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Current Transformer (1) Current Transformer Introduction Instrument Transformers Since relays cannot be connected directly onto the MV network, the information they receive comes from current transformers or CTs (see fig.) and from voltage transformers or VTs The main tasks of instrument transformers are: To transform currents or voltages from a usually high value to a value easy to handle for relays and instruments. To reduce both of cost and size of protection relays To insulate the relays, metering and instruments from the primary high voltage system which lead to protection to any voltage side from any fault due to fault in the other side . To provide possibilities of standardizing the relays and instruments etc. to a few rated currents and voltages (as example 5 or 1 A) . safety for persons from high power side The main types of instrument transformers are: Current transformers Potential transformer Matching transformer (value balance) Interposing transformer ( phase balance , used in delta to star( 30o) ) Intermediate transformer (Matching transformer+ Interposing transformer) Summation transformer General current protection information Protection devices have many functions since they have to: Protect equipment from destruction or damage as a result of faults (short circuit, overload.), Ensure normal operation of the installation and its equipment (control, load shedding...), Guarantee safety of personnel. Difference between current transformers and power transformers The main differences between power transformer and instrument transformer are: many Comparison face Power Transformer Current Transformer No. of cores One core Two cores or more No. of primary turns At least more than one turns one turns (often 2 turns ) Rating power MVA VA Changing of secondary Using tape changer Voltage reduction voltage Load connection Parallel Series When primary current is high, the CTs are of the cross bar type, and when it is low they are of the wound primary type. CTs have a number of roles to play in electrical networks: Current Transformer (2) supplying at their secondary a current exactly mirroring the one flowing in the HV conductor concerned, providing galvanic insulation between the HV and the measuring and protection circuits, Protecting the measuring and protection circuits from damage when a fault occurs on the HV network. Using this current image in the HV conductor, the relay generates in turn a tripping order according to the type of protection it provides and the values at which it has been preset [threshold(s), time delay(s)....]. This order is transmitted to one or more breaking devices (circuit-breaker, contactor, switch). CT configurations vary according to the type of protection to be provided. Current transformer Current transformer is an instrument transformer in which the secondary current, in normal conditions of use, is substantially proportional to the primary current and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections. Current transformers can perform circuit control, measure current for power measurement and control, and perform roles for safety protection and current limiting. They can also cause circuit events to occur when the monitored current reaches a specified level. Current monitoring is necessary at frequencies from the 50 Hz/60 Hz power line to the higher frequencies of switch mode transformers that range into the hundreds of kilohertz CTs construction 1- Primary winding 2- Core 3- Secondary winding Primary winding Has large cross sectional area and has two types Single Turn Multi turn Core Current Transformer (3) Consist of laminations of iron due the following reasons: Minimizing the eddy currents Minimize both iron and copper loss Secondary Winding Multi turns Burden (load) connected series in the circuit Current transformer theory Current Ie depend on: Material type of core Burden of load( how much the magnitude of Is ) Ie= Ia +Im Ia: Magnetization current copper loss = Ic2R Im: Iron loss = Iiron2 Xc Current Transformer (4) CURRENT-TRANSFORMER BURDEN What about the effect the transformer will have on the current it's monitoring? This is where the term burden enters the picture. Any measuring device alters the circuit in which it measures. For instance, connecting a voltmeter to a circuit causes the voltage to change from what it was before the meter was attached. However minuscule this effect may or may not be the voltage you read isn't the voltage that existed before attaching the meter. This is also true with a current transformer. The burden resistor on the secondary is reflected to the primary by (1/N2), which provides a resistance in series with the current on the primary. This usually has minimal effect and is usually only important when you are concerned about the current that would exist when the transformer isn't in the circuit, such as when it's used as a temporary measuring device Burden = The impedance of the secondary circuit in ohms and power factor. The burden is usually expressed as the apparent power (S) in volt-amperes absorbed at a specified power-factor at the rated secondary current. All CT accuracy considerations require knowledge of the CT burden. The external load applied to the secondary of a current transformer is called the “burden.” The burden is expressed preferably in terms of the impedance of the load and its resistance and reactance components. Formerly, the practice was to express the burden in terms of volt- amperes and power factor, the volt-amperes being what would be consumed in the burden impedance at rated secondary current (in other words, rated secondary current squared times the burden impedance). Thus, a burden of 0.5-ohm impedance may be expressed also as “12.5 volt-amperes at 5 amperes,” if we assume the usual 5-ampere secondary rating. The voltamperes terminology is no longer standard, but it needs defining because it will be found in the literature and in old data. Current Transformer (5) The term “burden” is applied not only to the total external load connected to the terminals of a current transformer but also to elements of that load. Manufacturers' publications gives the burdens of individual relays, meters, etc., from which, together with the resistance of interconnecting leads, the total CT burden can be calculated. The CT burden impedance decreases as the secondary current increases, because of saturation in the magnetic circuits of relays and other devices. Hence, a given burden may apply only for a particular value of secondary current. The old terminology of “volt-amperes at 5 amperes” is most confusing in this respect since it is not necessarily the actual voltamperes with 5 amperes flowing, but is what the volt-amperes would be at 5 amperes if there were no saturation. Manufacturers’ publications give impedance data for several values of over current for some relays for which such data are sometimes required. Otherwise, data are provided only for one value of CT secondary current. If a publication does not clearly state for what value of current the burden applies, this information should be requested. Lacking such saturation data, one can obtain it easily by test. At high saturation, the impedance approaches the d-c resistance. Neglecting the reduction in impedance with saturation makes it appear that a CT will have