INSTRUMENT TRANSFORMERS

Current and Potential Transformers

Instrument Transformers

The instrument transformers in a sub-station are:

Current transformers: Voltage transformer.

♣ Capacitive voltage transformers (CVTs )

♣ Voltage Transformers ( IVTS or PTs )

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 a system fault.

The errors in transformer output may abnormally delay the operation of the protection, or cause unnecessary operations.

Importance of CTs &PTs

Whenever the values of voltage or current in a power circuit are too high to permit convenient direct connection of measuring instruments or relays, coupling is made through transformers.

Such 'measuring‘ transformers are required to produce a scaled down replica of the input quantity to the accuracy expected for the particular measurement

TYPES OF CTS

Wound Type – Primary winding has more than one turn

Bar Type – Primary winding is a Bar of suitable size forming integral part of CT

Dry Type - A current transformer which does not require any liquid or semi-liquid Material Oil Immersed type - CT that has oil as insulating and cooling medium. Ring type – CT with opening in the center to accommodate the primary winding

Types of CTs

There are two types of current transformers

High reactance type: It has a wound primary with a considerable

separation between primary and secondary, which results in appreciable internal secondary leakage reactance.

Low reactance type: This type is a torroidaly wound transformer with a

bar primary and the leakage reactance being very low. This type is most commonly used

CT TERIMINOLOGIES

CT TERIMINOLOGIES (contd.)

Instrument transformer

Instrument transformer is a transformer intended to supply measuring Instruments, relays, and other similar instruments

CURRENT TRANSFORMER

An Instrument Transformer in which the secondary current , in normal condition of use is substantially proportional to the primary current with approximately zero phase angle error.

CT TERIMINOLOGIES (contd.) Rated transformation ratio

Ratio of rated primary current to the rated secondary current

Current Error /Ratio error Current Error /Ratio error = ( Kn . Is –Ip ) x 100 / Ip

Kn - Rated transformation ratio.Ip - Actual Primary current.

Is - Actual Secondary current.

Phase angle Error Phase difference in minutes between Primary and

secondary current- It is positive when Is leads Ip

CT TERIMINOLOGIES (contd.)

Accuracy Class A designation assigned to a current

transformer, the errors of which remains within specified limits ,under prescribed condition of use.

Rated output Value of the apparent power which the

current transformer intended to supply at rated secondary current at rated power.

CTS Insulation System

O.I.P:- Oil Impregnated Paper >>>Popular Expected to be popular for next 15-20 years.

SF6 :-Plain SF6 under pressureProducts available in International market

for 30 yearsBut still not popular. ( Less than 3% )Costlier than OIP

EHV Current transformers

EHV CTs

LIVE TANK DEAD TANK

CURRENT TRANSFORMERS

DIFFERENCE FROM A NORMAL TRANSFORMERCLASSIFICATIONERRORSBURDEN IN A CTKNEE POINT VOLTAGECALCULATION OF KNEE POINT VOLTAGECALCULATION OF STABILISING RESISTORSEFFECT OF SATURATIONEFFECT OF DC COMPONENTEFFECT OF RESIDUAL FLUXCONNECTION METHODS

DIFFERENCE BETWEEN A POWER TRANSFORMERAND AN INSTRUMENT TRANSFORMER

POWER TRANSFORMER INSTRUMENT TRANSFORMER

EFIICIENCY TRANSFORMATION RATIOLOSSES LINIARITY REGULATION SATURATION

FUNCTIONAL RANGE

CLASSIFICATION OF CTs

MEASURING SUITABLE FOR METERS - ACCURATE UPTO200% OF RATING.

PROTECTION SUITABLE FOR REALYSACCURATE UPTO 2000% OF RATING

SPECIAL (CLASS PS) FOR SPECIAL PROTECTIONS(DIFFERENTIAL / REF)EQUIPMENT/ FEEDER SPECIFIC

HOW CTs ARE SPECIFIED

FOR NORMAL PROTECTIONS:

5P10, 15 VA SUITABLE FOR BURDENS UPTO 15VA5% ACCURACY CLASS , 10 TIMES RATED CURRENT

5P20, 5 VA SUITABLE FOR BURDENS UPTO 5VA5% ACCURACY CLASS , 20 TIMES RATED CURRENT

FOR SPECIAL PROTECTIONS (DIFF / REF)

