# Lecture Methods of Electric power systems analysis - Lesson 6: Power operations, power flow

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Lecture Methods of Electric power systems analysis - Lesson 6: Power operations, power flow provide students with knowledge about three bus powerworld simulator case; basic power control; power flow in transmission line is limited by heating considerations; overloaded transmission line; automatic generation control;...

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## Nội dung Text: Lecture Methods of Electric power systems analysis - Lesson 6: Power operations, power flow

1. ECEN 615 Methods of Electric Power Systems Analysis Lecture 6: Power Operations, Power Flow Prof. Tom Overbye Dept. of Electrical and Computer Engineering Texas A&M University overbye@tamu.edu
2. Announcements • Read Chapter 6 from the book • The book formulates the power flow using the polar form for the Ybus elements • Homework 2 is due on Thursday September 17 1
3. Three Bus PowerWorld Simulator Case PowerWorld Case Name: B3Slow Load with Area Name: Home ACE: -15.5 MW green Home Area 25.4 MW MW Load: 316.2 MW MW Gen: 301.0 MW 25.5 MW MW Losses: 0.28 MW arrows Bus 2 5.3 Mvar A -4.9 Mvar A Bus 1 1.00 pu indicating 210.8 MW 105.4 Mvar MVA MVA slack 1.00 pu amount A A 115.4 MW -1.9 Mvar of MW 151.0 MW AGC OFF 34.3 MW MVA A A MVA 10.1 MW 100 MW AVR ON 10.6 Mvar 3.1 Mvar flow 121.3 Mvar 34.5 MW MVA MVA 10.1 MW -3.0 Mvar Other Area -10.0 Mvar Scheduled Transactions 1.00 pu 0.0 MW Bus 3 105.4 MW Note the Off AGC 150.0 MW 52.7 Mvar power Used 39.7 Mvar AVR ON AGC ON balance at to control each bus output of Direction of green arrow is used to indicate generator direction of real power (MW) flow; the blue arrows show the reactive power 2
4. Basic Power Control • Opening a circuit breaker causes the power flow to instantaneously (nearly) change. • No other way to directly control power flow in a transmission line. • By changing generation we can indirectly change this flow. • Power flow in transmission line is limited by heating considerations • Losses (I^2 R) can heat up the line, causing it to sag. 3
5. Transmission Line Limits • Power flow in transmission line is limited by heating considerations. • Losses (I2 R) can heat up the line, causing it to sag. • Each line has a limit; many utilities use winter/summer limits. 4
6. Overloaded Transmission Line Area Name: Home ACE: -263.1 MW Home Area MW Load: 559.2 MW -162.5 MW MW Gen: 301.0 MW 165.3 MW Bus 2 39.8 Mvar MW Losses: 4.91 MW -25.8 Mvar Bus 1 A A 1.000 pu 112% 112% 372.8 MW MVA MVA 186.4 Mvar slack 1.000 pu A A 363.0 MW MVA -52.3 Mvar MVA -59.2 MW 151.0 MW AGC OFF A A 97.7 MW 18.8 Mvar 100.0 MW AVR ON -26.5 Mvar 245.0 Mvar MVA MVA 59.8 MW -96.2 MW Other Area -16.9 Mvar 31.6 Mvar Scheduled Transactions Bus 3 1.000 pu 0.0 MW 186.4 MW Off AGC 93.2 Mvar 150.0 MW AGC OFF 107.9 Mvar AVR ON 5
7. Interconnected Operation Balancing Authority (BA) Areas • North American Eastern and Western grids are divided into balancing authority areas (BA) – Often just called an area • Transmission lines that join two areas are known as tie-lines. • The net power out of an area is the sum of the flow on its tie-lines. • The flow out of an area is equal to total gen - total load - total losses = tie-flow 6
8. US Balancing Authorities 7
9. Area Control Error (ACE) • The area control error is the difference between the actual flow out of an area, and the scheduled flow – ACE also includes a frequency component that we will probably consider later in the semester • Ideally the ACE should always be zero • Because the load is constantly changing, each utility (or ISO) must constantly change its generation to “chase” the ACE • ACE was originally computed by utilities; increasingly it is computed by larger organizations such as ISOs 8
10. Automatic Generation Control • Most utilities (ISOs) use automatic generation control (AGC) to automatically change their generation to keep their ACE close to zero. • Usually the control center calculates ACE based upon tie-line flows; then the AGC module sends control signals out to the generators every couple seconds. 9
