Tuesday, March 22, 2011

Circuit Breaker

1. General Background
       Every electric circuit needs a switching device and a protective device. The switching and protective device  have been developed in various forms. For example, everyone is familiar with low voltage switches and re-wirable fuses. The switch is used for opening and closing the electric circuit and the fuse is used for over-current protection.
·               A Circuit-breaker is "a switching and current interrupting device Basically, a C.B. comprise a set of fixed and movable contacts. The contacts can be separated by means of an  operating  mechanism. The separation of current carrying contacts produces an arc. The arc is extinguished by a suitable medium such as dielectric oil, air, vacuum, sf6 gas. The circuit-breaker serves tow basic purposes:
-         switching during normal operation conditions, for the purpose of operation and maintenance.
-         Switching during abnormal condition, such as short circuit. And interrupting the fault currents.
·               Protective relays are "automatic devices which can sense the fault and send instruction to the associated circuit breaker to open". Every part of the power system is provided with a protective relaying system and an associated switching devices.

ALL equipment associated with the fault clearing process are covered by the term "switchgear". In addition to circuit-breaker and protective relays, the associated equipment for controlling, regulating and measuring can also be considered as switchgear devices. 

Switchgear includes switches, fuses,circuit breakers, isolator, relays, control panels, lighting arresters, current transformers, and various associated equipments. Switchgear is an essential part of a power system and also that of any electric circuit. Switchgear are also necessary at every switching point in the power system see figure (1.1)).
Since, between the generation stations and final load point, there are several voltage levels and fault levels hence, in various application, the requirements of a switchgear vary according to :
·     Location
·   Ratings, and
·   Local requirements
    2. Sub-station Equipment
      In every electrical sub-station, there are generally various indoor and outdoor switchgear equipment. Each equipment has certain functional requirement (table 1.1).
  •    Isolator are "disconnecting switches which can be used for disconnecting a circuit under no current conditions". they are generally installed along with the circuit breaker. An isolator can be opened after the circuit breaker,
  •    After opening the isolator, the earthing switch can be closed to discharge the trapped electrical charges to ground
  •    Current transformer (CTs) and potential transformers (PTs) are used for transforming the current and the voltage to lower values for the purpose of measurement, protection, and control.
  •    Lighting arresters (surge arresters) divert the over-voltages to earth and protect the sub-station from over over-voltages.
Table 1.1  Various Substation Equipment
 

       3.Faults and abnormal condition 
             During a fault, the fault impedance is low and accordingly, the fault currents are relatively high.  since the fault currents being excessive, they damage the faulty equipment and the supply installation. During the faults, the power flow is diverted towards the fault;and the supply to the neighboring zones is affected.
      Faults can be classified as :
    •    single line to ground fault
    •    line to line fault
    •    double line to ground fault
    •     simultaneous fault
    •     three phase fault
    •     open circuit, etc.
      The other abnormal conditions include:   
    •    voltage and current unbalance
    •    under frequency
    •    over voltage
    •     temperature rise
    •     reverse of power
    •     power swing
    •     instability
       4. Fault clearing process
    1.  The protective relays are connected in the secondary circuits of current and/or voltage transformers.
    2.  The relays sense the abnormal conditions and close the trip circuit of the associated circuit breaker
    3.  The circuit breaker opens its contacts.
    4.   Arcis drawn between contacts as they separate.
    5.  The arc is extinguished by suitable medium and technique.
    6.   After final arc extinction, a high voltage wave appears across the circiut breaker contacts tending to re-establish the arc.
    7.   The transient voltage wave is called "Transient Recovery Voltage"(TRV)
       5. Protective Relaying 
               The power system is covered by several protective zones. Each protective zone covers one  or tow components of the system. The neighboring protective zones overlap so that no part of the system is left unprotected
       6. Neutral Grounding (Earthing) and Equipment Grounding :
               The term "Grounding" or "Earthing" refers to the connection of a conductor to earth. The neutral point of a generator or a transformer is deliberately connected to earth
      The neutral earthing has several advantages such as :
    •   It stabilizes the neutral point.
    •   It is useful in discharge over-voltages due to lighting to earth.
    •  Simplifies design of earth fault protection
    •   Grounded system require lower insulation levels as compared with ungrounded systems.
                On the other hand, the  "Equipment Grounding" refers to grounding of non-current carrying metal parts to earth. It is used for safety of personnel.
        7. Over-voltage and Insulation Co-ordination
               Over-voltage surges in power systems are caused by various causes such as ;
    •   lighting,
    •   switching
    •   resonance,etc.
                 The power system elements should be withstand the over-voltages without insulation  failure. The insulation level of a power system element refers to its values of power frequency and impulse voltage withstand.
                  The protective measures against over voltage due to lighting include ;
    •    use of overhead ground wires,
    •   low tower footing resistance, and
    •   use of lighting arresters (surge arresters)
                 The surge arresters offer low resistance to over-voltage and divert over-voltages to earth.

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