PS CLASS VAKNEE POINT VOLTAGEMAGNTISING CURRENTRct

ERRORS IN A CT

RATIO ERRORPHASE ANGLE ERRORCOMPOSITE ERROROPERATING RANGE

CHIEF CONTRIBUTORS FOR ERRORS IN A CT

- CORE QUALITY- CORE CROSS SECTION

BOTH ABOVE INFLUENCE THE EXCITATION CURRENT &THE LINARITY OF THE TRANSFORMER

RATIO ERROR IN A CT

CTR * I2 - I1 e = * 100 I1

e = RATIO ERROR %I1 = PRIMARY CURRENTI2 = SECONDARY CURRENTCTR = TRANSFORMATION RATIO

NORMAL RATIO ERRORS = ?

PHASE ANGLE ERROR IN A CT

- IS CAUSED BY THE QUADRATURE COMPONENTOF THE MAGNETISING CURRENT

- INFLUENCED BY THE CORE MATERIAL

- ORDER OF ERROR MIN

IqIr

Ie

PHASE ANGLE ERROR IN A CT

COMPOSITE ERROR A CT

DEFINED AS THE RMS VALUE OF THE DIFFERENCE BETWEENTHE IDEAL SECONDARY CURRENT AND ACTUAL SECONDARY CURRENT

(THIS INCLUDES THE RATIO & PHASE ANGLE ERRORS PLUS THE HARMONICS IN EXCITATION CURRENT)

USUALLY EXPRESSED AS THE % OF PRIMARY CURRENT

ERRORS IN CTs

RATIO ERROR IS CONSIDERED POSITIVE WHENACTUAL SECONDARY CURRENT IS HIGHER THAN THE CALCULATED SECONDARY CURRENT

OPERATING RANGE OF A CT

MEASUREMENT PROTECTION

20 TO 200% 50 TO 1000%OR50 TO 2000%

BURDENS IN A CT

- LOAD ON THE SECONDARY SIDE OF CT EXPRESSED IN VA-MAJOR INFLUENCE ON THE PERFORMANCE OF THE

PROTECTION SCHEME- SUM TOTAL OF THE INTERNAL IMPEDANCE,

IMPEDANCE OF CONNECTED RELAYS AND THE CONNECTING LEADS

- ALL ERRORS ARE AT A SPECIFIED BURDENWITHIN THE SPECIFIED BURDEN RATING,

LOWER THE BURDEN - HIGHER THE ACCURACY.- LOADING THE CT BEYOND THE RATED BURDEN,

WILL RESULT IN SATURATION OF THE CT AND HENCE HIGHER ERRORS.

- SIZE AND COST OF CT ARE DIRECTLY PROPORTIONAL TO BURDEN RATING

KNEE POINT VOLTAGE IN A CT

-RELATES TO THE VOLTAGE DEVELOPED ACROSSCT SECONDARY UNDER MAX FAULT CONDITIONS

-DEFINED AS THAT POINT IN THE EXCITATION CURVE OF CT, WHEN A 10% VOLTAGE INCREASE NEEDS 50% INCREASE IN EXCITATION CURRENT

-FOR PRACTICAL PURPOSESKPV IS TAKEN AS 2 X Vs, WHERE Vs IS THE CTSECONDARY VOLTAGE DURING A FAULT

KNEE POINT VOLTAGE

- PLAYS A MAJOR ROLE IN DIFFERENTIAL &REF PROTECTIONS

- KPV SHOULD BE SPECIFIED WHEN ORDERINGPS CLASS CTs FOR DIFFERENTIAL PROTECTION

- KPV SHOULD BE KNOWN, TO CALCULATE STABILISING RESISTOR IN REF PROTECTION

EFFECT OF SATURATION IN CTs

- IF A CT SATURATES , IT WILL NOT DEVELOPADEQUATE VOLTAGE AT THE SECONDARY SIDE TO OPERATE A CONNECTED RELAY.

