Differential Protection For Transformer And Generator

Is this like "Where's Wally"?

That was funny, cruel but funny

In the transformer incoming voltage protection and out going protection is given and you need both side protected. In the generator only the out going to be protected for a safe operation that is already controlled by the AVR and the out going MCCB.

I know about incoming and outgoing for transformer, but where is the exact location of CT nstallation

Three CTs in secondary bushings (these are immersed in transformer oil) To approach these oil has to be drained and then further long procedure - better hire expert.

Other three CTs on Neutral side of Generator on leads before the start point. These are also mostly bushing type dry CTs (Epoxy cast). Because for testing of Alternator, the Start point for large sized generators (for example hydrogen cooled Turbo alternators) are outside the frame of Generator, in a separate box.

Depending upon manufacturer and type of Differential Protection Relay, either the interposing CT is build in the relay or separate interposing CTs are required to match vector and ratio of both end CTs.

Hope above is enough for your understanding. For any work on CTs, inspection testing, connections and calibration of Differential Protection - please hire an expert. One needs good knowledge to do this job. (a little knowledge is a dangerous thing).

The Primary CT will be located inside the switchgear and the secondary CT will be located at secondary of transformer bushing, is that what do u mean?
For generator - all 6 cts in the panel/switchgear?

What are the ratings of

1.Generator?

2. Transformer?

3. Are these connected in Unit Scheme (No breaker between Alternator and Transformer)?

4. Do you plan to add protection scheme now or it already exists?

The CTs should be located wherever you want them to be to define the limits of what is protected by the differential.

For example, on a transformer, the smallest coverage is to put them at the transformer's primary and secondary bushings, so that only the windings of the transformer are protected. The bCTs typically are internal (around the inner part of the bushing), but can be on the outside of the tank if added after manufacturing. If you want broader coverage, to include the cables or bus that connect to the bushings in your differential, then you can install free-standing CTs farther from the transformer or put them in your switchgear.

The same with your generator. You can install the CTs right at the generator's external teminals, or further away at the switchgear, depending on what you want to protect. This is a decision that should be made by a competent electrical protection engineer or similar consultant.

The transformer is indoor type transformer, I never seen any CT installed at the busing except temperature winding resistance CTs which is embedded inside the bushing for indoor type( to be installed in small substation).You are correct there is no rules to which location the CTs to be installed, but what is the most practical way?, for the busduct, the ct can be installed anywhere, but for the cable, where to terminate the CT?, the practical way inside the switchgear.

Since you have an existing transformer, it is impractical to retrofit CTs inside the unit itself. Mounting on the outside, around the bushings, is usually a pain because of space considerations, mounting provisions, etc. Therefore your best bet is at the source of the cables feeding the transformer.

You should look in the compartment where the cables / bus enter your switchgear. You will need to choose the appropriate ratio, accuracy class, and insulation level CTs. If it is LV or shielded cables, you can usually use 600V rated donut CTs. The cables need to be termporarily disconnected to slip the CTs around them.

Use fiberglass (trade name "Glastic") strut or angle to build a mounting rack or frame to support the CTs. Secondary wiring should be run from the CTs' secondary screw terminals to a shorting terminal block in the instrumentation/control compartment of the switchgear, from which you can wire to your relays.

This includes the cables / bus duct in the differential, but that is not necessarily a bad thing. It means in the event of a fault in the cables/duct, it will also trip. It gives you a larger protected area, but then you have to diagnose whether the problem is the conductors or the transformer if you get a trip.