1732IEEE Transactionson Power Delivery,Vol. 5, No.4, November 1990 GAPPED CORE CURRENT TRANSFORMER CHARACTERISTICS AND PERFORMANCE An IEEE Power System Relay Committee Report* ABSTRACT The results of an IEEE working group's study of the gapped core current transformer (ct) are presented. The characteristics and performance of ct having small gaps are discussed. The effect of remanent flux in ct cores on ct performance, and of small gaps to control remanence for improved transient performance of ct are also discussed. Advantages and disadvantages of gapped core ct are enumerated. Proposed performance requirements for gapped core ct are presented and a method of specifying gapped core ct to produce the required performance for protective relaying application of such ct is proposed. A short description of terms are included for clarity of the discussion. Keywords: Protective relaying; instrument transformers. REMANENCE FLUX Remanent flux, or remanence, results from the interruption of the primary current of a ct when the core flux is not zero as can happen during severely offset faults. Careless use of dc in testing ct circuits or in determining ct polarity also may be s defined as responsible for remanence. Remanence i the magnetic flux density that remains in a magnetic circuit after the removal of an applied magnetomotive force, it should not be confused with residual flux density, which is the magnetic flux density at which the magnetizing force is zero when the core material is in a symmetrically cyclically magnetized ~0ndition.l~ Depending on the type of steel, the remanent flux may be as high as 90 percent of the saturation flux.6 The effects of remanence on the performance of protective relays should be fully understood and the need for its reduction stressed.2, lo During normal load, the core flux may contain a remanent component, its magnitude being dependent on the initial trapped flux and the magnitude of flux swing. Remanent flux does not gradually disappear, but remains constant once the appropriate equilibrium condition has been attained. The percentage decrease in the remanent flux for a given alternating flux depends upon the core material and the minor B-H loop around which normal operation occurs. It is not, therefore, possible to precisely predict the remanent flux level following a fault. In metering current transformers, the normal full load flux approaches the saturation level with the result that remanent flux is reduced to an insignificant level in a very short time following supply restoration. Remanence in relaying accuracy class ct can therefore remain for an infinite period. Once remanent flux is established in the core of a transformer, it will take special effort to remove. A voltage in excess of saturation voltage must be applied and gradually reduced to zero in order to reduce the remanent flux to less than 10 percent of saturation flux. Such high voltages are not ordinarily induced in the winding of current transformers under normal operating conditions, because the voltage across the ct winding is limited by the burden connected to the ct. The remanent flux, then, may remain in the ct core until the next occurrence of a high magnitude fault current or other transient condition which could provide the necessary flux to effect demagnetization. Demagnitization by this means is rather unlikely since one would expect the fault-clearing to occur at, or near, a current zero at which time the ct core will be near maximum excitation and will in all likelihood continue to have excessive remanent flux. A transient condition which produced a flux opposite in polarity to the remanent flux could possibly reduce the remanence to a more acceptable level, also an unlikely occurrence. Under operating conditions, remanent flux can be left in the core when the primary current is interrupted while the flux density in the core of the transformer is high. The flux in the core depends on many factors, the most important of which are the magnitude of the primary current, the magnitude of dc offset transient, and the impedance of the secondary circuit. INTRODUCTION Current transformers in general use today are commonly manufactured with continuous steel cores having no intentional gaps. High power system fault current levels have amplified problems in the response and performance characteristics of conventional ct partly because of remanence. A special concern is that due to remanence the ct may saturate before the high-speed protective relaying have a chance to operate. In order to overcome or minimize these problems, current transformers which have various sized intentional gaps in their steel cores are being manufactured. Such gaps may be filled with non-magnetic material and not necessarily air. Gaps in ct cores limit the remanence to acceptable levels. The physical and electrical characteristics of gapped core ct have not been standardized nor have the results of studies of the performance capabilities and characteristics been widely published. The intent of this document is to present the collective efforts of an IEEE working group formed to study the characteristics of current transformers having small gaps in their core, and to propose methods of classifying these ct and of determining their performance. This document also furnishes, to both the user and the manufacturer, a method for specifying gapped core ct. Gaps of 0.0001 to 0.0003 per unit of mean length of magnetic path will be considered. *Present Members of the PSRC Working Group are: B . Bozoki, Chairman, H . J . Calhoun, C . M . Gadsden, K . J . Khunkhun, W . C . Kotheimer, R . J . Moran. Former Members were: J . Berdy, F . B . Hunt, E . C . Wentz, C.F. Burke, J.M. Vanderleck, M . Rosen, J.W. Walton. A paper recommended and approved by the IEEE Power System Relaying Committee of the IEEE Power Engineering Society for presentation at the IEEE/ PES 1989 Summer Meeting, Long Beach, California, July 9 - 14, 1989. Manuscript submitted March 16, 1989; made available for printing May 19, 1989. 