# How can energy "physically" be fed back into the grid?

I have found some similar questions (below), but I am still having issues understanding what is "physically" happening when energy is "fed back into the grid" and the equipment necessary for this.

My question seems to have multiple parts, but I believe they are needed to understand the bigger concept that I want to comprehend. Please correct me if I am looking at this wrong, or I need to clarify something.

How does this physically happen?

The way I see it is that a utility is able to send 3 phase electricity over multiple lines, for example, 1 phase per line, for a 3 phase power supply. Most homes only need one phase, and therefore one line entering the home. But if electricity is traveling in AC form over long distance, wouldn't feeding the excess energy of a home (produced by PV) back into the one phase line that is entering the home then potentially cancel out the frequency of the energy coming into the home?

I cannot comprehend how, for example, multiple homes can feed their excess photo-voltaic energy (PV) into a grid using the lines of the public utility. To me it would seem there would have to be a dedicated "feed in" line where a home puts its excess PV energy back into the grid, and that this line would be shared by multiple homes, however, only one home could use it at a time, so that the proper phase and frequencies are maintained and congestion avoided.

Equipment

Another issue is that PVs are outputting DC, so an inverter would be necessary to convert the electricity to AC for the energy to then be able to go back into the grid. Can excess PV be fed directly into the grid using an inverter? Or is it necessary to go through a "middle man" like a battery, and from there the DC can be converted to AC.

Another issue is me wanting to know all the equipment required to do this. The minimum equipment I believe is necessary to feed energy back into the grid:

• PV panels (Or any form of a distributed energy resource (DER))
• Inverters (Convert PV panel DC to AC)
• Transformers (To step up the voltage to be sent back into the grid)
• Some sort of switch to determine if energy is going into the home, or back into the grid
• Battery (to work as a "middle man", or possibly deal with imbalances and smoothing)

What is a utility doing with this excess home energy?

Is the utility automatically distributing the excess energy from individual homes to other loads in the grid? Are they "bleeding" it off somewhere similar to a pressure valve in cases where they cannot handle all of it?

Another issue is there are multiple power producers that are using lines that stretch hundreds of miles, and now you have homes feeding energy back into these lines, so I am guessing that a single power producer needs to be in charge of an entire section, so there is no congestion.

I attached a figure of how I am imaging a small section of the grid where the homes are “feeding” energy back into the grid. Right now I only have the loads, substations, and power producer, but my hope is after getting answers to my question I may be able to add all the pieces necessary to help visualize the process.

If someone can point me to some sources with diagrams and equations, then perhaps that may help me with my questions. Please let me know if more clarification is needed.

• at abstract level, phase of voltage and current, determines the sign of work being done, i.e. direction in which power is being sent Jan 12 at 22:26
• Read up about "grid tie inverters" the boxes responsible for this job. They don't "cancel out frequency" but they track the incoming frequency precisely and apply a slightly higher voltage than the incoming voltage. That means the current flows out to the grid instead of into the house. (Assuming your house generates more power than it uses! If it generates exactly what you're using, no current flows to or from the grid) Jan 12 at 22:29
• The energy all has to balance out. It is like accounting. You have deposits (generators) and withdrawals (consumers). The net is always equal to zero. If a big consumer shuts off, the grid responds by ever so slightly increasing frequency and voltage. This increase causes some other consumers to use a bit more power, and it also causes the generators to back off a bit. Inverters do feed AC back into the grid. the utility company knows that this happens and prepares for it each day when the sun comes up by backing off other generators gradually. Jan 14 at 2:07
• If I can comment on the phrasing of the question: I suspect that the "physically" aspect comes from somebody trying to distinguish between having multiple generating plants supplying power to the transmission/distribution grid, and the fairly common case where some middleman claims to be selling "ethical electricity" with the claim that he only invests in renewable sources of energy (he isn't "physically" injecting power himself). Jan 14 at 15:18

I cannot comprehend how, for example, multiple homes can feed their ... energy into a grid using the lines of the public utility.

