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Infinite Bus / Synchronization
What is an Infinite Bus?

Hi folks,

I need your help in understanding what an infinite bus is? What bus bar system (arrangement) is adopted in Power Plant Switchgears (bus bars)?
Lastly, how are the different generators connected and synchronized in?

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Hi Wisdom,

An infinite bus is a a very large grid with tens or hundreds or even thousands of generators all synchronized together supplying a very large aggregate load. Think Western Europe, or the UK, or North America, or even Texas (yes; they have their own grid). In my mind's eye, an infinite grid is one in which when a generator is synchronized to a grid it has a negligible impact on grid frequency--because there are so many other generators and their prime movers synchronized to the grid that the incoming generator, even if it puts out full power, can't really change the grid frequency by an appreciable amount. (There are lots of reasons for this inability to affect grid frequency, but, suffice it to say if you connect even a 150 MW generator to a 6,000 GW grid and run the 150 MW generator up to full load the grid frequency isn't likely to change by more than a couple thousandths of a Hz--and that can't generally be detected.)

On a small, or "island," grid if one synchronizes a generator to the grid with other generators and starts increasing the power output of the incoming generator without doing anything to the power output of the other generator(s), the grid frequency will start to noticeably change. (There are operating modes, automatic modes, that can prevent manual intervention from being required, but if there was no automatic mode in use and someone synchronized a generator to a small, island grid and did nothing to any of the other generator outputs and the load (the number of motors and lights and televisions and computers and computer monitors) didn't change, the grid frequency would increase as the load of the incoming generator increased.)

So, to me, an infinite grid is one where no single generator can have any appreciable effect on grid frequency as the generator is loaded (or unloaded), and a small, "finite" or "island" grid is one in which any single generator can have an appreciable effect on grid frequency as it is loaded and unloaded if nothing is done to the other generator(s) to compensate for the change in power from the incoming generator.)

A power plant will have a breaker or breakers or switches that connect the plant to the grid, usually downstream of step-up transformer(s) that convert the generator voltage to something much higher, to reduce the current to reduce transmission losses. Usually, each generator in most power plants (but not all!) has it's own generator breaker across which synchronization is accomplished, and the "load" or "bus" or "running" side of the generator breaker is connected to the low-voltage side of the transformer, and the high voltage side is connected to the grid through another breaker or high-voltage switching arrangement.

The "load" side of the generator is always hot (as long as the grid is energized), and once the prime mover gets the generator breaker up to synchronous speed then the synchronization process takes over.

Busbar arrangements in plants can take all forms; I'm not clear about the question. Busbars aren't considered "infinite"--the grid is considered infinite,. or finite.

All synchronous AC generators (more correctly called alternators) and their prime movers have to connect to a grid with other generators (and their prime movers) by synchronizing to the grid. Synchronizing is a process of ensuring that the generator phases (the "incoming" phases) are rotating in the same direction as the grid phases (this is done before initial synchronization, and should be done if the bus bars or potential transformer wiring is disturbed during any outage).

The next part of synchronization involves what's called "speed matching"--or getting the frequency of the generator phases to be nearly equal to, and usually slightly greater than, the frequency of the grid.

And, the final part of synchronization (which can be accomplished at the same time as speed matching) is called "voltage matching." Voltage matching is adjusting the generator terminal voltage to be nearly equal to, but usually slightly greater than, grid voltage.

Every synchronous generator has a rotating magnetic field that, when synchronized, is locked into synchronous speed which is directly proportional to grid frequency per the formula:

     F = (P*N)/120
where F = Frequency (in Hz)
P = the number of poles of the generator rotor
N = Speed of the generator rotor (in RPM)
The generator prime mover CANNOT rotate the generator rotor any faster than synchronous speed once synchronized to the grid--because there is a large magnetic field created by the current flowing in the stator windings (which are connected to the grid through the generator breaker) which grabs on to the magnetic field(s) of the generator rotor and keeps the generator rotor from spinning any faster than the speed which corresponds to the grid frequency and the number of poles of the generator rotor. No single generator synchronized to a grid (under normal circumstances) can rotate any faster or any slower than the speed which corresponds to grid frequency. So, for example a 60 Hz, two-pole generator must rotate at 3600 RPM when connected to a grid operating at 60.0 Hz. If the grid frequency changes, EVERY generator on the grid also changes (hard to believe--but it's true).

When synchronizing a synchronous generator to a grid with other synchronous generators if the magnetic fields of the incoming generator don't match the phases of the grid when the generator breaker is closed then the magnetic field of the generator rotor won't be in the proper position--and we all know what happens when we try to force the North poles of two magnets together, they DON'T want to go together. They want to repel each other--with a lot of force, even with small magnets. And, if that happens during synchronization then one of two things happens: either the generator rotor wants to spin VERY fast in the proper direction to get the poles aligned with the stator poles, and then WHAM! the generator tries to stop VERY fast when the North pole of the rotor is aligned with the South pole of the stator. Not good for the coupling between then generator and its prime mover. Or, the generator rotor can actually try to rotate in the wrong direction until its poles are aligned with the stator poles, in which case it will still try to STOP very fast, but it will also have some really BAD effects on the coupling between the generator and it's prime mover.

