2004ABB Cover 1.qxp 12/17/2004 11:52 AM Page 1 Instrument ... · transformers are the most common...
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Instrument Transformers Technical Information and Application Guide
Transcript of 2004ABB Cover 1.qxp 12/17/2004 11:52 AM Page 1 Instrument ... · transformers are the most common...
Instrument TransformersTechnical Information and Application Guide
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2004ABB_Cover_1.qxp 12/17/2004 11:52 AM Page 1
Table of Contents
An introduction to instrument transformer fundamentals,a discussion on power quality, and a detailed explanation of design considerations to assist in selecting an instrument transformer.
Technology Review
Product Description
3
37A general overview of ABB’s current and voltage transformers.This section is complemented with photographs and describes the specific classes and uses for instrument transformers.
3
Introduction 4
Types of Construction 6
Performance Characteristics 8
Accuracy Classifications for Metering 12
Burdens 13
Current Transformers: Accuracy Classes for Relaying 15
Insulation Systems 16
Voltage Ratings 18
Power Quality 22
Service Conditions 24
Auxiliary Current Transformers 31
Fusing Voltage Transformers 32
New Design Technologies 33
Technology Review
Instrument transformers (ITs) are designed to transform voltageor current from the high values in the transmission and distribu-tion systems to the low values that can be utilized by low voltagemetering devices. There are three primary applications for whichITs are used: metering (for energy billing and transaction pur-poses); protection control (for system protection and protectiverelaying purposes); and load survey (for economic managementof industrial loads).
Depending on the requirements for those applications, the ITdesign and construction can be quite different. Generally, themetering ITs require high accuracy in the range of normal oper-ating voltage and current. Protection ITs require linearity in awide range of voltages and currents. During a disturbance, suchas system fault or overvoltage transients, the output of the IT isused by a protective relay to initiate an appropriate action (openor close a breaker, reconfigure the system, etc.) to mitigate thedisturbance and protect the rest of the power system. Instrumenttransformers are the most common and economic way to detecta disturbance. Typical output levels of instrument transformersare 1-5 amperes and 115-120 volts for CTs and VTs, respectively.There are several classes of accuracy for instrument transformersdefined by the IEEE, CSA, IEC, and ANSI standards. Figure 1presents a conceptual design of CTs and VTs.
Figure 2 shows how the polarity markers are used to keep thedirection of current flow in the meters exactly the same, as if theprimary circuit was carried through the meters. Grounding of thesecondary circuit is most important, but in complicated three-phase connections, the best point to ground is not always easily determined.
Introduction
Technology Review
4
Technology Review
V
V2
I2
I1
1
Voltage Operated Relay
CurrentOperatedRelay
Current-Current Relay
Current Transformer
VoltageTransformer
Lines in which either current or voltage is too high topermit running the lines directly to the meter or relay
Current-Voltage Relay
Volt Meter
Transformeror Power
Sub-Station
Watt Meter
H1 H2
X1 X2H1 X1
H2 X2
H1 H2
X1 X2
+-
+-
+-
+-
+-
+-
W
R
R
A B
V
R
RAmmeter A
Figure 2: Instrument Transformer Connections
A. The current transformer is designed to connect in series with the line to transform the line current to thestandard 5 amperes suitable for the meter or relay. The voltage transformer is designed to connect in parallel with the line to transform the line voltage to 115 or 120 volts suitable for the meter or relay. To keep thevoltage at the meters and relays at a safe value, the secondary circuit must be grounded.
B. The polarity markers indicate the relative instantaneous directions of current in the windings. The polarity, orinstantaneous direction of current, is of no significant difference for current-operated or voltage-operateddevices. Correct operation of current-current, voltage-voltage, or current-voltage devices usually dependson the relative instantaneous directions.
Figure 1: Current and Voltage Transformer Symbols and Simplified Concepts
5
I1
I2
V1
V2
Symbol of a Current Transformer
Symbol of a Voltage Transformer
Conceptual picture of a Current Transformer
Conceptual picture of a Voltage Transformer
The principal forms of construction used for instrument trans-formers, together with standard symbols according to IEEEStandard C57.13, are shown in Figure 3.
Types of Construction
Figure 3: Types of Instrument Transformer Construction
Simple Basic Forms
Current Transformer Voltage Transformer
X1H1
X2H2
SecondaryPrimary
Current Example5.5 Amperes
Ratio 1:1
H1
X1
H2
X2
CORE
Voltage ExamplePrimary 7200 Volts
Ratio 60:1 or 7200:120 Volts
Window-type Bar-type Wound
X1 X2 X3 X4 X5
20 10 50 40
600:5 Multi-RatioSecondary Winding
Primary Ampreres
Secondary terminals toget 5 Amps
50100150200250300400450500600
X2-X3X1-X2X1-X3X4-X5X3-X4X2-X4X1-X4X3-X5X2-X5X1-X5
600:5 Multi-Ratio
H1
X2 X1
H1
X2 X1X1X2
H1
H2H1
X3
X2
X1
200/400:5 Amperes200 Amperes in H1-H2will produce 5 Amperesin X2-X3 *
400 Amperes in H1-H2will produce 5 Amperesin X1-X3 *
* See nameplate for actual connections
Dual Ratio Example Multi-Ratio Example
Construction Types
Secondary Types
Technology Review
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In the 1970’s, the insulating medium for the higher voltage (5-34.5 kV) units was butyl rubber. The material itself is excellent,but the pressures and temperatures necessary to use it as a dielec-tric were not conducive to the exacting clearances and geometriesinside a voltage or current transformer. Without excess bracing,the core/coil assemblies would shift during molding and fail BILtesting. With enough bracing, the material flow inside the unit wasrestricted, increasing the possibility of voids. For these reasons,another dielectric insulating material was sought. Butyl rubber isstill used by some manufacturers.
Materials Used inConstruction
Butyl Rubbers
Cycloaliphatic epoxy (CEP) was first introduced in outdoor insula-tors in the late 1970's due to its very good resistance to humidity,ultraviolet (UV) radiation, outdoor pollutants, and chemicals. Itsoutstanding mechanical strength and dielectric properties werealso highly desirable.
Aromatic polyurethane (PUR) elastomers are another cost-effec-tive insulation for medium voltage electrical equipment. There areapproximately fourteen general types of PUR that are successfullycommercialized for a wide variety of applications, includinginstrument transformers. Thirteen of these types are called con-ventional rubber. That means they are mixed, milled, and moldedby techniques which have been in use by the rubber industry sincethe 1920’s.
Polyurethane rubber raw materials are liquid, which permits themto be pumped, metered, mixed, and dispensed by machines undervery precise control of temperature and ingredient proportions.The liquid mixture enters the mold at vacuum pressure and iscured at slightly elevated temperatures. This unique characteristicallows molding of large parts which are completely uniformthroughout. When compared to high pressure-molded butyl rub-ber and vacuum cast epoxies, PUR in general has the most forgiv-ing process.
The fully cured PUR elastomers possess a desirable balance ofease of manufacturing (via vacuum casting), mechanical toughness(it is, after all, a rubber) and very good electrical properties.
Cycloaliphatic Epoxies
Aromatic Polyurethanes
In the early 2000's, a global epoxy resin supplier introduced ahydrophobic version of CEP called Hydrophobic CycloaliphaticEpoxy (HCEP) to the market. HCEP is formulated to sustain sur-face hydrophobicity better than its CEP counterpart upon pro-longed exposure to aggressive outdoor environments withoutsacrificing other desirable chemical and mechanical properties.
A hydrophobic insulation surface is desirable for outdoor applica-tions because it prevents water from developing completely wetted,resistive conductive surfaces. Leakage currents are thereforereduced, which helps to reduce the flashover risk. The result isenhanced reliability. Furthermore, less discharge activity means lessattack and therefore less surface erosion, which extends the trans-former’s life.
