I’ve just bought a plot of land over near Capri (De Oude Weg), and intend to build a house on it.
One of the things I’d like to do is install an underwater heating system (hydronic heating) for the house. I almost installed one in my apartment in Shanghai a few years ago, and regret not doing it, especially when I visited friends who had done it!
Underfloor heating has literally been around for thousands of years, so its not a new option.
As the romans and greeks noted, heating is best situated at the ground level, as heat rises, and the area’s that we live in will stay warm.
So, thats what I intend to implement.
While I don’t even have foundations in place, or even architect’s plans, let alone any council bits done, I have already been looking at getting the underfloor bits ready, so I can order and ship a container over. Priorities!
Underfloor heating is best planned and implemented before/during a house build, so now is the perfect time for it from my perspective.
Underfloor heating essentially means running piping in a loop under the flooring.
The piping comes back to a central location to a manifold, and hot water (or cold water if you want to cool a house, eg in summer) is piped through the manifold to each room.
NB – Its better to run parallel loops to increase heat efficiency, rather than a long in / out loop.
This piping can be laid directly into the foundation slab, or on top of it, usually with insulation above the slab.
There are pro’s and con’s to both methods.
With the piping in the foundation, the concrete gets heated, and keeps its heat for a long period. The concrete holds the heat and gives it out much like a stone holds the heat in the sun.
European installs typically do underfloor heating
with a 50mm overlay concrete with insulation between the overlay and the slab (insulation above the slab).
This has three major advantages:
- Repair or replacement of the pipes is easier. With the pipes in the slab replacement is pretty much impossible.
- The floor temperature can be easier adjusted to changing conditions and less heat loss.
- Other services can be accommodated in the floor.
Interestingly its important to insulate the sides of the floor also, more heat escapes that way than up, as heat escapes to where the resistance is least.
This can be mitigated somewhat by laying the piping away from the exterior walls.
I’m probably going to use ICF (Insulated Concrete Formwork) for the outer foundation walls, with a concrete mix of 20 mpa, 100 slump, 13mm aggregate for the foundation and walls.
This is where the walls have an inner and outer styrofoam layer and the concrete is poured inside.
It provides great insulation and sound proofing, and speeds up build, plus its greener. Win/win!
I’ll then lay the pipe in the foundation or above a insulation layer above the foundation (as per the diagram above) in a parallel loop using PEX conduit routed to a central location.
or using a mesh
Each room will have a different pipe conduit (probably using 100M lengths) back to the central location (aka maintenance closet), this will allow me to use 20cm spacing (works out to roughly 5M of pipe per M2 of room).
Some calculations are available here for those interested – http://www.heatweb.com/techtips/Underfloor/underfloorheating.html
I’ll update more when I have shopped more!
As Eskom has again gone down the load shedding route, and it looks like there won’t be much respite until Medupi et al come online, I thought I’d look at how to take my grid-tied setup to the next level, and incorporate off-grid + batteries so that we have power when the rest of the suburb does not.
I haven’t done that so far, as thats by far the most expensive part of going off-grid!
A flash file explaining a possible setup below:
There are a number of issues to deal with first.
Currently we have a grid-tied PV inverter. This expects power to be present before it supplies power from the solar panels to our house (G83 / G59 standards as per NERSA specs etc). When power goes out from Eskom, then the inverter cuts off as well (by design). What that effectively means, is that when Eskom goes off, we go off, even though daytime we could still be running off the panels.
In order to prevent that happening, I need a battery inverter system in front of the grid-tied inverter, that can automatically switch over to battery when Eskom goes offline.
Then, when Eskom goes offline, it kicks in immediately to provide power, and the grid-tied inverter doesn’t go off. Sounds simple enough, but there are a few wrinkles.
Eskom obviously doesn’t want you feeding back into their grid when its off. With the above setup, there is nothing stopping the battery inverter system from feeding back out . Thats a no-no!
This is important to follow for 2 reasons.
First, and most importantly – safety. If a lineman is doing maintenance, its better not to try to electrocute them!
Second, you don’t want your house setup to be attempting to supply the neighbourhood with power when Eskom is offline, as its quickly going to run out!
So, we need another device to sit in front of the meter that will turn off a relay to stop power going outbound if Eskom has no power, and turn it back on when Eskom is back online to allow it again. Those devices are typically called ATS’s (Automatic Transfer Switches), and are useful to prevent anti-islanding (feeding back into the grid when the grid is off).
