Wednesday, March 23, 2011

Rated characteristic of circuit breakers

4. Rated characteristic of circuit breakers
        The rating of a circuit breaker denote its capabilities under specified condition of use and behavior. The following paragraphs are generally based on the recommendation of  IEC-56: "High Voltage Alternating Current Circuit-Breakers" and IS-2516: " Specifications of Alternating current circuit-breaker".
           The capabilities of a circuit breaker of a particular type are proved by conducting type tests as per the recommendations of the standards.
4.1 Rated Voltage
       The rated voltage of a circuit-breaker corresponds to the higher system voltage for which the circuit breaker is intended. The standards values of rated voltages are given in table 2. The rated voltage is expressed in KVrms and refers to phase to phase voltage for three-phase circuit. The earlier practice of specifying the rated voltage of a circuit breaker as nominal system voltage is no more followed.
Remarks
Design Feature
Voltage. Breaking capacity
Medium
Type
Used for medium and low voltage, A.C., D.C., Industrial circuit breakers. Have current limiting
Incorporates: Arc runners, arc splitters, magnetic coils
430-600 V, 5-15-35 MVA
 Recently 3.6-12 KV, 500 MVA
Air at atmospheric pressure
          Air- break circuit breaker
Used for low and medium voltage
Small size, current limiting feature
430-600 KV
Air at atmospheric pressure
Miniature C.B.
Getting obsolete, used up to 12 KV, 500 MVA
One tank up to 36 KV, 3 tank above 36 KV, fitted with arc control devices
12-36 KV
Dielectric oil
         Tnak type oil circuit breaker
Used for metal enclosed switchgear up to 36 KV. Outdoor type between 36 and 245 KV
The circuit breaking chamber is separate from supporting chamber. Small size, arc control device used
3.6-245 KV
Dielectric oil
          Minimum oil  circuit breaker
Suitable for all EHV applications, fast opening-closing. Also for arc furnace duty
Unit type construction, several units per pole, auxiliary compressed air system required
245 KV, 35.000 MVA up to 1100 KV, 50.000 MVA, also 36 KV, 500 MVA
Compressed air
(20-30 kgf/cm2)
         Air blast circuit breaker
Suitable for SF6 switchgear, and medium voltage switchgear. EHV circuit breaker. Maintenance free
Live tank/Dead tank design, single pressure type preferred
145 KV, 7500 MVA
245 KV, 10.000 MVA
12 KV, 500 MVA
36 KV, 2000 MVA
SF6 gas
(5 kgf/cm2)
         SF6 circuit breaker -single
Suitable for a variety of applications from 3.6 KV up to 36 KV
Variety of designs, long life, modest maintenance
Preferred for indoor switchgear rated up   to 36 KV, 750 MVA
Vacuum
        Vacuum circuit breaker
Recently developed, used in HVDC systems. Installed in USA
Artificial current zero by switching in capacitors
500 KV DC, 15 KA/20 KA
Vacuum or SF6
         H.V.D.C circuit breaker
Table  1   Comparison of circuit breakers

4.2 Rated Insulation Level
         The rated insulation level of a circuit breaker refers to the power frequency withstand voltage and impulse voltage withstand values which characterize the insulation of the circuit breaker.


Rated Voltage of Circuit Breaker
( KVrms )

Nominal System Voltage
( KVrms )

0.246
0.440
3.600
7.200
12.000
24.000
36.000
72.500
145.000
245.000
420.000
525.000
765.000
0.240
0.415
3.300
6.600
11.000
22.000
33.000
66.000
132.000
220.000
400.000
500.000
750.00
Table 2 : Rated Voltage of Circuit Breaker

 Power-frequency over voltages are due to regulation, ferranti effect, higher tap-setting, etc. The circuit breaker should be capable of withstanding the power frequency over-voltages which are likely to occur. These capabilities are verified by conducting power frequency voltage withstand tests and impulse voltage withstand tests. The circuit breaker is subjected to impulse over-voltage due causes like lighting surge and switching surge.
          During single-line to ground faults, the voltage of healthy lines to earth increases to √3 time the normal value in the system with insulated neutral. Hence higher values of insulation are recommended for circuit breaker connected in noneffectively earthed systems. The following insulations are provided in the circuit breaker :
  • Insulation between live parts and earth for each pole external and internal.
  • Insulation between poles.
  • Insulation between terminals of the same pole-external and internal
         The design of these insulations depends upon the structural form of the circuit breaker and the rated insulation level desired.

4.3 Rated Frequency
       The standard frequency for a three pole circuit breaker is the frequency of the power system (50/60 HZ). The characteristics like normal current breaking capacity etc. are based on the rated frequency.
       The frequency of the current influences the circuit breaker behavior as follows
  • The temperature rise of current carrying parts and neighboring metallic parts is influenced by eddy-current heating . The increase in frequency results in increased eddy currents. Hence, with specified limits of the temperature rise the rated current of a circuit breaker needs de-rating for application on higher frequency. 
  • The frequency corresponds to the number of current-zeros per second. Since the breaking time of the circuit breaker is associated with the time for half cycles during the arc extinguishing process, the breaking time is influenced  by the frequency of current. The breaking time increases with reducing in frequency. 
  • The increase in frequency influences the TRV and rate-of-rise TRV. Hence a circuit breaker designed and rated for a certain frequency cannot be recommended for other frequencies unless capabilities are proved for those frequencies. 
  • The d.c. circuit breakers generally adopt a different principle of arc extinction and have different construction than a.c. circuit breaker.
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