HydrophobicCycloaliphatic Epoxy
Technology Review
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The specific performance characteristics of instrument transformersare easily determined from the equivalent circuit. Figure 4 workswell for most instrument transformers. For current transformers,the value of the reactance X is determined in a special way sothat it represents the leakage flux. The flux flows in the part ofthe core represented by the left-hand exciting branch of theequivalent circuit shown in Figure 4.
An additional winding (or windings) placed over the outer leg(s)of the core and connected back in parallel with the secondarywinding, as shown in Figure 4, can keep the leakage flux out ofthe core. The leakage reactance is then effectively connectedahead of both exciting branches as shown in Figure 4. This dif-ference is important for current transformers because leakageflux in the core affects current ratio. It also improves the per-formance of current transformers and subjects their perform-ance to simple calculation.
Voltage transformers are designed so the through impedance (RS, R, XP, and X) is as low as possible, while current transform-ers are designed so the excitation impedance (Zo and Zi) is ashigh as possible. Neither transformer is very good at performingthe function of the other.
PerformanceCharacteristics
Equivalent Circuits
Rp Xp
Zo Zi
X Rs
Input Output
Exciting Impedance Perfect or IdealTransformer
ResistancePrimary Coil
SecondaryResistance
Coil
InternalReactances
Rp Xp X
Zo
Rs
Returning primarycircuit far enoughaway that leakagefrom it is negligible
Leakage fluxwhich flowsin the core
Primary windingenclosing oneside of the core
Secondarywinding
Leakage fluxnot reachingthe core
Core
D
Leakage flux not reaching the core
Primary winding
Secondary windingWorking flux
Leakage flux whichflows in the corerepresented byreactance "X"
CoreAuxiliary winding on outer legof core connected in parallel
A
B
C
Zi
Figure 4: Equivalent Circuits
A. A typical transformer and its equivalent circuit.The leakage flux is shown entering the outer part ofthe core and is represented by reactance X. The reac-tance develops voltage applied to the exciting branchZo, which represents the outer side of the core. Theseries impedance, RP + RS + j (XP + X), is responsi-ble for the loss of voltage in transformation. Thevoltage transformers are carefully designed to keepthis impedance as low as possible. The loss of cur-rent in transformation is due to current by-passed bythe exciting branches, Zo and Zi. Current transform-ers are specially designed to keep these by-pass exciting impedances as high as possible.
B. A common construction of HV or EHV currenttransformer. Leakage flux enters the core even thoughthe winding is uniformly wound over a ring core.The equivalent circuit is the same as for Figure A.
C. A construction used in HV or EHV currenttransformers. The parallel auxiliary winding effectively keeps the leakage flux out of the core sothat the leakage reactance in the equivalent circuit is effectively ahead of the exciting branches. This simplifies the calculation of the current by-passedthrough Zo and Zi.
D. A typical bushing current transformer.This resembles the transformer in B but has onlynegligible leakage flux in the core because the returnconductor is far away. This transformer still has agood deal of leakage reactance, but the leakage fluxdoes not enter the core in significant amount. Thereactance is ahead of the by-pass branches Zo and Ziso that the performance as a current transformer canbe easily calculated.
Technology Review
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The value of through impedance is constant, but the value of excitation impedance is variable. The exciting impedances representing the exciting currents for the two parts of the coredepend on the voltage applied to them, the current flowing inthem, or flux density in the core. The easiest way to understandZo and Zi, which are of primary importance in current trans-formers, is to draw “saturation” curves showing how the currentflowing into the exciting branch varies with the voltage applied,as shown in Figure 5. The curve is usually plotted for the combination of Zo and Zi in parallel.
Voltage transformers are designed such that the operating pointon the saturation curve, as shown in Figure 5, is typically at a relatively high voltage. This is subject to the limitation that thispoint must not be so high that the exciting current itself is exces-sive. Voltage transformers are designed to work without excessiveexciting current up to 110% of rated voltage. The IEEE stan-dard for performance requires good performance also at 90%voltage. Figure 5 shows that the exciting current will not reach ahigher per unit value, with consequent increase of voltage lossfrom exciting current, at voltages over the range of 5% to 110%.
Voltage transformers are designed such that theoperating point on the saturation curve is typi-cally at a relatively high voltage.
The voltage must not be so high that the excit-ing current itself becomes too high. This wouldcause a voltage drop in the primary impedancethat would bring about an excessive error inratio of phase angle. IEEE C57.13 requires per-formance standards to be met at 110% of ratedvoltage. The curve shows the per unit excitingcurrent, below which the error due to voltagedrop caused by the exciting current itself willequal that at 110% rated voltage. Performance at voltages down to 5% is not significantly different at the same burden connected to thetransformer secondary. The error limits requiredby C57.13 apply not only at a given burden butat zero burden.
The current transformer, on the other hand,is designed to operate at the low range of thecurve (see region marked on the curve) so thatthe exciting current by-pass will be as low asfeasible. The curve shows that as the voltage isreduced, the exciting current is not reduced inproportion. This means, in a current trans-former, that as the primary and secondary cur-rent decrease, the by-pass current which causesthe error actually increases in percentage. Theerrors in current transformation typicallyincrease at the lower currents.
Figure 5: Typical Transformer Saturation Curve
110100
50
20
10
5
2
1
0 21 5 10 20 50 100 300Exciting Current %
Volta
ge A
pplie
d to
Ter
min
als
%
Currenttransformers
normally operateat rated current
in this range
45° line representingconstant exciting reactance constant per unit exciting current
Technology Review
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Ideal transformers induce the same voltage per turn in the sec-ondary winding as that applied to the primary voltage transform-ers. They also produce the same ampere-turns in the secondaryas circulated in the primary current transformers, to deliver anydesired ratio of primary to secondary voltage or current. In theactual transformer shown in Figure 4, the secondary current output is deficient by the amount of current bypassed by theexciting branches, Zo and Zi, and the secondary output voltage is deficient by the voltage drop in the transformer through impedance.
The transformer nameplates show a “marked ratio,” usually aneven number, such as 20 to 1. The actual ratio of primary to sec-ondary quantity may be slightly higher or lower than the markedvalue by an amount1 called ratio error, which is defined in IEEEC57.13 as ratio correction factor (RCF). For instance, if theactual ratio is 20.2 to 1, then the RCF is 1.01 and the ratio erroris 1%. The secondary output may be slightly out of phase withthe primary input. This error is called phase angle (usually meas-ured in minutes) and is designated as positive if the secondaryoutput leads the primary input.
1. The ratio of turns is usually adjusted to “compensate” for the losses in thetransformer. Because of this “compensation”, the actual secondary output canbe higher than would be expected from the marked ratio.
Ratio Error andPhase Angle
The Effect of Ratio Errorand Phase Angle onWattmeter Readings
If the ratio correction factor exceeds 1.0, the meters will readlow and the readings should be multiplied by the correction factor. The effect of phase angle, however, is not as obvious.The transformer correction factor (TCF) determines reasonablelimits for RCF and phase angle. This depends on both RCF and phase angle and may be used to correct the reading of awattmeter. TCF is based on the fact that if the power factor of a metered power load is 60% lagging (represents the usualminimum power factor of actual power loads being metered bywatt hour meters), 2.6 minutes of phase angle in the current (or voltage) transformer output will cause 0.1% error in thewattmeter reading.
Technology Review
10
The RCF and phase angle must be determined at the specificburden involved. However, as with an actual measurement, thesevalues apply to the secondary voltage at the transformer termi-nals. If the leads from the transformer to the burden are verylong, they may have sufficient impedance to introduce additionalvoltage drop and error. The voltage drop in the leads can be calculated from the current drawn from the transformer and the impedance of the leads. If it is of appreciable magnitude in percent of secondary voltage, the addition to ratio and phaseangle error may be calculated according to the vector diagram in Figure 6 and the formulas:
Percent Ratio will be increased by:
Add this amount to the percent ratio of the transformer to getthe actual percent ratio of primary to burden voltage.
