Different Conditions in Which a Surge Divertor is Used ?

It was a surge diverter in a transformer feeder relay panel. I was told that it was used to supress the surges caused to transformer in-rush current. Are there any other specific reasons (like T/F in rush current ) to employ surge arrestor?

I'm with you redfred, I'd like to see the print too, but not until the 4th. Happy New Year!

Dear Sir,

Yes, it is installed in secondary CT terminal to protect relay against overvoltage due to CT circuit open or CT connection loose. Remember CT circuit shall be in close cirrcuit.

When the CT circuit open, CT see the very high impedance, then the excitation current try to develop voltage accross the open terminal.

Regards,

Well you've added more information, but still not enough information to be helpful to us or yourself. I'm not certain what a CT terminal is (Center Tap of a transformer, Cathode Tube terminal of an old computer network, Celsius Temperature probe terminal) or what kind of relay (network relay, form C relay, time delay relay, multiphase contactor relay) this thing has. I also do not understand how these mystery parts are connected to each other; in series, parallel, contact only, or just proximity. I do realise that English is not your primary language. You are not leaving these details out to tease us. This is why I had hoped you could scan or fabricate a diagram for us to see. A picture is worth a thousand words.

But only once I understand what you have, can I try to explain what you have.

Dear Redfred,

In the previous relay circuit, the SD have not implemented yet. They provided shorting switch between terminal Y1-Y2. It will be shorted before technician opening terminal T1 or T2. See simplified diagram of SD installation. This SD protect equipment and personal by diverting high voltage which may occured when terminal T1 or T2 open.

Regards,

Ok, now we're getting somewhere.

It appears to me that CT is a Current Transformer that is converting the possibly neutral but clearly center tap current of a split phase transformer. The output voltage of this current transformer is proportional to the current on this neutral leg of the power transformer. This current will be at a maximum when the load on the split phases are significantly imbalanced and likely during brief turn "ON" and "OFF" cycles of the user loads. Ignoring for the moment the surge diverter, when this voltage reaches the threshold voltage of your relay, the relay turns "ON". Presumably the contacts of the relay either just send the "load imbalance" signal somewhere for monitoring, but might reduce the current in the primary of the power transformer until the imbalance gets corrected. Now onto the surge diverter, this part will likely be a very non-linear device like a trans-orb that clamps the current transformer output voltage to a safe voltage for the relay but still above the relay threshold. Now depending on a variety of factors, the surge diverter may not be necessary for the peak voltage capable of the current transformer maybe always a safe voltage for the relay. Some but not all of the factors can be the saturation of the current transformer, current limitation of the power transformer and/or feeder power, different relay used.

But this is just my analysis of your schematic.

I hope this helped.

Dear Redfred,

Your explanation are correct for the CT circuit is in closed loop condition and the relay burden meet the specification. Old system protection never used the surge diverter, even the some new systems also disobey this SD, but some others aply it. I just to remind "Do not open the CT circuit, i.e. disconnecting T1 without shorting the CT terminal, while the current is flowing in primary circuit".

The simplified diagram is represent three line diagram, phase protection are indicated by 50/51 (ANSI Device No.), and Ground protection indicated by 50/51N. So, during normal condition current flow on each phase, while neutral current is depend on the unbalance condition or fault to ground in the system.

Sorry I cannot explain you better due to my English,

Regards,

Hi !! Belated happy ne year ... my computer got affected by virus ..so couldn't reply earlier... CT is current transformer here ..the one CT is across the 3 phase transformer terminals while other is Neutral Current Transformer (NCT)...in case of surges of order of 3KV due to transformer in rush current , the surge diverter helps to suppress the surges by offering a low resistance path at high voltages ...and the relay gets actuated at 0.2 % of CT secondary current through stablising resistor.. ..Thanks a lot for all the help !!!

"in case of surge of order of 3kV due to transformer in rush current, ..........and so on"

I do not understand the transformer inrush current may establish the 3 kV voltage in current transformer. As described above the high voltage in CT terminal will occur when the CT terminal open while the primary CT is flowing normal current. The transformer inrush current is approximately 6 to 12 times normal current, while the CT is designed to be able to draw current 20 times normal current with 5 or 10% error.

To understand the high voltage establish in secondary when CT circuit openned is as follows:

Initially, consider the burden impedance connected to the CT secondary to be adjusted low burden enough to allow the CT to operate on the linier portion of its saturation curve (no attachment of CT saturation curve). Increasing the impedance of burden with the CT load held constan, the secondary CT voltage requirements will increase untill the knee of saturation is reached. Further increases in the burden impedance would result in a reduction of CT secondary current and an increase of the CT excitation current. When the circuit open, means the burden become infinity, CT secondary current approaches zero. All of the CT primary current reflected to secondary is used as transformer excitation current. This means all current reflected from the primary to secondary absorbed by impedance of CT.

The CT secondary voltage can reach dangerously high levels of several hundred to several thousand volts. These voltages present serious personnel hazard as well as the possibility of overstressing CT secondary insulation and the insulation of connected burdens and interconnecting wires.

Therefore, a CT secondary never be operated with its secondary circuit open. A surge diverter install at CT secondary terminal to prevent high voltage establish due to human error or unskill personal to open the CT circuit.

the 3 KV may be developed across the secondary terminals due to internal faults and the surge divertor is used to limit the peak voltage .