Protection of HV Air Core Dry Type Reactors

I thought these were in fact a protection device...you say for voltage control, but you don't say what particular aspect of voltage control....

https://www.gegridsolutions.com/AlstomEnergy/grid/Global/Grid/Resources/Documents/Air%20core%20reactors%20Brochure%20GB-epslanguage=en-GB.pdf

The reactor will be connected to the line through a breaker (or circuit switcher) and shunted to ground at the station. The reactor will be switched in and out probably once a day based on the voltage on the line; such that when the line is lightly loaded the reactor will be switched in and when the line is more heavily loaded switched out.

Other type of reactors are used to limit currents (switching or fault currents) in a few applications. The link you provided shows current limiting and also shunt type reactors.

Thanks.

I've said this many times before, and I'll say it again.

You said, "It seems that they are not very common in the industry as it has been difficult to get feedback."

So, instead of going to the manufacturer, seeking out a recognized expert or at least consulting local authorities, you choose to go to an anonymous forum of total strangers about whom you know nothing and"consult" with a total stranger who may not know as much as you do, which is almost nothing.

Maybe they are not very common because they are inappropriate for that application?

Or, perhaps cost drove your decision.

In any case taking the word of an un-credentialed, total stranger, who may well be qualified to help, leaves you liable for any lapses in judgment you make.

Keep this picture in mind as you proceed.

Are you sure this is safe?

Hi Lyn,

Well I have done many of the things you say and as I said there has been little feedback using the contacts I have available. So I was hoping to reach someone that has experienced this kind of installation in this broader forum to share their experience.

I don't think I would follow blindly any suggestion I get. The responsibility of taking a decision of what to do is mine as part of my job.

So, I don't see anything wrong in looking for information from a forum. I think that this is actually one of the main purposes of having a forum. There may be more specialized forum for what I am asking but I haven't found them.

I'll keep your warning in mind.

Good luck!

It is most unlikely that this sort of thing would be a D_I_Y, home brew, piece of equipment. So the first course of action, always, is to seek advice from its manufacturer by approaching it, in the first instance, by telephone. The manufacturer will have Sales Engineers who have the right sort of advice if not written on the inside of their eyelids then very nearly so.

Seeking advice from random strangers over the internet carries inherent risks and liabilities that are well worth minimising, and the only true way to do this is to take the advice of the original equipment manufacturer. Doing otherwise is somewhere in the middle of abstruse/foolhardy/criminal/irresponsible; exactly where is unknown and of no particular interest.

<...We...> So, what do the design drawings and calculations say? Surely someone has done a "sanity check" on them? What does the most senior Electrical Engineer say? To what has the Project Manager committed, and why has a commitment not been made before now?

If I went to my Electrical Power Engineering mentors with these questions, I would be asked to answer the following prior to getting any assistance from them:

1) Why do you think that a dry reactor requires different protection than an oil-filled unit?

2) What are the consequences of not providing separate protection?

3) Is auto-reclose normally allowed onto a faulted reactor; if so, why?

4) From a system perspective, what are the differences between the two types, and what role does commonality play?

When you use your favorite search engine to answer them, you will be better equipped to discuss the matter with the person(s) who actually specified the devices.

NB- You would also do well to follow Lyn's advice and seek the services of an Electrical Power/Protection Engineer who has actually done this type of work. They are few and far between and like to be compensated for their work. You can use the PM system to contact one.

The replacement of dry type transformers and reactors in place of oil immerersed transformers and reactors are very common now . This is required to minimise the maintenance cost .Most of the transformer manufacturers are currently manufacturing dry type transformers. There is no special additional protection used, other than the inbuilt temperature sensers which are provided by the manufacturer and are used to switch off (or isolate by the operation of breaker) the equipment on over heating. The preventive maintenance is only periodical cleaning of dust . It is better to install them in dust and moisture protected location. You can always get further details from the manufacturers and their websites.

Knowing little about power electrical engineering, I still have learned a lot from your posts. I have not seen you post for some time, but it is good to know you are still around!

Hello,

Thanks for the general advice about talking to manufacturers and senior engineers here at my workplace. I would like to provide more information to explain better the application and the options I am looking at (probably should have done it form the start but didn't want to make it too long to read and discourage responses).

Again what I am trying to determine is cost effective protection for a shunted dry type air core reactor that will be connected to a 230kV Transmission Line through a circuit breaker or a circuit switcher (used for switching and not for fault clearing). This 30MVAR reactor will probably be switched daily.

The main reason we are going dry type reactor, as opposed to oil immersed, is that there is not oil involved, which means no risk of catastrophic explosion (safer), or risk of environmental damage and lower maintenance costs.

I have read papers and guides and also get advice from reactor manufacturers; from which I extracted the following considerations:

• Phase to phase faults may not be a concern because the distances between phases are very large; thus the usual current differential protection used in oil filled reactors to detect internal phase faults may not be necessary;
• Ground faults may not be a concern either because of the amount of insulation used to ground; so a ground differential or REF (Restricted Earth Fault) protection may not be necessary;
• Turn to turn faults may be developed as a result of switching over-voltages which are caused be re-ignition within interrupting device (breaker or circuit switcher) at de-energizing. This overvoltage would stress the windings insulation that is closer to the interrupting device causing degradation of insulation to the point of generating an arc between turns. If not detected on time the arc may extend to the whole reactor phase causing a phase to ground fault. Thus detection of turn to turn faults may be necessary; however it seems that reliable detection of this type of fault is very difficult. The SEL paper recommends:
• Directional ground overcurrent (67G) using a neutral CT for the overcurrent, and phase CTs and VTs for zero sequence impedance directionality; with addition of delays and some inrush suppression logic.
• Other option would be a distance element.
• One reactor manufacturer recommends only a phase overcurrent relay that will clear the fault only when the turn to turn fault already developed into a ground fault.

Another consideration regarding turn to turn fault detection is that once the fault occurs and is cleared, the phase reactor will be most certainly damaged and will need replacement. Thus early detection of a turn to turn fault may not provide a major advantage; also consider that there is no risk of explosion as there is no oil involved. However at the 230kV level two stacked reactor sections will be required per phase; thus if the turn to turn fault is detected on time, damage to the 2nd section may be avoided (this is not clear).

Thus considering the above, another possibility arises which would be not to provide dedicated protection for the reactor but use the line protection relays (POTT and maybe a time overcurrent). However, this line will have auto-reclosing and thus if no dedicated protection is provided for the reactor, then upon a fault in the reactor the Line protection will reclose into the faulted reactor. This may not be a great idea, however this option will be the most cost effective as no stand-alone CTs will be required.

I am thinking that providing directional ground overcurrent or at least phase overcurrent is the way to go. Any advice?

Thanks for your comments.

You've answered your own question, "...early detection of a turn to turn fault may not provide a major advantage..." Having directly witnessed the damages caused to transformers, breakers, generators, etc. by this type of fault, it is apparent that it is impossible to re-energize an already failed piece of equipment.

Similarly, reclosing on to a fault "...may not be a great idea..." is correct, but the reason given for allowing it is apparently based on being "...cost effective..." is not.

Doing so totally ignores the underlying real purpose of protective relaying; i.e., to remove faulted equipment from service so as to maintain the integrity and stability of the power system to ensure continuity of service to the customers. There are many other considerations besides initial cost, start evaluating those first, then the answer will be obvious.