instrument transformers presentation2011-2
TRANSCRIPT
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Instrument Transformers
For currents greater than 100A and
voltages higher than 500V, it is difficult to
construct ammeters and current coils of
wattmeters, energy meters and relays
carrying alternating currents greater than
100A.
Specially designed transformers knownas instrument transformers are usedfor this purpose.
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Agenda
Current TransformersVoltage Transformers
Required Information for Specifying CTs & VTs
Take Home Rules for CTs & VTs
Applications
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Why use Instrument Transformers?
Circuit Isolation
Reduce voltage and currentsto reasonable working levels.
Phasor combinations for
summing and measuringpower
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CURRENT TRANSFORMERS
TYPES OF C.T. CONSTRUCTION
The most common type of C.T. construction is the
DOUGHNUT type. It is constructed of an iron toroid, which
forms the core of the transformer, and is wound with secondary
turns.
Secondary Winding Primary Conductor
Iron Core
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Transformer ratio (TR)
Primary Current
(100 amps)
Secondary
Current
(5 amps)
Primary Current
Secondary CurrentTransformer Ratio =___________________
__
100
5___= 100:5 or 20:1
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Polarity
Direction ofPrimary Current
Direction of
Secondary Current
H1
X1
Primary current into polarity =
Secondary current out ofpolarity
P1
IEEE
IEC
PrimaryPolarity
Marks
IEEE
IECS1
Secondary
Polarity
Marks
Remember:
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Polarity
Direction of
Primary Current
Direction of
Secondary Current
H1
X1
P1
IEEE
IEC
Primary
Polarity
Marks
IEEE
IECS1
Secondary
Polarity
Marks
Primary current into non-polarity =
Secondary current out ofnon-
olarity
Remember:
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Generator typical wiring
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CT Rating Factor (RF) -- IEEE
Rated current x (RF) =Maximum continuous current carrying
capability:
Without exceeding temperature limits
Without loss of published accuracy
class
Typical rating factors for Metering CTs are:
1.0, 1.33, 1.5, 2.0, 3.0, 4.0
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Short-Time Thermal Current Rating
One (1) second thermal ratingExpressed as value of RMS primary current
Main influencing factor:
CT primary & secondary wire size
Can be converted to thermal rating for any
time period (t) up to five (5) seconds:
(New RF at new Ambient/Stated RF at 30 degrees C)2=
85-New Ambient/New Ambient
Example: CT with rating factor of 4 at 30 degrees = rating factor of 2.95 at55 degrees
X2/42=85-55/55 X=2.95
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Instrument Transformers
The instrument transformers in a sub-
station are:
Current transformers:
Voltage transformer.
Capacitive voltage transformers (CVTs )
Voltage Transformers ( IVTS or PTs )
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Importance of CTs &PTs
Many protection systems are required to
operate during the period of transient
disturbance in the output of the
measuring transformers that follows asystem fault.
The errors in transformer output may
abnormally delay the operation of theprotection, or cause unnecessary
operations.
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Importance of CTs &PTs
Whenever the values of voltage orcurrent in a power circuit are too high topermit convenient direct connection of
measuring instruments or relays,coupling is made through transformers.
Such 'measuring transformers arerequired to produce a scaled downreplica of the input quantity to theaccuracy expected for the particularmeasurement
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To provide insulation between primaryand secondary circuit for equipment and
personnel safety
To change the magnitude (but not thenature) of the quality (voltage or current)
being measured to a suitable level for
use with standard instruments (protectiverelays, metering equipment, etc).
Basic Function of Instrument
Transformers
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Instrument Transformers (DO)
The quality of instrument transformers
willaffect directly the overall accuracyand performance of these metering
and monitoring systems.
Instrument transformer performance is
critical in protective relaying, since the
relays can only be as good as theinstrument transformers.
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Basic Function of Instrument
Transformers
By using instrument transformers,electrical instruments have beenstandardized to operate on 110V and 5A
or 1A. They are essential parts of many
electrical systems, and are used for
Measuring (metering) andMonitoring (relaying) devices.
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Normally both the above functions arecombined in one unit in such apparatusused in power systems. Hence the
general terminstrument transformers.
There are occasions where these areused exclusively for commercialmetering and in which case they arecalled Metering Transformers.
