Classification and principle of operation of the current transformer
Current transformers are classified:
- on number of transformation ratioes: with one koeffitsiyoent of transformation; with several coefficients of the transforyomation received by change of number of rounds of primary or secondary winding, or both windings, or application of several secondary winding with different number of rounds, sootyovetstvuyushchy to different rated secondary current;
- on number of steps of transformation: one-stage; kasyokadny (multistage), i.e. with several steps of the transyoformation of current;
- on performance of primary winding: single-turn; multi-turn.
Single-turn transformatora of current have 2 разновидности:без own primary winding; with own primary winding. The single-turn current transformers which do not have own primary winding are carried out built-in, tire or connector.
The internal current transformer represents the magnetic conductor with the secondary winding which is reeled up on it. It has no own primary winding. Its role carries out the current carrying rod of the insulator through passage. This current transformer has no insulating elements between primary and vtoyorichny windings. Their role is carried out by isolation of the izoyolyator through passage.
Own primary winding TT - the current carrying rod of the insulator through passage (tire). In the busbar current transformer the role of primary winding is carried out by one or several tires of the distribution ustroyyostvo which are passed when mounting through the cavity of the izolyayotor through passage. The last isolates such primary winding from vtoyorichny.
The connector current transformer 2 has no own primary winding too. Its magnetic conductor consists of 2 parts which are pulled together by bolts. It can be disconnected and closed around the conductor with the current which is primary winding of it of the TT. Isolation between primary and secondary windings is imposed on the magnetic conductor with secondary winding.
The single-turn TT having own primary winding are carried out with bar primary winding or from U-shaped.
The current transformer 3 has primary winding in the form of the rod of the round or rectangular section fixed in the prokhodyony insulator.
The current transformer 4 has the U-shaped primary winding executed in such a way that almost all internal isolation of the TT is imposed on it.
Multi-turn current transformers are produced with the coil primary winding which is put on the magnetic conductor; with the lap primary winding 5 consisting of several rounds; with link primary obmotyoky 6, executed in such a way that internal isolation of the current transformer is structurally distributed between pervichyony and secondary windings, and the relative positioning of windings reminds chain links; with the rymovidny primary winding executed in such a way that internal isolation of the transforyomator of current is applied generally only on the primary winding having the eye form.
Key parameters and characteristics of the current transformer according to GOST 7746 — 78 "Current transformers. The general technical requirements" are:
- Rated voltage — the operating value of liyoneyny tension at which the current transformer is intended to work, specified in the passport table of the current transformer. For domestic current transformers the following scale of rated nayopryazheniye, kV is accepted: 0,66; 6; 10; 15; 20; 24; 27; 35; 110; 150; 220; 330; 500; 750; 1150.
- The rated primary current of I1n specified in the passport table of the current transformer, - the current passing on primary winding at which long operation of the current transformer is provided. For oteyochestvenny current transformers the following scale of rated primary currents is accepted, And: 1; 5; 10; 15; 20; 30; 40; 50; 75; 80; 100; 150; 200; 300; 400; 500; 600; 750; 800; 1000; 1200; 1500; 2000; 3000; 4000; 5000; 6000-8000; 10 000; 12 000; 14 000; 16 000; 18 000; 20 000; 25 000; 28 000; 32 000, 35 000; 40 000. In the current transformers intended for the turbo komplektovayoniya - and hydrogenerators, values of rated current over 10 000 And can differ from the znayocheniye given in this scale. The current transformers expected rated pervichyony current 15; 30; 75; 150; 300; 600; 750; 1200; 1500; 3000 and 6000 And, have to allow beyond all bounds the long time the greatest working primary current equal respectively 16; 32; 80; 160; 320, 630; 800; 1250; 1600; 3200 and 6300 A. V other cases the greatest primary current is equal to rated primary current.
- The rated secondary current of I2n specified in the pasyoportny table of current transformers - the current passing on secondary winding. Rated secondary current is accepted equal 1 or 5 And, and current 1 A is allowed only for current transformers with rated peryovichny current to 4000 A. In coordination with the customer production of the current transformer with rated secondary current 2 or 2,5 dopuyoskatsya And.
