Post on 24-Oct-2014
A SEMINAR REPORT ON
LESS KNOWN FACTS ABOUT CT's & PT'S
References:
http://encyclopedia2.thefreedictionary.com/Instrument+Transformer
http://en.wikipedia.org/wiki/Current_transformer
http://www.paranaelectrotech.com/technicalliterature/articleonferro-resonance.php
http://www.powerelectricalblog.com/2007/03/ferro-resonance-introductionclassificat.html
Instrument Transformer :
In electrical engineering, a current transformer (CT) is used for measurement of electric currents.
Current transformers, together with voltage transformers (VT) (potential transformers (PT)), are known
as instrument transformers. When current in a circuit is too high to directly apply to measuring
instruments, a current transformer produces a reduced current accurately proportional to the current in
the circuit, which can be conveniently connected to measuring and recording instruments. A current
transformer also isolates the measuring instruments from what may be very high voltage in the
monitored circuit. Current transformers are commonly used in metering and protective relays in the
electrical power industry.
an electrical transformer in which the current or voltage being measured acts on the primary
winding of the transformer; the secondary (step-down) winding is connected to measuring
instruments and protective relays. Instrument transformers are used primarily in power
switchboards and in high-voltage AC circuits to ensure safety in measuring current intensity,
voltage, power, and energy. One of the terminals of the secondary winding is grounded, as a
protective measure in cases of insulation breakdown on the high-voltage side. Instrument
transformers make possible the measurement of various magnitudes of electrical quantities with
devices whose range of measurement extends to 100 watts (W) and 5 amperes (A).
A distinction is made between instrument potential (used with voltmeters, frequency meters,
parallel circuits of wattmeter's, energy meters, phase meters, and voltage relays) and instrument
current transformers (used with ammeters, series circuits of wattmeter's, energy meters, phase
meters, and current relays). Connection diagrams of instrument transformers in electric circuits
are shown in Figures 1 and 2.
Figure 1. Connection diagram of an instrument potential transformer
Figure 2. Connection diagram of an instrument CT
In an instrument potential transformer (Figure 1), the voltage U1 being measured is fed to the
terminals of the primary winding; the winding W1 is connected in parallel with the load. A
secondary voltage U2 is fed from the winding W2 to a voltmeter or to the voltage circuits of
measuring instruments and protective relays. The accuracy of measurement is defined by a
percentage error, which determines the accuracy of reproduction for the amplitude of the voltage
being measured, and by the angle error in degrees. The angle error is equal to the angle between
the vector of primary voltage and to the vector of secondary voltage, rotated by 180°; it
determines the accuracy of phase reproduction. Most instrument voltage transformers for high
voltages are manufactured in a sectionalized, oil-filled design.
The primary winding W1 of an instrument current transformer (Figure 2) is connected in series
with the control circuit, which carries an alternating current I1 the secondary winding W2 is
connected in series with an ammeter or other measuring instrument. The accuracy of an
instrument current transformer is defined by a percentage ratio between the difference of the
value of the reduced secondary current and the value of the actual primary current to the value of
the actual primary current.
Fig 3. common use of instrument transformers
Constructional Features CT:
Like any other transformer, a current transformer has a primary winding, a magnetic core, and a
secondary winding. The alternating current flowing in the primary produces a magnetic field in
the core, which then induces a current in the secondary winding circuit. A primary objective of
current transformer design is to ensure that the primary and secondary circuits are efficiently
coupled, so that the secondary current bears an accurate relationship to the primary current.
The most common design of CT consists of a length of wire wrapped many times around a
silicon steel ring passed over the circuit being measured. The CT's primary circuit therefore
consists of a single 'turn' of conductor, with a secondary of many tens or hundreds of turns. The
primary winding may be a permanent part of the current transformer, with a heavy copper bar to
carry current through the magnetic core. Window-type current transformers are also common,
which can have circuit cables run through the middle of an opening in the core to provide a
single-turn primary winding. When conductors passing through a CT are not centered in the
circular (or oval) opening, slight inaccuracies may occur.