That's the way the power stations do it, so why not? All that is required is to keep the AC frequency correct and in-phase with the grid and then raise the voltage above the grid voltage. Current will flow from the higher voltage to the lower.

Can excess PV be fed directly into the grid using an inverter?

Yes, but a grid-tie inverter is required. It will maintain frequency and sync with the grid and also have features to disconnect in a guaranteed safe manner when the mains power is lost. This avoids you trying to power up the city with your little solar panel and also avoids electrocution of the lineman out doing the repairs.

Or is it necessary to go through a "middle man" like a battery, and from there the DC can be converted to AC.

Theoretically no, but there may be practical or separate reasons to have one there.

Is the utility automatically distributing the excess energy from individual homes to other loads in the grid?

You can consider it like water feeding into the national pipework. It will generally be drawn off by the nearest consumer. If the grid voltage starts to rise then the load schedulers need to reduce supply.

Are they "bleeding" it off somewhere similar to a pressure valve in cases where they cannot handle all of it?

In the case of steam power stations, yes, the excess steam will have to be vented. Wind turbines can be turned out of the wind. Hydro can be shut down quickly. Gas turbines can shut down quickly.

Another issue is there are multiple power producers that are using lines that stretch hundreds of miles, and now you have homes feeding energy back into these lines, so I am guessing that a single power producer needs to be in charge of an entire section, so there is no congestion.

It's more likely to be controlled by the network company these days.

• Unlikely that excess steam is vented. More likely the grid runs a teensy bit faster (and higher voltage) until the boiler controls are adjusted to burn less fuel. Jan 12 at 22:32
• What about pumped storage? Or is it too slow to be used for this kind of instantaneous balancing? Jan 13 at 8:46
• @Tomeamis the phrase to seach for is "frequency response": nationalgrideso.com/industry-information/balancing-services/… ; there is some pumped storage (Dinorwig, Cruachan) but it takes of the order of some seconds to spin up and down. However this only matters for discrepancies of the size of megawatts or above. One home's 4kw solar installation isn't going to be noticed or responded to. Jan 13 at 9:40
• @Tomeamis: Pumped storage would be included in hydro but with the advantage that it can be used to take energy from the grid as well as put energy in. Jan 13 at 10:07
• @BrianDrummond I can tell you that the nuclear power stations in France vent enormous amounts of steam at the end of the winter morning peak (when the electric home heating shuts down and the electric workplace heating reaches normal temperature). Nuclear and coal are slow to heat up and cool down and when there are step drops in consumption (or peaks in renewables supply), steam venting is necessary. Frequency is tightly controlled by law, voltage less tightly, but still controlled. Jan 14 at 8:30

The way I see it is that a utility is able to send 3 phase electricity over multiple lines, for example, 1 phase per line, for a 3 phase power supply. Most homes only need one phase, and therefore one line entering the home. But if electricity is traveling in AC form over long distance, wouldn't feeding the excess energy of a home (produced by PV) back into the one phase line that is entering the home then potentially cancel out the frequency of the energy coming into the home?

The inverter always locks onto the phase of the incoming supply before it starts generating. All the time it's generating it's (very nearly) in phase with the supply, so the power is additive.

I cannot comprehend how, for example, multiple homes can feed their excess photo-voltaic energy (PV) into a grid using the lines of the public utility. To me it would seem there would have to be a dedicated "feed in" line where a home puts its excess PV energy back into the grid, and that this line would be shared by multiple homes, however, only one home could use it at a time, so that the proper phase and frequencies are maintained and congestion avoided.

The wires running along the street can carry current in either direction. If you're generating more than you're using, power goes out. If you're using more than you're generating, power goes in.

The transformers can work both ways, too. In the UK, it would normally be 11kV in, 415/240V out. But if you push 415V (or 240V on one phase) into the secondary, you'll get 11kV out on the grid side.