89 SM 802-0 PWRD 0885-8977/90/1100-1732$01.00 Q 1990 IEEE consequently its decay will take a longer time . Gaps in the order of 0. the unfaulted phase(s) exciting current desensitizes the ground relays. For example. Similarly. The effect of adding gaps in the iron core of a ct is shown in Figure 2 . At the instant of fault clearing. thus improving the transient performance of the ct. With a closed core there is relatively little flux change to sustain a secondary current after the primary circuit opened. It should be recognized that because of the higher exciting currents of gapped cores at current levels below 20 times rated. the magnified effects of idle ct should be considered. Although the loss in the transformation of the dc transient may be quite large. The actual size of gaps depend on the physical dimensions of the ct core and the magnetic properties of the core material. because the remanent flux value is not much lower than the value at time of fault clearing. the ct used in the system would be operating at current levels well below those which might produce saturation in the ct cores. The distortion which is reproduced from reference 5 can have an adverse effect on the operation of the high-speed relays. such -200 (b) " e n t flux = 50%of saturation flu> I +ma I - I PRIMARY 1 1 (c) " n t flux = 75% of saturation flu: -200 0 I 2 CYCLES 3 4 Figure 1 Distortion of secondary fault current due to remanence for a ct operating under transient fault conditions. The amount of remanent flux can be limited to a low value. the flux in the core can be at saturation level.0. How air gap reduces remnant flux 1 I EFFECTS OF GAPS ON TRANSIENT PERFORMANCE Some loss in the transformation of the dc transient in fault current will result in gapped core ct. The reduction of remanence in the core provides a proportional increase in the amount of the core available for flux excursions. then the time-to-saturation can be substantially increased by the embodiment of larger gaps in the core. the steady state performance of gapped ct is of greater concern for instrumentation and measuring devices than for protective relaying. With the gapped core ct the flux is heading for a much lower remanence value so the secondary current is sustained by a larger flux change.0003 per unit of mean length of magnetic path will reduce the remanent flux to acceptably low levels. The distortion of the secondary current is greatly influenced by the amount of remanent flux as can be seen in Figure 1. bus differential relays are desensitized by ct on bus connections not contributing current to an internal fault.lo EFFECTS OF GAPS ON STEADY STATE PERFORMANCE It is assumed that steady state conditions in a power system involve normal current levels associated with loading and power flows within the system. 1. - . CONTROLLING REMANENCE IN CURRENT TRANSFORMERS The remanent flux in the core can be effectively reduced by inserting gaps in the magnetic circuit of the core. In general. A i r Gap Line LH L 2 .1733 Remanent flux in a ct contributes to distortion in the secondary current.0001 . For these conditions. such as on a ring bus. Increasing the gaps reduces the ct shunt impedance and magnetizing time constant. While this flux decays to the remanence level a secondary current will flow. errors approaching 10% in ratio and 3 O in phase angle may also occur over an operating range from one to 20 times rated current. by addition of quite small gaps in the core. Since the practice of inserting various measuring devices. particularly in the first few cycles after the inception of a fault. When a good replica of the dc transient in the primary of a ct is not needed in the secondary relaying circuits. . Undesired breaker failure relay operation can occur where two or more gapped core ct are paralleled. but this loss can be minimized by appropriate sizing the gap(s). such as 10% of the saturation flux. Loop for Iron B 4 . Loop with Gap (a) zero T-emnent flux -200 I Figure 1 1 2. the ct can be designed to conform to a standard relaying accuracy class. Since gapped cores cause higher exciting current. The physical position of the gap or gaps relative to the return conductor. the performance of the ct under steady state conditions merits consideration only in particular situations. but economics and physical size limitations impose practical limits to core area increases. There are several significant advantages of gapped ct over conventional ct.( The open circuit voltage across the secondary terminals produced by a sinusoidal primary current is smaller for a gapped ct. the discharge voltage is higher. both in ratio and phase angle.8 The size and shape of the gaps may change over a period of time without detection. gapped core ct can be used for metering purposes. 2 . such as on large generators. 3. consequently.lo Reduction of the time constant of the ct permits use of a smaller core cross section and. 5. Another risk associated with this phenomenon is the decay current following fault interruption may delay a current relay from dropping out to indicate that the circuit breaker is open and thus cause a false operation of the breaker failure protection . 