Electrical Questions and Answers
1) What
is the difference between earth resistance and earth electrode resistance?
20)
Why We use of Stones / Gravel in electrical Switch Yard
39)
How to measure Transformer Impedance?
- Only one of the terminals is evident in
the earth resistance. In order to find the second terminal we should
recourse to its definition:
- The resistance of the electrode has the
following components:
(A) the resistance of the metal and that of the connection to it.
(B) The contact resistance of the surrounding earth to the electrode. - Earth Resistance is the resistance
existing between the electrically accessible part of a buried electrode
and another point of the earth, which is far away.
- 4-20 mA is a standard range used to
indicate measured values for any process. The reason that 4ma is chosen instead
of 0 mA is for fail safe operation.
- For example: A pressure instrument gives
output 4mA to indicate 0 psi up to 20 mA to indicate 100 psi or full
scale. Due to any reason, if instrument broken wire, its output reduces to
0 mA. So if range is 0-20 mA then we can differentiate whether it is due
to broken wire or due to 0 psi.
- Since two bulbs are in series they will
get equal amount of electrical current but as the supply voltage is
constant across the Bulb (P=V^2/R).So the resistance of 60W bulb is
greater and voltage across 60W is more (V=IR) 60W bulb will glow brighter.
- Bulbs or devices for AC are designed to
operate such that it offers high impedance to AC supply. Normally they
have low resistance. When DC supply is applied, due to low resistance, the
current through lamp would be so high that it
- Mainly transformer has high inductance
and low resistance. In case of DC supply there is no inductance, only
resistance will act in the electrical circuit. So high electrical
current will flow through primary side of the transformer. So for this
reason coil and insulation will burn out
- When AC current flow to primary winding
it induced alternating flux which also link to secondary winding so
secondary current flow in secondary winding according to primary
current.Secondary current also induced emf (Back emf) in secondary winding
which oppose induced emf of primary winding and thus control primary
current also.
- If DC current apply to Primary winding
than alternating flux is not produced so no secondary EMF induced in
secondary winding so primary current may goes high and burn
transformer winding.
- Megger used to measure cable resistance,
conductor continuity, phase identification where as contact resistance
meter used to measure low resistance like relays, contactors.
- We connect capacitor bank in series to
improve the voltage profile at the load end in transmission line there is
considerable voltage drop along the transmission line due to impedance of
the line. so in order to bring the voltage at the load terminals within
its limits i.e (+ or – % 6 )of the rated terminal voltage the capacitor
bank is used in series
- Diversity factor is the ratio of the sum
of the individual maximum demands of the various subdivisions of a system,
or part of a system, to the maximum demand of the whole system, or part of
the system, under consideration. Diversity factor is usually more than
one.
- This sound is coming due to ionization
(breakdown of air into charged particles) of air around transmission
conductor. This effect is called as Corona effect, and it is considered as
power loss.
- We can have the frequency at any
frequency we like, but then we must also make our own motors, transformers
or any other equipment we want to use.
- We maintain the frequency at 50 Hz or 60hz because the world maintains a standard at 50 /60hz and the equipments are made to operate at these frequency.
- Generally capacitor gives infinite
resistance to dc components (i.e., block the dc components). It allows the
ac components to pass through.
- 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.
13) Why ELCB cannot work when Neutral input of ELCB is not connected to ground?
- ELCB is used to detect earth leakage
fault. Once the phase and neutral are connected in an ELCB, the current
will flow through phase and that same current will have to return neutral
so resultant current is zero.
- Once there is a ground fault in the load
side, current from phase will directly pass through earth and it will not
return through neutral through ELCB. That means once side current is going
and not returning and hence because of this difference in current ELCB
will trip and it will safe guard the other circuits from faulty loads. If
the neutral is not grounded fault current will definitely high and that
full fault current will come back through ELCB, and there will be no
difference in current.
- MCB is miniature circuit breaker, which
is thermal operated, and use for short circuit protection in small current
rating circuit.
- Normally it is used where normal current
is less than 100A.
