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Topic Title: Ze of Generator
Topic Summary: Ze of Generator
Created On: 08 June 2017 11:27 AM
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 08 June 2017 11:27 AM
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bream0504

Posts: 5
Joined: 04 May 2017

I am currently calculating a new temporary installation design, and I'm having trouble with the Ze of the Generator, used to find the Zs of each circuit, Zs=Ze+(R1+R2).

I have tested both the 32A and 16A 230v O/P socket of the Genny using a Metrix 4900-MOD Installation tester, and have found the Ze to be 1.6ohm.

This in turn puts all of my ZsMAX calculations out due to 32A and 16A CB's having ZsMAX values of 0.68ohm and 1.37ohm according to P.58 of the 17th Edition.

My question is, am I doing the test wrong? I've noticed there is a ZsLOOP and ZsLINE test function, but not currently able to access the kit to check which setting this was on, and if this would effect the test. Also, I was not able to null the leads before test, so took the resistance measurement of the Line and Earth test leads, but only being 0.2ohm, did not reduce the Ze enough to pass the requirement of the CB's ZsMAX value.

I am pretty sure my cable CSA and Length are suitable, for example a 32A 230v socket has a current draw of 10.43A on 4mm cable at 5m in length. I've calculated this at 0.061ohm R1+R2.
 08 June 2017 12:33 PM
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di515223

Posts: 337
Joined: 08 July 2010

I would suspect that the generator will not have a low enough Ze to operate your ADS as its PSC will likely be low enough to be less than the instantaneous triip level of a 32A breaker.
What is the output power of the Genny? I suspect it is too small to do what you are intending.
As an aside, what is the output breaker on the genny - I suspect that is what will trip in the case of an issue, rather than any of the downstream protection. Discrimination may well be non existant.

Dave
 08 June 2017 12:44 PM
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AJJewsbury

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Joined: 13 August 2003

Small generators will naturally have a reasonably large "Ze" - you simply can't get thousands (or even hundreds) of amps to flow when the windings are driven from a relatively puny internal combustion engine. Even your measured Zs reading might be optimistic as it'll assume that the source will behave linearly (i.e. it will test at 10A or 25A and come up with a resistance using V=IR, and you're assuming that the same apparent resistance will apply when much larger fault currents try to flow - whereas with a small generator the mechanical side of things will get in the way and it will more likely see a collapse in voltage and probably stall the engine).

The usual answer is to use an RCD for ADS when using a small generator (or RLV).

- Andy.
 08 June 2017 01:12 PM
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bream0504

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The generator is a Stephill SSD20KvA, the overall design will only draw around 6kW of power and the Generator can supply 15.2kW on the 230v side. It is quite a large generator in size, and the engine seems to be a fair size as well.

I myself am used to there being a seperate CB on each O/P, but this one seems to have a C25 and a C80, with 2x 100mA rcd's. From looking at the diagram it seems to show that the 230v side, which consists of 2x 32A and 1x 16A, is protected by the 80A CB.

I am redesigning a setup that has been used for some years now, and looking at previous calculatikns, can see that the apparent Ze should be 0.2ohms. This is why i came to the conclusion it was my test techniques rather than the genny?
 08 June 2017 01:16 PM
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bream0504

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I should mention there is a 3phase 63A O/P and another 16A 230v, as well as 3x 32A and 1x 16A 110v O/P. This caused confusion as to how all of the sockets are protected?
 08 June 2017 04:20 PM
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davezawadi

Posts: 3848
Joined: 26 June 2002

This may all get somewhat complex, I will attempt to explain.
It is not possible to get a proper reading of Zs with a generator as you describe. Your meter applies a load for a very short period (much less than a half cycle) and measures the change in output voltage. Normally the generator automatic voltage regulator (AVR) compensates for voltage change with load, but cannot respond to such load changes as made by the meter. Also unloaded the apparent Zs will be much higher than you expect because there will be very little excitation due to the AVR. A 20 kVA generator should trip a 32A type B breaker quite quickly off the AVR measured phase but maybe not the other two (depending on alternator design). If you think about it, your measured 1.6 ohm cannot be correct as it suggests a 50 volt drop on loading, which you will not observe. You can make an estimate by measuring the winding resistance, probably less than 0.5 ohms. As the supply is TN-S (with neutral - earth link somewhere) you should use an RCD to supply ADS for earth faults, and a suitable earth electrode to keep you somewhere ground potential.
Beware, the generator wiring may not be as you expect, most will provide 3 phase with one set of connections and single phase at full kVA output with another set. Yours may even have 3 options to give 110V too. How all the connectors are wired may not be important to you as long as you get the single phase output you require. Do check that an RCD operates with a simulated fault to real earth when all connected up. You are probably worrying about the exact design far too much. Overload should be controlled by checking the current meter when everything is working, although you will stall the engine if you go too far, but it is possible to damage the alternator with long term small overload (say 20%). Any MCBs are probably more a case of convenience as to not loosing all supplies than a very effective protection against overload, which is rather different to mains supplies.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 09 June 2017 04:49 PM
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kenelmh

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Hi,

The 'Ze' of a generator will be dominated by reactance, so this is where you need to look in terms of defining the external impedance (internal to the alternator).

