1600kva 11kv HT settings ,LT ACB settings calculation and formula

Write about the 1600kva 11kv HT settings calculation and formula

In electrical power systems, a 1600 kVA, 11 kV HT (High Tension) transformer is commonly used for stepping down high voltage to lower voltages for industrial or commercial applications. The settings for protection and operation of such a transformer typically involve key parameters like current, voltage, impedance, and protection relay settings (overcurrent, earth fault, etc.). Below are the important aspects and formulas for the calculations:

1. Full Load Current Calculation

The full load current (FLA) of the transformer is the current that the transformer draws when operating at its full capacity (1600 kVA) at the specified voltage level (11 kV). This is calculated using the formula:

$$\text{FLA} = \frac{\text{Transformer Rating (kVA)}}{\sqrt{3} \times \text{Voltage (kV)}}$$

For a 1600 kVA, 11 kV transformer, the full load current is:

$$\text{FLA} = \frac{1600}{\sqrt{3} \times 11} = \frac{1600}{19.05} \approx 84 \, \text{Amps}$$

2. Protection Relay Settings

High-tension transformers are equipped with various protective relays, such as overcurrent (OC) and earth fault (EF) relays, which safeguard the transformer from faults like short circuits and overloads. The relay settings are usually based on the full load current and should be adjusted accordingly.

a. Overcurrent Relay Setting (OC)

The OC relay is typically set as a multiple of the full load current to allow some margin for inrush currents or temporary overloads. The current setting $I_s$ can be expressed as a percentage of the full load current:

$$I_s (\%) = \text{Multiplier} \times \text{FLA}$$

For example, if the overcurrent setting is chosen as 125% of FLA, the relay setting would be:

$$I_s = 1.25 \times 84 = 105 \, \text{Amps}$$

b. Earth Fault Relay Setting (EF)

The earth fault relay protects against ground faults. The typical setting for the earth fault relay is around 20% to 40% of the full load current:

$$I_{EF} = \text{Percentage} \times \text{FLA}$$

For example, if the earth fault setting is 30%, the relay setting would be:

$$I_{EF} = 0.3 \times 84 \approx 25.2 \, \text{Amps}$$

3. Impedance of the Transformer

The transformer impedance (Z) affects the short-circuit current and the voltage regulation of the transformer. The impedance value is usually provided by the manufacturer as a percentage impedance (%Z). To calculate the short-circuit current (Isc) based on the impedance:

$$I_{SC} = \frac{\text{Full Load Current}}{\text{Percentage Impedance (as a fraction)}}$$

For example, if the transformer has a 6% impedance, the short-circuit current would be:

$$I_{SC} = \frac{84}{0.06} = 1400 \, \text{Amps}$$

This indicates the maximum current that would flow during a short-circuit condition, and it is critical for the sizing of the protection equipment.

4. Power Factor Consideration

The power factor (PF) of the system, typically 0.8 (lagging) for industrial loads, affects the real power and reactive power delivered by the transformer. The apparent power (S) is given by:

$$S = P + jQ$$

Where:

• $S$ is the apparent power in kVA,
• $P = S \times PF$ is the real power in kW,
• $Q = S \times \sqrt{1 - PF^2}$

For a 1600 kVA transformer with a power factor of 0.8:

$$P = 1600 \times 0.8 = 1280 \, \text{kW}$$ $$Q = 1600 \times \sqrt{1 - 0.8^2} = 960 \, \text{kVAR}$$

To calculate the settings for an Air Circuit Breaker (ACB) for a 1600 kVA transformer, we need to calculate the appropriate current values and protection settings. Below are the steps to determine the important settings and the formulae used:

Step 1: Calculate the Full Load Current (FLC)

The full load current is calculated based on the transformer's rated capacity, voltage, and phase.

For a three-phase transformer:

$$\text{Full Load Current (FLC)} = \frac{\text{Transformer Rating (kVA)}}{\sqrt{3} \times \text{Voltage (V)}}$$

If your transformer is rated at 1600 kVA and the voltage is 400V (typical for low voltage side of a transformer), the full load current would be:

$$FLC = \frac{1600}{\sqrt{3} \times 400} = \frac{1600}{692.8} \approx 2,31 A$$

If this is the high voltage (e.g., 11kV side), we use 11,000V:

$$FLC = \frac{1600}{\sqrt{3} \times 11,000} = \frac{1600}{19,052} \approx 83.96 A$$

Step 2: Protection Settings for ACB

The ACB protects the transformer and distribution system. The primary protection settings are:

1. Overload Protection (Ir):

• Overload protection should be set at 110% to 125% of the full load current, depending on the application and transformer specifications.
• Formula: $$\text{Overload Setting} (Ir) = FLC \times \text{Safety Factor}$$
• For example, if FLC = 2310 A and the safety factor is 1.10 (10% margin): $$Ir = 2310 \times 1.10 \approx 2541 A$$

3. Short Circuit Protection (Isd):

   • Instantaneous short-circuit protection should be set to 8 to 12 times the full load current to protect against high-current faults.
   • Formula: $$\text{Short Circuit Setting (Isd)} = FLC \times \text{Multiplier}$$
   • For example, using a multiplier of 10: $$Isd = 2310 \times 10 = 23,100 A$$

4. Ground Fault Protection (Ig):

• Ground fault settings typically range between 20% to 50% of the full load current to protect against ground faults.
5. • Formula: $$\text{Ground Fault Setting (Ig)} = FLC \times \text{Percentage (20\% to 50\%)}$$
   • Example, for 20%: $$Ig = 2310 \times 0.20 = 462 A$$

Step 3: Time Delay Settings

• Overload Time Delay (tr): The time delay for overload protection should be set long enough to avoid nuisance tripping but short enough to protect equipment. Typically set between 5 to 30 seconds.