- MCCB molded case circuit breaker and is
thermal operated for over load current and magnetic operation for instant
trip in short circuit condition. Under voltage and under frequency may be
inbuilt.
- Normally it is used where normal current
is more than 100A.
- 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.
- The main advantage of using the star
delta starter is reduction of current during the starting of the motor. Starting
current is reduced to 3-4 times of current of Direct online starting
Hence the starting current is reduced , the voltage drops during the
starting of motor in systems are reduced.
- HRC stand for “High Rupturing Capacity”
fuse and it is used in distribution system for electrical transformers
- The different methods of starting an
induction motor
- DOL: direct online starter
- Star delta starter
- Auto transformer starter
- Resistance starter
- Series reactor starter
- Vaiable Frequency drive
- Soft Starter
- 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.
However 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
- Reducing Step and Touch potentials
during Short Circuit Faults
- Eliminates the growth of weeds and small
plants in the yard
- Improves yard working condition
- Protects from fire, which cause due to
oil spillage from transformer and also protects from wild habitat.
- The miss concept is Line voltage is in
multiple of 11 due to Form Factor. 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 1.11.
- The Main reason is something historical.
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. This is the reason. 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. Now a days when, we have voltage correction systems,
power factor improving capacitors, which can boost/correct voltage to
desired level, we are using the exact voltages like 400KV in spite of
444KV
- Determine primary voltage and frequency.
- Determine secondary voltage required.
- Determine the capacity required in
volt-amperes. This is done by multiplying the load current (amperes) by
the load voltage (volts) for single phase.
- For example: if the load is 40 amperes,
such as a motor, and the secondary voltage is 240 volts, then 240 x 40
equals 9600 VA. A 10 KVA (10,000 volt-amperes) transformer is required.
- Always select Transformer Larger than
Actual Load. This is done for safety purposes and allows for expansion, in
case more loads is added at a later date. For 3 phase KVA, multiply rated
volts x load amps x 1.73 (square root of 3) then divide by 1000.
- Determine whether taps are required.
Taps are usually specified on larger transformers.
- Industrial control equipment demands a
momentary overload capacity of three to eight times’ normal capacity. This
is most prevalent in solenoid or magnetic contactor applications where
inrush currents can be three to eight times as high as normal sealed or
holding currents but still maintain normal voltage at this momentary
overloaded condition.
- Distribution transformers are designed
for good regulation up to 100 percent loading, but their output voltage
will drop rapidly on momentary overloads of this type making them
unsuitable for high inrush applications.
- Industrial control transformers are
designed especially for maintaining a high degree of regulation even at
eight time’s normal load. This results in a larger and generally more
expensive transformer.
- Transformers can be used at frequencies
above 60 Hz up through 400 Hz with no limitations provided nameplate
voltages are not exceeded.
- However, 60 Hz transformers will
have less voltage regulation at 400 Hz than 60 Hz.
- Voltage regulation in transformers is
the difference between the no load voltage and the full load voltage. This
is usually expressed in terms of percentage.
- For example: A transformer delivers 100
volts at no load and the voltage drops to 95 volts at full load, the
regulation would be 5%. Distribution transformers generally have
regulation from 2% to 4%, depending on the size and the application for
which they are used.
- It is used for determining the
interrupting capacity of a circuit breaker or fuse employed to protect the
primary of a transformer.
- Example: Determine a minimum
circuit breaker trip rating and interrupting capacity for a 10 KVA single
phase transformer with 4% impedance, to be operated from a 480 volt 60 Hz
source.
- Calculate:
- Normal Full Load Current = Nameplate
Volt Amps / Line Volts = 10,000 VA / 480 V = 20.8 Amperes
- Maximum Short Circuit Amps = Full Load
Amps / 4% =20.8 Amps / 4%= 520 Amp
- The breaker or fuse would have a minimum
interrupting rating of 520 amps at 480 volts.
