GEC CDG16 Puzzle 2

[chaseyama]

I have a problem trying to test the operation of a CDG 16 and similar relays on an teaching rack in our university. I do not register any current when I inject at pins 9 and 10. Before injection, I measure 0.8 ohm at these inputs which I thought would have been the relay impedance. On the drawing I see shorting contacts. How do they operate? Where are they? Is it possible that they have a connection with this problem. Or the relay coils are burnt? We have over 10 similar relays and they behave the same way. I have used similar relays elsewhere without problems. On the manual I also see a connection which looks like a thermocouple junction. If this part breaks, certainly no current will register. Please assist with all this.

 

[ScottyUK]

The CT shorting contacts allow the relay to be withdrawn safely from the casing with current passing through the CT primary. Without the shorting contact this would result in a dangerously high voltage on the CT secondary winding. The contacts open when the relay is fully inserted into the casing; if the relay isn't fully inserted then the contacts won't open.

Do your relays have the setting plug on the front panel? If the setting plug is missing then you won't pass current through the relay.

Pull the relay from its case and measure directly on the rear terminals of the relay. You should be able to measure the coil impedance from there. Don't touch the disc!

 

[chaseyama]

Thanks for the great response, especially about the functioning of the shorting contacts. Because the relay is built-in and wired into the training cabinet, I need to undo the wiring from the back of the panel and detach the relay and then do a direct measurement, and direct injection also. Meanwhile, I appreciate the response. I will update you after this further investigation.

 

[DTR2011]

I am not an expert in Alstom relays, but have run into them from time to time.
There is a special test plug used. Often I dont have that. In order to isolate
the CT shorting mechanism, a Zip Tie (Tie wrap, cable tie), whatever it is referred as
can be used to open up the CT shorting, if the test plug is not available.

Stick that in it (opening up the automatic shorting link), may help, without disturbing the wiring.

 

[chaseyama]

Ah, DTR2011, I will check that too. There is something which we used for current injection tests on substation relays while I was working on the Railways. It was nicknamed, "Mouth Organ". If you plugged this in, you had direct access, it seems to most of the terminals. I will go and find out about this too, in a few hours time.

 

[krisys]

Dear chaseyama,

Now the CDG 16 is totally outdated. Teaching how to test CDG 16 would not add good value to the students. The academic community should update themselves to match with the current trend.

Now the industry trend is to use microprocessor based digital relays. These are not that expensive for the institutes to buy on sample basis. May be the institutes get a special discount or free on promotional basis.

Hope I am not offending your institution!!!

 

[ScottyUK]

krisys,

I disagree - the old electro-mechanical relays are a great teaching aid. The modern relays give absolutely nothing away about how they operate because modern relays replicate the functions through mathematics. I'm not arguing that an electro-mechanical relay is a better relay than a modern numerical type, only that they are a lot easier for a student to get to grips with the concepts because they can see it in front of them.

 

[DiscoP]

Sometimes this problem is due to the injecting equipment.

Depending on the plug selected and the relay's burden, some test sets will fail to produce the current.

And I agree with ScottyUK. We train new staff with a CDG at first so they can visualise what a modern relay does.

 

[davidbeach]

Scotty is absolutely correct about the value of teaching the electromechanical relays. The connection to the electromechanical days is still there, deeply engrained in all of the fancy new electronics, why else would we talk about torque control in a numeric relay. There's no torque in a mathematical calculation, but there sure is on a rotating disk.

While I have no interest in ever installing another electromechanical protection relay, I have great admiration for the gurus that figured them out. The numeric relay requires a computational infrastructure, but once you have that any calculation can be performed. But to figure out how to make an impedance element, supervise it with a directional element, and then put three phases (or three zones) worth of that in a single case took some really outstanding engineering.

 

[ScottyUK]

David,

I have a book written by a guy named Kaufmann who was employed by Metropolitan-Vickers, which became GEC many years later. It might be an interesting read for you if you are interested in the history of our industry and the old ways of doing things.

http://www.amazon.co.uk/The-Protective-Gear-Handbook-Kaufmann/dp/B000QRELNO

 

[davidbeach]

Thanks, I'll have to check it out.

 

[chaseyama]

Thanks very much for the very useful suggestions. I did a bid of clean up have managed to push current through one of the coils. So far though, I have not gotten the disc to turn. I have not had problems before with previous GEC relays so I suspect went wrong over the years. But I agree with my colleagues about the merit of starting with the induction disc types since you can actually see the movement, for example low currents, low disc speed and the time dials are very realistic. We do have modern microprocessor types also and there is a danger that students set them like setting a clock or phone and not appreciate the real meaning of what they are doing.