2. smaller physical size. especially the dc component. Higher magnetizing current will be supplied to the secondary of a ct. APPLICATION CONSIDERATIONS The use of gapped core ct may application considerations: 1 . Excessive secondary burdens may cause steady state performance which is unacceptable in very sensitive differential relay circuits or other current balancing schemes. 4. . 2. by a parallel ct. could be mechanically weaker and more expensive than those with closed cores. whose primary is open-circuited. The magnetizing current is higher. The significant increase in the magnetizing current falsifies the secondary current.3 3. The secondary current error is serious if a resultant current is generated by paralleling two or more ct. In this case it is advisable to confirm that the stability of the differential relays are not affected. it is essential that practical evaluation of the finished product can be established. wattmeters and current transducers. 5 . . as shown in Figure 3 . The time of decay may be so long that high-speed reclosing can take place before the remanence level is reached. the energy stored in the magnetic circuit must be dissipated in the secondary circuit and results in a unidirectional discharge of long duration (approximately 1 sec). This current may cause the overcurrent relay of a breaker failure scheme to indicate that a circuit breaker is still closed. This latter case is an argument for using a differential restraint winding on each ct and not connecting ct in parallel to supply one restraint winding . Replacement of ct or interchanging ct must be done with caution because of the large effect the gap construction has on the electrical characteristics. results in improved transient performance of the ct .8. ADVANTAGES AND DISADVANTAGES OF GAPPED CORES The study of characteristics and performance of gapped ct can perhaps be best done by enumeration of their advantages and disadvantages.8 When the core flux decays to its final value after fault interruption. whether filled with air or nonmagnetic material. Measurement of the kneepoint. The criteria proposed for use in evaluating the performance of gapped core ct are listed following: 1. as follows: 1 . The effect of the gaps on performance is some loss of accuracy under steady state conditions. up to the normal rating of the ct. or close proximity to other phases can affect the accuracy during fault current conditions 4 . In limited applications. Increasing the core area partially offsets this effect. A gapped core ct provides a secondary current substantially representative of the primary current. resulting in greater ratio and phase angle error. However. Ideally. 4. For example. 2. The effect of burden power factor on oversizing to prevent saturation is less for gapped cores than for closed cores. The gaps in the core may increase the secondary leakage reactance and may adversely affect the settings of high impedance differential relays.10*11 3. or remanence. Since the change in flux required to reach the remanence level is greater for a gapped ct. if periodic excitation tests are not made. such situation might occur in a generator unit differential relay scheme in which gapped ct on the generator neutral bushings are used while the unit high voltage breakers may have continuous core ct. or may produce a false operation of a differential relay during an external fault. For closed core of the kneepoint is defined as the 4 5 " target point on the excitation curve. 5. in protective relaying circuits is fairly common. the exciting current of a CEO0 accuracy class ct must not exceed 10 A when producing 800 V at the secondary terminals for a burden of 8 ohms. or if Measurement of the secondary voltage and exciting current to establish the upper part of the excitation characteristics of a ct.1734 as ammeters. The disadvantages of gaped ct are as follows: 1 . gapped ct should not be mixed with conventional ct in differential circuits. require additional Reduction in the remanent flux. accuracy in zero crossing time is desired.lo Current transformers with core gaps. Similarly a breaker with gapped core ct might be added in a bus differential scheme which previously involved only continuous core ct.3 Less core oversizing is required to avoid saturation for a ct with gaps than for a closed core ct for the same primary current. PROPOSED PERFORMANCE REQUIREMENTS In order to provide a means of comparison of the performance characteristics of gapped core ct. Operation of gapped ct in parallel may produce more errors in the output than with closed core transformers even though remanence has been substantially reduced. Methods have been described in the literaturelo f o r the field measurement of ct remanence. The advantages of using gapped core ct must be weighed against possible disadvantages in any given protective relay application. The class K ct will be preferred for protective relaying applications. the kneepoint and the general shape of the curve below and above the kneepoint. . . with sufficient points measured to provide.13. It should be noted that all of the foregoing statements apply to the entire secondary winding if the ct being considered is multi-ratio. The effect of gaps on the excitation curves. 