Fault currents to high ? 1

[Gianoli]

I work for a utility that operates several large 138 kV systems. Several of these system have become too large in opinion. The most concerning is a 345kV/138kV substation sandwiched between a large urban area and a heavily industrialized area. The 138kV section consists of 16 outgoing circuits in a breaker and half switching scheme. Three 600 MVA power transformers link the 345kV section to the 138kv section. Fault currents on the 138kV bus are in excess of 70KA. Power flows through the substation often exceed 2000 MW.

Now an Independent Power Producer want to connect a 550 MW project to the 138kV bus. With the addition of the 550MW project fault current levels on the 138kV bus will approach the 80KA rating of approximately 32 breakers. I am not comfortable operating the 138kV section with fault current levels within 1 or 2 KA of the breaker ratings.

Any thoughts?

 

[davidbeach]

So, the IPP gets to pay for new breakers with higher interrupting ratings. No problem.

 

[slavag]

Hi Gianoli.
What is interupting rating of CB's today? 70kA?
David, Im not sure if possible get 138kV CB with interupting rating 80kA, don't know.
For my pinion reconfiguration of grid needed, open of couplers,additional reactors, etc.
Regards.
Slava

 

[davidbeach]

My point was that there is an existing system that works and the IPP wants to break it. The IPP then needs to pay to fix it. New breakers are one option, reconfiguration is another option. What ever is done to directly respond to the change in system conditions brought about by the connection of the IPP is on the IPP's nickle.

On the other hand, if I were the IPP I would be inclined to challenge your assessment that the present condition is acceptable. If you are running 80kA breakers at "in excess of 70kA" I'd say that you are already too close to having overdutied your breakers and my measly 550MW is not the (sole) cause of your trouble.

I have been involved in a project feasibility study that determined that our project would push someone else's breakers from 99% of rating to 100% of rating. My response is that isn't our problem, those breakers are already in trouble. In this case we both work off the same system model and I know it can't get within 1% of fault currents, so those 99% breakers may actually already be at 105% of their rated duty and we're not going to pay to fix that.

 

[RRaghunath]

To me, it looks appropriate to connect 550MW station to 345kV bus and not to 138kV bus.

This should be so, even if the 138kV fault levels are not an issue. What is the current rating required for 138kV breaker / switchgear if it has to handle 550MW - and how about the busbar and bus coupler ratings!

 

[slavag]

"To me, it looks appropriate to connect 550MW station to 345kV bus and not to 138kV bus."

Good point, Raghun.
Regards.
Slava

 

[Gianoli]

Thanks for the responses:

** What is interrupting rating of CB's today? 70kA? **

There are 32-138kV breakers rated at 80KA interrupting, 4000 Amp continuous.

**What is the current rating required for 138kV breaker / switchgear if it has to handle 550MW - and how about the busbar and bus coupler ratings! **

Current fault levels sit at 71.8KA. If the IPP project proceeds as planned fault levels will sit at 78.8 KA. Bus is 4” heavy wall copper pipe. Bus and bus fitting current ratings will not be exceeded.

**"To me, it looks appropriate to connect 550MW station to 345kV bus and not to 138kV bus."**

Connecting the IPP at 345kV was considered early. For various reasons including space concerns in the 345kV yard, removing some load from the sometimes overloaded 600MVA transformers and respecting the interrupting rating of a number of 345kV breakers it was decided to connect at 138kV.

My question is, is it a good practice to operate the 138kV yard with fault current levels within 1.2 KA of the breakers interrupting rating? We have done it before but only with a plan in place to migrate to high rated breakers. The breakers were just replaced 4 years ago to increase interrupting ratings from 63KA to the current 80KA. At present 80KA is the highest rated breaker available.

The argument I receive is that the breaker manufacture states as long as the nameplate ratings are respected the breakers are OK and when 100KA breakers appear on the market we will upgrade again. I say 78.5 KA is a very high fault current level and we need to explore other options.

 

[jghrist]

and my measly 550MW is not the (sole) cause of your trouble

I don't know if I'd call 550MW "measly" at 138 kV. It probable adds about 20kA to the fault level.

 

[davidbeach]

With three 600MVA transformers already feeding the 138kV bus, 550MW is not large compared to what's there already. If I were the IPP I'd be trying to downplay the impact of the new generation and from that standpoint they could well claim "measly".

In the end it is probably appropriate that the IPP pay some for the upgrades and the utility pay the remainder. Connection directly to the 345kV bus would help some, but the fault duty at the 138kV bus would go up and is already too close to the breaker ratings.

 

[scottf]

Gianoli-

Have you looked at putting in current limiting reactors?

Might be able to install them on low side of IPP to limit fault current enough to give you some breathing room.

Installing them at 138kV, 4000A cont, would likely be very expensive and need a LOT of space.

 

[stevenal]

You talked with the wrong folks at the breaker manufacturer. If you are getting close to the nameplate rating, you must adjust for X/R and reclosing prior to comparing to the symmetrical fault current rating of the breaker. See IEEE C37.010.

 

[QBplanner]

It seems that Davidbeach is a typical Utility guy. pay the CBs.
In the Utility I work is the same thing. IPP comes in hook up to our system trigger the CBs/disconnect switches replacements. Pay it.

but 550MW connect to 138kV unless it is a short distance. look the SIL of 138kV is only 50MVA. transfer 550MW at 100km distance depends on your system, it will at least be four - five lines. Not a good option.345kV make more sense at least single line works for 550MW within 100km.

 

[newpower]

"You talked with the wrong folks at the breaker manufacturer. If you are getting close to the nameplate rating, you must adjust for X/R and reclosing prior to comparing to the symmetrical fault current rating of the breaker. See IEEE C37.010. "

Great point, Stevenal

In the past, I worked on several feasibility studies involving new generators with breakers approaching their fault current limits. Thoroughly reviewed C37 and the breaker manufacturer's documentation. Check the contact parting times and the X/R ratios. They can make a difference.

In the area I was in, the generator paid for the breakers once they exceeded their ratings, but they always asked for our philosophy on rating breakers.

It is also very important to determine the fault current contribution for each contingency and the actual fault currents for a loss of a line. Of course, a stuck breaker at the bus for a close-in fault is the worst, but for most line faults, the fault current magnitudes acutally drop when dealing with networked transmission systems.

If the generator pays, replace the breakers. If it's not that simple, look at you substation configurations. If it's a breaker and a half with all of the breakers closed, see if you can segment your buses with opening tie breakers. You may want to take a close look at the fault contributions for each line and look at the configuration of the substations at the other end of the lines, too.

Reactors will definitely reduce fault currents, but be careful on their impact on system stability.

Wow, 3-600 MVA transformers. If they are in parallel, I am not surprised at the fault current levels. With the load levels you have, having the generators on the low-side will definitely help with future load flow problems and protect against transformer overloads.

Last thought - make sure you look at the fault current capability of all of your disconnect switches associated with the breakers.

Everyone - apologies for the very long email; I have not posted in several years and was very excited. If I said something wrong, please let me know!

 

[slavag]

Hi.
"Everyone - apologies for the very long email; I have not posted in several years and was very excited. "
We hope, you'll change it and will see you posts allways.
Best Regards.
Slava

 

[MxGxK]

Last thought - make sure you look at the fault current capability of all of your disconnect switches associated with the breakers.

Good point.

I seem to remember the rating on disconnect switches was not the RMS Symmetrical rating. I remember seeing 100kA ratings that were certainly not capable of thru-faults of 80kA.