MOTORS AND MCC
• What is Motor?
It is a device, which converts electrical energy into mechanical (rotating) energy.
Definition of terms used in Motor:
1) Duty Cycle rating: Most motor has a continuous duty rating to permit continuous
operation at a rated load. However motors may be rated as intermittent duty,
periodic duty or varying duty and must be turned off and allowed to cool after a
fixed operating time.
2) Full- Load current: The current required to produce full-load torque.
3) Jogging: The starting and stopping of a motor at frequent intervals.
• What is Motor controller?
A device that controls some or all of the following functions: starting, stopping,
overload protection, over current protection, reverses, changing of speed sequence
control and running/jogging.
• What is Motor speed?
The shaft speed of the three-phase squirrel cage motor is determined by the
frequency of the supply voltage and the number of poles in the motor. A two-pole
motor runs at a speed of 3000 rpm on 50 cycles per second.
rpm = cycles per second x 60 - slip
Poles
(Where slip is the difference between the speed of the rotating magnetic field and the
speed of the rotor.)
• Why Over current protection used?
A fusible disconnect or circuit breaker used to protect the branch circuit conductors,
control devices and the motor from grounds and short circuits. the over current
protection device must be capable of carrying the starting current to exceed 400% of
the motor full load current.
• What is Overload?
Any excessive amount of current drawn by the motor is called overload. Overloads
on a motor may be mechanical or electrical.
• What is Plugging?
The instant reversal of motor is called plugging. Damage to the driven machinery
can be result if plugging is applied improperly.
• What is Sequence control?
The control of separate motors to operate in a predetermined pattern.
• What is Service factor?
The amount of overload that may be permitted without causing significant
deterioration of the insulation on a motor. For example, if a 10 hp motor has a
service factor of 1.15, the motor can be safely be subjected to an 11.5 hp load.
• What is Starting current or Locked rotor current?
The current flow in the motor at the instant of starting. This current can be 4-10
times the full load current of the motor. The most common locked rotor current is
about 6 times the full load current. Such a motor will start with a 600% overload.
• What is Torque?
The twisting force produced by the motor is called torque. Its unit is in foot-pounds
(ft-lb.), torque is related to horsepower by the following formula.
Torque = horsepower * 5252
Revolution per minute (rpm)
• Write details MCC construction.
a) MCC are made up of sheet steel enclosure, indoor floor mounting and free
standing, Dust and vermin proof, modular type and of double front and single
front (X1, Y1).
b) Degree of protection is IP 50 as per IS 2147.
c) 0.9 * 0.8 * 2.4 meters size (double front and single front) and top entry of cables.
d) 0.9 * 0.6 * 2.4 meters size (single front) and bottom entry of cables.
e) Parts are incoming panel, Cable entry, TB compartment, MCC module
compartment.
f) MCC modules are fully drawn-out type.
g) Main buses are horizontally mounted and vertical buses are connected to MCC
cells.
h) Stab – in contacts are used for power and wipe – in contacts are used for control
circuits.
i) CT and PT are used for current and voltage measurements.
j) 3φ indication lamps are provided for identification.
k) Voltage meter and ammeter are provided.
l) Panel space heater and emergency push button key operated are provided.
m) Control building and SRPH MCC are safety related and SB, TB, RAB, CWPH,
DM Plant MCC’s are non safety related.
• What is the maximum load on MCC?
3 Phase load upto 90 kW are fed by MCC.
• What are the types of MCC?
Type Application Protection
A Receptacles, Cranes, Elevators, Local control panels Fuse
B Locally controlled heaters Fuse, 49
C Remote controlled heaters Fuse, 49
D Remote controlled loads <29 kW (49 in control panel) Fuse
F For valve motors Fuse, 49
G Locally controlled loads <29 kW Fuse, 49
H Locally controlled loads >29 kW (RTM installed) Fuse, 49
I DG MCC
J Remote controlled loads <29 kW Fuse, 49
K Remote controlled loads >29 kW (RTM installed) Fuse, 49
L Barring gear motor MCC Fuse, 49
SP For F/M supply and PHT S/D cooling pumps CT, PT Used
• What is the operating life of bearings?
a) Continuous 24 hrs operation – 40000 to 50000 hrs.
b) Affected by load axially or radial.
c) Operating temperature.
• Give the 415 V MCC bus ratings and cable used.
All MCC bus bars are made of aluminium. Short time current is 50 kA/sec and
momentary rating is 105 kA.
Type Bus Rating Class Location Cable Used
K1 1000 A IV TB 111 Mts.
K2 1500 A IV TB 111 Mts.
L1 1000 A IV RAB 108 Mts.
L2 1000 A IV SB 106.5 Mts.
L3 1500 A IV TB 111 Mts.
M1 1000 A IV SB 106.5 Mts.
M2 1500 A IV TB 111 Mts.
N1 1000 A IV TB 111 Mts.
N2 1000 A IV RAB 108 Mts.
W1 1500 A IV CCW PH 98 Mts.
W2 1000 A IV DM plant 98 Mts.
P1 1500 A III CB 100 Mts.
P2 1500 A III CB 106.5 Mts.
Q1 1500 A III CB 100 Mts.
Q2 1500 A III CB 106.5 Mts.
X1 600 A III SRPH 100 Mts.
Y1 600 A III SRPH 100 Mts.
PMCC S 630 A II CB 106.5 Mts.
PMCC T 630 A II CB 106.5 Mts.
• What are the types of isolators used?
Isolator Range Loads
32 A ≤ 9.3 kW
63 A > 9.3 kW and ≤19 kW
125 A > 20 kW and ≤ 47 kW
250 A > 48 kW and ≤ 110 kW
400 A > 111 kW and ≤ 134 kW
600 A ≤ 310 kW
• What are the ranges of fuse used?
Fuse Range Loads
2 A 10 to 280 watt
4 A 340 to 440 watt
6 A 500 to 700 watt
10 A 1000 to 1500 watt
16 A 1.8 kW to 2.25 kW
20 A 3 to 4 kW
25 A 5 to 8 kW
32 A 9 to 9.3 kW
50 A 9.6 to 15 kW
63 A 16 to 19 kW
80 A 22 to 24 kW
100 A 25 to 36 kW
125 A 38.6 to 45 kW
160 A 48 to 67.5 kW
200 A 72 to 80 kW
250 A 85 to 90 kW
• What is use of fuse in electric circuit, what are the materials used for fuse and what
are their melting points?
Fuse is a weakest point in an electrical circuit, which breaks the circuit when
abnormal current more than it’s rating flows through it. It works on principle of joule
law (I2Rt). HRC fuse is filled with quartz powder to extinguish the arc generated in
breaking the circuit or when fuse blown.
Current rating is depends on the type of material, cross section area, length and size
of terminal (large size terminal dissipates more heat).
Formulae
H = I2Rt/J
R = ρl/a
a = d2π/4
Material Melting point in °C
Silver 1830
Copper 2000
Aluminium 240
Zinc 787
Tin 436
Lead 624
• What are the materials made of thermal overload relay? How OLR are selected?
The bimetallic materials are Invar and brass. These materials having the differential
coefficient of expansion. All bimetallic relays incorporate additional built in single
phasing protection.
The range of the relay provided for the feeders are such that the full load rating of the
feeders is comfortably within the range of the relay (range will be at Centre) except
in very minimum loads ranging from 0.1 – 0.16 A.
• What are the functions of arc chute?
To increase the speed of rise of arc by magnetic action.
It splits the arc by this arc resistance increases.
Diagnosing the arc by cooling.
• What are IP (ingressive protection) and IC?
IP means ingressive protection to the motor against the dust and water entry.