Another issue is that PVs are outputting DC, so an inverter would be necessary to convert the electricity to AC for the energy to then be able to go back into the grid. Can excess PV be fed directly into the grid using an inverter? Or is it necessary to go through a "middle man" like a battery, and from there the DC can be converted to AC.

Batteries are entirely optional. They are nice for saving spare electricity for when it's dark. But the inverter will work fine without one.

Is the utility automatically distributing the excess energy from individual homes to other loads in the grid? Are they "bleeding" it off somewhere similar to a pressure valve in cases where they cannot handle all of it?

The electricity from homes adds to the grid in the same way as any other generator. However, 4kW from a home is so dwarfed by 4MW from a wind turbine, or 400MW from a big power station, that nobody even notices it. The grid as a whole is constantly monitored, and generators adjusted up and down in power as required to keep everything in balance.

And also

Some sort of switch to determine if energy is going into the home, or back into the grid

Not needed. It all sorts itself out. If you don't have a battery in the system, the inverter will always generate as much as it can. If that's more than the home is using, the excess will go out to the grid. If it's less than the home is using, then the home will be running off a mixture of generated and grid power.

If you have a battery, you need a more clever inverter. It will try to power the home if it can, putting any spare power into the battery. Once the battery is full, any spare will be exported. In the evening, the inverter will draw down the battery, producing just enough power to power the home. All this is done by a current sensing coil on the incoming supply, so it always knows whether power is going in or out.

Ah, the apparent paradox of energy. Don't forget, electricity goes equally back and forth. It's an AC system after all.

How then could we ever feed any electricity to any homes? Answer: we don't have to.

Instead, things become much clearer once we start start employing physics terminology. (Sometimes the answers even seem simple and obvious.) First, never say "electricity." If you mean energy, say energy, or perhaps "joules." If you mean charge, say charge, coulombs, or "electrons."

Instead of selling "electricity," the utility companies are actually selling electromagnetic energy. The electrons inside the wires may wiggle equally back and forth, but the energy behaves very differently.

As with any transmission line, the energy in the AC power grid is traveling out in the space surrounding the wires. You may already know that both signals as well as "electric power" travel at nearly the speed of light? That's because they're the same thing AS light; same as radio waves propagating along a waveguide, same as magnetic fields around inductors, or the e-fields between the plates of capacitors.

Unfortunately, all these correct ideas are only taught in engineering school, while back in K-12 grade school, instead we're all fed "lies to children," a collection of over-simplified explanations which have little to do with reality. Electrons zooming through solid copper at the speed of light? Batteries and capacitors "storing charge," etc.? These distorted ideas must first be abandoned, if we want to have any hope of deeply understanding the AC grid.

To understand basic EM physics, even before treating equations, we first need to realize:

• The word "conductor" means "materials which are full of mobile electricity." For example, metals contain an enormous amount of free electric charge, roughly 10K coulombs per cubic cm of copper. In copper, each atom of the metal donates one mobile electron to the "common sea." Physicists call it the "electron sea" or the "ocean of charge." A piece of wire is like a pipe that comes pre-filled with electricity, and no bubbles allowed.

• In AC systems, electricity moves back and forth only. It doesn't flow across the grid, and it doesn't flow across circuits. The electricity (electrons) were already inside the wire, even before the power supply was connected. Generators don't create any electricity, and loads don't consume any electricity. Instead, generators are charge-pumps. Instead, electrons inside wires just wiggle back and forth. (Or in DC circuits, all the electrons move slowly in closed circles, with no electricity being gained or lost. They move like a slow flywheel; a slow drive-belt.)

• Electrical energy flows almost instantly from a source to a load. In an AC circuit, this energy doesn't wiggle back and forth. It flows continuously forward, as waves.

• Electrical energy, or electromagnetic energy, is composed of e-fields and b-fields, corresponding to volts and amperes respectively. When electric energy is sent through coaxial cable, all the energy is located inside the plastic dielectric. When it's sent across circuits, or along transmission lines, it's located in empty space surrounding the wires (see diagram of fields below. That's a diagram of the energy flowing along your AC power line.) While the amperes may truly be inside the wires, the joules are all found in the space outside.