Principally the equation is:

Isc = FLC / Xd
Xd being the reactance factor.

There is sub-transient and transient reactance and manufacturers will quote factors for each by which you can calculate the fault current at the alternator terminals. Calculating from Sub-T will give you the very short term peak fault current to allow you to check the withstand of the switchgear (probably not an issue with the size of your genset) and the transient will give you the longer fault level that may be used to define your Zs via Ohms law. I would not be surprised if it was so high that ADS will not work and an RCD would then be a must.
 09 June 2017 05:42 PM
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williamjohn

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David
The short circuit current of a generator is even more complicated than this. The initial rms value of a three phase short ciurcuit is typically 7 times the full load current. But this armature current demagnetises the generator so the fault current quickly decays. At constant excitation, the short circuit current may fall to a value below the full load current. However the AVR tries to prevent the decay. How effective this is depends on the characteristics of the AVR and field circuit. The fall in magnetisation causes the reactance of the armature winding to increase from the saturated to towards the unsaturated value. Furthermore the fault current contains a dc component that depends on the instant in the ac cycle that the fault occurs. In the worst case it can nearly double the peak value of the fault current but it usually decays in a few cycles. A further complication is the transformer effect of the armature and field coils which share the same magnetic circuit. The analysis of a single phase fault current is usually done by symetrical components. The zero sequence reactance involves a magnetic circuit through the end covers so can bear little resemblence to the positive or negative sequence reactances. A full analysis is usually required for a generator of tens of MW but is hardly required for a generator of 20 kVA.

I agree, David, with your suggestion that you just try and see if the RCD operates when switched onto a short circuit. But you may have to do it more than once in case the RCD has been tripped by the dc component. The dc component may or may not be present on an actual fault.

WilliamJohn, MA, Ceng, MIEE, MIMechE
 09 June 2017 05:42 PM
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davezawadi

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Ken whilst it might be nice to have this data, it is difficult to get for small alternators. I think he will find my method and suggestions adequate. As I suggest overload protection is tricky, and RCD protection is the best way to deal with earth faults as you also say.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 09 June 2017 11:32 PM
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mapj1

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Depending on how the generator output voltage is stabilised, there is more than one right answer anyway, as although it looks like the supply impedance is high, once the load has been there a while the regulator will wind the output up - so over a long term test give a very low impedance. Machines with a brushed drive to an armature that is in effect a permanent magnet run at constant speed. Machines that are in induction motor being pushed have variable slip between physical speed, and output frequency, that is the AC frequency flowing in the core.
If the genset is a brushed machine and lightly loaded and you short it as the motor will be running starved of fuel it will likely lock and stall.
IF the machine is running hard and diesel is flowing like wine at a party, then you may lock the genset, but the engine keeps going. In good designs there is a shear pin as a deliberate weak link.
I don't recommend a short circuit test on such a machine.
On an induction type if you short the output, the load is removed from the generator and the engine races away until the governor pulls it back down to earth.

As David says, you are better assuming the circuit breakers are for overloads and only need to operate in slow time, and the fast ADS is RCD related. So check the NE bond is fitted.

-------------------------
regards Mike
 10 June 2017 09:56 AM
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davezawadi

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The alternator in the class of machine under discussion is often brushless but fully synchronous, just to make life difficult. For example Stamford alternators (a major player) have two sections, there being a DC generator with field on the stator driving the alternator rotor field, so that we can change the excitation of the DC machine to change the AC machine excitation on the rotor! It also has the advantage that less power is needed from the AVR (the DC machine doing the heavy lifting) and there are no brushes or slip rings to wear out. Small alternators may work as above (expensive) but synchronous with the engine speed so frequency control is possible, or be the asynchronous type which is basically a driven induction motor which has slip to provide the rotating field power. Accurate frequency control is not possible as slip varies with load. Accurate speed control is necessary for parallel operation and for loads where the frequency matters, and some machines have crystal frequency control to 1 ppm or so and genlock for television and film lighting. It is true that these requirements are changing somewhat with electronic lighting ballasts but is still needed to synchonise multiple machines or to the mains supply.

The engine power available is normally not much greater than the electrical output rating for economic reasons, and there may be other ways to protect the engine, for example if the AVR detects zero output voltage (short circuit conditions) it removes the excitation; the result being that no current is available to trip any circuit breakers or fuses! There may be more subtle feedback too, with output current transformers to monitor overload, and allow the output voltage to be reduced to limit the maximum load current.