The first digit indicates protection against accidental contact with live or moving
parts (solid particles).
The second digit indicates protection against ingress of water, foreign bodies (liquid
particles).
IC means instrument cooling to the motor (type of cooling)
• What are the classes of AC motors?
Depends on phases
a. 1φ.
b. 3φ.
Depends on construction
a. Squirrel cage induction motor for fixed torque.
b. Wound rotor motor for variable torque.
Depends on voltage
a. LT motor - <200 kW.
b. HT motor - >200 kW.
Depends on torque and current
a. Class – A (Normal torque and normal starting current. E.g. Fractional motors.) .
b. Class – B (Normal torque and low starting current).
c. Class – C (High starting torque and low starting current. E.g. Double sq. cage motor)
d. Class – D (high starting torque and high starting current).
Depends on mechanical characteristics
a. Drip proof (IP 54). Safety against water or dust.
b. Splash proof.
c. TEFC (totally enclosed fan cooled).
d. TEOV (totally enclosed open ventilated).
e. TETV (totally enclosed tube ventilated. Principle is thermosymphony E.g. - CEP).
f. Explosion proof.
• What is the PI value required for motors?
For class F insulation >2 and for class B insulation 1.5 to 2.
• What are the enclosures used for HT and LT motors?
LT motors (<200 kW)
a. Open drip proof.
b. TEFC.
c. Totally enclosed air over type.
HT motors (>200 kW)
a. Open drip proof.
b. Weather protected I
c. Weather protected II
d. Totally enclosed water-cooled.
e. Totally enclosed pipe ventilated.
• What are the causes of motor failure?
a. Corrosion or rust.
b. Excessive moisture (winding IR low and bearing lubrication loss).
c. High ambient temperature.
d. Poor ventilation.
e. Inadequate lubrication.
f. Misalignment.
g. Oil and dirt.
h. Excessive starts and repetitive surges.
i. Persistent over loads.
j. Shaft currents (bearing pitting).
k. Mis application.
l. Manufacture defect or wrong design.
m. Deterioration with age.
n. Maintenance improper.
• What are the effects of excessive starts and repetitive surges?
Repetitive surges may give impact to the insulation of the motor and dielectric
capability of the motor.
Excessive starts may subject stator winding to high current for more time.
Subsequently in HT motor due to High Mass rotor bar and rotor short ring may loose
or fail. Bearing also may damage.
• What are the effects of broken rotor bars and broken shaft parts?
Broken rotor bars
a. High stator current and over heat of stator winding.
b. More harmonic currents in end parts.
c. High vibration.
Broken shaft or parts
a. Stator winding loose bracing.
b. Rotor high vibration and bearing vibration.
c. Frame vibration and more harmonics in side bands.
• Give the relation between current and temperature in motors.
a. Winding temperature is proportional to square of the current.
b. 10% increase in current gives 30% increase in temperature.
c. 10°C rise in temperature makes 50% life reduced.
• What are the effects of imbalance stator winding resistance?
If the stator winding resistances are imbalance phase to phase give pulsating fluxes
and pulsating torque on rotor and vibration may increase. For accurate resistance
measurement Kelvin Bridge is used.
• What reflects the change in speed?
a. Supply frequency may vary the speed of the motor.
b. Load on the motor may vary the speed of the motor.
• What is use of BORESCOPE inspection?
BORESCOPE inspection method makes it easy to observe the end winding condition
of the motor. In this inspection winding ties, loose coils, dust etc can be observed.
• What you mean by CRAWLING and COGING?
Crawling
The motor fails to rotate at rated speed or motor rotates at … or 20% speed is called
motor crawling. This may be due to system imbalance or more pulsating torque.
Coging
Motor fail to start atoll is called motor coging.
• Why motor starting current is high compared to transformer charging current?
Transformer charging current is only 1% and that of motor starting current is 30 to
40%. Because of air gap between stator and rotor. If the air gap is more load taking
capacity increases and if air gap is less the load taking capacity reduces.
• State construction details of the motor.
Stator or rotor core
Built from high quality low loss silicon steel laminations and flash enameled on both
the sides made up of close-grained alloy cast iron.
Rotor conductor
Heavy bars of copper or aluminium alloy.
Stator
Copper conductor.
• What is the use of making rotor skewing?
1. To run motor quickly by reducing magnetic hum.
2. It reduces locking tendency with the stator.
• Why under voltage tripping of motor is incorporated in motor feeder breakers?
The under voltage can occur in case of bus fault. If the motors are kept connected
they will feed the fault which may cause the damage. Due to the back feeding from
the motor the motor will slow down very fast. Hence process system will come to
halt very fast. (In case pht motor will not rotate for the designed 3 minute period in
case of bus under voltage).
• What are the problems in station operation due to grid under voltage?
a) All the HT motors overloaded.
b) VAR load increases on generator leads to heating up of rotor
c) Stator current increases for same power export leads to stator over heating.
• What are the problems in station operation due to grid under frequency?
a) Turbine having under frequency limitation, house load happen if < 48 Hz
b) Due to under frequency PHT flow reduces, therefore reactor power reduces,
generator power reduces
c) If frequency is less than 48 Hz DG cannot be synchronised to grid, therefore DG
kept on isolation running
d) GT overfluxing.
• What is the difference between fixed trip and trip free?
Fixed trip: Breaker will trip only after closing even if trip impulses are existing.
Trip free: Breaker is free to trip at any position.
• What is the making current capacity of a 3-phase breaker as derived from its
symmetrical breaker capacity?
Making capacity = 2.55 times symmetrical breaking capacity.
• Why intermediate contacts in English electric breaker?
To prevent even slightest arcing on main contacts.
• Where preloaded ball bearings used?
If more vibration exists even when machine is not in running conditions.
• Why do we grease the bearings?
a) Grease lubricant gives good protection against ingress of moisture and dirt into
motor.
b) Easy to seal against leakage of grease into motor compared to oil.
c) Low friction torque at starting.
• Which bearings preferred for all large power motor?
Plain bearings
• Which is more dangerous alkali or acid?
It especially exposed alkali is more dangerous, use boric acid powder solution
immediately.
• What FCN was implemented to avoid reactor trip on 220V-DC failure of PHT and
PPP breakers?
The breaker close position supervision relay VAA 21 is changed by VAJC type,
contacts position do not change if 220V-DC is lost now.
• Where oil lubrication is preferred over grease lubrication?
a) Determined by speed and operating temperature.
b) Oil lubrication recommended.
c) When speed and temperature is high.
d) When heat to be conducted away from bearing.
e) When adjacent machine components are oil lubricated.
f) High viscous oil for low speed machine, low viscous oil for high speed machine.
g) At temp<125 ºC, synthetic oils recommended.
• What are the causes of failure of bearings?
a) Faulty mounting
b) Faulty lubrication
c) Foreign matter in lubrication
d) Water in the bearing arrangement
e) Vibration
f) Inoccurrences of form of shaft or housing seating.
g) Passage of electric current.
h) Metal fatigue.
• What does the bearing number mean?
7318 7 = single row angular contact ball bearings
3 = width of race
18 = 18 x 5 = 90 mm bore diameter.
6310 6 = single row deep groove ball bearings.
3 = width of race
10 =10 * 5=50 mm dia
• What is the purpose of static starter? How current setting adopted?
The static starter limits the starting current of the motor to 2.5 times the motor rated
current instead of 6 times the rated current. If the motor is directly on UPS, the UPS
fuse will blow, since the UPS cannot supply so much starting current. Hence the
static starter is set to limit the starting current. This is achieved by firing angle
control of back to back thyristor.