• Electricity wiggles slightly back and forth, while electrical energy flows forward. In other words, two separate things are flowing along any circuit. There is no single stuff called "electricity." This isn't a complicated concept; it's all about Waves-versus-Medium. In circuits, the slow electrons of the copper metal act as the medium for the traveling energy-waves, while the lightspeed waves themselves are located in the EM fields just outside the wires.

• During engineering school, in our fields/waves class about transmission-line theory, note well that transmission lines have no low-freq cutoff. They function the same at microwave freqs, HF, audio, and all the way down to DC. Transmission-line theory isn't just about radio, it's about all circuits everywhere. The EM energy-flow is composed of b-fields created by circuit currents, and e-fields created by circuit potentials. After all, a high-freq AC supply can drive an incandescent lightbulb, just as easily as it can power a dipole antenna. (But in the case of the dipole, rather than being absorbed by the lightbulb, the traveling EM fields instead leap off the wires and propagate away into empty space.)

Above: b-field of a circuit, e-field of a circuit, and the watts of EM energy-flow

.

What happens to excess energy fed to the power grid? Well, imagine that all the dynamos are just conventional DC supplies, or even batteries. Imagine that the loads are just resistors, or perhaps incandescent light bulbs. Now, what happens when excess energy is fed to the grid? How can we even do such a thing?! Easy: just slightly raise the voltage of one of your DC supplies. All the light bulbs will burn slightly brighter. Ideally, the added energy flows to all the bulbs on the entire grid (but if the grid conductors have slight resistance, the closer bulbs will receive a bit more of the overvoltage/ energy.) The same thing happens on the AC grid. A simple way to feed "excess energy" is to raise the voltage of one of the AC dynamos. The grid voltage will rise slightly, and the injected energy will end up flowing to all the connected loads.

Note that if we raise the voltage of one battery, then it fights with the other batteries, and it charges other batteries hooked to the grid. Or at least for wires having some ohms, it reduces their output current, if not actually reversing it. For an AC grid with dynamos, the same thing happens, and if one dynamo has a too-high voltage, it will spin the other dynamos as if they were AC motors, or perhaps just reduce their output-currents, if the grid wires aren't perfectly conductive.

Is there a minimum amount of (joules) which can be fed into the grid? No, not unless you're talking about single EM photons! For example, if your DC power supply is set to exactly the same voltage as the entire battery-grid (say 120VDC,) then its supply-current will be zero. The wattage at that DC supply will be zero, and no energy is flowing. Now try turning up the voltage of the one DC supply. This produces a current (as if the entire grid was a giant resistor,) and now there are some watts of energy-flow, directed out of your DC power supply and going out into the grid. There is no lower limit to this (you could adjust it to produce an amp, or a mA, or a picoamp.) Just multiply this current by 120V, to find the rate of energy-flow being injected into the grid.

Photovoltaics? Connecting DC supplies to AC systems is conceptually fairly complicated.

Therefore, in order to clarify, either pretend that the entire power-grid is batteries, or DC generators (and let the questions answer themselves.) That, or for an AC grid, first hook your PV panels to a DC motor, and connect the motor's shaft so it can spin a small AC dynamo that's connected to the AC grid. Note that such a dynamo will constantly spin at 3600RPM, "idling" or "freewheeling," with the DC motor acting as a generator, the DC motor-current ideally being zero, and no energy being sent into the AC grid.

With the above setup, whenever the PV panel is producing zero output current, the DC motor is spinning without torque, while your AC dynamo freewheels (no torque is on the connecting shaft, for ideal frictionless bearings.) In that case the AC dynamo is "idling" at 3600RPM/60Hz, and doesn't send any energy into the grid. The back-EMF of the spinning DC motor will match the PV panel's output-volts, with zero drive current. Next, add brighter sunlight, drive the freewheeling DC motor a bit. This pushes the AC dynamo slightly ahead, slightly increasing the AC output voltage. Math: if we add up the dynamo's instantaneous wattage over one AC cycle, we'll find that electrical energy is now flowing out of the dynamo and into the grid. (Heh, or instead put your thumb against the shaft between AC dynamo and DC motor when it's freewheeling, and instead, the energy-flow reverses, and the dynamo will start taking energy from the grid, functioning temporarily as an AC motor.)