11. Three Bus Case on AGC Area Name: Home ACE: -0.0 MW Home Area MW Load: 330.2 MW MW Gen: 330.6 MW -21 MW 21 MW MW Losses: 0.40 MW Bus 2 4 Mvar -4 Mvar A A Bus 1 MVA 220 MW MVA 110 Mvar slack A A 100 MW 158 MW MVA 2 Mvar 127 Mvar MVA -41 MW -21 MW AGC ON 13 Mvar A A 6 Mvar 100 MW 0 Mvar 41 MW MVA MVA 21 MW Scheduled Transactions -12 Mvar Bus 3 -6 Mvar Other Area 0.0 MW Area AGC Status: Part. AGC 173 MW 110 MW 37 Mvar 55 Mvar AGC ON Generation Net tie flow is is automatically close to zero changed to match change in load 10
12. Generator Costs • There are many fixed and variable costs associated with power system operation • The major variable cost is associated with generation. • Cost to generate a MWh can vary widely • For some types of units (such as hydro and nuclear) it is difficult to quantify • More others such as wind and solar the marginal cost of energy is essentially zero (actually negative for wind!) • For thermal units it is straightforward to determine • Many markets have moved from cost-based to price- based generator costs 11
13. Economic Dispatch • Economic dispatch (ED) determines the least cost dispatch of generation for an area. • For a lossless system, the ED occurs when all the generators have equal marginal costs. IC1(PG,1) = IC2(PG,2) = … = ICm(PG,m) 12
14. Power Transactions • Power transactions are contracts between areas to do power transactions. • Contracts can be for any amount of time at any price for any amount of power. • Scheduled power transactions are implemented by modifying the area ACE: ACE = Pactual,tie-flow - Psched 13
15. 100 MW Transaction Area Name: Home ACE: -0.0 MW Home Area MW Load: 335.8 MW MW Gen: 436.8 MW 35 MW -34 MW MW Losses: 1.01 MW Bus 2 -7 Mvar 7 Mvar A A Bus 1 MVA 224 MW MVA 112 Mvar slack A A 0 MW 226 MW MVA 28 Mvar 116 Mvar MVA -33 MW -66 MW AGC ON 10 Mvar A A 21 Mvar 100 MW 0 Mvar 33 MW MVA MVA 66 MW Scheduled Transactions -10 Mvar Bus 3 -19 Mvar Other Area 100.0 MW Area AGC Status: Part. AGC 211 MW 112 MW 28 Mvar 56 Mvar AGC ON Net tie-line Scheduled 100 MW transaction flow is now from the Home Area to the 100 MW Other Area 14
16. Security Constrained ED • Transmission constraints often limit system economic operation. • Such limits required a constrained dispatch in order to maintain system security. • In the three bus case the generation at bus 3 must be constrained to avoid overloading the line from bus 2 to bus 3. 15
17. Security Constrained Dispatch Area Name: Home ACE: 0.1 MW Home Area MW Load: 580.0 MW MW Gen: 685.9 MW -22 MW 22 MW MW Losses: 5.90 MW Bus 2 4 Mvar -4 Mvar A A Bus 1 MVA 387 MW MVA 193 Mvar slack A A -0 MW 223 MW 100% MVA 37 Mvar 246 Mvar MVA -142 MW -122 MW AGC ON A 49 Mvar A 41 Mvar 100 MW 100% 0 Mvar 145 MW MVA MVA 124 MW Scheduled Transactions -37 Mvar Bus 3 -33 Mvar Other Area 100.0 MW Area AGC Status: OPF 463 MW 193 MW 26 Mvar 97 Mvar AGC ON Dispatch is no longer optimal due to need to keep the line from bus 2 to bus 3 from overloading 16
18. Multi-Area Operation • If areas have direct interconnections then they may directly transact, up to the capacity of their tie-lines. • Actual power flows through the entire network according to the impedance of the transmission lines. • Flow through other areas is known as “parallel path” or “loop flow.” 17
19. Seven Bus Case: One-line Area Top System has three areas has five 44 MW A 42 MW 31 MW A 31 MW 80 MW 30 Mvar buses 1.05 pu Bus 1 MVA Bus 3 0.99 pu MVA Bus 4 1.00 pu 61 MW 105 MW 37 MW 110 MW 32 MW AGC ON 40 Mvar A A A A 93 MW MVA MVA Case Hourly Cost AGC ON MVA 38 MW MVA 16933 $/h A 14 MW 60 MW 33 MW 1.04 pu 79 MW MVA 77 MW 1.01 pu Bus 2 Top Area Cost Bus 5 8030$/h 40 MW 39 MW 130 MW 40 MW 40 Mvar 20 Mvar A A 170 MW AGC ON MVA MVA 40 MW 40 MW 20 MW A 20 MW 1.04 pu 1.04 pu MVA Bus 6 20 MW A 20 MW Bus 7 200 MW Left Area Cost MVA Right Area Cost slack 200 MW 0 Mvar 4189 $/h 4714$/h 0 Mvar 200 MW AGC ON 201 MW AGC ON Area Left Area has one Right has bus one bus PowerWorld Case: B7Flat 18
20. Seven Bus Case: Area View Actual Top Area Losses 7.1 MW flow System has 40.2 MW between -40.2 MW 40 MW of 0.0 MW 0.0 MW areas “Loop Flow” Scheduled Left Right Area Losses 40.2 MW Area Losses flow 0.3 MW 0.0 MW 0.7 MW 19