UNIT -4 Electrical Substation Practices(22633)
132KV Substation
Q 1 Write any four needs of 132/33 kV substation. ½ Mark foreach of
anyfour needs
= 2 Marks
Needs of 132/33 kV Substation:
1. For decreasing the voltage level from 132 kV to 33 kV for secondary transmission.
2. To receive power from primary transmission system and supply it for secondary
transmission.
3. Provides facility for switching when power is required to be turned “ON” or “OFF”.
4. To satisfy / supply the new load growth.
5. To accommodate new generation i.e., there is a need of collector substation to tie solar
/ wind generators etc. and connect them to the power grid.
6. To maintain reliability requirements in case of fault etc. on the other substations / lines.
7. To provide overall support to the power flow system i.e., for better management of
overall power flow system.
8. To connect communication signals to the circuits.
9. For regulating voltage to compensate for system voltage changes.
10. To measure electric power quantities flowing in the circuits.
11. Substations provide the necessary real-estate / infrastructure to install transformers and
associated substation equipment required for power transmission and distribution.
Q 2 Suggest the suitable method of neutral grounding in 132/33 kV substation with its
any twospecific reasons. 1 Mark foreach of
anytwo points
= 2 Marks
Ans:
Suitable Method of Neutral Grounding in 132/33 kV Substation with SpecificReasons:
1. Generally, one neutral ground is provided at each voltage level for better safety, operation,
protection and proper discrimination.
2. The neutral grounding is provided at source end and not at load end for more effective
protection.
3. The neutrals of auxiliary supply generators (if used) are grounded through resistance
grounding to limit the stator fault current.
4. The neutrals of synchronous motors and synchronous capacitors (if used) are grounded
through reactance grounding to limit the earth fault current.
5. For all circuits / equipment / machines (e.g. star point of distribution transformer, current
transformer, potential transformer etc.) which has the rated voltage in the range of 3.3kV
to 33 kV resistance or reactance grounding is used because by adjusting the value of
resistance / reactance the arcing grounds can be minimized.
6. Each major “Bus - Section” is provided with neutral grounding for safety and stability.
7. For all circuits / equipment / machines (e.g. star point of distribution transformer, current
transformer, potential transformer etc.) which has the rated voltage of below 600 Volts
and above 33 kV solid or effective grounding is used because effective grounding limits
Prof.Mrs.Godase .G.S (Electrical diploma dept)
UNIT -4 Electrical Substation Practices(22633)
132KV Substation
the voltages of healthy phases to line-to-neutral values in the event of ground faults.
Q 3 Define the terms Touch potential, Step potential, Mesh Potential and Transferred
Potentialin associated with substation.
Ans:
i) Touch Potential: The potential difference between any item of
metalwork and the soil around it (creating a hand-to-feet voltage difference)
during the time that fault current flows. 1 Mark
ii ) Step Potential: Step potential in a particular direction is defined as
the potential difference between two points a meter apart. 1Mark
iii) Mesh Potential: The maximum touch voltage within a mesh of a ground
grid is called Mesh Potential. 1 Mark
Transferred Potential: The potential which is transferred into or
out of the substation from or to a remote point external to the substation
transferred potential. 1Mark
Q 4 Draw and explain single line diagram of 132kV/33kV substation indicating major
equipment.
Ans: 3 Marks for diagram
3 Marks for explanation
Single line diagram of 132kV/33kV substation:
(Sample diagram is shown, any equivalent diagram is acceptable)
1. 132kV Incoming lines:
i) Two Incoming lines of 132kV voltage level.
ii) Capacitive Voltage Transformer is connected to both the lines.
iii) Major equipment like Lightning arrestor, Wave trap, Current Transformer,
Circuit Breaker, Isolator on both side of breaker are connected in each bay.
i) 132kV Busbar: Two bus system is present.
ii) Major equipment like, Power Transformer, Current Transformer, Circuit
Breaker, Isolator on both side of breaker are connected in each bay
2 . 132/33kV Transformer:
• 132/33kV Transformer is used to step down the voltage level.
Isolators with Earthing are provided on both side of transformer.
3. 33kV Outgoing lines ( Feeders):
i) Eight outgoing 33kV feeders are connected to 33kV Busbar which is common
to all feeders.
Supply will provide to all types of consumers through these feeders.
Prof.Mrs.Godase .G.S (Electrical diploma dept)
UNIT -4 Electrical Substation Practices(22633)
132KV Substation
Q 5 Justify the use of Earth Mat in 132 kV substation. 2 Marks for correct answer
ANS-
Use of Earth Mat in 132 kV substation:
(i) Primary requirement of earthing is to have a low earth resistance. Substation involves many
earthings through individual electrodes, which will have fairly high resistance.