- The burden of the current transformer z2n corresponds to the full soproyotivleniye of its external secondary circuit expressed in ohms with the indication of electrical power factor. The burden can be also characterized by the ultimate power in volt-amperes consumed by it at this electrical power factor and nominalyyony secondary current. The burden with cos electrical power factor sr2 = 0,8 at which the established class of accuracy of the current transformer or extreme frequency rate of primary current concerning its noyominalny value is guaranteed is called rated vtoyorichny load of the current transformer z2n. numbers For domestic current transformers are established sleduyuyoshchy values of the rated burden S2n .ny, vyrayozhenny in volt-amperes, at cos electrical power factor r2 = 0,8: 1; 2; 2,5; 3; 5; 7,5; 10; 15; 20; 25; 30; 40; 50; 60; 75; 90; 100; 120. The corresponding values of the rated burden (in ohms) are defined by expression of Z2n. number = S2n. ном/I2н^2.
- The transformation ratio of the current transformer is equal to the relation of pervichyony current to secondary. In calculations of current transformers 2 sizes are applied: valid transformation ratio of n and rated transformation ratio of nn. The valid koeffitsiyoent of transformation of n is understood as the relation of the valid primary current to the valid secondary. The nominalyyony transformation ratio of nn is understood as the relation of rated primary current to rated secondary.
- The resistance of the current transformer to mechanical and thermal influences is characterized by current of electrodynamic firmness and current of thermal firmness.
Current of electrodynamic firmness of Id is equal to the greatest ampyolituda of the short-circuit current for all the time of its course which the transformer maintains without damages interfering its further correct work. Current of Id characterizes ability of the current transformer to resist to mechanical (electrodynamic) vozdeyyostviye of the short-circuit current.
The electrodynamic stoyyokost can be characterized also by frequency rate of Cd, predstavlyayoyushchy itself the relation of current of electrodynamic resistance to amplitude of rated primary current. Requirements of elektroyodinamichesky firmness do not extend to the tire, built-in and connector current transformers.
Current of thermal firmness of Itt is equal to the greatest deystvuyuyoshchy value of the short-circuit current for the TT interval which the current transformer maintains during all period without heating of current carrying parts to temperatures exceeding dopuyostimy at short-circuit currents and without povreyozhdeniye interfering its further work.
The elements participating in the current preobrazoyovaniye are peryovichny 1 also the secondary 2 obyomotka who are reeled up on the same magnetic conductor 3. The primary winding turns on consistently (in the rasyosechka of the current-conducting wire of high tension 4), i.e. is flowed round by I1 line current. Measuring apparatuses (the ampermeter, current winding of the counter) or the relay are connected to secondary winding. At the current transformer rayobota secondary winding is always closed on loading.
Primary winding together with the chain of high tension is called the primary circuit, and the external circuit, poluchayuyoshchy measuring information from secondary winding of the transforyomator of current (i.e. loading and bonding wires), call the secondary circuit. The chain formed by the secondary obyomotka and the secondary circuit attached to it is called the branch of secondary current.
From the key diagram of the transformer it is visible that between primary and secondary windings there is no electric communication. They are isolated from each other on the full working napryayozheniye. It also allows to carry out the direct prisoyediyoneniye of measuring apparatuses or the relay to secondary winding and by that to exclude influence of high tension, priyolozhenny to primary winding, on service personnel. As both windings are imposed on the same magnetic conductor, they are magnetic and connected.
In fig. 1 only those elements of the current transformer which participate in transformation of current are represented. Of course, the current transformer has many other elements providing demanded уровень isolation, weather proofing, appropriate mounting and utilization properties. However they do not take part in transformation of current and will rassmatyorivatsya below in the corresponding heads.
Let's pass to consideration of the principles of operation of the current transformer. On primary winding of 1 transformer proyokhodit I1 current called by primary. It depends only on parameters of the primary circuit. Therefore in the analysis of the phenomena occurring in the current transformer, primary current can be considered the set size. When passing primary current on primary winding in the magnetic conductor the changed magnetic flux of F1 changing with the same frequency as I1 current is created. The magnetic flux of F1 covers rounds of both primary, and secondary windings.