To further complicate things, I have a 3 phase system.
Luckily ATS’s come in 3 phase varieties, and aren’t super expensive.
Having 3 phase means I need to consider whether I want a full 3 phase setup, or I can wing it with a battery inverter setup on 1 phase only to provide emergency power.
If I go the 3 phase route, then I can charge the system from solar even when Eskom is down, and I’ll have longer runtimes. This is obviously the best choice from a provisioning perspective. Costwise – not so much!
There is yet another issue though (aren’t there always!) – the battery inverter needs to be able to stop charging the batteries when they’re full.
If Eskom is off, and the battery inverter is running, and its daytime, our PV excess will be charging the battery / inverter. That needs to turn off charging to the batteries somehow when full, so that they don’t spontaneously combust. Some solutions do things like changing the frequency to the PV inverter so that it shuts off. The battery inverter then solely runs the house, and then that drains the batteries till they need more charge, and the battery inverter syncs again to provide power to the PV inverter, so that it kicks in and starts generating, and this lovely inefficient cycle of retardedness rinse / repeats ad nauseum stressing the system. I’d like to avoid that sort of solution, as it makes me, and the equipment unhappy.
That issue is described in much better detail here.
If I go single phase, then I can’t charge the system from solar when Eskom is down as the PV inverter will also be off, but I can keep one phase alive and running from batteries until Eskom deigns to do its job.
Single phase diagram (assume my inverter is 3 phase, image is snarfed from here)
Choices Choices Choices…
Obviously I would like to go the 3 phase route, but that means 3 x the cost in inverters, as there don’t seem to be 3 phase inverter chargers at the consumer level in the market yet! (pocket goes ouch, and thats before I even look at batteries!).
Those who’ve read this far, may be wondering why I didn’t get a hybrid grid-tied inverter / charger in the first place, instead of sticking more stuff in front of my existing setup.
2 main reasons.
Cost, and uh cost.
Seriously – the 3 phase hybrid solutions are in the R400K and up range!
Now I’ve explained some of the pitfalls, I’ll look at some of the solutions (to be expanded as I continue my research)
Comap Mainspro. $$ – This comes in a 3 phase and single phase switch setup, and can switch over automatically to prevent outage. I’d use one of these to prevent Eskom getting power, and also to switch inputs when the power goes off before any of our devices noticed there was an issue
ATS4BC0100. $ – This comes in a 3 phase switch setup (or single), and can be programmed to switch over as appropriate to inverter or mains to remove Eskom similar to the Comap.
GIS Control Module. ? – Fully automated integration of any Grid Tie Inverter System with any Battery Inverter System.Automated Load Dumping and Load Isolation based on battery charge state and solar array gain. Battery charge level monitoring and protection.
Magnum’s MS-PAE does AC coupled grid tie, but they do the aforementioned sillyness in turning off the PV inverter when their battery setup is full.
Studer Extender series +
Some info on what City of Cape Town has been doing regarding feeding back into the grid, for single phase users (hint, not much). As for three phase – nothing..
Info on why net generators get charged a base fee (NERSA!), why council has been dragging their feet on renewables, feeding back, and other issues (by the very helpful Brian Jones) –
Brian Jones_Challenges to get RE going in municipalities_CoCT
Latest SSEG application form Grid tied form SSEG
The system has been running nicely for the last 3 months without issue, and we’ve generated a little over 2MW so far!
We still don’t have all the panels on the roof – only the 16 ones we put up last year!
Quite impressed with the yield, and the inverter.
I did have one issue the other day though – the inverter fan came on late afternoon (which is unusual), and I could hear it working overtime.
As thats strange, I plugged in a computer, and took a look at the stats.
Our Eskom side was actually under supplying, and our inverter was working overtime to keep it running smoothly. I noted the issue, and went on with my day.
Guess what was in the news that evening – Eskom declares power emergency!
Was rather cool to troubleshoot an issue back to the Electrical provider, and find out I was right 🙂
Mine has been running for a week now, albeit with only half the panels mounted and installed, as I didn’t have time to mount the rest yet this visit – I’m back in Shanghai, China again now..