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Types of Instrument
Transformers
Instrument transformers are of twotypes, depending upon whether it is used
to excite the current or voltage coil of the
measuring instrument
Current Transformers- CTs
Voltage Transformers VTs (also
referred to as Potential Transformers,
PTs).
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Types of Instrument
Transformers (Contd)
Both of these types act as insulatorsbetween high-voltage primary and low-voltage secondary.
The ratio of primary to secondary voltageis in proportion to the turns of ratio and
will usually produce 110-120V at thesecondary terminals with rated primaryvoltage applied.
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Accuracy of Instrument
Transformers
To be a useful part of a measurement system,
instrument transformers must change the
magnitude of the quantity being measured
without introducing any excessive unknownerrors.
The accuracy of an instrument transformer
must either be of a known value, so that errors
may be allowed for, or the accuracy must besufficiently high that errors introduced by the
instrument transformer may be ignored.
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Factors Affecting Accuracy of
Instrument Transformers
Design of the instrument transformer
Circuit conditions such as voltage,current and frequency
Burden connected to the secondarycircuit of the transformer
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Burden of Instrument
Transformers
In instrument transformer operations, the
primary quantities are reduced by the
turns ratio to provide a secondary current
or voltage to energize protective relaysand other equipment.
The totality of the impedances of the
loads connected to current or voltagetransformers are referred to as burden.
B d f I
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Burden of Instrument
Transformers (Do)
The burden consists of the impedances
of the following:
Secondary winding of the instrument
transformer
Interconnecting leads
Relay and/or other connected devices.
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connections
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Current Transformers - CTs
Current Transformers are used whenever
the magnitude of the operating current
has to be reduced to the value for which
instruments, meters and protectivedevices are designed. At the same time
current transformers isolate metering and
protective devices from the systemvoltage.
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CTs
The essential requirement of a currenttransformers is to deliver on its secondarya quantity(current), which truly represents
the applied quantity on its primary. Thefailure of protective system to perform itsfunction correctly is due to incorrect
application of this transformers.
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CTs
The requirements of a protective currenttransformer are quite different from thatof a metering C.T. The metering C.T. is
only required to perform its functionover the normal range of load current,while the protective C.T is required to
give satisfactory protection over a widerange of fault conditions.
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CTs
Working ranges of CTs
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CTs Characteristics
The curve above describe a characteristic,
indicating three regions namely:
(i) Ankle point
(ii) Linear or straight line region(iii)Knee point
The working range of a metering C.T., isfrom the Ankle point to the Knee point and
slightly beyond it.
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CTs Characteristics
Thus the metering C.T., operates between 10%and 120% of the rated current and saturatesbeyond this in order to protect the metering
instruments.The working range of a protective C.T. extendsover the full range from the ankle point andbeyond. Generally the operating region of aprotective C.T. is beyond the knee point as it isrequired to operate at fault currents, which isseveral times the full load or rated current.
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CTs Characteristics
The knee point voltage of a metering C.T. isgenerally around 60 to 120V and is keptlow so as to protect meters.
The knee point voltages of protective C.T.saregenerally quite high varying from 200V to1900V depending upon the requirements of therelay. The upper limit of 1900V is specified
because the secondary cables from a C.T. aregenerally rated to withstand 2KV for about 1 or3 minutes and 660 volts or 1100 voltscontinuously.
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CTs Characteristics
The excitation voltages of metering and protective C.Ts is as follows:
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Theory of Current Transformers
The current transformer operates likeany other transformer in that the voltage
ratio and the reciprocal of the current ratio
are proportional to the turns ratio i.e.Ep = Np
Es NsWhere: p and s denote primary and secondary.
E VoltageI Current
N Number of turns
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Theory of Current Transformers
The primary winding is connected inseries with the load and it is the latter
which determines the current induced in
the secondary winding. The secondary is connected to a burden,
which does not vary, and the primary
current is not influenced by the magnitudeof the secondary burden.
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Theory of Current Transformers
The current in the secondary isdetermined by the current in the primary
winding.
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Phasor diagram of a C.T.
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Phasor diagram of a C.T.
The secondary currentIs lags behind thesecondary induced voltage, Es by an
angle . This angle is determined by the
impedance of the external burden and theimpedance of the secondary winding.
