Current transformers normally have a single conductor as a primary winding connected in the run of the line conductors. The voltage which appears across the primary terminals is very small and therefore the voltage at the secondary terminals is small.…
Before starting to write a switching program, it is important to understand and use standard terminology. The table below includes some common fields found on templates. Program Field Meaning Program number This is obtained from the operating authority Written…
High voltage switching is a high-risk task, and as such the use of switching programs is critical. A switching program is a written list of sequential logical operations to permit isolation and re-energisation of electrical plant and/or equipment. The prime…
Ring main units (RMU) are widely used in industry and for distribution systems. RMUs operate independently, which means that it is possible to earth an incoming supply cable while it is still energised from the other end or operate a…
The standard power transformer is “on-load tap changing” (OLTC) to ensure that the secondary distribution voltage is within limits although the primary voltage may change. Tap changer range is usually from 15% above normal to 5% below, and has up…
Voltage transformers carry full system voltage but no significant current. They have a fine wire of many turns on the primary winding (very high resistance and inductance) connected between the phase conductor and earth, and a secondary winding of only…
Earthing switches are usually installed in combination with an isolator and the two should be interlocked electrically or mechanically to ensure that the isolator is open before the earthing switch is closed, and conversely that the earthing switch is open…
Isolators are simple devices with contacts invariably comprising a moving blade going into two parallel fixed contact faces. The fixed contact faces are usually made up of a number of spring-loaded fingers. The best isolators employ a roll-over action at…
The following basic types are available, typically rated at 350 or 500 MVA at 11 or 22 kV respectively: bulk oil minimum oil air break vacuum SF6 Circuit breakers are designed to deliberately create an arc, and thereafter to control…
The main four different switches used in a power system are: Circuit breakers are designed for controlled closing and controlled or automatic opening of load and fault current. Their duties are to close onto a fault and to automatically open…