The best guide to normal load conditions are the output voltage current, voltage and frequency meters, which will give an immediate diagnosis of what is happening to the machine. Normal subcircuit OPD will work fine, but at maximum load levels it is very difficult for all the above reasons.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 11 June 2017 04:42 PM
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kenelmh

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Hi,
Understood but what should be used when recording Zs's for the circuits fed off of the generator? Clearly they would be less than 1667ohms etc so if fed via 30mA RCDs then all is compliant with respect to ADS but I would still expect a test sheet with accurate results. Is there a way of achieving that without some estimation or accurate measurement of the Ze?
 11 June 2017 07:55 PM
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davezawadi

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A measure of R1 + R2 will give you the a result to put on the results sheet. The Zs is as you say not very important. A suitable column is provided on the model form. The value of generator internal impedance measured several posts below above is not really relevant as described.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 12 June 2017 04:37 PM
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williamjohn

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David
On a large generator, the armature resistance is two orders of magnitude lower than the subtransient armature reactance. Hence the initial value of the fault current is E/Xd" where Xd" is typically 15%, hence the approximate initial value is 7 times full load. On a 20kVA generator the resistance is likely to be of similar order to the reactance, but the reactance cannot be ignored. Measurement of the armature reactance depends on the rotor position and on the saturation of the magnetic circuit . The rotor position giving the highest value is approximately Xq" and the lowest value is approximately Xd". Both Xq" and Xd" have saturated and unsaturated values.

As I said above, there is flux linkage between the field winding and the armature winding. A lagging armature current opposes the field and the current decays as the emf decays. (A leading armature current would boost the field). The short circuit current can theoretically fall to a value below full load current but this depends on the characteristics of the machine.

I am surprised that protection can cut the field if the voltage falls to zero as the only excitation will then be by the armature current. This sounds like an unstable situation. An increase in armature current will lead to an increase in excitation.

Andy
With a large generator, a short circuit is near zero power factor so it does not overload the prime mover. If the generator is on load, a short circuit will cause the generator to speed up as the system load is lost due to the low voltage. With a 20 kVA generator, the resistance is much higher and the prime mover may be overloaded by a short circuit but can you be sure about this.

I do not see an easy way to calculate the short circuit current on a 20 kVA generator.

WilliamJohn
PS Do not forget the dc component which may or may not be present on a short circuit depending on the position in the ac cycle that the short circuit occurs.
 12 June 2017 06:03 PM
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davezawadi

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WJ, whilst I am not going to disagree with your comments, I am afraid that my experience says that the biggest snag with smaller generators (<1MVA) is that they cannot supply anything like 7 times FLC even into a reasonable load such as a large motor with a similar rating to the alternator or a large incandescent lighting load. Both are likely to give the engine extreme difficulties or stall completely, as can short circuits under some circumstances. In all these cases the engine is not fueled to provide the necessary torque, and the sudden load change instantly reduces RPM to an unsustainable level. This situation is entirely different to a test set where the alternator is driven by an electric motor from the mains, where very large overload power is instantly available. Such loads can be started from generators with dimmers or motor drives as there is little excess current requirement.

Clearly subcircuit protection devices are likely to operate correctly as there is a large excess power available, and a short on a fully loaded generator will probably open a CPD correctly, contrary to expectations! All this makes understanding of the real situation quite difficult, and the machine design rules (such as the reactance of windings being 7 times the load current by rule of thumb) rather less usable. My post should enable the OP to obtain the properties he needs from his machine, as the full analysis is very complex, and needs loads of unavailable data, such as the flywheel momentum value and engine torque / speed characteristics. Generators tend to have very large flywheels compared with other engine applications, specifically to ameliorate this power deficit for surges, but it all depends on the exact design and machine cost.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 12 June 2017 11:22 PM
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williamjohn

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David
I think we agree. The 7xFLC is the initial (ie subtransient) value of the short circuit current on a large generator (eg 10 MVA). The armature resistance of a 10 MVA generator is negligible and therefore the short circuit current is at nearly zero power factor. The armature impedance of a 20 kVA generator will contain significant resistance and significant reactance. The impedance including resistance will be more than 15%. The power factor of a short circuit current will not be nearly zero and the prime mover may struggle. I think however that you need to consider the various reactances as well as R1 + R2 when assessing the impedance. The synchronous reactance can be as high as 200%.

To calculate the subtransient, the transient and the final steady state (usually called synchronous) current would require numerous measurements of the machine parameters and is hardly practicable.

WilliamJohn

Edited: 13 June 2017 at 10:18 AM by williamjohn
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