Losing excessive electricity

[Sinbad]

You are seeing readings of around 200V after loadshedding? Thats pretty crazy, for how long?

City power spec is 10% variance on 220V.
So anything between 198V and 242V is just fine according to them.

That being said, 99/100 when we get turned back on, I see 230V straight away. I check it, before I switch my mains back on. Too many neutral bars being nicked in our area during blackouts.

 

[mmacleod]

Starting to think I need a relay before the main switch that keeps the power off an extra 10 minutes after an outage or something, manually doing this is becoming a pain, any of our electricians care to comment on legal possibilities for that?

 

[thehuman]

Check ac/dc catalogue they have delay timers . Not sure about current rating

 

[savage]

Check ac/dc catalogue they have delay timers . Not sure about current rating

More than likely only 15A - they are really small stuff. You can however use the delay timer to power a contactor, and put your load on the appropriate size contactor (i.e. 63A or whatever).

 

[Geoff.D]

VA (Volt-Amps) is not the same as Watts as has been pointed out. The difference is taken into account by the PF ( Power factor)

With the increase in use of CFLs, there IS a change in the PF for a typical home.
Any device with a transformer in it, a motor, or a compressor ( fridges and freezers) will not have a PF of 1.
Incandescent lamps as are heaters, geyser elements, kettles are all assumed to have a PF of 1.

In the past there was an assumption that a residential home would have a PF of 1 ( loads being predominantly resistive).
Anyone know what the new assumption is for a typical residential home?

Lastly, very old "meters" were not always able to take into account PF, but I am pretty sure the new ones and pre-paid meters do --- can anyone confirm this?

If they do, then what some are seeing or getting a feeling that it is happening, is that the meters are taking into account the changes in PF, giving readings that do not quite stack up. It does not necessarily mean the meter is faulty however.

So correlating the readings on a pre-paid meter with those taken directly on a geyser ( which would have a PF of 1) will only be possible if ALL other loads are disconnected during the measurement.

Voltage Tolerances:
In the good old days one could expect in most parts of the country, Voltage would be controlled at 220V +/- 10% (EXCEPT in PTA where the line voltage was 240V) and Frequency would be held at 50Hz +/- 0.5 Hz. Even today our line voltage in PTA is still at 235V (what the tolerance is anyone's guess). (I stopped measuring it because it was just too disturbing for words). Line voltage today in PTA East at 09:57 this morning is 237V.

A recent trip to Zimmerpan in Germiston where Eskom's control room is, showed that Frequency seems to be last thing on Eskom's priority list anymore, with deviations of more the 5 Hz being basically accepted as normal.

The effects on appliances such as fridges, freezers, washing machines is really damaging. Fancy entertainment systems etc are "supposed" to be more tolerant, but a visit to your nearest TV repair shop will show you just how many flat screen TVs are being repaired and the faults are just about always with the PSUs.
The DSTV decoders are terrible in this regard, they pop PSUs at the drop of a hat .
There is a lot to be said for using contactors with delay timers to prevent appliances from automatically coming on as soon as power is restored after LS. (This will allow the supply to stabilise first, eliminating switching transients, etc before you connect that R150000 TV to the mains).

In the short term, the only way to handle this is to deliberately switch off everything before LS and deliberately restore 10-15 minutes after LS. -----

 

[P924]

You only pay for real power as a residential user, so the power factor does not play a role in this instance.

 

[mmacleod]

More than likely only 15A - they are really small stuff. You can however use the delay timer to power a contactor, and put your load on the appropriate size contactor (i.e. 63A or whatever).

Yes, but is this allowable? i.e. Is it okay to have a mains that has a 15 minute delay from grid to come on, or are there some obscure rules that might prohibit such a thing?

 

[Geoff.D]

Yes, but is this allowable? i.e. Is it okay to have a mains that has a 15 minute delay from grid to come on, or are there some obscure rules that might prohibit such a thing?

No there is nothing to stop you from switching off your mains switch whenever you want to.

 

[mmacleod]

No there is nothing to stop you from switching off your mains switch whenever you want to.

Yes but we are talking about an automated system here, not me manually doing it myself

I could envision some or rule existing saying you can't have a delay because "people may not be able to tell if the grid is off" or some other obscure reason - just want to be sure before I persue a dead end

 

[Geoff.D]

Yes but we are talking about an automated system here, not me manually doing it myself

I could envision some or rule existing saying you can't have a delay because "people may not be able to tell if the grid is off" or some other obscure reason - just want to be sure before I persue a dead end

A contactor in its simplest form will drop out on a power failure and will not connect automatically until you the users push it back on.
A contactor with a delay timer will automatically reset after power returns. It is a standard product used in many applications and there is no reason why you would not be able to use them in your home as well as far as I am aware.

