In a power system, information of equipment, such as location and state information equipment, needs to be transmitted by a switch or digital signal, especially between primary equipment, between secondary equipment, or between primary equipment and secondary equipment in a power plant and a transformer substation. Generally, this may be achieved by using a dry contact (i.e., a contact with no initial voltage applied thereto) at a signal terminal to switch a signal, which remains open when no signal input exists and becomes closed when an input signal comes in. Meanwhile, a switch signal acquisition circuit is arranged at a receiving terminal to form a loop between DC+, the dry contact, the switch signal acquisition circuit, and DC−. When there is no signal in this system, the contact is open; in this way, there is no current in the acquisition circuit. On the other hand, when there are signals, the contact is closed and the acquisition circuit is ON in order to output the correct signals. Nonetheless, the existence of distributed capacitance, AC interfusion, and/or control cable crosstalk may lead to wrong signal output in the transmission process of the switch signals due to interference. The general solutions for this problem are as follows:
1) Debouncing is added in the switch signal acquisition circuit to avoid the period of interference.
2) The drive current of the switch signal acquisition circuit is increased to inhibit interference.
However, both of these approaches have limitations. Thus, the interference problem cannot be fundamentally solved. For example, AC interfusion could not be completely solved.