more inaccuracy than it actually will have. Of course, if such apparently greater inaccuracy can be tolerated, further refinements in calculation are unnecessary. However, in some applications neglecting the effect of saturation will provide overly optimistic results; consequently, it is safer always to take this effect into account. It is usually sufficiently accurate to add series burden impedances arithmetically. The results will be slightly pessimistic, indicating slightly greater than actual CT ratio inaccuracy. But, if a given application is so border line that vector addition of impedances is necessary to prove that the CT’s will be suitable, such an application should be avoided. If the impedance at pickup of a tapped overcurrent-relay coil is known for a given pickup tap, it can be estimated for pickup current for any other tap. The reactance of a tapped coil varies as the square of the coil turns, and the resistance varies approximately as the turns. At pickup, there is negligible saturation, and the resistance is small compared with the reactance. Therefore, it is usually sufficiently accurate to assume that the impedance varies as the square of the turns. The number of coil turns is inversely proportional to the pickup current, and therefore the impedance varies inversely approximately as the square of the pickup current. Burden Burden = The impedance of the secondary circuit in ohms and power factor. The burden is usually expressed as the apparent power (S) in volt-amperes absorbed at a specified power-factor at the rated secondary current. Secondary winding impedance (internal burden) Z s Secondary load impedance Z 0 The secondary load S = V 0 Is (cos θ = 0.8 ind for example) As a matter of safety, the secondary circuits of a current transformer should never be opened under load, because these would then be no secondary mmf to oppose the primary mmf, and all the primary current would become exciting current and thus might induce a very high voltage in the secondary. Burden connection Current Transformer (6) Whether CT’s are connected in wye or in delta, the burden impedances are always connected in wye. With wye-connected CT’s the neutrals of the CT’s and of the burdens are connected together, either directly or through a relay coil, except when a so-called “zero phase-sequence-current shunt” (to be described later) is used. It is seldom correct simply to add the impedances of series burdens to get the total, whenever two or more CT’s are connected in such a way that their currents may add or subtract in some common portion of the secondary circuit. Instead, one must calculate the sum of the voltage drops and rises in the external circuit from one CT secondary terminal to the other for assumed values of secondary currents flowing in the various branches of the external circuit. The effective CT burden impedance for each combination of assumed currents is the calculated CT terminal voltage divided by the assumed CT secondary current. This effective impedance is the one to use, and it may be larger or smaller than the actual impedance which would apply if no other CT’s were supplying current to the circuit. If the primary of an auxiliary CT is to be connected into the secondary of a CT whose accuracy is being studied, one must know the impedance of the auxiliary CT viewed from its primary with its secondary short-circuited. To this value of impedance must be added the impedance of the auxiliary CT burden as viewed from the primary side of the auxiliary CT; to obtain this impedance, multiply the actual burden impedance by the square of the ratio of primary to secondary turns of the auxiliary CT. It will become evident that, with an auxiliary CT that steps up the magnitude of its current from primary to secondary, very high burden impedances, when viewed from the primary, may result. Note: The maximum input current of a CT can be increased by varying the ohms of the burden resistor. Lowering the ohms of the burden resistor will increase the maximum input of the CT, but it lowers the resolution. Also, the accuracy of the output voltage depends on the accuracy of the burden resistor. The burden resistor should never be used for more than 55 % of its wattage capacity, and thermal concerns of the surrounding materials should be considered to prevent over heating damage. For circuits requiring very accurate outputs, the CT should only be used up to 50 % of saturation line of the core. What happens if the burden resistor is left off or opens during operation? There will be no secondary e.m.f to oppose the primary e.m.f, and all the primary current would become exciting current, i.e. The output voltage will rise trying to develop current until it reaches the saturation voltage of the coil at that frequency. At that point, the voltage will cease to rise and the transformer will add no additional impedance to the driving current. Therefore, without a burden resistor, the output voltage of a current transformer will be its saturation voltage at the operating frequency. Current transformers generally work at a low flux density. The core is usually made of very good metal to give a small magnetizing current. When it is open circuit the secondary impedance now becomes infinite and the core deeply saturates. As the ac wave then moves from positive half cycle to the negative half cycle, the rate of change of flux dΦ/dt is so great that very high voltages are induced in the secondary winding. Current Transformer (CT) Magnetization Curve This curve is the best method of determining Current Transformer (CT) performance. It is a graph of the amount of magnetizing current required to generate an open-circuit voltage at the terminals of the unit. Due to the nonlinearity of the core iron, it follows the B-H loop characteristic and comprises three regions, namely the initial region, unsaturated region and saturated region. Current Transformer (7) Figure 2.2 Typical CT Magnetization Curve. Knee- Point Voltage The transition from the unsaturated to the saturated region of the open circuit excitation characteristic is a rather gradual process in most core materials. It is difficult to define this transition and use is made of the so-called "knee-point" voltage for this purpose. It is generally defined as the voltage at which a further 10% increase in volts will require a 50% increase in excitation current. For most applications, it means that current transformers can be considered as approximately linear up to this point. Metering CTs Instruments and meters are required to work accurately up to full load current, but above this it is advantageous to saturate to protect the instruments under fault conditions. They therefore have a very sharp knee-point and a special nickel-alloy metal is used, having a very low magnetizing current, in order to achieve the accuracy. It operate only at ankle point (directly after initial region) Protection CTs Protective gear, on the other hand, is concerned with a wide range of currents from fault settings to maximum fault currents many times normal rating. Larger errors may be permitted and it is important to ensure that saturation is avoided wherever possible to ensure positive operation of the relays. It operate from ankle point to knee point Definitions Composite error = under steady-state conditions, the r.m.s value of the difference between: a) the