## How do you find the Subtransient reactance of a generator?

John Archer

- impedance on new base = old impedance x (new base / old base)
- impedance on 100 MVA = 0.16 x (100 / 0.8)
- = 0.16 x (125)
- = 20 per unit ( or 2000%) on 100 MVA.

## What is Subtransient reactance?

The subtransient reactance is an impedance value that entirely neglects the resistance component. Although the resistance of the windings of a synchronous generator are generally negligible compared to their reactance, they do play a role in the decay rates of the transient currents in the form of L/R time constants.

## What is synchronous reactance of generator?

The Synchronous Reactance (XS) is the imaginary reactance employed to account for the voltage effects in the armature circuit produced by the actual armature leakage reactance and by the change in the air gap flux caused by the armature reaction.

## What is sub transient current?

• The AC current flowing in the generator during the sub-transient. period is called the sub-transient current and is denoted by I”. The time constant of the sub-transient current is denoted by T” and it can be determined from the slope. This current can be as much as 10 times the steady-state fault current.

## Why symmetrical fault is severe than unsymmetrical fault?

Obviously symmetrical fault is more severe than asymmetrical fault. During symmetrical fault all of the three phase of the transmission line is short circuited with each with other or all them are grounded . During this kind of fault huge short circuit current will flow from all the three phase to ground.

## What is the equation of direct axis short circuit transient time constant?

to 0.368 times its initial value. Direct-axis transient open circuit time constant (do): It is the time, in seconds, required for the transient alternating component of the open-circuit current to decrease to 0.368 times its initial value.

## How are symmetrical faults analyzed?

The analysis of symmetrical (L-L-L) faults includes the determination of the voltage at any point (or bus) in the power system network, the current in any branch and value of reactance necessary to limit the fault current to any desired value. The circuit breaker MVA breaking capacity is based on 3-phase fault MVA.

## What is steady state reactance?

Finally the transient dies out and the current reaches a steady sinusoidal state called the Steady State. The reactance in this state is called steady state reactance Xd. Since the short circuit current of the alternator lags behind the voltage by 90 degree, the reactance involved are direct axis reactance.

## What is the rated breaking capacity of the circuit breaker?

Breaking capacity or interrupting rating is the current that a fuse, circuit breaker, or other electrical apparatus is able to interrupt without being destroyed or causing an electric arc with unacceptable duration.

## What is transient reactance?

Reactance Defined Identifying a generator’s transient reactance helps the specifier approximate voltage dip when large motors are started. It also helps the specifier approximate the current in a three-phase short-circuit condition to specify correct circuit breaker protection.

## How do you find the short circuit current of a circuit breaker?

I S.C. for 3 Ph = I F.L /Z%; where I. S.C. is the short circuit current, and Z% is the transformer impedance; which usually can be obtained from the local utility company. I S.C. 3 ph for point 1 = 1202 /0.05 = 24,506 A or 24.5 KA (this is the available short circuit current at point 1).

## What is fault level calculation?

Fault level at any given point of the Electric Power Supply Network is the maximum current that would flow in case of a short circuit fault at that point. The circuit breaker should be capable of Breaking & Making current as per their ratings & should also have Rated short time capacity.

## What is the formula for short circuit current?

This applied voltage divided by the rated primary voltage (times 100) is the impedance of the transformer. Example: For a 480 Volt rated primary, if 9.6 volts causes secondary full load current to flow through the shorted secondary, the transformer impedance is 9.6/480 = . 02 = 2%Z.

## How do you calculate fault current in a circuit?

Fault current calculations are based on Ohm’s Law in which the current (I) equals the voltage (V) divided by the resistance (R). The formula is I = V/R. When there is a short circuit the resistance becomes very small, and that means the current becomes very large.

## What is maximum fault current?

Maximum fault currents help determine the required interrupting capacities of overcurrent protective devices. Minimum fault currents are used in coordinating operations of overcurrent devices, re-closers and relays.

## How much does it cost to fix short circuit?

Nationally, the average cost for having an electrician replace a bad circuit breaker switch is $150 to $200, including labor and materials.