Inter-Trip control circuit between MV Switchgears 2

[Bilegan]

One (1) vertical section of each bus of my existing MV switchgear, 4000A, 13.8kV, 60Hz. is to be modified to replaced the old air blast MV circuit breaker with new VCB. Since the CT's of the existing relays device 50/51 and 50G with zero sequence CT are under rated, we opted them to be removed as well and replaced with new individual relays, i.e., device 50/51 and 51N with only new 3 CT's.

The existing MV switchgear to be modified will feed a new MV switchgear, 1200A, 13.8kV, 60Hz., secondary selective, N.O. bus tie VCB, and have auto transfer scheme. Each of the incomer protection relays are device 51 and 51N only, no instantaneous overcurrent relay device 51 option.

A 3x1/C-240sqmm,XLPE,AWA per phase power cables between existing and new MV switchgears, without separate grounding wire. The distance between existing and new switchgear is approx. 1.5 kM, power cables will be installed underground of approx. 500m and then emerged aboveground in cable tray for a distance of approx. 1000m.

My senior suggest that there should be inter tripping control circuit between existing and new switchgear. According to him, an additional tripping relay device 94 and lockout relay device 86 shall be added for the inter-tripping requirement.

He also suggest to change the proposed new device 51N with 50G with a separate zero sequence CT in the existing MV switchgear.

I am totally confused, my understanding is, a transfer trip is applicable only between 13.8kV switchgear and 4.16kV switchgear via step-down trafo, 13.8kV pri. and 4.16kV sec.

I need your opinion guys!

Regards,
Bilegan

 

[slavag]

Hi again Bilegan.
More info needed, but I agree with your EE.
What is a info needed:
1. What is a type of system grounding.
2. What is a setting of ground fault ( 50N, 51N or G) you would put, of course size of phase CT needed.
About inter-trip. isn't must, but is good solution in many cases, for example if possible dual feeding ( once from exist SS, once to..). BTW, control cable for this distance work, but I would like recommend use something via fiberoptic.
Best Regards.
Slava

 

[Bilegan]

Slavag, the existing 13.8kV switchgear derive its source from an upstream transformer, 69kV delta-13.8kV wye. There is a neutral grounding resistor on the wye side of the transformer rated at 800A at 10sec. It is worth to mention that there is no grounding resistor connected in the bus of existing switchgear.

To simplify my concern based on the original post;

1. For the residual relaying scheme adapted as protection of power feeder cables between 13.8kV switchgear to switchgear without transformer in between, what is more appropriate;
1.a An application of device 50/51 with ground relay (device 51N) and common CT's.
1.b A device 50/51 with dedicated CT's and ground sensor relay (device 50G) with a separate zero sequence CT.

2. Based on the residual relaying scheme on my first concern, do we need an inter-tripping circuit between swithcgear to switchgear?

Regards,
Bilegan

Murphy's Law - If anything can go wrong..it will.

 

[slavag]

Hi Bilegan.
1.What used for the ground fault protection:
Residual connection or dedicated/separated ZC CT?
It's depend on the setting of protection
For example:
your grounding resistor is 800A for the 1sek
what is a setting of ground protection relay? about 10-20% of this current, 80-160A. What is a size of phase CT?
Recommended in the residual(Holmogren) connection used minimum 10-13% of phase CT size for avoid unwanted trips due the CT errors and partial saturation of CT.
Whith newest relays possible put 50-100A setting for the ground fault protection, this lower setting will prevent damage of your eqp. But it's request dedicated ZC CT.
2. No, you don't NEED inter-trip circuit. decades such systems in operation w/o inter-trip circuits.
But, you can fiber-optic cable between SS, save few fibers for protection and add binary IO to fiber optic
converters ( BTW, SEL have very nice relay for this purpose). In future you can use this ( I think ) 8 signals for the control between both SS's, include inter-triping, BFP, blocking scheme, etc..
Best Regards.
Slava

 

[Bilegan]

Slavag here are info.

Disregard the inter-trip control circuit issue, my senior also drop this suggestion.

We will focus on the proposed protection scheme on the branch tie-in of existing 13.8kV MVSG and the incomer of the new 13.8kV MVSG.

Total demand load is 18 MVA (when one incomer of the new 1200A, 13.8kV, 60Hz MVSG is open and the bus tie CB is closed).

Existing 4000A, 13.8kV, 60Hz. MVSG
The branch proposed relays are (tie-in point);
a. Phase O/C inst/time delay relay device 50/51
b. 3 x CT's = 800A/5A/each
c. Residual Ground O/C relay device 51N (see note)

Note; For a residual relaying scheme, which is more appropriate relay to be used, residual ground O/C relay device 51N or ground sensor relay device 50G with a separate zero sequence CT, to be sized at 100A/1A.

New 1200A, 13.8kV, 60Hz. MVSG (power supply shall be derived from existing branch of 4000A, 13.8kV MVSG)
Incomer proposed relays are;
a. Phase O/C time delay relay device 51
b. 3 x CT's = 800A/5A
c. Residual Ground O/C relay device 51N.

N.B.; Both existing and new MVSG are secondary selective, N.O. bus tie CB with auto transfer scheme. Hence, proposed protection relays are identical on both buses.

Additional info., please refer to my original post.

What is your opinion guys, please help!

Regards,
Bilegan

Murphy's Law - If anything can go wrong..it will.

 

[slavag]

Hi Bilegan.
From theoretical point of view best choice is separate ZC CT.
BTW, 100/1A is very right solution.
From selectivity point of view, possible use phase CT with 80-100A minimum setting of 51N, NOT LESS.
This type of CT is about 200-300$ of cost ( I hope so).
I always preferd separate CT, but it isn't MUST in your application.
Sorry for this expration again, for decades the MV application was with residual connection and work fine and will work fine.
Hope that help.
Best Regards.
Slava