Thyristor converters with metering capacitors in a power circuit were widely used in electrical installations demanding a continuity of energy metering when a load was changed [1]. Transistor capacitor converters of half-bridge and bridge types are used usually in installations under 10-15 kilowatts in power. If a power of installation was greater [2] then thyristor capacitor converter was more preferable. Ion nitriding furnaces, arc metal spraying pistols, and such like can served as examples of such loads.
Thyristor-capacitor converters for dc loads are executed of bridge circuit and for ac loads are executed of half-bridge circuit.
Unreliability of an overload protection was the general disadvantage of aforementioned thyristor capacitor converters when a thyristor commutation was failure. A pulse-width selector [2] was part of a protection circuit of common devices that accounted for a lag post triggering of protection and operable reliability of protection was reduced.
The proposed protection system [3] is shown in figure 1 and it are comprised of the three-winding transformer. The first winding of this transformer is connected between the side and the thyristor bridge common anode. The second winding was connected between a cathode one of thyristor bridges and an anode another thyristor bridge so that the midpoint of the second winding formed one of leads of the alternating current thyristor bridge diagonal. The second winding has twice number of loops than the first winding. It has an opposite connection with the first winding. And the third winding of the mentioned transformer is connected with the turn-off pulse former; output of it is connected with the driving point of the thyristor circuit-breaker.
That makes to exclude the comparator and, above all, the pulse-width selector from circuits of type devices. That permits to provide a performance and a high reliability of the protection system. Indeed, the preventive type of the protection system was provided in the proposed device, i.e. a current was not increased practically when the thyristor bridge commutation was break off.
The vital difference of the proposed solution consists in the following. A load current flowed, for example, through the first winding 12 and the half-winding of the second winding 13, i.e. through opposite connection windings having an equal number of loops and formative a bifilar when the thyristor bridge operated normally. Thus sum of ampere-turns is vanishing. Accordingly, inductance of these windings is vanishing too. An analogous picture was occurred when the diagonal of thyristors 3 and 4 was activated. The resistance can be neglected, i.e. the proposed device operates analogous type devices on an operation mode. The current induced by the supply Un must flowed through the first winding 12 and through the whole second winding 13 of the three-winding transformer 11 when a disruption of commutation was occurred, for example, when an activations of thyristors 1, 3 were occurred synchronous. This current had not surpassed the magnetizing current until the three-winding transformer is saturating, i.e. the short circuit being closed practically. Characteristics of the three-winding transformer choose sufficiently that its saturation time would equal the interrupting time of the thyristor circuit-breaker. A disconnect signal enter to this circuit-breaker 7 through the pulse former 15 from the third winding 14 in emergency state. An analogous picture would occurred too when thyristors 2 and 4 had been connected. In this case the magnetizing current flowing through the first winding 12 of the three-winding transformer 11 only. Note that since an interrupting time of the thyristor circuit-breaker 7 is less (tens of microseconds) that three-winding transformer gabarits are less too and connection of this with the power circuit of the thyristor capacitor converter is not a disadvantage. The demagnetizing of the three-winding transformer 11 would occurred when every disconnection of thyristor circuit-breaker was occurred. I.e. the three-winding transformer 11 is used according to total hysteresis curve and the proposed protection system is in ertialess and is restricted the current to a trifling load.
References
- Magazinnik L.T. Single-phase power supplies of inverter type with capacitors in power circuits. Electromechanics. Proceedings of universities №6, 2003. p. 21-23.
- Bulatov O.G. and other. Thyristor-capacitor power supplies for electrical installations. Moscow: «Energoatomizdat», 1989.
- Magazinnik L.T., Magazinnik A.G. Thyristor-capacitor converter. Patent RU 2320070 20.03.2008.
The work was submitted to international scientific conference «Priorities for Science, Technology and Innovation», Egypt (Sharm el-Sheikh), November 20-27, 2008, came to the editorial office оn 06.03.2009.