It finally went live on the 14th. Its working out well though, although I am getting slightly less output than I expected – panels are in theory 4800W total for 16 panels x 300w, but looks like they’re really 250w panels, as we get about 3.9x KW peak off the 16 we have mounted at the moment after inverter losses etc.
Everything survived the massive storm that hit Cape Town last weekend too, so that was a relief!
Still need to do paperwork for council approval, and arrange a new digital 4 quadrant Landis & Gyr meter so I can eventually “feed into the grid” as a SSEG (similar to Arthur), but we’re generating electricity on a separate circuit in the interim, and our power is down to pretty much zero usage daytime.
The Landis & Gyr people are a pleasure to deal with too. They answer questions, and are quite helpful. Thanks again to Arthur for their details!
Although the whole install so far was fairly painless given that it was all DIY, I did have one issue.
My DC switch for the panel side decided to fry itself almost immediately, and lose its magic smoke (black and stinky that it was). It was only there as an extra safety precaution, so it wasn’t a huge problem, I just wired MC4 connectors to the cable from the roof (after safely disconnecting from the roof), then plugged directly into the inverter.
I think the DC switch was just bad from the factory, and the inverter does have an off/on switch for DC, so it wasn’t a calamity.
Other than that oh faaark moment, its all been great.
I need to do an update on the blog to show current working status, then its the fun part of documenting all of it, getting council signoff’s, paying more money to get a bidirectional smart 3 phase meter (+-R4k with gprs and ethernet) , and becoming a small scale provider! (or not, depending on what the base charges will turn out to be).
Panels – approx R1600 / panel x 30
Inverter – approx R20,000
DC, AC Cabling, Mounting – approx R5600
Distribution panel side + 3 phase, 1 phase, dc switches etc – approx R500
Total – about 75k
I did get dinged for storage charges for 20k due to incompetence at freight forwarder, and clearance was expensive too, as the freight forwarder *****ed me on that too, that came to about 40-50k for that portion of the shipment, but I did have other stuff in the container, so its hard to calculate it out.
Assuming I use 40k for shipping, clearance, and taxes (no duties on panels or inverters), then looking at about R115,000
Installation took us 2 days (3-4 hours of work a day for 2 people), and there will be some ancillary costs for electrical signoff, and other paperwork bits n bobs, and of course a new meter, so for my 3 phase setup, total will probably be about R120,000 all in*
*At todays RMB-> Rand rate. I did buy most of it when the RMB was at R8.5 or so, but we are at 10.2 ish again, so used 1-> 1.6 for rmb->rand values.
Its grid-tied, and although I can go off-grid completely would probably cost me another R100,000 to do so at current prices in China for equipment + batteries.
Still, I think its a fair investment, as electricity is only going to go up in price in future, and if / when I do move, I can take it all with me! (Or sell it to the next buyer!)
I’ve learned from the experience though, and will probably be up for doing it again, as it was quite painless aside from the freight company royally *****ing me. There is a fair amount of interest in smaller (single phase) systems from everyone thats seen it, and I can put together a 4k + single phase inverter setup for reasonable prices for family+friends in future, and hopefully make a bit of cash doing it!
If you recall from my last update, I unpacked the Solar Panel crate, and moved the panels to the back garden. It does no good having a pile of panels in the garden, they need to be mounted!
Mounting is probably one of the lesser discussed area’s of installing Solar. Its almost an afterthought for most people, although it’s just as important as the rest of the system.
There are a few area’s of concern for mounting – first one is can your roof sustain the extra weight.
30 panels and mounting brackets will add another 850kg of weight onto the roof, albeit spread out over a large area. We had a look inside the roof, took some photos, and spoke to a structural engineer friend first – his take was that ours is an older victorian house, and as is quite common for older houses, its built reasonably well from substantial materials.
Essentially, its been quite substantially over-specced, and has more than enough beams for weight distribution, so there won’t be an issue.
Another major concern is wind. In Cape Town, its not uncommon to get extreme wind conditions – we’re not called the Cape of Storms for nothing! Any system used, needs to be sufficiently strong to withstand gale force winds on a semi regular basis, annually.
Our local conditions dictate that mounting needs to be extremely strong, as a flying panel can and *will* cause substantial damage. Surprisingly, given this, there are no real laid out Solar installation requirements, unlike other parts of the install, its quite unregulated.
