Governor Response Sequence

1. Governor Response Sequence ERCOT Future AS Workshop January 20, 2014 Bob Green Garland Power and Light

2. You get the idea: if the power generated exceeds the load, frequency rises. If the generator power is less than the total loads, frequency declines

3. Governor Response Issues • Traditional training simplifies the reason for frequency deviations from 60HZ. See “You get the idea” on the previous slide. In reality, power generated always equals the load. • This training module describes what actually causes frequency deviations: mechanical power into a turbine not being equal to the electrical power output of the connected generator. • The trip of a large generator is used to illustrate the process.

4. Governor Response Schematic Voltage Regulator Gross Metering Field Valves Step – Up Transformer To Load • Energy Source • Boiler • Reactor • Penstock Turbine Armature Transient Reactance Governor Net Metering Auxiliary Load

5. Loss of Generation - 1 • In response to a unit trip, each real generator in the interconnected system increases their MW output in proportion to their electrical distance from where the generator tripped. • The rotating kinetic energy in each tandem turbine-generator assembly supports the increased instantaneous power output. • With power output greater than the power input, shaft speed [frequency] decreases.

6. Torque and Inertial Energy • Steam input hits turbine blades creating torque that increases rotational speed of the T-G which in turn increases the kinetic (inertial) energy. • When the electrical torque is greater than mechanical torque, the rotation slows extracting stored inertial energy from the mass.

7. Loss of Generation - 2 • The governors sense the slower speed and open valves between the mechanical energy sources [boiler, etc.] and the turbine to balance the generator mechanical input with the electrical output. • When the mechanical input equals the increased electrical output, the frequency stop decreasing [this is called the C point]. • The frequency increases as the kinetic energy is replaced and settles at a new steady state frequency value [this is called the B point].

8. Loss of Generation - 3 • The transition from the initial steady-state frequency [the B point] to restoring frequency to the scheduled value normally consists of sustained power plant control and control from the center control [AGC/LFC]. • Plant controllers need to recognize the higher desired output [MW] due to low frequency. • Control Centers need to send AGC signals to increase the governor set-points and restore frequency to schedule [usually 60 HZ].

9. Loss of Generation - 4 • Traditional Control Areas and the ERCOT Zonal Market typically started sending LFC control within 10 seconds of a unit trip. • Control Areas responded to a steady ACE value. • ERCOT Zonal QSEs responded to a Responsive Reserve deployment. • ERCOT Nodal sends RegUp deployments to QSEs but waits approximately 60 seconds for SCED to calculate new Base Points utilizing capacity released by RRS deployments.

10. Loss of Generation - 5

12. Closing Misunderstandings • Frequency Nadir is where the total transient mechanical turbine input equals the total electrical generator output [not the system load]. • Slope of the dark green line is also influenced by whether the generator trips [steep slope] or if the turbine trips [generator stays connected to the grid for several seconds]. • Settling Frequency is where the governors have driven the steady-state mechanical turbine input equal to the electrical generator output.

13. Review of Frequency Control Steps • Electrical/mechanical energy imbalance caused by a change in load or change in generation • Turbine-generator shaft speeds [frequency] increase or decrease • Initial governor response due to the resulting change in frequency • Stabilizing frequency at new steady-state value • Restoring scheduled frequency [and interchange] using Automatic Generation Control • Preparing for next imbalance

14. Primary versus Secondary Frequency Control

15. After Governor Response to 7 MW of Load Added

16. After AGC Secondary Frequency Control