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Topic Title: Protection for a 400Hz Mobile Genset
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Created On: 05 December 2013 03:57 PM
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 05 December 2013 03:57 PM
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amulet

Posts: 4
Joined: 05 December 2013

Assuming I have a 75kVA, 204V, 400Hz, 0.8pf mobile generator with a 30m 4 core armoured cable to a 110V load system, is there still a need for RCD protection?

The load is operating on a 110V AC system. The voltage drop for a 400HZ system is higher due to skin effect.

The main question is if there is a need for a RCD in this case. The generator controller comes with a overcurrent protection, lightning arrestor, under frequency protection, over-frequency protection, under voltage and over voltage protection and others.

There will be a earth rod at both the generator and load system separately during deployment. Is it correct to state that in this case, this is a TT earth system?

In order to improve the safety of the personnel operating the system, is it advisable to:

1) Lay another CPC between the generator and load system 30m away. The idea is to change to a TNS system to ensure a good earth return as outfield deployment may yield differing earth resistance values at different sites. The 4 core existing armoured cable (to be laid on the ground) has a specified receptacle so unable to tap the armour as CPC.

2) Install a RCD to protect this armoured cable between the generator and load system.

3) Any experience with a 400Hz RCD? I am aware that a normal 50/60Hz RCD will not work on a 400Hz system.

Much appreciate your input and experiences in generator protection, if any.

Cheers!

Edited: 05 December 2013 at 11:32 PM by amulet
 05 December 2013 04:25 PM
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AJJewsbury

Posts: 11699
Joined: 13 August 2003

OK, silly questions first -

Assuming I have a 75kV, 204V, 400Hz, 0.8pf mobile generator with a 30m 4 core armoured cable to a 110V load system

Do you mean 75kA (rather than 75kV)?

204V generator and 110V load? Is there a transformer somewhere, or are we talking 3-phase generator and single phase star-connected load (sqrt-3 having gone a bit awry somewhere)? Or am I barking up the wrong tree....

There will be a earth rod at both the generator and load system separately during deployment. Is it correct to state that in this case, this is a TT earth system?

If there's a N-earth link at the generator then and the load is directly connected to the generator (no intervening transformer) then yes it would be TT, otherwise it it's just missing a N-earth link it could be IT with the generator's exposed-conductive-parts connected a separate electrode. Although I've no idea how ADS would work in such a TT system without an RCD (or insulation monitor or similar) though.

Lay another CPC between the generator and load system 30m away.

Anything wrong with using the existing armour? (isn't it already acting as a c.p.c. for the cable itself?)

- Andy.
 05 December 2013 05:20 PM
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AncientMariner

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Joined: 14 December 2004

400Hz suggests avionics to me?

Would be worth talking to those involved with ground power systems at airports etc to see what their equipment offers.

Certainly Googling aircraft ground power unit 400Hz brings up some similarly rated products.

Cheers!

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Clive S Carver GCGI IEng MIET
 05 December 2013 06:57 PM
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Jobbo

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Evening

I often work on 400Hz systems supplied by rotary gen-sets and generally RCD protection is specified as the majority of these units do not provide sufficient fault current. I normally use Houchin gen-set units as these are very reliable.

It is very important that the protection devices installed are suitable for 400Hz and that the characteristics of the devices are adjusted accordingly (@ 400Hz there is an increase in the magnetic tripping threshold). You will also find that the assemblies are not tested for use at 400Hz as there is no standard for the manufactures to test against (generally you need to get confirmation from the manufacturer for its use - they do this on a one to one basis. I would recommend Schneider and ABB for this)

For an RCD device I would suggest you use a Si type (iID RCCB)which are manufactured by Schneider - These will operate normally (although the test button will not operate) and Schneider will also provide you with verification of this - if you use other makes/type these will generally by unreliable. Note that the physiological effect of current at 400Hz is different by a ratio of 6, therefore a 180mA current will have the same effect to 30mA at 50Hz.