There are no substantive national standards for mounting compliance, or local ones, other than the requirement that things mounted on the roof can stick out no more than 600mm. I asked City of Cape Town what their rules are and received this:
All PV roof top installations: No building plans are required to be submitted provided the panel(s) in its installed position does not project more
than 1,5 metres, measured perpendicularly, above the roof and/or not more than 600mm above the highest point of the roof.
This is quite permissive, and looks like its aimed more at Solar Heating, rather than Solar PV.
Our panels will be flat mounted on the roof at a total height of about 100mm (including panel), so we’re well within compliance.
I did a bit of research on some options, and went with something thats German designed, but produced in China, from a company called NiceSolar.
Nice Solar has a number of different mounting choices for various roof types – The first choice of what you’ll use is dictated by what type of roof you have; in our case, its a galvanised steel/zinc sheeting, as opposed to roof tile.
The mounting system I chose mounts directly into the roof beams via screws, and is composed of aluminium mounting brackets. Its reasonably well thought out, and simple to install. We had most of the bracket mounting done in 2-3 hours, with just 2 people, and that included carrying everything upstairs onto the roof, and the usual going up and down to get the extra tools that you need. Finding the roof beams was quite easy from the roof – we just had to follow the existing screws holding the sheets in place, and mount accordingly.
Initially when I opened the box of component parts shipped for the mounting system, I was worried, it all looked horribly complicated – lots of different pieces.
Turned out to be quite simple though.
My system uses L shaped brackets to mount into the roof, then an aluminium mounting channel is screwed onto 3 brackets. Panels then sit on top of the mount and are held in place with a T piece or a C piece for ends. Mounting channels are joined together with a sliding clamp system.
Below is a shot of a T piece, and the L shaped bracket in a mounting channel
This is what the L shaped bracket looks like close up
My system needs 3 x L brackets per mounting channel, so the first thing we did was to setup a dummy channel with L brackets screwed in for sizing, and mark out our holes for drilling. You can see both in the shot below:
Unsuprisingly, this was the longest part of the job, as we had to think about and plan where the panels would go due to Chimneys, Skylights, Solar Hot water Heating and other obstructions getting in the way of a clean easy install.
Once we decided where things would go we went pretty fast.
You can see some of the L brackets already mounted below:
Once the L brackets were on, the next step was to mount the channels.
This is where my choice of system came in handy – mounting was a breeze!
Below is a channel waiting to be screwed into the mounts
Its obvious that some thought went into the design, as there are some design concepts that integrate together cleanly, and ensure both a strong connection, and ease of mounting.
The L mounts, and the mounting channels are corrugated so that you get a tight fit, and the L brackets mounting screws angle into the channel from any location and lock into place.
Below is a shot of one half of the install team in his farmer hat, busy fixing a channel onto the L mounts. Don’t forget the importance of protection from the sun when you’re on a roof!
Here’s the other half –
Once mounted, each channel was joined together with a slide in joiner bracket. This was a little fiddly, as some channels got slightly damaged in shipping and needed coaxing with a screw driver and pliers before we could slide them on. Luckily only 2 or 3 channels were affected. It was extremely minor damage though, and didn’t take us longer than a minute a channel to resolve.
Eventually, we had our mounting done, and had lines of mounting ready to roll
As you can see, we managed to drag one panel up onto the roof to test mounting.
I recommend involving additional friends and family if possible!
We brought our tools and equipment down, adjourned till the next day when we could rope in some rather reticent workforce, and continue.
The next day involved most of the hard labour – we had to carry 15 panels up onto our lower roof area, then from there up onto the roof.
Once we had a panel on the roof, we mounted it immediately using the T pieces, and things went rather rapidly.
T Piece, waiting for 2 panels –
Took another 3 hours to get to this point
You can see the roof line with the panels mounted below. We made the executive decision to mount the end flush with the mounting, as we had concerns about leaving a lip for the wind to get under. The way its mounted should negate that issue for the highest area’s. Cape Town has substantial wind conditions, and this needed to be addressed.
We’ll be inspecting the mounting in a day or so, then in a few weeks to ensure that the mounting is still secure, and there are no issues. Today has been fairly windy, but I’ll be on the roof tomorrow for final inspection, and hookup.
We’re still not live, as I haven’t hooked up the panels yet.
For safety reasons.
