11 Best EEE Job viva Question & Answer

1. Why star delta used in transformer

Answer: Star-delta connections are used in transformers for a variety of reasons, primarily related to voltage levels, grounding, and harmonic mitigation. Here’s a breakdown:

1. Voltage Transformation:

• Step-down: The most common use of star-delta transformers is to step down voltage for distribution. The star connection on the high-voltage side allows for a lower phase voltage, reducing insulation requirements and costs. The delta connection on the low-voltage side provides a stable voltage for three-phase power supply.
• Step-up: In some cases, delta-star connections are used to step up voltage at the beginning of transmission lines. The delta connection on the low-voltage side allows for higher current capacity, while the star connection on the high-voltage side provides the necessary voltage for long-distance transmission. In a star connection, the neutral point is available, which helps balance the phase voltages and results in better voltage regulation.

2. Grounding:

The star connection provides a neutral point that can be grounded. This is crucial for safety and for providing a reference point for voltage measurements. Grounding helps to prevent voltage surges and protects equipment and personnel from faults.

3. Lower Harmonics:

The delta connection helps reduce the harmonic distortion in the system. This is because the delta configuration allows for the cancellation of certain harmonic components. The star connection on the primary side can also help in maintaining a more balanced load.

4. Load Sharing:

The star-delta connection helps in effectively sharing the load between the primary and secondary windings, which results in more efficient operation.

 2. Why busbar used in electrical substation?

Answer: A busbar is a metallic conductor (often copper or aluminum) used in substations to collect and distribute electrical power from various sources (transformers, generators, etc.) and feed it to different parts of the distribution or transmission network. It essentially acts as a central point for power flow.

Busbars are an essential component in electrical substations due to several key functions they serve:

1. Centralized Connection: Busbars act as a central point for distributing electrical power from one or more sources to multiple circuits. They provide a common connection point for incoming and outgoing power, facilitating efficient power distribution.
2. Minimizing Complexity: Instead of having individual connections for each circuit, a busbar simplifies the wiring by consolidating the connections. This reduces the complexity and number of conductors required in the substation, making the setup more organized and manageable.
3. Reliability and Redundancy: Busbars are typically designed to be robust and highly reliable. In the case of a fault or maintenance requirement, a substation can be designed with multiple busbars to allow the system to continue functioning, minimizing downtime. The system can switch between busbars to maintain power supply without interruption.
4. Efficient Power Distribution: By acting as a main distribution point, busbars ensure that power is evenly and efficiently distributed to various parts of the substation or to different feeders. This helps maintain balanced loading and prevents overloading of any single circuit.
5. Safety: Busbars help in managing and controlling the flow of electricity within the substation, reducing the risk of electrical faults. They are often protected with fuses or circuit breakers to prevent damage to the system in case of overloads or short circuits.
6. Compactness and Space Saving: Busbars are usually compact and can handle large amounts of current in a relatively small space. This allows substations to be more space-efficient, especially in areas where real estate is limited.
7. Ease of Maintenance and Expansion: Busbars make it easier to isolate specific parts of the system for maintenance or replacement without affecting the rest of the substation. Also, they allow for future expansion or addition of new circuits without major modifications to the existing infrastructure.
8. Cost-Effectiveness: While the initial cost of busbars can be higher than cables, they offer long-term cost savings due to their durability, ease of maintenance, and flexibility. They also reduce the risk of costly downtime due to their reliability.

         3. What is the deference between magnetic conductor and circuit breaker?

• A magnetic conductor (in the sense of a conductor carrying current) deals with the flow and manipulation of electrical energy via magnetic effects.
• A circuit breaker is a protective device that automatically disconnects an electrical circuit in the event of a fault, ensuring safety in the electrical system.

While both may involve magnetic fields in some way, a circuit breaker is a safety device, while a magnetic conductor is more about the conduction and control of electrical energy.