instantaneous values of the primary current b) the instantaneous values of the actual secondary current multiplied by the rated c is generally expressed as a percentage of the r.m.s. values of the primary current according to the formula: K n= rated transformation ratio Current Transformer (8) I p= r.m.s. value of the primary current i p= instantaneous value of the primary current i s= instantaneous value of the secondary current T= duration of one cycle Rated instrument limit primary current = The value of the minimum primary current at which the composite error of the measuring current transformer is equal to or greater than 10%, the secondary burden being equal to the rated burden Secondary limiting e.m.f = The product of the accuracy limit factor, the rated secondary current and the vectorial sum of the rated burden and the impedance of the secondary winding. Instrument security factor = The ratio of rated instrument limit primary current to the rated primary current. Specification data Rated primary current Rated secondary current Turns ratio Actual transformation ratio (Ip Actual/ Is Actual) Burden(2.5-5-7.5-10-15-30 VA) Value above 30 VA may be selected to suit the application (Recommended values: 45, 60 VA) Errors in CT's Current Error (ratio error) The current error is the error which a transformer introduces into the measurement of a current and which arises from the fact that the actual transformation ratio is not equal to the rated transformation ratio. Current Transformer (9) The current error expressed in percent is given by the formula: where KN is the rated transformation ratio, IP the actual primary current and IS the actual secondary current when IP is flowing, under the conditions of measurement. Phase Angle error The phase angle error is the difference in phase between the primary and secondary current vectors, the direction of the vectors being so chosen that the angle is zero for a perfect transformer. The phase angle is said to be positive when the secondary current vector leads the primary current vector. The phase error is usually expressed in centiradians or minutes. Note:-This definition is strictly correct for sinusoidal currents only Types of current transformer All types of current transformeres1 are used for protective-relaying purposes. The bushing CT is almost invariably chosen for relaying in the higher-voltage circuits because it is less expensive than other types. It is not used in circuits below about 5 kv or in metal-clad equipment. The bushing type consists only of an annular-shaped core with a secondary winding; this transformer is built into equipment such as circuit breakers, power transformers, generators, or switchgear, the core being arranged to encircle an insulating bushing through which a power conductor passees. Because the internal diameter of a bushing-CT core has to be large to accommodate the bushing, the mean length of the magnetic path is greater than in other CT’s. To compensate for this, and also for the fact that there is only one primary turn, the cross section of the core is made larger. Because there is less saturation in a core of greater cross section, a bushing CT tends to be more Current Transformer (10) accurate than other CT’s at high multiples of the primary-current rating. At low currents, a bushing CT is generally less accurate because of its larger exciting current. The main data discovered from the ct type is the number of primary turns Wound type Through type Bushing C.T Window C.T Bar Type (Busbar & line) ( busbar only ) Phase displacement and accuracy class Phase displacement = The difference in phase between the primary and secondary current vectors, the direction of the vectors being so chosen that the angle is zero for a perfect transformer. Accuracy class = A designation assigned to a current transformer the error of which (Current error, phase displacement, composite error) remain with specified limits under prescribed conditions of use. Rated thermal and dynamic currents Rated short-time thermal current = The r.m.s. value of the primary current which a transformer will withstand for one second without suffering harmful effects, the secondary winding being short-circuited. Rated continuous thermal current = The value of the current which can be permitted to flow continuously in the primary winding, the secondary winding being connected to the rated burden, without the temperature rise exceeding the values specified. Rated dynamic current = The peak value of the primary current which a transformer will withstand without damaged electrically or mechanically by the resulting electromagnetic forces, the secondary winding being short-circuited. Measuring current transformer General Measuring current transformer = A current transformer intended to supply indicating instruments, integrating meters and similar apparatus. Current Transformer (11) Definitions Composite error = under steady-state conditions, the r.m.s value of the difference between: a) the instantaneous values of the primary current b) the instantaneous values of the actual secondary current multiplied by the rated c is generally expressed as a percentage of the r.m.s. values of the primary current according to the formula: K n= rated transformation ratio I p= r.m.s. value of the primary current i p= instantaneous value of the primary current i s= instantaneous value of the secondary current T= duration of one cycle Rated instrument limit primary current = The value of the minimum primary current at which the composite error of the measuring current transformer is equal to or greater than 10%, the secondary burden being equal to the rated burden Instrument security factor = The ratio of rated instrument limit primary current to the rated primary current. Accuracy requirements For measuring current transformers the accuracy class is designated by the highest permissible percentage current error at rated current prescribed for the accuracy class concerned. The standard accuracy classes for measuring current transformers are: 0,1 - 0,2 - 0,5 - 1 - 3-5 Accuracy +/- Percentage current error at +/- Phase displacement at percentage of class percentage of rated current rated current shown below shown below Minutes Centiradians 5 20 100 120 5 20 100 120 5 20 100 120 0,1 0,4 0,2 0,1 0,1 15 8 5 5 0,45 0,24 0,15 0,15 0,2 0,75 0,35 0,2 0,2 30 15 10 10 0,9 0,45 0,3 0,3 0,5 1,5 1,5 0,5 0,5 90 45 30 30 2,7 1,35 0,9 0,9 Current Transformer (12) 1 3,0 3,0 1,0 1,0 180 90 60 60 5,4 2,7 1,8 1,8 Class +/- Percentage current error at percentage of rated current shown below 50 120 3 3 3 5 5 5 Marking The rating plate shall carry the appropriate information in accordance general marking. The accuracy class and instrument security factor shall be indicated following the indication of corresponding rated output (e.g. 15 VA Class 0,5 F s 10). F s = instrument security factor Current transformers having an extended current rating shall have this rating indicated immediately following the class designation (e.g. 15 VA Class 0,5 ext. 150%). Protective current transformer General Protective current transformer = A current transformer intended to supply protective relays. Selecting core material As the Permeability of the core material is high and core loss low exciting current small I fe current error small. The exciting current determines the maximum accuracy that can