Operational Features of Thyristors in Regulated Magnetic-Valve Power Supplies

S. V. Selskii, Ya. A. Kantin

(Presented by the Scientific and Technical Seminar of the Research Institute of Nuclear Power Engineering)

At the present time, power circuits of regulated power supplies have become widespread which provide for the series connection of a thyristor and a transformer for matching with the load [1]. With the aid of such devices, which have come to be called valve-magnetic regulators, automatic programmed control of a technological process is carried out by current, voltage, or other parameters. They can be successfully used for supplying heating devices and for regulating thermal conditions in electrothermal installations. However, the combination of thyristors with a transformer load has a number of special features that sharply complicate the dynamic properties of such power supplies and create considerable technical difficulties in designing stably operating closed-loop automatic systems. Mathematical analysis and synthesis of the characteristics of such systems, as well as optimization of the principal parameters of the regulating device, constitute a very complex and, in a number of cases, insoluble technical problem. The most effective means of studying them is the method of physical modeling.

The block diagram of the stabilizer model is shown in Fig. 1, a. The stabilizer under study is constructed according to the classical principle of a static automatic-control system with continuous feedback by load current. Tests of the indicated model showed unsatisfactory operation of the stabilizer. An unstable, intermittent, and in some cases rectifier mode of operation was observed, with a sharp increase of the current in the primary winding of the transformer above the rated value. The main cause of the instability of the closed-loop system can be found by analyzing the oscillograms in Fig. 1, b, c, d, e, obtained during the investigation.

The instability of the system follows from the operating characteristics of the symmetrical thyristor through which voltage is applied to the transformer. As is known, a symmetrical thyristor is an element with limited control. It can be turned on by a control pulse only once during the positive or negative half-periods of the supply voltage. In a closed-loop control system, the control pulse by means of which the thyristor is turned on is shifted under the influence of the continuously varying feedback signal.

Therefore, the phase of the control voltage \(\varphi_1;\ \varphi_1';\ \varphi_2;\ \varphi_2'\) is a random function of time of a continuous character, caused by disturbing influences. Since commutation of the thyristor occurs discretely, between its turn-ons in the positive and negative half-periods there elapses a time \(\tau_1;\tau_2\). During this time the continuously varying mismatch signal shifts the phase of the control pulse by some angle \((\varphi_1-\varphi_1')\) or \((\varphi_2'-\varphi_2)\), thereby creating asymmetry of the positive and negative half-waves of the voltage on the thyristor. As a result of the nonsymmetrical voltage applied to the transformer, a current flows in its primary winding whose harmonic composition includes a dc component that magnetizes the core of the magnetic circuit. The inductance of the transformer, when the core is magnetized and saturated, drops sharply, and the current in the primary winding correspondingly increases to values exceeding the permissible ones, which leads to failure of the thyristor or disruption of the regulation mode. The degree of magnetization of the core of the power transformer increases with improvement of the quality indicators of the regulation system: response speed and accuracy of current stabilization in the load circuit.

Fig. 1

The analysis carried out of the operation of a closed thyristor–magnetic regulation system made it possible to formulate the basic requirements for the regulating device; if these requirements are met, the dc component of the current in the primary winding of the transformer is eliminated and stable regulation is ensured. The meaning of these requirements is as follows:

1. The phase angle of regulation \(\varphi\) of the control signal \(U_2\) must not vary in the time intervals \(T(k+1)<\tau<kT\), where \(T\) is the period of the mains voltage \((k=1,2,3\ldots n)\).

2. The change of the phase angle of regulation must be carried out by a jump at arguments \(\tau=kT\), where \(\tau\) is time \((k=1,2,3\ldots n)\), i.e., in the zones where the voltage sinusoid crosses through zero.

The conditions listed above can be fulfilled if the continuously varying voltage at the input of the phase-shifting device is converted into a stepped-form voltage whose amplitude would remain practically unchanged during a period. Such a functional transformation can be carried out with the help of

by means of a correcting memory device made in the form of a rectangular pulse element [2].

Fig. 2 shows the block diagram of a current stabilizer with a correcting device in the form of a rectangular pulse element, and voltage diagrams explaining its operation. The rectangular pulse element—RPE—consists of amplifier 2, pulse element 3, and zero-order hold 4.

The amplified mismatch signal \(U_1\) is converted, by means of pulse element 3, into a voltage \(U_2\), whose amplitude is equal to

Fig. 2.

the instantaneous value of the voltage \(U_1\) at the time \(\tau = kT\). The zero-order hold 4 stores the amplitude value of the pulse for the period \(T\) until the arrival of the next pulse \(U_2\), after which the storage process is repeated. The voltage \(U_3\) at the input of phase-shifting device 5 does not change during the period \(T\); and since the thyristor control angle \(\varphi\) is determined by this voltage, it likewise does not change during the period, thereby ensuring the symmetry of the positive and negative half-waves of the voltage \(U_\tau\) on the thyristor in any transient operating modes of the stabilized regulator.

Such a circuit in principle precludes the appearance of a dc component of the current in the winding of the power transformer and makes the control system dynamically stable and operational even with small time constants of the regulating device and a high degree of stabilization accuracy.

The experimental prototypes of single-phase valve-magnetic power supplies rated at 180 and 560 kVA, developed on the basis of the investigations carried out, successfully passed tests when operating with active and active-inductive loads in electrothermal installations of industrial frequency.

REFERENCES

1. N. Kh. Sitnik, V. S. Bogryi. Commutating and regulating devices based on symmetrical thyristors. Moscow, “Energiya,” “Elektrichestvo,” 1966, No. 5.

2. G. F. Zaitsev. Analysis of linear automatic control and regulation systems. Kiev, “Tekhnika,” 1967.