Shapes and sizes can vary depending on the end user or switchgear manufacturer. Typical
examples of low voltage single ratio metering current transformers are either ring type or plastic
molded case. High-voltage current transformers are mounted on porcelain bushings to insulate
them from ground. Some CT configurations slip around the bushing of a high-voltage
transformer or circuit breaker, which automatically centers the conductor inside the CT window.
The primary circuit is largely
unaffected by the insertion of
the CT. The rated secondary
current is commonly
standardized at 1 or 5 amperes.
For example, a 4000:5 CT
would provide an output
current of 5 amperes when the primary was passing 4000 amperes. The secondary winding can
be single ratio or multi ratio, with five taps being common for multi ratio CTs. The load, or
burden, of the CT should be of low resistance. If the voltage time integral area is higher
than the core's design rating, the core goes into saturation towards the end of each cycle,
distorting the waveform and affecting accuracy.
a.Window type b. Bar type
b. Bar type
c. Wound type
Fig 4. Types of current transformers
Principle of operation:
A current transformer is defined as "as an instrument transformer in which the secondary current
is substantially proportional to the primary current (under normal conditions of operation) and
differs in phase from it by an angle which is approximately zero for an appropriate direction of
the connections." This highlights the accuracy requirement of the current transformer but also
important is the isolating function, which means no matter what the system voltage the
secondary circuit need be insulated only for a low voltage.
The current transformer works on the principle of variable flux. In the "ideal" current
transformer, secondary current would be exactly equal (when multiplied by the turns ratio) and
opposite to the primary current. But, as in the voltage transformer, some of the primary current
or the primary ampere- turns is utilized for
magnetizing the core, thus leaving less than the
actual primary ampere turns to be "transformed" into
the secondary ampere- turns. This naturally
introduces an error in the transformation. The error
is classified into two-the current or ratio error and the
phase error.
Usage:
Current transformers are used extensively for measuring current and monitoring the operation of
the power grid. Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-
hour meter on virtually every building with three-phase service and single-phase services greater
than 200 amps.
The CT is typically described by its current ratio from primary to secondary. Often, multiple CTs
are installed as a "stack" for various uses. For example, protection devices and revenue metering
may use separate CTs to provide isolation between metering and protection circuits, and allows
current transformers with different characteristics (accuracy, overload performance) to be used
for the devices.
Safety Precautions:
Care must be taken that the secondary of a current transformer is not disconnected from its load
while current is flowing in the primary, as the transformer secondary will attempt to continue
driving current across the effectively infinite impedance. This will produce a high voltage across
the open secondary (into the range of several kilovolts in some cases), which may cause arcing.
The high voltage produced will compromise operator and equipment safety and permanently
affect the accuracy of the transformer.
Accuracy:
The accuracy of a CT is directly related to a number of factors including:
Burden
Burden class/saturation class
Rating factor
Load
External electromagnetic fields
Temperature and
Physical configuration.
The selected tap, for multi-ratio CTs
For the IEC standard, accuracy classes for various types of measurement are set out in IEC
60044-1, Classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1, and 3. The class designation is an approximate
measure of the CT's accuracy. The ratio (primary to secondary current) error of a Class 1 CT is
1% at rated current; the ratio error of a Class 0.5 CT is 0.5% or less. Errors in phase are also
important especially in power measuring circuits, and each class has an allowable maximum
phase error for a specified load impedance. Current transformers used for protective relaying also
have accuracy requirements at overload currents in excess of the normal rating to ensure accurate
performance of relays during system faults.
Burden:
The secondary load of a current transformer is usually called the "burden" to distinguish it from
the load of the circuit whose current is being measured.