be achieved with a current transformer Definitions Composite error = Under steady-state conditions, the r.m.s value of the difference between: a) the instantaneous values of the primary current b) the instantaneous values of the actual secondary current multiplied by the rated transformation ratio. The composite error εc is generally expressed as a percentage of the r.m.s. values of the primary current according to the formula: Current Transformer (13) K n= rated transformation ratio I p= r.m.s. value of the primary current i p= instantaneous value of the primary current i s= instantaneous value of the secondary current T= duration of one cycle Rated accuracy limit primary current = The value of primary current up to which the transformer will comply with the requirements for composite error. Accuracy limit factor = The ratio of the rated accuracy limit primary current to the rated primary current. Secondary limiting e.m.f = The product of the accuracy limit factor, the rated secondary current and the Victoria sum of the rated burden and the impedance of the secondary winding. Accuracy requirements The performance of the current transformers during overcurrent is given by the overcurrent accuracy class rating. The rating is based on the maximum secondary voltage the transformer can deliver without exceeding the given accuracy. For protective current transformers the accuracy class is designated by the highest permissible percentage composite error at the rated accuracy limit primary current Current Transformer (14) prescribed for the accuracy class concerned, followed by the letter "P" (meaning protection) at rate current prescribed for the accuracy class concerned. The standard accuracy classes for protective current transformers are: 5 P and 10 P Accuracy Percentage current Phase displacement at Composite error at rated class error at primary rated primary current accuracy limit primary current in % current in % Minutes Centiradians 5P +/- 1 +/- 60 +/- 1,8 6 10 P +/- 3 10 Marking The rating plate shall carry the appropriate information in accordance general marking. The rated accuracy limit factor shall be indicated following the corresponding output and accuracy class (e.g. 30 VA Class 5 P 10). 10 = Accuracy limit factor A current transformer satisfying the requirements of several combinations of output and accuracy class and accuracy class limit factor may be marked according to all of them. 15 VA Class 0,5 or 15 VA Class 0,5 30 VA Class 1 15 VA Class 1 ext 150% 30 VA Class 5 P 10 15 VA Class 5 P 20 Design Current transformer In current transformer design, the core characteristics must be carefully selected because excitation current I e essentially subtracts from the metered current and affects the ratio and phase angle of the output current. If it were possible to have no core or wire losses at all, the calculation for the ratio between primary and secondary; equals dividing the primary current by the turns on the secondary equals the secondary current , then take the secondary current and multiplied it by the number of ohms of the burden resistor across the secondary, which would equal the voltage output across the secondary burden resistor. Note: The maximum input current of a CT can be increased by varying the ohms of the burden resistor. Lowering the ohms of the burden resistor will increase the maximum input of the CT, but it lowers the resolution. Also, the accuracy of the output voltage depends on the accuracy of the burden resistor. The burden resistor should never be used for more than 55 % of its wattage capacity, and thermal concerns of the surrounding materials should be considered to prevent over heating damage. For circuits requiring very accurate outputs, the CT should only be used up to 50 % of saturation line of the core. Current Transformer (15) The higher the exciting current or core loss the larger the error Effect of Gapping Air gap increases the effective length of the magnetic path Measuring or protective current transformers Measuring current transformer Permeability of the core material high and core loss low => exciting current small (I fe<<) => current error small. The exciting current determines the maximum accuracy that can be achieved with a current transformer => Study accuracy classes Protective current transformer Selecting core material When choosing a core material a reasonable value for B m (0,2 ... 0,3 T) typically results in L c and R fe values large enough to reduce the current flowing in these elements so as to satisfy the ratio and phase requirements. Permeability of the core material is low When remanence is reduced to a lower level (increase the useful flux density, gapping), the voltage spikes produced by the leakage inductance due to the transformer saturation will be eliminated. In linear current transformers there are generally air gaps in the iron core to reduce the time constant and remanence. Such current transformers are used only to protect objects of major importance that require a short tripping time. Current Transformer (16) Window utilization factor The window utilization factor (K u = S 1 x S 2 x S 3 x S 4 ) is the amount of copper that appears in the window area or transformer of inductor. The window utilization factor is influenced by four different factors: (1) wire insulation, (2) wire lay (fill factor), (3) bobbin area and (4) insulation required for multilayer windings or between windings. In the design of high-current of low-current transformers, the ratio of conductor area over total wire area can vary from 0,941 to 0,673 depending on the wire size. The wire lay or fill factor can vary from 0,7 to 0,5, depending on the winding technique. The amount and the type of insulation are dependent on the voltage. A transformer intended to supply measuring instruments, meters, relays and other similar apparatus Effect of Gapping Air gap increases the effective length of the magnetic path Air-gapped current transformers Current Transformer (17) These are auxiliary current transformers in which a small air gap is included in the core to produce a secondary voltage output proportional in magnitude to current in the primary winding. Sometimes termed ´transactors´ or ´quadrature current transformers´, this form of current transformer has been used as an auxiliary component of unit protection schemes in which the outputs into multiple secondary circuits must remain linear for and proportioned to the widest practical range of input currents. Anti-remanence current transformers A variation in the over dimensioned class of current transformer has small gap(s) in the core magnetic circuit, thus reducing the possible remnant flux from approximately 90% of saturation value to some 10% only. These gap(s) are quite small, for example 0.12mm total, and so within the core saturation limits. Errors in current transformation are thereby significantly reduced when compared with those with the gapless type of core. Linear current transformers The ´linear´ current transformer constitutes an even more radial departure from the normal solid core CT in that it incorporates an appreciable air cap, for example 7.5-10mm. As its name implies the magnetic behavior tends to linearization by the inclusion of this gap in the magnetic circuit. However, the purpose of introducing more reluctance into the magnetic circuit is to reduce the value of