Key Differences Summarized:

Feature	Magnetic Contactor	Circuit Breaker
Primary Function	Frequent switching	Overload and short circuit protection
Operation	Electromagnetically controlled	Automatic tripping based on current sensing
Protection	Limited overload protection, requires additional devices	Built-in overload and short circuit protection
Resetting	Requires manual intervention after fault clearance	Can be manually reset
Typical Use	Motor control, lighting, etc.	Electrical panels, circuit protection

 

4. How to understand If a fault occur in transmission line from Dhaka to Chattogram ?

 

Answer:

Detecting a fault on a transmission line like the one from Dhaka to Chattogram involves a combination of monitoring, analysis, and communication. Here’s how it works:

1. Real-time Monitoring

• Sensors: Transmission lines are equipped with sensors that continuously monitor various parameters like voltage, current, and frequency. These sensors are strategically placed along the line to capture data at different points.
• SCADA Systems: The data from these sensors is transmitted to a central control center, often using a Supervisory Control and Data Acquisition (SCADA) system. SCADA systems provide real-time visualization of the transmission line’s status and any anomalies.

2. Fault Detection Techniques

• Abnormal Readings: When a fault occurs, it causes significant changes in the monitored parameters. For example, a short circuit can lead to a sudden drop in voltage and a surge in current. The SCADA system is programmed to detect such abnormal readings.
• Protection Devices: Transmission lines are equipped with protection devices like relays and circuit breakers. These devices are designed to automatically trip and isolate the faulty section of the line when a fault is detected.
• Fault Location Algorithms: Once a fault is detected, sophisticated algorithms are used to pinpoint its location. These algorithms analyze the data from the sensors to estimate the distance to the fault from a known point.

3. Communication and Response

• Alarms: When a fault is detected, the SCADA system triggers alarms to alert operators in the control center. The alarms may include audible signals, visual indicators, and text messages.
• Automated Actions: In some cases, the system can automatically initiate actions like opening circuit breakers to isolate the fault and prevent further damage.
• Dispatching Crews: Once the fault is located, maintenance crews are dispatched to the site to assess the damage and carry out repairs.

4. Advanced Techniques

• Traveling Wave Analysis: This technique analyzes the high-frequency transients generated by faults to detect and locate them quickly.
• Machine Learning: Machine learning algorithms can be used to analyze historical data and identify patterns that may indicate potential faults.

5. Challenges

• Long Distances: The Dhaka-Chattogram transmission line is a long one, making it challenging to monitor and detect faults across its entire length.
• Environmental Factors: Weather conditions, terrain, and other environmental factors can affect the performance of monitoring equipment and make it difficult to detect faults accurately.
• Cybersecurity: Transmission lines are vulnerable to cyberattacks that can disrupt monitoring systems and prevent fault detection.

6. Communication with Power Plants and Local Grid

• If the fault impacts the flow of electricity from the power plants in Dhaka to Chattogram, the operators will often notice irregularities in the power supply at receiving stations. Power plants and receiving substations can give feedback about the status of the transmission line.
• How it helps: Feedback from the grid can help operators confirm the fault and even give clues about the fault’s severity or location based on the drop in power supply.

7. Automatic Reclosers:

• Transmission lines often use automatic reclosers, which are devices that try to automatically restore power after detecting a temporary fault (e.g., a tree branch touching the line). If the fault is still present after a few attempts to reconnect, the recloser will lock out the circuit.
• How it helps: If the recloser attempts to reconnect and fails repeatedly, it signals that the fault is likely a permanent one, allowing the operators to focus on troubleshooting.

5. What are the main protection devices used in substations?

Answer: The main protection devices used in electrical substations include:

1. Relays:

1. • Overcurrent Relay: Protects the circuit from overload or fault currents.
   • Distance Relay: Protects transmission lines based on impedance, measuring how far a fault is from the substation.
   • Differential Relay: Detects faults within transformers and generators by comparing the input and output currents.

2. Circuit Breakers: As discussed earlier, they automatically disconnect the electrical supply during a fault condition.
3. Fuses: Fuses are used for protection against short circuits and overloads. They blow and disconnect the circuit when excessive current flows.
4. Surge Arresters: Protect equipment from voltage spikes or surges caused by lightning or switching events.
5. Grounding and Earthing Systems: Ensures that any fault current is safely dissipated into the ground, protecting the equipment and personnel from electrical shock.

