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Discuss Ze of Generator in the UK Electrical Forum area at ElectriciansForums.net
Since no one else is biting...I don't think I'm quite understanding the question...and don't think I'm completely alone in this.
Are we talking a generator wired as a TN-S setup, with one end of windings (neutral) earthed at the generator? Or is it some form of centre-tapped earth?
What is making this different from measuring Ze on a normal supply?
Is the 7.5 you mention the Zs@Db in ohms? Is this an 120v supply?
Or is the focus of the question that you are unable to disconnect loads to determine it?
Sorry for loads of questions, I'm hoping answering a couple of them will facilitate more responses for you!
This is probably a terminology thing - in my mind the term Ze meant the impedance of any external earth electrode and earthing conductor up to the point the distribution starts in the building. Is that not a fixed resistance whatever the generator is doing?
Or maybe I'm still missing the point of the question!
View attachment 85842
I understand now. So it comes down to whether the generator can generate sufficient fault current to trip the MCB in required time without stalling / frying, and as Lucien pointed out there are three states to consider.
This web page is a maybe a helpful starting point: Calculating the short-circuit current across the terminals of a synchronous generator - https://tinyurl.com/cbtsxvn9
As I think you've now realised, in simple terms you'll never achieve final circuit ADS times on a generator, hence reliance on RCD's for protection instead. The reason is simply down to the mechanical reaction time of the generator itself and the larger the set then the worse the problem, unless you happen to be running a chain of sets so high already that Ipfc = In. You need upstream electronic protection devices to interrupt the current flow, not simply the supposed Zs on a downstream MCB.
As I think you've now realised, in simple terms you'll never achieve final circuit ADS times on a generator, hence reliance on RCD's for protection instead. The reason is simply down to the mechanical reaction time of the generator itself and the larger the set then the worse the problem, unless you happen to be running a chain of sets so high already that Ipfc = In. You need upstream electronic protection devices to interrupt the current flow, not simply the supposed Zs on a downstream MCB.
Am I correct to say that a 400 amp breaker will trip for a fault within 5 seconds before a separately excited permanent magnet synchronous generator drops its excitation or shuts down?
It's not just about the generator actively changing the excitation, but also about the demagnetising effect of the fault current which in turn depends on the fault current pf. If low, the output winding reaction MMF more strongly demagnetises the machine with rapid reduction of terminal voltage during the transient period and also reduction of engine load. So it can maintain speed and deliver low real power in a kind of 'folded-back' state, which might occur while you are still waiting for the breaker to trip. Obviously the pf depends on the winding L and R and tends to decrease with increasing machine rating,
I don't have practical experience of speccing this size of unit for utility replacement duty and wouldn't like to call your particular situation. I would go from the actual curves of the specific machine and breaker rather than using any generalised multipliers.
It will, but whether it makes a significant difference is less clear.Regarding PF, does the jX of the external conductor or fault loop make a difference?
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