CIRCUIT BREAKER SIZING ON FAULT CALCULATIONS

Prior to designing substation, you should have an anticipation plan before its to late one of the most important is the protective devices to your equipment transformer etc. So this articles content the definition of circuit breaker, the breaker rating based on the standard, its breaker sizing, and with example.

A. CIRCUIT BREAKER
Is a device :
- to open and close a circuit by non-automatic means
- to open the circuit automatically on a predetermined overload of current or during faults without damage to itself when properly applied within its rating.
- When the transformer primary reference current is equal or greater than 45 amperes or when there is more than one transformer in a substation, the protection will be by a power circuit breaker........................IEC











B. BREAKER RATINGS

1. Voltage Rating (Rated Maximum Voltage)
* the highest rms(root mean square) voltage, above nominal system voltage, for which the circuit breaker is designed, and is the upper limit of operation.
* 15 kV, 72.5 kV, 145 kV, 245 kV

2.Continuous Current Rating (Rated Continuous Current)
* Rated Continuous Current
* the designated limit of current in rms amperes at rated frequency w/c it shall be required to carry continuously without exceeding any of the limitations/conditions in a specified ambient temperature.
* Ambient temp. from -30 deg. C to 40 deg. C
* Altitude is not above 3300 ft (1000m)

3.KAIC Rating (Rated Short Circuit Current, K=1)
* Kilo-Ampere Interrupting Capacity (KAIC)
* the maximum amount of symmetrical current that the breaker can safely interrupt at rated maximum voltage -without injuring or damaging itself.

* KAIC = k X Rated Short-Circuit Current
where:
k = is the ratio of rated maximum voltage to the lower limit of the range of operating voltage.
= Vmax / Vmin

Modern Circuit Breakers, k = 1.0

KAIC = k X Rated Short Circuit Current
= Rated Short Circuit Current

Old Circuit Breakers, k>1.0
Condition 1: If RMV > OV > (1/k) x RMV
then,
KAIC = Rated Isc x (Rated Maximum Voltage / Operating Voltage)

Condition 2: If OV < (1/k) x RMV then, KAIC is limited to Rated Isc x k KAIC = Rated Isc x k Example: Consider an indoor oilless circuit breaker having a rated short-circuit current of 37,000 A at rated maximum voltage of 15kV and K = 1.30. What is the symmetrical interrupting capability at an operating voltage at a.)13.2 kV? b.)11.5 kV? C. BREAKER SIZING
E/X Simplified Method


- E/X simplified procedure result may be compared with 100% of the circuit breaker KAIC rating where
X/R < 15 (X1/R1 for 3-phase faults) ((2X1 + Xo)/(2R1 + Ro) for SLG-faults) - If X/R is unknown, the E/X does not exceed 80% of the KAIC rating of the breaker. - For SLG – Faults, where it is greater than the 3-Phase Faults or Xo less than X1

C. SAMPLE PROBLEM

A SUBSTATION



1. Solve for the maximum fault duties at the 3-phase fault points.

2. Specify the Voltage Ratings, Continuous Current Ratings & KAIC Ratings of the circuit breakers.