Regards

Jobbo
 05 December 2013 11:46 PM
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amulet

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Joined: 05 December 2013

Dear Andy,

This is actually a 75kVA mobile generator.

For a 400Hz system, the voltage drop is higher as compared to conventional systems. Hence the difference between the generated and load voltage.

As for using the armor as cpc, we not not practice it normally. We only use it as a supplementary cpc. Moreover the 4 cores are terminated into a receptacle which makes tapping the armor difficult. My intention is to add in a cpc between the generator and load 30m apart to keep them at equipotential.

However, without a RCD, i suppose this length of 30m armor cable is unprotected during times of a damage/fault and the overcurrent does not operate in time.

Cheers,
Amulet

Edited: 06 December 2013 at 12:11 AM by amulet
 06 December 2013 12:13 AM
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amulet

Posts: 4
Joined: 05 December 2013

Originally posted by: AncientMariner

400Hz suggests avionics to me?

Cheers!


Good day!

Yes, 400Hz systems are mainly used in aircrafts and ground systems at airports.

As this generator is of military spec to be supply radars, it make googling that a bit more difficult.

Thus I'm just asking for more opinion and common practices elsewhere in the world.
 06 December 2013 12:19 AM
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amulet

Posts: 4
Joined: 05 December 2013

Originally posted by: Jobbo

Evening



I often work on 400Hz systems supplied by rotary gen-sets and generally RCD protection is specified as the majority of these units do not provide sufficient fault current. I normally use Houchin gen-set units as these are very reliable.



It is very important that the protection devices installed are suitable for 400Hz and that the characteristics of the devices are adjusted accordingly (@ 400Hz there is an increase in the magnetic tripping threshold). You will also find that the assemblies are not tested for use at 400Hz as there is no standard for the manufactures to test against (generally you need to get confirmation from the manufacturer for its use - they do this on a one to one basis. I would recommend Schneider and ABB for this)



For an RCD device I would suggest you use a Si type (iID RCCB)which are manufactured by Schneider - These will operate normally (although the test button will not operate) and Schneider will also provide you with verification of this - if you use other makes/type these will generally by unreliable. Note that the physiological effect of current at 400Hz is different by a ratio of 6, therefore a 180mA current will have the same effect to 30mA at 50Hz.



Regards



Jobbo


Good day!

Yes sir, I have gathered the above from google. For a 115V system, the risk of electrocution is already lowered compared to a conventional 230V system. With a more tolerant physiological effect up to 180mA, I suppose the risk is further mitigated.

But yes, my personal feel is that a RCD will still be a further improvement for safety.

However, the worry of mal-tripping due to a sensitive setting is always a possibility and with the test button may/may not operate, it is difficult to check if this RCD is actually working fine which just further complicates the whole scenario.

Cheers,

Amulet
 06 December 2013 09:43 AM
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OMS

Posts: 19861
Joined: 23 March 2004

Whatever you end up doing, the armour should be solidly bonded to earth (ideally at both the gen set end and the load end). If, you then feel there is a further impedance problem then you can add an additional supplementary CPC.

Ideally, you would use a 5 core cable.

You will almost certainly require an RCD at a 75KVA set as you will effectively have less than 250A of fault current - so clearing anything above a 400Hz rated 32A MCB is going to be impossible - and you must ensure all the protection devices are approved for 400Hz use - they can perform very differently over the magnetic part of the device compared to 50Hz operation

You must ensure you get manufacturers data and approval on RCD's for use at 400Hz - it may require you to deploy a purpose built earth fault shunt trip or a schneider or ABB product that Jobbo mentioned above. Do not use a Type AC RCD, it will demonstrate excessive sensitivity to the point of being almost impossible to use.

We usually get the set supplier to provide the earth fault and overcurrent protection as part of the set supply contract - as mentioned Houchin are ideally placed for this.

I'm suprised on a Mil Spec unit, the earth fault protection isn't already included as part of the set protection given that it can be deployed in very variable ground conditions/locations anyway

Regards

OMS

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