Ze of Generator

[Cookie]

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.

Picture it like this:

A 400kw unit has an output current of 1,110 amps. If the generator can export at least 300% current for 10 seconds then 1,110 amps x 3 = 3330 amps. It takes about 8 times the handle rating to trip a breaker in 5 seconds- so 3,330 / 8 = 416 amps. Thus we know that the maximum breaker that we can have the in emergency (life safety) switchboard is 400 amps.

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?


Maybe an example drawing will help clear things up (370kw is the prime rating, 400kw standby rating):

 

[Cookie]

From the site specs:

400REOZJC, 60 Hz | Industrial Diesel Generators | KOHLER

kohlerpower.com

 

[Lucien Nunes]

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.

 

[Cookie]

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.

You are big help none the less You know more about this stuff than I do.

Regarding PF, does the jX of the external conductor or fault loop make a difference?

 

[pc1966]

Regarding PF, does the jX of the external conductor or fault loop make a difference?

It will, but whether it makes a significant difference is less clear.

Your example 400 kcmill supply cable is a little bigger than our 185mm (for which I have a table handy) where the cable Z is about 20% higher than R due to inductance, where as the 4/0 AWG cables are a little bigger than our 95mm size where the difference is around 5%.

With large cables, transformers, etc, it is always best to run the calculations using the full information of X & R values and then see what comes out, even if in reality something like a 5% difference is likely lost in the uncertainty of fault magnitude, etc.

 

[Lucien Nunes]

The machine's own resistance and reactance would normally dominate for a bolted fault at the main panel. The stator reaction depends on the magnitude and phase of the current w.r.t the EMF, not the terminal voltage. The difference between the two is no longer nominal nor hidden behind the AVR.

 

[Cookie]

It will, but whether it makes a significant difference is less clear.

Your example 400 kcmill supply cable is a little bigger than our 185mm (for which I have a table handy) where the cable Z is about 20% higher than R due to inductance, where as the 4/0 AWG cables are a little bigger than our 95mm size where the difference is around 5%.

With large cables, transformers, etc, it is always best to run the calculations using the full information of X & R values and then see what comes out, even if in reality something like a 5% difference is likely lost in the uncertainty of fault magnitude, etc.

Alright and agree.

However, what R & X to I assume for the generator windings when doing the equation to get the total Zs? And what voltage? I know this is a dynamic 4-D scenario, but I want to keep it simple (if practicable) to a 2-D set of equations.

 

[Cookie]

The machine's own resistance and reactance would normally dominate for a bolted fault at the main panel. The stator reaction depends on the magnitude and phase of the current w.r.t the EMF, not the terminal voltage. The difference between the two is no longer nominal nor hidden behind the AVR.

As I am reading up on it, and I could be wrong, the kw on the prime mover actually drops due to the R going down and the X going up. The current is reactive, and as the voltage goes down the field is "forced" to its max excitation to keep voltage up.

 

[Cookie]

As I am reading up on it, and I could be wrong, the kw on the prime mover actually drops due to the R going down and the X going up. The current is reactive, and as the voltage goes down the field is "forced" to its max excitation to keep voltage up.

Anyone know? R goes up as shirt circuits move away from the generator.