Технические науки / 5. Энергетика

Breydo I.V., Makarenko N.V.

Karaganda State Technical University, the Republic of Kazakhstan

BOUNDARIES OF D.C. ELECTRIC DRIVE STABILITY AT POWER SUPPLY FROM COMPARABLE POWER SOURCE

Power supplying a thyristor drive of a number of mining machines is done from independent and dependent sources having the power comparable with an electric motor power. For such conditions a considerable (up to 40%) change of a.c. voltage amplitude in the mains is characteristic which results in the deformation of static and dynamic characteristics of the electric drive, as well as in pseudoswitchings-off connected with the breakdown of the thyristor converter operation.

The carried out investigations of a d.c. thyristor drive closed in speed and current, when power-supplied from a comparable power source, have shown that in addition self-excited oscillation processes can occur [1]. With the aim of estimating the influence of active and reactive components of a power source and non-linearities of a controlled rectifier on the dynamic electric-drive characteristics investigations of the above mentioned type of an electric drive have been fulfilled using simulating facilities. Simulating experiments were carried out on a standard electric drive the power part of which consists of a completely controlled three-phase rectifier, a compensated d.c. motor of separate excitation of 4PF132S type. The electric drive control is done by a single-circuit controlling system having a proportional speed controller (SC). From the results of simulating experiments it follows that at the regimes of starting the electric drive and at changing its load a non-sanctioned closing of thyristors due to the excess of the armature e.m.f. values (E_H), caused by the voltage drop in the mains (E₂), over the rectified one results in a break of the electric circuit. Besides the current sensor signal coming into the controlling system is equal to zero and the thyristors are closed. It results in the sharp increase of the speed-controller output signal. At the next moment when the voltage drop in the armature circuit is absent since the circuit is broken, the voltage of the mains E₂ starts to exceed E_H, the thyristors are opened if a control action exists and as the SC output voltage is very high it results in the increase of the armature e.m.f. and the excess of its value over the rectified one of the mains voltage. The latter again causes a non-sanctioned thyristors closing. The system comes into a self-exciting oscillation regime. A simplified model of the electric drive power part is given in Fig.1.

Fig.1. Equivalent circuit of electric drive power part

In the equivalent circuit E_AB - is an e.m.f. source, L_AB - is an equivalent inductance, S1 - is a key the control of which is fulfilled taking into account the conditions and the phase control system operation. Proceed from the thyristors commutation conditions the key S1simulating them (Fig.1) is closed when the instantaneous value of the motor armature e.m.f. and the voltage drop in its circuit E_H is less than the instantaneous e.m.f. value in the input of the controlled rectifier E₂. Thus the commutation conditions of the key S1 are described by the following relations:

Е₂ < Е_Н - the key S1 is broken; (1)

Е₂ > Е_Н - the key S1 is closed.

Consider a single-circuit system of speed controlling when the key is closed, i.e. Е₂ > Е_Н, then:

(2)

where ; ; =Е_АВ/Е_AN; =Е_A/ Е_AN; =i/i_N; r_SS=R_SАВi_N/Е_AN - is the relative resistance of the supply source; ; ; ; ; ; ; Е_AN=СФ_Nw_N, Ф_N, w_N, Е_N, i_N - are nominal values of the armature e.m.f., the magnetic flux, the angular velocity, the voltage and the motor armature current respectively; C - is a constructive motor constant; - is a total magnetic flux of the poles ; - is an angular motor velocity; M_R - is a moment of resistance of the motor; M_N - is a nominal value of the motor electric and magnetic moment; J - is a moment of inertia driven to the motor shaft; ; ; К_SC, К_ТC - are the coefficients of the speed controller and the thyristor converter.

At ; M_R=0, Т_SS=0, (2) takes on the form:

(3)

where K₁ = К_SC/ К_ТC - is a coefficient of the speed control system.

The system stability condition can be derived from solving the following inequality:

<0,

where К₂=Т_Мρ_A - is a constructive coefficient of the motor; К_SS=ρ_SS/ρ_A - is a coefficient of the supply source.

According to Gurvits number the system is stable at:

>0, (4)

For standard adjustments of a single-circuit speed control system having a P-controller condition (4) is impracticable. Increasing the stability region can be obtained by reducing the speed gain (K_SC≈0).

The stability condition for an open single-circuit system of the speed control can be obtained by solving the following inequality:

>0 (5)

where К₃₌;К₄=.

The system is stable at: К₄-1 <0, i.e. 0 < < 1.

In the case of the excess of the voltage drop in the mains over the armature e.m.f. the open system is unstable.

List of Publications Used

1. Breydo I.V., Makarenko N.V. Problem of D.C. Thyristor Electric Drive Stability at Power Supply from Comparable Power Source. Modern Power Engineering, Telecommunication and Higher Education: Publications of the 4^th International Scientific-Technical Conference (September, 23-24, 2004). Almaty, 2004. s.287-290.