- Example: Determine the
interrupting capacity, in amperes, of a circuit breaker or fuse required
for a 75 KVA, three phase transformer, with a primary of 480 volts delta
and secondary of 208Y/120 volts. The transformer impedance (Z) = 5%. If
the secondary is short circuited (faulted), the following capacities are
required:
- Normal Full Load Current =Volt Amps / √
3 x Line Volts= 75,000 VA / √ 3 x Line Volts √ 3 x 480 V =90 Amps
- Maximum Short Circuit Line Current =
Full Load Amps / 5%= 90 Amps / 5% =1,800 Amps
- The breaker or fuse would have a minimum
interrupting rating of 1,800 amps at 480 volts.
- Note: The secondary voltage is not used in the
calculation. The reason is the primary circuit of the transformer is the
only winding being interrupted.
- Flash-over causes are not always easily
explained, can be cumulative or stepping stone like, and usually result in
an outage and destruction. The first flash-over components are available
voltage and the configuration of the energized parts, corona may be
present in many areas where the flash-over occurs, and flash-over can be
excited by stepping stone defects in the insulating path.
- 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.
Taps are generally set at two and a half and five percent above and below
the rated primary voltage. (There are two types Online and Offline tap
changer)
- Insulating and isolating transformers
are identical. These terms are used to describe the separation of the
primary and secondary windings. A shielded transformer includes a metallic
shield between the primary and secondary windings to attenuate (lessen)
transient noise.
- A transformer is an electrical apparatus
designed to convert alternating current from one voltage to another. It
can be designed to “step up” or “step down” voltages and works on the
magnetic induction principle.
- A transformer has no moving parts and is
a completely static solid state device, which insures, under normal
operating conditions, a long and trouble-free life. It consists, in its
simplest form, of two or more coils of insulated wire wound on a laminated
steel core.
- When voltage is introduced to one coil,
called the primary, it magnetizes the iron core. A voltage is then induced
in the other coil, called the secondary or output coil. The change of
voltage (or voltage ratio) between the primary and secondary depends on
the turns ratio of the two coils.
- 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 — that is, when tested the
terminals of the high voltage and low voltage windings on the left hand
side are connected together, refer to diagram below. 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.
- Boucholz relay is a device which is used
for the protection of transformer from its internal faults,
- it is a gas based relay. whenever any
internal fault occurs in a transformer, the boucholz relay at once gives a
horn for some time, if the transformer is isolated from the circuit then
it stop its sound itself otherwise it trips the circuit by its own
tripping mechanism.
- The two types of earthing are Familiar
as Equipment earthing and system earthing.
- In Equipment earthing: body (non
conducting part) of the equipment should be earthed to safeguard the human
beings.
- The System Earthing : In this neutral of
the supply source ( Transformer or Generator) should be grounded. With
this, in case of unbalanced loading neutral will not be shifted. So that
unbalanced voltages will not arise. We can protect the equipment also.
With size of the equipment ( transformer or alternator)and selection of
relying system earthing will be further classified into directly earthed,
- NEUTRAL is the origin of all current
flow. In a poly-phase system, as its phase relationship with all the three
phases is the same, (i.e.) as it is not biased towards any one phase, thus
remaining neutral, that’s why it is called neutral.
- Whereas, GROUND is the EARTH on which we
stand. It was perceived to utilize this vast, omnipresent conductor of
electricity, in case of fault, so that the fault current returns to the
source neutral through this conductor given by nature which is available
free of cost. If earth is not used for this purpose, then one has to lay a
long. long metallic conductor for the purpose, thus increasing the cost.
- Ground should never be
used as neutral. The protection devices (eg ELCB, RCD etc) work basically
on principle that the phase currects are balanced with neutral current. In
case you use ground wire as the neutral, these are bound to trip if they
are there – and they must be there. at least at substations. And these are
kept very sensitive i.e. even minute currents are supposed to trip these.
- One aspect is safety – when someone
touches a neutral, you don’t want him to be electrocuted – do you? Usually
if you see the switches at home are on the phase and not neutral (except
at the MCB stage). Any one assumes the once the switch is off, it is safe
(the safety is taken care of in 3 wire system, but again most of the
fixtures are on 2 wire) – he will be shocked at the accidental
touching of wire in case the floating neutral is floating too much.