The Phase Angle will be increased by:
Add this amount to the phase angle of the transformer (alge-braically) to get the actual phase difference between primary andburden voltages.
Voltage Transformers
RL
Ep EsIs
XL
E InstrumentE
Instrument
Voltage Transformer
RsEs
Is
IsXL
IsRL
0
Figure 6: Effect of Leads in Voltage Transformers
RL and XL represent the resistance and reactance in the leads. Method for calculating impedance drops in theleads and resulting ratio and phase angle errors.
+ XL sin ) ES
IS (RL cos 0 0 x 100
- XL cos ) ES
IS (RL sin 0 0 x 3438 Minutes
The ratio correction factor and phase angle must be determinedat the specific burden and current involved. Current transformercharacteristics at special burdens can only be determined by anactual measurement. The test burden must duplicate the actualburden, including the secondary leads. The secondary terminalvoltage and power factor must be identical to that of the installa-tion. In addition, these measurements need to be made andapplied at the actual service currents.
Current Transformers
Technology Review
11
AccuracyClassifications forMetering
Figure 7: The Basic 0.3 Class Parallelogram for Current Transformers
5
1.001
10 1551015
1.002
1.003
0.999
0.998
0.997
B
A
Minus Plus
RCF
Phase AngleMinutes
15.6 = 0.6 x 26
The Figure 7 parallelogram outlines the area in which themeasurements of RCF and Phase Angle at 100% current mustplot to designate the transformer accuracy as 0.3 Class withTCF within the limits of 0.997 and 1.003. For example, if theRCF at a given burden at 100% current is 0.998 (99.8% Ratio)and the Phase Angle is 3.5 minutes, point A is seen to fall out-side the parallelogram.
Another example: RCF = 1.002, Phase Angle 5 minutes, rep-resenting greater absolute error, point “B” is now inside theparallelogram and meets the required limits for 0.3 AccuracyClass. In the second case, TCF is less than 1.003 because theeffect of phase angle on the wattmeter compensates for theerror in ratio.
IEEE C57.13 recognizes that current transformers naturallyhave greater errors at lower currents, and that error at low cur-rent does not usually represent significant error in total regis-tration of kilowatt hours. This permits twice the error at 10%that is permitted at 100% current. The error at the maximumcurrent permitted by the Thermal Rating Factor of the trans-former (a multiplier of 1.5 or 4.0 applied to many transform-ers) is limited to the same value at 100% current.
Other Accuracy Classes: In addition to the 0.3 Class, C57.13recognizes the 0.6 and 1.2 Classes in which the permissibleerrors are twice as great (0.6%) and twice again (1.2%) ascompared to the 0.3 Class. Any one of these classes may beselected for specification by the user depending on whether0.3%, 0.6%, or 1.2% seems reasonable for a given application.
The Figure 8 parallelogram outlines an area in which themeasurements of RCF and Phase Angle at 100% (also at110%) voltage must plot to designate the transformer accuracyas 0.3 Class with TCF within the limits of 0.997 and 1.003.For example, if the measured RCF at a given burden is 0.999and the phase angle is -8 minutes, point A is seen to fall out-side the parallelogram.
Another example: RCF = 1.002, Phase angle is -10 minutes,both representing greater absolute errors, but the point “B” isnow inside the parallelogram and meets the required limits forthe 0.3 Class. In the second case, the TCF is less than 1.003because the effect of phase angle on the wattmeter compen-sates for the phase angle.
The reason for the reversed appearance of Figure 8 comparedto Figure 7 is that phase angle in the current transformerbrings the secondary current more nearly in phase with theload voltage, increasing the wattmeter reading. In the voltagetransformer, the effect is just the opposite.
C57.13 requires that the limits also be met at 90% voltage; inreality, the performance at voltages down to 5% are not signif-icantly different at the same burden connected to the trans-former secondary. The error limits required by C57.13 applynot only at a given burden, but also at zero burden.
Other Accuracy Classes: In addition to the 0.3 Class, C57.13recognizes the 0.6 and 1.2 Classes in which the permissibleerrors are twice as great (0.6%) and twice again (1.2%) ascompared to the 0.3 Class. Any one of these classes may beselected for specification by the user depending on whether0.3% , 0.6%, or 1.2% seems reasonable for a given application.
Figure 8: The Basic 0.3 Class Parallelogramfor Voltage Transformers
1.003
1.002
1.001
0.999
0.998
0.997
15 10 5 5 10 15
B
A
PlusMinus
RCF
Phase AngleMinutes
15.6 = 0.6 x 26
Figures 7 and 8 provide an explanation of accuracy classes for cur-rent and voltage transformers. These figures show the AccuracyClasses as adopted by IEEE, as well as the special limitationswhich apply to current and voltage transformers. IEEE C57.13 hasrecognized 0.3% as a reasonable error limit and has designated thisas “Accuracy Class 0.3.”
Technology Review
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The errors in ratio and phase angle depend on the impedanceconnected to the secondary of the transformer. This impedanceis commonly referred to as “burden”. The calculations requiredfor determining the performance of a transformer when differ-ent burdens are applied are beyond the scope of this discussion.Therefore, the standard burdens as outlined in IEEE C57.13 areused to represent typical service conditions. Each transformer israted according to its performance at these standard burdens.
Burdens
Standard Burdens forCurrent Transformers
Actual Burdens forCurrent Transformers
Standard Burdens for Current Transformers with 5 A Secondaries *
BurdenDesignation +
Resistance(Ω)
Inductance(mH)
Impedance(Ω)
Volt Amperes(at 5 A)
PowerFactor
Metering BurdensB-0.1 0.09 0.116 0.1 2.5 0.9B-0.2 0.18 0.232 0.2 5.0 0.9B-0.5 0.45 0.58 0.5 12.5 0.9B-0.9 0.81 1.04 0.9 22.5 0.9B-1.8 1.62 2.08 1.8 45.0 0.9
Relaying BurdensB-1 0.50 2.3 1.0 25.0 0.5B-2 1.00 4.6 2.0 50.0 0.5B-4 2.00 9.2 4.0 100.0 0.5B-8 4.00 18.4 8.0 200.0 0.5
Many current transformers supply only a limited number ofwatthour meter elements with a limited number of runs. Formetering and relaying applications, IEEE C57.13 has establishedthe standard burdens as given in Figure 9.
Figure 9: Standard Burdens for Current Transformers
* If a current transformer is rated at other than 5 A, ohmic burdens for specification and rating may be derived by multiplying the resistance and inductance of the table by [5/(ampere rating)]2 the VA at rated current and the power factor remaining the same.
+ These standard burden designations have no significance at frequencies other than 60 Hz.
Actual devices connected to instrument transformers ofteninclude an inductor with an iron core, which usually means thatthe inductance is not constant but varies during the cycle, andvaries differently with different currents. Exact analysis of cur-rent transformer performance with such devices is difficult.Fortunately, the impedances of most instruments and meters aresufficiently constant that no appreciable error is introduced byconsidering them to be constant. Many electro-mechanical relays,however, have variable impedance. Analysis of the transformerperformance is usually based on an equivalent value at normalcurrent. This can be justified on the basis that the burden athigher current is usually less and thus the current transformerwill perform better than expected from the equivalent burden.
Technology Review
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Some relays operate from two or more sources of current: differ-ential (current-current), or power or impedance-measuring (cur-rent-voltage) relays. If the two circuits are magnetically coupledby the relay, the burden on one source is affected by the currentin the other source, and vice versa. Most of the two-source relaysact by balance-beam or other mechanical coupling, so that theburdens are fixed.
