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Paralleling Distribution Generators

Paralleling Distribution Generators

Paralleling Distribution Generators

(OP)
What are the typical commercial practices of paralleling distribution generators? In addition, what are the protection features used to prevent distribution generators from synching out of phase?

RE: Paralleling Distribution Generators

A synch check relay is the usual solution.

RE: Paralleling Distribution Generators

Who are you? Your profile provides no information...you may well be a student, and student postings are not allowed.

RE: Paralleling Distribution Generators

Base load?, peaking?, standby?, co-gen?, surplus energy?, manual sync'ing?, automatic sync'ing?, droop control?, load control panel?, and/or a couple of other questions to zero in on an answer for you.
What specific application are you concerned with?
And what specifically do you mean by distribution generators? Do you mean distributed generation?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Paralleling Distribution Generators

Well, when I paralleled two generators in the submarines, there was no "protection" except the panel operator reflexes.

RE: Paralleling Distribution Generators

Solutions would typically be:
  • Synchronising relay backed up by check-sync relay
  • Manual sync backed up by check-sync relay
  • Manual sync without backup
Manual synch could use a syncroscope, or phasing lamps, or a couple of moving coil meters in decreasing order of preference.

RE: Paralleling Distribution Generators

After the solutions suggested by Scotty the next step may be overcurrent protection. This may be a molded case breaker up to around 1 megaWatts to 2 megaWatts.
The next step may be the addition of a reverse current relay to protect the prime mover from damage that may be caused by motoring. This may use a shunt trip on a molded case breaker.
Back in the day protection would be added piecemeal with individual dedicated devices. Today, any more extensive protection would probably be provided by a sophisticated "Generator Protection Relay" that would almost all protection functions in one device.
A sophisticated load control panel may include both prime mover starting and automatic synchronizing in one device, as well as speed control and load sharing.
Nice paper Mike.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Paralleling Distribution Generators

(OP)
davidbeach: I have heard of using sync checks to help perform paralleling. However, looking at IEEE standard its hard to know what settings to apply, for example is a maximum of 10 degrees phase difference a typical reliable number to use for paralleling.

crshears: No I'm not a student, but I am new to this forum which is why not much information of me yet. I am a electrical engineer just curious to know how others have performed paralleling in their fields.

waross: manual synching, but have automation ensure it is safe to close breakers. The specific application i am concerned with is proper settings for synchronizing generators using automation and what generators are typically designed to for example are large generators typically designed to 10 degree phase difference or less? do you know of standard settings of voltage magnitude percentage difference, phase angle difference, frequency slip that are safe for most large generator application. Base load would be the preferable point of paralleling. Distribution generators, what i meant to say is parallel operation of generation on distribution systems, so yes distributed generation.

racookepe: thats interesting. So what would happen if there was reverse power?

catserveng: thanks, this is a nice piece of document , very helpful.

scottyuk: that is really good information. Looks like manual sync with sync check relay be of the most feasible. Are synchronizing relays similar to a sync check relay but it automatically generates the paralleling process?

waross: Yes overcurrent protection would be next things to consider. Have not used a molded case breaker, are they typically used for low power generation? generator protection relay I heard are really useful for paralleling.

RE: Paralleling Distribution Generators

Hello eepowersystemlogic, thanks for the info about yourself; I appreciate it.

I haven't had much to do with synchronizing for a few years, but here's some of what I remember about it; note that parts of this are anecdotal, but hopefully you'll find at least some of it worthwhile...

Back when I was a lowly [Thermal] Auxiliary Plant Operator, I was first exposed to 1960's-built 300 MW steam units without any auto-synch features at all; the equipment provided for manual synchronizing was incoming and running voltmeters, incoming and running frequency meters, twin synchroscopes and a back-up set of incandescent light bulbs wired so as to be at full brilliance when the incoming source and the system were 180° out of phase and fully dark when they were in synchronism. All of the foregoing were used in concert with the excitation control system, unit governor, field breaker and 230 kV ring bus breaker controls to synchronize the units.

One pair of units in that eight-unit plant did have something called ACTRUS, which stood for Automatically Controlled Turbine Run-Up System; I was told this system did include an auto-synch facility, but that the performance of the entire ACTRUS scheme had been lacklustre and the operational gains from its use minimal, and it had not been used in some time.

The emergency 7.5 MW combustion turbine generators on the site were equipped with fully automated synchronizing schemes built right into the auto-start system, meaning that if there was a 'green board' indicating all pre-start conditions were satisfied the operator could initiate a start of the unit and it would purge, ignite, complete its run-up, self-synchronize and go to minimum load, all without further intervention. There were also complete manual facilities installed; these had the same voltage and frequency meters and bulbs as the large units, but only one synchroscope. It was these 'little' units that were used to teach us junior staff how to perform manual synchronization.

