Thyristor Push-Pull AC Voltage Stabilizer
Abstract Generated abstract
This paper examines a push-pull thyristor AC voltage stabilizer using an autotransformer tap-commutation scheme intended to reduce the required rated power of the controlled valves. The authors derive relations for transformation ratios, commutated currents, output voltage, and permissible thyristor voltages, including the case with an added series resistance that permits stabilization when the input voltage exceeds the output voltage. Experimental tests on a model average-value stabilizer showed agreement with theoretical voltage results within 5 percent and current results within 20 percent. The analysis indicates that, for the proposed commutation method, the thyristor rated power need not exceed about 20 to 30 percent of the stabilizer output power.
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THYRISTOR PUSH-PULL AC VOLTAGE STABILIZER
L. A. Volynskaya, V. M. Razin
(Presented by the scientific seminar of the Department of VT, TPI)
Compensating AC voltage stabilizers consist of three main elements: a measuring element, a control element, and a regulating element. As regulating elements, two antiparallel-connected thyristors are widely used, forming a high-speed switch with controlled impedance, by means of which it is possible to commutate a single-phase AC circuit and to change the average values of the current in the circuit and of the voltage across the load. The key operating mode of the regulating element is the most economical; however, it leads to a change in the waveform of the output voltage. To improve the waveform of the output voltage, circuits with push-pull commutation are used, whose output voltage has a stepped-sinusoidal form [1]. A general drawback inherent in most thyristor stabilizers is that the rated power of the controlled valves of the regulating elements must be greater than half the power of the push-pull stabilizer [2].
The present work is devoted to questions of developing autotransformer push-pull AC voltage stabilizers in which the total rated power of the thyristors is 2–2.5 times less than the output power of the stabilizer. The block diagram of such a stabilizer is shown in Fig. 1, where 1 is the autotransformer; 2 is the thyristor regulating element controlled by the feedback circuit; 3 is the thyristor actuating element controlled by the synchronization circuit; 4 is the feedback circuit; 5 is the synchronization circuit; 6 is the additional resistance.
If the additional resistance is equal to zero, then, neglecting the no-load current, when commutating tap \(d\) of the autotransformer it may be assumed [3] that, for a step-up autotransformer, the following relations between the powers, currents, voltages, and numbers of turns of the winding sections are valid:
\[ U_1 I_1 = U_2 I_2; \qquad \frac{U_2}{U_1} = \frac{W_1 + W_2 + W_3}{W_2 + W_3} = K_1; \qquad \frac{I_2}{I_1} = \frac{1}{K_1}. \tag{1} \]
When commutating tap \(c\), the following relations hold:
\[ \frac{U_2}{U_1} = \frac{W_1 + W_2}{W_2} = K_2; \qquad \frac{I_2}{I_1} = \frac{1}{K_2}. \tag{2} \]
In the first case, the current flowing through the common part of the winding with number of turns \(W_1 + W_2\) will be equal to
\[ I_{01}=I_2(K_1-1)=I_2\frac{W_1}{W_2+W_3}. \tag{3} \]
In the second case, the current flowing through the common part of the winding is equal to
\[ I_{02}=I_2(K_2-1)=I_2\frac{W_1}{W_2}. \tag{4} \]
Thus, if the transformation ratios \(K_1\) and \(K_2\) are less than 2, then the currents flowing through the common part of the autotransformer winding will be less than the current flowing through the load.
Fig. 1. Block diagram of a push-pull thyristor stabilizer
Let us consider the operating principle of the stabilizer with push-pull commutation for the case in which resistance 6 is equal to zero. At the beginning of each half-period of the line voltage, the synchronizing circuit 5 switches on regulating element 3, which commutates tap \(d\) of the autotransformer. In this case the input voltage is transformed into the secondary circuit with transformation ratio \(K_1\). At the angle \(\alpha\), whose value is set by feedback circuit 4 with an average-value measuring element, regulating element 2 is switched on, commutating tap \(c\) of the autotransformer and ensuring an abrupt increase in the instantaneous value of the output voltage by a factor of \(K_2/K_1\). At the moment regulating element 2 is opened, regulating element 3 is locked by the reverse voltage arising in the section of the autotransformer winding.
If \(K_1 = 1.05\) and \(K_2 = 1.4\) are chosen, then, according to expressions (3) and (4), when the input voltage changes by a factor of 1.33 and the average value of the output voltage is stabilized, the current in the circuit commutated by element 3 will not exceed 5% of the load current, and in the circuit commutated by element 2, 40% of the load current. Direct connection of autotransformer tap \(b\) to the AC voltage source can take place only if \(U_1\) is less than the stabilizer output voltage. Connecting tap \(b\) to the AC voltage source through resistance \(R_1\) allows
carry out stabilization of the output voltage also in the case when \(U_1\) is greater than \(U_2\). With an active load \(R_2\), the relation between the output and input voltage of the stabilizer can be described by the following expression:
\[ U_{\mathrm{cp}2}=\frac{U_1}{2}\left[(S_2-S_1)\cos\alpha+S_1+S_2\right], \tag{5} \]
where
\[ S_1=\frac{K_1R_2}{K_1^2R_1+R_2},\qquad S_2=\frac{K_2R_2}{K_2^2R_1+R_2}. \]
The current through regulating element 3 will be maximal at the greatest input voltage
\[ I_{01}=I_2(S_1-1). \tag{6} \]
The current through the regulating element will be maximal at the smallest input voltage
\[ I_{02}=I_2(S_2-1). \tag{7} \]
With respect to the permissible voltage, the thyristors of regulating element 3 must be selected from the condition:
\[ U_{\mathrm{dop}1}>U_{1\max}\cdot S_2\frac{W_3}{W_1+W_2}, \tag{8} \]
and the thyristors of regulating element 2—from the condition:
\[ U_{\mathrm{dop}2}>U_{1\max}\cdot S_1\frac{W_3}{W_1+W_2+W_3}, \tag{9} \]
where
\[ U_{1\max} \]
is the maximum amplitude value of the input voltage.
Fig. 2. Dependences of the commutated currents on the magnitude of the input voltage
Experimental verification was carried out on a model of a thyristor push-pull autotransformer average-value AC voltage stabilizer, in which sealed contacts MKV-1 were used as regulating elements 2 and 3.
The autotransformer winding consisted of three sections with the number of turns \(W_1 = 65\), \(W_2 = 185\), \(W_3 = 500\). The load resistance was \(R_2 = 100\ \Omega\), and the resistance \(R_1 = 10\ \Omega\). As a result of experimental verification it was established that, when stabilizing the average value of the output voltage at a level of 20 V and with an output-voltage variation ratio of 1.25, the discrepancies between the experimental and theoretical results did not exceed 5.0% for voltage and 20% for currents. The dependences of the currents \(I_{01}\) and \(I_{02}\) on the magnitude of the input voltage at \(U_{\mathrm{av}\,2} = 20\) V are shown in Fig. 2. The output-voltage curve at \(U_1 = 18.3\) V is shown in Fig. 3.
Fig. 3. Output-voltage curve of the stabilizer
The analysis performed showed that, for the considered method of commutating the autotransformer taps, the rated power of the thyristors of the regulating elements should not exceed 20–30% of the output power of the stabilizer.
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