CT's for Differential Relays of 230kV Delta Pri X'former 69kV Wye Sec 4

[leur2011]

Dear fellow Power folks,

For a power transformer that is protected in the primary with a 230kV breaker and a half CB's and 69kV CB at the secondary as shown on the attached SLD, the differential zone should include the high voltage and low voltage side circuit breakers, by connecting the transformer ANSI device 87T differential relays to current transformers on the LINE side of the 230kV breaker and a half CB’s and on the LOAD side of the GIS 69kV incomer CB. However, I was advised to install the CT's of 87T at the primary of the transformer instead at the line side of the 230kV CB. The feeder between the 230kV Sub and 230kV/69kV transformer is via underground cu/XLPE/AWA/PVC cables at approx. 1.1kM. They keep telling me that the 87L line differential relay at Sub 69kV will line transfer trip between 69kV substation to 230kV Substation via F/O cable to the upstream 87L relay in case there is an internal fault within the transformer.

In my understanding, the line diffential is totally different function with transformer differential and there must be a separate dedicated CT's of 87T at the line side of the 230kV breaker and a half CB. Could you please share your expertise?

 

[DTR2011]

IN substations where the distance to the HV breaker to transformer is small, I've seen breaker CT's used in 87T Zone of protection, often. 1.1 kM is a long distance for 87T CT secondary leads and thus choosing to use Xfmr CT's will keep the CT lead resistance (and wire diameter) to a reasonable size, in order to avoid problems with 87T and lead resistance.

The 87L is properly protecting the cables and HV Breaker. In the 87T, the zone does include the LV breaker, but the distance is a very reasonable 20 meters.

87L and 87T are different functions. As I read the SLD, there are separate CTs for these functions (and I am assuming that CT's for other CB in breaker and half scheme are parallel or input to 87L relay). As I read SLD there defined Zones of protection for 87L and 87T and it looks like a standard solution, considering the distance from Xfmr to 230kV breakers.

Perhaps you could define your question better if my answer doesn't make sense.

 

[leur2011]

Smallgreek appreciate your response. Base on your explanation, there seems to be the main concerns for installing the CT's of 87T at the line side of 230kV breaker and a half CB's is due to the 1.1kM distance presumably large cross sectional area of copper conductors are required to compensate voltage drop and maitain CT lead resistance, in addition, to avoid mal-operation of 87T transformer differential.

We are planning to use line differential SEL 311L relays to protect our underground feeder cables between Sub 230kV and Sub 69kV via F/O cable. And SEL 387 relay for transformer differential protection.

Still not clear to me how to comply with IEEE Std 242 1986 edition requirement that recommends 87T transformer diffrential zone of protection current transformers must be at the line side of HV breaker (not on the load side).

Perhaps anyone could clarify how the line transfer trip between Sub230kV and Sub 69kV via SEL 311L that will disregard the requirement to install the CT's of 87T at the line side of 230kV breaker and a half CB.

Thanks.

 

[DTR2011]

I don't have IEEE Std 242 1986 edition handy, but I would guess that this recommendation is to cover the breaker as well in the zone of protection. What I don't see is both breakers in the scheme feeding the 87L relay. This would require CT's(other breaker - CTs toward other 230kV Bus) wired in parallel into one input of the SEL 311L. CT's must be the same type, class, rating, etc.

I would say as long as the 87T trips the HV & LV breakers it is doing its function within its zone of protection. How you get the 87T trip signal to operate the 52TC 1.1 km away is another question. You haven't mentioned, but may I assume that 87T has an input to 87L to DTT HV Breakers, via FO cables? Is there redundancy for the trips in the FO cables?

To clarify your question, 87T must clear both breakers. Normally, an 86T (LOR) is used. Likely, 86T trips and locks out 69kV Breaker. Likely, 86T also feeds input to SEL 311L, which has that input programmed to DTT SEL 311L at other end, which in turn trips HV Breakers 1&2, either directly or via a LOR.

Again, maybe I'm confused. From SLD, I see 87L protecting HV Cables and breakers, NOT 69kV Cables. 69kV Cables from Xfmr to breaker are in 87T Zone of protection. 69kV CB is protected.

I don't see any problem with this design (other than mentioned above), as long as everything is carried out properly.

Referencing a 1986 standard may be part of the confusion. 87L didn't really exist (or at least popular and economical) at that time.