• What is the speciality of the inverter output transformer? Why it is provided?
a) This eliminates all 3rd harmonics in the output voltage.
b) Solid earthed neutral is required for the inverter output, hence the interconnected
star winding is essential.
c) The primary has to be star (not delta), since 3 separate inverters operates on
isolated primary winding.
d) Delta connection will cause circulating current between inverters during
unbalanced faults. The inverters cannot withstand this.
• Both silver and copper oxidise in air. Then why copper contacts are silver-plated?
The silver plating avoids the oxidation of copper, especially in outdoors. Silver
oxidises very readily. But its oxide is a good electrical conductor but copper oxide
produce a film of insulation.
• When auto transfer is effected?
a) When any one module trips
b) When overload exceeds 175% for more than 40 msec. is existing.
c) When UPS output voltage varies beyond 415v +/- 10%
• When static bypass is fired? Why static bypass is required?
For the same above 3 conditions, static bypass is also simultaneously fired along
with a closing impulse.
• When the static bypass is blocked?
When the phase error is more than 20º.
• What is phase lock mode?
The inverter continuously follows the frequency and phase angle of classIII bus
supply.
• What is the difference between a contactor and a breaker?
Contactor is not designed to open on short circuit condition (fuse will take care of
this situation). Breaker is having complicated mechanism for closing and tripping.
• What is the difference between isolator and contactor?
Contactor is used for on load operation. Because they are fast acting devices. They
posses arc chamber and arc chutes. Arc chamber and arc chute make it easy to
extinguish the arc produced during on load operation.
Isolator is off load devices. Because they are slow acting devices. The arc time is
more in slow acting devices and operated only in off load.
• What are the protections provided for motor feeder?
Ith - Thermal over load
I2S - Unbalance load
I0S - Earth fault protection
I1t - Stalling protection
I1Inst. - Short circuit protection
• What are the protections provided in PMCC circuit breakers?
1. IDMT O/C (CDG 34).
2. IDMT E/F (CDG 11).
3. Under voltage (40% of 110V).
• What is requirement of having DG’s?
To establish class III supply when class IV fails.
Parallel operation with class IV 6.6 kV supply.
DG to DG parallel operation.
• What are the characteristics of CB, OLR and HRC Fuse?
CB characteristics (it is back up fuse)
Current
Fuse characteristics
Margin to avoid fuse operation OLR characteristics
During starting
Minimum fusing current
Staring current
Running current
Time
CB Protection
Fuse Protection
OLR Protection
I
Time
When CB is used the CB characteristics should be below the fuse, because the CB
should operate first and then fuse. Not vice-versa. Because CB is the main protection
or main breaking device.
• Why control transformer is earthed?
If it is not earthed grounding of control circuit at two different places can cause
bypassing of logics. In case of primary and secondary of the control transformer is
getting the main fuse will blow off. (If secondary is not grounded then 415v will be
superimposed in the control circuit during short circuit of primary and secondary
winding)
• When the fuse will take over?
When the current increases beyond 700% then the fuse will take over from the
thermal overload protection.
• What is interlocks provided for the valve MCC
a) Mechanical interlock, which will not allow the other contactor to close if one
contactor, is closed.
b) 42 auxiliary contacts are wired in the control ckt. 42-1 contact in 42-2 and 42-2
contact in 42-1.
• How to calculate the full load current of the motors?
If kW is given, full load current = 1.5 times kW rating.
If hp is given, full load current =2 times hp rating.
• What is the safety interlock provided in MCC cell?
The MCC cell door can't open if the cell is in on condition.
• How the fuse is selected?
Fuse rating should be 2.5 times the full load current.
• How will you improve the IR value of a motor?
By providing external heating. (By filament lamps)
By providing internal heating by applying the low voltage.
By circulating hot and dry air.
• Why 110V has been chosen in MCC cell?
To isolate control circuit from power circuit for Human safety at control circuit side.
• What is the purpose of DIODE across the interposing coils in PLC?
To dissipate the stored energy in magnetic field of the interposing coils (Free
wheeling action). If it is not provided the stored energy will affect the PLC card
circuit.
• What are the in-built protections provided in MCC cell?
Fuses for short circuit protection.
OLR for over load and single phasing protection.
Electrical and Mechanical interlock in valve cell against short circuit.
• What is the plugging of an induction motor?
It is an electrical braking of an induction motor by sudden reversal of phase
sequence.
• Why CT operated over load relay is using for loads of high acceleration time upto 30
seconds? How it getting back?
The saturable current transformers linearly transforms the current upto twice the set
current, but above this value the transformer core gets saturated and the secondary
current is proportionally less. Thus these relays permit heavy starting conditions of
motors and offer dependable protection against overload.
When current reduces the core gets de-saturated, as material design is such.
• How many earthing should be done for motors? Why?
Two. For reliability.
• What is the significance of frame size of motor?
In order to make practical choice, interchangeability and large scale production
possible.
• What is polarisation index?
a) It is defined as a ratio of 10 minutes resistance value to 1-minute resistance value.
b) It gives a quantitative information about the insulation with respect to moisture,
dirt and other contamination.
c) A PI value of less than 1.0 indicates a need for immediate reconditioning.
• Why megger value of 1 minute is less than 10-minute value?
After 10 minutes the high voltage applied make the molecules such a way that
stabilised in a good insulation. If insulation is weak it leads to more leakage current
due to high potential.
• What is the classification of duty of rotating electrical machines?
S1 – Continuous operation at rated load (MCR) in 40 °C
S2 – Short time operation (STR) for 5 minute or 15 minutes or 30 minutes.
S3 – Intermittent periodic operation (resting and loading e.g. cranes, lifts etc)
S4 – As for S3 but with starting
S5 – As for S3 with electric braking
S6 – Continuous cyclic operation.
• What should be the value of insulation resistance of induction motor?
In Rm = kV + 1 M OHMS.
Insulation resistance of any electrical machine (motor or generator) should be above
0.5-M ohms in all cases.
• What are the classes of insulation?
Y – 90οC (max) cotton, silk, paper, wood without oil impregnation
A – 105οC Materials of class Y impregnated with natural resins,insulating oils.
E – 120οC Synthetic resin enamels, cotton and proper laminations.
B – 130οC Mica, glass fibre, asbestos with suitable bonding substance.
F – 155οC Class B with more thermally resistant bonding materials.
H – 180οC Glass fibre and asbestos, mica with silicon resins.
C – >180οC Mica, ceramics, glass, quartz and asbestos without binders.
• What are the checks on the motor during the preventive maintenance?
IR Value
Resistance and Inductance measurement
PI value (should > 1.0)
• What are the tests pressures used in lyra contact testing?
125 A - 3 kg
250 A - 5 kg.
• What are the causes of motor vibration?
a) Broken rotor.
b) Slacked stator core.
c) Slacked rotor core.
d) Rotor winding unbalance.
• What are the causes for motor high current?
a) High frequency (51 Hz - 105% current)
b) Low frequency (48 Hz – 102% current)
c) High voltage
d) Low voltage
e) Mechanical over load
• What are the causes for motor unbalance current?
a) Loose connection
b) Voltage unbalance
c) Turns short circuit
• What are the sources of 240 V AC class I supply? What are the functions of each
part of UPS?
Six sources.
Three 20 kVA UPS for safety related loads.
Two 60 kVA UPS for non-safety related loads.
One 60 kVA UPS as a standby to safety related loads.
These all UPS are back up by 220V DC batteries.
Rectifier
This converts AC to DC supply for inverter.
Functions
1. Produces DC voltage.
2. Supplies trickle charge to batteries.
3. Full load boost charge capacity.
Inverter
This converts DC to AC supply for loads.
20 kVA inverter is transistor based and 60 kVA inverter is thyristor based.