- If you mean the percentage impedance of
the transformed it means the ratio of the voltage( that if you applied it
to one side of the transformer while the other side of the transformer is
short circuited, a full load current shall flow in the short circuits
side), to the full load current.
- More the %Z of transformer, more Copper
used for winding, increasing cost of the unit. But short circuit levels
will reduce; mechanical damages to windings during short circuit shall
also reduce. However, cost increases significantly with increase in %Z.
- Lower %Z means economical designs. But
short circuit fault levels shall increase tremendously, damaging the
winding & core.
- The high value of %Z helps to reduce
short circuit current but it causes more voltage dip for motor starting
and more voltage regulation (% change of voltage variation) from no load
to full load.
- The minimum transformer KVA rating
required to operate a motor is calculated as follows:
- Minimum Transformer KVA =Running Load
Amperes x 1.73x Motor Operating Voltage / 1000
- If motor is to be started more than once
per hour add 20% additional KVA. Care should be exercised in sizing a
transformer for an induction type squirrel cage motor as when it is
started, the lock rotor amperage is approximately 5 to 7 times the running
load amperage. This severe starting overload will result in a drop of the
transformer output voltage.
- When the voltage is low the torque and the
horsepower of the motor will drop proportionately to the square of the
voltage.
- For example: If the voltage were
to drop to 70% of nominal, then motor horsepower and torque would drop to
70 % squared or 49% of the motor nameplate rating.
- If the motor is used for starting a high
torque load, the motor may stay at approximately 50% of normal running
speed The underlying problem is low voltage at the motor terminals. If the
ampere rating of the motor and transformer over current device falls
within the motor’s 50% RPM draw requirements, a problem is likely to
develop. The over current device may not open under intermediate motor
ampere loading conditions.
- Overheating of the motor and/or
transformer would occur, possibly causing failure of either component.
- This condition is more pronounced when
one transformer is used to power one motor and the running amperes of the
motor is in the vicinity of the full load ampere rating of the
transformer. The following precautions should be followed:
- (1)When one transformer is used to operate
one motor, the running amperes of the motor should not exceed 65% of the
transformer’s full load ampere rating.
- (2) If several motors are being operated
from one transformer, avoid having all motors start at the same time. If
this is impractical, then size the transformer so that the total running
current does not exceed 65% of the transformer’s full load ampere rating.
- The following points need to check
before going for Neutral Grounding Resistance.
- Fault current passing through ground,
step and touch potential.
- Capacity of transformer to sustain
ground fault current, w.r.t winding, core burning.
- Relay co-ordination and fault clearing
time.
- Standard practice of limiting earth
fault current. In case no data or calculation is possible, go for limiting
E/F current to 300A or 500A, depending on sensivity of relay.
- 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.
- LA is installed outside and the effect
of lightning is grounded, where as surge arrestor installed inside panels
comprising of resistors which consumes the energy and nullify the effect
of surge.
- Transmission Line Lightning Protection:
- The transmission line towers would
normally be higher than a substation structure, unless you have a
multi-storey 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:
- 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 metres, with the electrode some 2 to 3 metres 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)
- 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).
- 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 independant 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,
- 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.
- 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.
- 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.
- , are not of the insulating or isolating
variety.
- 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.
- 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.
- No. Phase converters or phase shifting
devices such as reactors and capacitors are required to convert single
phase power to three phases.
- 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
- Near distribution transformers and out
going feeders of 11kv and incoming feeder of 33kv and near power
transformers in sub-stations.
- 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.
- 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.
- Micro processor 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.
- with a ground path, in case of short
circuit the short circuit current goes to the body of the equipment &
then to the ground through the ground wire. Hence if at the moment of
fault if a person touches the equipment body he will not get a shock cause
his body resistance (in thousands of ohms) will offer a high resistance
path in comparison to the ground wire. Hence the fault current will flow
thru the ground wire & not thru human body.
