Mains Signalling

The operating principle of the proposed modulator is shown in Fig. 1. Two thyristors, Thi and Th2, are connected in parallel across a variable inductor. Thi and Thz are controlled via optical fibres connected to the control centre. Normally, the thyristors are open, so that the total alternating supply current passes through them and the inductor. In this state, there is no modulation, since the modulation voltage, UQ, is nought. This changes when one of the thyristors is closed, since then Us - the transmitter is sen- ding a signal into the mains network. In the simplest case, the receiver detects UQ, and switches on a certain load, or group of loads. The second thyristor is included to provide a complementary- phase signal UQ that can be used for switching off the load(s). The main function of the variable inductance is to ensure that the amplitude of UQ is virtu- ally independent of the load current. To achieve this, the control centre sets the required inductance with the ai...