Crossing rounds of secondary winding, the magnetic flux of F1 at the change induces in it elektyorodvizhushchy force. If secondary winding is closed on nekotoyory loading, i.e. the secondary circuit, then in such system "secondary winding — the secondary circuit" under the influence of induced э is attached to it. of page will pass current. This current according to the Lenz law will have the direction opposite to the nayopravleniye of primary current of I1.
The current passing on secondary winding creates the variable magnetic flux of F2 which is sent to opposite magnetic flux of F1 in the magnetic conductor. Thereof the magnetic flux in the magnetic conductor caused by primary current will decrease. As a result of addition of magnetic fluxes of F1 and F2 in the magnetic conductor the resulting magnetic flux of F0 = — F2 making several percent of the magnetic poyotok of F1 is established by F1. F0 flow is also that link which posredyostvo carries out transmission of energy from primary winding to secondary in the course of transformation of current.
The resulting magnetic flux of F0, crossing rounds of both windings, induces at the change in primary winding protivo-e. of the village Ekh, and in secondary winding — э. of page. To it. As rounds of primary and secondary windings have approximately identical coupling with the magnetic flux in the magnetic conductor (if пренебречь dispersion), then in each round of both windings is induced same э. of page. Under influence э. of page E2 in secondary winding proceeds I2 current called by secondary current.
If to designate number of rounds of primary winding through W1, and secondary winding — through W2, then at course on them sootyovetstvenno currents of I1 and I2 in primary winding the magnitoyodvizhushchy force of F1 = I1*W1 called by primary magyonitodvizhushchy force is created (m c), and in secondary winding — F2 magnetopropellent = I2*W2 called vtoyorichny by m of of the village Magnitodvizhushchaya force is measured in amyoper.
In the absence of losses of energy in the course of transformation of current magnetopropellents of F1 and F2 have to be in number equal, but are sent to the opposite sides. The current transformer at which process of transformation of current is not followed by energy losses is called and and l ь N ы m. For the ideal current transformer fairly sleduyuyoshchy vector equality:
F1=-F2 or I1W1=I2W2
Follows from this equality that I1/I2=W2/W1=n i.e. currents in windings of the ideal current transformer are inversely proportional to numbers of rounds.
The relation of primary current to secondary I1/I2 or numbers of rounds of secondary winding to number of rounds of the primary winding W2/W1 is called transformayotion coefficient п the ideal current transformer. Considering this equality, it is possible to write I1=I2*W2/W1=I2*n i.e. primary current of I1 it is equal to the secondary current of I2 increased by the transformation ratio of the current transformer n.
In real current transformers transformation of current is followed by losses of the energy spent for creation of the magnetic flux in the magnetic conductor on heating and magnetic reversal of the magnetic conductor and also on heating of wires of secondary winding and secondary circuit. These losses of energy break the equalities established above for absolute values of m of of page of F1 and F2.
In the real transformayotor primary has to provide with m of of page creation necessary secondary to m with and also additional m with, raskhoduyeyomy on magnetization of the magnetic conductor and the covering of others потерь energy. Therefore, for the real transformer the uravyoneniye will have the following appearance:
where — full of m of of page of magnetization, Fo spent for the proyovedeniye of the magnetic flux on the magnetic conductor, on heating and its magnetic reversal.
According to it equality will take the form
where i0 — the magnetization current creating the magnetic flux of F0 in the magnetic conductor and being the part of primary current of 11 highways. Having separated all members of the equation into W1, we will receive i1=i2*W2/W1+i0. At primary current which is not exceeding rated current of the transformer, the magnetization current usually makes no more than 1 — 3% of pervichyony current, and it can be neglected. In this case I1=I2*n. Thus, secondary current of the transformer пропорционален to primary current. It is necessary for lowering of the measured current that the number of rounds of secondary winding was more number of rounds of primary winding.
The real transyoformator of current distorts observed datas a little, i.e. has errors. Sometimes use so-called reduction of current to peryovichny or secondary winding of I0 '=I0/n.
Part of the given primary current goes for magnetization of the magnetic conductor, and other part is transformed to the secondary circuit, i.e. primary current kind of branches on the 2nd branch circuits: on the chain of loading and the chain of magnetization. Resistance of primary winding of the current transformer on the equivalent circuit is not shown as it does not exert impact on operation of the transformer.Top