Standard Burdens forVoltage Transformers
Standard Burdens for Voltage Transformers
BurdenDesignation
VoltAmperes
PowerFactor
Burden Impedance120 V Burden 69.3 V Burden
MeteringBurdens
W 12.5 0.10 1152 384
X 25 0.70 576 192
M 35 0.20 411 137
Y 75 0.85 192 64
Z 200 0.85 72 24
ZZ 400 0.85 36 12
Figure 10: Standard Burdens for Voltage Transformers
The standard burdens to be used for testing and comparing volt-age transformers are rated at 120 volts and at 69.3 volts. IEEEC57.13 specifies that the 120 volt-rated burden will be used forany transformer with the secondary voltage in the range of115 to 120 volts, while the 69.3 volt burden will be used for anytransformer with the secondary voltage in the range of 65 to 72 volts. This means that the actual volt amperes in the burden in a given test may be different than the nominal value of theburden in volt amperes. For instance, if the standard burden is 25 volt amperes, the actual burden when it is used for testing atransformer with 115 volt secondary is (115/120)2 or .918 timesthe nominal value of 25.
The burdens rated 69.3 volts have an impedance only one-thirdof that of burdens rated 120 volts and they should not be used in testing or rating transformers rated at 115 to 120 volts.Transformers rated at 115 or 120 volts should be treated as 115 or 120 volt transformers, and if they are actually used atreduced voltage, the performance will not be different if the 120 volt burden is used as a basis for performance. This isbecause the performance of a transformer down to voltages of about 5% of its rating is not significantly different from theperformance at 100% voltage.
Refer to Figure 10 for standard burdens for voltage transformersas outlined in IEEE C57.13.
Technology Review
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Relaying accuracy classes for CTs are defined with a “C” or a “T”classification.
“C” indicates that the transformer ratio can be calculated. Theseare transformers which are constructed so that the effect of leak-age fluxes on its performance are negligible.
“T” indicates the transformer where the leakage flux has anappreciable effect on the ratio. Since the calculation of the excita-tion current by-passed is a tedious process, the performance ofthe transformer can only be determined by test.
The basis for classification of performance for relaying is anerror limit of 10% at any current from 1.0 to 20 times normal.The accuracy class is the description of how much voltage thetransformer can supply to the output circuit (burden), withoutthe CT core going into saturation.
For example, a transformer that can supply a 2 ohm output cir-cuit (burden) at 100 A [20 times normal current (5 A)] or 200 V,without saturating the core and within a 10% error limit, is classi-fied as 200 accuracy class. Refer to Figure 11.Standard accuracy classes, which may be assigned for a relayingcurrent transformer, are 50, 100, 200, and 800. If a C200 trans-former can supply 100 A secondary output at exactly 10% errorinto a 2 ohm burden, then the exciting branch is not over 10 amperes. If the current is lower, then the burden can behigher without exceeding the output voltage limit if a trans-former can carry 2 ohms at 50 amperes and deliver 200 volts.However, if the burden is 1 ohm at 200 amperes, it will not worksince the internal impedance will be significant in relation to the1 ohm burden.
Current Transformers:Accuracy Classes forRelaying
Figure 11: Accuracy Standard Chart for Class C Current Transformers
0
100
200
300
400
500
600
700
800
Seco
ndar
y Te
rmin
al V
olts
Secondary Amps
10 10090807060504020 30
C800
C100
C200
C400
1 Ohm
2 Ohm
4 Ohm
8 Ohm
Technology Review
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Insulation Systems
Partial Discharges inTransformer Insulation
Figure 12: Partial Discharges
Source ofVoltage
Electrode
Solid Insulation
Void Space
Source ofVoltage
CapacitanceRepresentingVoid Space
~~
Normal Charging Current
Applied Source Voltage
Cur
rent
Vol
tage
............................................................................................................... ........... .....
............................................................................................................... ........... .....
A B
C
A. A sample of solid insulationbetween electrodes, which forms acapacitor, can have a void as shown.
B. The electrically equivalent capaci-tance network.
C. Applied voltage and the correspon-ding PD current caused by breakingdown the void.
Partial discharges (PD) are minute electrical discharges that resultfrom the electric field stresses imposed on any insulation system.As the name suggests, they do not cause a complete electricalbreakdown of the insulation, so their short term effect is not cat-astrophic. Over the long term, if the electric field stresses arehigh, PD can slowly deteriorate the quality of the insulation. Insolid insulation systems (such as in instrument transformers), PDcan occur where a void or discontinuity in solid insulation isintroduced Figure 12. Because of the difference between dielec-tric properties of a void (filled with air or gas) and solid material,the localized electrical stress in the void can be higher than in asolid. This will cause a void to break down although the voltageacross the solid will remain (Figure 12). These localized voidbreakdowns, resulting in small, high frequency current impulses,can be detected by using sensitive instrumentation. Sophisticated,specialized analysis of PD patterns can then be used to gaininsight to the nature of PD, its possible location, and mitigation.PD is measured in the pico Coulombs (pC) unit of electricalcharge. After many years of deliberation, different standards fordifferent electrical equipment (ANSI, IEC, IEEE) do not consis-tently agree on the allowable or maximum limits of PD.Manufacturers of ITs use different ways of minimizing or con-trolling the level of PD:• Control manufacturing processes (casting, curing,
temperatures, vacuum, viscosities, etc.) to minimize the introduction of voids
• Develop shielding materials and techniques to minimizeelectric field stress enhancement
• Fill voids with dielectric gas to lower the risk of voidbreakdown
• Use insulation materials with similar dielectric constants forthe solid insulation system
Technology Review
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All insulation materials are deteriorated by the combination ofoverheating and exposure to moisture, oxygen in air, and UVradiation in outdoor conditions. ABB insulation systems aredesigned to withstand degradation caused by all of these environ-mental factors.
IEEE Standard C57.13 recognizes two classes of insulation inso-far as resistance to temperature is concerned: (a) 55°C tempera-ture rise and (b) 80°C rise, both over a 30°C daily averageambient temperature. These values are the average temperaturerise of the winding (as measured by rise of resistance) during thetemperature test at rated maximum continuous current. TheGuides for Loading recognize that these temperatures can beconsiderably exceeded for short periods of time without causingexcessive deterioration of the insulation (reference Section 10 ofIEEE C57.13).
Overloading, Overheatingand Aging
Maintenance andInspection Testing ofInsulation
Instrument transformer users routinely test new transformers, aswell as transformers in service, to ensure their adequacy for serv-ice. It is rarely possible for the end user to run complete series oftests, but there are some things the user can do for reassurance.
Measurement of the resistance of each winding to ground (when one winding is measured, ground all other winding termi-nals) with a megger will indicate if something has happened toreduce the resistance values. Such an incident is most improbableon encapsulated transformers. All ABB insulated current andvoltage transformers should have typical readings from the highvoltage winding to the low voltage winding, and ground above 1 Megohm per volt at 25°C.
Insulation resistance should be measured at ambient temperature(not over 30°C) because it decreases rapidly at higher temperatures.
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Voltage Ratings
Voltage Ratings forCurrent Transformers
Current transformers are always rated at the line-to-line voltageof the three-phase system on which they will operate. A 13.8 kVcurrent transformer, for example, is designed for use on a 13.8 kV three-phase system. The actual voltage from the currenttransformer primary winding to ground is only 13.8/e3 or 7.9 kV as shown in Figure 13.
13.8 kV
13.8 kV
13.8 kV
13.8/√3 kV
13.8/√3 kV
13.8/√3 kV
Line Capacitance
Figure 13: Equivalent System Diagram
Whether or not this neutral is actually connected to ground, the natural symmetry of the cir-cuit and the equal line capacitors to ground will cause the neutral to assume ground potential.
In the Transformer Standards IEEE C57.13, an insulation class,which has the appearance of a system voltage rating, is associatedwith each of the standard arrays of dielectric tests (60 Hertz andimpulse voltage). It has become standard practice to apply trans-formers on systems with actual voltage higher than the insulationclass value. This is done on the basis that if the power system isdesigned such (grounded, usually) that the line-to-ground voltagecan never be more than 70% or 80% of the line-to-line value,lower-voltage-rated lightning arresters can be used and the insula-tion is protected from all the higher voltages to which it mightotherwise be subjected.