 

[Rkootknir]

A recent trip to Zimmerpan in Germiston where Eskom's control room is, showed that Frequency seems to be last thing on Eskom's priority list anymore, with deviations of more the 5 Hz being basically accepted as normal.

Your post is mostly sensible except for the quoted part. There is no way that frequency can vary this much. You're basically saying that generators without fancy switchgear \ syncronising equipment (pretty much all of them except Koeberg which spins at 1500 rpm standard) are changing speed from their standard 3000 rpm to either 2700 rpm or 3300 rpm on a regular basis - we would be experiencing much bigger problems if this was the case. 5 Hz frequency variation will break the grid & machines.

I would guess a variance of 0.5 Hz is closer to the reality - that leads to generator speeds from 2970 rpm to 3030 rpm which are probably manageable.

 

[Gnome]

I would guess a variance of 0.5 Hz is closer to the reality - that leads to generator speeds from 2970 rpm to 3030 rpm which are probably manageable.

Having had my oscilloscope on the grid power many times, the frequency is incredible well regulated. Sits at 49.7-49.9Hz with very little variation. When there is variation it seems to be depending on the time of day.

If there were constant fluctuation it would make it nearly impossible to synchronize power stations.

 

[EchoZA]

We were down to 48Hz earlier today, the same time we experienced a voltage drop one of our phases. It was enough to give the generator the ****s and pop fuses in one of the UPS's.

 

[Geoff.D]

Your post is mostly sensible except for the quoted part. There is no way that frequency can vary this much. You're basically saying that generators without fancy switchgear \ syncronising equipment (pretty much all of them except Koeberg which spins at 1500 rpm standard) are changing speed from their standard 3000 rpm to either 2700 rpm or 3300 rpm on a regular basis - we would be experiencing much bigger problems if this was the case. 5 Hz frequency variation will break the grid & machines.

I would guess a variance of 0.5 Hz is closer to the reality - that leads to generator speeds from 2970 rpm to 3030 rpm which are probably manageable.

The above is by observing a "meter" on the display board, that was showing the Frequency as being 55Hz and not 50 Hz, and on asking the question why the frequency was not 50 Hz, being told, " we no longer control the frequency to be 50 Hz +/- 0,5 Hz".

Real simple.

I am not arguing that it should not be controlled as it was, I am arguing that Eskom does not "seem" to bother all that much about frequency control anymore ------

And as attested below, it WILL cause all sort of problems with synchronisation ----
I do not have access to an oscilloscope at the moment, so I can't tell you what it is now in my area.

In the "old" days we all had electric clocks (not battery driven), all dependent on the frequency and it was quite easy to then tell if the frequency was correct or not.

Right now the Voltage in PTA east is sitting at 239 volts ------ (12:12).

 

[richjdavies]

We were down to 48Hz earlier today, the same time we experienced a voltage drop one of our phases. It was enough to give the generator the ****s and pop fuses in one of the UPS's.

That might be something much more 'local' than you realise. Probably not the whole grid, maybe just the final-leg from the transformer to you. (or indeed your measuring equipment!)

 

[Sinbad]

That might be something much more 'local' than you realise. Probably not the whole grid, maybe just the final-leg from the transformer to you. (or indeed your measuring equipment!)

How though? How is a transformer going to output a different frequency to input?

 

[EchoZA]

Were on a direct Eskom feed, and this is not the 1st time it's happened.

 

[Gnome]

I don't monitor the power with my oscilloscope 24/7 but I do turn it on every now and then, I'll let the scope capture some data and have a look.

Unfortunately I only have a 2 channel scope, so I can only measure 2 phases at a given time.

Would be interesting to see what the data shows.

 

[JohnHay]

I had a look through the logs of a UPS for the last 3 years and the lowest frequency recorded was 49.25 Hz and the highest 50.5 Hz. It only write an entry every 5 minutes, so there might be lower and higher ones that that was missed. In the 3 years it logged 49.25 6 times and 50.5 12 times. The UPS is an APC, but I do not know how accurate or stable its frequency measurement is.

 

[Geoff.D]

I had a look through the logs of a UPS for the last 3 years and the lowest frequency recorded was 49.25 Hz and the highest 50.5 Hz. It only write an entry every 5 minutes, so there might be lower and higher ones that that was missed. In the 3 years it logged 49.25 6 times and 50.5 12 times. The UPS is an APC, but I do not know how accurate or stable its frequency measurement is.

Pretty good values. Are you able to tell us what the voltage range was as well? Nothing to do with where you work I suppose?
Speaks for very good power regulation.
Geoff D