Ask the expert: comparing ratings between oil filled and air core shunt reactors

Author: Jim McIver

01/01/2009 - Volume V - Issue I

Power transformer specifiers commonly ask, "I am looking for a way to rate reactors. Is there a difference on an oil filled reactor compared to an air cooled reactor?"

 

There are a lot of differences between these two types, including dielectric safety factors, levels of saturation and stray flux/heating during overvoltages, etc. An oil-filled reactor is typically an iron-core reactor and will have thermal performance similar to that of an oil-filled transformer. However, in the case of reactors, the "overload" occurs due to system over-voltage (since MVAr is not subject ot variable "loading" in the same way as MW of a transformer).

 

Thus, "thermal current" of a reactor rises in proportion to system voltage, and the oil-filled reactor will have short-term over-current capability similar to a transformer (i.e. 125% of rated voltage for four to six hours, before oil and windings begin to be thermally damaged.) This is usually addressed in iron-core reactor specification through designation of "maximum reactor linearity" (i.e. reactor expected to function as designed, only up to 115% of system nominal voltage.) Above this voltage, the iron-core reactor saturates and no "thermal overcurrent" can occur.

 

In the case of an air-core reactor, there is no inherent overload capability, and the air-core also does not have a saturation point which limits "maximum reactor linearity." So in theory, a system overvoltage could thermally overload an air-core reactor (because reactor current rises in proportion to system overvoltage.) However, I do not believe such thermal overload is normally encountered as a practical limit. This is due to other limiting factors for the air-core application.

 

If system overvoltage rises significantly, the air-core reactor's stray magnetic field also increases proportionally. In practive, this will create over-heating of fencing, rebar, and metal supports long before the reactor's conductors begin to overheat.

As you can see, a simple and direct comparison between the two types of reactors is difficult to do, since their performance varies so tremendously.