ECE 530 – Analysis Techniques for Large-Scale Electrical Systems Prof. Tom Overbye Dept. of Electrical and Computer Engineering University of Illinois at Urbana-Champaign 09/21/ Lecture 8: Power System Operation

Power System Operations Overview Goal is to provide an intuitive feel for power system operation Emphasis will be on the impact of the transmission system Introduce basic power flow concepts through small system examples 2

Power System Basics All power systems have three major components: Generation, Load and Transmission/Distribution. Generation: Creates electric power. Load: Consumes electric power. Transmission/Distribution: Transmits electric power from generation to load. – Lines/transformers operating at voltages above 100 kV are usually called the transmission system. The transmission system is usually networked. – Lines/transformers operating at voltages below 100 kV are usually called the distribution system (radial). 3

Metro Chicago Electric Network 4

Small PowerWorld Simulator Case Load with green arrows indicating amount of MW flow Used to control output of generator Direction of arrow is used to indicate direction of real power (MW) flow Note the power balance at each bus PowerWorld Case: B3NewSlow 5

Basic Power Control 6 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.

Overloaded Transmission Line 7

Interconnected Operation Power systems are interconnected across large distances. For example most of North America east of the Rockies is one system, with most of Texas and Quebec being major exceptions Individual utilities only own and operate a small portion of the system; this paradigm is now more complex with the advent of ISOs 8

Balancing Authority (BA) Areas 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 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 10

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 11

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. 12

Three Bus Case on AGC Net tie flow is close to zero Generation is automatically changed to match change in load 13

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 14

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. IC 1 (P G,1 ) = IC 2 (P G,2 ) = … = IC m (P G,m ) 15

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 = P actual,tie-flow - P sched 16

100 MW Transaction Scheduled 100 MW Transaction from Left to Right Net tie-line flow is now 100 MW 17

Security Constrained ED Transmission constraints often limit system economics. Such limits required 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. 18

Security Constrained Dispatch Dispatch is no longer optimal due to need to keep line from bus 2 to bus 3 from overloading 19

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 flows.” 20

Seven Bus Case: One-line System has three areas Area Left has one bus Area Right has one bus Area Top has five buses PowerWorld Case: B7Flat 21

Seven Bus Case: Area View System has 40 MW of “Loop Flow” Actual flow between areas Loop flow can result in higher losses Scheduled flow 22

Seven Bus - Loop Flow? 100 MW Transaction between Left and Right Transaction has actually decreased the loop flow Note that Top’s Losses have increased from 7.09MW to 9.44 MW 23

POWER TRANSFER DISTRIBUTION FACTORS (PTDFS) 24 PTDFs are used to show how a particular transaction will affect the system Power transfers through the 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 Later in the semester we’ll consider techniques for calculating PTDFs

PTDF EXAMPLE - NINE BUS CASE ACTUAL FLOWS 25 PowerWorld Case: B9

PTDF EXAMPLE - PTDFS: TRANSFER FROM A TO I 26 Values now tell percentage of flow that will go on line

PTDF EXAMPLE - PTDFS: TRANSFER FROM G TO F 27

Wisconsin to TVA Line PTDF Contour 28 Contours show lines that would carry at least 2% of a power transfer from Wisconsin to TVA

NERC Flowgates A convenient glossary of terms used for power system operations in North America is available at One common term is a “flowgate,” which is a mathematical construct to measure the MW flow on one or more elements in the bulk transmission system – Sometimes they include the impact of contingencies, something we will consider later in the semester A simple flowgate would be the MW flow through a single transmission line or transformer 29

NERC TLRs In the North American Eastern Interconnect (EI) transmission loading relief procedures (TLRs) are used to mitigate the overloads on the bulk transmission system TLRs consider the PTDFs associated with transactions on flowgates if there is a flowgate violation 30

Loop Flow Impact: Market Segmentation 31 During summer of 1998 con- gestion on just two elements pushed Midwest spot market prices up by a factor of 200: from $ 20/MWh to $ 7500/MWh! Large price rises have occurred in 1999 and 2000 as well

