
Page 32
Power Equipment; Protection and underground MV cables laying
Cables tests:
The most important tests of power cables are:
1.
2.
3.
4.
5.
6.
7.
Phase sequence
Insulation resistance test before and after VLF test.
High voltage test at very low frequency.
DC Conductor resistance test.
Partial discharge test.
DC Sheath Test
Capacitance Measurements
1. Phase sequence test:
Before proceeding in cable testing, the phase identification must be carried out to avoid
serious problems to personnel and equipment.
Phase identification can be carried out using a Megger as follows:
1. The screens of all cables at one end are to be shorted and grounded.
2. The conductor of the cable under test is to be connected to the negative pole of the Megger.
3. The positive pole of the Megger is connected to ground.
4. Through a switch ground the other end of this conductor of the cable under test and
measure the resistance.
5. If the resistance is negligibly small, the phase identification is correct. Confirm that the
cable is marked with the correct colour coding.
6. Similarly repeat the test for the other phases and verify the correctness of the colour
coding.
R
-pole
Y
Megger
B
+pole
Phase identification
Result:
Phase
Results
R
Y
B
Page 33
Power Equipment; Protection and underground MV cables laying
2. Insulation resistance test:
Using Megger the insulation level can be checked; this test is carried out before and after high
voltage test. For MV cables we apply 5KV DC for 60 seconds duration.
Phase
Cable length
(m)
Applied Voltage
(kV)
Measured value
(MΩ) after 60 sec.
Results
R to Earth
Y to Earth
B to Earth
3. Very low frequency test (VLF):
VLF testing is a modern method of cable testing, it guarantees damage-free of the cable under
test. A high voltage with very low frequency is applied to the cable to check the dielectric
strength; the voltage applied is three times of the cable phase voltage with 0.1HZ for 30 minutes
(e.g. 19kV at 0.1HZ is applied for 11kV cables)
KV
To the core
R
Y
mA
B
To armor/sheath
VLF HV Tester
Phase
Cable
length (m)
VLF Test
Applied voltage
(KV) at 0.1HZ
Measured current
(mA) after 30min.
R to Earth
Criterion
Results
To withstand
To withstand
Y to Earth
To withstand
B to Earth
4. DC Conductor resistance test:
For a cable it’s required to transfer the power from supply to the load without any significant
losses (ideal case zero losses) as the power losses in the cable appears in the image of heat which
deteriorate the insulation and consequently the cable life time; another point the DC resistance
causes voltage drop across the cable terminals, thus the DC resistance of the live part (cable core)
is required to be minimum.
In factory the cable is wound on the drum, hence the two terminals are accessible and by applying
DC current into the cable terminals and by means of the feedback voltage the DC resistance can
be measured and it’s usually so small (usually in mΩ).At site the cable is laid and the two
terminals are no longer accessible so the current is applied to tow phases and the remote terminals
Page 34
Power Equipment; Protection and underground MV cables laying
of the two phases shall be shortened together as shown. The same steps are repeated for other two
phases.
Connecting lead
Connecting Leads
Termination
Y
T
R
DC resistance test circuit
Ohmmeter
1. Resistance of the connecting lead to be measured and recorded (RCL).
2. Resistance of two cables, connected in series by the connecting lead, to be measured and
recorded (m).
3. Resistance of two cables, at ambient temperature to be calculated: R= Rm - Rcl and then
resistance to be corrected to the value of 20 ºC (R20).
4. Resistance per phase at 20 ºC to be calculated.
5. Resistance per meter at 20 ºC for each phase to be calculated.
Readings
Step
Phase
a
b
R&Y
c
Y&B
B&R
Measured Value
(Ω)
Phase
Length
(km)
R
Y
B
And the resistances can be calculated as follows:
R = 0.5 (a + c - b)
Y = 0.5 (a + b - c)
B = 0.5 (b + c - a)
R20 = Rt / L (1+0.00393 (t-20))
Page 35
Rt (Ω)
Corrected
Value
R20 (Ω/km)
Results
Power Equipment; Protection and underground MV cables laying
5. Partial discharge (PD) test:
A partial discharge (PD) is a dielectric breakdown of a small portion in the insulation; it causes
progressive deterioration of insulating materials, ultimately leading to electrical breakdown. The
effects of PD within high voltage cables can be very serious as it can lead to complete failure.
PD test is carried out in factory and it’s a good way to detect early deterioration in the cable due
to some contaminations in the insulation layer.
The test voltage is the same cable nominal voltage (e.g.
11kV for 11kV cables…etc)
The test is carried out by applying the voltage between
the
conductor and metallic screen (copper tape).The pass
criterion is the magnitude of the discharge at 1.73 U0
(1.5U0 HV & EHV cables) shall not exceed 3 PC
(picocoulomb).
Note: one coulomb (C) is equivalent to one amp of current flowing through a conductor for one second. It is also equal to
18
6.24 × 10 electrons.
A partial discharge within solid
Insulation When a spark jumps
the gap
6. DC Sheath Test:
Outer sheath is the last layer of the cable and usually made of PVC or PE, this layer is used for
cable protection against environmental conditions.
DC sheath test is used to make sure that no cracks or damage happened to the cable during cables
installations.
The test is implemented by applying 10kV DC for 1 minute to the metallic sheath and the ground
as shown below, the test is assumed successful as long as no breakdown occurs.
