Synchronization 1

[joan271273]

Could someone provide some insight on the difference between active and passive synchronization.

I have a project with closed transition were everything was designed around "pasive" synchronization and the Generator vendor is stating that they cannot do that and they only do "active" synchronization.

What are the differences?
Are there standard ways to bridge the gap or to make this work.

 

[davidbeach]

My guess would be that passive synchronization is waiting for the generator to drift into synch with the utility while active synchronization would have the synchronizer controlling the generator speed to drive it into synch. One requires rather more hardware and controls than the other.

 

[joan271273]

davidbeach: Yep something like that is what I am told. I am still reaserching the web so I can seen how you modify the pasive to go active without spending my whole job budget.

 

[davidbeach]

I doubt that you can. The least expensive I am aware of would be the Basler BE3-25A. You have to get the right version to go with the type of governor on the gen set.

 

[joan271273]

Thanks I will be looking at the Basler site

 

[joan271273]

Generally this means allowing the two sources to come in synchronization without forcing a change in the speed or voltage of one of the sources. Passive sync with the utility is often difficult since we normally want the softer(smaller) source to come in with a slightly faster frequency and slightly higher voltage than the bus-or in this case utility. The loaded generator system is probably in droop mode meaning both the frequency and voltage are slipping behind and below the utility. The second problem with paralleling with the utility is the necessary shift in generation from following the KW and KVAR demands of the load to serving the grid. When we parallel with the utility we want to select either Var control or power factor control as a set level of excitation.

davidbeach, I received this input offline and this is a way he sees through this. I am sharing this info to hear your thoughts.
..."First I would set the generator system to run at slightly above 60 Hz; maybe 60.2 or 60.5 Hz. This would give you a natural slip between the two sources but not cause a problem for any equipment. I would then raise the nominal generator voltage to above 480V. If I could I would try to push the utility voltage down a bit, maybe 2.5% or so. Finally I would set the sync check relay window to about +/-5 degrees with a dwell of about .5 sec. You will want to do the transfer with no more than a one second overlap. This will result in a fast load of the utility but you should be able to make the re-transfer without going to active sync"....

 

[waross]

What size is your generator? Techniques that work well below 500 KVA, and can be used carefully up to about 1000 KVA are unacceptable on larger generators. Do you have an electronic, hydraulic or mechanical governor? Do you have human operators or is the installation meant to be completely automatic?
respectfully

 

[davidbeach]

What kind of closed transition are you doing? Is this a transfer switch with closed transition or are you breaker switching? Are you trying to cause a closed transition from utility to generator or only from generator back to utility? Coming in a bit fast and high is nice when paralleling an unloaded generator with the utility but not necessary when paralleling a loaded generator to the utility. If you are working with a closed transition transfer switch, or you have a breaker scheme that will do the closed transition in less than a half second or so you probably don't need to worry about it either.

 

[rbulsara]

I have a feeling that "passive" is referring to using a closed transition automatic transfer switch, which does not force or manipulates any source to force a sync but waits for two sources to come to synch when the generator is drifting. The transfer is typically completed within 100ms once the sources are in sync. Most ATS will open one of the source contacts if the closed transition time goes beyond 300ms. There is also a requirement by utilities to provide a watchdog timer to open one of the upstream breakers if the paralleling time exceeds 0.5 sec. This is commonly applied, most cost effective method for momentary closed transition transfers. This requires no special devices on part of a generator vendor.

Active paralleling required a whole complement of generator paralleling synchronizer, protective relays, load share modules etc, and allows for extended period (more than 0.5 second) of paralleling time. This is much more sophisticated approach for larger installations.

 

[joan271273]

The system uses a Main-Tie-Main Breaker arrangement controlled by a PLC. The generator is connected to one of the breakers.

 

[rbulsara]

The the main-tie-main arrangement must meet the requirements of the ATS (UL 1008) as described in my previous post.

YOu need to clarify the situation a bit more. Is this a speciofication provided by a designer/engineer and a vendor is saying he cannot meet it? What is your role in this?

It sounds like the engineer who wrote the spec need to answer vendors questions.

 

[matrixabc]

My interpretation of closed transition "Passive "would mean the generator is set as close to the mains voltage as possible, voltage and frequency, normally just a bit higher, the switch will detect when they drift in to synch then fire the breaker closed in parallel then open the other all within milliseconds. In UK a well known switch is made by "ASCO" (not sure of the spelling).

