Zero-Sequence CT and regular CT 3

[a10jp]

Does anyone know how to tell whether a currnet transformer is for zero sequence sensing and just a normal CT? For example, I am looking at the product page for one mfr. The product page did not mention anything related to zero sequence, so how do I know if it is for the particular application i am looking for or not, even though it is lised under ground sensing product category. I observed that the few product data sheets where it specifically says for ground fault, the ratio rnage is like 50:0.025A, which is very sensitive. My app is looking for 50:5 ZCT, do they make that?

 

[oldfieldguy]

Several manufacturers make 50:5 CT's. The CT doesn't care if it's a single phase or zero sequence. YOU need to make sure that the window of the CT is large enough to accomodate all the cables (at least three) that go through the window.

The 50-0.025 CT's must be interfaced with protective devices designed to accomodate that ratio.

old field guy

 

[slavag]

Hi.
What OFG saied.
No problem with this ratio of CT ring type.
If you are located in Europe, ABB Poland have a serious range of such CT's. Very important point is only size of window.

Good Luck.
Slava

 

[PHovnanian]

By "normal CT" you do mean protection class CT? Make sure you're looking at the correct type, as a 50:5 ratio could very well be metering class (no good for protection).

 

[a10jp]

Hi all, what have a situation in the field where the relay 750 requires an input (G10/H10) for gnd current input. Those inputs according to the original dwg should be the output of a 50:5 ZCT. My understanding is that a ZCT is different than a regular CT in that the transfomrer is wound differently, such that the ZCT only sees a ground fault currnet and not the normal "imbalance" current going through. Once the ZCT picks up the fault current, the CT must sense the direction if it is a correct GF current, or in a reverse direction, whereby other relay protection can take action. Reading through the MFR cutsheets, under the ground sensing category, osme says specifically for GF applications, or zero sequence, while others stated none. My questions, if the 2 typs of CT are different, shouldn't it be stated clearly it is for a certain type? Perhaps my understanding is wrong.

 

[racobb]

I think your understanding is wrong. A CT just measures current and it is up to you to determine what the CT is measuring and kind of relay you attach it to and how you set the relay. Direction is a specific type of relay, not a CT function or CT design/connection.

Is it residual phase input or separate CT input. The "ZCT" or residual will always see the imbalance as well as the phase to ground fault current, but this is also imbalance. Generally when I have a separate neutral CT I use a dedicated relay for the input. Some relays have what is called a "sensitive earth" input. Dedpends on the application. In either case, you must decide what you have and how to set the relay.

Sounds like you might need some help?

Hope this post helps!

Alan

 

[a10jp]

Hi RACOBB, what you said makes sense. I think I am confused with the term ZCT as being a different type. But since all GF current are also considered imbalances, together with other input such as polarizing voltages, and time delay should be used together as trip setting. I am attaching a sketch of the connection diagram, and based on discussion above, the 50:5 ZCT is just another regular CT?

 

[racobb]

Looks as though you have insulated 15 kV cable with all three phases passing through the CT. This connection will see the imbalance in the phases. Passing the neutral through would also allow you to detect ground current. Don't have access to the manual now but I am fairly sure that the 750 also internally calculates the residual (imbalance) of the phase currents(most microprocessor relays do this). I believe the manual will call the calculated residual 50/51N as opposed to 50/51G. Or something like that.

Alan

 

[oldfieldguy]

On the SR-750 relay you show, you are monitoring zero-sequence current from the ZCT. You set this element up as "Ground Current" in the relay setpoints. This is 50/51G.

The 750 also allows you to setups 50/51N elements using a derived value from the three phase inputs. These are set up as "Neutral Current" in the setpoints.

The ZCT is treated like any other CT: ratio, polarity, accuracy class (saturation). The only consideration that is often different is the physical size of the window since you're going to run all three phase cables through that window.