13-197812 for relaying requirements of gapped core current transformers. An example of such curves is shown in Figure 4 . Routine excitation tests on Every unit should be made to verify the excitation curves provided by the manufacturer. For example. a K800 ct will have a kneepoint at 0. CONCLUSIONS In some cases the use of gapped core ct can offer a significant physical size reduction and accordingly provide an economic advantage.13 an additional relaying rating will be introduced. Exciting Current (Amperes) Figure 3. and the flux has subsided to a constant value after removal of the magnetizing force. In the next edition of ANSI C57. a class C ct. which may contribute to greater reliability in relay operation. Typical gapped ct excitation curve. at a minimum. Routine remanence test should be conducted on every unit.1735 For gapped ct this dtfinition was changed to a 30° point because it may not be possible to find a 45O tangent point on the gapped ct's excitation curve. the ratio error must be limited to 10% at any current from 1 to 20 times rated secondary current at the standard burden or any lower standard burden used for secondary terminal voltage rat io. The inability to draw a 30° tangent to the excitation curve constitutes noncompliance to the proposed limitations on excitation characteristics. Furthermore. they must be specified as a class C or class K ct with the following additional requirements: 1.7 x 800 V or higher. The curves and tolerances shall conform to ANSI C57.13-1978. Excitation curves on log-log coordinate paper shall be made available by the manufacturer. or better than. The method of specifying gapped core ct presented in this paper should supplement the relaying requirements now contained in ANSI C57. designated as ' K ' classification. Gapped core ct can offer improved transient performance over that available from similarly sized closed core ct. Perhaps in the future a special classification should be developed for gapped core ct which will include these requirements. Measurement of the remanent flux should be conducted after the core has been subjected to a magnetizing force of 500 ampere-turns per meter and the flux has been allowed to subside to a constant value after removal of the magnetizing force. TERMINOLOGY Accuracy Class Secondary Voltage The voltage the ct will deliver to a standard burden at 20 times rated secondary current without exceeding 10% ratio error. 12 2. The remanent flux in the core shall not exceed 10% of the saturation flux after the core has been saturated with a magnetizing force of 50b ampere-turns per meter. which can be used to determine conformance to specification. A test procedure is described in reference 10. The chief advantage of gapped core ct is that remanence can be kept to a low level resulting In improved transient performance. Figure 4. The ' K ' rating is the same as the 'C' rating except that the kneepoint voltage is not to be less than 70% of the accuracy class secondary voltage. This is significant where consistant high speed operation of the protection is important. and may be used in specifying gapped core ct for relaying applications. In all other respects the class K ct is equal to. ADDITIONAL SPECIFICATION Since there is no special classification in ANSI C57. I A " . lo.9 E f f e c t i v e Flux C a p a c i t y The p o s s i b l e f l u x e x c u r s i o n s from t h e remanent f l u x v a l u e t o a maximum n e a r s a t u r a t i o n is t h e s i g n i f i c a n t quantity which determines the actual transient performance o f a c t . Maximum Flux D e n s i t y ANSI C57. and between each t a p . can b e measured by t h e v o l t a g e induced by p u l s a t i n g d c . C o n t r o l l e d Remanent Flux A c t c o n s t r u c t e d t o meet c e r t a i n r e q u i r e m e n t s f o r f l u x e x c u r s i o n from maximum down t o a remanence v a l u e a f t e r t h e primary c u r r e n t i s i n t e r r u p t e d . When a c t h a s small g a p s i n t h e c o r e t o c o n t r o l t h e remanent f l u x . t h i s f l u x may b e i n t h e o r d e r o f 1 0 % o f t h e f l u x produced by a n e x c i t a t i o n t e s t c u r r e n t o f 1 0 amperes. f o r e a c h p u b l i s h e d r a t i o . The d e g r e e of o f f s e t i s t h e f r a c t i o n o f t h e maximum p o s s i b l e c u r r e n t t h a t a c t u a l l y a p p e a r s or i s assumed f o r d e s i g n purposes. a c t having small gaps i n t h e c o r e t o l i m i t t h e remanent f l u x t o no more t h a n 10% o f t h e maximum f l u x . Where t h e l e a k a g e f l u x is n e g l i g i b l e .13 I d l e CT c t having no primary c u r r e n t . which c a u s e s a f l u x v a r i a t i o n from remanent t o a maximum value. ( i n d u c t i o n ) a t which t h e magnetizing f o r c e i s z e r o when t h e material is i n a s y m m e t r i c a l l y c y c l i c a l l y magnetized c o n d i t i o n . Residual Flux Density.6 P e r c e n t Remanent Flux The p e r c e n t of t h e peak f l u x remaining when t h e primary c u r r e n t r e t u r n s t o z e r o . Usually. F l u x Excursion Measurement The f l u x e x c u r s i o n . t h e t y p i c a l e x c i t a t i o n c u r v e s are p l o t t e d w i t h log-log c o o r d i n a t e s .I2 at a The e x t e r n a l l o a d i n ohms or volt-amperes s p e c i f i e d c u r r e n t . The e x c i t a t i o n c u r r e n t produces t h e f l u x required t o induce t h e voltage f o r transformer action. making t h e l e a k a g e r e a c t a n c e n e g l i g i b l y small and normally c o n s i d e r e d zero. A c t w i t h no c o r e gaps may have a remanent f l u x a s high as 90% of a r e c e n t e x c i t a t i o n peak. a d e n s i t y t y p i c a l l y i n t h e neighborhood of 1. a c t having p r i m a r y and secondary c u r r e n t . This is t h e f l u x d e n s i t y a t t a i n e d when 10 amperes rms e x c i t i n g c u r r e n t is c i r c u l a t e d i n t h e secondary winding. It is i n the core .6 F u l l y D i s t