If we connect the shaft of a DC motor to an AC dynamo, that's called a Rotary Converter, and it performs the same task as modern solid-state inverters. To understand energy-flow, it pays to first understand DC motors which drive dynamos. Then later, figure out the details of the transistor-switching circuitry which simulates a motor-driven AC dynamo.

PS

In physics, what is Electricity? What's the narrow technical definition of the word "Electricity?" We could just ask JC Maxwell, Einstein, Faraday, that whole crowd. They all agree: Electricity isn't the flow of the charges, and Electricity is not a form of energy. Traditionally, Electricity is the coulombs! That way, a flow of coulombs is the same as a "flow of electricity," or in other words, an electric current. Current isn't electricity. Current is a motion of some pre-existing electricity.

• I notice that you wrote "never say 'electricity'" and then you called electric charge "electricity" 7 times after that. Maybe you want to edit those to say "charge" instead? Jan 13 at 19:07
• Yes, I habitually employ certain "errors" in order to attract certain comments. -PS "electricity" as a quantity in historical physics has a narrow strict meaning: the quantity measured in Franklins (or in Faradays!), the quantity "Q" in Coulomb's Law. Historically, the word "charge" was not used: CRC Handbook's physics glossary defined "Coulombs" without using the word charge!! So, never say "electricity" ...UNLESS making a point via intentionally adhering to the narrow and venerable technical definition (e.g. Maxwell's usage,) since it still appears widely in older texts Jan 13 at 21:13
• Re "As with any transmission line, the energy in the AC power grid is traveling in the space between the wires." No. It would still work if the wires were 2,000 km apart. It is a circuit here, not a transmission line. Jan 13 at 23:08
• @PeterMortensen would still work if the wires were Of course, since it still obeys xmission line physics, regardless of distance between wires. ALL circuits are xmission lines (the physics of circuits involves the nearfield, which is a special case of general transmission-line theory.) Note well: inside metal wires, the amount of axially-propagating electrical energy is exactly zero. Even at DC, "the wattage" is located where ExH energy density is located, which is outside the metal surfaces. (Perhaps you missed this part of your engineering courses? I think many do.) Jan 14 at 1:53
• @PeterMortensen to clarify: Do currents carry energy inide metal wires? Nope. Amps at zero volts give zero watts, just as volts at 0 amps give 0 watts. Voltage cannot carry energy along. Current cannot carry energy along. Electrical energy only travels across circuits as I*V. Similar: the enegy of capacitors is in the field between the plates, NOT inside the metal. The energy of coils is in their magnetic field, NOT in the currents inside the coil. In physics, circuit-loops are 1-turn inductors, but sliced up, so they have multiple "capacitor plates" at various potentials. EM energy flows. Jan 17 at 5:25

In AC network power flow is controlled mainly by phase angle of voltage (not magnitude as in DC systems).

To send power to the grid an inverter must generate EMF shifted relative to the mains voltage. To achieve this you may have an inverter with internal frequency generator. The control circuitry will slowly increase the generator's frequency until power flow to the grid reaches desired value. When power flow goes above the value, it will start to decrease the frequency. All the frequency deviations are in a very small range around 60 Hz (or 50 Hz in Europe) and are only needed to achieve the voltage phase angle shift (the shift is an integral of the frequency deviations). The inverter always stays in sync with the grid voltage.

The similar control system is installed on every power generator. When there is an excess power in the grid, generators start to accelerate. The system notices this and reduces amount of steam/water feed into turbine, thus slowing generator to the normal speed/frequency.