The burden, in a CT metering circuit is the (largely resistive) impedance presented to its
secondary winding. Typical burden ratings for IEC CTs are 1.5 VA, 3 VA, 5 VA, 10 VA, 15
VA, 20 VA, 30 VA, 45 VA & 60 VA. As for ANSI/IEEE burden ratings are B-0.1, B-0.2, B-0.5,
B-1.0, B-2.0 and B-4.0. This means a CT with a burden rating of B-0.2 can tolerate up to 0.2 Ω
of impedance in the metering circuit before its output current is no longer a fixed ratio to the
primary current. Items that contribute to the burden of a current measurement circuit are switch-
blocks, meters and intermediate conductors. The most common source of excess burden in a
current measurement circuit is the conductor between the meter and the CT. Often, substation
meters are located significant distances from the meter cabinets and the excessive length of small
gauge conductor creates a large resistance. This problem can be solved by using CT with 1
ampere secondaries which will produce less voltage drop between a CT and its metering devices
Rating Factor:
Rating factor is a factor by which the nominal full load current of a CT can be multiplied to
determine its absolute maximum measurable primary current. Conversely, the minimum primary
current a CT can accurately measure is "light load," or 10% of the nominal current (there are,
however, special CTs designed to measure accurately currents as small as 2% of the nominal
current). The rating factor of a CT is largely dependent upon ambient temperature. Most CTs
have rating factors for 35 degrees Celsius and 55 degrees Celsius. It is important to be mindful of
ambient temperatures and resultant rating factors when CTs are installed inside pad-mounted
transformers or poorly ventilated mechanical rooms. Recently, manufacturers have been moving
towards lower nominal primary currents with greater rating factors. This is made possible by the
development of more efficient ferrites and their corresponding hysteresis curves.
Short Time Rating:
The value of primary current (in kA) that the CT should be able to withstand both thermally and
dynamically without damage to the windings, with the secondary circuit being short-circuited.
The time specified is usually 1 or 3 seconds.
Instrument security factor (factor of security):
This typically takes a value of less than 5 or less than 10 though it could be much higher if the
ratio is very low. If the factor of security of the CT is 5, it means that the composite error of the
metering CT at 5 times the rated primary current is equal to or greater than 10%. This means that
heavy currents on the primary are not passed on to the secondary circuit and instruments are
therefore protected. In the case of double ratio CT's, FS is applicable for the lowest ratio only.
Summation CT:
When the currents in a number of feeders need not be individually metered but summated to a
single meter or instrument, a summation current transformer can be used. The summation CT
consists of two or more primary windings which are connected to the feeders to be summated,
and a single secondary winding, which feeds a current proportional to the summated primary
current. A typical ratio would be 5+5+5/ 5A, which means that three primary feeders of 5 are to
be summated to a single 5A meter.
Core balance CT (CBCT):
The CBCT, also known as a zero sequence CT, is used for earth leakage and earth fault
protection. The concept is similar to the RVT. In the CBCT, the three core cable or three single
cores of a three phase system pass through the inner diameter of the CT. When the system is
fault free, no current flows in the secondary of the CBCT. When there is an earth fault, the
residual current (zero phase sequence current) of the system flows through the secondary of the
CBCT and this operates the relay. In order to design the CBCT, the inner diameter of the CT, the
relay type, the relay setting and the primary operating current need to be furnished.
CT Classification for relaying
Over the years many standards for CT classification have been developed in North America and
Europe. Protection class CT’s are assumed to be able to supply 20 times its rated secondary
current to the relay. That means for a 5 amp rated secondary the CT must be able to supply 100
Amps of current, and for a 1 amp rated secondary the CT must be able to supply 20 Amps of
current.
10 C 400
The operating principals of CT’s are specified in a format such as this.
The first number represents the maximum amount of error, listed in as a percentage, that this CT
will produce. Therefore, the 10 in our example stands for no more than 10 percent error.
The second item, which is always a letter, can either be a T, C, K, L, or H.
• If the letter is a T which stands for “test”, it means that the CT accuracy can only be
determined by testing the CT. Current transformers with non-distributed windings fit in
this category.
• If the letter is a C or a K which stands for “Calculated”, it means the CT accuracy can be
determined by performing calculation using given excitation characteristics. CTs with
fully distributed windings, (bushing CT’s for instance) fit in this category.