The transformer test voltages consist of a full wave impulse,chopped waves, 60 Hertz applied, and induced voltage test accord-ing to the schedule outlines in IEEE C57.13
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The voltage which may be applied to a voltage transformer islimited not only by the permissible voltage to ground (as it is in acurrent transformer) but by the insulation between turns,between layers, and coil sections. It is also limited by the ability ofthe core to carry enough magnetic flux to induce the voltage.The voltage transformer is somewhat different from the distribu-tion transformer, and certainly from the power transformer, inhaving a very limited capacity to store energy.
Voltage Ratings forVoltage Transformers
Over Voltage Limits
Figure 14: Application Limitation - Group 1 Voltage Transformers
Normal Operation Transformer Primaries in Delta
Normal Operation Transformer Primaries in WyeTransformer Neutral Grounded.
Fault to Ground must be cleared quickly because Group 1 transformers are good for only 125% voltage for emergency service.
Fault to Ground may be permitted to exist as far as the transformers are concerned.
Grounded or Ungrounded
System
Grounded or Ungrounded
System
SystemLine-to-Line
Voltage
TransformerRated Voltage
SystemLine-to-Line
Voltage√3 x Transformer
Rated Voltage
Primar
yW
indi
ng
H1
H1
H1
H1
H1
H1
H2
H2
H2
H2H2
H2
IEEE C57.13 recognizes five groups of voltage transformers fordifferent capabilities and connections. These groups areexplained by Figures 14 through 18.
All transformers, according to IEEE C57.13, are capable ofoperating continuously, and of maintaining their accuracy at110% of rated voltage. As indicated in the figures, Group 2transformers need do no more than this, and Group 1 trans-formers must be able to operate (but not necessarily maintainaccuracy) at 125% voltage during emergencies, while Group 3and 4 transformers must be able to operate for one minute atline-to-line voltage, which is e3 times their rating. Group 5 mustbe able to operate (but not necessarily maintain accuracy) at140% voltage for one minute.
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Figure 15: Application Limitation - Group 2 Voltage Transformers
Normal Operation Transformer Primaries in Delta
Transformer Neutral Grounded
Normal operation transformer primaries in Wye
at on each winding Fault to Ground may be permitted to exist as far as the transformers are concerned.
Fault to Ground must be cleared fairly quickly because the insulation at the terminals of Group 2 transformers is not good for rated winding voltage to ground continuously.
Grounded or Ungrounded
System
Grounded or Ungrounded
System
SystemLine-to-Line
Voltage
TransformerRated Voltage
SystemLine-to-Line
Voltage
TransformerRated
Voltage
Primar
yW
indi
ng
H1
H1
H1
H1
H1
H1
H2
H2
H2
H2H2
H2
Rated Voltage__________√3
Figure 16: Application Limitation - Group 3 Voltage Transformers
Fault to Ground must be cleared within one minute because Group 3 transformers are good for more than 110% voltage for only one minute: 173% for 1 minute for ratings thru 92,000 for 161,000 Ground Y, 140% for 1 minute for ratings above this.
These terminals are insulated only for 5 kV class (to permit measurement of insulation power factor) and must be connected to ground for normal operation. Typical transformer rating 14,400 for 24,940 Ground Y.
GroundedSystem
SystemLine-to-Line
Voltage
√3 x Transformer Rated Voltage
These transformers have two secondary windings, one with a rating of 115 volts (except for the lowestrated 14400 for 25000 ground wye which is rated at 120 volts) and another secondary winding rated atapproximately 115 V/e3. The value is not always exactly 115 V/e3 because, for simplicity, the primary/secondary voltage ratio is adjusted to a round number.
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Figure 17: Application Limitation - Group 4 Voltage Transformers
Fault to Ground must be cleared within one minute because Group 4 transformers are good for 125% voltage for only one minute.
These terminals are insulated only for 1.2 kV class (to permit measurement of insulation power factor) and must be connected to ground for normal operation. Typical transformer rating 7,200 for 12,470 Ground Y.
GroundedSystem
SystemLine-to-Line
Voltage
√3 x Transformer Rated Voltage
Figure 18: Application Limitation - Group 5 Voltage Transformers
Fault to Ground must be cleared within one minute because Group 5 transformers are good for 140% voltage for only one minute.
These terminals are insulated only for 1.2 kV class (to permit measurement of insulation power factor) and must be connected to ground for normal operation. Typical transformer rating 14,400 for 24,940 Ground Y.
GroundedSystem
SystemLine-to-Line
Voltage
√3 x Transformer Rated Voltage
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Power Quality
Voltage Sags and Dips
Momentary Interruptions
Figures 19A and 19B: Distribution of RMS Events
SinglePhases
68%
TwoPhases
19%
ThreePhases
13%
(V< 0.1)9%
0.1< V< 0.510%
0.5<V<0.981%
Interruption
19A. Pie-chart showing the statistics of voltage sags and interruptions. Note most events (81%) are sags (between 0.5 pu and 0.9 pu).
19B. Pie-chart showing the statistics of all power quality eventtypes. Note most events (68%) are single phase.
Power quality is an important consideration in designing anypower system. Distribution systems are especially vulnerable topower quality problems. These problems should be taken intoaccount when selecting and purchasing equipment. Distances forpower delivery, density of loads, and customer concentrations areamong the aspects that differ between the US ANSI market andthe European IEC market. Power quality problems include:• voltage sags and dips• momentary interruptions (flicker)• harmonics (harmonic current and/or harmonic voltages)
Voltage sags and dips are associated with switching or faultevents in the power system that cause voltages on adjacent orneighboring circuits to partially collapse. These events can lastfrom a few milliseconds to more than a second.
Momentary interruptions are the same type of power systemevents as voltage sags and dips, but are caused by lightning andother transients. The primary difference is that momentary interruptions occur primarily in the circuits directly involved in the event rather than in an adjacent circuit. Momentary inter-ruptions are more severe power quality problems than voltagesags and dips.
Figures 19A & 19B show statistics of voltage sags and momen-tary interruptions in a typical ANSI power system.
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Harmonics are caused by non-linear loads. The nature of non-linear power equipment causes a perfectly sinusoidal voltagewaveform to result in currents containing other frequencies. Asinusoidal current can cause the generation of non-sinusoidalvoltages. Non-linear power equipment includes saturated trans-formers, motors, and generators, but is primarily associated withpower electronics. The act of triggering or switching a SiliconControlled Rectifier (SCR), a diode, an Insulated Gate BipolarTransistor (IGBT), or a Gate Turn-Off device (GTO), is in prin-ciple a non-linear operation. Power electronics devices such asAdjustable Speed Drives (ASDs), or Variable Speed Drives(VSDs) can exhibit a high level of harmonics, often more than100% Total Harmonic Distortion (THD). In the case of ASDs,this level of harmonics varies with the selected rpm of themotor, and mechanical load on the shaft.
Harmonics are unwanted events in distribution systems. They cancause excessive heating and damage to neutral connections andcables, and can saturate instrument transformers. Harmonics mayalso precipitate from the original location of the non-linear equip-ment to other locations like feeders and loads. They also maycause false tripping or malfunctioning of equipment, false read-ings from the CTs and VTs especially sensitive relays, computerloads, other ASDs, and Programmable Logic Controllers (PLCs).
There are a variety of techniques to mitigate and control the levelof harmonics, and industry standards regulating harmonics (seeIEEE 519, Standard Practices and Requirements for HarmonicControl in Electrical Power Systems, or IEC 555/1000-3).Possible mitigation procedures include reconfiguring the system,re-sizing the cables or transformers to include additional loadingdue to harmonic currents, or installing filters and harmonicblockers. It is important to remember that these measures workfor some, but not for all harmonics. Often a comprehensivestudy has to be performed to determine the level of harmoniccurrents, their impact on the power system, and possible suppres-sion measures.
