Islanded Generating Station 6

[Mbrooke]

How does an Islanded 600-2000MW generating station behave? What does typical critical clearing time look like? What is the voltage and frequency divergence during and after a fault for various fault scenarios? How is active and reactive power dispatched/controlled? How does generation behave during trip and reclose events? And how do you "block load"?

 

[crshears]

doing anything without semiconductors is inconceivable to me...

Not to me it isn't; I'm a bit of a paleotechnophile that way... [ whaddaya thinka that word I just made up? ]



CR

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

 

[Mbrooke]

I like it. There is one hit on Google on my end:

https://www.google.com/search?sourc...&ved=0ahUKEwj6yu_S5snoAhVmdt8KHZEtDQsQ4dUDCAs

Claim it before anyone else does!


I find a time dial relays more fascinating than an SEL relays.

 

[crshears]

One thing I'm still trying to wrap my head around is the uses, intricacies and nuances of, and the physics behind, saturable reactors and magnetic amplifiers...I had previous exposure to these when working in generating stations of various sorts.

I previously wrote about where I found these still in use; see thread

https://www.eng-tips.com/viewthread.cfm?qid=417149

Incidentally, Still Watch is for sale at the asking price of CDN K$250...see

http://www.scrutonmarine.com/p2421.htm

CR

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

 

[waross]

That is very similar to the hydraulic governor on a diesel CR.

This is a very good illustration for explanation but some refinements are missing.

The horizontal arm across the top is the droop feedback.
Not shown; A mechanism to vary the mechanical advantage and so adjust the percentage droop.

Yellow is hydraulic oil.
Not shown; an oil pump. Some used engine oil pressure. Some had an oil reservoir and a small gear pump to supply operating oil pressure.
A further common refinement was the addition of four check valves so that the governor could be rotated in either direction depending on the application and the pump would still deliver operating oil.
The leftmost yellow port is the supply port for pressure oil.
The lowest yellow port in the green body is the discharge port.

Not shown: A method of varying the pressure on the speeder spring and so adjust the speed setpoint
The speeder spring is the conical spring above the centrifugal weights.
The blue block on the upper left supports the fulcrum of the droop arm.
Moving this fulcrum up or down is one way of adjusting the bias force on the speeder spring.
A stand-alone generator:
This may have a screw adjustment, locked with a lock nut.
A universal governor, meant for general application but suitable for generator use may have a speed adjusting knob.
A small electric motor may be used to adjust the bias force on the speeder spring.
A parallel operated set;
This will have either a speed adjusting knob or an electric motor to adjust the bias force on the speeder spring and thus the speed setpoint.
In our plant we had electric motors on the governors. The control was a three position spring return to center switch on the switchboard. Faster-Off-Slower.

The power piston on the right, which controlled the fuel rack on the engine, was controlled by volume, not pressure.
As you can see, the pilot valve, the lowest red part, covers the discharge port very closely.
With any slight increase in speed the centrifugal weights will raise the pilot valve and allow some of the oil supporting the power piston to be discharged, thus lowering the piston and reducing the throttle position.
With any slight decrease in speed the centrifugal weights will lower the pilot valve and allow pressure oil to be admitted to raise the power piston, thus increasing the throttle position.

Leakage on an older, worn, governor.
Any leakage would allow the power piston to slowly reduce the fuel delivery.
The governor would react by allowing more oil past the pilot valve to maintain the position of the pilot valve.
There may be a small percentage error in the speed.
However the operator would typically be setting the speed by the frequency, not the physical position of the adjustment and so the error would tend to be self correcting.
An exception would be a cold start. If an engine was started cold and put online, the leakage or bypass of the cold oil would be less.
When the oil warmed up to normal operating temperature the operator may tweak the setting a few percent.

Next up: mechanical versus hydraulic governors.




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

 

[Mbrooke]

Is this how a stanadyne governor (if I have the name right) works in standby diesel gensets?

 

[waross]

How did it respond to faults where the gen is speeding up then slowing down? Would it be sufficient in Island mode?

Time sequence.
1. Fault occurs. See subtransient reactance.
2. AVR responds to the fault. See transient reactance.
3. Governor responds to the fault. See steady state reactance.
Note: the fault location is critical.
For close in faults the governor will typically reduce power.
For distant faults the governor may increase the power.

How fast does the governor respond?
Fast.
DOL motor starting, the governor will be responding as the motor accelerates.

A real world example.
A remote sawmill on diesel generator power.
Two generators.
The mill may often run on one generator, but one large motor requires both generators in parallel to start, one to run.
This was a hammer hog of about 300 HP.
One generator had an electronic governor and one had a hydraulic governor.
In the morning, both generators are started cold and paralleled.
The large motor is started.
The electronic governor controlled engine responds faster and hogs the load and the generator trips out on overload.
This throws the load on the other set which trips out on overload.
Solution.
The sets are started and the load, minus the large motor, is picked up by the hydraulic controlled set.
After about 20 minutes the second set is paralleled.
Now the large motor is started.
The oil has warmed up and with the reduced viscosity of the warm oil the hydraulic set can respond fast enough to share enough of the starting load to allow a successful start.
This example illustrates a real world response of generators including governors in one case of overload.


