1、1,How the Power Grid Behaves,Tom Overbye Department of Electrical and Computer Engineering University of Illinois at Urbana-Champaign,2,Presentation Overview,Goal is to demonstrate operation of large scale power grid. Emphasis on the impact of the transmission syste. Introduce basic power flow conce
2、pts through small system examples. Finish with simulation of Eastern U.S. System.,3,PowerWorld Simulator,PowerWorld Simulator is an interactive, Windows based simulation program, originally designed at University of Illinois for teaching basics of power system operations to non-power engineers. Powe
3、rWorld Simulator can now study systems of just about any size.,4,Eastern Interconnect Operating Areas,Ovals represent operating areas,Arrows indicate power flow in MW between areas,5,Zoomed View of Midwest,6,Power System Basics,All power systems have three major components: Generation, Load and Tran
4、smission. Generation: Creates electric power. Load: Consumes electric power. Transmission: Transmits electric power from generation to load.,7,One-line Diagram,Most power systems are balanced three phase systems. A balanced three phase system can be modeled as a single (or one) line. One-lines show
5、the major power system components, such as generators, loads, transmission lines. Components join together at a bus.,8,Eastern North American High Voltage Transmission Grid,Figure shows transmission lines at 345 kV or above in Eastern U.S.,9,Zoomed View of Midwest,Arrows indicate MW flow on the line
6、s; piecharts show percentage loading of lines,10,Example Three Bus System,Generator,Load,Bus,Circuit Breaker,Pie charts show percentage loading of lines,11,Generation,Large plants predominate, with sizes up to about 1500 MW. Coal is most common source, followed by hydro, nuclear and gas. Gas is now
7、most economical. Generated at about 20 kV.,12,Loads,Can range in size from less than a single watt to 10s of MW. Loads are usually aggregated. The aggregate load changes with time, with strong daily, weekly and seasonal cycles.,13,Transmission,Goal is to move electric power from generation to load w
8、ith as low of losses and cost as possible. P = V I or P/V = I Losses are I2 R Less losses at higher voltages, but more costly to construct and insulate.,14,Transmission and Distribution,Typical high voltage transmission voltages are 500, 345, 230, 161, 138 and 69 kV. Transmission tends to be a grid
9、system, so each bus is supplied from two or more directions. Lower voltage lines are used for distribution, with a typical voltage of 12.4 kV. Distribution systems tend to be radial. Transformers are used to change the voltage.,15,Other One-line Objects,Circuit Breakers - Used to open/close devices;
10、 red is closed, green is open. Pie Charts - Show percentage loading of transmission lines. Up/down arrows - Used to control devices. Values - Show current values for different quantities.,16,Power Balance Constraints,Power flow refers to how the power is moving through the system. At all times the t
11、otal power flowing into any bus MUST be zero! This is know as Kirchhoffs law. And it can not be repealed or modified. Power is lost in the transmission system.,17,Basic Power Control,Opening a circuit breaker causes the power flow to instantaneously(nearly) change. No other way to directly control p
12、ower flow in a transmission line. By changing generation we can indirectly change this flow.,18,Flow Redistribution Following Opening Line Circuit Breaker,Power Balance must be satisfied at each bus,No flow on open line,19,Indirect Control of Line Flow,Generator MW output changed,Generator change in
13、directly changes line flow,20,Transmission Line Limits,Power flow in transmission line is limited by a number of considerations. Losses (I2 R) can heat up the line, causing it to sag. This gives line an upper thermal limit. Thermal limits depend upon ambient conditions. Many utilities use winter/sum
14、mer limits.,21,Overloaded Transmission Line,Thermal limit of 150 MVA,22,Interconnected Operation,Power systems are interconnected across large distances. For example most of North American east of the Rockies is one system, with most of Texas and Quebec being major exceptions Individual utilities on
15、ly own and operate a small portion of the system, which is referred to an operating area (or an area).,23,Operating Areas,Areas constitute a structure imposed on grid. 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.
