What happens to excess energy fed into the power grid?

Question

The closest question to this is Linear useage of excess power generation.

I'm no engineer so I may not be able to phrase this correctly and would appreciate an answer which assumes minimal background knowledge (I have only a basic understanding of voltage, transformers, etc). The question arises from all this talk of the variable wind and power potentially disrupting the grid.

For example see the 2012 Electrical Connection article Rapid increase in solar installations potentially overloading the grid which discusses the potential for "reverse power flow" and also talks about some sort of "network protector" device. Also there is a or similar article about Hawaii The Interconnection Nightmare in Hawaii and Why It Matters to the US Residential PV Industry , which says the only "concrete concern identified by the Hawaii experience is the potential for transient overvoltage on the feeder – essentially a short duration voltage spike".

I'm curious about what happens here both with regard to a large grid and in a micro environment. For example, let's say I have a fully charged battery and I keep flowing electricity into it. What happens? Are there devices which will divert or dissipate the electricity as heat without damaging anything? I found a few similar questions online but the answers weren't too clear.

Answer 1

The most simple and direct answers to the main question depend on how "excessive" it is. Since most equipment is designed to operate within +/- 5% of nominal, the "extra energy" usually gets dissipated as heat, in the device itself. In the case of a light bulb (for example), it produces more light and heat. If the excess energy goes beyond the tolerance of the devices, they will overheat and/or burn (cause damage). These results will be obtained regardless of what causes the "excess energy" on the grid (lightning, solar installations, wind power, etc.).

For the last two questions, if you are charging a 12v battery with a 13v source, the extra 1v will keep the battery "warm" after it is charged to 12v. If you are charging it with an 24v unregulated supply, the battery will overheat, burn up, and possibly explode. If you charge it with an over-voltage and current-limited supply, the battery will be charged to 12v and the extra energy will be dissipated as heat in the supply regulators. One way you can make "efficient" use of any "extra energy," would be to use a bank of batteries and a "smart" charger, which would switch the charging to another battery when one is charged, and shut off (disconnect) when all the batteries in the bank are charged. If there is no interest in saving the extra energy, it can be "dumped" into an appropriate load and converted to heat.

Answer 2

This is, as you might imagine, not something that has just one solution and the problem in itself is pretty complex as well. Let's break it down.

The power grid as it exists now in most civilized countries has a hierarchical structure: on top there are the large centralized power stations, beneath that are the large-scale MV distribution networks or distribution rings, then come the city grids (usually about 400kV) which are usually underground HV, neighborhood networks (20kV or multi-phase mains voltage) and then the low-voltage 'postal code' nets which distribute 115/230V. Of course, as your question already implies, this hierarchy presumes a net energy flow from power station to home, and not the other way around.

Most decentralized power generation - non-commercial solar panels, wind turbines and the like - happens at the house level, i.e. it produces 115/230VAC and pumps it into the mains supply. Most of the time this is fine because power generated is much less than power consumed and the net energy flow is still in the right direction. Rarely, but more often nowadays because of the low price of solar, the amount of power generated is more than the power consumed on the postal code level. For basically all power nets this is not that much of a problem actually. The transformers used to convert MV into 115/230V are just linear transformers and they work just as well in one direction as they work in the other. They almost never have PFC or other flow direction dependent parameters so it's fine.

The problem that most power grids are coping badly with, is what happens on one step above that. Here we arrive at the conversion step from the underground city grid to smaller blocks, and these transformer stations nowadays often have PFC or at least some kind of decoupling mechanism to make sure that interference from the city grid doesn't travel back to the HV power lines as it would through a linear transformer. If this unit generates more power than it consumes, that energy cannot (generally) go anywhere, or at least it is stopped from doing so by very expensive, not-that-easy-to-replace-everywhere electronics. The reflex response of the system is to throw a switch and separate this unit from the rest of the grid. Of course, this won't 'kill' this unit; the power generated will simply pump up the voltage on this grid up to the safety limit of power inverters (usually nominal voltage + 5-7%) and very often it will destabilize the AC frequency. But the power will continue to be there until a cloud passes, the grid drops below brownout voltages and the solar inverters all switch themselves off. This problem is called the island generation problem and is very hard to solve without some additional intelligence in the power grid and inverters (i.e. smart grids).

However, as you can see in this previous paragraph the extra energy doesn't necessarily go anywhere. If an island situation occurs, inverters are required not just to dump all their available energy on the grid, but to modulate themselves when the grid reaches a certain voltage. When that cloud eventually passes over, they will switch themselves off and the situation is resolved.

