Power-cuts/Loadshedding - looking for some answers

[noob_saibot]

Hello there,

The following is to anybody that is involved with power-generation and/or electricity (most likely aimed at heavy-current electrical engineers).

When Eskom declares a state of emergency, they normally say that they need to do this due to not having reserve capacity (meaning that they always like to keep an extra 2-5% "in reserve"). They then use load-shedding as a means of cutting electricity consumption.

Some of my questions are:

By implementing load-shedding, is Eskom basically cutting demand so that they can continue generating electricity at the same levels, but without increased demand (which allows them to build up a reserve of sorts)?

How is this "reserve" stored?

Based on the structure of the electricity grid, can the end-user of the grid reverse the process and become a supplier to the grid?

 

[Sinbad]

Hello there,

The following is to anybody that is involved with power-generation and/or electricity (most likely aimed at heavy-current electrical engineers).

When Eskom declares a state of emergency, they normally say that they need to do this due to not having reserve capacity (meaning that they always like to keep an extra 2-5% "in reserve"). They then use load-shedding as a means of cutting electricity consumption.

Some of my questions are:

By implementing load-shedding, is Eskom basically cutting demand so that they can continue generating electricity at the same levels, but without increased demand (which allows them to build up a reserve of sorts)?

How is this "reserve" stored?

Based on the structure of the electricity grid, can the end-user of the grid reverse the process and become a supplier to the grid?

The reserve isn't stored, it just means their generation equipment is not running at 100% capacity.
Yes, grid tie is possible, but not many municipalities have implemented the framework needed for this. I mean joburg's billing is in enough of a mess already, imagine adding a 4-quadrant meter to the works.

 

[noob_saibot]

The reserve isn't stored, it just means their generation equipment is not running at 100% capacity.
Yes, grid tie is possible, but not many municipalities have implemented the framework needed for this. I mean joburg's billing is in enough of a mess already, imagine adding a 4-quadrant meter to the works.

So what you're saying is that even if people reduce consumption during off-peak times, it doesn't matter, as Eskom works on a scale-up/scale-down methodology? In other words, even if we all cut our consumption by 10% during off-peak times, it won't matter, as their power-plants won't be able to handle peak demand(5pm-9pm)?

I wonder if they are using this though:

https://en.wikipedia.org/wiki/Grid_energy_storage

 

[MickZA]

Years ago Eskom were using pumped storage from the Vaal dam to another upstream (forget the name), don't know if this is still the case.

 

[Denyed]

There are various control loops that adjust the power output of a generator. These respond on demand to any change in load. The coal plants ramp relatively slowly so they use ocgts and pumped storage to handle the rapidly changing load demands and peaks.

All of the in service power stations that aren't down for maintenance can generate about 35GW when running at 100%. If this capacity is exceeded the frequency will drop and the grid will become unstable as supply doesn't meet demand. Once this happens they will load shed to prevent a black out of the entire power system which would take weeks to restore from.

 

[Sinbad]

So what you're saying is that even if people reduce consumption during off-peak times, it doesn't matter, as Eskom works on a scale-up/scale-down methodology? In other words, even if we all cut our consumption by 10% during off-peak times, it won't matter, as their power-plants won't be able to handle peak demand(5pm-9pm)?

I wonder if they are using this though:

https://en.wikipedia.org/wiki/Grid_energy_storage

Correct, the problem is the peak. Usage off peak is completely irrelevant.
They do use pumped storage but it's nowhere near enough to fill the gap.

 

[Scooby_Doo]

There are various control loops that adjust the power output of a generator. These respond on demand to any change in load. The coal plants ramp relatively slowly so they use ocgts and pumped storage to handle the rapidly changing load demands and peaks.

All of the in service power stations that aren't down for maintenance can generate about 35GW when running at 100%. If this capacity is exceeded the frequency will drop and the grid will become unstable as supply doesn't meet demand. Once this happens they will load shed to prevent a black out of the entire power system which would take weeks to restore from.

How does this work? Do the generators just trip off or what? Why would it take so long to power back up?

 

[Denyed]

How does this work? Do the generators just trip off or what? Why would it take so long to power back up?

There is under frequency protection to prevent damage to the generating units.

Power stations require about 40MW of power to start up a unit and only a few units are black start capable. Eskom does a black start test every few years where they use the pumped storage in the Drakensberg to start up a unit in mpumalanga.

The grid needs to be restored slowly as more units get connected and the load must be increased to match the generation. This means switching the grid on section by section.

 

[Franna]

Was thinking the other day: some mines have been down for 5 months since the strike started. If there is already not enough power generated what will happen will all the mines come online again...

 

[Denyed]

I also realise how much the DA must lie on other topics when they say they will fix Eskom and reduce the electricity price at the same time. Its the one topic I am pretty knowledgeable on and I can tell you that fix Eskom and reduce the electricity price cannot really be spoken in the same sentence. Truthfully at least

 

[Scooby_Doo]

There is under frequency protection to prevent damage to the generating units.

Power stations require about 40MW of power to start up a unit and only a few units are black start capable. Eskom does a black start test every few years where they use the pumped storage in the Drakensberg to start up a unit in mpumalanga.

The grid needs to be restored slowly as more units get connected and the load must be increased to match the generation. This means switching the grid on section by section.

How would lower frequency affect the generators if they were forced to stay on or did not have safety measures?

 

[Denyed]

How would lower frequency affect the generators if they were forced to stay on or did not have safety measures?

The low frequency is caused by generators not being able to meet demand. Operating them at more than 100% of their rating will damage them. The under frequency protection prevents billions of rand in damage. Rather trip the plant and cause a blackout than destroy equipment.

