1) What do AC meters show, is it the
RMS or peak voltage?
- AC voltmeters and
ammeters show the RMS value of the voltage or current. DC meters
also show the RMS value when connected to varying DC providing the DC is
varying quickly, if the frequency is less than about 10Hz you will see the
meter reading fluctuating instead.
2) In the transmission tower
construction Middle arm is longer than the upper and lower Arm.
- Conductor of Upper
Arm and Lower Arm will stay apart.
- To prevent big
birds (Ostriches etc.) from bumping their heads against the conductor
above when they sit on the wire below.
- Designed to
maintain the mechanical requirement to prevent arching between conductors
while maintaining a tower height that is manageable, and of course
preventing head injuries to birds
- The arms are of
different links to prevent a broken upper line from falling on one or more
of the phase lines below.
- The clearance from
other phase.
- Mutual inductance
minimization.
- Preventing droplet
of water/ice to fall on bottom conductor.
3) Why transmission line 11Kv OR 33KV,
66Kv not in 10kv 20kv?
- The form factor of
an alternating current waveform (signal) is the ratio of the RMS (Root
Mean Square) value to the average value (mathematical mean of absolute
values of all points on the waveform).
- In case of a
sinusoidal wave, the form factor is approximately 1.11.
- The second reason
is something historical and contradicts to the above statement. In olden
days when the electricity becomes popular, the people had a misconception
that in the transmission line there would be a voltage loss of around 10%.
So in order to get 100 at the load point they started sending 110 from
supply side. It has nothing to do with form factor (1.11).
- Nowadays that
thought has changed and we are using 400 V instead of 440 V, or 230 V
instead of 220 V.
- Also alternators
are now available with terminal voltages from 10.5 kV to 15.5 kV so
generation in multiples of 11 does not arise.
4) What is the difference between Surge
Arrester & Lightning Arrestor?
- Transmission Line
Lightning Protection – General:
- The transmission
line towers would normally be higher than a substation structure, unless
you have a multi-story structure at your substation.
- Earth Mats are
essential in all substation areas, along with driven earth electrodes
(unless in a dry sandy desert site).
- It is likewise
normal to run catenaries’ (aerial earth conductors) for at least 1kM out
from all substation structures. Those earth wires to be properly
electrically to each supporting transmission tower, and bonded back to the
substation earth system.
- It is important to
have the catenaries’ earth conductors above the power conductor lines, at
a sufficient distance and position that a lightning strike will not hit
the power conductors.
- In some cases it is
thus an advantage to have two catenary earth conductors, one each side of
the transmission tower as they protect the power lines below in a better
manner.
- In lightning-prone
areas it is often necessary to have catenary earthing along the full
distance of the transmission line.
- Without specifics,
(and you could not presently give tower pictures in a Post because of a
CR4 Server graphics upload problem), specifics would include:
- Structure Lightning
Protection – General:
- At the Substation,
it is normal to have vertical electrodes bonded to the structure, and
projecting up from the highest points of the structure, with the location
and number of those electrodes to be sufficient that if a lightning strike
arrived, it would always be a vertical earthed electrode which would be
struck, rather than any electrical equipment.
- In some older
outdoor substation structures, air-break isolator switches are often at a
very high point in the structure, and in those cases small structure
extension towers are installed, with electrodes at the tapered peak of
those extension towers.
- The extension
towers are normally 600mm square approximately until the extension tower
changes shape at the tapered peak and in some cases project upwards from
the general structure 2 to 6 meters, with the electrode some 2 to 3 meters
projecting upwards from the top of the extension tower.
- The substation
normally has a Lightning Counter – which registers a strike on the
structure or connected to earth conductors, and the gathering of
that information (Lightning Days, number per Day/Month/Year, Amperage of
each strike)
5) How Corona Discharge Effect Occur in
Transmission Line?
- In a power system
transmission lines are used to carry the power. These transmission lines
are separated by certain spacing which is large in comparison to their
diameters.
