Download - Instrument Transformers 1
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).