Parallel Operation

[Guardiano]

Hi,
I am facing a dilemna.We are planning to operate our generator, a 5 MW steam turbine driven one,in parallel with the grid. The 5 MW plant is actually providing power to a cement factory. Our intention is to stay connected with the grid and import around 500-800 kW.Previously we were operating in island mode.We are also implementing a load shedding plan which will shed most non-critical loads in the plant should the 5 MW plant trip. The intention is to avoid importing too much from the grid and to keep the peak kVA to a safe limit as we are actually charged by the utility on both kWh imported and a fixed kVA charge. We wish to use the grid only as a facility which will provide power to the services of the energy plant for starting-up and then use the energy plant power to start other services of the factory. The auxiliaries of the energy plant require around 600 kW. My concern is as follows. I wish to limit kW import as far as possible when the factory is started.
Thanks for any input.
Guardiano

 

[7anoter4]

IMO the power you deliver to the grid has to be independent of your income power. The mains have to provide all the power you need.
In order to adjust the required power to the maximum possibility of your generator you have to state which of your consumers will stay connected and the minimum level of frequency your consumers could withstand. For the redundant power consumer an underfrequency relay which has to be provided will disconnect it in time in order to limit the required power to the maximum available.

 

[mgtrp]

From my understanding of the problem, an under-frequency scheme probably wouldn't provide the intended results. As I understand it, Mr. Guardiano wishes to load-shed when he loses his generator supply, not his grid supply, which is an event that should have negligible effect on the frequency assuming that the grid is relatively large in comparison to the generator (e.g., almost any national grid with the possible exception of a small island or similar).

One option might be to ensure that your start-up auxiliaries are on a dedicated bus supplied from the grid (perhaps with an intertie to your generator-supplied bus).

Another simple low-cost option might be to provide a transfer trip from the auxiliary of the generator circuit breaker to trip non-essential plant loads whenever the generator breaker is open. Another option might be a forward power relay on the grid supply to activate load-shedding when the grid power exceeds the amount that you want to limit it to (by time staggering the trips to different loads, you could even limit the amount of equipment that gets tripped to roughly the right amount to balance the grid supply).

Please bear in mind that there's a very good chance that your demand charge is based on a 15-minute average rather than instantaneous peak, so you might want to take this into consideration when setting up your scheme to avoid operating on inrush currnets.

 

[squeeky]

Dear Guardiano,

Firstly if I were you, I would look at your co-generation agreement that you have with the Utility to ensure that both sides have the correct protection and that they have removed any special operational requirements on their side like auto re-closers. If they open under a fault, then close again, you could close unsynchronized and this could cause some serious damage to your machine.

You need to ensure that you see any faults on their side very quickly so you should have dV/dt (ROCOF) or vector shift. Under / over frequency and voltage is also a good idea. You don’t want to feed their faults and trip on under frequency.

When you connect to the Utility, your Alternator mode of operation needs to change from Voltage mode to pf correction mode or at worse case droop mode. This is set in your AVR (Automatic Voltage Regulator) on the Alternator. The former is better. It looks at the status of the breakers to determine what mode you should be in.

Then your operational philosophy will have to include putting your machine demand control into base load to ensure it always delivers the maximum power and thus minimizing your energy bill. This would be the disadvantage of drop mode. You cannot affect the voltage of the grid so the only thing you can do to operate efficiently within the capability of your machine is to alter the power factor.

Your load shedding will have to be extremely fast and I have never seen one that operates fast enough to shed internal load instantaneously with loss of mains if you are running well over on your demand. We seem to have everything and we still black out. Normal hydraulic governors will not be fast enough. You will have to look at the electronic ones. Dawson and Woodward. I am more familiar with the Dawson system which will do everything you require and also warm your machine up to OEM specifications and do all the switching between modes (on and off the grid) change your load profiles Base Load / demand load etc. Has lots of protection features. I know Woodward would have the same.

The tariff was mentioned. Watch out for that maximum demand. Look at your tariff structure carefully as you may have to trim production during those periods.

 

[tinfoil]

Speaking from a utility perspective:

A modern DG (distributed generation) facility with 5 MW capacity connected in parallel to a medium voltage ( <50 kV ) 'grid' would very likely also require a monitored 'transfer-trip' component to ensure that any grid loss-of-supply would send a signal to trip and LOCK OUT your utility interconnection device, until the utility eventually permits a sync/close in once the grid system is restored to normal.

This way, you are protected from whatever reclose (79) sequence they have on their breaker / reclosers, and they are protected from any possible backfeed / islanding issues you might create.

If you want to keep your plant generator 'up', your control system will have to be pretty quick to adjust for a sudden 800kW 'shortfall'.

You may also have to conform to a variety of technical standards as well, such as harmonic content and maximum allowable energy inrush when connected to the grid, to control utility 'flicker'.

From a tariff point of view, a 15-minute average would most likely be used to calculate your monthly peak. Where I work, our meters actually integrate over 5-minute intervals and then average three-in-a-row to derive the 'peak'. I am aware of a few utilities that have an ANNUAL ratchet on the peak demand component if your peak coincides with the utility's own peak period.


My advice would be to speak directly with a suitable technical person at your local utility to identify and address these issues.
Some utilities may even not allow you to do what you are proposing!

 

[crshears]

Our utility refers to this sort of configuration as 'load displacement generation' since in this instance the ST output will never normally exceed the cement plant's consumption.

We would definitely want to see some sort of remote or transfer tripping in place to separate the facility from the grid in the event of a feeder auto-reclosure or lockout [auto-reclose-auto]. Permission to reconnect to our grid post-event must always be obtained by the customer, and this requirement is clearly spelled out in the customer-specific Connection Agreement.

I ditto the recommendation to confer with your local utility to ensure all of the issues mentioned above are addressed - - or at least discussed. It would also be very advisable to make a record of which technical requirements the utility waives; the ones they insist on will for sure make it into the connection agreement [their lawyers will see to that], but it's better to have something in place about what wasn't asked for as well, just in case of a future issue arising...

FWIW we have a very few customers with generating capacity only slightly in excess of their own internal consumption who will deliberately separate themselves from the grid and go into 'island' mode whenever they deem the grid to be operating at too high a risk level for their comfort, for example when there is lightning activity anywhere within 2oo km of their site.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[squeeky]

South Africa, Malawi, Zambia, Lesthto, Mozambique and Swaziland work on a 30 minute cycle. As you say they ratchet up everytime you exceed your maximum demand and this becomes your new notified demand. During the Offcrop or Shutdown period we then have to pay the minimum of 70% of notified demand even though you may only be using say 20%. On an annual basis you can negociate to reduce it. crshears is correct about the protection. You really have to be carefull. As tinfoil stated understanding your agreement and keep the douments. Auto disconnection requires that you have electronic governors to "catch" the change and keep you up and running. It will be a process of continuous improvement to perfect it.