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