Alex Tsukanov

Faculty: Electrical

Speciality: Electrical Power Plants

Theme of master's work:

Research Influence Unbalance of Motors Accuracy Determine Location of the Defect in the Insulation System's Own Needs of Electrical Power Plant

Scientific adviser: Grebchenko Nickolay

Summary on the topic of master's work

Introduction

At the present stage of development, energy in Ukraine is facing serious problems. Operating life of the equipment at thermal power stations, which were built in the Soviet Union, is gradually coming to an end, and in some cases have long ended. Depreciation of equipment leads to frequent injuries and emergencies. To ensure uninterrupted supply of quality electricity consumers, it is necessary, first of all, to ensure efficiency auxiliary power plants.

According to statistics, the most common faults of induction motors:

1. Overload or overheating of the electric motor stator - 31%.

2. Interturn closure - 15%.

3. Damage to bearings - 12%.

4. Damage to the stator windings or insulation - 11%.

5. Uneven air gap between stator and rotor - 9%.

6. Looseness of the stator windings - 4%.

7. The imbalance of the rotor motor - 3%.

8. Misalignment of shafts - 2%.

Faults and damage to electric cars can not always be detected by external examination, as some of them (mainly electric) are hidden and can be detected only after appropriate testing and disassembly of the machine.

Topical issue

The emergence of much of the damage caused by the deterioration of electric insulation of the windings, so the continuous development of methods for diagnosis in our time requires special attention. Continuous diagnostics would detect damage to motors in the early stages of their development.

Relationship of

At the moment there are a number of methods that allow, under certain assumptions, without shutting down the connection, to determine the condition of the windings of electric motors.

The method proposed in [4], is based on solving systems of linear algebraic equations. The coefficients are determined periodically based on continuous measurement of phase currents and voltages phase of accession on the ground. To implement the method does not require the creation of additional conditions for the measurement and installation of additional instrument transformers.

In modeling the active and capacitive cross-conduction of electric motors to regard the findings focused on their windings (basic assumptions), and defects in insulation accession simulated changes in values of the admittance of isolation phases.

The disadvantage of the method is that the method has been developed taking into account assumptions about the symmetry of the longitudinal conductivity of controlled joining load. When unbalance method gives an error. The error arises from the fact that the parameters of the regime except components caused defects in isolation, there are components due to the asymmetry of accession. Therefore, to improve the accuracy of the method of determining the parameters of isolation in the presence (appearance) of asymmetry is necessary to determine the components of emergency caused by the asymmetry and exclude them from the profile used as input in the parametric method. Another possible way to improve accuracy is to introduce a scheme of replacement accession (a mathematical model of accession, in accordance with which compiled the current state of the equation) parameters, which reflect the degree and type of asymmetry. The real value of the input parameters should be determined for the time at which the system of equations solved by the current state. It should be noted that the direct determination of the longitudinal conductivity of the phase of accession to the operating mode is impossible, since the measured current phase accession consists currents due to transverse conductivity phase of accession, the values are unknown.

Objective

The aim is to develop a method that would enable the timely identification of place of origin of the defect isolation connection cable engine without disconnecting from the power source, including the asymmetry that erupted Coiled closures. The method is as follows. The values of phase current and voltage phases read from the measuring current and voltage transformers, and then, through a device, transformation to a digital signal and sent to the computer. The computer calculates the system of linear algebraic equations, the results of measurements are fed in Slough after the calculation which determines the values of the admittance of winding insulation. Next, a comparison of current values with predetermined nominal value. If the current value is within acceptable limits, the result is reset and the calculation is repeated for the new measurements. If the current value of the conductivity more than nominal, it indicates a decrease in resistance, then the signal is formed of a defect. 

The object of research and development

The object of investigation used the accession of the cable-electric motor, own needs of electrical power plant.

Consider the equivalent circuit (Fig. 1) joining the cable-electric motor (EM). The currents I_A, I_B, I_C are determined by measuring current transformers TA, under certain voltages phases in normal U_A, U_B, U_C. By the first law of Kirchhoff currents I_A, I_B, I_C respectively:

where I_AN, I_BN, I_CN, - vectors of phase currents of the load connection; I_AI, I_BI, I_CI, - vectors of currents through isolation phase of accession;

where Y_AN, Y_BN, Y_CN, and Y_AI, Y_BI, Y_CI - respectively, longitudinal and transverse conductivity of accession; U_A, U_B, U_C - the stress vectors of phases relative to ground; U_N - voltage neutral connection to earth.

For equations (1) - (3) modes of operation can be continuously determined by the vectors of phase currents I_A, I_B, I_C, voltage phase to earth U_A, U_B, U_C stress vector neutral connection to earth U_N. The unknowns are the longitudinal and transverse conductivity Y_AN, Y_BN, Y_CN, and Y_AI, Y_BI, Y_CI.

