What is short circuit fault level in 230/110 KV substation? How it arrived?

The short circuit fault level in a 230/110 kV substation is the highest current that can pass through the electrical system during a short circuit. This fault level is an important criterion in the design and management of electrical substations because it specifies the capacity and equipment requirements needed to properly handle such fault occurrences.

## Determining the Short Circuit Fault Level

## Network Configuration

### Source Impedance

The power sources impedance (generators and transformers) that supply the substation is critical. This includes the impedance of both high-voltage (230 kV) & low-voltage (110 kV) sources.

### Transformer Impedance

The transformers impedance that connect the various voltage levels (230/110 kV) is important in estimating the fault current. Transformer impedance is commonly expressed as a percentage of the transformer’s base rating.

## System Impedance Calculation

The total impedance seen at the fault point is calculated by adding the source & transformer impedances. For the purpose of simplicity, per-unit (pu) computations are commonly used.

Impedance calculations entail converting all impedances to a standard base (often the substation’s power base, such as MVA base) and summing them in series (or) parallel, based on the network arrangement.

## Short Circuit Current Calculation

The fault level in MVA (Mega Volt-Amperes) is easily computed using the following formula:

**Fault Level (MVA) = System Voltage (KV)2/Total System Impedance (Ω)**

The short circuit current (Isc) for a certain point in the network is:

**Isc = Vsys/Ztotal**

Where

**Vsys**- represents the system voltage at the fault site and**Ztotal -**represents the total impedance up to that point.

## Example Calculation

### Assumptions

Transformer rating: 100 MVA at 230/110 kV and 10% impedance.

Source impedance is equivalent to 5% on a 230 kV, 500 MVA system.

### Impedance Calculation

Transformer impedance in pu at 100 MVA base: 0.1 pu.

Source impedance in pu at 500 MVA base: 0.05 pu.

Convert the source impedance to 100 MVA base.

Zsource (pu) = 0.05 x ( 100 500 ) = 0.01 pu

Z source(pu) =0.05 x ( 500 100)=0.01pu

Total Impedance in PU

Total impedance

Total_pu = 0.1 pu (transformer) + 0.01 pu (source) = 0.11 pu.

## Fault Current Calculation

**At 230 kV side:**

Isc (230 kV) = 230 /(√ 3 x 230 x 0.11) = 230 / (√3 x 25.3) = 4.27 kA.

I = √3 x 230 x 0.11 = 4.27 kA at 230 kV voltage.

**At 110 kV side:**

Isc (110 kV)) = 110/ ( √3 x 110 x 0.11) = 110 / (√3 x 12.1) = 5.25 kA

Isc(110kV) = √3 x 110 x 0.11 = 5.25 kA.

## Significance

- The computed fault level helps to define the specifications for circuit breakers, transformers, & other protective equipment, ensuring that they can withstand the maximum fault current without damage.
- Accurate fault level computation is critical for the design of the protection systems, as it ensures timely and effective fault isolation to reduce damage and system stability.
- Ensuring that equipment can withstand & interrupt fault currents is essential for both human safety and power system reliability.

In conclusion, the short circuit fault level is an important design parameter for substations, defined by the combined impedance of the sources & transformers, and it influences the selection & rating of electrical equipment & protective systems.