(ii) If these individual electrodes are inter linked inside the soil, forming earth mat, it increases
the area in contact with soil and creates number of parallel paths.
(iii) Hence the value of the earth resistance using earth mat will be much lower
than the individual value. So earth mat is used in 132 kV substation.
Q 6) State the meaning and functions of PLCC used in 132kV / 33 kV substation.
1 Mark formeaning
Ans:
PLCC: PLCC means Power Line Carrier Communication. It refers to a technique in which
power transmission line is used as communication line for transferring communication / telephone
signals from one substation to the other.
Function of PLCC:
3 Marks for
any 3 functions (Diagram is optional)
Prof.Mrs.Godase .G.S (Electrical diploma dept)
UNIT -4 Electrical Substation Practices(22633)
132KV Substation
1) Telecommunication & Telemonitoring: Power-line carrier communication (PLCC) is mainly
used for tele-communication, and tele-monitoring between electrical substations through power
lines at high voltages, such as 110 kV, 220 kV, 400 kV.
2) Telephony: PLCC can be used for interconnecting private branch exchanges (PBXs) for voice
messaging.
3) Line Protection: For the purpose of carrier aided protection, PLCC channels use modulation
schemes namely the Amplitude modulation (AM) for blocking schemes and Frequency Shift
keying (FSK) for unblocking, permissive and direct-trip schemes.
Telemetering: To monitor electrical quantities like voltage, current, power (kW, kVA, kVAR) etc.
at remote locations.
Q 7) Write the name of relevant fire-fighting equipment used in case of powertransformer
fire and justify its use.
1 Mark for name
+
3 Marks for justification
= 4 Marks
Ans:
Relevant fire-fighting equipment used in case of power transformer fire:
1. Deluge System: This system uses open spray heads attached to a piping system connected to
a water supply through a valve that is opened by means of a detectionsystem installed in the
same area as the spray heads. When the valve opens, water flows into the piping system and
discharges through all spray heads attached to thesystem. This type of system uses high
volumes of water which may lead to other post fire clean up issues or contaminated runoff.
2. Fixed Water Spray System: This system is similar to a deluge system; however, the water
discharge points are designed to create a spray pattern unique to the particular area or equipment
being protected. The water spray head locations and spray pattern are tailored to the asymmetrical
shape of the equipment being protected. The added benefit of the equipment specific design is
the control of fire spread through increased wetting of the equipment under the influence of the
fire. Electrical transformers containing oil are commonly protected by fixed water spray systems.
3. Water Mist System: This system is similar to the fixed water spray system with an added
benefit of using significantly less water by employing special discharge heads creating water
Prof.Mrs.Godase .G.S (Electrical diploma dept)
UNIT -4 Electrical Substation Practices(22633)
132KV Substation
4. droplets called mist. A water mist system is generally defined by the droplet size created.
Droplet sizes typically are less than 1000 microns and delivered by a high-pressure pump. The
water droplets create a mist that allows a given volume of water to create a larger surface area
exposed to the fire. Smaller droplets facilitate larger heat absorption to cool the fire.
5. Pre-action Water Mist System: This system uses automatic sprinklers attached to a piping
system containing air with a supplemental detection system installed in the same areas as the
sprinklers. System activation may require both a signal from the detection system and the heat
activation of a sprinkler. Pre-action systems are used to protect areas where the risk of false
discharge or leakage must be kept to an absolute minimum. This system has recently seen
increased occurrence of use in transformer fire suppression because of water conservation, greater
immunity to false initiation and greater ability to control fire spread.
Q 8) Describe the causes of hot spot formation in transformer and state the methods of
identification.
3 Marks
Ans:
Causes of hot spot formation in transformer:
1) The winding is heated up by I2R losses, the core is heated up due to the core losses (hysteresis
and eddy losses) inside the CRGO lamination stack. Losses will not be uniform in the core and
the joints between limbs and yokes will have more losses (10-20 %) than the average unit
weight loss (w/kg). Hence the temperature hot spot in the core will be near these core joints. In
three-phase three limbed cores, this hot spot will be in the middle of the top yoke above the
middle limb. In other types of cores, the hot spot will be slightly down in the upper part of the
middle limb portion.