- In Extra High
Voltage system (EHV system ) when potential difference is applied across
the power conductors in transmission lines then air medium present between
the phases of the power conductors acts as insulator medium however the
air surrounding the conductor subjects to electro static stresses. When
the potential increases still further then the atoms present around the
conductor starts ionize. Then the ions produced in this process repel with
each other and attracts towards the conductor at high velocity which
intern produces other ions by collision.
- The ionized air
surrounding the conductor acts as a virtual conductor and increases the
effective diameter of the power conductor. Further increase in the
potential difference in the transmission lines then a faint luminous glow
of violet color appears together along with hissing noise. This phenomenon
is called virtual corona and followed by production of ozone gas which can
be detected by the odor. Still further increase in the potential between
the power conductors makes the insulating medium present between the power
conductors to start conducting and reaches a voltage (Critical Breakdown
Voltage) where the insulating air medium acts as conducting medium results
in breakdown of the insulating medium and flash over is observed. All this
above said phenomenon constitutes CORONA DISCHARGE EFFECT in electrical
Transmission lines.
6) Methods to reduce Corona Discharge
Effect:
- Critical Breakdown
voltage can be increased by following factors
- By increasing the
spacing between the conductors:
- Corona Discharge
Effect can be reduced by increasing the clearance spacing between the
phases of the transmission lines. However increase in the phases results
in heavier metal supports. Cost and Space requirement increases.
- By increasing the
diameter of the conductor:
- Diameter of the
conductor can be increased to reduce the corona discharge effect. By using
hollow conductors corona discharge effect can be improved.
- By using Bundled
Conductors:
- By using Bundled
Conductors also corona effect can be reduced this is because bundled
conductors will have much higher effective diameter compared to the normal
conductors.
- By Using Corona
Rings or Grading Rings:
- This is of having
no greater significance but I presented here to understand the Corona Ring
in the Power system. Corona Rings or Grading Rings are present on the
surge arresters to equally distribute the potential along the Surge
Arresters or Lightning Arresters which are present near the Substation and
in the Transmission lines.
7) How to test insulators?
- Always remember to
practice safety procedures for the flash-over voltage distance and use a
sturdy enclosure to contain an insulator that may shatter, due to steam
build-up from moisture in a cavity, arcing produces intense heat, an AM
radio is a good RFI/arcing detection device, a bucket truck AC dielectric
test set (130KV) is a good test set for most pin and cap type insulators.
A recent article said the DC voltage required to “search out defects can
be 1.9 times the AC voltage.
- Insulators have a
normal operating voltage and a flash-over voltage. Insulators can have
internal flash-over that are/are not present at normal operating voltage.
If the RFI is present, de-energize the insulator (line) and if the RFI
goes away, suspect the insulator (line). Then there can be insulators that
have arcing start when capacitor or other transients happen, stop when the
line is de-energized or dropped below 50% of arc ignition voltage. Using a
meg-ohm-meter can eliminate defective insulators that will immediately
arc-over tripping the test set current overload.
8) How to Check Capacitor by Multi Meter.
- Most troubles with
Capacitors — either open or short.
- A multi meter is
good enough. A shorted C will clearly show very low resistance. An open C
will not show any movement on ohmmeter.
- A good capacitor
will show low resistance initially, and resistance gradually increases.
This shows that C is not bad. By shorting the two ends of C (charged by
ohmmeter) momentarily can give a weak spark.
9) How to identify the starting and ending
leads of winding in a motor which is having 6 leads in
the terminal box
- If it is a single
speed motor then we have to identify 6 leads.
- Use IR tester to
identify 3 windings and their 6 leads. Then connect any two leads of two
winding and apply small voltage across it and measure the current.
- Then again connect
alternate windings of same two windings and apply small amount of voltage
(same as before) and measure current.
- Check in which mode
you get the max current and then mark it as a1-a2 & b1-b2. You get max
current when a2-b1 will be connected and voltage applied between a1-b2.
