1. Field of the Invention
The present invention relates to insulated caps and more particularly to insulated caps for high-voltage loadbreak bushings.
2. Description of the Prior Art
Insulating caps for use on high-voltage (15 and 25 KV) transformers and switchgear are well known in the utility industry. Such caps are used to deadfront loadbreak bushing interfaces when energized. A typical prior art high-voltage bushing insulator cap 10 is illustrated in FIG. 1.
Prior art insulating caps generally include a top portion having an eyelet 12. The eyelet 12 is dimensioned for receiving a removal tool, such as a "hot stick" to facilitate removal of the insulating cap from the loadbreak bushing. Accordingly, an operator may remove the insulating cap from a safe working distance.
The insulating cap 10 further includes an outer shield 14 which substantially surrounds the insulating cap. The outer shield 14 is generally formed from a conductive material, such as a conductive rubber, thereby forming a conductive shield for the cap. The outer shield also includes an integral tab 16 having a hole therein for attachment of one end of a ground wire, the ground wire having its other end connected to a system ground. The inner portion of the outer shield 14 has a substantially smooth, rounded dome-like section 19 at a top portion thereof.
The insulating member 18 may be made from any suitable insulation material of high quality having a high dielectric value. The insulating member 18 forms a portion of the insulating cap receiving well 20 dimensioned to fit over a loadbreak bushing. The remainder of the bushing well is formed by a conductive insert 22.
The conductive insert 22 is generally positioned within the dome-shaped portion 19 of the insulating member. The conductive insert 22 has a corresponding smooth, dome-shaped outer surface 23 which is designed to control electrical stresses in the insulating cap. The conductive insert interior surface 24 is shaped to receive the mating bushing exterior. The conductive insert 22 extends to a point below a loadbreak bushing shield housing or "can" (FIG. 2) to shield the bushing assembly. Neither the conductive insert 22 of the insulating cap or the shield housing carries current, but merely provide electrical stress relief.
The insulating cap of the prior art further includes a high potential rod or probe 26 extending through a central axis of the cap. The probe at one extends slightly below a base of the cap and at the opposite end extends into the conductive insert. The probe mates with a current carrying contact assembly 48 (FIG. 2) in the loadbreak bushing 50 (FIG. 2). Accordingly, this high-voltage connection brings system voltage through the probe to the conductive insert, which as previously noted is smoothly shaped to control electrical stresses.
A shortcoming of presently available insulating caps for use with high-voltage loadbreak bushings is that as the insulating cap is removed, system voltage appears "outside" the bushing opening providing an opportunity for a system voltage flashover from the probe to ground. Accordingly, it would be advantageous to provide a high-voltage insulating cap which eliminates the possibility of flashover upon removal from the loadbreak bushing.