The shaft voltage of rotating generator is produced due to magnetic unbalance. The value of the shaft voltage will be changed with the voltage and load of the generator. If the shaft voltage exceeds the destruction value of oil film, the shaft current will increase rapidly and thereby cause severe damage to the bearing within a very short time. The current may exceed 100A. Actually, even the small current about several amperes will greatly damage the bearings.

To prevent the shaft current, usually insulation strength will be adopted at the bearing end as a measure, but it can’t solve the problem substantially. In case the bearing insulation is broken, serious consequences may happen. It will cause burnout of bearing bush, deterioration of oil quality and enlarging the vibration of bearing. A new type of AECM-III shaft current protector is developed, which is one kind of perfect solution. Because the damage degree of bearing bush of the generator is determined by the amplitude of shaft current and the effective time, it is very important to find and monitor the change trend in early time. First, split shaft current mutual inductor is used to detect the value variation of the shaft current, and send the detected current value to the shaft current protection device. The detection range for the shaft current mutual inductor is 0.25-5A and it will be sent out by 0.5~10mA AC current through working winding. The AECM-III shaft current protector can strictly monitor the shaft current value. When it exceeds the designed value, it can send out the signal correctly and realize protection. To satisfy the need of automation and monitoring in power plant, this device can show the value of shaft current directly or by external instruments. The current ring output (4-20mA or 0-5V) interfaces are set in this device for selection.