- Follow the same
process to identify a1-a2, b1-b2, c1-c2.now we will be able to connect it
in delta or star.
10) Why the up to dia 70mm² live conductor, the earth cable
must be same size but above dia 70mm² live conductor the earth conductor need
to be only dia 70mm²?
- The current
carrying capacity of a cable refers to it carrying a continuous
load.
- An earth cable
normally carries no load, and under fault conditions will carry a significant
instantaneous current but only for a short time most
Regulations define 0.1 to 5 sec before the fuse or breaker trips. Its size
therefore is defined by different calculating parameters.
- The magnitude of
earth fault current depends on:
- (a) the external
earth loop impedance of the installation (i.e. beyond the supply
terminals)
- (b) the impedance
of the active conductor in fault
- (c) The impedance
of the earth cable.
- i.e. Fault
current = voltage / a + b + c
- Now when the active
conductor (b) is small, its impedance is much more than (a), so the earth
(c) cable is sized to match. As the active conductor gets bigger, its
impedance drops significantly below that of the external earth loop
impedance (a); when It is quite large its impedance can be ignored. At
this point there is no merit in increasing the earth cable size
- i.e. Fault
current = voltage / a + c
- (C) is also very
small so the fault current peaks out.
- The neutral
conductor is a separate issue. It is defined as an active conductor and therefore
must be sized for continuous full load. In a 3-phase system,
- If balanced, no
neutral current flows. It used to be common practice to install reduced
neutral supplies, and cables are available with say half-size neutrals
(remember a neutral is always necessary to provide single phase voltages).
However the increasing use of non-linear loads which produce harmonics has
made this practice dangerous, so for example the current in some standard
require full size neutrals. Indeed, in big UPS installations I install
double neutrals and earths for this reason.
11) How to measure Transformer Impedance?
- Follow the steps below:
- (1) Short the secondary side of the transformer with current
measuring devices (Ammeter)
- (2) Apply low voltage in primary side and increase the
voltage so that the secondary current is the rated secondary current of
the transformer. Measure the primary voltage (V1).
- (3) Divide the V1 by the rated primary voltage of the
transformer and multiply by 100. This value is the percentage impedance of
the transformer.
- When we divide the primary voltage V1 with the full load
voltage we will get the short circuit impedance of the transformer with
refereed to primary or Z01. For getting the percentage impedance we need
to use the formula = Z01*Transformer MVA / (Square of Primary line
voltage).
12) Why Bus Couplers are normally 4-Pole. Or When Neutral
Isolation is required?
- Neutral Isolation is mandatory when you have a Mains Supply
Source and a Stand-by Power Supply Source. This is necessary because if
you do not have neutral isolation and the neutrals of both the sources are
linked, then when only one source is feeding and the other source is OFF,
during an earth fault, the potential of the OFF Source’s Neutral with
respect to earth will increase, which might harm any maintenance personnel
working on the OFF source. It is for this reason that PCC Incomers &
Bus Couplers are normally 4-Pole. (Note that only either the incomer or
the bus coupler needs to be 4-pole and not both).
- 3pole or 4pole switches are used in changing over two independent
sources ,where the neutral of one source and the neutral of another source
should not mix the examples are electricity board power supply and
standalone generator supply etc. the neutral return current from one source
should not mix with or return to another source. As a mandatory point the
neutral of any transformer etc. are to be earthed, similarly the neutral
of a generator also has to be earthed. While paralling (under uncontrolled
condition) the neutral current between the 2 sources will crises cross and
create tripping of anyone source breakers.
- Also as per IEC standard the neutral of a distribution system
shall not be earthed more than once. means earthing the neutral further
downstream is not correct,
13) Why Three No’s of Current transformer in 3 phase
Star point is grounded.