The panels haven’t been connected together, as they shouldn’t be connected while live.
As they’re live when there’s sun, I’ll need to go up at dusk or early evening to connect up.
I also still need to run DC cables to the roof from the Solar Board.
The astute among you will have noted that there are only 16 panels on the roof currently.
The other side of the roof is at an extreme angle, and we’ll need to hire safety harnesses, and possibly a scaffold. Due to that, I’ll be hooking those up at a later date, so will only have half the system up and running this week.
I’ll be running the panels in 2 x 15 sets (strings).
Each string will be running at 562.5v @ 8A for a total of 4500W per string.
(Panels are 37.5v @ 8A / 300W each nominal voltage)
My inverter has MPPT inputs for 2 separate strings, so this works out nicely.
A voltage of 562v is also under its maximum of 1000v per string.
If everything goes well today, and I get time to install the cabling, and hookup the panels early evening, I’ll be able to test the system on Tuesday morning, when the sun rises.
The inverter won’t power up until it see’s at least 300v DC from its inputs, so I’ll need to wait till about 6am when there is sufficient sun to see my creation come to life!
Many thanks to my cousin Joel for his assistance in mounting all of this on the roof, to Wesley for taking a look at the cabling to double check I haven’t done anything silly, and to Angie & my brother Jerome for their assistance in getting the panels on the roof.
As my system is 3 Phase, I thought I’d talk about some of the different 3 Phase standards for wiring.
3 Phase, as you may or may not know, is better for transmission of power.
Power stations generate electricity at 22 000 volts (3 phase 50Hz). To transmit this power over long distances, Eskom steps up the power to to the following voltages for transmission: 220kV; 275kV; 400kV or 765kV. This electricity now goes into our national grid.
When it gets to the end user it is stepped down. This could be 11kV for a large factory or 400V(380V) for shops/homes. If you take a phase to neutral (single phase voltage) i.e. 400V/sqrt(3) you will get 230V single phase @ 50Hz.
When it gets to the house, it generally gets split up into single phase, and different circuits get each phase. So, the lights might be on one phase, the plugs on another, and heavy equipment may use all three (eg an old 1950’s Oven dating back to the Union of South Africa!).
Plug sockets at home are single phase 230VAC 50Hz.
There are two main connection standards for 3 phase; Delta wiring – which uses 3 wires, and Y wiring (also known as wYe), which uses 4.
Delta has one wire for each phase so 3 wires total.
Y wiring has one wire for each phase, plus a neutral, for 4 wires.
In my house, we have 4 wires, so its the Y standard.
You can check this by looking at how many wires go from the house to the street.
In our case, this is 4 separate wires, as you can see below:
So, what exactly is 3 phase?
AC current runs in a sine wave. This sine wave runs at 50hz for South Africa (50 ups and downs a second). In 3 phase, each phase is run at an offset of 120 degrees, so each phase peaks at an offset of the other. The 3 phases add up to a total of 380v – although these days is more likely to be closer to 400v, as the rest of the world has migrated to that. Either makes no difference, as they’re both values within the margin of error for provision of electricity.
Its extremely important to know what you have with regards to wiring, as it involves large amp circuits, and you don’t want to wire things incorrectly and cause Eskom to come smack you for tripping the street transformer!
Three phase is relatively easy to turn back into 1 phase – I’ll be doing that on one leg to provide an additional “solar” circuit for our laundry room.
Those of you who like the technical aspects should take a look here –
Some basic calculations for 3 phase below. These can be used to work out your maximum load or other important wiring details, like how thick your electrical wire should be if you’re carrying whatever max current your supply provides…
Basic electrical calculation:
Volts = Watt ÷ Amps
Volts = Ampere x Ohms
Amps = Volts ÷ Ohms
Amps = Watt ÷ Volts
Ohms = Volts ÷ Amps
Volt-Ampere (VA or Watt) = Volts x Ampere
For 3 phase, we need to use the square root of 3 for our calculations as an additional factor.
The square root of 3 is 1.73 (rounded off to 2 digits).
