Reverse current into 20MVA power transformer 3

[Rodmcm]

An industrial site has a 14MVA 11kV load and 6MVA of embedded generation. The site is supplied from 2 x 20MVA transformers
The 11kV breaker on each transformer is fitted with a 260A directional OC (back into transformer) which is graded with a 260A directional OC on the site incomer.

Why would this be? Obviously to stop reverse power to the transformers, but the generators rely on the load for the energy to run.
Also why 260A ( about 5MVA).....?

 

[davidbeach]

Hopefully there's a file somewhere with the setting calcs. There's no magic formula, that 260A probably just fit well with what's on either side of it.

 

[ScottyUK]

Does the utility have an export limit? Many older systems were designed for power flow from top to bottom, i.e. from large central generating plant to passive loads. Adding generation low down in the distribution system can cause problems with protection, system stability and so on.

 

[waross]

The issue is to limit reverse power into the grid. The transformer is incidental. The utility may have demanded the reverse power protection and dictated the setting. Too much reverse power available may make fault clearing difficult and may also cause issues with voltage regulation.
The file that David suggests may be part of the terms of service contract with the utility.
As an anecdotal example of possible utility issues with reverse power limits and support of Scotty's post:
I encountered a co-generatoin installation with problems. The installation was approved subject to limits on the voltage and the PF at the Point of Common Coupling.
These limits effectively limited the plant output (and the revenue) to about 80% of approved capacity.
When the utility dictates a condition "It is what it is."

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DTR2011]

Is the issue to prevent reverse power or to prevent feeding into a fault. I believe a Directional OC relay is not the correct choice for reverse power protection / detection. Basler has some good application notes on the differences, hows and whys, etc.

The generators are a source of fault current on the HV side and I would think the utility requires this in order to run the generators in parallel with their system. I have seen much more stringent protection requirements (Distance, DTT, 27/59, etc) required when operating in parallel with utility (IEEE 1547 as an example).

 

[crshears]

I've mostly seen the term "reverse power" applied to steam turbine generators, the purpose of the scheme being to prevent the turbine from being motored by the system with all steam valves closed and hence with no steam flow present to provide ventilation; very bad.

More commnly we use the term 'line back-up protection' using impedance/distance/21 relaying to "back up" the transfer tripping scheme that normally clears the line for faults.

With distributed now becoming widely connected to the [Ontario, Canada] grid, such problems as Bill has described have become major headaches; more relaxed conditions are sometimes applied where a generator is for load displacement purposes only, and not of sufficient capacity to export power to the grid.

It is so, so important to do your homework before embarking on this type of undertaking...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[arrehman1]

@DTR2011,

Can you please point to the Basler application notes/references that you alluded to for Directional OC relay & reverse power protection / detection?

Thanks!

http://GeneralPAC.com

- Awesome video tutorials for power systems PAC!
The Wye Wye Transformer Connection video:

http://gpac.link/1GdGRSQ

 

[EP007]

Does the power supply company allow the site to provide or sell power to the grid?

 

[DTR2011]

It took some searching to find this and you will need to register with Basler to download.
"Myths of Protecting the Distributed Resource To Electric Power System Interconnection
by
Gerald Dalke, Basler Electric Company"

https://www.basler.com/ResourceDownload.aspx?id=63

Hope this helps.

 

[ppedUK]

Rodmcm

What your describing would be a pretty standard application of DOC applied to paralleled transformers fed from grid, although you wouldn't normally have 2 x 11kV breakers in series on the secondary of the TX (but the breaker at the TX is probably the metering breaker owned by your utility). The 260A looks like a 25% setting of the TX FLC, which would be chosen to grade with the paralleled incoming 11kV breaker forward protection and the upstream grid protection. It might not be anything to do with export limiting.

 

[sarathchan]

I cannot say for sure without a single line diagram. But as someone who has experience in grid level protections, I think it is for the purpose of fault discrimination.

It is assumed the two transformers are connected to an 11 kV bus bar system through cables (say A & B). Now imagine a fault occurring in cable A (or anywhere in the section between the HV breaker of transformer B and the 11 kV CB B at the bus bars). Fault current would flow from transformer B directly through the transformer B and the cable. The problem is that a fault current would flow from the transformer A, through the healthy cable A to the bus bar and back to the fault. Thus, if DOC is not installed, both 11 kV feeds would trip.

With DOC this is prevented because the cable A DOC will sense a forward power flow and not trip. On the other hand the DOC on cable B will sense a reverse power flow and trip even at a low MVA level. At the same time the HV breaker of Transformer B also will trip, clearing the fault. Thus power continuity will be maintained to the bus bars through the healthy transformer A.