The metallic sheath of the cable under test shall be connected to the high voltage connection
point. The metallic sheaths of the other cables shall be grounded. (See IEC 60502-2_1)
KV
To outer sheath
mA
Power cable
HV DC Tester
DC Sheath Test
Page 36
Power Equipment; Protection and underground MV cables laying
7. Capacitance Measurements
 The capacitance of every complete cable circuit including the termination shall be measured between
phase and metallic sheath for each core. Value of capacitance shall be then calculated per metre of
cable.
 Test Circuit:
2.6.2. Fiber optic cables
An optical fiber is a thin, flexible, transparent fiber that acts as a waveguide, or "light pipe",
to transmit light between the two ends of the fiber.
Optical fiber typically consists of a transparent core surrounded by a cladding material
Optical fibers are widely used in fiber-optic communications, which permits transmission
over longer distances and at higher bandwidths (data rates) than metal wires because signals
travel (light) along them with less loss and are also immune to electromagnetic interference.
Fiber Optic Cable
Optical Fibers
Page 37
Power Equipment; Protection and underground MV cables laying
No.
1
2
3
4
5
6
7
8
9
10
11
Description
Material
Central element
GRP (glass reinforced plastic)
Fiber
SM (single mode)
Tube filling compound
Jelly
Loose buffer tube
Thermal plastic polyester
Core filling jelly
--------------Wrapping tape
Polyester tape
Ripcord
--------------Inner sheath
PE (polyethylene)
Armouring
Longitudinal corrugated steel tape
Outer sheath
HDPE (high density polyethylene)
Conductive layer
Semi-conductive
12 buffer 144 Fiber Optic Cable Cross Section
Types of optical Fibers:
There are two types of optical fibers:
 Single mode optical fiber (SM)
Main features of Single mode optical fibers are:
1. Small cores (8 to 10 microns)
2. Used for long distances transmission
3. Carry only a single ray of light with only a single wavelength (typically
1310 or 1550nm).

Multi-mode optical fiber (MM)
Main features of Single mode optical fibers are:
1. Large cores (50 & 62.5 microns)
2. Used for short distances transmission
3. Carry multiple rays with multiple of wavelengths
The color code for fiber and tubes is shown below.
1. Blue = BU
2. Orange = OR
3. Green = GR
4. Brown = BN
Page 38
Power Equipment; Protection and underground MV cables laying
5. Slate = SL
6. White = WT
7. Red = RD
8. Black = BK
9. Yellow = YW
10. Violet = VL
11. Rose = RS
12. Aqua = AQ
Fiber optic cable tests:
 HV test for outer sheath
 Attenuation versus wave length at wavelengths of 1310 and 1550 nm
 Backscatter measurement
1. HV test for outer sheath :
10kV DC is applied between steel-tape and grounded outer sheath for 60 sec.
Cable pass criterion is to withstand.
2. Attenuation test:
Optical time domain reflectometry
(OTDR) is used to measure the total loss
at 1550& 1310 nm and the attenuation
per kilometer at 1550 & 1310 nm
OTDR
Page 39
Power Equipment; Protection and underground MV cables laying
Test result form:
Core no.
As tested at 1310 nm
A (dB)
L (m)
As tested at 1550 nm
A
(dB/km)
A (dB)
L (m)
A
(dB/km)
1
2
3
4
5
6
7
8
….
….
….
….
Pass criteria:
Attenuation at 1310 nm & 1550 nm:
on drum at factory or at
site, before installation
after installation including
all splices
Attenuation at 1310 nm
(dB/km)
<=0.35
Attenuation at 1550 nm
(dB/km)
<=0.4
<=0.25
<=0.3
3. Backscatter:
Backscatter is the reflection of waves, particles, or signals back to the direction they came
from.
Light is weaker after
scattering
As light passes through a particle part of it is scattered in all directions. The part that returns to
the source is called backscatter.
Backscatter is directly related to the level of light in the test pulse. As the level of light in the
pulse width decreases with distance, so does the backscatter it produces.
Page 40
Power Equipment; Protection and underground MV cables laying
3. Underground MV Cables Works
 Excavation
1m depth
Fine sand with
maximum 120˚C
cm/W thermal
 Bedding sand
Page 41
Power Equipment; Protection and underground MV cables laying
Max. 2 m spacing
between rollers
 Rollers queuing inside trench
Rollers in front of cable
drum
 Drum preparation
Page 42
Power Equipment; Protection and underground MV cables laying
Pulling robe
 Laying the cable and PVC pipes
PVC pipes for
fibre optic cables
* Cables spacing
40cm for 11kV &
22kV and 50cm for
33kV cables
* Min. Bending radius
= 12 x cable diameter
Page 43
Power Equipment; Protection and underground MV cables laying
 Fine sand over the cable
Fine sand till concrete tiles
 Interlocked concrete cover tiles
50x250x500 interlocked
tiles
Page 44
Power Equipment; Protection and underground MV cables laying
Warning tape (1 tape
per cable)
 Warning tape
 Final backfilling
Sieved soil
Route marker
every 30m
Page 45
Power Equipment; Protection and underground MV cables laying
 Cable joint marker
Cable Joint marker
at each cable joint
 Bentonite injection into cables concrete ducts (for cable cooling)
Page 46
Power Equipment; Protection and underground MV cables laying
Page 47