Synchronous Generator: Will these loops interact? 5

[williamlove]

I am tasked with designing a PID control strategy for a system that will use fluid power to turn the shaft of a synchronous generator. My question is whether my planned control strategy will work, and specifically, will my two PID loops interact and therefore not work well.

The generator will be connected to the local grid. The fluid power will be created by a new innovative system that extracts energy from the temperature differential of hot water heated by a massive solar array and cold sea water and creates pressured flow of hydraulic oil. The fluid power from the hydraulic oil will be transferred by using it to spin the shaft of a synchronous generator. Before turning the shaft it passes through a control valve.

There are two measured variables: Kilowatts (kW) and power factor (PF) are measured at a control device where the power is transferred to the grid and are available to our control system as 4-20mA input signals. There are two control variables our control system can manipulate with analog outputs: the position of the control valve that regulates flow of the hydraulic oil, and the excitation current to the synchronous generator.

CONTROL LOOPS:
1. Kilowatt (kW) controller whose PV is kilowatts (available as an analog input from the device connecting the generator to the grid) and whose CV is the position of the valve allowing fluid power to be delivered to the generator shaft. (The setpoint is 250kW which is what the generator is designed to produce.) This will be a relatively slow acting loop.
2. Power Factor (PF) controller whose PV is the power factor (available as an analog input) and whose CV is the excitation current to the synchronous generator. This will be a relatively fast acting loop.

CONTROL STRATEGY:
• Ramp open the valve until the generator RPM is at the design point (1800). No excitation is applied to the generator and the generator is not connected to the grid yet.
• Apply a specified excitation and put the generator online.
• With the kW controller in manual, ramp open the valve slowly to a specified value (say 80%) that is known from design and experimental confirmation to result in 250kW. During this time the excitation controller is allowed to operate in auto. It is expected that it will raise the excitation current as valve opens and kW increases. So in effect, both the valve position and excitation are ramped up.
• The kW controller is placed in auto. When cloud cover results in slowly dropping kW, the KW controller will slowly open the valve. When fluid power recovers due to restored sunlight the KW controller will slowly close the valve. It is expected that the PF controller will not interact with the kW controller since the latter is slow acting.
• Additional algorithms will determine when to take the generator offline, usually due to nightfall.

 

[davidbeach]

Why reinvent the wheel? Small gen control systems that can do this are widely available. You're looking for an AVR (excitation) and a governor (throttle). Generally a whole lot more than just one simple control loop in each though.

 

[waross]

Use speed instead of kW. Set 3% proportional band. (Fast acting to prevent over speeds) Use an initial set point of 60 Hz. to synchronize. Then raise the set point. until you have the loading you want. If your grid connection fails your speed will be limited to 103% or 61.8 Hz.
This is standard droop operation. This is available from a standard governor. The set will lock on to the grid frequency. The set point offset will control your loading. Forget PID, just use P. You may want to add a kW loop to avoid having the prime mover overloading the alternator.
You only have to worry about too much input power and throttle your control valve if your input power goes above the alternator rating. If you have limited input power, leave the valve wide open. The electrical output of the alternator will correspond to the prime mover input power less losses.
Your energy source sounds interesting. Without a lot more information about the characteristics of the system we may not be able to give complete recommendations.
There are experts here who are familiar with steam, diesel, gas turbines, combined cycle, large installations, and small installations. Most of us don't know what to expect a normal operating characteristics of a thermal/hydraulic system.
I am wondering why you develop energy and then try to throttle it in the hydraulic loop instead of not developing the energy in the first place, possibly by diverting part of the solar input? Once the energy is in your system it may be difficult to control it without dissipating a lot of energy somewhere in your system.
Our recommendations are, for the most part, based on basic principles. We would very much like to learn as much about your system as you are at liberty to share.
Thanks.

Bill
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"Why not the best?"
Jimmy Carter

 

[itsmoked]

Besides following Bill's advice your initial starting recipe lacked synchronising before grid connection.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[williamlove]

Mr. Beach I thank you for the comment. The few hours I spent designing this were very enjoyable and so even if I’m reinventing the wheel it is no waste. But I’m interested to know more about the algorithm(s) you mentioned, and compare them to my design. Will you please elaborate on them so I can search?

Waross thank you also. We initially were going to use speed as the PV of a loop that controls the valve, but then we were told that when a synchronous generator is put online the grid will keep it at speed, and that what will vary is the power factor and KVAR (as well as KW of course). If you have time and interest, will you scan my control algorithm overview again and see if it makes sense in light of that?
I’m not really allowed to say much about the part of the system that creates the fluid power, but I will say that it uses a massive solar array to create hot water, and cold sea water to cool another water source, and the temp differential is used in a large scale new, proprietary, innovative system to create the fluid power.

Itsmoked thank you too. Can you please elaborate on your comment? Synchronize what? I am very interested to know more about what you have in mind. Thanks.

 

[itsmoked]

williamlove; Just making sure you realize that you must have your generator's output power waveform synchronized with the grid's power waveform before closing the interconnect or you can destroy your generator.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[williamlove]

Based on thinking about the information provided so far, I’d like to ask you to help me validate two primary assumptions. Can you please tell me if you know or don't know if the assumptions below are correct?