- For CT’s either you use for 3 phase or 2 phase or even if you
use only 1 CT’s for the Over current Protection or for the Earth Faults
Protection, their neutral point is always shorted to earth. This is NOT as
what you explain as above but actually it is for the safety of the CT’s
when the current is passing thru the CT’s.
- In generally, tripping of Earth faults and Over current
Protection has nothing to do with the earthing the neutral of the CT’s.
Even these CT’s are not Grounded or Earthed, these over current and the
Earth Faults Protection Relay still can operated.
- Operating of the Over current Protection and the Earth Faults
Relays are by the Kirchhoff Law Principle where the total current flowing
into the points is equal to the total of current flowing out from the
point.
- Therefore, for the earth faults protection relays operating,
it is that, if the total current flowing in to the CT’s is NOT equal total
current flowing back out of the CT’s then with the differences of the
leakage current, the Earth Faults Relays will operated.
14) What is tertiary winding of Transformer?
- Providing a
tertiary winding for a transformer may be a costly affair. However, there
are certain constraints in a system which calls for a tertiary transformer
winding especially in the case of considerable harmonic levels in the
distribution system. Following is an excerpt from the book “The J&P
Transformer Book”.
- Tertiary winding is
may be used for any of the following purposes:
- (A)To limit the
fault level on the LV system by subdividing the indeed that is, double
secondary transformers.
- (B)The
interconnection of several power systems operating at different supply
voltages.
- (C) The regulation
of system voltage and of reactive power by means of a synchronous
capacitor connected to the terminals of one winding.
- It is desirable
that a three-phase transformer should have one set of three-phase windings
connected in delta thus providing a low-impedance path for third-harmonic
currents. The presence of a delta connected winding also allows current to
circulate around the delta in the event of unbalance in the loading
between phases, so that this unbalance is reduced and not so greatly fed
back through the system.
- Since the
third-order harmonic components in each phase of a three-phase system are
in phase, there can be no third-order harmonic voltages between lines. The
third-order harmonic component of the magnetising current must thus flow
through the neutral of a star-connected winding, where the neutral of the
supply and the star-connected winding are both earthed, or around any
delta-connected winding. If there is no delta winding on a star/star
transformer, or the neutral of the transformer and the supply are not both
connected to earth, then line to earth capacitance currents in the supply
system lines can supply the necessary harmonic component. If the harmonics
cannot flow in any of these paths then the output voltage will contain the
harmonic distortion.
- Even if the neutral
of the supply and the star-connected winding are both earthed, then
although the transformer output waveform will be undistorted, the
circulating third-order harmonic currents flowing in the neutral can cause
interference with telecommunications circuits and other electronic
equipment as well as unacceptable heating in any liquid neutral earthing
resistors, so this provides an added reason for the use of a delta
connected tertiary winding.
- If the neutral of
the star-connected winding is unearthed then, without the use of a delta
tertiary, this neutral point can oscillate above and below earth at a
voltage equal in magnitude to the third-order harmonic component. Because
the use of a delta tertiary prevents this it is sometimes referred to as a
stabilizing winding.
- When specifying a
transformer which is to have a tertiary the intending purchaser should
ideally provide sufficient information to enable the transformer designer
to determine the worst possible external fault currents that may flow in
service. This information (which should include the system characteristics
and details of the earthing arrangements) together with a knowledge of the
impedance values between the various windings, will permit an accurate
assessment to be made of the fault currents and of the magnitude of
currents that will flow in the tertiary winding. This is far preferable to
the purchaser arbitrarily specifying a rating of, say, 33.3%, of that of
the main windings.
15) Why do transformers hum?
- Transformer noise is caused by a phenomenon which causes a
piece of magnetic sheet steel to extend itself when magnetized. When the
magnetization is taken away, it goes back to its original condition. This
phenomenon is scientifically referred to as magnetostriction.
- A transformer is magnetically excited by an alternating
voltage and current so that it becomes extended and contracted twice
during a full cycle of magnetization. The magnetization of any given point
on the sheet varies, so the extension and contraction is not uniform. A
transformer core is made from many sheets of special steel to reduce
losses and moderate the ensuing heating effect.