r i b u t e d Windings Core O v e r s i z i n g The d e s i g n of a c o r e l a r g e r t h a n n e c e s s a r y f o r r a t e d s t e a d y s t a t e o p e r a t i o n i n o r d e r t o produce a c c u r a t e operation during t r a n s i e n t conditions. Residual Induction The magnetic f l u x d e n s i t y .13 now s p e c i f i e s i n e f f e c t a maximum f l u x d e n s i t y a t which c t i s u s a b l e . b u t having i t s s e c o n d a r y i n p a r a l l e l w i t h . on a c t is p r o p e r l y c a l l e d t h e burden. DC O f f s e t T r a n s i e n t The magnitude of d c o f f s e t c a n v a r y between z e r o and t h e p e a k v a l u e of t h e t r a n s i e n t ac c u r r e n t O K f a u l t current. a c c u r a c y c a l c u l a t i o n s a r e p e r m i t t e d and a C class is used. I t must b e k e p t i n mind t h a t t h e c u r r e n t shown on t h e c u r v e i s n o t even a p p r o x i m a t e l y s i n u s o i d a l . Leakage F l u x C T a r e c l a s s i f i e d on t h e b a s i s o f l e a k a g e f l u x . The c o r e w i l l s a t u r a t e i n a much s h o r t e r time i f t h e remanent f l u x is n o t n e g l i g i b l e ( w i t h remanent f l u x i n t h e u n f a v o r a b l e d i r e c t i 0 n 1 . t h e k n e e p o i n t is d e f i n e d a s a p o i n t on t h e e x c i t a t i o n c u r v e where a s t r a i g h t l i n e a t 30° is t a n g e n t when p l o t t e d on a log-log c o o r d i n a t e s l2 h a v i n g s q u a r e d e c a d e s .7 D e t e r m i n a t i o n of t h e E x c i t a t i o n C h a r a c t e r i s t i c s The w i n d i n g s o f a c t a r e e v e n l y d i s t r i b u t e d around t h e c i r c u m f e r e n c e o f t h e t o r o i d a l c o r e . e x t e n d i n g from 1% of t h e accuracy c l a s s secondary terminal voltage t o a voltage t h a t w i l l c a u s e a n e x c i t a t i o n c u r r e n t o f 5 times r a t e d secondary current. The v o l t a g e s h a l l be measured by means of a n a v e r a g e v o l t a g e r e a d i n g When t h e v o l t m e t e r and by a n rms r e a d i n g v o l t m e t e r . s u p p l y is measured w i t h o u t l o a d t h e measured v a l u e on t h e a v e r a g e v o l t m e t e r s h a l l n o t d e v i a t e from t h a t measured on t h e rms v o l t m e t e r by more t h a n 2% o f t h e measured v a l u e o v e r t h e i n t e n d e d measuring range. c o n v e n i e n t l y f u r n i s h e d by a half-wave r e c t i f i e r . E x c i t a t i o n Curve An e x c i t a t i o n c u r v e is a p l o t of e x c i t i n g c u r r e n t versus voltage applied t o t h e secondary with t h e p r i m a r y open. and a d e n s i t y a t which t h e f l u x e x c u r s i o n from remanence t o maximum is meaningful. The d c c u r r e n t is r e q u i r e d t o match ( w i t h o p p o s i t e p o l a r i t y ) any c u r r e n t t h a t t e n d s t o change t h e e x i s t i n g c u r r e n t a t t h e i n s t a n t o f f a u l t . Yet this s o r t o f c u r v e h a s found so many p r a c t i c a l u s e s t h a t i t i s recognized a s a useful c h a r a c t e r i s t i c of a transformer. The g a p is sufficient to absorb about twice as many ampere-turns as t h e i r o n c o r e .0005 p e r u n i t o f t h e mean c i r c u m f e r e n c e . w i t h s q u a r e d e c a d e s . l ~ E f f e c t i v e Remanent Flux The remanent f l u x t o u s e f o r c a l c u l a t i o n s . I n normal s e r v i c e t h i s c u r r e n t i s s u p p l i e d by t h e p r i m a r y winding and i n a c t i t is t h e e r r o r current. a d e n s i t y n o t g r e a t l y a f f e c t e d by r a t h e r l a r g e changes i n e x c i t i n g c u r r e n t . A Kneepoin t An e x c i t a t i o n c u r v e s h a l l e x t e n d a t l e a s t up t o t h a t p o i n t a t which a 10% i n c r e a s e i n t h e v o l t a g e r e s u l t s i n a 100% i n c r e a s e i n c u r r e n t .1736 Burden For a Class C c t . and e x c i t e d by. For a T ( f o r t e s t ) class c t t h a t h a s a p p r e c i a b l e l e a k a g e or s t r a y f l u x i t is n o t p r a c t i c a l t o r e p r e s e n t t h e c t by a n e q u i v a l e n t c i r c u i t and c a l c u l a t e accuracy. Large Gaps One or more g a p s i n t h e c o r e having a t o t a l l e n g t h of a t l e a s t 0. such a s a bushing c t . The s u p p l y s o u r c e s h a l l be c o n s i d e r e d a d e q u a t e i f t h e e r r o r between i n s t r u m e n t r e a d i n g s d o e s n o t exceed 10% of t h e measured v a l u e s o v e r t h e range o f a p p l i e d v o l t a g e s . For a gapped c t .8 T. remanent f l u x t o maximum. W. Antiremanence Gapped Cores or Linear Cores for Current .' Western Protective Relay Spokane. Power System Relaying Committee Report 76-CH1130-4 PWR. The maximum intrinsic value of induction possible in a material. E. pp. 'Remanent Flux in Current TKanSfOKmeKS. 'Control of Residual Flux in Current Transformers. pp. May 1966. pp. 1974. Iwanusiw. N. Conner. 1970. "Requirements for 13.000 Gauss). 3 . 18-21. the symmetrically 10. IA-15 13. R. 1979. pp. Third Quarter. E .' IEEE Transactions on Power Apparatus and Systems. 0 . 'Transient Conditions in Current Transformers and Their Repercussion on the Reduction of Operating Time of Protective Devices. pp. 5. W. Note: This term is often used for the maximum value of induction at a stated high value of field strength where further increase in intrinsic magnetization with increasing field strength is negligible. 14. 'Remanent flux in current-transformer Cores. E. pp. B. That value of flux below which the corresponding exciting current can be considered negligible. WA. ANSI/IEEE Std 100-1988. Proc. 