magnetizing reactance, this in turn reduces the secondary time-constant of the CT thereby reducing the over dimensioning factor necessary for faithful transformation. The time constant of the circuit depends on the inductance of the coil and on the resistance in the circuit in accordance to the following simple formula: Rating and performance requirements Standard values of rated currents and outputs The standard values of rated primary current are: 10 - 12,5 - 15 - 20 - 25 - 30 - 40 - 50 - 60 - 75 amperes And their decimal multiples of fractions. The preferred values are those underlined. The standard values of rated secondary current are:1,2 and 5 amperes, but the preferred value is 5A. Short-time current ratings Current Transformer (18) Current transformer supplied with a fixed primary winding of conductor shall comply with the requirements of rating below. Thermal rating = A rated short-time thermal current shall be assigned to the transformer. Dynamic rating = The values of the rated dynamic current shall normally be 2,5 times the rated short-time thermal current and it shall be indicated on the rating plate when it is different from this value. Limits of temperature rise The temperature rise of a current transformer when carrying a primary current equal to the rated continuous thermal current, with a unity power-factor burden corresponding to the rated output, shall not exceed the appropriate value given in the table below. Class of insulation (in accordance with IEC Publication Maximum temperature rise 0 85) C All classes immersed in oil 60 All classes immersed in oil and hermetically sealed 65 All classes immersed in bituminous compound 50 Classes not immersed in oil or bituminous compound Y 45 A 60 E 75 B 85 F 110 H 135 Terminal markings - general rules The terminal markings shall identify: the primary and secondary windings; the winding sections, if any; the relative polarities of windings and winding sections; the intermediate tapings, if any. Core saturation and Burden Core saturation level is an important consideration when specifying current transformers. The maximum volt-microsecond product specifies what the core can handle without saturating. The burden resistor is one of the factors controlling the output voltage. There's a limit to the amount of voltage that can be achieved at a given frequency. Since frequency = 1/cycle period, if the frequency is too low (cycle period too long) so that voltage-time product exceeds the core's flux capacity then saturation will occur. The flux that exists in a core is proportional to the voltage times cycle period. Most specifications provide a maximum volt- microsecond product that the current transformer can provide across the burden resistor. Exceeding this voltage with too large a burden resistor will saturate the transformer and limit the voltage. . Current Transformer (19) For complete accuracy There are factors in the current transformer that affect efficiency. For complete accuracy, the output current must be the input current divided by the turns ratio. Unfortunately, not all the current is transferred. Some of the current isn't transformed to the secondary, but is instead shunted by the inductance of the transformer and the core loss resistance. Generally, it's the inductance of the transformer that contributes the majority of the current shunting that detracts from the output current. This is why it's important to use a high-permeability core to achieve the maximum inductance and minimize the inductance current. Accurate turns ratio must be maintained to produce the expected secondary current and the expected accuracy. The value of the current transformed is smaller than the input current by: ITRANSFORMED=IINPUT - ICORE - jIMAG Notice the four loss components in the circuit of Fig. 2. The resistance of the primary loop (PRIDCR), the core loss resistance (RCORE), the secondary DCR (RDCR) is reduced by 1/N2, and the secondary burden resistor RBURDEN is also reduced by a factor of N2. These are losses that affect current source (I). The resistances have an indirect effect on the current transformer accuracy. It's their effect on the circuit that they are monitoring that alters its current. The primary dc resistance (PRIdcr) and the secondary DCR/N2 (RDCR/N2) don't detract from the Iinput that is read or is affecting the accuracy of the actual current reading. Rather, they alter the current from what it would be if the current transformer weren't in the circuit. With the exception of the burden resistor, these loss resistors are the components that contribute to the loss in the transformer and heating. Current Transformer (20) This wasted energy is usually small compared with the power in the circuit it's monitoring. Usually, the design of the transformer and choice of the burden resistor will be within the maximum energy loss the end user can allow. As battery-operated devices come into wider use and power consumption contributes to the energy crisis — even this power may be of concern. Under these circumstances, it may require special design attention to power consumption Measuring or protective current transformers Measuring current transformer Protective current transformer Marking *Terminal markings - general rules The terminal markings shall identify: the primary and secondary windings; the winding sections, if any; the relative polarities of windings and winding sections; the intermediate tapings, if any. Current Transformer (21) CT Specification A current transformer is normally specified in terms of • A rated burden at rated current. • An accuracy class. • An upper limit beyond which accuracy is not guaranteed. (Known as the Accuracy Limit Factor, ALF). Special (Class X) Current Transformers These are normally specified for special purpose applications such as differential protection, where it is important that CTs have matching characteristics. For this type of CT an exact point on the Magnetization Curve is specified, e.g. • Rated primary current • Turns ratio • Rated knee point e.m.f. at maximum secondary turns • Maximum exciting current at rated knee point e.m.f. • Maximum resistance of secondary winding. In addition, the error in the turn's ratio shall not exceed +/- 0.25%. Current Transformer (22) Basic Calibration Circuit Fig 1: Basic calibration circuit Basic Calibration Theory: Figure 1 shows the basic calibration circuit for the passive compensated current comparator designed by Kusters and Moore [2], with a test current transformer connected in series. The compensation winding, which has the same number of turns as the secondary winding, will carry a current which is the difference between the comparator secondary winding current and the test transformer secondary current. If i is the nominal secondary current and a and b are respectively the errors of the comparator and the test transformer secondary currents, the compensation winding current is (i + a) - (i + b) which is equal to (a - b). In linking the detector core, the compensation winding ampere-turns due to a will subtract from those of (i + a) due to the secondary winding, giving a resultant due to i, which exactly cancels the primary ampere-turns. The detector core is therefore magnetized only by a current b, the error of the test current transformer. An equal and opposite current from the balance control circuit is injected into the