 

[supersunbird]

Hello there,

The following is to anybody that is involved with power-generation and/or electricity (most likely aimed at heavy-current electrical engineers).

When Eskom declares a state of emergency, they normally say that they need to do this due to not having reserve capacity (meaning that they always like to keep an extra 2-5% "in reserve"). They then use load-shedding as a means of cutting electricity consumption.

Some of my questions are:

By implementing load-shedding, is Eskom basically cutting demand so that they can continue generating electricity at the same levels, but without increased demand (which allows them to build up a reserve of sorts)?

How is this "reserve" stored?

Based on the structure of the electricity grid, can the end-user of the grid reverse the process and become a supplier to the grid?

The load shedding is to keep the grid up and running. If there was no load shedding the grid would crash and everyone would have 0% power. Most of the power stations need electricity to start up, while a handful can start themselves (due to having large diesel generators?). So if the grid goes totally down, big areas would have days or even more than a week without power.

 

[HavocXphere]

<<Not a elec expert - some I'd love to hear corrections where applicable>>

No reserve in the sense of what you're thinking.

Essentially supply must equal demand at all times. If it does not then they still supply everyone with power...but the frequency departs from the 50Hz we need and then really bad things happen (e.g. computers die etc).

So usually they keep a bit of spare supply capacity in the form of power they can ramp up at will (open cycle turbines, pumped storage)...stuff that can kick in fast if needed.

We don't have any spare capacity left...so the only option to keep the frequency stable is to forcefully get rid of demand. And thats why you're sitting in the dark.

If they don't do that then the power stations and the grid are no longer at the same frequency & the connection trips disconnecting that plant from the grid. That in turn causing a bit of a chain reaction with the frequency getting progressively worse. Thats what they mean with the grid will collapse. Fixing that requires weeks because most power plants can't "start" themselves...they pull power from the grid to kickstart. Now if the grid is dead you can see how thats an issue. That means couple of plants that can self-start (via gas turbines) need to be started and then start the others in turn. Hence the country being dark for weeks.

Pretty sure the plants / grid has a way to "waste" excess power though I don't know how.

 

[Space_Chief]

So has a two tier pricing system been considered/tested? Price of electricity is reduced between 22h00 and 04:00 or something along those lines? If you want to heat up your geyser you turn it on at that time or install a small water heater to take instantly heated showers at 0:00, instead of 6:00? Maybe run your pool pump at midnight and do any baking/grilling late at night?

 

[noob_saibot]

I'm just wondering why they never considered using some type of storage as an interim/mid-term solution, while they built up capital to build newer plants.

Perhaps the cost is the biggest issue, but if they considered using this mid-term method:

People could literally switch off a lot of power-consuming things in their homes (like geysers) during the day and late at night, which would build up a massive stored capacity, which can be used first during the peak time, so that they always "remain ahead" of the demand.

This capacity for stored electricity could also be used for 100s of years, whenever they need to make upgrades. Just by increasing capacity before an upgrade and implementing some reduction strategy (like making people switch off their major energy-consuming products), that stored capacity will always keep them in a load-shedding-free capacity.

Again though, I suspect costs and possible energy loss through mega-storage are some of the reasons why they haven't implemented the storage idea.

I see this as an important thing not for the general consumer, who can live without lights for a little while, but for critical infrastructure like hospitals.

Perhaps we should all consider the reality of a solar geyser. Not only for us, but for the people that really need the energy (imagine ICU at a public hospital without electricity )

 

[HavocXphere]

So has a two tier pricing system been considered/tested? Price of electricity is reduced between 22h00 and 04:00 or something along those lines?

Already in force at some commercial 3 phase systems...its implemented in a pretty harsh way though. If you draw too much juice in any given day during peak then you overall rate goes up...i.e. it acts as a multiplier on your entire bill.

Residential...they're busy installing "smart" prepaid meters that can do this type of thing. Pretty sure the fake ripple tech thread we had the other day is also being actively trialed.

 

[chicken247]

I want to be part of the solution. I want to rent out my roof for solar to an enterprising company. I want options like that, that are available in other countries. I am tired of feeling like a hobbo every winter. I wanna buy enough shares in eskom to offset my monthly bill. Privatize it already.

 

[HavocXphere]

I'm just wondering why they never considered using some type of storage as an interim/mid-term solution

Cost. Storing energy on the scale need for a national grid is stupidly expensive. You can store a bit to help things a bit for peak but thats about it.

http://en.wikipedia.org/wiki/Ingula_Pumped_Storage_Scheme

^^ 1.3GW pumped storage. Thats massive by storage standards...and still only like 3% of grid capacity. You need to push in a couple times that 1.3GW in energy during off peak just to have that 1.3GW available during peak. i.e. You pump the water up...then it flows down through the turbines...and in that process you lost most of the energy due to inefficiencies (even with good tech).

Storing energy on a mass scale is a mission and I'm not aware of any "good" solutions.

 

[HavocXphere]

I want to be part of the solution. I want to rent out my roof for solar

Solar is the headache SA is going to be facing in 5 years time. Hopefully politicians can see this....preferably before its too late this time round.

Germany recently manage 50%+ of their grid on solar. Sounds good, right? Yes it is...until the sun goes down. Now you've got a grid that just lost 50% of its generation capacity within hours. Dealing with that from a national keeping the grid stable perspective is a major headache...to the point where some countries are just refusing to connect solar in suburbs that are loaded with solar already. Plus the entire grid is set up for energy to flow from plant to house not other way round.

Privatize it already.

Please don't. Its bad enough that these government okes think their mission in life is to make a profit instead of keeping the lights on. Privatize it and you can expect 100% worse.