ABSTRACT AFTER MASTER'S WORK THEME
(limited version)

Manin Sergey

Faculty: Electrotechnical

Speciality: Electrical networks and systems

Theme of master's work: Transformers diagnosis

DESIGNATION OF THE NEW POWER TRANSFORMERS MAGNETIC SYSTEM DIAGNOSTIC METHOD

Leader of work: Rogozin George

Email: manin06@rambler.ru

Introduction. Theme urgency

       Magnetic system of power transformers alongside with active conductor materials and isolation of windings it is necessary to carry to the major elements of a design. A number of damages in elements of magnetic system inevitably leads to switching-off of the transformer and long repair in conditions of the specialized enterprises.
       It is necessary to carry a local fire in steel with burning out of a part of sheets of steel laminated core to number of characteristic damages of magnetic system, short circuit of separate sheets among themselves, damage of isolation coupling core studs. Presence of defects of isolation (a varnish covering) between sheets or deterioration of its isolating properties leads in operation to decrease in specific resistance of intersheet isolation of core packages and to increase in open circut losses of the transformer.
       Meanwhile, the control of a condition of intersheet isolation is connected with carrying out of the special tests [1] including performance of following measurements on extraction part of the transformer:

• definition of open circut losses at winding the control winding necessary for creation of a level of a working induction in the transformer core;
• measurement of open circut losses of i at short circuit of extreme core sheets on its external surface;
• measurement of a voltage between extreme sheets and core packages on its external and internal surfaces;
• measurement of voltage on packages (for detection of packages with the short-circuited sheets);
• measurement of specific resistance to a direct current for separate packages to a method of the ammeter and the voltmeter.

       Labour input and significant expenses of time for carrying out of the specified tests specify presence of the certain problem connected not only with the control of working capacity of magnetic system, but also prevention of energy losses growth in this system while in service transformers. It is necessary to note that the majority of transformers in power supply systems of Ukraine is in operation tens years and their substitution sequence should be caused, in particular, and the requirement of decrease in losses of energy of its transformation.
       It is obvious, that introduction of new system of diagnosing should prove an opportunity of recognition not only growth of open circut losses, but also an establishment of the damaged element of a design of transformer magnetic system (laminated core steel, coupling studs, displacement of windings as a result of electrodynamic influences at through short circuits).

The purpose and research problems

        The purpose of work is development throretical positions and research of a new way experemental definitions of the power transformers electromagnetic parameters, differing the increased accuracy of identification of the power transformer magnetic system and including an experemental basis in the form of the transitive functions, which reflecting results of imitating modelling of damages of the power transformer constructive elements and a final matrix of symptoms for diagnosis of a technical condition of last.
       The primary problem of work:

• development of a synthesis technique of an equivalent circuit of the transformer core on experemental transitive functions;
• development of a experemental modelling technique of characteristic damages of the transformer magnetic system elements;
• definition of a experemental transitive functions of the transformer at imitation damages in magnetic system;
• definition of parameters of recognition of simulated images of magnetic system in time and frequency areas of conditions vectors;
• development ot the symptoms matrix for diagnosis a condition of elements of a design of the transformer magnetic system;
• research of the developed methoderrors.

Results of researches

       Definition of transitive functions was made for transformers TM-100/6 and ТМ-240/6 from windings of the lowest voltage 0,4 кV at the disconnected primary winding. Experiences of a direct current attenuation were carried out at initial values of a direct current of the order 10 A.
       Registration of falling off process of a free current was carried out with use of the multichannel registering device of type «Rekon-08МС». Values of currents in phase windings LV of the transformer it was expressed in shares of their initial values. Time of supervision of transient was limited 6 sec. Frequency of interrogation of a signal was installed equal to 10 кHz. At approximation of skilled curves by the exponential components sum uniform division of time coordinates of a current curve within the limits of the separate intervals installed according to preliminary experiments was accepted. With the specified purpose according to decomposition of a current curve on making exhibitors the uniform time scale for ten values of a current in each of following time intervals was installed: 0 - Т1; Т2 - Т1; Т3 - Т2; Т4 - Т3. Values of constants of time (T), and also other parameters of a separate components for transformer TM-100/6 for a phase A are resulted in tab.1.
        Table 1 - Decomposition of a attenuation direct current curve in a phase A of the transformer TM-100/6 on exponential components



       From the results analysis of decomposition of transitive function in winding LV of the transformer TM-100/6 on making exhibitors follows:

• the range of essential changes of the transitive functions, determined in the size of the greatest time constant of an exponential component, which equivalent a skilled curve, is within the limits of 3 sec.;
• there is an essential distinction in parameters of transitive function exponential components of an average core phase winding of an three-rod core transformer in comparison with windings of extreme cores;
• time constants of greatest of two quick damping exponential components of a current are within the limits of 0,05 and 0,2 sec.;
• value of two quick damping components of a current during the initial moment of transient is within the limits of 0,8 d.u.

       Allocation of four parameters essentially differing on values of an exponential components in transitive function of a transformer current specifies presence in transitive functions of the information reflecting transients in separate elements of magnetic system: laminated core steel sheets; massive elements of a design, in particular, in studs and other similar core elements; the elements of a design creating ways of magnetic streams of dispersion through a transformer tank.
       With a view of a technique perfection of experiment carrying out and fuller use of opportunities of the registering device it is expedient to carry out following recommendations:

• for stabilization of a transformer core magnetic condition (at carrying out reference and the subsequent periodic measurements of transitive functions) before carrying out of experience should be translated one of possible curves of magnetization of the transformer B(H) on a falling down site of a curve limiting hysteresis cycle by realization of 7-10 consecutive asymmetrical cycles magnetization-demagnetization (during repeated recurrences of experiences of attenuation of a direct current). Transition to the steady specific cycle providing adequacy of measurement results at constant magnetic conductivity of core steel, is shown on fig. 2 a,b [4];
• fegistration of transitive functions to spend at frequency of interrogation?equal 20 кHz;
• to increase initial value of an entrance signal of the registering device up to 225 mV (instead of 75 mV in the lead experiences), that will allow to guarantee minimal mean-square error of measurements of 0,8 % in a range of measurement of current transitive function from 1,0 up to 0,5 d.u.;
• to increase quantity of a current transitive function values in the ranges allocated on the basis of change of time constants of approximating exhibitor.

       Figure 2 - Stabilization of a transformer core magnetic condition : a - process of reusable core magnetization; b - transition from a initial magnetization curve to an asymmetrical specific cycle at repeated carrying out of experience of attenuation of a direct current.