2) Moisture and oxygen cause the oil to decay much faster than the normal rate and form acid and
sludge. Sludge settles on windings and inside the structure, causing transformer temperature to
rise. If temperature increases, then conductor resistance increases and consequently
transformer Output voltage and load voltage decreases. So, under voltage occurs if
transformer temperature rises. Moisture lowers the dielectric strength of oil. Thus, insulating
property decreases. So, breakdown voltage also decreases with increase of moisture content in
oil, Moisture raises the temperature and hence dielectric power factor and increases the risk of
thermal breakdown of solid insulation.
3) When the windings are loaded, leakage flux from the windings under the rated load will
impinge on the outermost packets of the core, heating the surface due to the eddy current losses.
This temperature rise can be a few kelvins to several tens of kelvins over the adjacent oil
depending on the transformer winding, core, and tank shielding design. This creates hot spots.
4) In short, the real core hot spot temperature is a combination of the hot spot from the main flux,
core surface heating from winding leakage flux, and temperature rise of the oil around the area
of the surface hot spot.
Identification of Hotspot in transformer- 3 MARK
There are few methods to identify and measure the hotspot temperature.
1) The conventional method of finding the hotspot temperature is indirect calculation method.
Adding the temperature difference between the hot spot temperature in the transformer tank and
the temperature of the top oil, the temperature rise of the top oil in the tank and the ambient
temperature, the sum is equal to the hot spot temperature.
2) Other method is using fibre optical temperature sensors positioned at the predicted hotspot of
the windings. The thermal sensors, attached to the end of optical fibre are usually placed
Prof.Mrs.Godase .G.S (Electrical diploma dept)
UNIT -4 Electrical Substation Practices(22633)
132KV Substation
3) between the insulated conductor and spacer, and their signals via optical fibre transmitted out of
the tank.
4) When hotspots due to faults are created in the transformer winding, they lead to formation of
hydrocarbon gases (H2, CH4, C2H4, C2H6, and C2H2), whereas faults involving the solid
insulation of the transformer result in the formation of oxide gases (CO and CO2). Oil samples
are taken from transformers and are analyzed for the existence of these gases. Then the
approximate location of hot spots is
found by using infrared images of the external surfaces of the power transformer
Q9) Describe “Safety practices” followed during breakdown maintenance of 132kV
substation. 6 Marks
Ans:
Safety practices to be followed during breakdown maintenance of 132kV
1. substation: Properly isolate the equipment under breakdown.
2. Do the local earthing.
3. No unsafe operation should be permitted.
4. Interlocks should not be bye-passed unless it is very essential.
5. Operation & maintenance staff should be authorized to work on the breakdown equipment.
6. Operation & maintenance staff should be familiar with the station layout and operating limits
of different equipment such as breakers, transformers, Isolators, CTs, PTs, etc. A person
should be allowed to operate or take over the equipment only after he has acquired adequate
knowledge of the equipment.
7. Operations and maintenance manuals/manufacturers instructions for different equipment
should be available and should be referred to before carrying breakdown maintenance.
8. It is the responsibility of the supervisor to interpret correctly and explain safety rules and
regulations to all the persons concerned and ensures that they thoroughly understand the same.
9. Supervisor shall guard against the use of defective safety appliances, tools and materials.
10. In case of any emergency, in which quick action is necessary in order to safeguard life or
property, persons are authorized to take necessary action, which is thoroughly understood by
them. Under no circumstances attempt shall be made to carry out operations which are not
safe.
11. All persons must use the standard protective equipment intended for the job.
12. All protective equipment should be periodically tested.
13. Metal ladders should not be used in switchyard.
14. Adequate number of first aid and fire-fighting equipment shall be maintained.
15. Every person should be familiar with fire-fighting and should know how to operate fire-
fighting equipment, so that fire can be extinguished promptly minimizing damage.
16. In the event of fire on electrical installation, the affected part shall be immediately switched-
off and isolated from all the sources.
17. First aid and artificial respiration chart should be exhibited.
18. While working on the breaker, its operating mechanism should be de-energizedsuch as
discharging spring, release air pressure etc.
19. Transformer should be discharged and grounded from all sides (windings).Neutral
grounding of the transformer should not be treated as grounding.
20. Current transformer secondaries should never be left open circuited.
21. After disconnection of capacitor bank, it should be allowed to discharge through discharge PT
Prof.Mrs.Godase .G.S (Electrical diploma dept)
UNIT -4 Electrical Substation Practices(22633)
132KV Substation
for about 5 minutes. The bank should be grounded with hot stick before commencing the work.
Apparatus, frame work and other non-current carrying metal parts associated with power system
are to be effectively grounded.
Prof.Mrs.Godase .G.S (Electrical diploma dept)