- IT IS NECESSARY TO ENSURE THAT CT DOES NOTSATURATE AT MAX FAULT CURRENTS(BY CHOOSING PROPER CORE MATERIAL & ADEQUATE CROSS SECTIONAL AREA)

- CTs CAN SATURATE IF THE BURDEN OFFEREDBY THE RELAY IS HIGHER THAN THE RATED

BURDEN OF CT(USE LOW BURDEN RELAYS)

EFFECT OF DC COMPONENT IN CTs

- RESULTS DUE TO INDUCTIVE NATUREOF CTs

- WILL CAUSE SATURATION IN CTsDURING FIRST FEW CYCLES OF FAULT

- RATIO ERROR WILL INCREASE INFIRST FEW CYCLES

Instrument Transformers - Current TransformersInstrument Transformers - Current TransformersEFFECT OF DC COMPONENTSEFFECT OF DC COMPONENTS

F lu x

P rim ary cu rren t

P rim ary cu rren t

S eco n d a rycu rren t

EFFECT OF REMNANT FLUX IN CTs

- RESULTS DUE TO INDUCTIVE NATUREOF CTs

- WILL CAUSE SATURATION IN CTsDURING FIRST FEW CYCLES OF FAULT

Instrument Transformers - Current TransformersInstrument Transformers - Current Transformers

R em an en t F lu x = 0 %

R em an en t F lu x = 5 0 %

R em an en t F lu x = 7 5 %

Transient Response of a Current Transformer

POTENTIAL TRANSFORMERS

BUS PTs STAR / STARUSED FOR :U/V OR O/VU/F OR O/F SYNCHRONISINGPOLARISING

OPEN DELTA PTs STAR (Py) / OPEN DELTA(Sy)USED FOR :NUETRAL DISPLACEMENT

CVTs HT FEEDERS

HOW PTs ARE SPECIFIED

- Py / Sy VOLTAGE- BURDEN- ACCURACY CLASS

Indoor Resin cast Instrument Transformers up to 33kv

Combined CT/PT units up to 33kV Voltage Class

CT PT&CT

Precision Grade Current & Potential Transformers

Precision grade CTs are supplied in portable model in wooden boxes, fitted with specially designed terminals for easy use.

They can supply portable standard CTs with accuracy class upto 0.1. For CT ratios above 100 Amps, a bore of 75mm dia is provided for external primary turns.

Precision grade standard PTs are supplied in oil cooled model upto 132kV voltage class with accuracy class 0.2.

CT Test Benches fitted with CT/PT test set, standard CTs,Burden Boxes and current source are also available.

Presition CTs

Typical modern CT for use on MV systems

A current transformer is overloaded while system short-circuit currents are flowing and will be short-time rated.

Standard times for which the CT must be able to carry rated short-time current (STC) are 0.25, 0.5, 1.0, 2.0 or 3.0 seconds.

Dead Tank Design(220kV CT)

Live Tank Design (132kV CT)

Vacuum Drying Of Instrument Transformers

Outdoor oil cooled instrument transformers are hermetically sealed type in construction.

The whole CT/PT unit is dried in air heating oven under very high vacuum & strictly controlled conditions.

Measurement of amount of moisture taken out from the CTs/PTs is done at regular intervals during the vacuum drying process. After the process is over, the CTs/PTs are filled with filtered, de-aerated EHV grade transformer oil under vacuum.

The oil filtration plant, the processed oil storage tank & the vacuum drying chamber are all interfaced together to ensure that the CTs/PTs are filled with oil untouched by human hand.

To seal the CTs/PTs, the space left for expansion on the top is filled with dry & pure nitrogen gas through non-returnable valve at pre-determined pressure.Manufacturers cancan also supply on request CTs/PTs with stainless steel bellows instead of nitrogen gas cushioning.

High permeability CRGO silicon steel is used as core material. High quality crepe insulating paper is used to build up main insulation of CT/PT. The main insulation is build up with fine grading, the grading ensures proper distribution of electrical stresses .

Vacuum Drying

Definition of knee-pointof excitation curve

That sinusoidal voltage of rated frequency applied to the secondary of transformer , all other windings being open-circuited, which when increased by 10 percent ,causes the exciting current to increase by 50 percent.