So, to calculate VA its: Volts x Ampere x 1.73
KVA = (Volts x Ampere x 1.73) ÷ 1000
= (400 volts x 60 amps x 1.73) ÷ 1000
= 41520 ÷ 1000
= 41.52 KVA
We can also work out the Amps as below:
KVA = (Volts x Ampere x 1.73) ÷ 1000
41.52 KVA = (400 volts x A x 1.73) ÷ 1000
41.52 x 1000 = 400 x A x 1.73
41520 ÷ (400 x 1.73) = Amps
41520 ÷ 692 = 60 Amps
If we wanted to convert KVA to KW, we need to use a power factor (this represents losses in transmission). The figure used for this is typically 0.85, so
KW ÷ 0.85 = KVA
KVA x 0.85 = KW
A more detailed explanation of power factor losses is here – http://www.energyaction.com.au/australian-energy-market/power-factor.html
As its been a while (understatement!), I thought I’d update on the progress.
Well, I finally shipped my container of goodies all the way from China to SA, encountering a few issues on the way. A groot fok jou goes to DN Freight / Temoore Freight for being complete doos’s – Shawn Patience, Elize Werner you know exactly what I’m talking about; managing to screw up, get a charge of an extra R20k+ in Storage fee’s, *then* having the audacity to try bill another R10k on top for fee’s *already paid* in Shanghai. The sea freight pretty much turned into air freight pricing… Grrr.
They tried to cover it up too, then backtracked and changed story. Couldn’t even be bothered to show up for a meeting they booked, then literally blackmailed me into paying the charges – if you don’t pay, fee’s go up daily. I’m still considering getting lawyers involved in that..
The good news is that despite the unexpected extra charges, all the stuff arrived in one piece, and the panels even made it without a breakage.
The forklift did have issues though in our front yard – it got completely stuck, and had to be towed out!
Next up, was wiring.
As I was already redecorating the house, wasn’t too bad – I had the builder’s electrician pull the 3 phase to the front for me, and a separate 1 phase to the laundry room, so I could terminate (puts on best Arnie voice and asks – “are you Sarah Connor?”) at my leisure.
I also had my builders mount the Inverter, and my electrical boxen, as I’m rather lazy. The inverter is also bloody heavy at 40kg, so made sense to get stronger people than me to mount that 🙂
Laying out how much cable is needed:
Running it into the conduit that I had builder put in for this very purpose:
The astute will note that I have 4 cables there. I have 3 phase in the house, so thats Phase 1, Phase 2, Phase 3, and Neutral. The smarter folks than me will ask what about ground?
Yup, I forgot! Doh. Running ground from a separate wall plug in the room though, so wasn’t a major disaster, although the electrical gods may frown upon me..
This is what it looked like pre-termination.
<img src="http://farm3.staticflickr.com/2826/10573372014_665f3e57e5 viagra se vende sin receta medica.jpg” />
…and this is what it looks like after a few hours of my time (wiring the MC4 connectors is fiddly business!)
Once I had that done, then it was a matter of waiting till a less rainy day, so that I could open my magic box of panels 🙂
An hour of hard labour later, and all 800kg of panels were in the back garden!
…and thats where we’re up till currently.
Next steps will be to mount everything (hopefully this weekend), then do a test of the system to see that the panels can power up the inverter.
If thats all good, then I’ll pay for the electrician to come back and sign off on the setup, so I can go all Frankenstein and turn it on for a few minutes to configure, and check everything before I shut it down again, and start on the paperwork side with the City of Cape Town.
Luckily I snarfed a pre-made SSEG (Small Scale Electricity Generator) document from Arthur @ MyBroadBand, so I have a lot less prep work to do. Yay!
Not many of us know that the power company / municipality also uses in-line signalling (aka ripple control) to implement power control and load shedding, so I thought I’d do a little writeup on that.
Many of us have noticed that streetlights don’t always come on, or go off when its light or dark – they appear to be on a timer system.
What most people don’t know is that the timer system controls are actually implemented centrally at substations, and these add signals to the power lines to tell the equipment to turn off / on when instructed.
This is done using ripple control codes.
With ripple control, a small signal is added to the incoming A/C at a distribution location – eg a substation. This signal is read by a special relay in place on the larger circuits (typically the Geyser), and turns power off or on when the electricity company requires – usually when power is scarce, and they need to shed some load.
As this signalling can work on multiple channels, each listening relay can be set to listen to a specific channel, and used to power specific things on / off remotely (e.g. Streetlights).
In South Africa, we use DECABIT signalling to tell things to turn off and on, as well as the older K22 signalling standard.
