LCC Quick Notes, Generation Turbine Governor Overspeed Mitigation


    Unlike their mechanical-hydraulic ancestors, digital turbine governors can respond to speed excursions to prevent excess over-speed in instances of sudden load disconnect from unmonitored causes (like transformer failures or off-plant disconnects).  While the ensuing excursion is typically defended by over speed trip devices closing stop valves, a governor intervention can reduce the excursion speed,  but  it takes a fast digital governor to be of value.  Many turbine governors are not.  This Quick Note arms the engineer with the calculations to find out what's needed in minimum response for effective speed limiting.


 

(1)    We're NOT Talking About Load Drop Anticipators

    This topic is frequently confused with Load Drop Anticipator (LDA) functions.  LDA systems have feedback from sensors or process transmitters, over-speed anticipators do not.  LDA can work off position switches on main disconnect breakers and use an almost immediate logic input to induce a valve closure.  Steam pressure mismatches can also be used in this manner.  With over-speed anticipation all the governor has to work on is a series of increasing speed sensing feedback values.  The timing of the speed monitoring and the valve closure signal processing become the key issues whether a governor can successfully defend against a damaging speed excursion.

(2)    Calculations

    In the specifications of a digital governor one should be able to locate a value for speed sampling period or frequency.  This period defines the time lag between resolver algorithms taking a reading of the speed probe feedback signals and converting it to a value for the governor's use. 

    In determining the minimum response time of the governor, we usually assume the worst case scenario, that of the load loss and speed excursion beginning just after a governor speed sample.  Since we can't guarantee this will never be the case, and as luck would have it this is probably best.  Next we need to add two additional speed sample periods to represent two increasing speed excursion samples.  We can't use only one sampling because unit load and system frequency upsets can scatter one reading momentarily, and it is not advisable to close the governor valves whenever that happens.  The next processing time is consumed by the sequence of the governor calculating the speed acceleration, comparing it with an Excess Acceleration setpoint, then outputting a valve closure command.  The final processing time is the response speed of the governor valve operator in closing (but not tripping) the valve/s.  This gives us:

             Pex = 3 x Pgv + Pgp

                where Pex = Excursion Period,  Pgv = Governor Speed Sampling Period,  Pgc = Governor Processing Period

    Plugging in some values, Governor Processing Period, Pgc, is very short on dedicated governors (less than .005 second) but may be relatively long on DCS-based systems with busy networks, often 0.100 to 0.250 second.  Pgv runs from .0166 second on fast dedicated governors to .060 second on general purpose dedicated governors to 0.100 second on DCS or PLC.  Using these numbers, a range of Pex can be calculated for different governor types:

    Dedicated Fast Governor                        Pex = 3 x .0166 + .005  =     0.055 seconds

    Dedicated Commercial Governor           Pex = 3 x .050 + .005   =      0.155 seconds

    Typical  DCS or PLC                                Pex = 3 x 0.150 + 0.100  =   0.600 seconds

    Note that the Dedicated Fast Governor uses a Pgc of .0166 seconds or the 60 Hz period.  This limit is based upon measuring one full shaft rotation of a timing wheel.  It is possible to increase the speed resolver rate by counting less than all the timing wheel teeth.  Partial revolution counts are somewhat controversial, however, in that to be accurate they must use very high precision machined timing wheels.

    Most steam turbine governor valve operators have been designed for a full scale response in 1.0 seconds.  This has been a design basis for decades. The unit acceleration requires a bit more detail to calculate and is simpler to present by example.  Specific unit values can later be substituted for the example values to yield results for other units.

        A Westinghouse 165 MW turbine-generator set has mechanical and electrical load parameters of:

        Rated KW: 165,000

        Total WK: 312,000 lb/ft*

        Rated Speed: 3600 RPM

* Total WK calculated based upon rotor weights and estimated diameters from longitudinal section.

      The first set of overspeed calculations is based upon a trip occurring at rated load with the primary steam reduction being the throttle valve trip through         overspeed trip input.  The speed excursion can be evaluated in two segments, before and after the over speed trip.

        Stored Energy (KWsec) = (2.31 * WK * RPM)/107

        = 934,053

        H = (KWsec(stored)/(Rated KW) = 5.66

        Delta Speed (per rated unit) = (Delta Time) * (Accelerating Torque)/(2*H)

        = (1 * 1)/(2 * 5.66)

        = 0.088, or 8.8% of rated speed per second

    Given the throttle valves closure time of 800 milliseconds and the overspeed trip is initiated at a setpoint of 5% with 60 milliseconds processing time to autostop oil loss

        Delta Speed before trip = 5.0%

        Delta Speed after trip = 0.088 * (0.800+0.060)

        = 7.57%

        Total Overspeed excursion without governor intervention = 5.0% + 7.57% = 12.57%

        Now, to evaluate the governor mitigating influence:

        Time from load loss to over speed trip without any governor throttling =    0.088 %/second x 5.0% =   0.44 seconds

        If this is compared to a maximum governor valve closure rate of 100%/second we can only achieve a governor closure of 44% if the governor were instantaneous (which it's not).  Therefore this turbine cannot be kept from tripping, but still might be helped by fast governor action to limit total overspeed from a full load trip. 

                 Initial Unthrottled Period, 0 to Pex produces a turbine speed increase =     .088 x 3600 x Pex     

                                                                                                                                        =    .088 x 3600 x 0.055    = 17.4 RPM for dedicated fast governors

                                                                                                                                        =    .088 x 3600 x 0.155    = 49.1 RPM for dedicated commercial governors

                                                                                                                                        =    .088 x 3600 x 0.600    = 190.08 RPM for DCS/PLC governors

    The net reduction in excursion speed is then easily compared using the above values of speed increase by Pex since all subsequent closure is operator speed limited and the same for all governor types.

    Thus the DEDICATED FAST GOVERNOR allows (49.1 - 17.4) = 31.7 RPM less speed excursion than the DEDICATED COMMERCIAL GOVERNOR,

    and (190.08 - 17.4) = 172.68 RPM less speed excursion than the DCS/PLC GOVERNOR.

 

(3)    Conclusion

    Since over speed operation is the number one contributor to turbine failures, limiting excursion speed in full load trips is greatly beneficial and therefore demands dedicated fast governors to protect against excess over speed excursions.


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