Vector diagram for current

transformer (referred to secondary)

Protection CT error limits for classes 5P and 10P

CT error classes

Classifications of CT requirements

For Metering CT’s Class of accuracy Rated VA burden Instrument security factor Rated primary current & CT ratio Rated short-time thermal current (STC) & rated time. Rated continuous thermal current or rated extended primary

current Insulation level and system frequency. Reference to relevant IS 2705 Parts I to IV & IS-4201 –

Application guide for CT’s

Classifications of CT requirements For Non-unit protection system:

Eg: Instantaneous & IDMT relays etc. Class of accuracy Accuracy limit factor Rated VA burden Rated STC & rated time. Rated continuous thermal current (if a value

different from rated primary current is required). Rated primary current & CT ratio.

Classifications of CT requirements For Unit protection system

Eg: Differential protection. Knee point voltage Rated primary current & CT ratio Exciting current at knee point voltage and/or at a stated

percentage there of. Resistance of secondary winding Rated STC & rated time. Rated continuous thermal current (if a value different from

the rated primary current is required). Turns ratio and error in turns ratio

•The power system impedance governs the current passing through the primary winding of the current transformer.•This approach is developed in Figure, taking the numerical example of a 300/5A CT applied to an 11kV power system. •The system is considered to be carrying rated current (300A) and the CT is feeding a burden of 10VA.

CASE STUDIES

Secondary burden and Primary current

Secondary burden and Primary current

Summation current transformers

The summation arrangement is a winding arrangement used in a measuring relay or on an auxiliary current transformer to give a single-phase output signal having a specific relationship to the three-phase current input.

Air-gapped current transformers

These are auxiliary current transformers in which a small air gap is included in the core to produce a secondary voltage output proportional in magnitude to current in the primary winding.

Sometimes termed 'transactors’ and 'quadrature current transformers', this form of current transformer has been used as an auxiliary component of unit protection schemes in which the outputs into multiple secondary circuits must remain linear for and proportioned to the widest practical range of input

currents.

Bushing or bar primary type The bushing of a circuit breaker or power

transformer is used for this purpose. Many current transformers have a ring-shaped

core,sometimes built up from annular stampings, but often consisting of a single length of strip tightly wound to form a close-turned spiral.

The distributed secondary winding forms a toroid which should occupy the whole perimeter of the core, a small gap being left between start and finish leads for insulation.

Harmonics during the Transient Period When a CT is required to develop a high secondary e.m.f.

under steady state conditions, the non-linearity of the excitation impedance causes some distortion of the output waveform; such a waveform will contain, in addition to the fundamental current, odd harmonics only.

When, however, the CT is saturated uni - directionally while being simultaneously subjected to a small a.c.quantity, as in the transient condition discussed above, the output will contain both odd and even harmonics.

Usually the lower numbered harmonics are of greatest amplitude and the second and third harmonic components may be of considerable value.

This may affect relays that are sensitive to harmonics.

Optical Instrument Transducers

Optical sensor concepts

Certain optical sensing media (glass, crystals, plastics) show a sensitivity to electric and magnetic fields and that some properties of a probing light beam can be altered when passing through them.

One simple optical transducer description is given here in Figure in the next slide

Schematic representation of the concepts behind the optical sensing of varying electric and magnetic fields

Optical sensor concepts

Optical converters and optical glass fibre channels implement the link between the sensor and the low

voltage output. The fundamental difference between an instrument

transducer and a conventional instrument transformer is the electronic interface needed for its operation.

This interface is required both for the sensing function and for adapting the new sensor technology to that of the secondary output currents and voltages.

Optical current sensor basedon the magnetic properties of optical materials

Novel instrument transducer concept requiring an electronic interface in the control room

Conceptual design of a double-sensor optical CT

Optical voltage transducer concepts,using a ‘full-voltage’ sensor

Similar to conventional instrument transformers there are‘live tank’ and ‘dead tank’ optical transducers.

Typically, current transducers take the shape of a closed loop of light transparent material, fitted around a straight conductor carrying the line current (Figure).

In this case a bulk-glass sensor unit is depicted along with an ‘all-optical’ sensor example, as shown in Figure

Field installation of a combinedoptical CT/VT

Although ‘all-optical’ instrument transformers were first introduced 10-15 years ago, there are still only a few in service nowadays.

Figure shows a field installation of a combined optical CT/VT.

Other Sensing Systems

Zero-flux (Hall Effect) current transformer the sensing element is a semi-conducting

wafer that is placed in the gap of a magneticconcentrating ring. This type of transformer is also sensitive to d.c. currents.