The primary currentIp is the resultant of -
Is andIo the exciting current. The exciting
currentIo consists of two components
namelyIcthe core-loss component andIm
the magnetizing component.
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Phasor diagram of a C.T.
The angle between Ip and (-Is) is thephase displacement error between theprimary and secondary currents. This
angle is expressed in minutes of arc.
The difference in lengths between Ip and
(-Is) is called the Ratio Error. When this
ratio error is expressed as a percentage of
the primary current Ip, it is calledPercentage Ratio Error.
Kn = Ip = N2 (the transformation ratio).
Is NI
Common definition of terms used
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Common definition of terms usedwith Current Transformers
Composite error C Composite error C is the difference
between the ideal secondary current and
the actual secondary current under steady-state conditions. It includes amplitude
(Ratio) and phase errors and also the
effects of any possible harmonics in theexciting current
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Common definitionThusEc= 1001o
t[(Kn is - ip)2dt]Ip T
Where
Ec is the composite errorT the time of one periodIp the rms value of the primary current in Ampsip the instantaneous value of the primary current in Amps.is the instantaneous value of the secondary current in
Amps.Kn the transformation ratio = Ip
Is
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Common definition
At rated frequency and with rated burden connected, theamplitude error and phase error and composite error of a CT shall
not exceed the values given in the following table.
Marking: The accuracy class of a CT is written after the rated power. E.g.
10 VA 5P10, 15 VA 10P10, 30 VA 5P20
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Common definition
Rated BurdenThis is the apparent resistance of the secondary circuit
expressed in ohms together with the power factor for
which the specified accuracy limits are valid.
Burden Zb = rb + jXb
Secondary winding impedance Zs = rs + jXs
Total secondary impedance Zt = Zb + Zs
= (rb + rs) + j(Xb+Xs)
Es =Is (Zb + Zs)
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Common definition
Rated OutputThe rated output of a current transformer is theapparent power expressed in VA together with thepower factor, which the C.T. can deliver to thesecondary circuit at rated current and burden while stillmaintaining its accuracy in the specified class.
The rated output is equal to the product of the ratedsecondary current and the voltage drop in the
external secondary circuit due to this current.The standardised values of rated outputs are 2.5, 5, 7.5,10, 15, 30, 45, 60, 90, & 120 in VA.
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Common definition
In BSS, the VA output is specified along
with the accuracy class. For example
30 5P 10 means a protection C.T. of
accuracy class s having a total error of5% with a VA of 30. The number 10 is
the ALF defined later However in IEC,
the VA is specified separately.
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Common definition
Accuracy Limit Factor (ALF) The accuracy limit current is the highest
primary current at which a current
transformer still meets the specifiedrequirements as regards total error. Theaccuracy limit factor is the ratio of theaccuracy limit current to the rated
primary current. The standardized accuracy limit factors
are 5, 10, 15, 20 and 30.
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Common definition
For example, as per IEC 5 p 20 means aC.T. for protection having maximumtotal error of 5% at 20 times the rated
current. Marking: Accuracy limit factor is written
after the accuracy class.
E.g. 10 VA 5P10, 15 VA 10P10, 30 VA5P20.
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Common definition
Instrument Security Factor (ISF)The rated instrument security factor is thesmallest primary current at which an
instrumentation core exhibits a currenterror of 10%.The Instrument Security Factor ISF is the
ratio of the rated instrument safety currentto rated primary current.
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Common definition
The instrument security factor defines thebehavior of a metering C.T. core underover-current conditions. The ISF is
specified to protect instruments connectedto the metering C.T. core from systemshort circuit currents. The ISF to bechosen should be as low as possible.It is expressed as a number n 5 or n10.
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Common definition
Knee point voltage (Vk)This is the sinusoidal e.m.f of ratedfrequency applied to the secondary
terminals of the C.T., with all otherwindings being open circuited, which whenincreased by 10% causes the exciting
current to increase by 50% or more. Thisis illustrated below:
f
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Common definition
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Common definition
ExampleV1= 100 VV2= 110 V
Percentage Increase = 10%
Corresponding currents C1= 0.35AC2= 0.7A
Percentage increase = 50%V is the knee point voltage Vk.