There are alternative protection mechanisms. Some countries have shorting switches that can be engaged with special (DTMF) signals over the power line. When an island is created, they can short out the power grid to ground and black out a section of the grid immediately. This however is not a very safe practice, as this often causes inductive spikes on the power grid which can damage both the grid and household electronics. Nowadays this is rarely used. It is however an important protection mechanism for power generators that don't regulate their output well and may cause an overvoltage situation.

Answer 3

In Germany this May, the price paid for renewable energy actually swung negative, as they had too much of it. In other words, they were charging producers to take the excess energy. So they dealt with the excess of energy by incentivizing the producers to not shove it onto the grid- which is easy with solar, and possible with wind power.

Different generating methods have different time constants- nuclear plants like to run flat-out and start-up and shut-down take a lot of time. Hydroelectric can be quickly altered in output by redirecting or choking the water flow. Thermal plants (I used to have one nearby) have a longer time constant so if you suddenly lose the load (wot is slowing the turbines down), the stored energy in the steam has to be vented (loudly!) to keep the generators from spooling out of control. They don't attempt to absorb the electrical energy, as far as I know, though I did a feasibility study on instrumentation for a massive energy sink that would absorb huge amounts of energy (it's fun making instruments that work with common-mode voltages of 100's of kV).

Storing energy in large amounts reasonably efficiently is a very difficult problem, with no obvious solution. Distributed batteries/inverters and the old-school method of pumping water uphill into a dam to store it, and letting it rush out through turbines and generators to recover (some of) it are a couple methods.

Answer 4

Let me rephrase these articles in terms that make it easier to understand and to put it in context. I see these articles as the equivalent of " I just bought a new Ferrari, there is a serious problem in that I keep having to replace the brake pads as the power output from my engine is too much when I approach a stop light".

The simple answer is -"take your foot off the accelerator". i.e stop producing power when you can't use it.

There really isn't problem with over production, there is a problem with over delivery they just need to signal back to the producers "stop putting power on the grid". In fact some solar panel controllers use the cloud shadowing to predict how much power is going to be produced in the next 10 or 15 minutes and signal that forward to the grid authority.

These sorts of articles are not helpful. There are serious issues with the main grid and interconnect ties which can simply be solved by passing laws and spending money. Having wind power producers over running your control system has far simpler solutions.

Answer 5

It's a complicated problem with a variety of answers.

Even with no solutions in place, there is some tolerance for a supply-demand mismatch. Too much demand/too little supply) will drop the voltage and frequency on the grid from its usual spot of 50hz/60hz/whatever your country's mains is. Conversely, too much supply/too little demand will increase frequency. A small amount of frequency deviation is not a significant problem. In New Zealand, mains is 50 hz, but the grid is fine with frequencies ranging from around 49 - 52 hz. Outside of this, you can have serious problems. Most specifically, if you go below 49 hz, this can damage generators, which will automatically turn off or isolate themselves. This means that the grid frequency drops even more, as there is less supply, causing a chain reaction and eventually a total grid collapse.

In order to prevent this from happening, market operators pay people to perform a variety of services. These differ country to country, but again, I will use NZ as an example.

Frequency Keeping - this acts to both increase and decrease grid frequency, as required. To use a driving analogy, watch someone while they steer. They are constantly making tiny movements with the wheel, they probably aren't conscious of these, they react to the position of the wheel to keep the car straight as it goes over small bumps in the road. This has traditionally been performed by generators, running at less than 100% capacity, capable of varying their output with a sub second response time.

Reserves - In New Zealand, 'reserves' must be procured at all times in order to maintain the grid in the event of an N-1 situation - either the loss of the largest generator, or the loss of the transmission lines between the North and South Islands. In Europe, the continent as a whole operates on an N-2 situation, representing the loss of 2 large nuclear plants. These reserves can either take the form of generators running below capacity and able to ramp up quickly, or (more cheaply and quickly) demand response resources - sites that are willing to reduce load as required to maintain the grid. These resources are usually segregated by response time and amount of time they can sustain the change for. NZ has a fast market (1 second response time for loads, 6 s response time for generators sustained for 1 minute), and a sustained market (60 seconds response time but sustained for longer - up to about 30 minutes). Going back to the car analogy, this is where your car hits a large bump, swerving you towards a tree - you have to wrench the wheel back the other way to get back on the road (but don't turn too far or you'll end up hitting a tree on the other side of the road).