- The extensions and contractions are taking place erratically
all over a sheet and each sheet is behaving erratically with respect to
its neighbour, so you can see what a moving, writhing construction it is
when excited. These extensions are miniscule proportionally and therefore
not normally visible to the naked eye. However, they are sufficient to
cause a vibration, and consequently noise. Applying voltage to a
transformer produces a magnetic flux, or magnetic lines of force in the
core. The degree of flux determines the amount of magnetostriction and
hence, the noise level Why not reduce the noise in the core by reducing
the amount of flux? Transformer voltages are fixed by system requirements.
The ratio of these voltages to the number of turns in the winding
determines the amount of magnetization. This ratio of voltage to turns is
determined mainly for economical soundness. Therefore the amount of flux
at the normal voltage is fixed. This also fixes the level of noise and
vibration. Also, increasing (or decreasing) magnetization does not affect
the magnetostriction equivalently. In technical terms the relationship is
not linear.
16) How can we reduce airborne noise?
- Put the transformer in a room in which the walls and floor
are massive enough to reduce the noise to a person listening on the other
side. Noise is usually reduced (attenuated) as it tries to pass through a
massive wall. Walls can be of brick, steel, concrete, lead, or most other
dense building materials.
- Put the object inside an enclosure which uses a limp wall
technique. This is a method which uses two thin plates separated by
viscous (rubbery) material. As the noise hits the inner sheet some of its
energy is used up inside the viscous material. The outer sheet should not
vibrate.
- Build a screen wall around the unit. This is cheaper than a
full room. It will reduce the noise to those near the wall, but the noise
will get over the screen and fall elsewhere (at a lower level). Screens
have been made from wood, concrete, brick and with dense bushes (although
the latter becomes psychological)
- Do not make any reflecting surface coincident with half the
wave length of the frequency. What does this mean? Well, every frequency
has a wave length. To find the wave length in air, for instance, you
divide the speed of sound, in air (generally understood as 1130 feet per
second) by the frequency. If a noise hits a reflecting surface at these
dimensions it will produce what is called a standing wave. Standing waves
will cause reverberations (echoes) and an increase in the sound level. If
you hit these dimensions and get echoes you should apply absorbent
materials to the offending walls (fibreglass, wool, etc.)
17) What is polarity, when associated with a transformer?
- Polarity is the
instantaneous voltage obtained from the primary winding in relation to the
secondary winding. Transformers 600 volts and below are normally connected
in additive polarity. This leaves one high voltage and one low voltage
terminal unconnected. When the transformer is excited, the resultant
voltage appearing across a voltmeter will be the sum of the high and low
voltage windings. This is useful when connecting single phase transformers
in parallel for three phase operations. Polarity is a term used only with
single phase transformers.
18) What is exciting current?
- Exciting current is the current or amperes required for
excitation. The exciting current on most lighting and power transformers
varies from approximately 10% on small sizes of about 1 KVA and less to
approximately 2% on larger sizes of 750 KVA.
19) Can a three phase transformer be loaded as a single
phase transformer?
- Yes, but the load cannot exceed the rating per phase and the
load must be balanced. (KVA/3 per phase)
- For example: A 75 kVA 3 phase transformer can be loaded up to 25 kVA on
each secondary. If you need a 30 kVA load, 10 kVA of load should be
supplied from each secondary.
20) What are taps and when are they used?
- Taps are provided
on some transformers on the high voltage winding to correct for high or
low voltage conditions, and still deliver full rated output voltages at the
secondary terminals.
- Standard tap
arrangements are at two-and-one-half and five percent of the rated primary
voltage for both high and low voltage conditions.
- For example, if the
transformer has a 480 volt primary and the available line voltage is
running at 504 volts, the primary should be connected to the 5% tap above
normal in order that the secondary voltage is maintained at the proper
rating.