Static switch.
To take stand by UPS into service.
Manual bypass
To take main UPS to maintenance by putting stand by UPS into service.
• What is station Black out condition?
Simultaneous failures of class IV and class III supply is called Station Black out. In
this condition class II power UPS will feed the necessary loads for a 30 minutes of
duration. After that supplementary control room (SCR) 5 kVA UPS is used for
secondary shut down system (SSS) ion chamber amplifier.
220 kV SYSTEM
• What is meant by Dielectric strength?
The maximum electrical potential gradient that a material can withstand without
rupture usually specified in volts/millimeter of thickness. This also has known as
electric strength.
• Give switchyard specification.
1. Type : Out door.
2. Scheme : Double main bus bar with bypass switching scheme is provided.
This allows maintenance of one bus or one CB without interruption.
3. Normal voltage : 220 kV.
4. Rated voltage : 245 kV (400 kV)
5. Impulse voltage : 1050 kV (peak)
6. One-minute level : 460 kV (rms.)
7. Dynamic current capacity: 102 kA (peak) and 40 kA for one sec.
8. Rated current capacity : 2000 A for main and 1600 A for feeder bus.
9. Clearances : Phase to earth – 2100 mm.
Phase to phase – 2100 mm.
Phase to ground – 5500 mm.
Sectional clearance – 4300 mm.
Creepage clearance – (Total) 5600 mm.
– (Protected) 2800 mm.
10. Maximum temperature rise above ambient - 45°C.
11. CB – SF6
12. Isolator – motor operated rotating type.
13. Number of bays – 16 Nos.
• Give the details of switchyard 220 kV CB, Isolator, CT, CVT and lightning arrestor.
220 kV SF6 Circuit Breaker
1. Make – ABB
2. Air pressure blocking a. Close Block – 17.3 bar.
b. Open block – 16.7 bar.
c. Auto reclose block – 19 bar.
3. SF6 pressure block a. Alarm – 5.2 bar.
b. Rated – 6 bar.
c. Limit – 5 to 6 bar.
d. Open block – 5 bar
4. Weight of gas / pole : 20 kgs.
5. Closing time : 130-milli sec.
6. Method of closing : Electro-pneumatic.
7. Compressor pressure : 20.5 kg/cm2.
Isolator
1. Type : High-pressure pressure relieving isolator (HPPR) central pole double
break.
2. CB and Isolator clearances : Phase to Phase – 4500 mm.
Phase to earth – 2300 mm.
Current transformer
1. Make : TELK made hermetically sealed.
2. Type : Single pole dead tank.
Capacitor voltage transformer (CVT)
This is capacitive potential divider and inductive medium mineral oil sealed.
ABB. make 245 kV/110 √3 V.
3 cores for metering and protection.
Lightning arrestor
Type : WS surge arrestor of ZODIVER type and SMX style.
Gapier zinc oxide arrestor. Multi unit construction for transport, storage and erection.
Rated voltage : 216 kV rms.
Operating voltage : 184 kV rms.
• For a fault in switchyard lightning arrestor, what protection will act?
Bus bar differential protection.
• What is the purpose of the CVT?
To provide synchronising signal
To provide voltage indication
To facilitate the carrier communication
• What is the purpose of wave trap?
Carrier communication signals are sent through the lines. These are high frequency
signals. This signal should be prevented from entering the switchyard. The wave trap
is LC ckt, which is tuned for 50 Hz. Since it is connected in series with the line it
will effectively block the carrier signal entering into the switchyard.
• What is the purpose of lightning arrestor?
Due to lightning and switching surges high voltages are induced in the lines. If
equipment’s. Connected is subjected to this high voltage the insulation will fail. In
order to avoid the failure of insulation the LA is used. When the la is subjected to
high voltage it will conduct and discharge the current to the earth.
(It will divert the over voltages to earth and protect the substation)
• What is meant by restriking voltage?
The high voltage that will appear across the contact just after the quenching of the
arc is called restriking voltage.
• What does switching surges mean?
When a line is switched on high voltage will appear on the line due to its inductance
and capacitance. This voltage is known as switching surges.
• What is the purpose of compressed air in SF6 breaker?
This used for drive for opening and closing of the contacts.
(Arc quenching is taken care by SF6 gas)
• Why switchyard is located indoors of coastal plants?
Saline atmosphere will deposit on the insulators causing its flashover. The building
kept under positive pressure compared with outside thus preventing the (saline) air
entering from outside to inside the building.
• Why disc insulators grooved at bottom?
To increase the creepage distance, reduce the chances of flash over.
• How cap and pin attached to insulator?
By cementing.
• What is the material of cap, pin, and insulator?
Cap = galvanised cast iron
Pin = forged steel pin
Insulator = porcelain.
• Why insulators are glazed?
If not glazed, it will absorbs water, resistance comes down, leakage current through
porcelain, temperature increases till porcelain is puncture
• What is the station ground resistance?
Less than 0.5 ohms.
• What is the various design of CT's in switchyard?
Bus coupler CT's- live tank design 2000-1000A/1A
All other CT's- dead tank design 800-600-400A/1A - lines and GT.
125A/1A – SUT
• Advantage of CVT over EMPT.
Used as coupling capacitors for PLCC.
• What are the main parts of 220 kV Circuit Breaker?
Pole column filled with SF6
Pneumatic drive system with compressed air circuit
Control cubicle unit
• What is the type of 220 kV circuit breaker?
220 kV, SF6 breaker, single pole, puffer type.
• What are the levels of SF6 gas in 220 kV breaker and their significance?
7 kg/cm2 - normal pressure
5.2 bar - alarm
5.0 bar - closing/tripping operation blocked.
• What is the difference between circuit breaker and isolator?
Isolator is a disconnecting switch which is not having the making and breaking
capacity.
Bus coupler - 2000A
Feeders - 1600 A
• What is the purpose of ground switch?
To discharge the trapped electrical charges to ground to give complete isolation.
(To discharge the residual potential)
• What is the type of Lightning Arrestor?
Station type, heavy duty, gap less zinc oxide.
• What is the purpose of grading ring?
This assembly is provided to have uniform voltage gradient.
• What are the properties of SF6 gas?
Physical properties
1. Colourless
2. Odorless
3. Non-toxic. Pure SF6 gas is not harmful to the health.
4. Non-inflammable.
5. Density- more gas density, 5 times that of air at 20°C and at atmospheric
pressure. The gas starts liquefying at certain low temperature. The temperature of
liquefaction depends on pressure. At 15 kg f / cm2 the gas starts liquefying at
10°C. Hence this gas is not suitable for high pressures >15 kg f / cm2
6. The heat transferability of SF6 gas is 2 to 2.5 times that of air at same pressure.
Hence for equal conductor size the current carrying capacity is relatively more.
Chemical properties
1. Stable upto 500°C.
2. Inert gas due to the chemical inertness. The life of the metallic parts, contacts is
longer in SF6 gas. The components do not get oxidised or deteriorated. Hence the
maintenance requirement is reduced. However moisture is very harmful to the
properties of the gas. In the presence of the moisture, hydrogen fluoride is formed
during arcing which can attract the metallic and insulating parts in the circuit
breaker.
3. Electro negative gas – Ability of an atom to attract means carrying a negative
electric charge.
These advantages offer increased safety, reduction in size, weight, noiseless
operation, easy installation, handling and maintenance.
• What are the protections are there for BUSBAR?
Instantaneous over current protection
Bus bar differential protection
Local breaker back up protection
• What are the protections are there for lines?
Directional earthfault protection
Directional over current protection
Local breaker back up protection
Pole discrepancy
Main protection (distance protection)
Directional OverCurrent
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• What is Motor?
