Mains line inductance limits SMPS inrush current?

[eem2am]

Hello,

We have bought this offline switch mode power supply.

SMPS; LPS205-M ; Emerson Netwrok Power.....
**broken link removed**

You can see on the first page of its datasheet that the 'switch-ON' inrush current can be up to 50 Amps.

......The thing is, we plug this supply in to the mains in Cambridge, UK....
-our nearest power station is Little Barford Power Station which is 20 miles away.
...this of course means that this 50 Amps of inrush current comes along 20 miles of power transmission cable.

-even though there are step down transformers in the path, which make the upstream cable inductance appear smaller than it really is, i would still expect 20 miles of power transmission cable to have a large inductance.

So.....

1. Can you estimate what the line inductance is from Little Barford Power Station to Cambridge?

2. Isn't 50 Amps of inrush current far too much to expect?, since the 20 miles of line inductance would never allow it to get this high?

 

[DXNewcastle]

What a great question!

Of course the 50 Amps figure is the manufacturer's worst-case current intended to assist you in selecting appropriate fuses or MCBs. In practice I can see no reason why your supply cannot provide 50Amps, either instantaneously or continuously, unless the distribution arrangements limit that current through protection devices. When the night storeage heaters here come on at night or the plant next door is powered up by day, they draw much more than 50amps and all that happens is that there is a visible dimming of the lights. Its brief, because its not that more voltage is 'lost' due to the increased current through a finite resistance, but that the local transformers are adequately designed to respond to much higher demands than that.
In fact the routine application of Power Factor correction circuits probably applies a more challenging load.

However, I don't believe that inductance (even 20 miles of it!) would be enough of an impediment to 50 Amps of a mere 50Hz. But how to measure it? [Usual warning about not performing these experiments at home and not making connections to the mains supply which are hazardous or outwith the terms of a consumer's agreement with a supply company go here]. Well I guess you could apply a signal of several frequencies from a suitable low-impedance-output power amplifier via a suitable capacitor and then measure and plot the attenuation (either the attenuation across the capacitor or across a current sensing resistor). But this would only give you an indication of the 'lumped' inductance and capacitance of the entire distribution network at that instant in time, including all other consumers' installations.
To measure the point to point impedance would require access to the network at both ends, and again, the reading would be skewed by the 'lumped' inductance, and capacitance, of all the distribution network, but at least it would give you a figure to start from. You would then estimate the total fan out of the network and adjust, though I'm struggling to see how we can then isolate YOUR inductance from everyone elses' capacitance.

But I don't believe that even your interesting and hopefully hypothetical question adequately covers the supply network's response to your connection of a high load. Little Barford's turbines will be running in close synchronisation with the other turbines across the national grid, and any increase in local demand will have a barely perceptable impact on the rotational speed of those turbines, knocking them behind the reference phasing of the grid (by a tiny fraction of a degree), which in turn, will increase the proportion of energy drawn from the rest of grid, until the burners at Little Barford have been opened up to increase the pressure on the blades and thereby bring the turbines back in sync and up to full power.

Have you ever been using significant amounts of power when the local supply has been re-connected after a power outage? There would be a lot more than 50Amps being pulled locally! Or what about the impact of some of the electric trains running in the area. When they suddenly enter your local power network they slam over a MegaWatt onto your network. They don't seem to cause much local effect, do they?

 

[Orson Cart]

50 amps may be the peak current based on 265 or 275VAC, and an NTC inrush initial temp of 0 degC, the inductance of the supply line seen looking into your wall plug is sufficently low at 50Hz that this peak (fraction of a half cycle with smaller peaks over the next few half cycles) can be supplied without much of an effect on the supply system, don't forget that in stepping down the local voltage, the supply inductance is reduced by turns ratio squared...

Regards, Orson Cart.

 

[FvM]

There may be cases, where the grid impedance will considerable reduce inrush currents, e.g. with long overland lines after the last transformer, as it can be found on the countryside. In most uptown places, a grid short circuit capability of several kA would be expectable.

You are basically able to estimate the total Z from voltage drop of loaded grid and the L from voltage harmonics observed with harmonic currents.

 

[eem2am]

Thankyou for the great answers!

I notice that you each have spoken of the inrush current in a way that suggests that you believe that there is a 50Hz frequency involved with it.

..though the inrush surge current is more like a spike of current lasting about 100us or less.

..if mains power happens to be switched on at the mains peak then the inrush into the SMPS input capacitors will be very high.......................unless the line inductance forbids it.............

...the thing is that this "spike" of current has very high frequency harmonics, and these will surely see high inductance in the incoming line?

..though i see your point DXNewcastle about the difficulties of measuring it with the other customers capacitance there etc


...and though i see your point about the high current loads that you kindly mentioned

 

[FvM]

..though the inrush surge current is more like a spike of current lasting about 100us or less.

