1. Lecture 22 Power System Protection, Transient Stability Professor Tom Overbye Department of Electrical and Computer Engineering ECE 476 POWER SYSTEM ANALYSIS

2. 1 Announcements Be reading Chapters 9 and 10 After exam read Chapter 11 HW 9 is 8.4, 8.12, 9.1,9.2 (bus 2), 9.14; do by Nov 10 but does not need to be turned in. Start working on Design Project. Firm due date has been extended to Dec 1 in class Second exam is on Nov 15 in class. Same format as first exam, except you can bring two note sheets (e.g., the one from the first exam and another) Exam/solution from 2008 will be posted on website shortly Exam covers through Chapter 10

3. 2 In the News: Boulder municipalization Last week Boulder, CO narrowly voted to move forward with municipalization of their electric grid Currently Boulder is in the Xcel Energy electric service territory (Xcel is a large Investor Owned Utility) Xcel has recently decided not to continue funding the Boulder “SmartGridCity” initiative, which has cost $45 million, triple its original cost. Xcel does not wish to sell its electric grid in Boulder, saying it would be extremely expensive for Boulder to go on their own. Source: NY Times 11/3/11; Thanks to Margaret for pointing out this story

4. 3 Power System Protection Main idea is to remove faults as quickly as possible while leaving as much of the system intact as possible Fault sequence of events 1. Fault occurs somewhere on the system, changing the system currents and voltages 2. Current transformers (CTs) and potential transformers (PTs) sensors detect the change in currents/voltages 3. Relays use sensor input to determine whether a fault has occurred 4. If fault occurs relays open circuit breakers to isolate fault

5. 4 Power System Protection Protection systems must be designed with both primary protection and backup protection in case primary protection devices fail In designing power system protection systems there are two main types of systems that need to be considered: 1. Radial: there is a single source of power, so power always flows in a single direction; this is the easiest from a protection point of view 2. Network: power can flow in either direction: protection is much more involved

6. 5 Radial Power System Protection Radial systems are primarily used in the lower voltage distribution systems. Protection actions usually result in loss of customer load, but the outages are usually quite local. The figure shows potential protection schemes for a radial system. The bottom scheme is preferred since it results in less lost load

7. 6 Radial Power System Protection In radial power systems the amount of fault current is limited by the fault distance from the power source: faults further done the feeder have less fault current since the current is limited by feeder impedance Radial power system protection systems usually use inverse-time overcurrent relays. Coordination of relay current settings is needed to open the correct breakers

8. 7 Inverse Time Overcurrent Relays Inverse time overcurrent relays respond instan- taneously to a current above their maximum setting They respond slower to currents below this value but above the pickup current value

9. 8 Inverse Time Relays, cont'd The inverse time characteristic provides backup protection since relays further upstream (closer to power source) should eventually trip if relays closer to the fault fail Challenge is to make sure the minimum pickup current is set low enough to pick up all likely faults, but high enough not to trip on load current When outaged feeders are returned to service there can be a large in-rush current as all the motors try to simultaneously start; this in-rush current may re-trip the feeder

10. 9 Inverse Time Overcurrent Relays Relays have traditionally been electromechanical devices, but are gradually being replaced by digital relays Current and time settings are ad- justed using dials on the relay

11. 10 Protection of Network Systems In a networked system there are a number of difference sources of power. Power flows are bidirectional Networked system offer greater reliability, since the failure of a single device does not result in a loss of load Networked systems are usually used with the transmission system, and are sometimes used with the distribution systems, particularly in urban areas

12. 11 Network System Protection Removing networked elements require the opening of circuit breakers at both ends of the device There are several common protection schemes; multiple overlapping schemes are usually used 1. Directional relays with communication between the device terminals 2. Impedance (distance) relays. 3. Differential protection

13. 12 Directional Relays Directional relays are commonly used to protect high voltage transmission lines Voltage and current measurements are used to determine direction of current flow (into or out of line) Relays on both ends of line communicate and will only trip the line if excessive current is flowing into the line from both ends – line carrier communication is popular in which a high frequency signal (30 kHz to 300 kHz) is used – microwave communication is sometimes used

14. 13 Impedance Relays Impedance (distance) relays measure ratio of voltage to current to determine if a fault exists on a particular line