It is a device, which converts electrical energy into mechanical (rotating) energy.
Definition of terms used in Motor:
1) Duty Cycle rating: Most motor has a continuous duty rating to permit continuous
operation at a rated load. However motors may be rated as intermittent duty,
periodic duty or varying duty and must be turned off and allowed to cool after a
fixed operating time.
2) Full- Load current: The current required to produce full-load torque.
3) Jogging: The starting and stopping of a motor at frequent intervals.
• What is Motor controller?
A device that controls some or all of the following functions: starting, stopping,
overload protection, over current protection, reverses, changing of speed sequence
control and running/jogging.
• What is Motor speed?
The shaft speed of the three-phase squirrel cage motor is determined by the
frequency of the supply voltage and the number of poles in the motor. A two-pole
motor runs at a speed of 3000 rpm on 50 cycles per second.
rpm = cycles per second x 60 - slip
Poles
(Where slip is the difference between the speed of the rotating magnetic field and the
speed of the rotor.)
• Why Over current protection used?
A fusible disconnect or circuit breaker used to protect the branch circuit conductors,
control devices and the motor from grounds and short circuits. the over current
protection device must be capable of carrying the starting current to exceed 400% of
the motor full load current.
• What is Overload?
Any excessive amount of current drawn by the motor is called overload. Overloads
on a motor may be mechanical or electrical.
• What is Plugging?
The instant reversal of motor is called plugging. Damage to the driven machinery
can be result if plugging is applied improperly.
• What is Sequence control?
The control of separate motors to operate in a predetermined pattern.
• What is Service factor?
The amount of overload that may be permitted without causing significant
deterioration of the insulation on a motor. For example, if a 10 hp motor has a
service factor of 1.15, the motor can be safely be subjected to an 11.5 hp load.
• What is Starting current or Locked rotor current?
The current flow in the motor at the instant of starting. This current can be 4-10
times the full load current of the motor. The most common locked rotor current is
about 6 times the full load current. Such a motor will start with a 600% overload.
• What is Torque?
The twisting force produced by the motor is called torque. Its unit is in foot-pounds
(ft-lb.), torque is related to horsepower by the following formula.
Torque = horsepower * 5252
Revolution per minute (rpm)
• Write details MCC construction.
a) MCC are made up of sheet steel enclosure, indoor floor mounting and free
standing, Dust and vermin proof, modular type and of double front and single
front (X1, Y1).
b) Degree of protection is IP 50 as per IS 2147.
c) 0.9 * 0.8 * 2.4 meters size (double front and single front) and top entry of cables.
d) 0.9 * 0.6 * 2.4 meters size (single front) and bottom entry of cables.
e) Parts are incoming panel, Cable entry, TB compartment, MCC module
compartment.
f) MCC modules are fully drawn-out type.
g) Main buses are horizontally mounted and vertical buses are connected to MCC
cells.
h) Stab – in contacts are used for power and wipe – in contacts are used for control
circuits.
i) CT and PT are used for current and voltage measurements.
j) 3φ indication lamps are provided for identification.
k) Voltage meter and ammeter are provided.
l) Panel space heater and emergency push button key operated are provided.
m) Control building and SRPH MCC are safety related and SB, TB, RAB, CWPH,
DM Plant MCC’s are non safety related.
• What is the maximum load on MCC?
3 Phase load upto 90 kW are fed by MCC.
• What are the types of MCC?
Type Application Protection
A Receptacles, Cranes, Elevators, Local control panels Fuse
B Locally controlled heaters Fuse, 49
C Remote controlled heaters Fuse, 49
D Remote controlled loads <29 kW (49 in control panel) Fuse
F For valve motors Fuse, 49
G Locally controlled loads <29 kW Fuse, 49
H Locally controlled loads >29 kW (RTM installed) Fuse, 49
I DG MCC
J Remote controlled loads <29 kW Fuse, 49
K Remote controlled loads >29 kW (RTM installed) Fuse, 49
L Barring gear motor MCC Fuse, 49
SP For F/M supply and PHT S/D cooling pumps CT, PT Used
• What is the operating life of bearings?
a) Continuous 24 hrs operation – 40000 to 50000 hrs.
b) Affected by load axially or radial.
c) Operating temperature.
• Give the 415 V MCC bus ratings and cable used.
All MCC bus bars are made of aluminium. Short time current is 50 kA/sec and
momentary rating is 105 kA.
Type Bus Rating Class Location Cable Used
K1 1000 A IV TB 111 Mts.
K2 1500 A IV TB 111 Mts.
L1 1000 A IV RAB 108 Mts.
L2 1000 A IV SB 106.5 Mts.
L3 1500 A IV TB 111 Mts.
M1 1000 A IV SB 106.5 Mts.
M2 1500 A IV TB 111 Mts.
N1 1000 A IV TB 111 Mts.
N2 1000 A IV RAB 108 Mts.
W1 1500 A IV CCW PH 98 Mts.
W2 1000 A IV DM plant 98 Mts.
P1 1500 A III CB 100 Mts.
P2 1500 A III CB 106.5 Mts.
Q1 1500 A III CB 100 Mts.
Q2 1500 A III CB 106.5 Mts.
X1 600 A III SRPH 100 Mts.
Y1 600 A III SRPH 100 Mts.
PMCC S 630 A II CB 106.5 Mts.
PMCC T 630 A II CB 106.5 Mts.
• What are the types of isolators used?
Isolator Range Loads
32 A ≤ 9.3 kW
63 A > 9.3 kW and ≤19 kW
125 A > 20 kW and ≤ 47 kW
250 A > 48 kW and ≤ 110 kW
400 A > 111 kW and ≤ 134 kW
600 A ≤ 310 kW
• What are the ranges of fuse used?
Fuse Range Loads
2 A 10 to 280 watt
4 A 340 to 440 watt
6 A 500 to 700 watt
10 A 1000 to 1500 watt
16 A 1.8 kW to 2.25 kW
20 A 3 to 4 kW
25 A 5 to 8 kW
32 A 9 to 9.3 kW
50 A 9.6 to 15 kW
63 A 16 to 19 kW
80 A 22 to 24 kW
100 A 25 to 36 kW
125 A 38.6 to 45 kW
160 A 48 to 67.5 kW
200 A 72 to 80 kW
250 A 85 to 90 kW
• What is use of fuse in electric circuit, what are the materials used for fuse and what
are their melting points?
Fuse is a weakest point in an electrical circuit, which breaks the circuit when
abnormal current more than it’s rating flows through it. It works on principle of joule
law (I2Rt). HRC fuse is filled with quartz powder to extinguish the arc generated in
breaking the circuit or when fuse blown.
Current rating is depends on the type of material, cross section area, length and size
of terminal (large size terminal dissipates more heat).
Formulae
H = I2Rt/J
R = ρl/a
a = d2π/4
Material Melting point in °C
Silver 1830
Copper 2000
Aluminium 240
Zinc 787
Tin 436
Lead 624
• What are the materials made of thermal overload relay? How OLR are selected?
The bimetallic materials are Invar and brass. These materials having the differential
coefficient of expansion. All bimetallic relays incorporate additional built in single
phasing protection.
The range of the relay provided for the feeders are such that the full load rating of the
feeders is comfortably within the range of the relay (range will be at Centre) except
in very minimum loads ranging from 0.1 – 0.16 A.
• What are the functions of arc chute?
To increase the speed of rise of arc by magnetic action.
It splits the arc by this arc resistance increases.
Diagnosing the arc by cooling.
• What are IP (ingressive protection) and IC?
IP means ingressive protection to the motor against the dust and water entry.