'Transient Response of Current Transformers'. pp. 569-74. IEEE Trans. Vol.' IEEE Transactions on Power Apparatus and Systems. 'The Transient Behaviour and Uses of Current Transformers". Greb and E. E . Paris. 5 .Current Transformer Burden and Saturation. 1951.1737 distinguished from remanence by cyclic requirement. R.. IEEE Standard Dictionary of Electrical Electronics Terms. Avent._ Transformers. 94 11 Jan/Feb. May/June. C. C. 1656-61. W. Instrument Transformers'. ANSI/IEEE Standard C57. IEEE Special Publication. 'Design Considerations in the Application of ct for Protective Relaying Purposes. Korponay. Wentz. .L. Wright. PAS 96. CIGRE Paper 131-07. Nov/Dec 1969. Vol. 6 . 11. 4 . 113.. L. Bruce and A . A.. Mowa and Wuss. Brown Boveri Review. Jul/Aug. 1809-1814.. Powell. 294-302. . Smolinski.13-1978. . 'Current Transformer Response. E . IEEE Transactions on Power Apparatus and Systems. As summary report and discussion.Methods for Estimating Transient Performance of Practical ct for Relaying. REFERENCES 1 .l4 Saturation Flux That value of flux in a core which is arbitrarily determined as 10% greater than the flux at the kneepoint of the excitation curve. Jagsich. 1968 N . Allen. Wentz and D. and 2 . J. of IEE. J. March/April 1978 pp. 1226-33.' AIEE Transactions on Power Apparatus and Systems. E . Pfuntner.G.. 8. . 597-608. 7. 'Non-gapped Cores. 915-920. IEEE Transactions on Power Apparatus and Systems. Korponay. 116-122. 12.' IEEE Transactions on Industry Application. W. 1973. 9. pp. Power Apparatus and Syst. 1975. Transient Conference. R. No. E. 1973. JUl/AUg. Nov/Dec 1977. Conner. Vol.14 Saturation The state of a ferromaanetic substance placed in a field so strong that the intensity of magnetization becomes independent of the field: the substance is then said to be saturated. 'The Accuracy of Current Transformers Adjacent to High Current Busses. pp. Oct. 1329-36. Ontario Hydro Research Quarterly.9 The effective saturation or maximum flux density in silicon iron alloys is about 2 T (20. HENVILLE B. Experience in this regard extends back 20 years and includes virtually all 500 kV cts associated with circuit breakers on a system of over 5000 km of 500 kV lines. 1. The possible misoperation of a breaker failure relay due to magnetizing current in an idle ct is real. C. It is theoretically possible for the decay current to cause misoperation of breaker failure protection. and the core material performance is determined by type test. which allow the determination of ct performance when dissimilar cts are parallelled. Canada This Paper is a useful addition to the literature for dissemination of information about the application of cts with small air gaps. 4 5 ~ 2. It would be better to adopt the transient specification terminology of IEC in their classes TPS. 3 of the Paper. C. 4. 3. However. Fig. 1 of this discussion. gapped core cts are B. including.o 0 5 . using the transient performance specification of Ref. any gap problems in new cts are discovered and corrected before they leave the factory. The breaker failure protection B. C.. C. Following. This has already been done successfully for Canadian Standard CSA CAN3-C13-M83 (Ref l ) . . Hydro has purchased cts in accordance with this standard since its publication in 1983 with little difficulty. The comment numbers refer to the disadvantage numbers in the Paper. Hydro routinely neglects leakage reactance when setting high impedance bus differential relays with no known problems so far. The comment numbers refer to the consideration numbers cited in the Paper. is not a disadvantage. the discussors feel the authors have painted an unnecessarily pessimistic picture of the drawbacks. Section 8 of that standard covers cts for which transient performance is important. C. because the difference in magnetizing currents up to the knee point is less than 1% of the fault current. B. The fact that the flux in a gapped ct may not have decayed to its steady state remanence level before high speed reclosing takes place. O . Although all the application concerns mentioned in the Paper are true to a degree. L. routine remanence measurements seem superfluous. Could the authors please explain the need for the proposed routine remanence tests? It is appreciated that this should be a type test. This has been demonstrated by several direct transient performance tests which show that actual performance corresponds well with performance predicted from the excitation curves. In practical terms the increased ratio and phase angle error caused by small gaps is usually negligible except for very low ratio cts. but there have been negligible problems in the field with stability in the last eight years. Since B. B. but not limited to. and have been for about 15 years. C. TPX. C. C. In practice small gaps have not been found to significantly affect leakage reactance. but B. The discussors are not aware of any serious error in secondary current caused by paralleling two or more sets of three phase cts. Hydro routinely does. Hydro. B. Vancouver. gapped cts. are some comments on the application considerations cited. are some comments on the disadvantages cited. Computer programs exist. providing both sets are gapped. The flux in the gapped ct will still decay to a lower level than in a nongapped ct. transformer bushing cts and lower voltages are ungapped so transformer differentials routinely include both. We find the mismatch no more severe than that for the much different W and LV solid core cts normally found in such a differential circuit. 