Also, you might want to note that the shield grounding is correctly routed. There is a picture in the SR-750 manual that shows this. Failure to properly route the shield grounding will disable ground detection.

old field guy

 

[a10jp]

Hi RACOBB, the system is only 3-phase ungrounded, so there is no neutral to pass through.

I am attaching the 750 connection diagram, it shows the connection method which matches our installation with the shield grounding routing inside the CT window. Thanks for your tip, oldfieldguy!

Because what we have is a 3-ph delta system, why would you want to setup the relay to detect a calculaed 50/51N vs a straightforward 50/51G?

The original reason I made this post was that during a recent ground fault in the system, we discovered that some of the relays were tripping incorrectly (or mysteriously), that is, some of the upstream breakers were tripped thinking it was a forward fault current, when in fact, and we thought, it should be reverse (i.e. opposite to from source to fault location). And yes, we were investugating the polarity of the setup of the whole syste, and so far, everything checks out, and then we come to the ZCT and thought perhaps somehow the ZCT element was selected incorrectly which was my original post(but, polarity of installaion checked out).

 

[racobb]

I guess I did a bad thing and "assumed" that you were operating a 4 wire system.
No diagram attached?
Are you supplied from a utility?
Is the supply truly a 3 wire or 4 wire system?
How are you detecting the ground faults?
Radial or networked system?

Please describe your system in more detail.

Alan

 

[Pam60]

Hi

I noticed in this post that a metering class cannot be used for protection. What problems could possibly either way if the CT's are replaced in applications.

Thanks

 

[oldfieldguy]

Short answer:

"Protection" CT's are constructed in order to maintain accuracy at current levels far in excess of their nominal values, typically 20 times nominal, in order to insure proper representation of currents under fault conditions.

Metering CT's don't have to work under these conditions.

old field guy

 

[slavag]

Hi.
a10jp,please provide addtional info:
1. What is a setting of 51N function, current and time.
2. What is a lenght of cable, that protected by 750 relay and with is a setting range of 51N function in this relay.

It saied once, but I ask again, are you check that grounding concentrator of cable back to grounding via ZCT?

Best Regards.
Slava

 

[a10jp]

Hi, sorry I forgot to include the attachment. I am including them here. The PDF file shows the 750 manual installation method. It is shown in page 4 red circled diagram. The remaining 3 pages are our CT cut sheets, the 592 and 145 are installed in the current setup. The excel file show a very simplified conection diagram. It shows the system is 13.2KV, radial system. The new system which we worked on was connecting to the existing bus system. The existing system has a neutral ground resistor of 100A, 8KV class. The way ground fault is detected is via 50G (Pickup=2A, TD=around 2sec) and also via the 59G and 67G, however the designer explains the 59G/67G, is only for backup protection. The primary protection should still be 50G, which somehow generated multiple GF when phase B was faulted (as shown in the relay log), for which the sytsme was registering mutilple ground faults "mysteriously" elsewhere downstream of the tie breaker (diagram). But before I digressing into different topics, I want to point out based on the system I have described so far, are we using the right CT for this applications? Thanks.

 

[a10jp]

Here is the diagram, it wonlt let me attach 2 at the same time.

 

[a10jp]

Hi Slavag, sorry I may have misunderstood you before. What is a ground concentrator? Do you mean the ground connection on the load side of the CT? Thanks.

 

[slavag]

Hi a10jp .
I meant, grounding concentrator or shield of 15kV cable.
Firstly check, if it back via ZCT or not touched grounding between ZCT and bushing.
Second, check if capacitive current of cable is less than your setting. 2A seems is low setting, for cable size, from 120mm^2 is less than 1km lenght.

Best Regards.
Slava

 

[a10jp]

Hi Slavag, I understand your concern now. Think you have a valid point on the pickup. It will take some time to verify. Our cable size ranges from 150sqmm for spare circuits, 200sqmm and 2sets of 325sqmm which ties to other source buses. They are all some distances away. Also vaguely remember you mentioned about the torque angle. We have 30deg lead, does that seem reasonable? Your input has been most valuable.