It is offered to use an electronic voltage stabilizer on change expensive and bulky ferroresonance voltage stabilizers of small capacity and the voltage stabilizers of the big capacity executed on the basis of the autotransformer.
Nowadays a phase i.e. smooth regulation inside steps by means of impulse-phase controlled thyristors is replacing expensive and bulky ferroresonance voltage stabilizers of small (up to 1 kWA) capacity based on an autotransformer with step regulation of voltage adding welded points employing a semiconductor commutator. However, voltage distortions in the net are inevitable in the process and it is not always ac- ceptable.
An electronic voltage stabilizer with a voltage adding transformer, filter condensers connected in parallel with loading and a regulating key with a control system is offered [1].
The scheme of the device offered is shown in Figure.
An electronic stabilizer with minimal voltage distortion of a power line
The device functions as follows. If automatic switch 1 is on and the voltage below nominal is applied to a stabilizer, the signal equal Uc of power unit 13 is also below the operation level of semiconductor relay 14, accordingly relay 8 is off and contacts 5, 6 are closed. In this case transformer 2 is used as voltage adding, i.e. the voltage of transformer secondary winding 11 is added to the voltage of power line Uc. Let Uc have plus on the top terminal of loading 12 at the considered moment of time. We name this half-cycle positive. In this case a network voltage «positive» signal and a current zero signal in transformer 2 primary winding 3 enter one output of logic circuit. At the same time transistor is unlocked and winding 3 is short-circuited via voltage gauge, therefore the whole voltage of network Uc is applied to the loading .The positive signal of voltage and zero signal of current form a logic unit at the control unit of key 15 and the very first pulse from a pulse-width modulator comes to the base of transistor 9 unlocking it. The voltage at transistor 9 falls practically up to zero, transistor is locked and the voltage of network Uc is then applied to primary winding 3 of transformer 2.
At secondary winding 11 of this transformer there develops the voltage which adds to the network voltage. The resulting voltage Un average by condenser not shown for simplicity by a module extruder also having a condenser at the output, is compared to the reference voltage Us supplied by power unit 13.
If Δ = Us - Un > 0, i.e. the network voltage is i.e. with pulse width modulation below the nominal one, the pulses from unit 17, extend, increasing the average by voltage adding, otherwise pulses of pulse width modulation are compressed, i.e. the signal from comparator determines the porosity of pulse width modulation. During a negative half-cycle the scheme functions similarly with the described one, the only difference is that key 16 is unlocked and winding 3 is periodically short-circuited by transistor. As logic circuit carries out the elementary function of summation and is realized on typical triggers, elements AND, OR, for simplicity it is designated only structurally. We´ll notice that positive and negative working cycles can be displaced depending on the character of loading 12: at inductive loading the transition point of a current sinusoid through zero is displaced to the right and, accordingly, the unlocking moment of logic keys 15 and 16 is displaced as well. Condenser serves for compensation of input inductance.
If the network voltage exceeds the rating value, relays 14 and 8 will be on. Contacts 5 and 6 will open and contacts 4, 7 will close. Winding 11 of transformer 2 becomes voltage adding. Thus, the stabilizer carries out its function at fluctuations at network voltage both downwards and upwards from the nominal value. As the modulation frequency can be high enough, e.g. 1-12 kc, voltage pulses filtration of voltage adding does not represent a problem and is carried out by means of condenser of a small capacity and also due to dispersion inductance of transformer 2 designed for 50-60 cycles, i.e. is a typical stepping down transformer. However, it is necessary to note that in this case the transformer has alternatively two modes: the mode of an autotransformer when it is connected for a network and the mode of a current transformer when primary winding 3 is short-circuited by transistor. In the first mode the current in transistors and in a primary winding is equal to the magnetization current which is an additional advantage of the scheme and high frequency of modulation allows us to get rid of distortions. Semiconductor relay 14 is necessary to provide the high factor of return, i.e., the necessary accuracy of operation of relay 8.
Thus, the device offered provides high frequency of modulations with filtration of impulses of voltage adding by means of a condenser of small capacity and also due to dispersion inductance of a transformer. It allows us to exclude voltage distortion in a network.
References
1. The patent № 2123717 Russian Federation. An electronic voltage stabilizer / L.T. Magazinnik, J.J. Magazinnik // Bul.№№5,20.12.1998.