In real systems, one disturbance in the system can cause a cas-cade of other disturbances. For example, a lightning surge (fastovervoltage transient) traveling along an overhead feeder line cancause a flashover and subsequent short circuit in a substationsupplying an industrial plant, which in turn can cause abnormalacceleration of a local generator and so on.
Harmonics
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A voltage transformer is connected across the line or line-ground, and is loaded to a greater or lesser degree depending onthe number of devices connected in parallel at the secondary ter-minals (Figure 20). As the load is increased, the curves for ratioerror and phase angle will show how the accuracy is affected.
If accuracy is not important, the load can be increased to thethermal volt-ampere rating, the maximum which can be carriedwithout overheating. Voltage transformers must be able to with-stand an accidental short circuit for one second.
A current transformer’s primary is connected in series with theline and must carry whatever current flows in the line. The bur-den impedance connected to the secondary terminals affects theaccuracy, as shown by the curves for ratio error and phase angle,but generally has no significant effect on temperature.
Because watthour meters are usually capable of carrying at least400% of rated current continuously, many of those currenttransformers used almost exclusively with watthour meters havebeen designed specifically to carry two to four times normal cur-rent. Also, because many of those current transformers are usedalmost exclusively in enclosed switchgear, they must be designedto operate in a high ambient temperature (55°C), they will carry1.33 times normal current in a normal (30°C daily average) ambi-ent temperature. The factor designating the continuous currentcapacity in terms of rated current at 30°C ambient is called ther-mal current rating factor. Standard values are 1.33, 1.5, 2.0, 3.0and 4.0. The continuous thermal rating factor is based on 30°Caverage ambient temperature unless otherwise noted.
The IEEE Guides for Loading recognize that transformer insula-tion can withstand a considerable degree of overheating for ashort time without severe deterioration. For example, in the eventof a line short circuit, the fault current may easily be fifty timesthe rated current of the current transformer, but will probablyflow for not over one second. IEEE C57.13 permits a 250°Ctemperature for this very short time (compared to 95°C averagecontinuous winding temperature). All current transformers areassigned a one-second thermal current limit which denotes howmuch current they will carry (usually denoted in terms of timesnormal rated current) for one second. For durations up to fiveseconds, current transformers will carry currents lower than this.
The transformer will carry more current for a time less than onesecond according to the same rule, up to the mechanical currentlimit (which is also given for standard current transformers.) Atthis limit of current, the electro-mechanical forces tending toseparate the primary and secondary coils, become high enough todamage the transformer.
Service Conditions
Overload and Short CircuitCapability
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The mechanical current limit is specified in terms of times nor-mal rated current, as is the thermal one-second limit, but it isalways assumed that the current may be fully offset initially. If itis known that the current cannot be initially fully offset, the cur-rent transformer will be able to withstand mechanically a largerRMS value of current, larger than its rated mechanical limit.
The temperature at these high currents and short times (less thanfive seconds) cannot be measured, but is always calculated on thebasis that all heat generated by the current is stored in the copperfor the duration (not over five seconds) of the high current.
Currents higher than the rated value, but less than the five sec-ond limit, can be determined by the rules set forth in the IEEEGuides for Loading. The calculation for any given transformer islengthy, and as a general guide for standard transformers, thecurves of Figure 20 can be used.
At temperatures over the standard 30° C, daily average ambienttemperature current transformers should be derated 1% for eachdegree over (up to 55°C ambient). At temperatures under 30°C,they may be uprated 0.75% for each degree (down to 0°C).Special application CTs are available where bus bar temperature isas high as 135°C (refer to Figure 21).
Higher and Lower Ambient Temperatures
Figure 20: Overload Capability of Current Transformers
20
15
10 9876
5
4
3
2
1.5
11 2 3 5 7 10 20 30 7050 100Time in hours, minutes, and seconds following full load
Tim
es M
axim
um C
ontin
uous
The
rmal
Cur
rent
Hours
Minutes
Seconds
Recommended guide for short time loading of current transformers following rated loadfor 0.1% loss of life. Transformers so loaded will reach temperatures in excess of 55°Crise over ambient. Loading according to this curve is not safe if the ambient exceeds 30°Con the average, or if the overload occurs more often than once a day.
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Open-Circuit Voltage inCurrent Transformers
Figure 2 (page 5) shows that the line current must flow throughthe current transformer, and Figure 4 (page 8) shows that if thesecondary circuit is accidentally opened, all the current will haveto pass through the exciting current branches of the equivalentcircuit. This will develop a high voltage across the excitingbranch, which will appear as a high voltage at the secondary ter-minals. Because this voltage is limited by saturation of the core,the RMS value measured by a voltmeter may not appear to bedangerous. As the current cyclically passes through zero, the rateof change of flux at current zero is not limited by saturation, andis very high indeed. This induces extremely high peaks or pulsesof voltage.
These high peaks of voltage may not register on the conventionalvoltmeter, but they can break down insulation and are dangerousto personnel. Current transformers are insulated to withstand, foremergency operation, secondary peak voltages up to 3500 volts.This takes care of the smaller transformers with relay accuracyclass under T200, but if open-circuit of larger transformers isprobable, some protective circuit should be permanently con-nected to the secondary terminals. In general, open-circuit of thesecondary terminals should be considered as a serious accident.
The actual open-circuit voltage peak is difficult to measure accu-rately because it exists only as very short peaks. The method out-lined in the Test Methods Section of IEEE C57.13 represents anexcellent compromise between precision and feasibility.
Figure 21: 55°C Rise Current Transformer Thermal Loading Guide
Average Ambient Cooling air temperature for 24 hour period, degrees C.(Maximum ambient air temperature shall not exceed average by more than 10°C.)
0 10 20 30 40 50 600
50
100
150
200
250
300
350
400
450
500
1.0
1.33
1.5
2.0
3.0
4.0
Per
cent
of
rate
d pr
imar
y cu
rren
t
Curve designations arecontinuous thermal current rating factorsat 30°C ambient air temperature.
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If a system short-circuit occurs, with a current of several timesnormal, the voltage at the burden may be rather high. The fluxdensity in the equivalent exciting reactance (the core of the cur-rent transformer) may be high enough that if the fault current isabruptly interrupted, the core may be permanently magnetized ata fairly high flux density. This is often referred to as residualmagnetization.
If the secondary circuit of the current transformer is accidentallyopened, the flux density will become very high and even if thecircuit is immediately closed again, the core may be left with per-manent magnetization.
When normal current and flux variation is restored, the flux variation starts from the residual value and varies as shown inFigure 22. If the flux starts to increase from point a, the fluxvariation cannot be maintained in loop a-A in Figure 22, becausesuch a loop would require direct current to maintain it in its off-set position. The flux loop must shift down to the symmetrical(around the vertical axis) loop c-C. As it shifts down it actuallygenerates a small direct current in the secondary circuit. The sec-ondary burden will establish the rate of change. The flux varia-tion will stay in this loop indefinitely.
The slope of the loop c-C will be less than the slope of the nor-mal, completely symmetrical loop at the origin The peak excitingcurrent S1´ will be higher than the normal exciting current S1. Ifthe alternating flux density increases, the hysteresis loop movesto d-D and the slope of this loop becomes nearly equal to thesymmetrical loop. The exciting current S2´ is still greater than thesymmetrical value, but not as much greater in percent, as the dif-ference between S1´ and S1.
The final result is that the effective exciting current, which causesratio error and phase angle, is increased if the core becomes per-manently magnetized. This will usually cause no more than 0.1%and 3 minutes of additional error in practical metering trans-formers at metering burdens10. The difference is less dependenton burden and current than might be expected. This is becausethe increase in exciting current from permanent magnetism is lessat low flux variation, and moves to a constant ampere-turn valueas the flux variation increases.