The knee point voltage indicates the voltage above which
the C.T. enters into saturation and exciting current
increases rapidly with a very little increase in voltage
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Common definition
The Vk is also limited by practicaldesign and manufacturing considerationas:
Vk = Rated output in VA x ALFSecondary rated current
d fi i i
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Common definition
Rated Short Time Thermal Current (Ith)This is the rms value of the primary current,which the C.T. will withstand for one secondwithout suffering any internal damage or otherharmful effects with the secondary being short-circuited.
This rating is for a very short time and it is
usually assumed that the entire heat generatedis stored in the primary winding itself.
C d fi i i
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Common definition
Rated short time thermal current isexpressed in KA. It is related to themaximum short circuit current at the
point of installation of the C.T., and alsoon the duration of the breaking time ofthe short circuit current
C d fi i i
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Common definition
The following condition should be met with
Ith Iscx[t + 0.05 x 50] KA rms.f
WhereIth- Rated short time thermal current for 1 sec.Isc- Short circuit current at C.T. location in KA rmst - short circuit duration in sec.f - Rated system frequency.
For system frequency of 50 Hertz
Ith Isc[t + 0.05] KA rms.
C d fi iti
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Common definition
Rated Dynamic Current (I dyn)It is the peak value of the primary current, whichthe transformer will withstand without
being damaged electrically or mechanically by theresulting electromagnetic forces, the secondarywinding being short- circuited.The maximum value of this current can be 2.5times the rated short time thermal current(Ith)
I dyn = 2.5 Ith
C d fi iti
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Common definition
Rated Primary and Secondary CurrentThese are the values of the primary and secondarycurrent on which the performance of the currenttransformer is based.
Standard values of primary currents are:5, 10, 15, 20, 30, 60, 75, 50, 100, 150, 200, 300, 400,600, 800, 1000, 1500, 2000, 3000, 4000 and above.
Standard values of secondary currents as per BS 3938are 5A, 2A and 1A and as per IEC, 5A or 1A. Howeverthere are cases where occasionally ratings of 0.577A,0.866A or 2.87A have been used.
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Tips on CT selection
The selection of the primary current of a C.T. shall always beadopted as closely as possible to the full load or rated current of theinstallation by rounding off to the next higher standard. However theC.T must be capable of continuously carrying the maximum expectedcurrent in service. It is advisable to consider a permitted overload of20% of the full load current while deciding the rated current.
Another factor to be considered is also the load growth and theincrease in capacity of an installation. It is for this reasonthat multi ratio primary currents are adopted like 800- 400 - 200 - 100 A.
The selection of the secondary current depends upon the secondarycurrent of the equipment already in service where interchangeability isa consideration.
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The following are the advantages and disadvantages ofCTs with 5A and 1A secondary currents.
The number of turns required on the secondary side is less
for a 5A C.T. than for a 1A C.T. for a given primary current.
A thicker gauge wire is required for a 5A C.T than for a 1AC.T.
Both the above factors contribute to the cost reduction of a
5A C.T. when compared to a 1A C.T.
Since the number of turns is less for a 5A C.T, the voltage
induced on the secondary side during secondary saturation
or secondary open circuit is less when compared to a 1A
C.T.
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The lead burden, however, becomes excessive for a 5A C.Tsince the same is proportional to the product of the square
of the current and resistance of the lead wire. The lead
burden in a 1A CT. will be very low.
In view of the reduced number of secondary turns in a 5AC.T., it is difficult to provide for turns compensation to
design and manufacture low current higher accuracy class
CTs. However in a 1A C.T. it is possible to achieve
the desired accuracy class because of the increased
number of turns and by providing compensating turns. The internal resistance of a 5A C.T. is comparatively less (
1 ohm) when compared to that of 1A C.T. (Generally 3 to
12 ohms)
E l
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Examples
A case study of Estimation of Burden, Knee PointVoltage, Accuracy Class etc of a Protective CurrentTransformer
Requirement of a C.T. to protect a 15 MVA, 132/33 KVDelta/Star connected transformer.
Data available
% Impedance of Transformer = 10Fault level at 132KV side = 1400 MVA
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Examples
Transformer full load current per phase= 15 x 106_____
3 x 132 x 103= 65.61 A
Hence select primary current = 100 A
i.e. Ip = 100 A
Select secondary current Is as 5A. A 5A C.T secondary
has a winding resistant of less than 1.0 ohm. A typical
value may be chosen as 0.601 ohms.