A few years later during my retraining to become a power system operator I once had the task of manually synchronizing a tiny little hydraulic plant consisting of three 600 kW units tied to a common bus and thence to the grid by a single breaker [other than for maintenance outages, the number of units selected to run was based on the amount of water available]. The units were equipped with DC-motor-driven gate positioners; the units to be run were spun up from rest to roughly one-half or so of synchronous speed, then field was applied to the units to draw them into synchronism. Once the rotors had been electrically locked together, the units were gardually brought to synchronous speed, voltage was matched, and the plant was manually synchronized using a 1920's vintage synchroscope. The operator had to give the synchronizing breaker a close well in advance of the 'scope reaching the twelve o'clock position, since there was a delay between giving the close impulse and the breaker contacts actually making. Unfortunately I misjudged this and 'made a bad shot,' as it was called, or at least I thought so, since I succeeded in shaking a quantity of dust loose from the building's girders; but I was told that the shot was a good one as long as the units din't trip back out, and they did hang in there, although I could hear a series of diminishing wow-wow-wows as [due to hydraulic lag] the mass of the rotating units first imparted excess energy to the grid then motored; it took about ten or fifteen seconds for the units to settle out.

Another small hydraulic single-unit plant had a generator equipped with amortisseur windings; to start this unit, it was accelerated from rest to near-synchronous speed by opening its wicket gates part way, at which point the unit breaker was closed and the generator was connected to the line as an induction motor. Field was then applied gradually until the unit pulled into synchronism with the grid, at which point its real and reactive power outputs could be adjusted to the required values.

I heard of one plant consisting of four horizontal-shaft 900 kW units where the manually-closed unit breakers were some physical distance from the swivel-mounted synchrosope, which posed difficulties for myopic operators. It didn't take long for the operators to notice that once the units were synchronized, the spokes of the salient-pole rotors not only appeared to be stationary due to stroboscopic effect, but that one of the six spokes always appeared to be perfectly plumb. The smart and lazy operators soon figured out that after laboriously synchronizing the first unit to the system using the synchroscope they, once having very closely matched the speed and voltage of subsequent units to the required values from the switchboard, could observe the visual 'drift' of the rotor's spokes, and when any spoke was plumb, close the unit breaker to place the unit on line without reference to the synchroscope; it worked flawlessly and every time.

CR

RE: Paralleling Distribution Generators

The size of your units may make a difference to the level of sophistication of the controls and protection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Paralleling Distribution Generators

Very true, Bill; the more units are worth, and the greater the impact they can have on the local electrical area or grid reliability, the more complex the protection and control schemes typically become.

Choosing a manual synchronizing process with synchro-check supervision does appear to be a reasonable middle-of-the-road approach, particularly since the qualifications of operators can vary widely. As to the settings of the various parameters within the S/C relay, I'll leave that to those better qualified to answer than I am.

For synchronous units, there will typically be an interlock to prevent unit breaker closure if the field breaker is not closed, as well as loss of field protection to immediately remove the unit from the line for field failure. It may or may not be necessary to also trip the prime mover; this has to be assessed on a case-by-case basis.

Reverse power protection can be provided if needed to protect the prime mover; for instance a steam turbine's supply valves cn be fitted with limit switches to detect any combination of valve closure that would interrupt the steam flow to the turbine. This is commonly supervised with a time delay so that the unit will not immediately trip if the valves close for only a moment in response to governor control actions due to system conditions.

CR

RE: Paralleling Distribution Generators

A check-sync relay is normally a permissive relay, i.e. it gives a go / no-go signal which either allows or blocks action by another relay or by human intervention. They normally monitor voltage mis-match and phase mis-match as a minimum. Some include slip frequency too, although in my experience that is a feature which is often present and rarely implemented.

The synchronising relay is a command relay which can raise and lower volate by interfacing to the generator's automatic voltage regulator, and raise and lower and lower frequency by interfacing to the turbine speed controller. A well-adjusted unit will typically close the breaker within a few degress of phase match and within a couple of % voltage match. It is normal for the relay to have a predictive function to compensate for the breaker closing time, i.e the relay issues the breaker close command slightly early.

The main function of the check-sync relay is to prevent the synchronising relay from doing anything untoward if it suffers a fault or internal failure, or to prevent careless operators from closing the breaker out of step. I've experienced the latter once from the substation end and the noise from the busbars was scary, although I'm told it was considerably worse on the turbine floor where the whole massive structure seemed to rock. That was a combination of broken synchronising relay, check-sync permissive set far too wide and an inexperienced operator.

RE: Paralleling Distribution Generators

For distributed generators, the synchronization tolerances are provided in IEEE 1547. The smaller the amount of generation, the larger the tolerance windows become. Up to 0.5 MVA, for example, the standard says that 20 degrees is an acceptable phase difference. The standard covers DG in aggregate up to 10 MVA.

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