The transformer requires a power supply that is fed from the line or from a separate power supply. The sensing current is typically 0.1% of the current to be measured.

In its simplest shape, the Hall effect voltage is directly proportional to the magnetising current to be measured.

For more accurate and more sensitive applications, the sensing current is fed through a secondary, multiple-turn winding, placed around the magnetic ring in order to balance out the gap magnetic field.

This zero-flux or null-flux version allows very accurate current measurements in both d.c. and high frequency applications.

Rogowski coil

Schematic representation of a Rogowski coil, used for current sensing

Voltage transformers

Voltage transformers

They are connected to the operating voltage of the circuit.

The voltage transformers are classified as magnetic type voltage transformers & capacitive type.

The magnetic type voltage transformers work on the same principle as the power transformers .

Parameters to be specified for VTs Following points to be specified for VT’s suitable for

metering and protection: Class of accuracy Ratio Rated VA burden Rated voltage factor & time Rated residual secondary voltage, rated output and

accuracy class. System and VT earthing conditions Special requirements if any eg: In case of VT’s used for

unbalance protection of capacitor banks. Insulation level & system frequency. Reference to relevant IS 3156 parts I to IV & IS-4146 –

Application guide for VT’s.

Capacitive voltage transformers They work on the capacitive divider principle and these

are used for higher system voltages upto 765KV. Capacitive voltage transformers can be connected to

all customary measuring instruments and protection relays.

They are also authorised for tariff metering purposes. CVTs have the added advantage that they can be

used for coupling high frequency, power line carrier systems eq. for telephony, telecontrol and so on

Development of capacitorvoltage transformer

Simplified equivalent circuitof capacitor voltage transformer

The principle points to be considered when selecting Instrument Transformers

Standards. Principle parameters Technical requirements. Tests

Standards related to Insteument Transformers

IS-2705 – 1992 IEC – 185 (1987) Current Transformers IS-3156 – 1992IEC – 186 (1987) Voltage Transformers IS –

4379 (1981 Reaffirmed 1992) IEC – 358 (1990)Coupling capacitors and capacitor dividers

IEC – 44-4 Instrument Transformers:Measurement of partial discharges IEC – 481 (1974) Coupling devices for Power Line Carrier

Systems. ANSI – C5713 Requirements for Instrument Transformers. ANSI – C92.2 Power Line Coupling Voltage Transformers ANSI – C93.1 Requirements for Power Line Carrier

Type tests:

CT’s Radio interference test Seismic withstand test Thermal stability test Thermal co-efficient test (measurement

of tan-delta as a function of temperature

VT’s and CVT’s

High frequency capacitance and equivalent series resistance measurement.

Seismic withstand test Stray capacitance and stray conductance

measurement of the low voltage stability. Determination of temperature co-efficient test.

Routine and acceptance test.

CTs: High voltage power frequency withstand test. Oil leakage test. Measurement of tan-delta Measurement of partial discharge. Measurement of ratio error and phase angle error. Insulation resistance test for primary and secondary. Polarity test Di-electric test of oil. Magnetising characteristic test Secondary winding resistance

CT’s and CVT’s:

Capacitance and loss angle measurement before and after voltage test.

Partial discharge test on capacitor dividers. Sealing test. Insulation resistance test of primary and secondary. Polarity test. Ratio test. Secondary winding resistance measurement. Tan-delta and capacitance measurements between HV and HF point HF point and ground point of intermediate transformer. HV and ground point of intermediate transformer.

Potential Transformer Design (132kV PT)

Vector diagram for voltage transformer

The ratio and phase errors of the transformer can be calculated using the vector diagram of Figure 6.2.

The ratio error is defined as:

( Kn .Vs -Vp).100 / Vp

Where: Kn is the nominal ratio Vp is the primary voltage Vs is the secondary

voltage

Measuring voltage transformer error limits

Additional limits for protection voltage

transformers.

Voltage transformers: Permissible durationof maximum voltage

Residual voltage connection

Schematic diagram of typical cascadevoltage transformer

The capacitor VT (section 6.3) was developed because of the high cost of conventional electromagnetic voltage transformers but, as shown in Section 6.3.2,

the frequency and transient responses are less satisfactory than those of the orthodox voltage transformers. Another solution to the problem is the cascade VT (Figure 6.5).