The first digit indicates protection against accidental contact with live or moving
parts (solid particles).
The second digit indicates protection against ingress of water, foreign bodies (liquid
particles).
IC means instrument cooling to the motor (type of cooling)
• What are the classes of AC motors?
Depends on phases
a. 1φ.
b. 3φ.
Depends on construction
a. Squirrel cage induction motor for fixed torque.
b. Wound rotor motor for variable torque.
Depends on voltage
a. LT motor - <200 kW.
b. HT motor - >200 kW.
Depends on torque and current
a. Class – A (Normal torque and normal starting current. E.g. Fractional motors.) .
b. Class – B (Normal torque and low starting current).
c. Class – C (High starting torque and low starting current. E.g. Double sq. cage motor)
d. Class – D (high starting torque and high starting current).
Depends on mechanical characteristics
a. Drip proof (IP 54). Safety against water or dust.
b. Splash proof.
c. TEFC (totally enclosed fan cooled).
d. TEOV (totally enclosed open ventilated).
e. TETV (totally enclosed tube ventilated. Principle is thermosymphony E.g. - CEP).
f. Explosion proof.
• What is the PI value required for motors?
For class F insulation >2 and for class B insulation 1.5 to 2.
• What are the enclosures used for HT and LT motors?
LT motors (<200 kW)
a. Open drip proof.
b. TEFC.
c. Totally enclosed air over type.
HT motors (>200 kW)
a. Open drip proof.
b. Weather protected I
c. Weather protected II
d. Totally enclosed water-cooled.
e. Totally enclosed pipe ventilated.
• What are the causes of motor failure?
a. Corrosion or rust.
b. Excessive moisture (winding IR low and bearing lubrication loss).
c. High ambient temperature.
d. Poor ventilation.
e. Inadequate lubrication.
f. Misalignment.
g. Oil and dirt.
h. Excessive starts and repetitive surges.
i. Persistent over loads.
j. Shaft currents (bearing pitting).
k. Mis application.
l. Manufacture defect or wrong design.
m. Deterioration with age.
n. Maintenance improper.
• What are the effects of excessive starts and repetitive surges?
Repetitive surges may give impact to the insulation of the motor and dielectric
capability of the motor.
Excessive starts may subject stator winding to high current for more time.
Subsequently in HT motor due to High Mass rotor bar and rotor short ring may loose
or fail. Bearing also may damage.
• What are the effects of broken rotor bars and broken shaft parts?
Broken rotor bars
a. High stator current and over heat of stator winding.
b. More harmonic currents in end parts.
c. High vibration.
Broken shaft or parts
a. Stator winding loose bracing.
b. Rotor high vibration and bearing vibration.
c. Frame vibration and more harmonics in side bands.
• Give the relation between current and temperature in motors.
a. Winding temperature is proportional to square of the current.
b. 10% increase in current gives 30% increase in temperature.
c. 10°C rise in temperature makes 50% life reduced.
• What are the effects of imbalance stator winding resistance?
If the stator winding resistances are imbalance phase to phase give pulsating fluxes
and pulsating torque on rotor and vibration may increase. For accurate resistance
measurement Kelvin Bridge is used.
• What reflects the change in speed?
a. Supply frequency may vary the speed of the motor.
b. Load on the motor may vary the speed of the motor.
• What is use of BORESCOPE inspection?
BORESCOPE inspection method makes it easy to observe the end winding condition
of the motor. In this inspection winding ties, loose coils, dust etc can be observed.
• What you mean by CRAWLING and COGING?
Crawling
The motor fails to rotate at rated speed or motor rotates at … or 20% speed is called
motor crawling. This may be due to system imbalance or more pulsating torque.
Coging
Motor fail to start atoll is called motor coging.
• Why motor starting current is high compared to transformer charging current?
Transformer charging current is only 1% and that of motor starting current is 30 to
40%. Because of air gap between stator and rotor. If the air gap is more load taking
capacity increases and if air gap is less the load taking capacity reduces.
• State construction details of the motor.
Stator or rotor core
Built from high quality low loss silicon steel laminations and flash enameled on both
the sides made up of close-grained alloy cast iron.
Rotor conductor
Heavy bars of copper or aluminium alloy.
Stator
Copper conductor.
• What is the use of making rotor skewing?
1. To run motor quickly by reducing magnetic hum.
2. It reduces locking tendency with the stator.
• Why under voltage tripping of motor is incorporated in motor feeder breakers?
The under voltage can occur in case of bus fault. If the motors are kept connected
they will feed the fault which may cause the damage. Due to the back feeding from
the motor the motor will slow down very fast. Hence process system will come to
halt very fast. (In case pht motor will not rotate for the designed 3 minute period in
case of bus under voltage).
• What are the problems in station operation due to grid under voltage?
a) All the HT motors overloaded.
b) VAR load increases on generator leads to heating up of rotor
c) Stator current increases for same power export leads to stator over heating.
• What are the problems in station operation due to grid under frequency?
a) Turbine having under frequency limitation, house load happen if < 48 Hz
b) Due to under frequency PHT flow reduces, therefore reactor power reduces,
generator power reduces
c) If frequency is less than 48 Hz DG cannot be synchronised to grid, therefore DG
kept on isolation running
d) GT overfluxing.
• What is the difference between fixed trip and trip free?
Fixed trip: Breaker will trip only after closing even if trip impulses are existing.
Trip free: Breaker is free to trip at any position.
• What is the making current capacity of a 3-phase breaker as derived from its
symmetrical breaker capacity?
Making capacity = 2.55 times symmetrical breaking capacity.
• Why intermediate contacts in English electric breaker?
To prevent even slightest arcing on main contacts.
• Where preloaded ball bearings used?
If more vibration exists even when machine is not in running conditions.
• Why do we grease the bearings?
a) Grease lubricant gives good protection against ingress of moisture and dirt into
motor.
b) Easy to seal against leakage of grease into motor compared to oil.
c) Low friction torque at starting.
• Which bearings preferred for all large power motor?
Plain bearings
• Which is more dangerous alkali or acid?
It especially exposed alkali is more dangerous, use boric acid powder solution
immediately.
• What FCN was implemented to avoid reactor trip on 220V-DC failure of PHT and
PPP breakers?
The breaker close position supervision relay VAA 21 is changed by VAJC type,
contacts position do not change if 220V-DC is lost now.
• Where oil lubrication is preferred over grease lubrication?
a) Determined by speed and operating temperature.
b) Oil lubrication recommended.
c) When speed and temperature is high.
d) When heat to be conducted away from bearing.
e) When adjacent machine components are oil lubricated.
f) High viscous oil for low speed machine, low viscous oil for high speed machine.
g) At temp<125 ºC, synthetic oils recommended.
• What are the causes of failure of bearings?
a) Faulty mounting
b) Faulty lubrication
c) Foreign matter in lubrication
d) Water in the bearing arrangement
e) Vibration
f) Inoccurrences of form of shaft or housing seating.
g) Passage of electric current.
h) Metal fatigue.
• What does the bearing number mean?
7318 7 = single row angular contact ball bearings
3 = width of race
18 = 18 x 5 = 90 mm bore diameter.
6310 6 = single row deep groove ball bearings.
3 = width of race
10 =10 * 5=50 mm dia
• What is the purpose of static starter? How current setting adopted?
The static starter limits the starting current of the motor to 2.5 times the motor rated
current instead of 6 times the rated current. If the motor is directly on UPS, the UPS
fuse will blow, since the UPS cannot supply so much starting current. Hence the
static starter is set to limit the starting current. This is achieved by firing angle
control of back to back thyristor.