21) What is the difference between “Insulating,” “Isolating,
“And “Shielded Winding” transformers?
- Insulating and isolating transformers are identical. These
terms are used to describe the isolation of the primary and secondary
windings, or insulation between the two.
- A shielded transformer is designed with a metallic
shield between the primary and secondary windings to attenuate transient
noise.
- This is especially important in critical applications such as
computers, process controllers and many other microprocessor controlled
devices.
- All two, three and four winding transformers are of the
insulating or isolating types. Only autotransformers, whose primary and
secondary are connected to each other electrically, are not of the
insulating or isolating variety.
22) Can transformers be operated at voltages other than
nameplate voltages?
- In some cases, transformers can be operated at voltages below
the nameplate rated voltage.
- In NO case should a transformer be operated at a
voltage in excess of its nameplate rating, unless taps are provided for
this purpose. When operating below the rated voltage, the KVA capacity is
reduced correspondingly.
- For example, if a 480 volt primary transformer with a 240
volt secondary is operated at 240 volts, the secondary voltage is reduced
to 120 volts. If the transformer was originally rated 10 KVA, the reduced
rating would be 5 KVA, or in direct proportion to the applied voltage.
23) Can a Single Phase Transformer be used on a Three Phase
source?
- Yes. Any single phase transformer can be used on a three phase
source by connecting the primary leads to any two wires of a three phase
system, regardless of whether the source is three phase 3-wire or three
phase 4-wire. The transformer output will be single phase.
24) Can Transformers develop Three Phase power from a Single
Phase source?
- No. Phase converters or phase shifting devices such as reactors
and capacitors are required to convert single phase power to three phases.
25) Can Single Phase Transformers be used for Three
Phase applications?
- Yes. Three phase transformers are sometimes not readily available
whereas single phase transformers can generally be found in stock.
- Three single phase transformers can be used in delta
connected primary and wye or delta connected secondary. They should never
be connected wye primary to wye secondary, since this will result in unstable
secondary voltage. The equivalent three phase capacity when properly
connected of three single phase transformers is three times the nameplate
rating of each single phase transformer. For example: Three 10 KVA single
phase transformers will accommodate a 30 KVA three phase load
26) Difference between Restricted Earth Fault &
Unrestricted Earth Fault protections?
- Restricted earth fault is normally given to on star connected
end of power equipment like generators, transformers etc. mostly on low
voltage side. For REF protection 4 no’s CTs are using one each on phase
and one in neutral. It is working on the principle of balanced currents
between phases and neutral. Unrestricted E/F protection working on the
principle of comparing the unbalance on the phases only. For REF
protection PX class CT are using but for UREF 5P20 CT’s using.
- For Differential Protection CTs using on both side HT &
LV side each phase, and comparing the unbalance current for this
protection also PX class CTs are using.
27) Can transformers be operated at voltages other than
nameplate voltages?
- In some cases, transformers can be operated at voltages below
the nameplate rated voltage. In NO case should a transformer be operated
in excess of its nameplate rating unless taps are provided for this
purpose. When operating below the rated voltage the KVA capacity is
reduced correspondingly.
28) How many types of cooling system it transformers?
- ONAN (oil natural, air natural)
- ONAF (oil natural, air forced)
- OFAF (oil forced, air forced)
- ODWF (oil direct, water forced)
- OFAN (oil forced, air natural)
29) What is the function of anti-pumping in circuit breaker?
- when breaker is close at one time by close push button, the anti-pumping
contactor prevent re close the breaker by close push button after if it
already close.
30) There are a Transformer and an induction machine. Those
two have the same supply. For which device the load current will be maximum?
- The motor has max
load current compare to that of transformer because the motor consumes
real power.. And the transformer is only producing the working flux and
it’s not consuming. Hence the load current in the transformer is because
of core loss so it is minimum.
31) Where the lighting arrestor should be placed in
distribution lines?