 

[joan271273]

All , thanks for your input. After several meetings between gear vendor and gen-set vendor.
This is what I can summarize:

The specified and designed transfer system was intended to provide a closed transition from generator to utility power using a passive synchronization strategy. The concept, when the transfer controls determine a common voltage value for the two sources, the breakers are closed connecting the two sources in parallel. The utility source will dominate the generator source forcing that system into complete synchronization, and then the generator source can be disconnected without any interruption of power. In the unlikely event that the controls cannot find a common voltage point (which I think is generally at zero volts) within a set time limit, the control then defaults to an open transition.

Unfortunately,our project uses a Gen Set unit ( the green one as to avoid naming manufacturer) with a proprietary generator control system. This unit will not allow passive synchronization since it will see any attempt to connect to a source not already in complete synchronization as a reverse power flow into the generator and the generator system will trip the generator breaker.
This will then create an unintended loss of generator power and an undesired open transition.

The gen-set vendor has refused to allow any changes to their controls.

To allow for the proper closed transition back to utility power, additional components and revisions to the transfer controls will be required. In this case, we believe the proper strategy is to show the utility source as a generator, and to use the Gen Set system to actively synchronize to that “generator”. This active synchronization strategy uses standardized features of the Gen Set control system.

I hope this clarifies better were do I stand.

 

[rbulsara]

In the USA:

Passive sync and transfer cannot be any longer than 0.5 seconds in the worst case. As I described before it usually is completed within 100ms (before two sources drift much further as they are always drifting). At 0.3s (300ms) the ATS (or breaker) would separate the sources and you need a compeltely independent watchdog timer for not exceeding 0.5s. All this will have to be approved by the utiltiy co.

This is no different than a "Closed transition" ATSs.

If still confused seek help of professionals with experience in this area.

 

[rbulsara]

joan:

Are you the design engineer?

 

[joan271273]

Guys, I am not the design engineer, but the design engineers are stating that its the generator problem and viceversa.

All of the documentation even of the gen set brochures state Active Synchronization, not passive.

So what I am trying to get is an idea of the path we should follow to get this resolved.But I also do not want to get cornered with a "gold" plated design. It was nice when engineers use to stand up for their design and not just point out that "submittals" do not relieve a vendor or sub!!!!. When in particular I paid for a system design!!!

 

[rbulsara]

So called passive synch does not require anything special from the generator vendor. Its all the function of the automatic transfer device meeting the requirements as I stated.

Looks like you need opinion and help of other professionals and some re-design.

 

[waross]

It sounds like more than one engineer didn't get his reality cheque in the mail last month.
If the genset does active synchronization then it should be a simple matter of setting the parameters of the proprietary gen-set control for active synchronization. This will avoid defaulting to an open transition anyway. If the set defaults to fast open transition, it may cause machine damage.
back to your original question, what is the difference between active and passive synchronization?
With active synchronization, the control adjusts the throttle setting to bring the set into sync with the bus as soon as possible.
Passive takes advantage of the fact that generator frequency is seldom exactly 60 Hz. If the set is running at 61 Hz. it will drift into sync once a second. The fast transfer is made at the moment that the set is in sync.
I doubt that the ATS can tell the difference between active and passive sync. Either way, when the generator is in sync, the ATS should recognize the synchronization and transfer.
Passive sync is often used on smaller residential and commercial sets with basic governors. The governors are usually running in droop mode and will drift into sync quite quickly.
If your vendor is supplying a set with an autogenous governor, and it is well set up, it may take so long to drift into sync that it defaults to open transition on virtually every transfer. If you have any large motors, you will probably eventually damage one, one day.
It may be that the vendor is trying to supply a "Cadillac" system and your engineer is desperately trying to get a "Volkswagen" system (at a Cadillac price).The engineer specifying passive sync. has probably copied someone else's spec. and is afraid to change it because he doesn't understand it. I have seen some expensive installations specified that way.
The challenge is to get the engineers to stop shouting "Active" "Passive" at each other and sit down and make the system work. As I suggested, if you let the set pull itself into sync when it wants to, the transfer switch will never know the difference.
If the gen-set has an active sync feature, you may have to run two or three wires to provide a grid reference voltage to the gen-set control. The gen-set controller may also need to send an inhibit signal to the transfer switch to delay transfer until the gen set is ready. That should not be a budget breaker.
Oh and by the way, if you are using motor operated breakers you definitely want active sync. With passive sync, the gen-set may be drifting too far out of sync. again before the motor completes closing the breaker. That's really bad, and that's an understatement.
respectfully

 

[Skogsgurra]

It may take some time to read your posts, Bill. But always worth the effort. PLS for you.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Thank you very much Gunnar. I enjoy and respect your posts also.
Respectfully