This is why it's so important to get the incoming generator phases to be at or very nearly the exact same frequency as the grid (which is a function of the speed of the generator rotor per the formula above) before trying to close the generator breaker. And, actually, most power plants synchronize the generator breaker with the generator frequency just slightly higher than grid frequency (0.1-0.3% or so for many machines). This means that the prime mover is putting a little more torque on the generator rotor than is required to keep it spinning at synchronous speed, and when the generator breaker actually closes the generator--and its prime mover--actually slow down slightly to exactly match grid frequency and the extra torque produces a very slight positive power (watts) flow out of the generator on to the grid.

In the same way that speed matching is done at a slightly higher speed than synchronous speed to ensure positive real power flow out of the generator at the time of synchronization, voltage matching is usually done with the generator terminal voltage at a slightly higher voltage than grid voltage to ensure that reactive power (VArs) flows out of the generator at the time of synchronization when the generator breaker closes. (VArs flowing out of a generator are considered to be positive VARs, and VARs flowing into a generator are considered to be negative VArs).

If a prime mover does not provide sufficient torque to keep a synchronized generator spinning at synchronous speed the grid (other generators and their prime movers) will cause amperes to flow in to the generator stator to keep it spinning at synchronous speed. This is called "reverse power" and represents a load on the grid, and the generator actually becomes a motor. And most prime movers DO NOT like being spun by their generators and can be severely damaged if spun by their generators for long. (Gas turbines are generally the exception to this rule.) Reverse power makes the generator (and its prime mover) become a load on the grid and that's not a desirable thing, either. So, there are reverse current or reverse power protections on the generator output--which is another reason that positive power is desirable when synchronizing a generator to a grid and why the generator frequency is usually slightly higher than grid frequency during synchronization.

During typical synchronization, the energy flow-rate into a generator's prime mover is slightly higher than is required to keep the generator spinning at synchronous speed. And, when the generator breaker closes the energy flow-rate into the prime mover either remains the same, or in some cases, the prime mover's governor will slowly start increasing (ramping) the energy flow-rate into the prime mover to cause the generator power output to start increasing at a similar rate.

If the voltage of a generator is lower than the grid voltage it is being synchronized to then when the generator breaker closes the grid will cause reactive power (VArs) to flow in to the generator stator, which creates a reactive load on the grid. And, most grids have a reactive load that requires VArs from the generator to maintain a stable and desired grid power factor, so increasing the reactive load on a grid when a generator is being synchronized to the grid isn't good. So, by having the incoming generator voltage be slightly higher than grid voltage during synchronization a small amount of reactive current will flow out of the generator when the generator breaker closes--and the generator voltage will become equal to grid voltage (the difference causing positive VARs, just like the slightly higher speed/frequency difference during synchronization causes a slight positive real power (watts) to flow out of the generator when the generator breaker closes).

The descriptions above refer to synchronizing a synchronous generator and its prime mover to an infinite grid with many other synchronous generators and their prime movers. Again, when the generator breaker closes and the generator immediately slows to synchronous speed and there is no appreciable effect on grid frequency or grid voltage. And, as the generator is loaded and unloaded there is no appreciable effect on grid frequency. And, during synchronized operation no single generator and its prime mover can run any faster or slower than its synchronous speed--which is a function of the grid frequency to which all the generators are synchronized to. Synchronism is the key word, and it's all about magnetism--two magnetic fields being locked into synchronism and the prime mover being unable to spin the generator rotor faster or slower than the generator's synchronous speed, which is a function of the frequency of the grid it is synchronized to. Often, synchronizing is referred to a "paralleling," and while parallel operation is a correct term for how the generators work together (as opposed to being in series with each other) paralleling doesn't really do justice to what's happening during synchronization, or what happens during synchronized operation. There are some VERY LARGE mechanical forces at work inside every generator synchronized to a grid with other generators (and their prime movers).

This is as opposed to synchronizing a synchronous generator and its prime mover to a small grid with one or several other synchronous generators and their prime movers and when the generator breaker closes there may be no immediate appreciable affect on grid frequency--but as the incoming generator is loaded or unloaded the grid frequency may be impacted unless something is done to the other generator(s) to compensate for the change in load supplied by the incoming generator. All the generators and their prime still operate at synchronous frequency (none faster or slower than its synchronous speed), but if the total generation exceeds the total load the grid frequency will increase, sometimes appreciably depending on the relative sizes of the generators involved and the total load. Total generation must always match total load, or the grid frequency will deviate from desired, and so will the synchronous speeds of all the generators and their prime movers--together.