Dealing with peaks - peaking Generation or traditional demand response - to use our car analogy, there is a corner in the road. We can see it coming from a long way away, and we need to do a huge turn to stay on the road. This is summer heatwaves, winter cold snaps, evening peaks etc. This can be met with a variety of technologies. Usually, the bulk is from peaking generators, which are only run a few days a year. Again, demand response comes in to play - it is often cheaper to shut down a factory for 20 hours a year than it is to build an entire new peaking generator and upgrade transmission lines

Answer 6

I work in the subject, and I think I can be of help explaining this.

I'll explain it using the water analogy:

Electric current flow -> Water flow

Voltage -> pressure

Said this,

If you have a network with nodes, and branches; the nodes are where the water is injected and subtracted from the network, and the branches are the pipes.

(In electrical grids, the pipes are transformers and lines, while the nodes are the nodes or busbars)

If you have "water" injection in a node that originally was designed for consumption, then the pressure in the pipes might increase up to a level where the pipes break. (This would be solar production at household level) The same way, too much consumption at a node might lower the pipes pressure too much and the system will not work.

The way of dealing with this is to store the surplus of energy and supply it when needed, that is why batteries are the holly grail of renewables.

Huge renewable penetration is a situation that grid operators and electrical companies are against because it forces them to adopt new approaches to a job they've been doing for a century with few radical changes like the ones they need to make. (My opinion)

I hope this is clear enough, otherwise I can explain things further since this is my daily work.

[EDIT: Why do the pipes break?]

Well as you requested, I'll go a bit more in detail here:

Each branch element (lines & transformers) has a limit in the amount of current can go though it without overheating and set on fire. This nominal current can be overpassed for a limited amount of time, so an overload is not a life or death event, if it does not last too long (Also overloads diminish the elements life)

On the other hand, the voltage should be within a +-5% of the nominal voltage of a node, this is 230V +-5% per phase (In Europe, in US is 125?). Generating power in a node increases the voltage in that node and in the neighbour nodes (For the same load situation) Increment in demand in a node decreases the voltage in that node and its neighbours). This is why if I put a massive amount of solar panels at home I might get voltage issues at my house and at my neighbours houses. This issue can be mitigated by proper inverter firmware programming, but there is no regulation on that in many countries, so there are this problems people haven't heard about but are very real.

But why does the voltage has to be in such a limit? Well this limits is a security constraint set by grid operators. If the voltage in the sockets of your house is too high it might break the power electronics of your devices (PC's TV, etc..) if the voltage is is too low, electronic devices might not work or even break as well. An incandescent light bulb shines brighter on high voltage, and less brighter on lower voltage.

Tell me if more details are needed. Santi.

Answer 7

I think another good analogy is that you can think of a large (base load) power plant like a car that is being driven up hill at full throttle. It will attain a certain speed (grid voltage) and at that point it will require you keep the pedal to the floor to maintain that speed indefinitely. Now if the hill starts to level out and you leave your foot on the floor the speed will increase and you will need to lift off the gas to bring the speed back down. This would be like the grid voltage increasing and power generation would be reduced (peaking units shut down). On the the other hand, if the hill gets steeper (load on the grid increases) the car slows down (voltage drops) but you are already at full throttle. The only thing you can do now to get back up to speed is have another car push. That would be a peaking unit coming on line.

Answer 8

We have high voltage levels to transport energy and low volatge levels like 230V for distribution of power. As the grid was build and most time today, the power goes from the high to low volatge part of the grid. One tarnsformer distributes the power to several houses in an village or town. At this low voltage there is no N-1-saftey, there is just one transformer and a lots of houses around it. Because current goes from high to lower voltage, the highest voltage is at the transformer. At most(any that i know), old tranformers this voltage is constant. To fully use the +/-5% Range, the voltage at the tarnsformer is about +4/5%. On the way to the houses, the voltage can drop up to 10% and with -5% all is ok. If now a lot of Photovoltaik produces more power than consumed in this area, the Power has to go into the grid over the transformator. But yet, the current flows towards the transformer, which means, that it is the point with the lowest, not the highest voltage. Therefore the voltage can easiely be to high and the photovoltaiks have to shut down (to high voltage could damges any conteced device in this area). By using/installing adjustable transformers, this case make no problems, the voltage at the tarnsformer just need to be adjusted to e.g. -4%. But they are quite expensive.