• What is the speciality of the inverter output transformer? Why it is provided?
a) This eliminates all 3rd harmonics in the output voltage.
b) Solid earthed neutral is required for the inverter output, hence the interconnected
star winding is essential.
c) The primary has to be star (not delta), since 3 separate inverters operates on
isolated primary winding.
d) Delta connection will cause circulating current between inverters during
unbalanced faults. The inverters cannot withstand this.
• Both silver and copper oxidise in air. Then why copper contacts are silver-plated?
The silver plating avoids the oxidation of copper, especially in outdoors. Silver
oxidises very readily. But its oxide is a good electrical conductor but copper oxide
produce a film of insulation.
• When auto transfer is effected?
a) When any one module trips
b) When overload exceeds 175% for more than 40 msec. is existing.
c) When UPS output voltage varies beyond 415v +/- 10%
• When static bypass is fired? Why static bypass is required?
For the same above 3 conditions, static bypass is also simultaneously fired along
with a closing impulse.
• When the static bypass is blocked?
When the phase error is more than 20º.
• What is phase lock mode?
The inverter continuously follows the frequency and phase angle of classIII bus
supply.
• What is the difference between a contactor and a breaker?
Contactor is not designed to open on short circuit condition (fuse will take care of
this situation). Breaker is having complicated mechanism for closing and tripping.
• What is the difference between isolator and contactor?
Contactor is used for on load operation. Because they are fast acting devices. They
posses arc chamber and arc chutes. Arc chamber and arc chute make it easy to
extinguish the arc produced during on load operation.
Isolator is off load devices. Because they are slow acting devices. The arc time is
more in slow acting devices and operated only in off load.
• What are the protections provided for motor feeder?
Ith - Thermal over load
I2S - Unbalance load
I0S - Earth fault protection
I1t - Stalling protection
I1Inst. - Short circuit protection
• What are the protections provided in PMCC circuit breakers?
1. IDMT O/C (CDG 34).
2. IDMT E/F (CDG 11).
3. Under voltage (40% of 110V).
• What is requirement of having DG’s?
To establish class III supply when class IV fails.
Parallel operation with class IV 6.6 kV supply.
DG to DG parallel operation.
• What are the characteristics of CB, OLR and HRC Fuse?
CB characteristics (it is back up fuse)
Current
Fuse characteristics
Margin to avoid fuse operation OLR characteristics
During starting
Minimum fusing current
Staring current
Running current
Time
CB Protection
Fuse Protection
OLR Protection
I
Time
When CB is used the CB characteristics should be below the fuse, because the CB
should operate first and then fuse. Not vice-versa. Because CB is the main protection
or main breaking device.
• Why control transformer is earthed?
If it is not earthed grounding of control circuit at two different places can cause
bypassing of logics. In case of primary and secondary of the control transformer is
getting the main fuse will blow off. (If secondary is not grounded then 415v will be
superimposed in the control circuit during short circuit of primary and secondary
winding)
• When the fuse will take over?
When the current increases beyond 700% then the fuse will take over from the
thermal overload protection.
• What is interlocks provided for the valve MCC
a) Mechanical interlock, which will not allow the other contactor to close if one
contactor, is closed.
b) 42 auxiliary contacts are wired in the control ckt. 42-1 contact in 42-2 and 42-2
contact in 42-1.
• How to calculate the full load current of the motors?
If kW is given, full load current = 1.5 times kW rating.
If hp is given, full load current =2 times hp rating.
• What is the safety interlock provided in MCC cell?
The MCC cell door can't open if the cell is in on condition.
• How the fuse is selected?
Fuse rating should be 2.5 times the full load current.
• How will you improve the IR value of a motor?
By providing external heating. (By filament lamps)
By providing internal heating by applying the low voltage.
By circulating hot and dry air.
• Why 110V has been chosen in MCC cell?
To isolate control circuit from power circuit for Human safety at control circuit side.
• What is the purpose of DIODE across the interposing coils in PLC?
To dissipate the stored energy in magnetic field of the interposing coils (Free
wheeling action). If it is not provided the stored energy will affect the PLC card
circuit.
• What are the in-built protections provided in MCC cell?
Fuses for short circuit protection.
OLR for over load and single phasing protection.
Electrical and Mechanical interlock in valve cell against short circuit.
• What is the plugging of an induction motor?
It is an electrical braking of an induction motor by sudden reversal of phase
sequence.
• Why CT operated over load relay is using for loads of high acceleration time upto 30
seconds? How it getting back?
The saturable current transformers linearly transforms the current upto twice the set
current, but above this value the transformer core gets saturated and the secondary
current is proportionally less. Thus these relays permit heavy starting conditions of
motors and offer dependable protection against overload.
When current reduces the core gets de-saturated, as material design is such.
• How many earthing should be done for motors? Why?
Two. For reliability.
• What is the significance of frame size of motor?
In order to make practical choice, interchangeability and large scale production
possible.
• What is polarisation index?
a) It is defined as a ratio of 10 minutes resistance value to 1-minute resistance value.
b) It gives a quantitative information about the insulation with respect to moisture,
dirt and other contamination.
c) A PI value of less than 1.0 indicates a need for immediate reconditioning.
• Why megger value of 1 minute is less than 10-minute value?
After 10 minutes the high voltage applied make the molecules such a way that
stabilised in a good insulation. If insulation is weak it leads to more leakage current
due to high potential.
• What is the classification of duty of rotating electrical machines?
S1 – Continuous operation at rated load (MCR) in 40 °C
S2 – Short time operation (STR) for 5 minute or 15 minutes or 30 minutes.
S3 – Intermittent periodic operation (resting and loading e.g. cranes, lifts etc)
S4 – As for S3 but with starting
S5 – As for S3 with electric braking
S6 – Continuous cyclic operation.
• What should be the value of insulation resistance of induction motor?
In Rm = kV + 1 M OHMS.
Insulation resistance of any electrical machine (motor or generator) should be above
0.5-M ohms in all cases.
• What are the classes of insulation?
Y – 90οC (max) cotton, silk, paper, wood without oil impregnation
A – 105οC Materials of class Y impregnated with natural resins,insulating oils.
E – 120οC Synthetic resin enamels, cotton and proper laminations.
B – 130οC Mica, glass fibre, asbestos with suitable bonding substance.
F – 155οC Class B with more thermally resistant bonding materials.
H – 180οC Glass fibre and asbestos, mica with silicon resins.
C – >180οC Mica, ceramics, glass, quartz and asbestos without binders.
• What are the checks on the motor during the preventive maintenance?
IR Value
Resistance and Inductance measurement
PI value (should > 1.0)
• What are the tests pressures used in lyra contact testing?
125 A - 3 kg
250 A - 5 kg.
• What are the causes of motor vibration?
a) Broken rotor.
b) Slacked stator core.
c) Slacked rotor core.
d) Rotor winding unbalance.
• What are the causes for motor high current?
a) High frequency (51 Hz - 105% current)
b) Low frequency (48 Hz – 102% current)
c) High voltage
d) Low voltage
e) Mechanical over load
• What are the causes for motor unbalance current?
a) Loose connection
b) Voltage unbalance
c) Turns short circuit
• What are the sources of 240 V AC class I supply? What are the functions of each
part of UPS?
Six sources.
Three 20 kVA UPS for safety related loads.
Two 60 kVA UPS for non-safety related loads.
One 60 kVA UPS as a standby to safety related loads.
These all UPS are back up by 220V DC batteries.
Rectifier
This converts AC to DC supply for inverter.
Functions
1. Produces DC voltage.
2. Supplies trickle charge to batteries.
3. Full load boost charge capacity.
Inverter
This converts DC to AC supply for loads.
20 kVA inverter is transistor based and 60 kVA inverter is thyristor based.
