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Switched mode power supplies and grid instability

 
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fuzzy



Joined: 29 Nov 2013
Posts: 593
Location: The Marches, UK

PostPosted: Fri May 22, 2015 10:50 am    Post subject: Switched mode power supplies and grid instability Reply with quote

Since we have replaced our resistive loads [filament bulbs, heating etc] with switching loads [gadgets, LEDs etc] any supply voltage dips will have a much stronger destabilising effect on the grid, so supply stability is ever more critical. I saw this in action with large backup supplies in datacentres where those paid the big bucks couldn't work out why you couldn't changeover heavy switching loads that were supposedly within the nameplate ratings of the emergency systems.

Edit by admin, this post and those following has been split from an existing thread about a likely shortage of generating capacity. The original thread may be found here http://www.powerswitch.org.uk/forum/viewtopic.php?t=22392
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kenneal - lagger
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Location: Newbury, Berkshire

PostPosted: Fri May 22, 2015 3:18 pm    Post subject: Reply with quote

We have to watch this with our off grid genny supply and also from the battery/inverter supply. We can't start our saw table when we have a lot of other stuff on, say the washing machine, although if I spin the saw blade first and then switch on the load kick is much less.
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adam2
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PostPosted: Mon Jun 01, 2015 1:28 pm    Post subject: Reply with quote

Switched mode power supplies (SMPS) are now in very general use and have a number of advantages over older technologies including greater efficiency, lower weight and bulk, and the ability to accept a very wide range of input voltages.
Apart from the obvious application of transforming AC mains supplies into much lower DC voltages for electronics, variable speed motor drives and electronic lamp ballasts are also types of SMPS.

Unfortunately widespread application of SMPSs is liable to contribute to instability of grid systems under fault conditions.
Under steady state conditions, SMPS are a load like any other and all should be well, they are after all, in very general use and the lights have not gone out.
Consider however what happens in the event of say a cable fault in a grid system. A short circuit or gross overload may persist for a second or two before fuses or circuit breakers open.
During the fault, consumers nearby will receive a much reduced voltage for a second or two. In the past, with resistive loads, the current drawn would also reduce until the fault was cleared.
Now however with a lot of SMPS, during a voltage dip the current drawn might double, thereby causing a further drop in the voltage. This will increase the stress on the system, perhaps leading to otherwise unaffected consumers receiving a reduced voltage and drawing more current.

What would once have been a localised cable fault, perhaps blacking out a dozen consumers, and subjecting a few hundred consumers to a brief voltage dip, can now escalate to a wider problem.

This is NOT THE SAME as a shortage of generating capacity, it can occur when margins seem ample.
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PS_RalphW



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PostPosted: Mon Jun 01, 2015 2:32 pm    Post subject: Reply with quote

So you are saying that SMPS provide a positive feedback on current fluctuations, and if the percentage of demand from SMPS type loads on a given circuit gets too large, it can overwhelm the negative feedback provided by resistive loads, leading to runaway over or under voltage.

This is a bit like the problem with grid stability on the supply side caused by intermittent renewable sources, only potentially worse.

The two together are even harder to solve.
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Catweazle



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PostPosted: Mon Jun 01, 2015 7:46 pm    Post subject: Reply with quote

PS_RalphW wrote:
So you are saying that SMPS provide a positive feedback on current fluctuations,


It's true. SMPS will try to deliver the required output power, if the input voltage drops they will take more current to keep the power constant. Many are rated to run from approx 90 - 260 volts, so they won't just trip out when mains voltage is low.
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Pepperman



Joined: 10 Oct 2010
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PostPosted: Tue Jun 02, 2015 12:06 pm    Post subject: Reply with quote

Surely they'll only try and deliver current up to their maximum rated amps won't they? That's probably typically less than 50% more than their average current and they're found on comparatively low power devices (not washing machines, kettles, showers etc) so I'd be surprised if you'd see major spikes.

Would be interesting to see any research that's out there on this though. Anyone got any links?
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adam2
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PostPosted: Tue Jun 02, 2015 12:29 pm    Post subject: Reply with quote

Pepperman wrote:
Surely they'll only try and deliver current up to their maximum rated amps won't they? That's probably typically less than 50% more than their average current and they're found on comparatively low power devices (not washing machines, kettles, showers etc) so I'd be surprised if you'd see major spikes.

Would be interesting to see any research that's out there on this though. Anyone got any links?


Yes, the SMPS will only try to deliver up to it rated amps, but the input amps will rise substantialy if the supply voltage be low.
Consider as an example a SMPS with a rated output of 10 amps at 12 volts DC. With a 240 volt AC input it will draw about half an amp.
Now suppose that the supply voltage briefly drops to 100 volts as a result of a fault.
The input current will now more than double to well over an amp, and if many such SMPS are in use a severe grid disturbance could result.

Whilst most SMPS are indeed small in wattage, they tend to be long hour loads such as lighting and IT equipment that are used throughout the working day, rather say a kettle or shower that is used only briefly.
More and more washing machines BTW, DO use a SMPS to drive the motor.
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