compensation winding to give null deflection of the detector. The error b is given by ir(G + jwC), allowing the current errors and phase angle errors of the test transformer to be determined. Range of Measurements Routine current transformer calibrations can be carried out over a large range of primary to secondary current ratios, generally to rated secondary currents of 1 A and 5 A. Measurements are made at defined frequencies, burdens and power factors according to the customer's requirements. Current Transformer (23) Range of measurements Frequency/Hz 0.25 - 10000/5 50 5 - 1000/5 50 - 100 The table above gives a simplified range of the ratios available for calibration, although others outside of this range can be calibrated. The range of current ratios available for calibration should be increased in the near future following the commissioning of a 20 000 A energy regulator which has recently been installed. Uncertainties The approximate uncertainties associated with the measurement of the current and phase angle errors are given in the table below. Uncertainties will be increased if a particular transformers errors are high or if the transformer exhibits poor repeatability. Class Uncertainties Current Error Phase Error 0.01, 0.02, 0.03 10 ppm 10 µrads 0.1 and higher 30 ppm 30 µrads The reported uncertainties are based on a standard uncertainty multiplied by a coverage factor of k = 2, which provides a level of confidence of approximately 95 %. Current Transducer Measurements Current transducers, which give a voltage output, can be calibrated over a wide range of current levels. Instruments calibrated include current transformers with resistive loads connected across their secondary winding and Rogowski coils. NPL's UKAS accreditation covers the calibration of current transducers that have an output voltage greater than 0.25 V at 50 Hz and have a base measurement uncertainty of 0.05%. NPL certificates can be issued for the calibration of transducers that have smaller output voltages. NPL certificates can also be issued for the calibration of standard resistors and shunts where the impedance is measured using calibrated digital multimeters to measure the output voltage and calibrated current transformers to supply the current. These measurements can be carried out for a wide range of current levels at various frequencies up to several kHz. Current transformer test sets Calibration of both manual and automated current transformer test sets are carried out at 50 Hz, over the range: Ratio error: 10 ppm to 20 % Phase error: 10 µrad to 10000 µrad Current Transformer (24) Uncertainties are dependant upon the type of instrument calibrated. Selective Error Voltage Transformers A selective error voltage transformer consists of a transformer with a nominal ratio of unity. It has a series of switch positions that are used to introduce a fixed combination of voltage (ratio) and phase angle errors. To calibrate the transformer, these fixed errors are measured for each switch position in accordance with the Electricity Act 1989 [3]. Selective error voltage transformers are usually calibrated with an applied voltage of 230 V (110, 240, 250 V if specified by the customer), at a frequency of 50 Hz and at burdens of zero and 1 VA. Graphic symbols of current transformers current transformer: one output at two coils with the same double core current the secondary core transformer two alternative symbols Current Transformer Saturation Learning guideline After these lectures you should be able to understand and apply the following: i. CT saturation on resistive burden - Derivation of secondary current waveform based on CT magnetisation curve based on single slope magetising curve ii. CT saturation on inductive burden. 1 Output from a C.T. with a pure resistive burden For simplicity, it will be assumed that the magnetising component of the exciting current remains zero until saturation is reached, and no increase of flux above the saturation level is possible as shown in Fig. 1. In addition, it will be assumed that the core loss may be neglected in both the saturated and unsaturated states. Behavior during steady state conditions Under the above assumptions, i2 = - i1 ( N1 / N2 ) e2 = - i1 N1 R2 / N2 If i1 = I1pk sin t i2 = - I1pk ( N1 / N2 ) sin t during steady state conditions. Current Transformer (25) C.T. saturation If, however, a transformer possesses such a small core, or has a burden which is so great that the flux variations needed to provide the ideal undistorted output exceed the value needed, then saturation will occur sometime before the peak value of the flux wave. At this time the core flux density will become constant at the saturation value, no further e.m.f. will be produced, and the secondary current will collapse instantly to zero, the whole of the primary current then being available to hold the core in saturation. The core will remain in this state even when the exciting current falls below its saturation value. The core thus cannot desaturate before the primary current zero, at which time the e.m.f. required to support the secondary current to decrease at the instant of primary current zero from the saturation value, at exactly the same rate as it would have done had saturation not occurred. Current Transformer (26) This expression shows that, the smaller the core-flux saturation level, the shorter is the part of each half cycle of secondary current which would be produced. The amount of chopping of the secondary current wave which will occur in any given transformer is dependent on the primary current level and the magnitude of the secondary circuit impedance. Performance of C.T. during magnetic saturation When the primary current and secondary burden are such that the required secondary voltage is in excess of the knee point voltage, a C.T. will produce a secondary current of distorted waveform. This secondary current will contain a high proportion of odd harmonics, will have a larger ratio error, and may have zero points considerably displaced from those of the primary current. Such steady state saturation must, in general, be avoided up to the maximum value of through current in balanced and phase comparison systems of protection. In high speed protective systems the requirements for transient conditions automatically cater for this. In non-balance systems the results of saturation, while not so serious, still require some consideration. The harmonic content and limitation of output may modify time/current characteristics of overcurrent relays, directional relay characteristic, and the accuracy of distance protection. Current Transformer (27) Saturation with Resistive Burden The effect of saturation when the burden is a pure resistance is shown by the graphical analysis as shown in Fig. 6. In Fig. 6, the case is simplified by assuming that the slope of the excitation curve is zero in the saturated region. The analysis is started at any point in time and the circuit conditions are changed when the exciting current passes through the values corresponding to the onset of saturation. It can be seen from the analysis that the distortion resulting from saturation is generally in the form of a loss of the trailing part of the half-cycles of secondary current. This gives rise to a general loss of output, considerable harmonic content, and a possible large shift in the zeroes of