Explain about the Air Circuit Breakers Settings Calculation

Air Circuit Breakers (ACBs) are used for the protection of electrical circuits in medium-voltage and low-voltage applications. They protect the circuits by breaking the current flow during overcurrent, short-circuits, or any fault condition. Calculating the settings for an ACB involves determining the appropriate values for protection functions like overload, short-circuit, instantaneous trip, and others. Here’s a breakdown of how the settings are calculated:

1. Overload Protection (Long-time Setting)

Long-time pickup (Ir): This setting defines the current value above which the breaker will start the timing for tripping in case of an overload. It is usually set as a percentage of the ACB's rated current (In).
- Formula: $\text{Ir} = \text{In} \times \text{Long-time pickup setting (e.g., 0.8)}$
- For example, if the ACB is rated at 1000 A and the pickup setting is 0.8, the long-time setting would be $1000 \times 0.8 = 800 \ \text{A}$ .
Long-time delay: Determines how long the breaker will wait to trip after detecting a current that exceeds the long-time pickup setting. It is adjustable and expressed in seconds. The delay allows for short-duration overloads (e.g., motor starting currents).

2. Short-Circuit Protection (Short-time Setting)

Short-time pickup (Isd): This setting defines the current level at which the breaker will trip due to a short-circuit condition. It is typically set as a multiple of the ACB’s rated current.
- Formula: $\text{Isd} = \text{In} \times \text{Short-time pickup setting (e.g., 5x)}$
- For example, if the ACB is rated at 1000 A and the short-time pickup setting is 5, then the short-time setting would be $1000 \times 5 = 5000 \ \text{A}$ .
Short-time delay: The time delay before the ACB trips after detecting a current that exceeds the short-time pickup. It is used to allow coordination with downstream devices and avoid unnecessary tripping.

3. Instantaneous Protection (Ii)

Instantaneous pickup: This setting determines the threshold at which the ACB trips immediately without any intentional delay. It is used to provide fast disconnection during a severe short-circuit condition.
- Formula: $\text{Ii} = \text{In} \times \text{Instantaneous pickup setting (e.g., 10x)}$
- (e.g., 10x)
- For example, if the ACB is rated at 1000 A and the instantaneous pickup setting is 10, the setting would be $1000 \times 10 = 10,000 \ \text{A}$ .

4. Ground Fault Protection (Ig)

Ground fault pickup: This setting detects current imbalances that indicate a ground fault condition. It is typically set as a percentage of the ACB's rated current.
- Formula: $\text{Ig} = \text{In} \times \text{Ground fault pickup setting (e.g., 0.2)}$
- For example, if the ACB is rated at 1000 A and the ground fault pickup setting is 0.2, the ground fault setting would be $1000 \times 0.2 = 200 \ \text{A}$ .
Ground fault delay: The time delay before the breaker trips after detecting a ground fault. This setting helps in coordination with other protection devices to ensure selective tripping.

5. Adjusting the Settings

The above parameters are adjusted based on:

System parameters like the full load current, fault levels, and the characteristics of connected equipment (e.g., motors).
Selectivity and coordination with other protective devices downstream to ensure that only the faulty section is isolated.
Manufacturer recommendations and protection studies performed for the specific system.

Summary of Key Formulas

Overload Pickup (Ir): $\text{Ir} = \text{In} \times \text{Long-time pickup setting}$
Short-circuit Pickup (Isd): $\text{Isd} = \text{In} \times \text{Short-time pickup setting}$
Instantaneous Pickup (Ii): $\text{Ii} = \text{In} \times \text{Instantaneous pickup setting}$
Ground Fault Pickup (Ig): $\text{Ig} = \text{In} \times \text{Ground fault pickup setting}$

These settings need to be programmed into the ACB's protection unit, either manually or using specialized software. Proper calculation and adjustment of these parameters help ensure safety, reliability, and continuity of electrical systems.

Example:

Explain about the 1600kva transformer 11kv/415v ACB air circuit breakers settings calculation

A 1600 kVA transformer with a primary voltage of 11 kV and a secondary voltage of 415 V is commonly used in industrial or commercial power distribution. Understanding the ACB (Air Circuit Breaker) settings involves determining various protection parameters like overload protection, short-circuit protection, and other settings to ensure the transformer and the connected system operate safely and efficiently.

Key Aspects of 1600 kVA Transformer with ACB Settings

Transformer Full Load Current Calculation:
- Calculate the full load current for both the primary and secondary sides.
Secondary side full load current (415 V):
$I_{secondary} = \frac{\text{kVA} \times 1000}{\sqrt{3} \times V_{secondary}}$

Isecondary=3

Air Circuit Breaker (ACB) Settings: The ACB on the secondary side (415 V) is generally set to protect the transformer and connected loads from overloads and short circuits. Key settings include:
- Overload Protection (Long Time Setting - LTS):
  - Typically set between 80% to 100% of the transformer's full load current.
  - For a 1600 kVA transformer, the setting range can be: $\text{LTS} = 2226 \, \text{A} \times 0.8 \, \text{to} \, 1.0 = 1781 \, \text{A to} \, 2226 \, \text{A}$
  - Generally, you can set the ACB at 90% to 100% of full load current to allow some headroom for temporary surges in current.
- Short-Circuit Protection (Instantaneous Setting - I):
  - This setting should account for fault current and the transformer's short-circuit withstand capacity.
  - A typical range is between 6 to 10 times the full load current.
  - For a 1600 kVA transformer: $\text{Instantaneous Setting} = 2226 \, \text{A} \times (6 \, \text{to} \, 10) = 13356 \, \text{A to} \, 22260 \, \text{A}$
  - The setting should be chosen based on the fault level of the system and coordinated with upstream and downstream protection devices.
- Short-Time Protection (STS):
  - This setting is used to allow the breaker to hold for a short time during high inrush currents, such as when starting large motors.
  - Usually set between 2 to 4 times the full load current.
  - The time delay can be adjusted based on the coordination with other breakers.
- Ground Fault Protection (G):
  - Ground fault protection helps in detecting low-level ground faults.
  - It is typically set between 20% to 40% of the full load current.
  - For a 1600 kVA transformer, this could be: $\text{Ground Fault Setting} = 2226 \, \text{A} \times (0.2 \, \text{to} \, 0.4) = 445 \, \text{A to} \, 890 \, \text{A}$
Coordination with Upstream and Downstream Devices:
- The ACB settings should be coordinated with upstream protection (such as HV side circuit breakers or relays) and downstream breakers (MCCBs or other ACBs for distribution boards).
- This ensures that in case of a fault, only the faulty section is isolated, and the rest of the system continues to operate.
Thermal Magnetic or Electronic Trip Unit:
- Modern ACBs often have electronic trip units that allow more precise settings for overload, short-circuit, and ground fault protections.
- Thermal magnetic trip units are simpler but less flexible compared to electronic trip units, which allow better coordination and detailed adjustments.

Summary of ACB Settings for 1600 kVA Transformer 11 kV/415 V:

Overload (LTS): ~1781 A to 2226 A.
Short-circuit (I): ~13,356 A to 22,260 A.
Short-time (STS): 2x to 4x full load current with a time delay.
Ground Fault (G): ~445 A to 890 A.

These settings should be adjusted based on the specific site conditions, fault levels, and coordination requirements of the power system. Consulting with electrical engineers or using specialized software can help refine these values for optimal performance.