When load shedding needs to occur, the electricity distribution system needs to act fast to avoid system failures. Most things are automated, and happen in order of timing.
Implementations of the protection mechanisms in place have a specific time to occur – eg a latency. Responses to conditions also have a latency – eg getting additional idle power plants online to provide more power when needed, so its important to the grid to have multiple control and response mechanisms to respond to loads. Each response mechanism also has a different cost impact, so its also important to the electricity provider to best manage these.
A diagram of this is below (Excerpted from http://www.anime-za.net/tech/literature/Enermet_Farad.pdf ):
For light variances in load, frequency changes as generators speed up or slow down to supply enough electricity to the supply. If there isn’t enough supply to meet load, then frequency drops, and large scale equipment will disconnect until load decreases. This happens almost instantaneously – responses to these issues resolve with a latency of within a few milliseconds to a second. This is called Under-Frequency Load Shedding (UFLS).
As seen in the diagram above Eskom implements automated under frequency load shedding in an increasing percentage margin based off frequency rates.
(Additional details are in the PDF below)
The next set of load shedding is the one we’re interested in – ripple signalling. If the system still has too much load after 1 second, then it sends out a signal over DECABIT to turn off more equipment. DECABIT signalling has a latency of about 7 seconds – a minimum DECABIT signal frame is 6.6 seconds, so this is a second stage response to issues.
As each substation can be connected to up to 20,000 homes/customers (depending on substation load capacity), this allows localized load shedding where its needed, when its needed.
Eskom calls this Demand Market Participation, and has roughly 800MW of systems added into this mechanism. Municipalities are particularly keen on putting loads onto these mechanisms via DECABIT compliant relays, as this saves them peak power fee’s when loads are high – if they can temporarily cut off power to consumers for 10 seconds – 10 minutes for non-essential high loads, then they can substantially reduce what power costs them from Eskom, and make additional profits.
A good writeup on Demand Market Participation is below:
Eskom benefits as they can temporarily avoid adding more infrastructure to cope with growth.
This has been the case for a few years now, but it only delays the inevitable – you do need to invest in infrastructure, not incentivize clients to use less.
Eskom also has a secondary mechanism (using the same theory – lets encourage you to turn off power) called VPS. They have an additional 50,000MW of connections using this on a contractual basis – typically industrial users., and are looking to increase this number.
Its only been through introduction of these mechanisms that we’ve been able to stave off grid collapse. Its gotten so bad, that industrial users have been looking closely at what they can do to provide their own power when Eskom can’t.
Other countries – notably Germany, and the UK, have allowed consumers to become producers, by encouraging localized small scale production of electricity, thus helping the grid without requiring additional investment from the incumbents. This is called net metering – where both inputs and outputs are metered.
Eg – if you have a solar system that provides excess power during the day, it can feed into the grid – (when it needs it most), and they’ll credit you for your participation.
So far, South Africa has been rather reticent to implement this, as the short sighted vision is that its “stealing” from the incumbents profits.
A choice excerpt from that PDF is this –
Residential load can also be incorporated within the VPS, particularly when integrated with Smart Metering systems. Numerous pilot and small scale projects are being undertaken within both Municipalities and Eskom in response to the DOE’s Regulation 773 of 18 July 2008.
The Department of Energies regulation can be found here –
These state that all systems over a certain size require that smart metering be installed by 2012. As you may have guessed, quite a few municipalities have not met this deadline, and Eskom has been dragging its feet on that too.
Ironically, introduction of smart metering would actually help the grid here in South Africa, as IPP’s (independant power producers) would make the grid more stable by providing additional energy when needed, and at a lower cost than the incumbents can create it for.
This however does have its issues – most municipalities generate revenues from Electricity, and so are loath to change the status quo, even when it would benefit the country from a whole.
So, its unlikely to be implemented in the short-medium term, unless the government drags them kicking and screaming through the process.
In summation, this –
http://www.enerweb.co.za/brochures/AMEU%20Conference%20-%20Enerweb%20VPS%20Paper%20-%20201109%20-%20%20V1.0.pdf – Demand Market Participation
http://www.systemoperator.co.nz/f3210,36010947/Appendix_A_-_A_Collation_of_International_Policies_for_Under_Frequency_Load_Shedding.pdf – Load Shedding in International operators