 

[slavag]

Actually possible to use 87L with power transformer into line zone.
A lot of newer protective terminals is included such option.

Check SEL411L, if I'm remember right, this terminal is included inline transformer option.

 

[bacon4life]

In IEEE 242-2001 I saw an example using the breaker terminals instead of the transformer terminals, but having 1 km of cable is not specifically covered by the standard. As long as your zones of protection overlap, it is fine to use the transformer high side CT's for both the line differential and the transformer differential. All three circuit breakers must trip for a fault in either zone and for a bushing failure.

In a recent substation with a similar setup we used an SEL 487 & 387E for the transformer differential primary & backup, then used two sets of 311Ls for the line differential protection. We used mirrored bits to send the trip signal from the transformer lockout to the remote 230 kV substation.

 

[davidbeach]

If you're using SEL relays already, why not the SEL-411L and do both line and transformer differential in one zone with no need to long distance CT leads?

 

[leur2011]

Smallgreek, you deserved a star.

For clarity, I omitted in the SLD the parallel CT's input of the SEL 311L relay in the upstream. Also omitted, are the protection relays for cable short circuit/ground fault/overload and bus fault at the 69kV secondary side of the transformer.

This means there is a possibility to send direct trip signal (DTT) in the upstream HV CB's for let's say transformer primary protection SEL 387 output trip signal to 86T LOR at Sub 69kV, then inter-trip the SEL 311L (both programmed with DTT), probably a device 94 relay or 86 LOR will trip the upstream CB's at Sub 230kV?

The FO is a 24 channel single mode cable, basically it can accomodate trip signals.

 

[leur2011]

Davidbeach, a star for you. That is what I want to know. A relay that protects both line and transformer differentials.

I assume the 24 channel single mode FO cable is sufficient for the SEL 411L application, or I need to use dual mode?

 

[davidbeach]

One fiber pair per pair of relays. Various single mode and multi-mode fiber options.

 

[stevenal]

I don't believe it would be necessary to parallel the CTs as smallgreek suggested if the 411L is used. By the way, slavig mentioned the 411L application first. One fiber pair per channel. Might want two channels for redundancy.

 

[DTR2011]

I checked SEL website and do not believe the 411L will do 87T. It's a nicer 311L, with more CT inputs (paralleling not required), however it uses the alpha plane and not a restrained diff characteristic. I think the OP has 311L's & 387 selected.

A 411L that could handle 87T functions would be nice.

Wonder if GE has a UR relay that can do this with a brick?

 

[davidbeach]

Having programmed the 411L to perform 87T, I kinda think it has that capability.

Multiterminal lines and transformers in the differential zone are protected.

 

[DTR2011]

davidbeach-

Correct you are. I was looking right at that!

 

[leur2011]

With reference to the figure 36 of the attached SEL 411L data sheet, the functions of the advanced line differential protection, automation, and control system SEL 411L as a current differential relay for the combined line and transformer zones is well suited to my 230kV underground 1.1kM transmission lines and transformers.

Anybody has experience for other relays with similar functions as SEL 411L relays, I just want to explore other options, schemes and set-up.

BTW, thanks to smallgreek, slavag, bacon4life, davidbeach and stevenal for sharing their ideas.

 

[slavag]

ABB REL316*4, RED670
Siemens 7SD522
Im not remember, but 90% sure Alstom P545.

 

[leur2011]

Stevenal, I think Smallgreek is correct when he insisted CT's should be parallel input for 87L line differential relay at Sub 230kV with breaker and a half CB. Attached data sheet requires parallel CT's input for SEL 411L relay with 87L function as well.

 

[davidbeach]

No, each CT is connected to the relay independently and the relay sums them numerically for the 87L function. If paralleled outside the relay you lose many of the capabilities of the relay for no gain.

 

[slavag]

Possible connect CT's in parallel for one and half CB.
But such connection have a disadvantage in case of external fault on the 220kV buses, fault current flow via both CT's are connected in parallel, for avoid maloperation in this case, setting of 87L are increased. In case of separated connection of CT's, terminal is included separate restraint capability.

 

[davidbeach]

Yes, the relay will go to a more secure mode when it detects fault current flowing across one terminal, but to do that the two currents have to be wired in separately. No reason that I know of to externally parallel CTs when using SEL 400 series relays.