Power-system protection

Power-system protection is a branch of electrical power engineering that deals with the protection of electrical power systems from faults through the isolation of faulted parts from the rest of the electrical network. The objective of a protection scheme is to keep the power system stable by isolating only the components that are under fault, whilst leaving as much of the network as possible still in operation. Thus, protection schemes must apply with very pragmatic and pessimistic approach to clearing system faults. The devices that are used to protect the power systems from faults are called protection devices.

Components

Protection systems usually comprise five components:

For parts of a distribution system, fuses are capable of both sensing and disconnecting faults.

Failures may occur in each part, such as insulation failure, fallen or broken transmission lines, incorrect operation of circuit breakers, short circuits and open circuits. Protection devices are installed with the aims of protection of assets, and ensure continued supply of energy.

Switchgear is a combination of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switches are safe to open under normal load current, while protective devices are safe to open under fault current. [1]

Protective device

A digital (numeric) multifunction protective relay for distribution networks. A single such device can replace many single-function electromechanical relays, and provides self-testing and communication functions.

While the operating quality of these devices, and especially of protective relays, is always critical, different strategies are considered for protecting the different parts of the system. Very important equipment may have completely redundant and independent protective systems, while a minor branch distribution line may have very simple low-cost protection.

There are three parts of protective devices:

Advantages of protected devices with these three basic components include safety, economy, and accuracy.[2][3]

  • Safety: Instrument transformers create electrical isolation from the power system, and thus establishing a safer environment for personnel working with the relays.
  • Economy: Relays are able to be simpler, smaller, and cheaper given lower-level relay inputs.
  • Accuracy: Power system voltages and currents are accurately reproduced by instrument transformers over large operating ranges.

Types of protection

Coordination

Protective device coordination is the process of determining the "best fit" timing of current interruption when abnormal electrical conditions occur. The goal is to minimize an outage to the greatest extent possible. Historically, protective device coordination was done on translucent log–log paper. Modern methods normally include detailed computer based analysis and reporting.

Protection coordination is also handled through dividing the power system into protective zones. If a fault were to occur in a given zone, necessary actions will be executed to isolate that zone from the entire system. Zone definitions account for generators, buses, transformers, transmission and distribution lines, and motors. Additionally, zones possess the following features: zones overlap, overlap regions denote circuit breakers, and all circuit breakers in a given zone with a fault will open in order to isolate the fault. Overlapped regions are created by two sets of instrument transformers and relays for each circuit breaker. They are designed for redundancy to eliminate unprotected areas; however, overlapped regions are devised to remain as small as possible such that when a fault occurs in an overlap region and the two zones which encompass the fault are isolated, the sector of the power system which is lost from service is still small despite two zones being isolated.[5]

Disturbance-monitoring equipment

Disturbance-monitoring equipment (DME) monitors and records system data pertaining to a fault. DME accomplish three main purposes:

  1. model validation,
  2. disturbance investigation, and
  3. assessment of system protection performance.[6]

DME devices include:[7]

Performance measures

Protection engineers define dependability as the tendency of the protection system to operate correctly for in-zone faults. They define security as the tendency not to operate for out-of-zone faults. Both dependability and security are reliability issues. Fault tree analysis is one tool with which a protection engineer can compare the relative reliability of proposed protection schemes. Quantifying protection reliability is important for making the best decisions on improving a protection system, managing dependability versus security tradeoffs, and getting the best results for the least money. A quantitative understanding is essential in the competitive utility industry. [8][9]

Performance and design criteria for system-protection devices include reliability, selectivity, speed, cost, and simplicity.[10]

See also

Notes

  1. Alexandra Von Meier (2013). Electrical Engineer 137A: Electric Power Systems. Lecture 14:Introduction to Protection Systems, Slide 3.
  2. Alexandra Von Meier (2013). Electrical Engineer 137A: Electric Power Systems. Lecture 14:Introduction to Protection Systems, Slide 12.
  3. Glover J. D., Sarma M. S., Overbye T. J. (2010) Power System and Analysis 5th Edition. Cengage Learning. Pg 525.
  4. "Restricted Earth Fault Protection". myElectrical.com. Retrieved 2 July 2013.
  5. Glover J. D., Sarma M. S., Overbye T. J. (2010) Power System and Analysis 5th Edition. Cengage Learning. Pg 548-549.
  6. "System Protection Manual" (PDF). New York Independent System Operator. Retrieved 2011-12-31.
  7. "Glossary of Terms Used in Reliability Standards" (PDF). North American Electric Reliability Corporation. Retrieved 2011-12-31.
  8. E. O. Schweitzer, J. J Kumm, M. S. Weber, and D. Hou, “Philosophies for Testing Protective Relays,” 20th Annual Western Protective Relay Conference, Spokane, WA. Oct. 19-21, 1993.
  9. J.J. Kumm. E.O. Schweitzer, and D. Hou, “Assessing the Effectiveness of Self-Tests and Other Monitoring Means in Protective Relays,” 21st Annual Western Protective Relay Conference, Spokane, WA. Oct. 18-20, 1994.
  10. Glover J. D., Sarma M. S., Overbye T. J. (2010) Power System and Analysis 5th Edition. Cengage Learning. Pg 526.

References

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