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Examples
Assume(a)Distance from C.T to Relay control panel as 100 metres and
C.T. secondary leads of 10 sq mm. (RL = 0.1627 ohms for 100
metres)
(b)Connected relays are GEC CDG 11 over-current and earthfault relays with VA burden of 1.8 and 4 respectively.
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Examples
Relay burden =IS2RS + 2IS
2RL + VA of (OCR + EFR)
=(5) 20.601 + 2(5) 20.1627 + (1.8 + 4)
= 15.0 + 8.135 + 5.8= 28.935 VA
Hence select relay burden or output as 30 VA
Select Accuracyclass 5 P 20
Examples
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Examples
Knee point voltage
Vk = VA x ALF__
Sec. current
= 30 x 20
5
= 6005
= 120 V
Examples
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Examples
Fault current at C.T. installation = 1400 x 106
___3 x 132 x 103
= 6123.6 A
or 6.124 KA = Isc
IthIsc[t + 0.05] KA rms for 1 sec
Assume operating time of breakers, relays etc = 1 sec
Ith6.124[1.05]6.275 KA rms
Select Ithas 10 KA rms for 1 sec
Examples
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Examples
Ith short time rating = 10 KA rms for 1 sec.
Idyn = 2.5 Ith =2.5 x 10 = 25 KA for 1 sec.
Hence complete specifications for this protection C T will be:
Voltage class: 132 KVPrimary current: 100 A
Highest System Voltage: 145 KV
Secondary current: 5 A
Accuracy class: 5 P 20
Vk: 120 V.Ith: 10 KA rms for 1 sec.
Idyn: 25 KA for 1 sec.
Polarity and Markings
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Polarity and Markings
Polarity and Markings
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Polarity and Markings
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Voltage Transformers
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Voltage Transformers
Types of VTs
Electromagnetic VT
Capacitive VT
Types of VTs for Protective
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Relaying.
Voltage transformers have woundprimaries that are
Either connected directly to the power
systems (VTs)
Or across a selection of capacitor string
connected between phase and ground,
that is, coupling-capacitor voltagetransformers (CCVTs)
VTs For Relay Applications
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VTs For Relay Applications
Voltage transformers, which step downsystem voltages to sufficiently low, safer,
measurable values, are required for
Indication of the voltage conditions. Energy meters and watt meters (kWh
and kW meters)
Protective relays Synchronizing
Points To Note About VTs
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Points To Note About VTs
VTs are used at all power system voltages,andare usually connected to the bus.
Usually the CCTVs are connected to the line,
rather than to the bus, because the couplingcapacitor devices may also be used to couple
radio frequencies to the line for use in pilot
relaying
At about 115kV, the CCVT types becomesapplicable and generally more economical
than VTs at the higher voltages.
Points to Note About VTs (Do)
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Points to Note About VTs (Do)
Either type of voltage transformer (VT orCCVT) provides excellent reproduction ofprimary voltage, both transient and steady-state, for protection functions.
Saturation is not a problem because powersystems should not be operated above normalvoltage, and faults result in a collapse orreduction in voltage.
VTs are normally installed with primary fuses,which are not necessary with CCTVs. Fusesare also used in the secondary.
Voltage Transformers
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Voltage Transformers
HV and EHV Capacitor-coupled VT (CVT)
C1 & C2 are adjusted, so that a few kVs of voltage is obtains
across C2
Then, stepped down by T
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Example: To calculate the capacitance requirements for aCVT to be used on a 132KV system.
Let (1) Total capacitance of capacitor be 20,000pF
(2) Burden requirement 100 VA
(3) Magnetic transformer designed for a standard primary
voltage of 10/3 KV
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C1 = E2C2 E1
= 10/3_________
132/3 10/3
C1 = _10____ x C2 = 10_ x C2
132 10 122
or C2 = 122 C1
10
Also 1_ + 1 = 1C1 C2 C
Or C = C1C2_
C1 + C2
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Substituting for C2 in the above eqn.
C = C1 x 122 C1
10____
C1 + 122 C1
10
= C12 x 122
10____10C1 + 122C1
10
= 122 C1
132
C1 = 132 C
122= 132 x 20000 = 21639.34pF
122
C2 = 122 x 21639.34 = 264000pF
10
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