Mechanical versus Digital Hydro Governors

[esee135]

So the endless question in the generator control world, what do you think is the better governor, the classic Woodward style mechanical or various offerings of the new digital high pressure types?

I was pro-digital, then after some thinking I'm now semi-pro-digital, maybe because it's the easy way out. It may not be pretty but I think the mechanical governor with service lifes of near 80-100 years in the utility I'm employed by speaks volumes.

Which one actually has the best response? It always seems to come down to 'what method is used to measure speed'.

 

[byrdj]

From my Steam background, I would also be concern with the obsolencance life of digital components. Most of the digital FPT controls that I saw go in during the 80's have been upgraded (OEM no longer supports original version), or outright replaced several times now(OEM no longer suports original design!).

 

[ScottyUK]

The digital-hydraulic systems such as Westinghouse's (Emerson's) Digital Electro-Hydraulic can offer tighter control and easier interfacing to a supervisory load dispatcher or economiser for the plant than the mechanical equivalents which can be important for precise load control if the unit is working under a generating regime such as exists in the UK. The hydraulics is usually the limiting factor in response speed, with the control loops being much faster than the final actuator. The digital loops are much easier to tune for optimal valve dynamic response, resulting in better ability of the machine to handle contingencies such as a load dump or major grid disturbance without tripping.

The obsolesence problem is certainly greater with the digital / electronic systems. We have just been through an upgrade on a Westinghouse DEH system brought about by obsolesence: it was 'interesting' to say the least.

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Sometimes I only open my mouth to swap feet...

 

[esee135]

I've noticed similar in my utility. We're hitting the end of life of a set of Vevey digital governors (E-Prom type), and anticipate a replacement in a <10 year period. Those governors were installed in 1991. I think the argument still stands, yes you may get better response, but is it worth changing out the electronics every 15-20 years, versus replacing some linkages and valves?

I've noticed that one of the common discussions about the new digital governors on top of the speed measurement (either PT/VT or capacitive mag-probe) is the uneven scan times of a PLC.

I agree with the load rejection cushioning close rate as well. Much simpler to program a closing rate during a specific situation (ie: load rejection).

 

[byrdj]

ScottyUK,
My limit dealings with Mechanical converted to electroninc steam turbines has another view. Do they really have a governor that will control a sudden loss of load, with emergancy speed protection? Trying to talk with the convertsion techs while trouble shooting problems, I don't get a warm fuzzy feeling that they do, its more like a valve positioner with emergancy speed backup. This is more so with the digital type packages.

(my background is FSE for the MHC and the older EHC for utilities. I guess I'm also pieved that some of the MHC units I've grew up with no longer call.)

 

[ScottyUK]

Hi byrdj,

My experience is on big steam plant, not hydro. The Emerson WDPF / DEH controller certainly does a fair job of catching a load load dump. Some of it is a little crude in the way that the governors are rapidly closed above the controller overspeed setpoint to cut off the steam until the speed drops to a more acceptable level, then transferring to throttle control so the unit stays or near synch speed. It isn't a perfect system by any means and we still sometimes see the backup electrical or the mechanical overspeed trip devices tripping the unit outright rather than leaving it in a recoverable state. I'm fairly sure the DEH system can react more quickly than an equivalent mechanical system, unless the mechanical system is maintained in truly exceptional condition. The processor running the overspeed calculation has a loop time of 100ms, and the electronic I/O card responds to a received command from the processor almost instantaneously. The dominant response time in the overall system is the speed the governor valve actuator can respond

For a full load rejection I'm not sure any governor is capable of preventing an overspeed - there is just too much energy locked up in the steam in the turbine and steam chest where it is beyond the control of the governors to prevent an overspeed occuring.

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Sometimes I only open my mouth to swap feet...

 

[byrdj]

off on a tangent, but:

" sure the DEH system can react more quickly than an equivalent mechanical system, unless the mechanical system is maintained in truly exceptional condition" --Agree abosolutely, it is the upgrades and conversions that I don't feel that positively about (my company sells such and I am not convienced)

"For a full load rejection I'm not sure any governor is capable of preventing an overspeed" -- They should and for what I consider the glory days (60 thru 90s), they did. During my EHC commissioning days (80s), some offshore customers insisted on 100% load dump testing. I was greatly impressed even with the lowest inertia highest energy density turbine code (G3). From MHC maintenance days, I got report back how the operators of the unit I just went through got bonuses for their excellent performance in getting thier unit back on line from major upset, while the other 3 units at station went down on OST. ( they were also G3s, but greater inertia since older)

Thanks for letting me reminnis,

 

[ScottyUK]

Hi byrdj,

Reminisce all you like!

What I meant when I said '... preventing an overspeed ...' under full load rejection was that governor will allow the machine will overspeed to the extent that the OPC operates to catch it, rather than the speed controller loop. Apologies for my inability to exress myself properly!

Our overspeed protection within the DEH system is set very marginally below the backup electrical protection setpoint, and the mechanical OST's tolerance puts it in the frame too. In my opinion the setpoints of the backup protections are too close to the setpoint for the DEH to allow it time to do its job when the machine is accelerating rapidly after a full load trip. I guess I have found another task to add to my ever-growing pile!


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Sometimes I only open my mouth to swap feet...