1.) Before connecting a synchronous generator to a utility power grid, we believe there are only two things we can and should do prior to making the connection:
A.) Make the shaft spin at the proper RPM
B.) Apply some minimum specified excitation to the generator.

2.) Apart from startup and trip situations, we believe that the generator RPM will be not vary; the grid will keep it going at 1800 RPM if the grid is at 60Hz, and the RPM will fluctuate with the grid frequency. We believe actions we take with the valve position (controlling flow to the generator shaft to spin it) or with excitation affect kW, KVAR, and PF, but not RPM.

 

[davidbeach]

If all you do is just 1)A) and 1)B) it will be very, very interesting. You definitely need 1)C) as outlined by itsmoked.

 

[ScottyUK]

2) Your basic assumption is more-or-less correct, but the governor does not affect the reactive load, at least not in any significant way. There are some second order effects which do cause some interaction but ignore that for now. To a fairly good first approximation your governor regulates the power output in kW. The field current of the generator regulates the kVA[sub]R[/sub]. Power factor is a measure of the ratio of kW and kVA[sub]R[/sub], so a change in either one will change the PF.

David and Keith's 1C) is to use a synchronising relay or a synchroscope to ensure that the breaker closes when the utility and the machine are electrically in synchronism, which means that speed, terminal voltage, and phase angle are all adjusted within acceptable tolerances. I think you are overlooking phase angle. The relay performs its task by automatically by raising or lowering the field regulator and governor position. A synchroscope provides indication which allows an operator to manually perform the raise and lower functions. An out-of-sync breaker closure is arguably the worst fault the machine can experience in terms of mechanical shock to the generator because the utility will force the generator into synchronism very very quickly causing huge torsional forces to act on the generator and prime mover. Things tend to break - on big machines shafts 2' thick have snapped during this kind of fault. As David says it could be very. very interesting.


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

or fatal..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jeebusmn]

This may be a dumb question and I am green. Every time someone talks about connecting a generator of some kind to the utility grid someone brings up that, they should have synch-phase-over/under voltage relays. The question I have is do they ever initially drive the generator with the utility grid before engaging the prime move to synch and phase match?

 

[davidbeach]

Some gas turbines are brought up to speed using the generator as a motor, but you couldn't do that with reciprocating engine driven generators, hydro generators, or steam turbine driven generators, nor with any gas turbine not designed to be motored by the generator. When the generator is used as a starting motor there is usually a VFD to bring the speed up gradually.

 

[ScottyUK]

By the time the generator is at synch speed the static start will have disengaged. Starting devices are usually not needed much beyond about 60% speed because the turbine's compressor is starting to build combuster pressure by then and the machine can become self-sustaining. At low speed the compressor is more like an inefficient fan.


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If we learn from our mistakes I'm getting a great education!

 

[williamlove]

I didn’t discuss synchronicity because we don’t control the synchronizing relay. The system that provides us the analog signals containing kW and PF (and KVAR which I don't use) has a synchronizing relay. Our job is simply to control once online.

In light of that, do you think my control strategy will work? I am not an electrical engineer my background is manufacturing and some process control Thanks!

 

[ScottyUK]

Having had more than a little involvement in the control of gas and steam turbines, I would say keep away from any form of open loop control. I'm not from a hydro generation background so these are my thoughts from the world of thermal plant. I hope they aren't too far out:

- Accelerate up to about 98% nominal speed in closed loop speed control.
- Raise excitation and allow the AVR to set nominal voltage.
- Allow the synchronising relay to nudge speed setpoint up and down to match grid frequency. Simultaneously allow synchronising relay to nudge AVR voltage setpoint up or down to match grid voltage.
- Allow synchronising relay to close breaker.
- Detect breaker closure (not the command to close!) and use that as a transfer signal to move from speed control into load control. You need kW measurement to close the load control loop. Think about how to implement a bumpless transfer between modes.
- Ramp load setpoint to a stable minimum point and hold, usually about 5% would be a good bet as a starting point.
- Set target load and ramp to setpoint.

In my opinion you either need the synchronising relay, or you use the above scheme but perform the synchronising functions using raise and lower buttons controlled manually. For this you need the synchroscope I mentioned earlier. You should have an independent check synch relay as a breaker close permissive, just to back up the synchroniser on the day it suffers a failure and issues a close command at the wrong time, or the day when the operator presses the breaker close demand without paying attention. Saves an awful lot of problems.


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Nice recipe Scotty. I think I'm ready to give it a try!

How do you prove the breaker, a position switch, or some sort of electrical change in the line?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mgtrp]

Breaker closure is indicated by auxiliary contacts on the breaker, wired back to the governor/AVR.

 

[itsmoked]

Thanks.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

Even better than the aux contacts is the presence of current through the breaker.

 

[PHovnanian]

ScottyUK touches on an interesting point. The need for PID vs P control will depend on the characteristics of the prime mover. In particular, the throttle response.

The PID praameters may also have to change once the generator is connected to the grid, as the inertia of a free spinning rotor will behave quite differently than one that is generating power.