It's all really a balancing act--even on infinite grids. The grid operators have to be able to load and unload some, or many, generators and their prime movers as load on the grid changes. As people wake and begin turning on their lights and coffee makers and televisions and load increases, grid operators must increase the power output of one or more generator(s) to match the change in load--or the grid frequency will start to drop. Conversely, as people shut off their televisions and lights and computers and computer monitors at night if the grid operators don't reduce the loads of some generators by an equal amount the grid frequency will start to increase. The amount of generation on a grid must exactly match the amount of load on the grid--or the grid frequency will deviate. Grid operators don't know how many lights and motors and televisions and computers and computer monitors are going on or off at any given time, but they CAN see the changes in grid frequency at any given time. And, they can predict, based on historical norms, when grid frequency will start to change and can anticipate changes by slowing increasing or decreasing generation to keep grid frequency. It's when they don't properly anticipate changes, or large blocks of load suddenly drop off the grid, or large generators suddenly trip off the grid.

And, it can be even more "interesting" for small, islanded "finite" grids.

But, every generator has to be synchronized, as noted above. And the smoother the synchronization the less effect on the grid frequency and voltage. And, the less damage caused to the generator, its breaker, the coupling between the generator and its prime mover, and the prime mover. That's why synchronization is so critical, to protect both the grid and the generator and its prime mover.

Hope this helps! I hope I understood (most of) the questions correctly.

CSA

thanks for the great lecture, you are a walking encyclopedia! By breaker, I believe you are alluding to generator protection (Relay) breaker?

I am an Electrical Engineering Technician based in Ghana. For most of my career, I have been installing Low Voltage Switchgears. In future I want to work in one of the new Independent Power Plants being built to address the energy deficit here. I would like to keep in touch

Plaudits
Wisdom
facebook -wise samlafo

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Widsom,

I was referring to a three-pole, high-voltage breaker (11,000 Volts to as much as 13,800 volts). The generator outputs of many power plant-sized generators are in the range of approximately 11,000 to 13,800 volts.

There are many relays, or sometimes one single multipurpose relay, used to protect the generator and auxiliary equipment.

I prefer to keep in touch via this forum, as when I answer your question(s) many people get to see and read from the exchange. This is much better than one person answering the question(s) of one person via email or some other limited forum.

There is no such thing as a dumb question--just a dumb answer. It takes a little something different to explain things to people some times (we answer the SAME questions many times over here on control.com, with slightly different twists on the question and/or the answers--to try to help people understand their situations).

There is a wealth of information available here on control.com through the 'Search' function at the top of every control.com webpage. The search syntax is a little different for control.com, so it's strongly suggested that you use the Search 'Help' feature the first few times you search for information, but it's very fast and, again, there is a LOT of really good information here on control.com. And, one of the best things about reading many of the posts on control.com is that we ask posters if they find the information valuable or useful--or not, as the case may be--that they right back to let others know if they found the information useful or not. It's that feedback that makes the posts useful to MANY people. And, it's one of the things that sets the quality of information available apart from other sites--people get to see if the information provided was useful or not.

Welcome to the forum! Use the 'Search' feature to peruse the Archives (as they're called), and let us know what you think about the information. Even if it's just a click on the 'Thumbs-up' or 'Thumbs-down' for a post. It all helps. And, remember, respondents don't get paid for their time and knowledge--so, feedback is VERY helpful and courteous for them to know if they've been helpful or not.

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From the electric utility operations perspective, generating units can and do affect the "infinite bus" grid frequency.

There is a recent, real-life example almost exactly like CSA describes, although in a 6000 MW system (perhaps 6000 GW is a typo?): on 02-jan-2017 22:13 local time in New Zealand (roughly 6000 MW peak system), a 140 MW unit trip (I'm assuming the unit was at/near full load) brought system frequency to 49.4 Hz (NZ is a 50-Hz system). The pre-disturbance frequency isn't stated, but supposing it was even 49.9, that 140 MW loss dropped it 0.5 Hz. You can look this up at https://www.transpower.co.nz/power-system-live-data

In the system I'm in (Western third of North America), one real-life, typical example: when total generation was about 135,000 MW, a 700 MW unit trip step-changed the system frequency down 0.08 Hz, from 60.00 to 59.92. That's large enough to draw attention, and utilities here must demonstrate that their aggregate generation response due to governor action (droop) contributed their share of MW in response to the under-frequency. We are all in this together.

The reason that normal generation output changes and unit startups/shutdowns don't cause these effects in my part of the world is because there are one or more central control systems that adjust generating unit outputs to compensate. The error, or difference between total desired and actual generation, is computed every few seconds and MW change controls issued when the error exceeds a threshold. It is possible to accomplish this manually; depends on the amount and duration of frequency deviation your system (generators and customers) can tolerate. And in systems where utilities exchange, or buy/sell power, there is greater effort to maintain the actual power transfers close to their desired values.

In the end, as CSA writes, it's simple: Power equals torque multiplied by angular speed. Electric power generated equals electric power consumed, otherwise angular speed (system frequency) isn't what it should be.