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

 

[waross]

What are the governor like in coal plants? I've heard this debate about running boilers with the steam valves fully open. What is that about?

The market for export power is high for some reason.
Foe example, you have a contract to supply a fixed amount of power, but you have an outage and cannot fulfill your contract.
You must purchase power from a competitor to meet your obligations.
A few plants on the grid are down for turnarounds, the grid is experiencing peak loading and the price of excess power becomes exorbitant.
I have heard of peak power hitting $2 per KWHr for a short time.
Whatever the price your manager wants a piece of the windfall profits.
Sacrifice some long term life for a large short term gain for a few hours.
The order comes down:
Give me the absolute maximum possible.
Overloading is permissible.
Hence, steam valves at 100%.
The power output will be limited by the capacity of the steam turbines.
Or,
You have a plant go down unexpectedly and you are pushing the remaining sets to the max to avoid paying exorbitant spot power prices.

Still to come; mechanical versus hydraulic governors.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Bill has explained the engineering behaviour of unit response to faults; I can only recount my memory of how I observed units behaving...

I was once at work on a dark and stormy night, so stormy, ferocious and violent in fact that the apocalyptic lightning flashes seemed virtually incessant. My inspection tour had me a level or two above the turbine floor, looking down across the turbine hall to the windows to the outside, so I had a clear view of the brightening effect of all the lightning strikes. More than once a particularly powerful and close lightning bolt struck one of the OHLs just beyond the 230 kV switchyard, with a strike or two even landing in the swyd itself, overwhelming the sky wires and blasting right down to the phase conductors. There were several units on line at the time, and the simplest way to describe the way they sounded was to say they grunted, very loudly. I paid a visit to my instructing turbine boiler operator, who showed me the way the analog needles for the generator terminal voltages, line loadings, unit reactive output, etc. would jump wildly with each one of these strikes. I was told the fault paths created by the lightning strikes were collapsing the local system voltage, causing the fast-acting AVRs to cause shots or pulses in the excitation current and field strength. I was also told the "jerk" associated with these strikes contributed to that grunting sound, but that the speed with which a strike and its subsequent re-clearing occurred were far too fast for the governor to respond.

CR

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

 

[waross]

"Not that the answer has to be long, but it may take awhile to make it short" (Mark Twain?)

Stanadyne LLC is a developer and manufacturer of fuel pumps and fuel injectors for diesel and gasoline engines

The best way I can explain this is to work backwards from the injectors.
The injector introduces fuel into the cylinder of a diesel emgine.
The power developed depends on the amount of fuel.
One method of varying the amount of fuel is to vary the amount of fuel admitted to the injector.
Some engines used mechanical injectors.
A part of the injector may be rotated.
The position of the rotatable part determines the amount of fuel admitted to the injector.
There is a spur gear on the injector and a rack gear passing all the injectors in a bank to vary the injectors in unison.
The governor must move these racks to control the engines.
The injector was operated by a push rod from the camshaft in a similar manner as the valves.
The injector made a full stroke each cycle and delivered the pre determined fuel charge to the cylinder.
Some engines used injection pumps.
A special high pressure pump delivered a metered amount of fuel to each fixed injector at the proper time, similar to the distributor on a spark engine.
This may be called an injection pump or a distributor pump.
A shaft and lever on the side of the distributor pump was used to control the amount of fuel delivered at each injection cycle.
This lever was operated by linkage from the governor.
This brings us to the Stanadyne.
This is an ingenious combination of a governor and a distributor pump in one small housing.
This is suitable for smaller generating sets.
I have installed over a dozen small sets with a Stanadyne type governor/distributor pump.
The only failure I experienced was a time when the customer did not have enough fuel in the water.
After the hurrican the generator failed. (Water in the fuel).
I was away on the island getting the utility and two seafood processing plant up and running.
Customer called in a "diesel engine expert".
Diagnosis?
Engine needs a rebuild.
They rebuilt an engine with less than 100 operating hours.
It wouldn't run.
Diagnosis?
Bad injectors.
The injectors were sent to an injector lab and rebuilt and tested.
Injectors replaced.
Still wouldn't run.
Diagnosis? Just confusion.
By this time I had arrived and had acquired a replacement Stanadyne.
I had the fellows remove the injectors but connect the fuel lines.
We cranked the engine.
One injector produced a small amount of fuel, the others no fuel.
I gave them a few moments to digest this and then showed them the new Stanadyne pump.
Their eyes lit up. I had found a "teachable moment".
The set was running a short time later.

It may be a little while but I will get to the comparison between hydraulic and mechanical governors.
Just now, a busted toilet in a rental house is taking precedence.

