ECEN 460 Power System Operation and Control Lecture 22: Transient Stability Prof. Tom Overbye Dept. of Electrical and Computer Engineering Texas A&M University overbye@tamu.edu

Announcements Read Chapters 11 and 12 Design project is due on Dec 5; details on the website Homework 8 is 11.12, 11.18 (part a only), 11.19, 11.21, 11.25, 11.26; due on Thursday Nov 30 Lab 10 is on Nov 27/28, both in WEB 115; no lab week of Dec 4

Lab 9 Results: Marginal Prices 2

Lab 9 Results: Profit 3

Earlier Experiment Results: Group A, LAO 4

Earlier Experiment Results: Group B, LAO 5

Earlier Experiment Results: Group A, PAO 6

Earlier Experiment Results: Group B, PAO 7

Earlier Experiment Results: Offer Movies 8

Back to Transient Stability Example 11.10 Determine the initial conditions for the Example 11.3 case with the classical generator replaced by a two-axis model with H = 3.0 per unit-seconds, D = 0, = 2.1, = 2.0, = 0.3, = 0.5, all per unit using the 100 MVA system base First determine the current out of the generator from the initial conditions, then the terminal voltage Gen is delivering S=1+j0.3286 to bus 2 9

Example 11.10, cont. We can then get the initial angle, and initial d and q values 10

Example 11.10, cont. The initial state variable are determined by solving with the differential equations equal to zero. The transient stability solution is then solved by numerically integrating the differential equations, coupled with solving the algebraic equations 11

PowerWorld Solution of 11.10 12

Generator Exciters and Governors The two-axis synchronous model takes as an input the field voltage and the mechanical power. The next section discusses how these values are controlled 13

Generator Exciters The purpose of the exciter is to maintain the generator terminal voltage (or other close by voltage) at a specified value. Input is the sensed voltage Output is the field voltage to the machine, Efd Physically several technologies are used. Older generators used dc machines with brushes transferring the power With the newer brushless (or static) exciters power is obtained from an “inverted” synchronous generator whose field voltage is on the stator and armature windings are on rotor; output is rectified to create dc. 14

ABB UNICITER Example Image source: www02.abb.com, Brushless Excitation Systems Upgrade, 15

Exciter Block Diagrams Block diagrams are used to setup the transient stability models. The common IEEE Type 1 exciter is shown below (neglecting saturation); this is a dc type exciter. Initial state values are determined by knowing Efd and the terminal voltage Vt. 16

Exciter Block Diagram Example Consider again the Example 11.10 case, with an IEEE T1 exciter with Tr = 0, Ka = 100, Ta = 0.05, Vrmax = 5, Vrmin = -5, Ke = 1, Te = 0.26, Kf = 0.01 and Tf = 1.0. Determine the initial states. Initial value of Efd = 2.9135 and Vt = 1.0946 17

PowerWorld Example 12.1 Solution 18

Generator Governors The other key generator control system is the governor, which changes the mechanical power into the generator to maintain a desired speed and hence frequency. Historically centrifugal “flyball” governors have been used to regulate the speed of devices such as steam engines The centrifugal force varies with speed, opening or closing the throttle valve Photo source: en.wikipedia.org/wiki/Centrifugal_governor 19

Isochronous Governors Ideally we would like the governor to maintain the frequency at a constant value of 60 Hz (in North America) This can be accomplished using an isochronous governor. A flyball governor is not an isochronous governor since the control action is proportional to the speed error An isochronous governor requires an integration of the speed error Isochronous governors are used on stand alone generators but cannot be used on interconnected generators because of “hunting” 20

Generator “Hunting” Control system “hunting” is oscillation around an equilibrium point Trying to interconnect multiple isochronous generators will cause hunting because the frequency setpoints of the two generators are never exactly equal One will be accumulating a frequency error trying to speed up the system, whereas the other will be trying to slow it down The generators will NOT share the power load proportionally. 21

Droop Control The solution is to use what is known as droop control, in which the desired set point frequency is dependent upon the generator’s output R is known as the regulation constant or droop; a typical value is 4 or 5%. 22

Governor Block Diagrams The block diagram for a simple stream unit, the TGOV1 model, is shown below. The T1 block models the governor delays, whereas the second block models the turbine response. 23

Example 12.4 System Response 24

Problem 12.11 25

Restoring Frequency to 60 Hz In an interconnected power system the governors to not automatically restore the frequency to 60 Hz Rather this is done via the ACE (area control area calculation). Previously we defined ACE as the difference between the actual real power exports from an area and the scheduled exports. But it has an additional term ACE = Pactual - Psched – 10b(freqact - freqsched) b is the balancing authority frequency bias in MW/0.1 Hz with a negative sign. It is about 0.8% of peak load/generation 26

2600 MW Loss Frequency Recovery Frequency recovers in about ten minutes 27

Lab 10 Intro: Running Transient Stability and Critical Clearing Time 28

Lab 10 Intro: Changing Transient Stability Model Parameters 29

Lab 10 Intro: TSGC Frequency Response Use to retire generators 30

In the News: Luminant Coal Plant Closings Eliminate 600 Texas Jobs Dallas-based electricity generator Luminant is cutting about 600 jobs as it closes three coal-fired power plants and a mine in Texas Texas Workforce Commission said Nov 20, 2017 that Luminant plans to make the layoffs in January Previously announced closing are at Monticello, Big Brown and Sandow power plants Closings are due to cheap natural gas, low wholesale power grids and increase in renewable generation Monticello is 1880 MW by Mount Pleasant, Big Brown is 1150 MW by Fairfield, Sandow is 1137 MW by Rockdale Source; DallasNews.com, Nov 21, 2017 31

Our Fictitious Texas Grid Company Generator Plant Locations Red is nuclear, black is coal, brown is natural gas, green is wind, yellow is solar 32

Lab 10 Intro: PowerWorld DS 33