Line Reactors

[mull982]

Can someone explain to me scientifically how a line reactor is used to limit current. We have several line reactors (480V) and Current limiting reactors (CLR's) here at an industrial facility and was wondering how they operated and what they are used for. I believe they are used to limit current but was not sure how this worked. I was told this was done by introducing reactance into the circuit, but was a little fuzzy on how this worked and was hoping to get an explanation. Does the reactor limit steady state current, or only transient current since it is basically an inductor?

I would appreciate it if someone could give me a scientific explanation or point me to some reference material online.

Thanks

Mull982

 

[kingpb]

In short, you can think of it as a transformer with a 1:1 voltage ratio. They are used to limit fault current by, as you said, adding reactance to the line. This is becoming more common as system available short circuit current increase, without having to increase the bus fault current capability.

Using the transformer analogy, the impedance in a transformer is almost all reactance, this is why a fault current through a transformer is limited to approximately the rated current divided by the impedance. So in some respects it acts as a fault current choke.

So, take the reactor, it does the same thing but on a smaller scale. Smaller scale because you only introduce enough reactance so that the fault current is reduce to a desired level. Unfortunately a by-product of this can be a voltage regulation issue, whereby during say a large motor start, the voltage may be momentarily dragged down to an unacceptable level. e.g. below 85% terminal voltage, and unlike a transformer, there are no taps to adjust the low side voltage.

You can also think of a cable. Given the same amount of fault current for two cables, one being say a 500kcmil, and the other being a #10AWG, that the impedance of the cable higher for a smaller cable, which means that less fault current can flow due to the higher impedance.

Purposely introducing impedance, or reactance in this case, will then effectively reduce the amount of fault current that can flow. But, again, the smaller the wire (higher impedance) the more voltage drop there is, thus the same as with the reactor being added.

Hope that helps.

Refer to "Haefely Trench" as a manufacturer for additional info.

 

[mull982]

kingpb

Thanks for the explanation, its starting to become a little clearer. You mentioned that we purposley introduce reactance into the line, but I was curious why we introduce reactacne into the line rather than resistance as a source of impedance. Does the reactance impede the current differently than a resistance or Resistor would.

Is the reactor impeding current all of the time or does it only respond to a change in current, in this case a fault current. I believe that the reactor is basically an inductor, which I understand respond to a change in current. Is there a mathmatical way of looking at how this added reactance limits the current?

I guess basically what I am trying to understand is what it is about the reactive properties of the inductor that limit the current as opposed to resistive ones?

Thanks for the help.

 

[rcwilson]

Fault Current Reducing Reactors

Fault Current Reducing Reactors

A 1-ohm resistor or a 1-ohm reactor or 1 ohm of extra cable will all reduce fault current in a circuit the same amount. They all have the same impedance. The difference is the resistor will cause watt losses during load current but the reactor only creates var "losses".

A reactor is a large coil of heavy gauge wire wound to create a magnetic field that opposes the flow of current. Sometimes the coil is wound on a core like a transformer, other times it is an air-core reactor. The coil wire is sized to handle full load current with minimum losses, just like a transformer winding. It has a low R and a high X.

Remember, Z= R + jX. Impedance is composed of resistance and reactance. Current flowing through an impedance creates a voltage drop (V = IZ), watt losses (watts = I x I x R) and vars ( vars = I x I x X). In a current limiting reactor we want to keep the watt losses as low as possible (low R) and use the Var "losses" to restrict current flow. (Vars aren't really lost. They just move from one place to another every cycle, but that's another story.)

The impedance "Z" of the reactor is mostly the coil inductance with a small resistance so under normal load current the reactor has minimal losses. When a fault occurs, the magnetic field of the reactor coil restricts the current flow.

They are usually applied when existing switchgear is being expanded or other changes increase the available fault current. Adding a reactor can allow system growth without stressing the equipment’s interrupting rating.


The magnetic field of the reactor coil can cause problems by inducing currents in adjacent structures, ground wire loops and other conductive objects.

 

[mull982]

The impedance "Z" of the reactor is mostly the coil inductance with a small resistance so under normal load current the reactor has minimal losses. When a fault occurs, the magnetic field of the reactor coil restricts the current flow.

Assuming normal load current is flowing through the reactor (200A), what would happen at the reactor if a fault occured downstream of the reactor? When the fault occurs downstream of the reactor, and tries to pull additional current through the reactor, does the reactor block this additional current due to its magnetic field? Does it actually block the current, or does it just delay it (due to phase shift and reactance)? With reactance, isnt the current delayed with the reactance taking current/power out of the circuit then releasing it back into the circuit?

As far as limiting avaliable fault current does it just add impedance into the circuit thus dimishing the avaliable fault current I=V/R (bigger R)? Does it allow a breaker upstream to clear the fault, or just limit the fault current onto the secondary bus off the reactor?


Thanks for the help this is becoming much clearer.

Mull982

 

[kingpb]

Because of the inductor characteristics, it does not allow an instantaneous change in current, thereby reducing the fault current through it that would normally be available without it. So, in essence both things happpen, lower fault current and protective devices operation will clear the ensuing fault condition.

 

[Jraef]

Search for information on the "Inductive Time Constant", it may help to explain the principal behind it all.