4. B. there were some manufacturing problems in maintaining stability of the gaps. Incomplete decay of flux can be catered for by specifying the reclosing duty of the ct. as B. s 5 . which the discussors recommend for consideration. Hydro's experience is that cts with anti-remanence gaps can be applied with little concern about the drawbacks. neither the new "K" accuracy class nor the existing "C" accuracy specifies the transient performance.COMPARATIVE PERFORMANCE OF SOLI0 h QAPPEO CORE CTS There is no doubt about the stated need for proposals to specify gapped ct performance requirements. Hydro has no known such misoperations with the conventional breaker failure protection routinely applied at 132230 kV.1738 DISCUSSION B. AVENT and C. W and E W cts at circuit breaker positions are normally gapped. 1. Hydro's standard for circuit breaker associated cts above 138 kV. but remanence is only a function of the core material (hysterisis) and excitation characteristic. it should be). C. C. One way to avoid the problem is to size the cores such that the magnetizing current is below the sensitivity of breaker failure relays at the voltage of the shared burden. Since the excitation characteristic is proposed to be a routine test (and indeed. Several years ago. Many year's of experience with numerous installations has not produced identifiable problems. C. B. The difference in performance of the two cts cannot be easily seen until after saturation.) FIGURE I . Hydro provided on 500 kV breakers is designed to be immune to this decay current. There could be significant neutral current error if dissimilar cts are connected in a three phase set. However. ~ m x ~ ~ 4 4 TI* I. In addition. Following. TPY and TPZ. 1 of this discussion shows the calculated comparative performances of two of the cts with the approximate excitation characteristics of Fig. Hydro has routinely mixed gapped core cts with solid core cts in differential circuits. Hydro specifies excitation tests as routine production tests. F. lacks clarity in some parts and in the terminology section. Labaj Ontario Hydro Research Division Toronto. the worst case. This is the need for protection systems to be secure during external faults. B. some elaboration on the subject of driving a gapped-core ct into saturation may be helpful. With regard to metering ct operating at a level approaching saturation under normal full load. more use could be made of existing instrument transformer standards along with the IEEE and IEC dictionaries. Secondary Current 'rated B. suitable kVA rated supply with low source impedance. then twice as much. there may be some serious drawbacks in its implementation. CAN3-Cl3-ME3.001 pu gap. if 500 At/m were required to fully saturate a given closed core. Hydro is firmly convinced that the advantage of improved transient performance outweighs the disadvantages. The paper. An illustration of this fact is the accuracy curve which normally takes the following form: While the committee's reference 6 appears to be a fairly simple and straightforward means to measure flux excursion and thereby remanence. 7. Canada 5. 1 which has been tested and found adequate as a means of defining et transient performance requirements. With regard to the additional specification on testing for remanent flux. DISCUSSION By: P. Mr Labaj asked why we did not discuss issues related to the use of gapped ct for meterin?. 4. For this reason. As an experienced user of gapped core cts. Instrument Manuscript received July 12. There may be advantages to using a different core material having inherently lower remanence. such as an amorphous metal (metallic glass) or hot rolled silicon steel. In the protection business. or 1000 At/m would be required to fully saturate the same core with a 0. This is particularly important when reclosing onto a permanent fault with a breaker which is part of a ring bus.0001-0. There viability as alternatives to gapped GOSS cores should be considered. there is a wider variation in the excitation curves for gapped core ct.00033 pu gap. their accuracy varied significantly depending upon the location of the gap in relation to the return conductor. 2000 At/m would be required to fully saturate the same core with a 0.00033 pu adds as much additional reluctance as the steel itself provides. and selection of suitable diodes.~ 1739 It should be noted that there is an additional important reason to try to reduce the premature ct saturation caused by remanence. 1989. Should caution not be exercised when applying gapped-core ct for metering purposes? Based on our limited accuracy measurements on minimally-gapped (0.000 (11 mil Allegheny M4 @ 500 At/m) a gap equivalent to 0. REFERENCES 2. we have found that their ratio error and phase angle are significantly different than their metering councerparts. We have found up to this point in time. Also. with the same metering burden connected. The committee members are to be commended for producing a paper on a subject that is long overdue.0003 pu length) core protective ct. there should be further consideration of existing IEC terminology and Ref. 1. (possibly due to the initial fault) its performance could be degraded enough to cause misoperation of the adjacent line protection zone. The definition given for saturation flux appears to be inconsistdntwithmany standards. the accuracy curve would look more like a parabola. Manuscript received July 17. can you explain why this definition was chosen? A definition of saturation flux arbitrarily determined as 50% greater than the flux at the kneepoint might be more appropriate than 10%. If there was remanence in this ct.13-1978 and Canadian Standard CAN3-C13-M83. For example. while full of useful information. The discussers contributions significantly increased the value of our paper. Ontario. all fault current flows through one ct set. good ct transient performance is the name of the game and cts with antiremanence gaps (Type TPY) offer more advantages than disadvantages. BOZOKI: The interest expressed and the informative comments included in the discussions is greatly appreciated by the working group. . With permeability on the order 3. C.W. We did not consider these issues because it If the ct were operating beyond the point of maximum permeability. that dc methods as those discussed in references 5 and 10 and Canadian Standard CAN3-C13-M83 can be universally applied and are quite reliable. 