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Low grid voltage


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Yesterday the street whatsapp group lit up with complaints about very low incoming grid voltage. Like > 190 V low. People were saying that their lights wouldn't work properly, that anything that heated via an element was taking longer. More generally they were worried about damage to their appliances.

City Power actually reacted quite quickly and went to inspect the substation feeding our street. They said (and I'm quoting 2nd hand here) that it was only the even numbered properties affected, and that the problem was a bent feeder cable.

I wasn't sure about the bit about the bent cable, but the voltage came up to about 215V.

Until later that evening when it got down into the 180s again. Then came up about 23:00 (+200).

This morning we sit at about 205 - 208 V AC, but I have reason to think that the voltage dropped a bit earlier this morning. I say this because it looks like my inverter disconnected from the grid for a while. The usual little "sips" of grid power that it usually takes throughout the night were greatly reduced, and battery SOC at 7:00 was lower than usual.

OK... so it seems to me that if voltage goes DOWN, then current must go UP if the load remains the same. This means that for keeping the same devices running, our circuit breakers are getting closer to tripping.

But some questions
1) I know that voltage IS important, but not why. Is it because of transformers that precede rectifiers, and expect a certain incoming voltage so that they can deliver the correct output DC. OK... that's for televisions and such. What about AC stuff like motors in appliances?

2) What can cause the voltage to drop? (I suspect I might regret asking this). It seems to me - on a small sample so far - that the voltage gets lower during Eskom's period of highest demand. Also I have repeatedly seen that when power is restored after a load shed, then voltage is initially low and climbs to something approaching 230 over 45 minutes or so. 

This is really about gathering information so that we can help City Power to help us.

TIA

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OK.... So City Power have been out twice to look at this. Yesterday they cut our street off for a couple of minutes whilst they did some work.

The situation persists, and it may be out of their control. We generally get 205 to 215 V during the day, but every evening it dips and then recovers. The timing seems to correspond with Eskom's reported time of heaviest usage.

I am going to monitor the voltage in another suburb (where my mother lives, and where she has an inverter box that displays the incoming voltage). That will be another clue, though I sense that City Power are now filing this under "first world problems".

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Possible to much load been pulled out of that circuit. The transformer in that minisub cannot handle the load.

They could possibly change the tap settings on the transformer, if they are not already at maximum.

Cable bend will never have such an effect. That is a big joke, lol

 

 

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10 minutes ago, Antonio de Sa said:

Possible to much load been pulled out of that circuit. The transformer in that minisub cannot handle the load.

They could possibly change the tap settings on the transformer, if they are not already at maximum.

Cable bend will never have such an effect. That is a big joke, lol

 

 

I raised an eyebrow at the bent cable explantion, but I did not hear it from City Power but from somebody who was quoting what they understood City Power to have done (and who assumed that they were actually addressing the low voltage problem), and so it has to be taken with a pinch of salt.

I have long suspected that there is something up on our little hoekie of the grid, with one or more properties demanding a lot of power. Often after a load shed our street is left in darkness whilst everybody else's power is restored. Or so it seems. Since I got my solar system, and since a couple of folks bought inverters, what we see is that the power is back on but the voltage is very low. Voltage slowly climbs after restoration.

This is why Eskom recommend that people turn off everything except light circuits on the DB during a load shed, and then turn them back on once power is restored. It's an inconvenience (especially if the load shed is 2am - but then who is awake to have a moan?), and we should not be happy with it, but it will reduce the problem of sub stations tripping because of a huge surge in demand, so it's a lesser of evils. This is a constant problem not far away from our street, but try to get those folks to turn off their geysers...

Two things
1) City Power have ripple switches installed (this is mandated) but I guess these are easily bypassed, or ignored when people add extra geysers.
2) Attitudes aren't always sensible. I know folks whose response to a load shed is to turn on EVERYTHING in their house when power is restored, just to demonstrate to Eskom that nobody is going to tell them when they can use power and how much.

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51 minutes ago, WannabeSolarSparky said:

That is code for "we have no [email protected]@ing clue what we are doing"

Low voltages are fridge killers :(

Well they did respond, and even knocked on somebody's door to say they would be looking into the matter. But when there are suburbs with no power for days, one street complaining about low voltage is probably not what they are most concerned with.

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PS: The larger than usual drop in my battery's SOC over night may have been due to a person who decided to use "free electricity" to run an electric blanket. The problem is confusing "from the sun" with "unlimited". This is a battle I find myself fighting over and over again. The concept of "finity" seems very hard for some to grasp.

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Their are actually a few reasons this can happen.

Load to high for Transformer/Minisub to handle.

Cable not able to handle the load.

Phase loss on the 11KV side.

Internal winding fault on transformer(Doubt there protection will pick it up if any is still working.)

Tap changer fault inside transformer.

Loose connection on HT or LV side.

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That makes sense, but a burned cable would possibly have a dead shorth circuit, that would trip the breaker at the minisub, in my humble opinion low voltage means that there is not enough " Juice" to supply the load.  the transformer at the minisub for that specific suburb cannot cope. 

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Many reasons given but due to ageing network this low voltage could come all the way from the 88/132kV network. All the transformers will have their own tap changers.

Further due to the state of the network the voltage could also be lowered in order to draw lower amps from all the equipment in use. I am not indicating it is done but can be used as a tool in peak times when more heating=resistive loads are connected while preparing meals and heating of water.

I would also take a flyer and say that about all the ripple controllers are no longer in use. This is the case in Tshwane. We all paid to have them fitted and they out of order.

These ripple relays can easy save Eskom about 2 stages if all Munics that have them fitted were to do the repairs and have them actively used during LS. Yes most are bypassed or just set to be off(permanent on for geyser) and will stay off unless a carrier is sent to put them back in use. This is one of the 10 plans to lower LS if there was a will to improve the supply of power. Not rocket science.

 

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Just thinking out loud. Since everyone and his neighbour seems to have installed an inverter, I wonder what's the demand for battery charging directly after a round of loadshedding. One or two or more x 1C batteries at 5kW charge rate each, times by how many houses in the street?

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6 minutes ago, GreenFields said:

Just thinking out loud. Since everyone and his neighbour seems to have installed an inverter, I wonder what's the demand for battery charging directly after a round of loadshedding. One or two or more x 1C batteries at 5kW charge rate each, times by how many houses in the street?

Not sure in your area but the penetration of PV systems is still very low. May be less than 5%. Yes a lot of batteries are charged but also some might only be charged from PV the next day. 5kw inverters do not use more power than say a oven and geyser. Items that might be on if LS ends during meal preparing times in the evening.

Edited by Scorp007
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