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Looking for Line Frequency Monitor. 6

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itsmoked

itsmoked

Electrical
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Feb 18, 2005
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I've tried all the search subjects I can think of and have found lots of really nice phase-loss protection relays but not a single one will trip on frequency deviation. You'd think no one used non-utility power.

Does anyone know of a line freq monitor?

240V, 3ph, no neutrals. It can, of course, be just a single phase.

It will be used to inform a PLC.

Keith Cress
kcress -
 
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Can't do it with a (relatively generic) power meter, either with a discrete alarm contact, or via some form of communications?

I've seen similar things done with over / under speed relays and a magnetic pickup, but that assumes that you are dealing with a rotating machine and have access to it. I'm aware its not the same thing, although for practical purposes it can be close enough depending on the application.

Crompton's 250 series might well have something, its been a while since I looked though, and I was always trying to take them out, not put them in.

Edit: Crompton has such a thing, looks like they've redone their 250 series, look for a 'PHD' part number.

EDMS Australia
 
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Generator problems, Keith?
Some issues that i have encountered and possible solutions.
How much frequency deviation is too much in your application?
Frequency deviation is normal for a standby generator.
Unloaded they will produce 61.8 Hz., at full, load 60 Hz and motor starting, a combined frequency and voltage dip.
A/C units will generally cause a dip when starting.
Often the off frequency is not damaging, just a nuisance.
Some models of UPS have quite a tight frequency monitoring circuit and will spend a lot of time alarming when on generator power. My customers have all decided that they could live with it, rather than spend money.
If you have a UPS in a public area, or if there are too many affected people to educate you may consider disabling the audible alarm.
You may be able to find a more frequency agile UPS.

If there is a possibility of real damage, move to plan "B".
Try to avoid complete shutdowns if possible.
Protect critical circuits with a UPS.
Use a frequency relay and a timer to ride through short frequency excursions.
Alarm on longer frequency excursions. Possible one minute or 5 minutes.
Shut down either the critical circuits or the entire set on sustained low frequency.

Anecdote warning:
I saw this done at the mill associated with one of the worlds largest open pit mines.
The incoming service was at 140,000 Volts. The main mills totaled 36,000 HP plus several thousands of HP more on the flotation skimmers.
The mine was on a large island with hydro generation and also undersea cables from the mainland.
If the undersea links were lost, the island could become "Islanded" and may not have enough local capacity for the load.
Two under-frequency relays were installed.
At 58 Hz. severe load shedding was automatically applied.
At 56 Hz the whole mine went down and could only be reconnected on the permission of the utility generation dispatcher.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Thanks Flexo! Doesn't look like it's easy to lay hands on and the Crompton's 250 series is also proving really hard to come by.. Dang.

Thanks Cat but I haven't brought myself up to playing Basler's nasty bait-and-switch registration scheme yet, though at this rate I'll probably have to.

Bill; Yeah, generator duty on a private rail car. Two 10hp refrigeration compressors. Picture them struggling to keep the dome car habitable while in motion in the southern desert on a 120F day. Everything under maximum stress and the generator deciding to drop a couple of hertz. The compressors would toast in a minute or two. If I can sense the drop I can dump some loads in hopes of keeping things running and maybe the generator will recover. Maybe I can figure out a way to measure the frequency with the on-board PLCs. There is a UPS on-board, a big one, but it doesn't give any problems with generator power. I have seen lots of smaller ones decide a nominally running generator as being 'foul' and proceeding to run down their batteries pointlessly. Seen that lots actually.

Keith Cress
kcress -
 
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Part of the reason I kept looking at removing things like the Crompton units (they do the job they're supposed to quite well, but there are easier ways of achieving things these days) is that the all in one controllers do most of the work without custom coding. The approach of using a PLC was better placed when the controllers weren't that capable, but usually had the issues of having to process signals (i.e. using signal processors to bring AC voltage down to a signal level the PLC can handle, either using pulse counters or discrete relays to work out when to stop the engine cranking solenoid).

If you have the opportunity, I'd look at things like the lower end Comap (or even Deep Sea) controllers, the high end ones are way more expensive, but the lower end ones often end up comparable to things like the Crompton relays, most of them have setpoints for both frequency trip, and load shed. If the PLC is already installed and operational, its a different argument to completely replace it of course.

EDMS Australia
 
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I appreciate your concern, Keith. The situation may not be quite as bad as you fear. The Under Frequency Roll Off feature of the AVR will give you some protection. The UFRO drops the voltage proportionally to a frequency drop to relieve some of the load on motors and give the generator a chance to recover. While UFRO is great for overcoming motor starting dips, in your place I would probably also be looking at some type of load shedding.

Here is relay that you may like. I've never used one, I'm just going by the spec sheet.
Link
Here is the parent site:
Link

You could also use a tach on the engine.
Alternate solutions:
Voltage, when the frequency drops below 57 Hz the AVR will start to drop the voltage. Cheaper but not as dependable nor as precise as a frequency based solution.
Current: Monitoring the current would be more precise and more dependable than a voltage indication, but still not as good as frequency based protection. As it is kW and not KVA that drags the frequency down there may be PF issues.
Tried and found wanting, stay with a frequency or RPM based solution.
PC or PLC; If you have a PLC dedicated to the system you could use a simple one-shot circuit to input cycle pulses from the 60 Hz and count the number of pulses in a second.
If there is a PLC, consider using it. Myself, I would not add a PLC just for frequency monitoring, unless I was going to use it to control some timing and some fancy load shedding.