If precision measurement of ratio and phase angle is important,especially if ratio and phase angle measurements made at differ-ent times or in different laboratories are to be compared, demag-netization of the core by the conventional method of applyingan alternating voltage high enough to circulate rated current inthe secondary winding, reducing it gradually to zero, is desirable.(See IEEE C57.13).
10. At 100% current
Permanent Magnetization ofCurrent Transformer Cores
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Figure 22: Permanent Magnetism in Current Transformers
Flux
Amperes Exiting Current
Equal FluxVariation
a
c
d
S1
D
C
A
Y
Y
S2'
S1'
To allow loop a-A to exist, a direct current must flow, requiring a DC voltage. This in turn requires adownward change in flux to induce it, with the result that the loop shifts down to c-C which is symmetri-cal about the vertical axis and requires no direct current. Operation is stable over this loop. The iron ispermanently magnetized by having been at point a, and the loop c-C will not of itself shift down further.Imposition of a still higher AC flux variation will cause the larger loop d-D, shifted down from c-C. Avery high flux density will restore the original symmetrical loop y-Y.
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Technology Review
An ungrounded system is always actually grounded, although perhaps very poorly, by the capacitance and resistance of its insulation-to-ground, as shown in Figure 24. If a voltage trans-former is connected from one line-to-ground it is, in effect,connected in parallel with a capacitor. The equivalent circuit isshown in Figure 24. Figure 24 also shows how parallel resonancecan occur between transformer and capacitor, and how it cancause a very high voltage with destruction of the voltage trans-former. If examination of a voltage transformer which has failed,shows only the primary winding uniformly roasted from end-to-end, gross over-excitation is undoubtedly the cause and “ferro-resonance” can be suspected. It is called ferro-resonance becausethis resonance depends on partial saturation of an iron core.
The number of combinations of constants which can causeferro-resonance is large, and the analysis of the circuit and pre-diction of possible destructive voltage is not simple. For ABBtransformers of present and foreseeable future manufacture, thisworks out as shown in Figure 23.
These high loadings may cause errors greater than 0.3%, buttransformers connected line-to-ground on an ungrounded systemare rarely used for metering in the ANSI market (units connectedline-to-line are more common). If three transformers are con-nected in Y on the primaries, the secondaries should never beconnected in Delta.
Voltage TransformerConnected Line-to-Groundon an OtherwiseUngrounded System
Figure 23: Watt Loading - Ferro-resonance Prevention
Voltage Class(Primary)
Watt Loading onSecondary to Prevent
Ferro-resonance
Approximate OhmsEquivalent
Resistance perPhase (120 V)
Connected AcrossCorner of
Broken Delta
To 500 200 72 216
7200-15000 500 29 87
25000 and up 750 20 60
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Technology Review
Figure 24: Ferro-resonance Voltage Transformers
Line-to-Ground Capacitance Circuit
A
C
B
Voltage
Total Current
Possible Over VoltageWith Very HighExciting CurrentNormal Voltage
Transformer Lagging Current
Lagging CurrentLeading Current
Capacitor Leading Current
Ep
A. Line-to-ground capacitance circuit
B. Equivalent Y circuit
C. Approximate possible curve of current (neglecting resistance components) showing that at some particular value of voltage (EP) the capacitor and transformer’s reactive current total zero, representing infinite impedance, with the result that the voltage-to-ground on this line may reach EP. This is a high enough value that excessive exciting current will burn out the primary winding.
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Many circuit problems can be solved by connecting one currenttransformer to supply another current transformer, usually calledan Auxiliary current transformer, shown in Figure 25.
Auxiliary current transformers perform like other current trans-formers, but there are certain problems associated with their usewhich merit discussion.
First, the auxiliary current transformer constitutes an additionalburden on the main current transformer, which usually increasesthe errors of the main current transformer.
Second, the performance of auxiliary current transformers is notusually as good as the performance of main current transform-ers. This is because the auxiliary current transformer is designedto impose as small a burden as possible on the main currenttransformer. This means its own burden capacity must be rela-tively low.
These two considerations mean that the errors of transforma-tion, when an auxiliary current transformer is used, will typicallybe three times the value which might be expected with a singletransformer. However, the special functions which can be per-formed by the auxiliary current transformer, as indicated inFigure 25, often dictate their use.
Auxiliary CurrentTransformers
Figure 25: Auxiliary Current Transformers
Adjustable RatioAuto-Transformer to Get
any Special Ratio Desired
High Ratio Transformerto Produce Low Current
(0.1 Amp Typical) Suitablefor Long Distance Transmission
Transformer to Have 1/√3 Ratio or Multiple Thereof for Certain
Y-Delta Circuitry
MainCurrent
Transformer
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Primary fuses are primarily used with voltage transformers to takethe transformer off the line in the event of an internal failure.This prevents a failed VT from becoming an L-G fault, whichrequires the breaker to interrupt the customer’s electric service.
Since modern transformers are much more reliable than olderones, the fuses may never operate due to internal transformer fail-ure. Transformer failure may still be caused by overload or shortcircuit on the secondary, however. If such failure occurs it mayinvolve other apparatuses, perhaps causing an outage.
Modern fuses are more substantial and reliable than older fuses, sothe chance of fuse failure is minimized. Only the exceptionalinstallation will be so dependent on continuity of voltage that thetransformer must be connected solidly to the primary circuit. Withmodern fuses, the principal disadvantages are the cost of fusesand mounting and the space they require.
Most primary fuses will also protect the transformer against par-tial short circuit in the primary winding and against secondaryshort circuit. Some operators choose fuses which will not operateon secondary short circuit conditions.
All voltage transformer fuses will interrupt only a certain maxi-mum fault current, therefore current limiting fuses are utilized.These current limiting fuses have higher interrupting ratings andcan, in most cases, be used without external resistors to limit the current.
Primary fuses may be mounted on indoor transformers at the factory or they may be in separate mountings. Generally, separatemountings increase the cost and the space required for installation.
Whether to use fuses or not is determined by the operator’s established practice, but operation without fuses has proven satis-factory in the majority of installations. The same considerationsapply for outdoor transformers, but separately mounted fuses intheir own mountings must be used. They are not mounted on the transformer.
The use of secondary fuses is relatively rare and is a matter ofpersonal choice with the operator. Such fuses should be rated sothat they will carry the secondary current indicated by the thermalVA rating of the transformer, but should fault in a short time atany higher current to protect the transformer. If mechanical fail-ure of a fuse occurs and is not quickly detected, a considerableloss of revenue or protection may occur.
Fusing VoltageTransformers
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New DesignTechnologies
New Process Technologies
Cost-effective, highly reliable electrical equipment is the result of adesign process that combines optimized electrical and mechanicaldesign features, efficient use of the best insulation material available,and highly refined manufacturing processes. This innovative approachis used on the next generation of ABB instrument transformers.
New Material Technologies
Wider availability of advanced simulation software has providedengineers with many tools to develop new products and associatedtooling without incurring large prototyping expenses.
In the next generation design of instrument transformers, ABBuses Ansoft's Maxwell® 3D electrostatic simulation software. Thisprogram calculates electric field stress distribution on the surfaceand inside the cast electrical device. For each insulation materialcandidate, different field stress distribution is obtained dependingon the dielectric properties of the material. Furthermore, the engineers can vary the conditions of the environment outside thesimulation model to obtain field stress distribution specific to the environment.
The 3D-based design features are then imported into an ABB-proprietary software package (Reactive Molding RAMZES).This program optimizes the casting process and parameters,and calculates the resulting mechanical stresses and strains thatdevelop inside the casting. Optimum mechanical robustness can be designed into the product through the variations of these casting parameters.