Pricing Electricity Cost to supply electricity to bus is called the locational marginal price (LMP) Presently some electric makets post LMPs on the web In an ideal electricity market with no transmission limitations the LMPs are equal Transmission constraints can segment a market, resulting in differing LMP Determination of LMPs requires the solution on an Optimal Power Flow (OPF) 32

3 BUS LMPS - OVERLOAD IGNORED Line from Bus 1 to Bus 3 is over-loaded; all buses have same marginal cost Gen 1’s cost is $10 per MWh Gen 2’s cost is $12 per MWh PowerWorld Case: B3LP 33

LINE OVERLOAD ENFORCED Line from 1 to 3 is no longer overloaded, but now the marginal cost of electricity at 3 is $14 / MWh 34

MISO LMPs 35 Five minute LMPs are posted online for the MISO footprint Source:

MISO LMP Volatility! 36 This is how the LMP contour looked for the next 5 minute update!

37 Bus Example Design Case This is Design Case 2 From Chapter 6 of Power System Analysis and Design by Glover, Sarma, and Overbye, 4 th Edition, 2008 PowerWorld Case: TD_2012_Design2 37

Good Power System Operation Good power system operation requires that there be no reliability violations for either the current condition or in the event of statistically likely contingencies – Reliability requires as a minimum that there be no transmission line/transformer limit violations and that bus voltages be within acceptable limits (perhaps 0.95 to 1.08) – Example contingencies are the loss of any single device. This is known as n-1 reliability. 38

Looking at the Impact of Line Outages Opening one line (Tim69-Hannah69) causes an overload. This would not be allowed 39

Contingency Analysis Contingency analysis provides an automatic way of looking at all the statistically likely contingencies. In this example the contingency set Is all the single line/transformer outages 40

Power Flow And Design One common usage of the power flow is to determine how the system should be modified to remove contingencies problems or serve new load – In an operational context this requires working with the existing electric grid – In a planning context additions to the grid can be considered In the next example we look at how to remove the existing contingency violations while serving new load. 41

An Unreliable Solution Case now has nine separate contingencies with reliability violations 42

A Reliable Solution Previous case was augmented with the addition of a 138 kV Transmission Line 43 PowerWorld Case: TD_2012_Design2_ReliableDesign

Generation Changes and The Slack Bus The power flow is a steady-state analysis tool, so the assumption is total load plus losses is always equal to total generation – Generation mismatch is made up at the slack bus When doing generation change power flow studies one always needs to be cognizant of where the generation is being made up – Common options include system slack, distributed across multiple generators by participation factors or by economics 44

Generation Change Example 1 45 PowerWorld Case: TD_2012_37Bus_GenChange

Generation Change Example 1 Display shows “Difference Flows” between original 37 bus case, and case with a BLT138 generation outage; note all the power change is picked up at the slack 46

Generation Change Example 2 Display repeats previous case except now the change in generation is picked up by other generators using a participation factor approach 47

Generator Reactive Limits Generators are P-V buses (P and V are specified). Q Gi of generator i must be within specified limits During the PF solution process the bus is now a P-Q bus and the originally specified V at this bus is relaxed and calculated. 48

Voltage Regulation Example: 37 Buses Display shows voltage contour of the power system, demo will show the impact of generator voltage set point, reactive power limits, and switched capacitors 49 PowerWorld Case: TD_2012_37Bus_Voltage

Remote Regulation and Reactive Power Sharing It is quite common for a generator to control the voltage for a location that is not its terminal – Sometimes this is on the high side of the generator step-up transformer (GSU), sometimes it is partway through the GSU It is also quite common for multiple generators to regulate the same bus voltage – In this case only one of the generators can be set as a PV bus; the others must be set as PQ, with the total reactive power output allocated among them – Different methods can be used for allocating reactive power among multiple generators 50

Multiple PV Generator Regulation 51 PowerWorld Case: B7Flat_MultipleGenReg In this case both the Bus 2 and Bus 4 gens are set to regulate the Bus 5 voltage. Note, they must regulate it to the same value!!