THE USE OF IMPULSE ACTIONS AT CHECKING FOR SHORT CIRCUITS IN ROTOR TURBINE.

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Abstract: Unbalanced magnetic attraction of the rotor and its uneven heating is caused by coil short circuit in the rotor winding, which causes its bending. These processes increase the vibration due to the composing reverse speed. The damage of the coil isolation can be caused both by the movement of the windings in the slot and the displacement of insulating spacers between the turns of the rotor coils.

Keywords: Rotor, turbine, impulse action, short circuits.

1. Introduction

The coil short circuit remained insignificant for a long time unless due to thermal imbalance it caused the shift of the vibration level. For a long time turbogenerators (up to 120 MWh) in most of power systems worked with coil short circuit in the rotor, some coil shorts were detected just by advanced methods. But after the invention 800 MWh turbo-generators and higher coil short circuits detection became essential due to the large loss during the possible outage in power supply caused by coil short circuit.
Therefore, the purpose of this work is to develop the interturn insulation field winding control procedures using the impulse actions. This method is more efficient comparing to the traditional method, which is associated with the time consumption and application complexity in an emergency.

2. Schematic diagram of the diagnostic tests

Schematic diagram of the diagnostic tests for the methods associated with video pulse filing on the excitation winding turbogenerator is shown in (Fig. 1).

[pic]

Fig.1. Scheme for coil short circuit diagnosis in the winding rotor

The impulse impact generator includes low-power DC source UG, allowing you to install the current not more than 10-15 A, capacitor C, connected to the source through the excitation winding; VT - diode, that may influence the presence of only the positive half-wave current excitation; S - electronic key, switching transient in the circuit. On the scheme turbogenerator rotor is submitted by the following elements: LG - field winding, LD1, LD2 - the equivalent damping contours representing an array of rotor. LS turbogenerator stator winding is connected to a "star" and is plugged to T block transformer disconnected by QF switch from the system. Free change of eddy currents in the rotor elements induced by impulsive action is recorded by PS automated measuring set. The purpose of this complex is to collect data and initial processing of the diagnostic experiment results.

3. Physical processes in the structural rotor elements under submitting of video pulse

Diagnostic experiment with the impact of unipolar pulse is performed considering the electromagnetic nature of physical processes and is illustrated in (Fig. 2).

[pic]

Fig.2. Currents and voltages graphs

Changes in the electrical parameters characterize the following typical time intervals.
1. The stage of establishing the transitional process after the power source is switched in the circuit (is not considered in the present work as it does not bear useful information). At the end of this stage if current flows through the coil excitation equal to 10 - 15 A, currents in the rotor array are absent (Figure 2. Time interval t t1). Herewith the register device records the change in Uf voltage, what allows evaluate the presence of coil circuit in the winding rotor. To eliminate the influence of the measuring complex on the electromagnetic transient formation the value of its resistance and capacitance should be limited to appropriate values.

4. Compilation of differential equations

The calculation of transient currents with an intact rotor winding is shown in (Fig. 3). Coil resistance of excitation and damping circuits are not shown.

[pic]

Fig.3. The equivalent circuit calculation scheme for intact rotor winding
The differential equation (1) of the transition process is formed until the time t1 when the current in the field winding drops to zero:

[pic] (1)

The system of differential equations (2) for eddy currents free change phase in the array of rotor:

[pic] (2)

The equivalent circuit scheme shown in (Fig.4) is used to calculate the transition process in the rotor with coil circuit

[pic]

Fig.4. The equivalent circuit calculation scheme for coil circuit in the winding rotor stimulation

The system of differential equations (3) on the pulse formation stage:

[pic] (3)

The system of differential equations (4) on the stage of eddy currents free change in the rotor's array:

[pic](4)

5. Transform the original differential equations to normal form for the solution

For solving differential equations systems is necessary to adapt them to the standard form. This requires the derivatives of the currents are placed in the left side of the equations. Let us write the differential equations system in matrix form. And take a capacitor charge as an additional variable. Thus we receive the following system of differential equations (5):

[pic] (5)

Compiled L, R, S, E coefficients matrix (6):

[pic] (6)

for the system (7):

[pic] (7)

for the system(8):

[pic] (8)

for the system (9):

[pic] (9)