Is there any choice in fuel supply? You want the heaviest fuel available.
Up here in the winter, summer diesel fuel may gell in extreme cold, shutting down vehicles. Winter diesel fuel is about 10% lower specific gravity. That translates to about a 10% loss in power. It sounds as if you can't stand any power loss at all. It's good to be aware of the effect of a lower specific gravity fuel even if you can't do anything about it.
Keep us posted on your final solution.
Yours
Bill


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Unfortunately UFRO is hell on hermetic compressors, they stall and go to locked rotor current rather than smoothly cutting back on current draw. UFRO works great with loads that appear to be constant impedance; puts up with loads that exhibit some degree of constant power, can’t deal with loads that stall under reduced voltage. Utilities see this in FIDVR (Fault Induced Delayed Voltage Recovery) events where a fault causes a momentary voltage sag that causes the compressors to stall, drastically increasing the current and thus making the sag worse. Full recovery can be delayed until all the compressor thermal cut outs take them off. Then they randomly restart at full voltage and do just fine.

In Keith’s case I’d try to get rid of enough compresss long enough to let the frequency recover and then bring them back as soon as practicable. Keep the fans running as much as absolutely possible.
 
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The UFRO is not a only voltage reduction. It keeps a safe V/Hz ratio as the frequency drops. You don't have that in most of your grid faults.
A hermetic doesn't depend on flywheel effect nearly as much as a belt driven air compressor. They can be slowed down quite a bit as long as a suitable V/Hz ratio is maintained.
Also a voltage sag induced by an increasing load is not the same as the voltage sag caused by a severe fault.
I have had a lot of hands on experience starting A/C units on smallish standby sets. An A/C unit starting can pull the frequency down quite a bit without the refrigerators stalling.
Still, I wouldn't like to count on the UFRO alone.
Before UFRO was implemented in AVRs the issue with under-frequency was saturation. The old AVRs would keep the voltage up as the frequency dropped. The effect had some similarities with an under-voltage stall. Excess current and overheating. Wait for the thermal protection to take the unit offline.
UFRO acts similar to a VFD. It works well with most motor loads. As the frequency and voltage drop, the kW damand drops, allowing the prime mover to recover gracefully. The motors still may develop full torque without over-current.


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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I am SHOCKED! Simply shocked by all the great suggestions here! After Bill mentioned "Frequency Relay" I've been able to find quite a few different ones, but you guys have found even more. Really cool.

The car has two PLCs installed (or will momentarily) Freddy. Not sure how to get them to monitor 16ms events for a tiny variation without maybe impacting the ongoing scan periods. These are CLICKs so they aren't real agile with servo inputs etc. Probably much easier to just add one of these cool freq relays I'm being shown and get on with all the other stuff that absolutely requires engineering time.

I hear you David. I can see stalls happening since the only flywheel is the motor armatures. I can also see them shedding some load with slower speed which would help. I also will be controlling the head unloaders (6 cylinder comps) so there is some sequential shedding available.

YIKES!! Earthquake.
URL]
Damn, that was nasty. The Berkley quake. Hit me as a single massive P wave that came thru the house. I heard it 400ms before it hit - always that brief moment in time where your mind is parsing the sound as "what is that?" and pow!. Bet that woke you up Jeff.
URL]


Anyway, DYFI report submitted.


Jeff; great links! I love that meter with 4-20mA output. I'll probably go with that since it's lovely accessible by the car managers, if it's not excessively priced.

Cat that's a great little site link. Thanks.

I'll keep you all in the loop. Gotta head to Spokane for a couple of days. Back soon.




Keith Cress
kcress -
 
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Well, Bill, I'll defer to your experience with hermetic compressors on small generators. You're right that utility scale power system events generally don't include a frequency change the way a small generator would when being overloaded.
 
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I should have thought of this yesterday.
Your generator sets should be rated for prime power.
If the sets are standby rated they should be re-rated as prime power sets.
When a prime power set is loaded to the point of losing frequency, it is at least 10% overloaded.
This load is typically allowed for only one hour out of twelve hours.
Worse, if the prime mover is oversized, or if the load has a PF of less than 0.8 your current may be above the 10% allowable and be exceeding the maximum allowable full load current.

Depending on your load time/kW profile and PF you may want to consider using a kW or current signal to control the load shedding scheme.
One compromise may be to allow an overload of 5% for several hours rather than the 10% overload for one hour.
One of these may be suitable for current based control.
Link
Before using the current output I would consider dropping the output of a CT across a resistor and using the resulting voltage as the input to the signal conditioner. I believe that the signal conditioner may take a greater percentage over-voltage than percentage over-current. (Verify this)

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Keith,
Last night I woke up thinking one of the kids was jumping on the bed, then remembered that my kids haven't lived here for over a decade! (And hadn't been bed jumpers for another decade before that)


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
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Jeff; You got off easy then as you mentally missed it - low terror factor. LOL

Bill! Bill, Bill. Bill... I'm not expecting an overload to ever cause the generator to sage. The car is set up to prevent that -always- with interlocks and such so heating can't run with refrigeration etc. It's about a 90kW prime set. No, I'm looking at a malfunction like some bad fuel, psychotic governor, sticky linkage, pathological sage brush, bird nest, etc. I am including three phases of current monitoring in this service cycle, been contemplating three phases of voltage monitoring but since I can't really get the instantaneous values I can't come up with watts so I'm probably going to pass on the voltage this time around and use an ICM450 for everything but the freq problem.
ICM450_fbyinr.png




Keith Cress
kcress -
 
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Good thinking Keith. A frequency monitor is a good choice. Or a tachometer with alarms.
Here's a colorful unit from Automation direct. Configure as either a frequency meter or as a Tach.
Avalable with two lo alarm outputs.

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