To solve the system (1) - (3) the relative conductivity of isolation is necessary to know the values of the longitudinal conductivity of accession or equal to each other, thus eliminating them from the system. In the second case, the actual ratio between the values of the longitudinal conductivity of the phase-controlled load accession has a great influence on the accuracy of the conductivity phase insulation to the ground in solving the system of equations (1) - (3).

In the method of determining the complex conductivity insulation in operating modes, electrical connections using a continuous solution of the system current state.

System (1) - (3) can be transformed, if we accept the assumption of equality of the longitudinal conductivity phase load. This makes it possible to exclude the longitudinal conductivity of the equations.

However, the real value of the longitudinal conductivity of the phase loads are slightly different from each other, which is why there is error in the determination of the admittance of isolation phase in relation to the earth.

The asymmetry in the form of short-circuit between phases or breaks in the phases in the method is not considered, because is short. In the event of such damages EM quickly turned off the relay protection.

Coiled-circuit in the stator EM belong to the species of static unbalance, consider the case when the asymmetry arose only in the phase A, and the complex conductivity of phase A will differ from the healthy phases B and C:

where the K_NA - the complex coefficient of asymmetry phase A.

Y_N - average value of load conductance.

Substituting (4) (5) (6), (1) (2) (3) we obtain:

Formulation of the problem lies in determining the K_NA, if we know the currents of phases, phase voltage and conductivity isolation phases Y_I, the process of defining detailed in [1]. As a result of the analysis and mathematical modeling established pattern of change of longitudinal conductivity of the phases of EM in the presence of static unbalance. In accordance with these laws the value of complex longitudinal conductivities of phases of EM are:

where k_SA, k_SB, k_SC - the complex coefficients of static unbalance of phases A, B, C;

Y_A = Y_B = Y_C - nominal value of complex longitudinal conductivity phases EM.

As a result of static unbalance appears current reverse sequence, in case of static unbalance its amplitude and phase did not change over time. When the static unbalance is determined by the phase in which the asymmetry arose, for example, if the asymmetry in the phase A, the coefficients are determined by the expressions:

where I₁ and I₂ - vectors of the current direct and reverse sequence.

In the simulation of a local defect in the stator winding of electric motor equivalent circuit is divided into two parts: the place of the defect and then place the defect (Figure 3). Each part of the cable, the engine is provided L-shaped pattern of substitution. Changing these parts is proportional to the distance (b) from the beginning of the element to the place of the defect (in proportion to the number of wiring from the beginning to the place of the defect) corresponds to the change in the location of the defect isolation. Figure 3 shows the equivalent circuit of phase A, where Z_AK and Z_AD - комплексніе resistance appropriately cable and motor, and Z_AIK Z_AID - complete insulation resistance of phase A relative to ground, respectively, the cable and the engine, Z_D - impedance isolation phase defect A.

Figure 2 - Equivalent Circuit of phase A with the presence of insulation defects in the stator winding of electric motor.

After completing the conversion scheme obtain expressions for determining the conductivity of phase A:

For phases B and C, the conductivity determined by the same expressions.

As a result of transformation scheme has the form (Figure 3):

Figure 3 - Modified equivalent circuit of a node circuit with motors.

Such an equivalent circuit connection with defect isolation allows you to simulate insulation defects in any phase, also allows you to simulate multiphase defect and balance the asymmetry of longitudinal and transverse parameters of the phase of accession.

 In writing this essay master's work is not completed, the final version of the work can be obtained from the author or a supervisor after December 2010.

References

1. Grebchenko N.V., Sidorenko A.A., Belchev I.V., Method of determining the insulation of electric connections with static and dynamic asymmetry / / scientific work of Donetsk National Technical university Series: Electrical engineering and power, edition 9 (128): onetsk: DVNZ "DonNTU". - 2009. - S. 58-65.

2. Grebchenko N.V. Mathematical modeling of the local distribution of defects and electrical insulation in the units of electrical systems with engines / scientific work of Donetsk National Technical university Series: Electrical engineering and power, edition 79: Donetsk: DVNZ "DonNTU". - 2004. - S. 55-62.

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8. Swee P.M. Control of isolation high voltage equipment. - M.: Energoatomizdat, 1998.

9. Path. 213601 RU. Method for determining active and capacitive components of the insulation resistance phase of the network on the ground / K.V. Lapchenkov, A.I. Sidorov. - BI, 1999, № 24.

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11. Continuous evaluation of insulation condition accessions own needs 6 kW of thermal power stations Grebchenko N.V., Sidorenko O.A., Spiridonov M.V., Belchev I.V. -

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