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

 

[waross]

Thanks for that CR.
In addition it is possible that any part of the fault path passing through a winding would create a powerful torque transient.
This could be positive or negative.

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

 

[waross]

A similar graphic of a governor.

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

 

[waross]

Many hydraulic governors are arranged so that the spool of the pilot valve spins with the flyweights. This gives zero sticktion to the spool operation.

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

 

[Mbrooke]

Lots to read- and lots to take in. I'm digging in!

 

[Mbrooke]

Just want to ask- how is the boiler fire rate adjusted? And how is the output of a nuclear reactor adjusted? The governor operating for more load would cause the steam pressure to drop.

 

[crshears]

CANDU nukes can be set up for turbine-follows-boiler / feed forward operation, meaning governor itself still responds to system frequency changes but the output of a pressure transducer on the boiler is sent to the speeder gear controller, loading and unloading the turbine as needed to maintain steady steam pressure.

I'm no nuclear operator, but I've picked up some snippets of info over the years...to my understanding, if operating in boiler-follows-turbine mode, the steam production rate of nuclear reactors can be controlled by various means, depending on design; control rods are inserted into or withdrawn from the reactor core so as to alter the reactivity by absorbing some of the particles released by the chain reaction, leaving less of them to cause fission; absorbing "poisons" may be injected into the moderator to accomplish the same thing; the level of normal [not heavy] water within trim tanks can be altered so as to trim the overall reactivity profile within the core; etc., etc.

Speaking in generalities, the firing rates of fossil fuel boilers are adjusted by opening or closing fuel admittance valves, or speeding up or slowing down oil pressurizing pumps so as to vary the oil feed rate; pulverized-coal-fired boiler firing rates are altered by changing the number of pulverizers in service.

In all cases air flow is adjusted so as to supply no more and no less air than needed for optimum combustion; during such adjustments, the cardinal rule is, "When increasing the firing rate, FIRST increase the air flow, THEN the fuel flow; when decreasing the firing rate, FIRST decrease the fuel flow, THEN the air flow," as it is much safer and cleaner to have an excess of air than a deficiency of it.

CR

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

 

[Mbrooke]

Alright and alright. I'll digest this.

Is there such a thing as manual excitation control and manual governor control- in that a human is always tweaking these values? Or is it not humanly possible? Or simply not practical?

 

[crshears]

Is there such a thing as manual excitation control?

Yes; the hydraulic units I used to operate had two different means of manual voltage control. The first and really crude one involved a DC source and an adequately rated [read: honkin' big!] resistor, capable of being adjusted either from local control via handwheel or from the control room via motor positioner. The second involved using the AVR but under manual control, so that it responded only to whatever setpoint and therefore output the operator chose, with no automatic response to unit var output or terminal voltage.

manual governor control?

Ambiguous and possibly oxymoronic question; the manual controls sometimes applied to a governor [ which cannot fulfil its function if not operating automatically] can include but are of course not limited to: [1] the speeder gear and [2] a load limiter. Dashpot bypasses are also sometimes provided to temporarily disable the governor compensation, for example during large load changes to a synchronized generator.

The Woodward "cabinet" governors I'm familiar with also included a change-over handwheel which transferred control of the unit wicket gates from the governor to a separate gate positioning handwheel that, other than for one notable exception I participated in, was only ever used for maintenance purposes with the unit dewatered and under test so the wicket gate linkages themselves could be adjusted for "pinch."

not humanly possible? Or simply not practical?

Almost anything is possible, depending upon the circumstances; practical is another question, depending on if you define it as do-able or advisable...compare a water-wheel-driven flour or grist mill with a modern generator, and the differences, consequences and severity readily become apparent.

As an example, over the course of my time in the hydraulic plant where I served, it became prohibited to place a unit on line without a fully capable AVR in service on automatic, since the perceived risk started to be viewed as alarmingly intolerable...which sort of makes sense if one is concerned about the generator voltage going sky-high if the unit breaker opens for some reason that doesn't also trip the field breaker; prior to that, running with less than a fully-functional-on-automatic AVR was considered routine, as if it occurred a defect report would be submitted to have whatever issue there was with the AVR auto function corrected during normal working hours. The only change we might make in operating regime in way of risk mitigation was to make that unit last on, first off.

CR

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

 

[Mbrooke]

More thanks. Still digesting- will ask more questions are they come.

I hear good stuff about the separate Dc generator on the end of the turbine and honking big resistor in that the field does not collapse during a fault?


I like the idea of power plants being controlled by guages and handle switches like they were in the 50s, 60s and 70s.

 

[crshears]

What a paleotechnophile you are!!!

I feel the same way.

CR

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

 

[Mbrooke]

I'll let you in on a little secret. In my own time I'm evaluating a relay logic generating station. That and how to control one. In addition two concepts: 1) Feeding most of the load in the US through radial islanded generating stations 2) Breaking up systems to their ISOs. Modelling the behavior of such along with the economics is fun.

If you'd like I can use New Jersey, NY, Connecticut or the Carolinas as an example.