- Near distribution
transformers and outgoing feeders of 11kv and incoming feeder of 33kv and
near power transformers in sub-stations.
32) Why Delta Star Transformers are used for Lighting Loads?
- For lighting loads,
neutral conductor is must and hence the secondary must be star winding. And
this lighting load is always unbalanced in all three phases.
- To minimize the
current unbalance in the primary we use delta winding in the primary. So
delta / star transformer is used for lighting loads.
33) NGR grounded system vs. solidly grounded system
- In India, at low voltage level (433V) we must do only Solid
Earthing of the system neutral. This is by IE Rules 1956, Rule No. 61 (1)
(a).Because, if we have opt for impedance earthing, during an earth fault,
there will be appreciable voltage present between the faulted body &
the neutral, the magnitude of this voltage being determined by the fault
current magnitude and the impedance value.
- This voltage might circulate enough current in a person accidentally
coming in contact with the faulted equipment, as to harm his even causing
death. Note that, LV systems can be handled by non-technical persons too.
- In solid earthing, you do not have this problem, as at the
instant of an earth fault, the faulted phase goes to neutral potential and
the high fault current would invariably cause the Over current or short
circuit protection device to operate in sufficiently quick time before any
harm could be done.
34) Why do not We Break Neutral in AC Circuits?
- Neutral is connected to earth at some point, thus it has some
value as a return path in the event of say and equipment earth being
faulty. It’s a bit like asking ‘why don’t we break the Earth connection’
- It was stupid and dangerous, as it was possible for the
neutral fuse to blow; giving the appearance of ‘no power’ when in fact the
equipment was still live.
35) What are Minimum Value of Insulation Resistance /
Polarization Index?
- Motor Insulation Resistance:
- The acceptable meg-ohm value = motor KV rating value + 1 (For
LV and MV Motor).
- Example, for a 5 KV motor, the minimum phase to ground (motor
body) insulation is 5 + 1 = 6 meg-ohm.
- Panel Bus Insulation Resistance:
- The acceptable meg-ohm value = 2 x KV rating of the panel.
- Example, for a 5 KV panel, the minimum insulation is 2 x 5 =
10 meg-ohm
- IEEE 43 – INSULATION RESISTANCE AND POLARIZATION INDEX (min IR at 400C
in MΩ)
Minimum Insulation Resistance
|
TEST SPECIMEN
|
R1 min = kV+1 R1 min =
100
|
For most windings made
before about 1970, all field windings, and others not described below For
most dc armature and ac windings built after about 1970 (form wound coils)
|
R1 min = 5
|
For most machines with
random -wound stator coils and form-wound coils rated below 1kV
|
36) What is service factor?
- Service factor is the load that may be applied to a motor
without exceeding allowed ratings. For example, if a 10-hp motor has a
1.25 service factor; it will successfully deliver 12.5 hp (10 x 1.25)
without exceeding specified temperature rise. Note that when being driven
above its rated load in this manner, the motor must be supplied with rated
voltage and frequency.
- Keep in mind, however, that a 10-hp motor with a 1.25 service
factor is not a 12.5-hp motor. If the 10-hp motor is operated continuously
at 12.5 hp, its insulation life could be decreased by as much as
two-thirds of normal. If you need a 12.5-hp motor, buy one; service factor
should only be used for short-term overload conditions.
37) Calculate the size the CT on the neutral point of the
secondary side of 11/0.415 kV Transformer
- For high impedance relays (differential or restricted earth
fault relays), ‘Class X’ current transformers are recommended to be used.
- Please note that both CTs (neutral & phase) shall have
the same characteristics. The following is an example to size the CT:
- Input data: 11/0.415 kV ,2500 KVA Power transformer ,Transformer
impedance is 6% ,Length of cable from neutral CT to the relay is 200 m
,Cross section of CT cable to be used is 6 mm² -copper and resistance is
0.0032 Ω/m
- Step 1: Calculation of CT Rated Primary Current
- I = kVA/ (0.415×1.732) = 2500/ (0.415×1.732) = 3478.11 A, CT
with primary current of 4000 A to be selected.