15. 14 Impedance Relays Protection Zones To avoid inadvertent tripping for faults on other transmission lines, impedance relays usually have several zones of protection: – zone 1 may be 80% of line for a 3  fault; trip is instantaneous – zone 2 may cover 120% of line but with a delay to prevent tripping for faults on adjacent lines – zone 3 went further; most removed due to 8/14/03 events The key problem is that different fault types will present the relays with different apparent impedances; adequate protection for a 3  fault gives very limited protection for LL faults

16. 15 Impedance Relay Trip Characteristics Source: August 14 th 2003 Blackout Final Report, p. 78

17. 16 Differential Relays Main idea behind differential protection is that during normal operation the net current into a device should sum to zero for each phase – transformer turns ratios must, of course, be considered Differential protection is used with geographically local devices – buses – transformers – generators

18. 17 Other Types of Relays In addition to providing fault protection, relays are used to protect the system against operational problems as well Being automatic devices, relays can respond much quicker than a human operator and therefore have an advantage when time is of the essence Other common types of relays include – under-frequency for load: e.g., 10% of system load must be shed if system frequency falls to 59.3 Hz – over-frequency on generators – under-voltage on loads (less common)

19. 18 Sequence of Events Recording During major system disturbances numerous relays at a number of substations may operate Event reconstruction requires time synchronization between substations to figure out the sequence of events Most utilities now have sequence of events recording that provide time synchronization of at least 1 microsecond

20. 19 Use of GPS for Fault Location Since power system lines may span hundreds of miles, a key difficulty in power system restoration is determining the location of the fault One newer technique is the use of the global positioning system (GPS). GPS can provide time synchronization of about 1 microsecond Since the traveling electromagnetic waves propagate at about the speed of light (300m per microsecond), the fault location can be found by comparing arrival times of the waves at each substation

21. 20 Power System Transient Stability In order to operate as an interconnected system all of the generators (and other synchronous machines) must remain in synchronism with one another – synchronism requires that (for two pole machines) the rotors turn at exactly the same speed Loss of synchronism results in a condition in which no net power can be transferred between the machines A system is said to be transiently unstable if following a disturbance one or more of the generators lose synchronism

22. 21 Generator Transient Stability Models In order to study the transient response of a power system we need to develop models for the generator valid during the transient time frame of several seconds following a system disturbance We need to develop both electrical and mechanical models for the generators

23. 22 Example of Transient Behavior

24. 23 Generator Electrical Model The simplest generator model, known as the classical model, treats the generator as a voltage source behind the direct-axis transient reactance; the voltage magnitude is fixed, but its angle changes according to the mechanical dynamics

25. 24 Generator Mechanical Model Generator Mechanical Block Diagram

26. 25 Generator Mechanical Model, cont’d

27. 26 Generator Mechanical Model, cont’d

28. 27 Generator Mechanical Model, cont’d

29. 28 Generator Swing Equation

30. 29 Single Machine Infinite Bus (SMIB) To understand the transient stability problem we’ll first consider the case of a single machine (generator) connected to a power system bus with a fixed voltage magnitude and angle (known as an infinite bus) through a transmission line with impedance jX L

31. 30 SMIB, cont’d

32. 31 SMIB Equilibrium Points

33. 32 Transient Stability Analysis For transient stability analysis we need to consider three systems 1. Prefault - before the fault occurs the system is assumed to be at an equilibrium point 2. Faulted - the fault changes the system equations, moving the system away from its equilibrium point 3. Postfault - after fault is cleared the system hopefully returns to a new operating point

34. 33 Transient Stability Solution Methods There are two methods for solving the transient stability problem 1. Numerical integration this is by far the most common technique, particularly for large systems; during the fault and after the fault the power system differential equations are solved using numerical methods 2. Direct or energy methods; for a two bus system this method is known as the equal area criteria mostly used to provide an intuitive insight into the transient stability problem

35. 34 SMIB Example Assume a generator is supplying power to an infinite bus through two parallel transmission lines. Then a balanced three phase fault occurs at the terminal of one of the lines. The fault is cleared by the opening of this line’s circuit breakers.

36. 35 SMIB Example, cont’d Simplified prefault system

37. 36 SMIB Example, Faulted System During the fault the system changes The equivalent system during the fault is then During this fault no power can be transferred from the generator to the system

38. 37 SMIB Example, Post Fault System After the fault the system again changes The equivalent system after the fault is then

39. 38 SMIB Example, Dynamics