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Home / Download Center / Electrical Engineering Books and Technical Guides / Relay control and protection guides / The basics of power system protection that every engineer should know about

Introduction to relay protection

Protection is the branch of electric power engineering concerned with the principles of design and operation of equipment (called ‘relays’ or ‘protective relays’) that detects abnormal power system conditions, and initiates corrective action as quickly as possible in order to return the power system to its normal state.

Power system protection basics you really should know about
Power system protection basics you really should know about (photo credit: SEL)

The quickness of response is an essential element of protective relaying systems – response times of the order of a few milliseconds are often required. Consequently, human intervention in the protection system operation is not possible.

The response must be automatic, quick and should cause a minimum amount of disruption to the power system. The entire subject is governed by these general requirements: correct diagnosis of trouble, quickness of response and minimum disturbance to the power system.

To accomplish these goals, we must examine all possible types of fault or abnormal conditions which may occur in the power system. We must further examine the possibility that protective relaying equipment itself may fail to operate correctly, and provide for a backup protective function.

It should be clear that extensive and sophisticated equipment is needed to accomplish these tasks.


Nature of relaying

We will now discuss certain attributes of relays which are inherent to the process of relaying. In general, relays do not prevent damage to equipment: they operate after some detectable damage has already occurred.

Their purpose is to limit, to the extent possible, further damage to equipment, to minimize danger to people, to reduce stress on other equipment and, above all, to remove the faulted equipment from the power system as quickly as possible so that the integrity and stability of the remaining system is maintained.

Example of relay protection logic
Figure 1 – Example of relay protection logic

Reliability, dependability and security

Relays, in contrast with most other equipment, have two alternative ways in which they can be unreliable: they may fail to operate when they are expected to, or they may operate when they are not expected to.

This leads to a two-pronged definition of reliability of relaying systems: a reliable relaying system must be dependable and secure.

  • Dependability: the measure of the certainty that the relays will operate correctly for all the faults for which they are designed to operate.
  • Security: the measure of the certainty that the relays will not operate incorrectly for any fault.

Example

Consider the fault F on the transmission line shown in Figure 1 above. In normal operation, this fault should be cleared by the two relays R1 and R2 through the circuit breakers B1 and B2. If R2 does not operate for this fault, it has become unreliable through a loss of dependability.

If relay R5 operates through breaker B5 for the same fault, and before breaker B2 clears the fault, it has become unreliable through a loss of security.

Title: The basics of power system protection that every engineer should know about – Ahmed Fouad Atta Helwan at University, Faculty of Engineering (Electric Power Department)
Format: PDF
Size: 2.0 MB
Pages: 58
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The basics of power system protection that every engineer should know about
The basics of power system protection that every engineer should know about

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3 Comments


  1. Joseph Arthur
    Feb 20, 2020

    Please advice me. On a completely Rocky area, how do you achieve good and effective earth system (TT system) thank you.


  2. Salah M.Sallam
    Jan 07, 2020

    Dear :
    1- A – Dependability: the measure of the certainty that the relays will operate correctly for all the faults for
    which they are designed to operate.-
    B – Security: the measure of the certainty that the relays will not operate incorrectly for any fault.
    – Are these a definition or description of performance, or what does it mean exactly?
    C – The SLD page 2 does not match the fault analysis and the analysis is also incorrect

    2- Why is the word (Transducers) instead of the word (Current transformer) used in components?
    Where each of them has a different performance and composition in addition to identifying it in all books
    and) * {3. Elements of a protection system -page 7}
    3- 2.2. SELECTIVITY OF RELAYS AND ZONES OF PROTECTION
    Relays usually have inputs from several current transformers (CTs)
    Also We have voltage,Frequency (U\O)relays and etc.,
    4- Example:
    This fault lies in a closed zone, and will cause circuit breakers B1 and B2 to trip. The fault at F2,
    being inside the overlap between the zones of protection of the transmission line and the bus, will
    cause circuit breakers B1,B2,B3 and B4 to trip, although opening B3 and B4 is unnecessary. Both
    of these zones of protection are closed zones.(page 3)
    There is a big difference with F2 before the CB OR AFTER CB, but within the two ZONES in the
    TRIP ordering , therefore, the F2 malfunction analysis is not identical as written.
    PL. CLEAR THAT ITEMS AND I HAVE ANOTHER
    Dr salah sallam
    01003825824

     


    • Salah M.Sallam
      Jan 07, 2020

      We have many comment
      Any technical document offering to be reviewed by specialists in the same field must be reviewed before it is published

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