- Select the secondary current of the CT 1 or 5 A. selecting
1 A secondary current, as the cross section and length of pilot wires can
have a significant effect on the required knee voltage of the CT and
therefore the size and cost of the CT. When the relay is located some
distance from the CT, the burden is increased by the resistance of the
pilot wires.
- Step 2: Calculation of maximum Fault Current
- Ift = kVA/ (0.415×1.732x Z)
- Ift = 2500/ (0.415×1.732×0.06) = 57968.59 A (say 58000 A)
- Step 3: Calculation of the Knee Voltage of the CT (Vkp)
- Vkp = (2x Iftx (Rct+Rw)/CT transformation ratio)
- Where: Rct is the CT resistance (to be given by the
manufacturer), Here Rct is1.02 Ω.
- Rw: total CT cable resistance= 2x cable length (200 m)
x wire resistance= 2x200x0.0032= 1.28 Ω
- CT transformation ratio = CT Primary Current/CT Secondary
Current
- CT transformation ratio = 4000/5= 800 A, for CT with 5 A
secondary current; or,
- CT transformation ratio = 4000/1= 4000 A, for CT with 1 A
secondary current. We will use 1 A in this example.
- Vkp = (2x58000x (1.02+1.28)/4000)= 66.7 V.
- The Vkp of the CT should be higher than the setting of relay
stability voltage (Vs), to ensure stability of the protection during
maximum Through fault current.
- To calculate the stability voltage, we should follow the
related formula given by the relay manufacturer, as each relay
manufacturer has its own formula.
- We may calculate the Vkp as above using a CT with secondary
current of 5 A, and you will notice the difference in the Vkp.
38) When should we use Molded Case Circuit Breakers and Mini
Circuit Breakers?
- MCB is Miniature Circuit Breaker, since it is miniature it
has limitation for Short Circuit Current and Amp Rating MCB:
- MCB are available as Singe module and used for :-
- Number of Pole :- 1,2,3,4 – 1+ N , & 3 + N
- Usually Current range for A.C. 50-60 HZ, is from 0.5 Amp – 63
Amp. Also available 80A, 100A, and 125 Amp.
- SC are limited 10 KA
- Applications are as: – Industrial, Commercial and Residential
application.
- Tripping Curve:
- (1) B Resistive and lighting load,
- (2) C Motor Load,
- (3) D Highly inductive load.
- MCCB:
- MCCB: – Moulded Case Circuit Breaker.
- MCCB are available as Singe module and used for :-
- Number of Pole :- 3 pole , & 4 Pole
- Current range for A.C:
- For 3.2 /6.3/12.5/25/50/100/125/160 Amp and Short Circuit
Capacity 25/35/65 KA.
- For 200 250 Amp and Short Circuit Capacity 25/35/65 KA
- For 400 630/800 Amp and Short Circuit Capacity 50 KA
- Protection release :
- Static Trip :- Continuous adjustable overload protection
range 50 to 100 % of the rated current Earth fault protection can be add
on with adjustable earth fault pick up setting 15 to 80 % of the current.
- Microprocessor Based release:
- Over load rated current 0.4 to1.0 in steps of o.1 of in trip
time at 600 % Ir (sec) 0.2.0.5,1, 1.5 , 2 ,3
- Short Circuit :-2 to10 in steps of 1 lr , short time delay
(sec) 0.02.0.05,0.1, 0.2 ,0.3
- Instantaneous pick up :2 to10 in steps of 1 in Ground fault
pick up Disable: 0.2 to 0.8 in steps of 0.1 of in Ground fault delay
(sec): 0.1 to 0.4 in steps of 0.1
- MCB (Miniature Circuit Breaker) Trip characteristics normally
not adjustable, factory set but in case of MCCB (Moulded Case Circuit
Breaker) Trip current field adjustable.
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