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Topic Title: G59/2 Query
Topic Summary: Definition of low or high impedance networks
Created On: 11 September 2012 03:06 PM
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 11 September 2012 03:06 PM
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descole

Posts: 4
Joined: 03 December 2002

Ladies and Gents G59/2 provides a table of settings for the protection at the PCC for long term parallel operation of generators Ref Clause 10.5.7.1. For Loss of Mains it advises factors K1 and K2 depending on whether the network is low or high impedance. The definition of high impedance given is not well defined. It says 'A fault level of less than 10% of the system design maximum fault level should be classed as high impedance' I assume that this probably means that if the the fault contribution due to the generator is less than 10% of the system fault level (grid fault level) then the system is defined as high hmpedance. Could anyone offer their opinion on this? many thanks
 11 September 2012 04:38 PM
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ArthurHall

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Joined: 25 July 2008

I dont know for certain, but I think it could refer to the means of earthing of the HV network. For example in some parts of the country the 11kV network is impedance earthed while in other parts it is directly earthed.
 11 September 2012 10:36 PM
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JonathanHill

Posts: 225
Joined: 09 September 2002

In general, in the UK (which is the geographic area for which G59/2 is aimed), declared DESIGN Fault Levels @ 11kV are 250MVA. Therefore, if the FL at the PCC of an embedded generator is 25MVA or less, then it'll be classed as high impedance.

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Jonno
 12 September 2012 01:21 PM
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aroscoe

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Interesting ... I suspect Jonno's explanation is spot on, although I do not know where written confirmation of the 250MVA figure might be. It might be specific by region so best contact the DNO to be sure.

Certainly, it could NOT mean that if the new generator fault level is less than 10% of the ACTUAL system fault level, that it would be a "high impedance" network, because that logic would be back-to-front. If the generator fault level was MORE than 10% of the actual system fault level, that might be used as a definition of a "high impedance" network, but thats not what's stated in G59/2. So, I suspect Jonnos explanation is the correct one. It would be helpful if the exact definition was included in that section or table, or at least with guidance on the maximum design fault level values.

Andrew

-------------------------
Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
 24 September 2012 05:23 PM
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fredman

Posts: 221
Joined: 26 July 2007

From my experience the DNO's al declare a 250MVA fault level if you ask a basic question. If asked specifically then they can tell you the expected true value. Unless you are sat on top of a main supply then 250MVA is not a probability. The reality will be lower, I know of sites where the declared was down as low as 50MVA when calculated but when asked the answer was 250MVA.
 25 September 2012 10:37 AM
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JonathanHill

Posts: 225
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The 250MVA is the design FL - ie they may reinforce their network up to that level so therefore it makes sense to size connected equipment (especially switches & circuit breakers) to meet that rating.

Existing FL, especially the minimum value this may be when the network is configured abnormally (eg for maintenance or fault recovery), is needed for establishing appropriate overcurrent & earth fault protection settings. I think the DNO would want to use normal FL in establishing the K factors for the G59/2 settings, but perhaps might be persuaded to reconsider if frequent nuisance trips were experienced.

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Jonno
 01 October 2012 04:08 PM
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iamck

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Joined: 16 September 2001

Back in the 1960's, when I was a young engineer, there was a big programme to rid the UK public system of switchgear that either did not have a fault rating, or whose fault rating was below the fault level at the point where it was located. My memory is that replacement switchgear was rated at 250 MVA as standard. There may have been some rated at 350 MVA for use in generation switchboards, but I can't confirm that. With so much new 250MVA switchgear introduced at that time, I suspect that became the benchmark for the system.

If anyone has more accurate memories of those days, I'd like to hear their experiences.

Iain McKenzie

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iamck
IET » Energy » G59/2 Query

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