16、 The flow out of an area is equal to total gen - total load - total losses = tie-flow,24,Three Bus System Split into Two Areas,Net tie flow is NOT zero,Initially area flow is not controlled,25,Area Control Error (ACE),The area control error mostly the difference between the actual flow out of area,
17、and scheduled flow. ACE also includes a frequency component. Ideally the ACE should always be zero. Because the load is constantly changing, each utility must constantly change its generation to “chase” the ACE.,26,Home Area ACE,ACE changes with time,27,Inadvertent Interchange,ACE can never be held
18、exactly at zero. Integrating the ACE gives the inadvertent interchange, expressed in MWh. Utilities keep track of this value. If it gets sufficiently negative they will “pay back” the accumulated energy. In extreme cases inadvertent energy is purchased at a negotiated price.,28,Automatic Generation
19、Control,Most utilities use automatic generation control (AGC) to automatically change their generation to keep their ACE close to zero. Usually the utility control center calculates ACE based upon tie-line flows; then the AGC module sends control signals out to the generators every couple seconds.,2
20、9,Three Bus Case on AGC,With AGC on, net tie flow is zero, but individual line flows are not zero,30,Generator Costs,There are many fixed and variable costs associated with power system operation. Generation is major variable cost. For some types of units (such as hydro and nuclear) it is difficult
21、to quantify. For thermal units it is much easier. There are four major curves, each expressing a quantity as a function of the MW output of the unit.,31,Generator Cost Curves,Input-output (IO) curve: Shows relationship between MW output and energy input in Mbtu/hr. Fuel-cost curve: Input-output curv
22、e scaled by a fuel cost expressed in $ / Mbtu. Heat-rate curve: shows relationship between MW output and energy input (Mbtu / MWhr). Incremental (marginal) cost curve shows the cost to produce the next MWhr.,32,Example Generator Fuel-Cost Curve,Current generator operating point,Y-axis tells cost to
23、produce specified power (MW) in $/hr,33,Example Generator Marginal Cost Curve,Current generator operating point,Y-axis tells marginal cost to produce one more MWhr in $/MWhr,34,Economic Dispatch,Economic dispatch (ED) determines the least cost dispatch of generation for an area. For a lossless syste
24、m, the ED occurs when all the generators have equal marginal costs. IC1(PG,1) = IC2(PG,2) = = ICm(PG,m),35,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 transac
25、tions are implemented by modifying the area ACE: ACE = Pactual,tie-flow - Psched,36,Implementation of 100 MW Transaction,Net tie flow is now 100 MW from left to right,Scheduled Transaction,Overloaded line,37,Security Constrained ED,Transmission constraints often limit system economics. Such limits r
26、equired a constrained dispatch in order to maintain system security. In three bus case the generation at bus 3 must be constrained to avoid overloading the line from bus 2 to bus 3.,38,Security Constrained Dispatch,Net tie flow is still 100 MW from left to right,Gens 2 &3 changed to remove overload,
27、39,Multi-Area Operation,The electrons are not concerned with area boundaries. Actual power flows through the entire network according to impedance of the transmission lines. If Areas have direct interconnections, then they can directly transact up their tie-line capacity. Flow through other areas is
28、 known as “parallel path” or “loop flows.”,40,Seven Bus, Thee Area Case One-line,Area “Top” has 5 buses,Area “Left” has one bus,Area “Right” has one bus,ACE for each area is zero,41,Seven Bus Case: Area View,Actual flow between areas,Scheduled flow between areas,42,Seven Bus Case with 100 MW Transfe
29、r,Losses went up from 7.09 MW,100 MW Scheduled Transfer from Left to Right,43,Seven Bus Case One-line,Transfer also overloads line in Top,44,Transmission Service,FERC Order No. 888 requires utilities provide non-discriminatory open transmission access through tariffs of general applicability. FERC O
30、rder No. 889 requires transmission providers set up OASIS (Open Access Same-Time Information System) to show available transmission.,45,Transmission Service,If areas (or pools) are not directly interconnected, they must first obtain a contiguous “contract path.” This is NOT a physical requirement. U
31、tilities on the contract path are compensated for wheeling the power.,46,Eastern Interconnect Example,Arrows indicate the basecase flow between areas,47,Power Transfer Distribution Factors (PTDFs),PTDFs are used to show how a particular transaction will affect the system. Power transfers through the
32、 system according to the impedances of the lines, without respect to ownership. All transmission players in network could be impacted, to a greater or lesser extent.,48,PTDFs for Transfer from Wisconsin Electric to TVA,Piecharts indicate percentage of transfer that will flow between specified areas,
33、49,PTDF for Transfer from WE to TVA,100% of transfer leaves Wisconsin Electric (WE),50,PTDFs for Transfer from WE to TVA,About 100% of transfer arrives at TVA,But flow does NOT follow contract path,51,Contingencies,Contingencies are the unexpected loss of a significant device, such as a transmission
34、 line or a generator. No power system can survive a large number of contingencies. First contingency refers to loss of any one device. Contingencies can have major impact on Power Transfer Distribution Factors (PTDFs).,52,Available Transfer Capability,Determines the amount of transmission capability
35、 available to transfer power from point A to point B without causing any overloads in basecase and first contingencies. Depends upon assumed system loading, transmission configuration and existing transactions.,53,Reactive Power,Reactive power is supplied by generators capacitors transmission lines
36、loads Reactive power is consumed by loads transmission lines and transformers (very high losses,54,Reactive Power,Reactive power doesnt travel well - must be supplied locally. Reactive must also satisfy Kirchhoffs law - total reactive power into a bus MUST be zero.,55,Reactive Power Example,Reactive
37、 power must also sum to zero at each bus,Note reactive line losses are about 13 Mvar,56,Voltage Magnitude,Power systems must supply electric power within a narrow voltage range, typically with 5% of a nominal value. For example, wall outlet should supply 120 volts, with an acceptable range from 114
38、to 126 volts. Voltage regulation is a vital part of system operations.,57,Reactive Power and Voltage,Reactive power and voltage magnitude are tightly coupled. Greater reactive demand decreases the bus voltage, while reactive generation increases the bus voltage.,58,Voltage Regulation,A number of dif
39、ferent types of devices participate in system voltage regulation generators: reactive power output is automatically changed to keep terminal voltage within range. capacitors: switched either manually or automatically to keep the voltage within a range. Load-tap-changing (LTC) transformers: vary thei
40、r off-nominal tap ratio to keep a voltage within a specified range.,59,Five Bus Reactive Power Example,Voltage magnitude is controlled by capacitor,LTC Transformer is controlling load voltage,60,Voltage Control,Voltage control is necessary to keep system voltages within an acceptable range. Because
41、reactive power does not travel well, it would be difficult for it to be supplied by a third party. It is very difficult to assign reactive power and voltage control to particular transactions.,61,Conclusion,Talk has provided brief overview of how power grid operates. Educational Version of PowerWorld Simulator, capable of solving systems with up to 12 buses, can be downloaded for free at 60,000 bus commercial version is also available.,