1. such as applied voltages exceeding the ct insulation strength. During reclosing onto a permanent fault. The 50% figure appears to be a reasonably obtainable flux that would still ensure that a ct core was fully saturated for all practical purposes. 1989. The discusser has several points to raise with regard to this committee report. CSA Standard Transformers. and in fact the normal operating flux is usually quite low. In ring bus arrangements the adjacent line protection is also a bus protection system for the line terminal. Similarly. this has never been our experience. These standards only allow for +25% variation in the excitation current below the kneepoint. From what we have observed. 6. Ct saturation problems will manifest themselves for heavy close-in faults. This would suggest that wider acceptance criteria may be advisable for gapped-core ct than is allowed in the currentANSI/IEEE C57. 3. however. In the Canadian Standard a close tolerance is placed on the calculated excitation curve. therefore. This question underscores the conclusion of our paper that there is a need for a special classification in ct standards for gapped ct. a test based on the CSA standard is sufficient to verify the existence of the gap in the core. The magnetizing current required to bring the flux level 10% above the kneepoint is 2 A. The answer depends on the accuracy limits placed on the excitation test. the the the . The tolerance on the excitation curve in ANSI C57. In our opinion the definition given in the paper is in agreement with the IEEE dictionary.1740 was assumed that most utilities would use gapped ct only for relaying and would specify a separate closed core ct for metering. BOZOKI: The interest expressed and comments included in informative discussions is greatly appreciated by B. the kneepoint of the ct described in Figure 4 is approximately 800 V on the 1200-5 A tap. Avnet and Henville questioned the need for routine remanence tests. Our recommendation was based on the assumption that it is more economical to request routine remanence tests than to specify close tolerance on the calculated excitation curve.13. only limits the magnetizing current in the positive direction. however. Avnet and Henville questioned the need for routine remanence tests. The discussers contributions significantly increased the value of our paper. Labaj also asked why the saturation flux was defined differently from the standard definitions. Mr Labaj asked why we did not discuss issues related to the use of gapped ct f o r meterinq. The magnetizing current required to bring the flux level 10% above the kneepoint is 2 A. The answer depends on the accuracy limits placed on the excitation test. To raise it 50% above the kneeDoint value would require approximately 100 A-magnetizing current. In the Canadian Standard a close tolerance is placed on the calculated excitation curve. Messrs. To raise it 50% above the kneepoint value would require approximately 100 A magnetizing current. therefore. the kneepoint of the ct described in Figure 4 is approximately 800 V on the 1200-5 A tap. where a note allows for a "stated high value" of flux to be defined as a practical value of the saturation flux. For example. Messrs.13. Our recommendation was based on the assumption that it is more economical to request routine remanence tests than to specify close tolerance on the calculated excitation curve. where a note allows for a Itstated high valuell of flux to be defined as a practical value of the saturation flux. The tolerance on the excitation curve in A N S I C57. working group. This question underscores the conclusion of our paper that there is a need for a special classification in ct standards for gapped ct. In our opinion the definition given in the paper is in agreement with the IEEE dictionary. consepently a routine test based on it may not indicate if a closed core ct is supplied in place of gapped ct. The flux level 10% above that at the kneepoint was selected in order to limit the magnetizing current to a relatively low value during testing. The flux level 10% above that at the kneepoint was selected in order to limit the magnetizing current to a relatively low value during testing. We did not consider these issues because it was assumed that most utilities would use gapped ct only for relaying and would specify a separate closed core ct for metering. consepently a routine test based on it ?ay not indicate if a closed core ct is supplied in place of gapped ct. a test based on the CSA standard is sufficient to verify the existence of the gap in the core. Mr. Mr. only limits the magnetizing current in the positive direction. Labaj also asked why the saturation flux was defined differently from the standard definitions. For example.
Report "Gapped Core Current Transformer Characteristics and Performance"