The primary of the CT is in series with the power circuit being measured. The current in the secondary of the CT is proportional to the current in the primary. The CT secondary will generate enough voltage to cause the secondary current to flow or it will saturate trying. Prior to saturation, and even into saturation, extremely high voltages can be developed across the open secondary, often leading to flash over.
Depends on who you are explaining it to. When teaching electricians basic protective relaying I basically said:
The secondary of the CT knows one thing, "I need to produce X Amps (Usually 5A)", based on the resistance of the secondary circuit it will produce the necessary voltage to maintain that current flow. Under normal conditions the resistance is only a few ohms, so only a few volts is necessary, if you lift the leads (Open the circuit) the resistance incleases as you pull the lead away, the voltage increases, and you will draw an arc. Then I show the interlocks and shorting block/contacts for a 50/51 relay (Hands on, pass around the class).
Not 100% acurate but gets the point and basic theory across, I would use a different one for Engineers and another one for intro students.
I take a little different tack with the explanation to technicians.
"If I have a transformer with ten primary turns and ten secondary turns and I put a volt in the primary, what's the secondary voltage?"
"Now, if I have a transformer with ten primary turns and and TWENTY secondary turns and I put a volt int he primary, what's the voltage on the secondary?"
"Now, if I have ONE primary turn and 120 secondary turns, and I put 4160 volts on the primary, what do you think the secondary voltage will it TRY to produce?"
Yes, it bends the reality just a little bit, but most techs get the idea...
The CT has to produce enough voltage across the secondary winding to create a current flow in the proper ratio to the primary current. As the burden of the secondary circuit increases, the voltage required to produce the same current increases.
The higher the secondary circuit impedance, the more voltage, so if you open the secondary side, impedance goes to infinity and the voltage tries to go to infinity as well.
My instructor used a much more graphic explanation. He told us the story of the electrician who opened a CT secondary under load, and ended the story;
"The electroician said that the last thing he saw was a big orange basket ball!"
In the rest of the course he often reminded us of
"The big orange basket ball."
Bill
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"Why not the best?"
Jimmy Carter
I think I've mentioned on here once before that I was present at the time a big 18000/5 CT went O/C. We disturbed a bad crimp lug while commissioning a generator auxiliary transformer. The CT was part of the overall diff scheme on the generator isophase bus duct which is why it is such a big ratio. I vividly remember the big blue arc and the hissing noise, but I didn't remember vaulting over the guard rail of the platform about 8' up in the air to get away from it: I was told about it afterwards, but it was just pure instinct at the time.
The transformer we were commissioning was the replacement for one which blew up 11 months earlier killing three of our friends so at the slightest hint of a problem we ran like hell. Looking back it was slightly irrational but we were terrified when that CT circuit lit up. There was no big bang or any other huge drama but the terminals were in a real mess and about 3" of conductor was missing, just completely gone. It wasn't even a field crimp, it was wrong when it left the factory.
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If we learn from our mistakes I'm getting a great education!
I was, however, a bit surprised to see the heading "Control Transformer circuit". A control transformer is NOT a CT. CT is reserved for use with Current Transformers. And Current transformers are what the answers are about.
Opening the secondary of a Control Transformer is absolutely non-critcal. Please, Pup7, do not create confusion bu telling your pupils that they are not supposed to open the secondary of a Control Transformer - and DO NOT tell them to keep the secondary of it shorted. That is not good at all.
Yes, Gunnar beat me to it (as usual).
A "Control Transformer" is better described as a Control Power Transformer abbreviated CPT so as to avoid this very confusion. The term "CT" is reserved for Current Transformer, and THAT is the one you should never open the circuit on. No problem opening the circuit on a CPT!
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