Core Balance CT

[zimmerDN]

Is there a difference between Core Balance CT for ground fault detection and a phase CT used for o/c protection?

I understand for ground fault detection, you have to run all three phase conductors through the CT but does it require a special window type CT?

 

[dpc]

There is no fundamental difference, but the core balance CT must be toroidal and large enough to allow all the conductors to pass through. GE has (or used to have) a special CT specifically designed to connect to their relays - it has a very non-standard ratio. The accuracy of the core balance CTs is not a critical as phase CTs in many applications that use instantaneous trip elements for the ground protection. The CT design and secondary current must be compatible with whatever relay you are connecting to it.

 

[juleselec]

You can hook up three phase CT's in a residual connection and use that as the input to the ground fault relay. However, it's less sensitive than a core balance toroid (since the phase CT's won't be perfectly matched). Whether or not you need core balance CTs will probably depend on the neutral grounding method - high impedance and ungrounded systems typically require the sensitivity of core balance CTs to properly detect ground faults.

 

[zimmerDN]

What's are the names of some CT manufacturers for ground fault / core balance ct?

I am having a difficult time locating a suitable ct.

 

[scottf]

zimmerDN...where are you located?

 

[ssantoshr]

Below are some manufacturers who offer the ground sensor CT (zero sequence CT).

GE/ITI, Link

ABB, Link

Schneider Electric, Link

 

[zimmerDN]

Why do manufacturers have specific ground fault / zero sequence ct if they are contructed and the same as any other toridal (window/donut type) ct?

Even with a zsct you still have to choose the appropriate window size / burden and ratio.

Am I missing something here?

 

[zimmerDN]

Here's another thought, correct me if I'm wrong.

Core Balance CT
Ground Fault CT
Zero Sequence CT
Window CT

These names does not implied how a CT is constructed because they are all the same phsyically (other than ratio, accuracy, burden and window size).

The name only tells you how a CT is applied, in this case the first three names are for ground fault detection and the last name is for phase current detection.

If a window CT has all phase conductors going through it and its used with a ground fault relay, then we can it CBCT, ZSCT or Ground Fault CT?

Is that correct?

 

[scottf]

Technically, they are all basically the same construction. However, ground-fault CTs normally have a larger window and/or different mounting to allow all 3 phase conductors to pass through. Also, they typically are available only at lower ratios, e.g. 50:5A, 100:5A and with relatively low protection ratings (IEEE C15, C30, C50, etc...).

 

[rasevskii]

When using core balane CTs it is important that the cable sheath (armor) conductor passes backward through the CT window to its ground bus connection. THis to avoid sheath currents from causing a false operation of the protection. Cable sheath currents can be caused by third harmonic currents originating from generators connected to the busbars ( a typical Diesel Powerplant method) rather than via individual transformers.

rasevskii

 

[randyman]

zimmerDN,

I cannot speak for all CT makers but I can speak for those that I have personally been involved with - with regards to zero sequence GF CTs they are wound in a different manner than a typical CT. In ZSGF all three phases pass thru the window, and it is possible to have unequal line currents that balance in the neutral, and since placement of those conductors may or may not be controlled, the winding must be structured such to prevent nuisance tripping. This is typically true of low ratio units (50/5, 100/5). So due to the unequal line currents you will have unequal flux entering the core at different sections of the core. Depending on the sensitivity it can create issues.

Further, depending on the window size and shape, not all cores will be same. Some may be continous tape wound (toroidal or oblong) and others may be stacked laminations (real long windows). So my suggestion when using a single core for zero sequence apps (core balance CT), specifiy the application as such.

From my perspective, core balance and zero sequenece have same meaning. GF may or may not be the same. BUT they all can be window-type CT.

 

[ArchieMedes]

To rasevskii,

Can you please explain more by what you meant on your post below?

When using core balane CTs it is important that the cable sheath (armor) conductor passes backward through the CT window to its ground bus connection. THis to avoid sheath currents from causing a false operation of the protection. Cable sheath currents can be caused by third harmonic currents originating from generators connected to the busbars ( a typical Diesel Powerplant method) rather than via individual transformers.

What does "passes backward through the CT window" mean? If you have a drawing/ circuit diagram to illustrate, please share. We are encountering some false operation of protection and would like some ideas. Thanks in advance.

 

[davidbeach]

The CT needs to be looking ONLY at the current in the three phase conductors. At the point where the CT can be installed, the cable includes the shields/neutrals and the current on those needs to not be included in what the CT measures. So, at the termination point where the shield/neutral is separated from the center conductor, they need to be wrapped back and pass back through the CT so that the current that came in through the CT on those conductors then goes back out through the CT, summing to zero. All of the shields/neutrals can be tied together and a single conductor taken back through the CT, but they can't be grounded on the cable termination side of the CT.

 

[trosepe]

As mentioned CTs used for zero sequence detection schemes typically have a much lower ratio than the of the CTs used for the phase over current. As such the secondary current in the zero sequence CT is much higher and the CT core requires more iron to prevent saturation. They are significantly larger and heavier than the phase CTs.

 

[ArchieMedes]

Thanks for the reply.

What if the CBCT is on a busbar with cables? Does it matter if the CBCT is above the cables or on the cables? Should both the cable screens on these 2 scenarios be looped back to the CT? Attachment here.

 

[trosepe]

Figure 'A, shows the zero sequence CT located above the stress cone and the shield connection routed back through the CT which will result in the current flowing out to the fault to be cancelled by the same current flowing back on the shield. Typically the zero sequence CT is 600 volt rated and installed on the shielded portion of the cable (below the stress cones). Then you need to loop the shields back through the zero sequence CT as indicated in the post by David Beach. As for figure 'B'; I don't think I have ever seen a zero sequence CT installed around bus bars on a medium voltage system.

 

[LionelHutz]

No, you have it backwards. The screens would not go through the CT in your picture and the CT being located on the busbar or right at the ends of the cables would be considered the same. The grounds for the screens would be passed through the CT if the CT was located in the same area as the text "A:CBC on cables" is located. The ground wire would also pass through the CT in the opposite direction. The GE Multilin relay manuals (369/469) illustrate this connection.