Constant Settable droop Design Standard of GE

 

1.0          System Summary

 

Constant Settable Droop Speed/Load control represents a method of formulating the gas turbine droop response as a function of the unit power output.  This method of speed/load control is applied to units where the fuel stroke reference (FSR) is not predictable as a function of the gas turbine output power.  Standard droop control utilizes the approximate linear relationship between FSR and the gas turbine power output as the basis for reacting to variations in electrical grid frequency.   When gas fuel heating value is subject to variation due to fuel composition  changes,  the FSR may not be predictable with load.  Constant Settable Droop Speed/Load Control is a method where gas turbine megawatt output is used as a control parameter to formulate the turbine droop response to electrical grid perturbations.

 

2.0                Application

 

Constant Settable Speed/Load Control should be applied when the Pegasus code option BP5B is called for on a requisition and on all DLN units.  This option is used instead of Standard Droop Speed/Load Control (BP1G) or the optional Non-Linear Droop Speed/Load Control (BP5D). 

 

NOTE:  The Constant Settable Droop and Non-Linear Droop options share some of the same software algorithms (BBLs), signal names, and constant settings.  These options, however, are documented in the standards as separate approaches. A modification to this standard should prompt a review of the BP5D standard.

3.0          Design Standard

 

3.1                Hardware explanation

 

Dual redundant megawatt transducers are required at a minimum to provide megawatt feedback to the Constant Settable Droop sequencing.

 

3.2          Scope of Responsibility

 

Not Applicable

 

3.3          Software Description

 

A droop governor response is used in turbine generator controls to help maintain an electrical grid at constant frequency.  Generally, a drop in electrical grid frequency indicates that the power generation capability of the grid is less than the load demand on the grid.  Conversely, if the electrical grid frequency is above the nominal, the power generation capability supplied to the grid is greater than the load demanded.  Droop governor response attempts to correct these situations by changing the power output of a turbine inversely proportional to the electrical grid frequency departure from nominal.  If the grid frequency drops below rated frequency, the turbine will be commanded to increase its power output.  If the grid frequency increases above the rated frequency, the turbine will be commanded to reduce its power output.

 

The droop response of a turbine generator governor is typically referred to in terms of the percent frequency variation required to cause a 100% turbine load output change.  The standard droop response configuration for GE gas turbine generator applications is a 4% droop response.  This means that the gas turbine load output will change by 100% with a 4% grid frequency change.  In other words, a 4% droop governor will modify the gas turbine output by 25% per every 1% grid frequency change (or, per every 1% turbine shaft speed change since grid frequency and turbine speed are proportionally related). 

 

The lines drawn in Figure 1 represent a 4% droop governor response.  The control command signal TNR is the gas turbine speed/load reference in terms of percent.  During startup, this command varies from 0% to 100% speed.  After synchronization to an electrical power grid, TNR becomes a load reference and varies between 100% and 104%  to command the load output from  0% to 100%.  The lines drawn in Figure 1 disregard any other facet of the gas turbine controls, such as exhaust temperature control, which could limit the maximum power output of the turbine.  The family of curves in Figure 1 is intended to represent the relationship between TNR, percent load output,  the electrical grid frequency and the percent turbine speed for a 4% droop configuration.  For example, if the gas turbine controls command a TNR setpoint of 102%, the unit will output a power level equal to 50% of rated base load for a normal 100% turbine speed operating point (Point “A”).  If the electrical grid frequency where to decrease and cause the turbine to operate at 99% speed, the power output would be increased to 75%  of rated base load (Point “B”) with the same TNR setpoint of 102%.

FIGURE 1

The Constant Settable Droop software used to implement the droop response characteristic is functionally shown in Figure 2 of paragraph 4.0.  Constant Settable Droop Control features an inner speed control loop and an outer megawatt control loop.  The inner speed loop is a proportional plus integral control whose mission is to make the turbine speed TNH match the called for reference speed command TNRL.  The outer megawatt loop formulates the droop governor response by creating a speed bias as a function of  unit power output.  When the turbine speed is held fixed by an electrical grid, the turbine fuel consumption and megawatt output is modified (or ‘Constantly Set’) such that the TNRL reference speed command is made to equal the turbine speed TNH.   The scaling of the turbine power output DWATT to the speed bias signal DWDROOP defines the droop governor response in terms of megawatt output change per percent grid speed change.

 

The control constant DWKDG uses the rated Base Load megawatts as a basis to scale the megawatt feedback to a percent speed bias resulting in the nominal droop governor response.  The formula for calculating DWKDG is:

 

 

 

7.1          Default Control Constant Settings

 

NAME

VALUE

UNITS

SCALE TYPE

DESCRIPTION

CODE

DWKDG

Note 1

%/MW

PC2MW

Speed Control Droop Reference Gain

C

DWKTC

Note 1

sec

SEC64

Speed Control Droop Reference Time Constant

-

DWKMN

Note 1

MW

MWATT

Minimum Megawatt for Droop Response

-

FSKNTC

Note 1

sec

SEC64

FSR Speed/Load Control Integral Time Constant

-

FSKNG

Note 1

%/%

PC7PC

FSR Speed/Load Control Proportional Gain

-

Code:  C = Constant is calculated per unit

            F = Constant can be field adjusted

 

Note:  Constants above are set for 4% droop over the entire speed range as a default for the database settings.

Note 1 :  Default constant settings are listed in the Constant Settable Droop  Design Standard BP5B:

Frame Size

DWKDG Value

DWKDTC Value

DWKMN Value

FSKNG Value

FSKNTC Value

MS3002

0.500

5.0

0.0

15.0

2.5

MS5002

0.250

5.0

0.0

12.5

2.5

MS5001P

0.150

2.5

0.0

10.0

2.5

MS6001B

0.100

2.5

0.0

10.0

2.5

MS7001EA

0.050

2.5

0.0

10.0

2.5

MS9001E

0.035

2.5

0.0

10.0

2.5

MS9001EC

0.024

2.5

0.0

10.0

2.5

MS6001FA

0.060

2.5

0.0

10.0

2.5

MS7001FA

0.025

5.0

0.0

10.0

5.0

MS9001FA

0.020

5.0

0.0

10.0

5.0

 

7.2          Control Constant Calculation Methods

 

 

        Software Signal Definitions

 

SIGNAL

UNITS

SCALE TYPE

DESCRIPTION

DWDROOP

%

PCT

Turbine Load Droop Reference

FSRN

%

PCT

Speed Control Fuel Stroke Reference

TNH

%

PCT

HP Turbine Speed

TNR

%

PCT

Turbine Speed/Load Reference

TNRL

%

PCT

Load Turbine Speed Reference

 

 

10.1        System Description Text

 

Constant Settable Droop Speed/Load control represents a method of formulating the gas turbine droop response as a function of the unit power output.  This method of speed/load control is applied to units where the fuel stroke reference (FSR) is not predictable as a function of the gas turbine output power due to varying fuel heating value or where fuel is switched between different combustion system injection nozzles. 

 

Constant Settable Droop Control features an inner speed control loop and an outer megawatt control loop.  The inner speed loop is a proportional plus integral control whose mission is to make the turbine speed TNH match the called for reference speed command TNRL.  The outer megawatt loop formulates the droop governor response by creating a speed bias as a function of  unit power output.  When the turbine speed is held fixed by an electrical grid, the turbine fuel consumption and megawatt output is modified (or ‘Constantly Set’) such that the TNRL reference speed command is made to equal the turbine speed TNH.   The scaling of the turbine power output DWATT to the speed bias signal DWDROOP defines the droop governor response in terms of megawatt output change per percent grid speed change.