In the early stages, a vacuum-encapsulating process was mostcommonly used for epoxies. This more forgiving process elimi-nated the costly mechanical reinforcement of the core-coil assem-bly as required with the high pressure molding of butyl rubber.During the 1980's, liquid injection casting of epoxies was intro-duced. The process was further automated and called automaticpressure gelation (APG). The APG process effectively shortensthe encapsulating cycle time and has become the process ofchoice for epoxies.
Advanced DesignTechnologies
To yield the most reliable and affordable transformers, hydropho-bic cycloaliphatic epoxy (HCEP) was configured into the electro-static simulation model for product optimization.
HCEP was developed by a global epoxy supplier using a propri-etary formulation and special processing techniques to produce animproved outdoor epoxy. It sustains surface hydrophobicity betterthan its cycloaliphatic (CEP) counterpart upon prolonged expo-sure to aggressive outdoor environments, without sacrificing otherdesirable chemical and mechanical properties. (The advantageouscharacteristics of hydrophobic insulation surfaces have been previ-ously discussed in the Technology Review section.) The resultingoutdoor equipment possesses the following attributes:• Improved weatherability and outdoor aging• Better performance in heavily polluted environments• Enhanced reliability and life expectancy of the product
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The best design utilizing the best material still does not guarantee a reliable and long-lasting performance if it is not properly manufactured.
Although the APG process has been applied to epoxy casting formany years, the large window of casting parameters such as tem-perature, pressure, and curing time still presents a great challengeon how to optimize the process to obtain a superior product.A trial and error approach is one way, but this leads to high prototyping costs and considerable evaluation time.
As previously mentioned, the use of advanced simulation software(RAMZES) greatly shortens this exercise and allows engineers toefficiently optimize the casting process. When the APG process iscoupled with the latest resin handling, mixing, and process controltechnologies, the result is a reliable, robust, and cost-effectiveproduct.
Voltage Sensing
Parallel to the instrument transformers, the new techniques ofmeasuring voltage and current are becoming commercially available.
Resistive voltage dividers are becoming a commercially viablealternative to measuring voltage. Although they draw slightlylarger (resistive) current, they can be embedded in a cast of soliddielectric material (such as epoxy or polyurethane) and provide astable output in a wide frequency range, including high order har-monics. The voltage from the resistive divider is equal to:
Capacitive voltage dividers have been in high voltage testing for along time. They provide a stable output proportional to the ratioof capacitances.
Capacitive voltage dividers have very high inherent impedance sothey have to be connected to a high-impedance (light) burden.With the advancement of electronic relays and meters, this isbecoming a commercially viable option for some applications.
Optical voltage sensors are available for high voltage applicationswhere the insulation line-to-ground is a major concern. Opticalvoltage sensors are based on the electro-optical phenomenoncalled Kerr effect by which a light-wave is polarized dependingon the electric field generated by the voltage in the system. Thetechnology is available to be used in distribution systems as well,but some improvements in cost and complexity of the opticalsystems are still needed.
VSEC = VPRI * R SEC
R PRI + R SEC
VSEC = VPRI * C PRI
C PRI + C SEC
New Sensing Technologies
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Rogowski coils or linear couplers are essentially air-core trans-formers that use a power conductor as a primary winding, and anair-core coil wrapped around it as a secondary. The difference isthat the primary current induces a secondary voltage signal in theRogowski coil (not a current), which is a proportional derivativeof the primary current dI/dt. This requires some careful process-ing of the dI/dt signal to convert to the true measurement ofthe primary current. Again, with the advancement of the elec-tronic signal processing, these devices are now commerciallyavailable. Rogowski coils are very linear and do not saturate, evenunder extreme current conditions and high frequencies.
Magneto-optical phenomenon, called Faraday’s effect, can beused to measure the current as well. When polarized light passesaround the conductor with the current, the magnetic field associ-ated with the current changes the polarization angle of the light.The change can be measured optically and converted to the elec-trical signal, proportional to the primary current. Optical trans-ducers are inherently good for very high voltages since they donot require any electrical connection to the primary phase con-ductors. The devices are linear over their entire current range.
Current Sensing
Technology Review
35
37
General Description 38
Contemporary Design 39
Indoor Medium Voltage 39
Outdoor Medium Voltage 39
600 V Indoor (Plastic Case) 40
600 V Indoor/Outdoor (Thermoplastic Rubber (TPR)) 40
Bushing Current Transformers (BCT) 40
Summary 40
Product Description
General Description ABB offers a complete line of instrument transformers from 600 V to 34.5 kV. In the 600 V class, ABB manufactures currenttransformers (CTs) and voltage transformers (VTs), using boththermoplastic rubber and plastic casings. In the 5 to 34.5 kV classes, ABB provides a wide range of indoor CTs and VTs cast inpolyurethane, using a state-of-the-art casting process. The outdoormedium voltage CTs and PTs are cast in HCEP (hydrophobiccycloaliphatic epoxy) using an automatic pressure gelation process.Specialty items such as linear couplers and bushing-type CTs arealso offered.
ABB instrument transformers are manufactured in Pinetops, NorthCarolina. The Pinetops operation began in 1978 and is considered aworldwide center of excellence for instrument transformers. Theoperation is widely recognized for its success in on-time deliveries,short lead-times, focus on quality, and customer responsiveness.On-going operations initiatives include the following:
• Customer responsiveness• Focused factory• Investments in new equipment• Productivity improvement• Inventory reduction• Manufacturing cycle time reduction• Front-end optimization
Transformers are built to a number of specifications, includingANSI, IEEE, CSA, IEC, and Australian. ABB-Pinetops receivedISO-9001 certification from UL on November 29, 1995, and ISO-14001 certification on August 31, 2000.
38
Product Description
The ABB instrument transformer family of products consists ofthe following:
• Indoor Medium Voltage (polyurethane)• Outdoor Medium Voltage (HCEP)• 600 V Indoor (Plastic Case)• 600 V Indoor/Outdoor (Thermoplastic Rubber)• Bushing Current Transformers
Voltage ranges are defined as low voltage under 700 V andmedium voltage 1.2 kV through 36 kV.
Indoor Medium Voltage
Contemporary Design
Outdoor Medium Voltage
39
Indoor medium voltage transformers are encapsulatedin polyurethane using a vacuum casting process.Current and voltage transformers are available in volt-age classes ranging from 5 kV to 34.5 kV. Typical appli-cations include switchgear and metalclad enclosures forprimary and revenue metering and protection.
Voltage units are available for line-to-line applications,designated by Y, and for line-to-ground applications,designated by GY.
Outdoor medium voltage transformers are encapsulatedin hydrophobic cycloaliphatic epoxy (HCEP) using theautomatic pressure gelation (APG) process. Current andvoltage transformers are available in voltage classesranging from 5 kV to 34.5 kV. These units are typicallyused by utilities in substations.
Voltage units are available for line-to-line applications,designated by Y, and for line-to-ground applications,designated by GY.
Product Description
600 V Indoor (Plastic Case)
600 V Indoor/Outdoor (Thermoplastic Rubber (TPR))
Bushing Current Transformers (BCT)
40
BCT units are 600 V class ring-type current transform-ers. They can be wrapped with polyester tape or encap-sulated in polyurethane. Typical applications includehigh voltage circuit breakers and power transformers.
TPR 600 V transformers are available in bothcurrent and voltage designs. They are suitablefor use in a variety of applications includingsecondary revenue metering.
Plastic case 600 V window-type current transformersare offered in a variety of internal window diametersand are used in various switchgear and outdoor vacuumbreaker applications.
Summary A number of considerations are involved in selecting properinstrument transformers. Many of the transformer types have sim-ilar characteristics, but each type has its own unique combinationof features that best meet the application for which it is designed.For more specific information, please refer to the descriptive bul-letins which can be found at www.abb.com/mediumvoltage orcontact your ABB sales representative.
Product Description
Instrument TransformersTechnical Information and Application Guide
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ABB Inc.3022 NC 43 NorthPinetops, NC 27864Tel: +1-252-827-3212www.abb.com/mediumvoltage
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