The inrush current waveform will be generated when precharging the primary side storage capacitor to the input peak voltage. I would expect rather 1 to 2 ms than < 100 us transient duration. It's true of course, that the grid inductance effect can be expected higher than for the 50 Hz fundamental. I guess, the 50A inrush current specification is a maximum value, that will be achieved with a low impedance grid, so the actual peak current can be lower.

 

[eem2am]

...actually i was am talking about say if the on/off switch is pressed at the instant of the mains peak................

-then you suddenly have 330V placed across the input DC bus capacitance (which are discharged to 0V), and their will be a huge inrush current

-the only thing to stand against it being the resistance of any NTC's (if present), the ESR of the DC Bus Caps, and of course, the line inductance.

...the line iinductance, and its effect in staunching this initial switch-on inrush current, is what i am interested in here.

 

[Orson Cart]

You can pretty much discount the line inductance as a means to limit inrush current, psu's up to 300W or so mainly use NTC's to limit the inrush to lelvels tolerable by the bridge rectifier and main storage caps, above 300W (or so) it is more usual to use a PTC or straight resistor which are then shorted by a relay when the bus caps get to a certain voltage, in this way the inrush can be limited to an amp or so, latching relays are used in some low standby psu's.
Regards, Orson Cart.

 

[FvM]

the only thing to stand against it being the resistance of any NTC's (if present), the ESR of the DC Bus Caps, and of course, the line inductance.

Not correct for an active PFC supply, where the storage capacitor is charged through the boost inductor. The input filter capacitor will be a few uF maximum.

 

[eem2am]

yes , i agree, the boost PFC input stage is a special case for inrush current....because at switch on, there is the situation of vast surging resonations (both voltage and current) due to the resonance between the boost inductor and the boost PFC output capacitor.
-thus with boost PFC stages, it is normal to put a diode across the boost inductor (anode to mains connector) so that the boost inductor is "shorted out" by this diode during the inrush event.

anyway, i still think that in a typical large , domestic housing estate, where there is no industrys around, that the line inductance could be a few miles of cable which could be significant.

Domestic houses do not comprise equipment which puts significant amounts of capacitance on the mains phases.

...so the inductive effect of miles of cable will be felt?

 

[FvM]

...so the inductive effect of miles of cable will be felt?

Yes, surely. But I won't expect the grid impedance to be much higher than the IEC 725 reference value for 230V/50 Hz power distribution system of 0.4 + j 0.25Ω (the reactance corresponds to 0.8 mH).

 

[DXNewcastle]

. . . . anyway, i still think that in a typical large , domestic housing estate, where there is no industrys around, that the line inductance could be a few miles of cable which could be significant.

Domestic houses do not comprise equipment which puts significant amounts of capacitance on the mains phases.

...so the inductive effect of miles of cable will be felt?

In that context I guess you might be correct.
If it helps, the only actual figures I'm aware of apply to Power Factor correction, in which banked capacitors are switched in/out of circuit (measured in KVAR) between the phases with typical equivalent capacitance values of 300 - 3000uF each.
These are, of course, deliberately introduced capacitance in addition of the reactive nature of the distribution network; simple wide-area domestic supplies do add to the 'lumped' network capacitance (due to the large fan-out of low current or unused wiring), and there is generally more capacitance in underground distribution than overground.
I regret that I don't have any figures for inductance but it certainly will be non-zero!

 

[Orson Cart]

Even if the line inductance was as high as 1mH, this still represents an ability for current to ramp to ~325amps in 1mS (for peak mains), for a house drawing maximum demand of 50amps AC say, this will present an inductive volt drop of 15V - which is a bit on the high side. So for more realistic inductances at 1mH or below there is very little inrush limiting effect of the mains line inductance (which is as it should be for an efficient distribution system).
Regards, Orson Cart.

 

[FvM]

Your calculation shows, that the IEC 725 assumed impedance is quite reasonable. For the high voltage feeder, a pure reactant impedance is usually assumed. The resistive part in the specification is apparently considering limited cross section of 16 A single phase wiring, but won't be measured with this amount near the switch board.

The said 15V inductive voltage drop won't be seen by a real load, however.

By charging a 220 uF capacitor through a 0.8 mH inductance, you get an about 1 ms 120 A current peak - and a huge overvoltage, if the capacitor is low ESR (0.5 ohm).

 

[chuckey]

0.4 + j 0.25Ω = Z of .47 Ω, now my house is fed via a 100A fuse so does this mean that at full load the mains to the house drops by 47V? Also domestic switchgear is rated at 3KA, so if .47Ω is a typical impedance (I ~500A), why?. I think that the mains has a substantially lower impedance, more like .15 Ω .
Frank

 

[FvM]

I didn't read the IEC 725 text, I understand it as specification for a 16A circuit. As I mentioned before, the real impedance can be expected lower, the reactive part would be O.K. for higher load currents.