- Providing a ground path helps in
clearing the fault. A CT in the ground connection detects the high value
fault current hence the relay connected to the CT gives breaker a trip
command.
- Grounding helps in avoiding arcing
faults. IF there would have been no ground then a fault with the outer
body can cause a arcing to the ground by breaking the air. This is
dangerous both for the equipment & the human beings.
- Distribution Transformers are designed
for a maximum efficiency at 50% of load. Whereas power transformers are
designed to deliver max efficiency ay 90% and above loads.
- The distributions transformers have low
impedance so as to have a better regulation power transformers have higher
so as to limit the SC current.
- Power transformers are used to step up
voltages from 11 KV which is the generating voltage to 132 or whatever
will be the transmission voltage levels. Power transformers are having
Star-Delta connection. It will be located at power generating stations.
- Distribution transformers are used to
step down voltages from transformer levels to 11 KV/415 V. Will be having
Delta-Star. It will be located in substations near load centers.
- The main basic difference lies in the
Design stage itself as power transformer are to operate at near full load
so there sensing is such that they achieve equal. of copper losses &
iron losses at full loads whereas this is achieved in the design itself at
about 50% loading in dist transformer but friends there is a dilemma as
our dist. transformer are almost fully loaded & beyond so they never
go operate at their full eff. & also poor voltage regulation.
- The difference between power and
distribution transformers refers to size & input voltage. Distribution
transformers vary between 25 kVA and 10 MVA, with input voltage between 1
and 36 kV. Power transformers are typically units from 5 to 500 MVA, with
input voltage above 36 kV. Distribution transformer design to have a max
efficiency at a load lower than full load. Power transformer design to
have a max efficiency at full load
- During normal condition the neutral
isolating switch should be kept close. In case it is kept open, under
balanced load conditions the current through neutral will not flow &
nothing harmful will take place but in case an earth fault takes place
then there will be no earth fault current flowing through the system &
the generator will run as a ungrounded generator. Thus the earth fault
will not be cleared.
- If more number of generators are
connected parallel. We will have a close loop and hence negative sequence
current will flow. This will increase the rotor temperature. Hence if more
number of generators are connected then only one is earthed and others are
open.
- In case of Two or more generators
connected to a common bus without a transformer in between, basically in
hydro stations, one of the Neutral Isolation Switch(NIS) is kept closed
& rest are opened to prevent circulating currents to flow between
generators. Hence the above explanation will not be valid for such
systems.
- Sometime we may want to test generator
and may want to isolate the neutral from ground. like for example
meggaring etc. In such case we would like to open ground connection cable
in case we want to remove the NIS? we will certainly not like to open all
the bolted connections for just a small test like checking insulation with
a meggar etc. for such things we need a NIS.
- Neutral isolator is required if we have
delta transmission system and at the time to connection with the Grid
Neutral isolation is required.
- If we ungrounded the neutral then the
generator is connected to the ground via Phase to earth capacitances.
Hence during faults arcing grounds can take place. Which are dangerous
both to human & equipment.
- When we provide earthed neutral, for a
fault, earth fault current will start flowing through the neutral, which
we can sense thru a CT & relay & hence can immediately identify
& clear the fault in about 100 ms by opening the associated
breaker/prime mover/excitation. Quicker the fault clearance less is the
damage.
- During maintenance or secondary
injection you will need to bypass the CT & for the same you need
shorting link. During sec. injection you will short circuit the main CT
& bypass it. Open circuiting the CT will saturate it & damage it.
- Fuse if given for CT blows off due to a
fault then rather than protecting the CT it will make it open circuited
hence it will be saturated & damaged. For PT it gives overload &
SC protection.
- The LA is provided with a dedicated
Prper earthing which may be in the form of a buried treated electrode next
to it.LA connection is securely made
with the electrode via a surge counter. If we directly earth the LA
through structure then the surge counter will not be able to measure the
no of surges. For lesser rating the counter is not provided, hence we can
bypass the insulated base. But then proper earthing has to be assured.