the secondary current. This latter effect is especially important with respect to phase-comparison systems of protection. Fig. 6 Saturation with Resistive Burden (Zero slope in saturation) Self Evaluation i. What is the meaning and difference between an ideal and practical B-H magnetic characteristic? ii. Do you understand the reason why the CT secondary current waveforms appears like that under ideal magnetic characteristic? iii. What is the difference between resistive, inductive, and combination between resistive and inductive burden? iv. Do you understand how to estimate the secondary current waveform under resistive burden taking into account the effect of magnetizing current in normal conditions? Understanding Ratio Error UNDERSTANDING CURRENT TRANSFORMER RATIO ERROR AND EXCITATION CURVES A current transformer follows all the standard physical laws for electrical transformers. The primary winding is usually a very low impedance and therefore treated as a "brute force" constant current source. Faraday's law of ampere-turn balance states that the number of turns in the primary winding times the primary current must equal the number of turns in the secondary winding times the secondary current. Therefore, since the primary is a constant current source, the secondary becomes a constant current source proportional only to the turns ratio. Other factors come in to play that affect the basic Faraday's relationship, such as the non-linear properties of the core material, eddy current, hysteresis and IR losses. As Figure 1 illustrates, the eddy current and hysteresis losses act to shunt current across the transformer secondary and are defined as excitation losses IE. Since the excitation losses are non-linear, they are determined from an Excitation Curve provided by the transformer's manufacturer. The IR losses act as a resistance RS in series with the secondary winding. As Figure 2 illustrates, the secondary voltage Es is found on the vertical axis and the secondary exciting current IE can be found on the horizontal axis. This exciting current can best be described as the current that contributes to the current transformation ratio error. Power transformers use the terms "Load" and "Regulation" to describe their operation. Current transformers use the terms "Burden" and "Accuracy" respectively to describe similar functions. Burden defines the connection made to the secondary winding to differentiate it from the primary connection that is generally described as the Load. Current transformers use the term Accuracy to describe what would Current Transformer (28) generally be considered Regulation with a power transformer. It is important to remember that Burden and Accuracy are interdependent; generally the lower the Burden resistance, the better the Accuracy. Designs that have the current transformer separate from the instrumentation resistor RI need to consider transformer ratio error. An example would be an ampere meter that uses an external current transformer. The transformer must have an accurately-defined current ratio to allow for interchangeability with other transformers of the same rating. Designs that have the current transformer as an integral part of the instrumentation can place less emphasis on ratio error and consider more on the transformer's linearity. An example would be a printed- circuit-board-mounted current transformer that inputs into an operational amplifier circuit. Ratio error can generally be minimized during calibration with adjustment to the offset and gain controls. The major concern to the overall accuracy of the design would then be linearity of the transformer through out the operating range. In practice, the designer must consider various factors in selecting a current transformer: since the secondary is operating as a constant current source, a Burden resistor of lower value will provide improved accuracy but decrease instrumentation voltage (V=IR). As the instrumentation voltage is increased with a high Burden resistor, the power dissipated may become a factor (P = I2 R). Generally the designer determines the lowest voltage the electronics can handle considering such parameters as circuit noise and gains. Then the value of the burden resistor can be determined, knowing the characteristics of the current transformer and overall design requirements. An example of calculating the actual secondary current, instrumentation voltage and error percentage is as follows: Determine the total burden terminal resistance RB across the secondary of the current transformer.This includes the secondary instrumentation resistance RI and any resistance in the interconnecting leads RL. For: RI = 0.02 ohm & RL = 0.01 ohm RB =.02 +.01 = .03 ohm Add the total burden resistance to the secondary winding DC resistance RS.From figure 2 for a 200:5 current ratio transformer: RS = 0.034 ohms.03 + 0.034 = .064 ohms Select a value of secondary current at a point you desire to determine the ratio error. For: IS = 3.75 A Calculate the secondary voltage ES required for the current to flow through the total secondary resistance. ES = IS x R ES = 3.75 x 0.064 = .24 V Find the secondary voltage ES on the vertical scale of the excitation curve and read over to the 200 line and down to the horizontal scale for the secondary exciting current IE. IE = .013 A Current Transformer (29) The primary current will be the turns ratio times the sum of the exciting current and the secondary current IP = NS / NP x (IE + Is). IP = 40 x (.013 + 3.75) = 150.52 A7. The voltage developed across the instrumentation resistor will be the secondary current times the instrumentation resistor EI = IS x RI.EI =3.75 x .02 = 0.075 V To calculate the percentage ratio error,divide the exciting current by the secondary current times 100. IE / IS x 100..013 / 3.75 x 100 = 0.35 % Current Transformer (30) Glossary of Terms The degree of uncertainty with which a measured value agrees with the ideal Accuracy values. Accuracy class of instrument transformers are defined by the requirements of ANSI standard number C57.13. Standard metering accuracy classes are 0.3, 0.6 and 1.2 Ambient Temp Temperature of the surrounding air. The product of the applied voltage and current in ac circuit. Apparent power, or volt- Apparent amps, is not the true power of the circuit since power factor is not considered in the Power calculation. Auxiliary A power source, other than that producing the measured input quantity, which supplies Power the power necessary for the correct operation of the transducer. The measurement of an AC voltage or current obtained using a DC instrument with a Average rectifying input circuit that converts AC energy to DC. The meter scale or readout is Responding usually calibrated in terms of the corresponding RMS values, but is accurate only for pure sinewave inputs. In current or potential transformers burden in VA is the maximum load the transformer Burden can support while operating within its accuracy rating. Adjustment of a transducer so the output is within a specified range for particular values Calibration of the input. An instrument transformer used to accurately scale ac currents up or down, or to provide isolation. Generally used to scalelarge primary or bus currents to usable values for measuring (or control) purposes. The current measurement range is expressed as the ratio of full scale primary