• If the letter is an L, this indicates that the CT has a “Low internal secondary impedance,”
• If the letter is an H, this indicates that the CT has a “high internal secondary impedance,”
Tests
A number of routine and type tests have to be conducted on CT s before they can meet the
standards specified above. The tests can be classified as :
a. Accuracy tests:-
To determine whether the errors of the CT are within specified limits.
b. Dielectric insulation tests:-
Such as power frequency withstand voltage test on primary and secondary windings for
one minute, inter-turn insulation test at power frequency voltage, impulse tests with
1.2u/50 wave, and partial discharge tests (for voltage >=6.6kv) to determine whether the
discharge is below the specified limits.
c. Temperature rise tests.
d. Short time current tests.
e. Verification of terminal markings and polarity.
Typical specification for a 11 kV CT
System voltage:11 kV
Insulation level voltage (ILV) : 12/28/75 kV
Ratio: 200/1 - 1 - 0.577 A
Core 1: 1A, metering, 15 VA/class 1, ISF<10
Core 2: 1 A, protection, 15 VA/5P10
Core 3: 0.577 A, Class PS, KPV>= 150 V, Imag at Vk/2 <=30 mA, RCT at 75 C<=2
ohms
Short time rating:20 kA for 1 second
Principle of operation VT:
The standards define a voltage transformer as one in which "the secondary voltage is
substantially proportional to the primary voltage and differs in phase from it by an angle which is
approximately zero for an appropriate direction of the connections."
This, in essence, means that the voltage transformer has to be as close as possible to the "ideal"
transformer. In an "ideal" transformer, the secondary voltage vector is exactly opposite and equal
to the primary voltage vector, when multiplied by the turns ratio.
In a "practical" transformer, errors are introduced because some current is drawn for the
magnetization of the core and because of drops in the primary and secondary windings due to
leakage reactance and winding resistance. One can thus talk of a voltage error, which is the
amount by which the voltage is less than the applied primary voltage ,and the phase error, which
is the phase angle by which the reversed secondary voltage vector is displaced from the primary
voltage vector
Rated burden VT:
This is the load in terms of volt-amperes (VA) posed by the devices in the secondary circuit on
the VT. This includes the burden imposed by the connecting leads. The VT is required to be
accurate at both the rated burden and 25% of the rated burden.
Accuracy class required:
The transformation errors that are permissible, including voltage (ratio) error and phase angle
error. Phase error is specified in minutes. Typical accuracy classes are Class 0.5, Class 1 and
Class 3. Both metering and protection classes of accuracy are specified. In a metering VT, the
VT is required to be within the specified errors from 80% to 120% of the rated voltage. In a
protection VT, the VT is required to be accurate from 5% up to the rated voltage factor times the
rated voltage.
Rated voltage factor:
Depending on the system in which the VT is to be used, the rated voltage factors to be specified are
different. The table below is adopted from Indian and International standards.
Rated voltage
factor
Rated time Method of connecting primary
winding in system
1.2 Continuous Between phases in any network
Between transformer star-point
and earth in any network
1.2
1.5
Continuous
for 30 seconds
Between phase and earth in an
effectively earthed neutral
system
1.2
1.9
Continuous
for 30 seconds
Between phase and earth in a
non-effectively earthed neutral
system with automatic fault
tripping
1.2
1.9
Continuous
for 8 hours
Between phase and earth in an
isolated neutral system
without automatic fault tripping
or in a resonant earthed
system without automatic fault
tripping
Temperature class of insulation:
The permissible temperature rise over the specified ambient temperature. Typically, classes E, B and F.
Residual Voltage Transformer (RVT):
RVTs are used for residual earth fault protection and for discharging capacitor banks. The secondary
residual voltage winding is connected in open delta. Under normal conditions of operation, there is no
voltage output across the residual voltage winding. When there is an earth fault, a voltage is developed
across the open delta winding which activates the relay. When using a three phase RVT, the primary
neutral should be earthed, as otherwise third harmonic voltages will appear across the residual winding.
3 phase RVTs typically have 5 limb construction.