- 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 Cts 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.
- 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.
- 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)
- When breaker is close at one time by
close push button, the anti pumping contactor prevents re close the
breaker by close push button after if it already closed.
- Transformers 1 KVA and larger, rated at
60 Hz, should not be used on 50 Hz service due to higher losses and
resultant heat rise. However, any 50 Hz transformer will operate on 60 Hz
service.
- Single phase transformers can be used in
parallel only when their voltages are equal. If unequal voltages are used,
a circulating current exists in the closed network between the two
transformers which will cause excess heating and result in a shorter life
of the transformer. In addition impedance values of each transformer must
be within 7.5% of each other.
- Dry type distribution transformers can
be reverse connected without a loss of KVA rating, but there are certain
limitations. Transformers rated 1 KVA and larger single phase, 3 KVA and
larger three phases can be reverse connected without any adverse effects
or loss in KVA capacity.
- Basically during welding we force a
short-circuit at the electrode tip. The fault condition produces large
magnitude currents. Greater the Current value have greater I2R heat
produced. The arcing energy elevates the temperature & hence melts the
electrode material over the joint.
- The transformer is designed to withstand
such high currents. But welding is a very complex & detailed
phenomenon. Besides there are many principles on which welding operates.
Some may be a welding, dc welding, arc, constant voltage, constant current
etc
- Breaker is one which disconnects the
circuit in fault condition and It is similar for all equipment. Based on
the equipment voltage and maximum short circuit current the ratings will
be decided. For better understanding we call generator or transformer or
line etc breakers.
- Generally the class indicates the
accuracy with which the meter will indicate or equipment will measure with
respect to its input.
- The accuracy of different equipment will
depend on number of factors.
- For example for a PT accuracy will
depend on its leakage reactance & winding resistance. For a PT
accuracy gives the voltage & phase error & it varies with the VA
burden of secondary. Also better core material will give better heat
dissipation & reduce error. class of accuracy will give the voltage
error for a PT
- different type of PTs available are:0.1,
0.2, 0.5, 1, 5 & error values will be: class% voltage error(+/_) phase
displacement
Similarly indicating instruments shall have accuracies & accordingly application as depicted below for testing the following values are generally used: - for routine tests : accuracy class 1
- for type tests : accuracy class 0.5 or
better.
- indicating meters generally will have
accuracy of 1.
- Once the current is obtained we can then
calculate the electrostatic KVA from the current multiplied with voltage.
- reason we have seen only 23/24 discs in
400 kv line cause in that case the creep age obtained must have been
enough & also the strain requirement.
- 33kv insulators are generally used in a
vertical installation & are not stacked together because that will
make the suspension very rigid
- Taps are normally in the primary winding
to adjust for varying incoming voltage. If the transformer is reverse fed,
the taps are on the output side and can be used to adjust the output
voltage.
- Transformer terminals are marked
according to high and low voltage connections. An H terminal signifies a
high voltage connection while an X terminal signifies a lower voltage
connection. A common misconception is that H terminals are primary and X
terminals secondary. This is true for step down transformers, but in a
step up transformer the connections should be reversed. Low voltage
primary would connect to X terminals while high voltage secondary would
connect on the H terminals.
- The voltage rating of LA is selected as:
Line voltage x sqrt(2)/ sqrt(3) so for 11kV line its 9kV
- In that case also the values would not
differ much if We takes the TOV factor as 1.4. However, we can take the
value of 1.56 as TOV to be more precise.
- Low frequency (50 – 60 Hz) AC currents
can be more dangerous than similar levels of DC current since the
alternating fluctuations can cause the heart to lose coordination,
inducing ventricular fibrillation, which then rapidly leads to death.
- However any practical distribution
system will use voltage levels quite sufficient to ensure a dangerous
amount of current will flow, whether it uses alternating or direct
current. Since the precautions against electrocution are similar,
ultimately, the advantages of AC power transmission outweighed this
theoretical risk, and it was eventually adopted as the standard.
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