current to full scale secondary current. The primary winding is connected in series with the conductor carrying the current to be measured or Current controlled. There are two classification of current transformers. Window type and Transformer Wound Primary type. In Window type current transformers the primary winding is provided by the line conductor and is not an integral part of the transformer. In Wound Primary type the primary winding is an integral part of the transformers and usually consist of more that one turn. Wound Primary transformers are used in applications that require very high accuracies or where high voltage isolation is required. Delay on A term describing a mode of operation relative to timing devices. Delay begins when the Energization initiate switch is closed, or on application of power to the input. Same as Delay on Make. Delay on Make Same as delay on energization. Dielectric Strength The continuous voltage a dielectric can withstand without deteriorating. Effective Power In ac measurements, effective power (measured in watts) equals the product of voltage, current and power factor (the cosine of the phase angle between the current and the voltage). Full Scale (F.S.) The specified maximum value of the input quantity being measured that can be applied to a transducer without causing a change in performance beyond specified tolerance. Full Scale Output The specified maximum output value for which the stated accuracy condition applies. Guaranteed Range Refers to a range of adjustment or operating range whereby the control device must at least operate or Current Transformer (31) cover the "guaranteed" range. Hysterisis An error resulting from the inability of an electrical signal or mechanical system to produce identical readings or position when approached slowly from either direction. Also referred to as deadband. Impedance The opposition in an electrical circuit to the flow of alternating (AC) current. Impedance consists of ohmic resistance (R), inductive reactance (XL), and capacitive reactance (XC). Inrush The initial surge of current through a load when power is first applied. Lamp loads, induction motors, solenoids, contactors, valves, and capacitive loads all have inrush currents higher than the normal running or steady state currents. Resistive loads, such as heater elements, have no inrush. Instrument Transformer A transformer which is intended to reproduce in its secondary circuit, in a definite and known proportion, the current or voltage of its primary circuit with the phase relations substantially preserved. Isolation To be electrically separate. A measure of the strength of the dielectric providing the electrical division or separation. Linearity A measure of departure from straight-line response in the relationship of two quantities, where the change in one is directly proportional to a change in the other. Normally expressed as a maximum percentage. Loop Powered The transducer uses the power supplied to the output current measuring loop. No auxiliary power supply is required. Loop Resistance The electrical resistance, in ohms, of a complete transducer circuit exclusive of an instrument's internal resistance. Non-Linearity In an ideal system, the input-output relationship between variables is linear(i.e. straight line) Any departure from straight line is expressed as non-linearity. Operating Voltage A nominal voltage with a specified tolerance applied. The design voltage range to remain within the unit's operating tolerances. Phase Angle The difference in time by which an alternating signal lags or leads another signal. Phase angle may be a measure of power factor when used to indicate the relationship of a voltage to current signal for a non-resistive load. Phase angle may also be used to measure the different in phase between the primary and secondary of an current or voltage transformer. Polyphase Wattmeter A wattmeter consisting of 2 or 3 single phase wattmeters mounted in the same package. The watt sensing elements can be electronic transducers. A dual element wattmeter will measure power in a 3 phase system regardless of power factor, voltage or current variations between phases. Most common types are 2,2« or 3 element forms. In 4 wire circuits, with the 4th wire carrying current, the 2« or 3 element type is used. If there is voltage imbalance, only the 3 element units Current Transformer (32) can be employed. Power A source or means of supplying energy. The unit of measurement is the watt. 1 Horsepower is equal to 745.7 Watts. Range Nominal operating limits, specified by the lowest calibration point to the highest calibration point. Rated Output The output at standard calibration Ratios The relationship between the primary input value divided by the secondary output value. For example: a current transformer that has a primary input value of 100 Amps and a secondary value of 5 Amps will have a Current Ratio of 100:5 and a Turns Ratio of 20:1. It is important to use the term Current Ratio for most applications because it defines the current handling capacity of wire used in the secondary winding. The Turns Ratio only refers to the winding ratio and does not define the current handling capacity of the either primary or secondary windings. Real Power Same as Effective Power. Reactive Power A component of apparent power (volt-amps) which does not produce any real power (watt) transfer. Repeat Accuracy The maximum deviation from one timing operation to the next. Self Powered The power required for correct operation of a transducer is supplied via the line being measured. Separately Powered The power required for correct operation of a transducer is supplied via an external or auxiliary power source, rather than via the line being measured. Setting Accuracy The ability to accurately set a knob, switch, or other adjustment to the time delay, or other monitored parameter. Snubber Network A form of suppression network which consists of a series connected resistor and capacitor connected in parallel with the output device. Helps to limit the maximum rate of rise of a voltage. Used to prevent false turn-on of solid state outputs. Snubber A resistance/capacitor or diode/resistor circuit used to dissipate transient energy peaks. Transducer A device for converting an electrical signal into a useable direct current or voltage for measurement purposes. RMS The effective value of alternating current or voltage. The RMS value equates an ac signal to a dc signal which provides the same power transfer. True RMS Amps The effective value of an ac signal. For an amp signal, true RMS is a precise method of stating the amp value regardless of waveform distortion. An ac measurement which is equal in power transfer to a corresponding dc current. True RMS Volts The effective value of an ac signal. For a voltage signal, true RMS is a precise method of stating the voltage value regardless of waveform distortion. An ac measurement which Current Transformer (33) is equal in power transfer to a corresponding dc voltage. Unbalanced Loads Refers to an unequal loading of the phases in a paleface system (current and/or phase angle) Watt Unit of electrical power. WATTS=E*I*PF VA The product of the RMS voltage applied to a circuit and the RMS current, in amperes, flowing through it. VAR(Volt-Amperes Reactive) The unit of reactive power as opposed to real power (watts)