• Short Circuit Time Delay (tsd): This is usually set to instantaneous (no time delay) or a very short delay (milliseconds), as short circuits need to be interrupted immediately.

• Ground Fault Time Delay (tg): The ground fault protection delay can vary but is typically set between 0.1 and 0.5 seconds.

Example Summary for 400V, 1600kVA Transformer

• Full Load Current (FLC) = 2310 A
• Overload Setting (Ir) = 2541 A (1.1 times FLC)
• Short Circuit Setting (Isd) = 23,100 A (10 times FLC)
• Ground Fault Setting (Ig) = 462 A (20% of FLC)
• Time Delay Settings: tr = 10 seconds, tsd = instantaneous, tg = 0.2 seconds

These settings should be confirmed based on system design standards, transformer specifications, and the applicable electrical code.

Air Circuit Breakers (ACBs) Settings Calculation and Formula

Air Circuit Breakers (ACBs) are essential protective devices in electrical distribution systems, used for switching and protecting electrical circuits from overloads, short circuits, and ground faults. Setting these breakers correctly is critical to ensure they function properly and provide protection without nuisance tripping. Key settings to configure include overload, short circuit, and ground fault settings.

1. Overload Protection (Long-Time Delay)

Overload protection safeguards the system from currents exceeding the normal operating levels over an extended period. This setting is typically adjusted to match the current rating of the equipment being protected.

Formula for Overload Protection: $I_{\text{long-time}} = I_{\text{rated}} \times S_{\text{long-time}}$

• $I_{\text{long-time}}$: Overload protection setting.
• $I_{\text{rated}}$: Rated current of the equipment.
• $S_{\text{long-time}}$: Overload protection setting factor (typically set between 0.8 and 1.0).

Long-Time Delay: This defines how long the breaker can tolerate an overload condition before tripping. The delay is usually adjustable in seconds.

2. Short-Circuit Protection (Short-Time Delay and Instantaneous Settings)

ACBs have two types of short-circuit protection: short-time delay and instantaneous settings.

• Short-Time Delay: Protects against moderate short circuits and is adjustable to allow for coordination with downstream devices.

  Formula for Short-Time Setting: $I_{\text{short-time}} = I_{\text{rated}} \times S_{\text{short-time}}$

  Where $S_{\text{short-time}}$ is typically between 2 and 12 times the rated current.

• Instantaneous Trip: This setting responds to very high short-circuit currents, causing immediate tripping without intentional delay.

  Formula for Instantaneous Trip Setting: $I_{\text{instantaneous}} = I_{\text{rated}} \times S_{\text{instantaneous}}$

  The instantaneous trip factor $S_{\text{instantaneous}}$ can be set between 2 and 15 times the rated current, depending on the breaker and application.

3. Ground Fault Protection

Ground fault protection prevents damage from faults between phase and ground. This setting should be coordinated with the system's grounding scheme and can be set based on system requirements.

Formula for Ground Fault Setting: $I_{\text{GF}} = I_{\text{rated}} \times S_{\text{GF}}$

Where $S_{\text{GF}}$ typically ranges from 0.2 to 0.6 times the rated current, depending on the sensitivity needed.

Ground Fault Delay: This setting allows for short delays in the trip action to avoid nuisance tripping from transient faults. Ground fault delays are usually measured in milliseconds.

4. Coordination and Selectivity

Proper coordination ensures that only the breaker nearest to the fault trips, maintaining the continuity of power to other parts of the system. This requires careful adjustment of the time-current curves for overload, short-circuit, and ground fault settings.

Summary of Formulas:

• Overload (long-time): $I_{\text{long-time}} = I_{\text{rated}} \times S_{\text{long-time}}$
• Short-circuit (short-time): $I_{\text{short-time}} = I_{\text{rated}} \times S_{\text{short-time}}$
• Instantaneous trip: $I_{\text{instantaneous}} = I_{\text{rated}} \times S_{\text{instantaneous}}$
• Ground fault protection: $I_{\text{GF}} = I_{\text{rated}} \times S_{\text{GF}}$

Practical Considerations:

• System Load: When setting ACBs, consider the actual load current and potential future expansions.
• Coordination: Ensure that the ACB settings coordinate with upstream and downstream protective devices to avoid unnecessary outages.
• Testing: Regularly test and adjust ACB settings to ensure continued reliable operation.

In summary, the correct setting of ACBs involves calculating the appropriate current levels for overload, short-circuit, and ground fault protection based on the system's rated current and operational needs, while allowing for proper selectivity and coordination with other devices.