Tests:
A number of routine and type tests have to be conducted on VT s before they can meet the
standards specified above. The tests can be classified as:
a. Accuracy tests:-
To determine whether the errors of the VT are within specified limits
b. Dielectric insulation tests:-
Such as power frequency withstand voltage test on primary and secondary windings for
one minute, induced over-voltage test , impulse tests with 1.2u/50u wave, and partial
discharge tests (for voltage>=6.6 kV) to determine whether the discharge is below the
specified limits.
c. Temperature rise tests
d. Short circuit tests
e. Verification of terminal markings and polarity
Typical specification for a 11 kV VT:
System voltage: 11 kV
Insulation level voltage (ILV) : 12 /28/75 kV
Number of phases: Three
Vector Group: Star / Star
Ratio: 11 kV/ 110 V
Burden: 100 VA
Accuracy: Class 0.5
Voltage Factor: 1.2 continuous and 1.5 for 30 seconds
Ferro-resonance:
The failure of single phase transformers (VTs) operating in unearthed power system has
remained a mystery for many years to designers as well as system engineers. The phenomena
known as “FERRO-resonance” or “neutral inversion” or “Neutral instability”
The term "Ferro-resonance ", which appeared in the literature for the first time in 1920, refers to
all oscillating phenomena occurring in an electric circuit which must contain at least:
a non-linear inductance (ferromagnetic and
saturable),
a capacitor,
a voltage source (generally sinusoidal),
low losses.
Power networks are made up of a large number of saturable inductances (power transformers,
voltage measurement inductive transformers (VT), shunt reactors), as well as capacitors cables,
long lines, capacitor voltage transformers, series or shunt capacitor banks, voltage grading
capacitors in circuit-breakers,metalclad substations). They thus present scenarios under which
ferroresonance can occur.
The main feature of this phenomenon is that more than one stable steady state response is
possible for the same set of the network parameters. Transients, lightningovervoltages,
energizing or deenergizing transformers or loads, occurrence or removal of faults, live works,
etc...may initiate ferroresonance. The response can suddenly jump from one normal steady state
response (sinusoidal at the same frequency as the source) to an another ferroresonant steady state
response characterised by high overvoltages and harmonic levels which can lead to serious
damage to the equipment.
A practical example of such behaviour (surprising for the uninitiated) is the deenergization of a
voltage transformer by the opening of a circuit-breaker. As the transformer is still fed through
grading capacitors accross the circuit-breaker, this may lead either to zero voltage at the
transformer terminals or to permanent highly distorted voltage of an amplitude well over normal
voltage.
To prevent the consequences of ferroresonance (untimely tripping of protection
devices,destruction of equipment such as power transformers or voltage transformers, production
losses,...), it is necessary to:
understand the phenomenon,
predict it,
identify it and
avoid or eliminate it.
Little is known about this complex phenomenon as it is rare and cannot be analysed or predicted
by the computation methods (based on linear approximation) normally used by electrical
engineers. This lack of knowledge means that it is readily considered responsible for a number of
unexplained destructions or malfunctionings of equipment.
A distinction drawn between resonance and ferroresonance will highlight the specific and some
times disconcerting characteristics of ferroresonance.
Practical examples of electrical power system configurations at risk from ferroresonance are used
to identify and emphasise the variety of potentially dangerous configurations.Well-informed
system designers avoid putting themselves in such risky situations.
Difference between a ferroresonant and linear resonant ccircuit:
The main differences between a ferroresonant circuit and a linear resonant circuit are for a given
ω :
Its resonance possibility in a wide range of values of C,
the frequency of the voltage and current waves which may be different from that of the
sinusoidal voltage source,
the existence of several stable steady state
responses for a given configuration and values
Contents
Instrument Transformer Current Transformers
I. Constructional Features
II. Principle of operation
III. Usage
IV. Safety PrecautionsV. Accuracy
VI. BurdenVII. Rating factor
VIII. Short time ratingIX. CT Classification for relaying
X. Tests
Voltage TransformerI. Principle of operation
II. Rated burden
III. Accuracy class required
IV. Rated voltage factor
V. Residual Voltage Transformer
VI. Tests
VII. Ferro-resonance