Static switch.
To take stand by UPS into service.
Manual bypass
To take main UPS to maintenance by putting stand by UPS into service.
• What is station Black out condition?
Simultaneous failures of class IV and class III supply is called Station Black out. In
this condition class II power UPS will feed the necessary loads for a 30 minutes of
duration. After that supplementary control room (SCR) 5 kVA UPS is used for
secondary shut down system (SSS) ion chamber amplifier.
220 kV SYSTEM
• What is meant by Dielectric strength?
The maximum electrical potential gradient that a material can withstand without
rupture usually specified in volts/millimeter of thickness. This also has known as
electric strength.
• Give switchyard specification.
1. Type : Out door.
2. Scheme : Double main bus bar with bypass switching scheme is provided.
This allows maintenance of one bus or one CB without interruption.
3. Normal voltage : 220 kV.
4. Rated voltage : 245 kV (400 kV)
5. Impulse voltage : 1050 kV (peak)
6. One-minute level : 460 kV (rms.)
7. Dynamic current capacity: 102 kA (peak) and 40 kA for one sec.
8. Rated current capacity : 2000 A for main and 1600 A for feeder bus.
9. Clearances : Phase to earth – 2100 mm.
Phase to phase – 2100 mm.
Phase to ground – 5500 mm.
Sectional clearance – 4300 mm.
Creepage clearance – (Total) 5600 mm.
– (Protected) 2800 mm.
10. Maximum temperature rise above ambient - 45°C.
11. CB – SF6
12. Isolator – motor operated rotating type.
13. Number of bays – 16 Nos.
• Give the details of switchyard 220 kV CB, Isolator, CT, CVT and lightning arrestor.
220 kV SF6 Circuit Breaker
1. Make – ABB
2. Air pressure blocking a. Close Block – 17.3 bar.
b. Open block – 16.7 bar.
c. Auto reclose block – 19 bar.
3. SF6 pressure block a. Alarm – 5.2 bar.
b. Rated – 6 bar.
c. Limit – 5 to 6 bar.
d. Open block – 5 bar
4. Weight of gas / pole : 20 kgs.
5. Closing time : 130-milli sec.
6. Method of closing : Electro-pneumatic.
7. Compressor pressure : 20.5 kg/cm2.
Isolator
1. Type : High-pressure pressure relieving isolator (HPPR) central pole double
break.
2. CB and Isolator clearances : Phase to Phase – 4500 mm.
Phase to earth – 2300 mm.
Current transformer
1. Make : TELK made hermetically sealed.
2. Type : Single pole dead tank.
Capacitor voltage transformer (CVT)
This is capacitive potential divider and inductive medium mineral oil sealed.
ABB. make 245 kV/110 √3 V.
3 cores for metering and protection.
Lightning arrestor
Type : WS surge arrestor of ZODIVER type and SMX style.
Gapier zinc oxide arrestor. Multi unit construction for transport, storage and erection.
Rated voltage : 216 kV rms.
Operating voltage : 184 kV rms.
• For a fault in switchyard lightning arrestor, what protection will act?
Bus bar differential protection.
• What is the purpose of the CVT?
To provide synchronising signal
To provide voltage indication
To facilitate the carrier communication
• What is the purpose of wave trap?
Carrier communication signals are sent through the lines. These are high frequency
signals. This signal should be prevented from entering the switchyard. The wave trap
is LC ckt, which is tuned for 50 Hz. Since it is connected in series with the line it
will effectively block the carrier signal entering into the switchyard.
• What is the purpose of lightning arrestor?
Due to lightning and switching surges high voltages are induced in the lines. If
equipment’s. Connected is subjected to this high voltage the insulation will fail. In
order to avoid the failure of insulation the LA is used. When the la is subjected to
high voltage it will conduct and discharge the current to the earth.
(It will divert the over voltages to earth and protect the substation)
• What is meant by restriking voltage?
The high voltage that will appear across the contact just after the quenching of the
arc is called restriking voltage.
• What does switching surges mean?
When a line is switched on high voltage will appear on the line due to its inductance
and capacitance. This voltage is known as switching surges.
• What is the purpose of compressed air in SF6 breaker?
This used for drive for opening and closing of the contacts.
(Arc quenching is taken care by SF6 gas)
• Why switchyard is located indoors of coastal plants?
Saline atmosphere will deposit on the insulators causing its flashover. The building
kept under positive pressure compared with outside thus preventing the (saline) air
entering from outside to inside the building.
• Why disc insulators grooved at bottom?
To increase the creepage distance, reduce the chances of flash over.
• How cap and pin attached to insulator?
By cementing.
• What is the material of cap, pin, and insulator?
Cap = galvanised cast iron
Pin = forged steel pin
Insulator = porcelain.
• Why insulators are glazed?
If not glazed, it will absorbs water, resistance comes down, leakage current through
porcelain, temperature increases till porcelain is puncture
• What is the station ground resistance?
Less than 0.5 ohms.
• What is the various design of CT's in switchyard?
Bus coupler CT's- live tank design 2000-1000A/1A
All other CT's- dead tank design 800-600-400A/1A - lines and GT.
125A/1A – SUT
• Advantage of CVT over EMPT.
Used as coupling capacitors for PLCC.
• What are the main parts of 220 kV Circuit Breaker?
Pole column filled with SF6
Pneumatic drive system with compressed air circuit
Control cubicle unit
• What is the type of 220 kV circuit breaker?
220 kV, SF6 breaker, single pole, puffer type.
• What are the levels of SF6 gas in 220 kV breaker and their significance?
7 kg/cm2 - normal pressure
5.2 bar - alarm
5.0 bar - closing/tripping operation blocked.
• What is the difference between circuit breaker and isolator?
Isolator is a disconnecting switch which is not having the making and breaking
capacity.
Bus coupler - 2000A
Feeders - 1600 A
• What is the purpose of ground switch?
To discharge the trapped electrical charges to ground to give complete isolation.
(To discharge the residual potential)
• What is the type of Lightning Arrestor?
Station type, heavy duty, gap less zinc oxide.
• What is the purpose of grading ring?
This assembly is provided to have uniform voltage gradient.
• What are the properties of SF6 gas?
Physical properties
1. Colourless
2. Odorless
3. Non-toxic. Pure SF6 gas is not harmful to the health.
4. Non-inflammable.
5. Density- more gas density, 5 times that of air at 20°C and at atmospheric
pressure. The gas starts liquefying at certain low temperature. The temperature of
liquefaction depends on pressure. At 15 kg f / cm2 the gas starts liquefying at
10°C. Hence this gas is not suitable for high pressures >15 kg f / cm2
6. The heat transferability of SF6 gas is 2 to 2.5 times that of air at same pressure.
Hence for equal conductor size the current carrying capacity is relatively more.
Chemical properties
1. Stable upto 500°C.
2. Inert gas due to the chemical inertness. The life of the metallic parts, contacts is
longer in SF6 gas. The components do not get oxidised or deteriorated. Hence the
maintenance requirement is reduced. However moisture is very harmful to the
properties of the gas. In the presence of the moisture, hydrogen fluoride is formed
during arcing which can attract the metallic and insulating parts in the circuit
breaker.
3. Electro negative gas – Ability of an atom to attract means carrying a negative
electric charge.
These advantages offer increased safety, reduction in size, weight, noiseless
operation, easy installation, handling and maintenance.
• What are the protections are there for BUSBAR?
Instantaneous over current protection
Bus bar differential protection
Local breaker back up protection
• What are the protections are there for lines?
Directional earthfault protection
Directional over current protection
Local breaker back up protection
Pole discrepancy
Main protection (distance protection)
Directional OverCurrent
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