6. Explain the working principle of a multi-meter.

Answer: A multimeter is a versatile instrument that can measure multiple electrical quantities. Here’s a simplified explanation:

• Basic Principle: Most analog multimeters use a moving coil meter as the core. Current flowing through the coil in a magnetic field causes it to deflect, and this deflection is indicated on a scale.
• Measurement of Voltage: To measure voltage, the multimeter uses a high resistance in series with the moving coil. This limits the current through the coil, and the deflection is proportional to the voltage.
• Measurement of Current: To measure current, the multimeter uses a low resistance in parallel with the moving coil. This allows most of the current to bypass the coil, and the deflection is proportional to the current.
• Measurement of Resistance: To measure resistance, the multimeter uses an internal battery to supply a current through the unknown resistance. The resulting current through the moving coil is inversely proportional to the resistance.
• Digital Multimeters: Digital multimeters use electronic circuits to convert the measured quantity into a digital value, which is displayed on a screen.

 

7. What is a circuit breaker, and why is it important in a substation?

Answer: A circuit breaker is an automatic electrical switch used to protect the electrical circuit from overloads, short circuits, and fault conditions. When the current exceeds a safe level, the circuit breaker trips and disconnects the affected section of the network to prevent damage to equipment and ensure safety.

In substations, circuit breakers are important because:

• They isolate faulty circuits to prevent damage to other components.
• They help in the safe operation of the grid by automatically opening the circuit during faults and closing it once the fault is cleared.
• They provide a means to safely switch circuits on and off for maintenance or control.

8. What is the purpose of an ammeter and how is it connected in a circuit?

Answer: An ammeter is an electrical measuring instrument used to measure current in a circuit. The main purpose of an ammeter is to monitor the flow of electric charge (current) in a conductor or circuit, which is typically measured in amperes (A).

How an ammeter is connected:

• An ammeter is always connected in series with the circuit or component through which current needs to be measured. This is because current must flow through the ammeter for it to measure the total current in that part of the circuit.
• It has low resistance to minimize the impact on the overall circuit and to avoid disrupting the current flow.

Important considerations:

• The ammeter’s current range must be chosen to match the expected current in the circuit to avoid damaging the instrument.
• Never connect an ammeter in parallel with a power source or load, as it could lead to short circuits or damage to the ammeter.

 

9. How does a voltmeter work, and where is it connected in a circuit?

Answer: A voltmeter is an electrical measuring instrument used to measure the potential difference (voltage) across two points in a circuit. It provides readings in volts (V), which is a measure of the electrical potential energy per unit charge.

Working of a voltmeter:

• A voltmeter works by measuring the difference in electric potential between two points in the circuit.
• It operates based on the electric field between two terminals, which causes a current to flow through the meter. The meter then uses this current to determine the voltage difference.

How a voltmeter is connected:

• A voltmeter is always connected in parallel across the two points where the voltage is to be measured. For example, if measuring the voltage across a resistor, the voltmeter would be connected across its two terminals.
• The voltmeter has a high internal resistance to ensure that it does not draw significant current from the circuit, which would affect the measurement.

Important considerations:

• The voltmeter must be chosen with a voltage rating higher than the expected voltage to prevent damage.
• Never connect a voltmeter in series, as it would create an open circuit and prevent current flow.

 

10. Explain the function of a relay in a substation.

Answer: Relays are crucial protection devices in a substation. Their primary function is to detect abnormal conditions, such as overcurrents, short circuits, or earth faults, and initiate the tripping of circuit breakers to isolate the faulty section of the network. They act as the “brains” of the protection system, ensuring rapid and selective clearing of faults to minimize damage and maintain system stability. Different types of relays exist, each designed to detect specific types of faults.

11. What are the key components of a substation?

Answer: A substation typically consists of the following key components:

• Transformers: For voltage transformation.
• Circuit Breakers: For interrupting fault currents and switching circuits.
• Isolators: For disconnecting equipment for maintenance.
• Relays: For sensing faults and initiating protective actions.
• Current and Potential Transformers (CTs and PTs): For measuring current and voltage.
• Busbars: For connecting various equipment and distributing power.
• Lightning Arresters: For protecting equipment from surge voltages.
• Earthing System: For providing a safe path for fault currents.
• Control and Protection Panels: For housing control and protection equipment.

 

For More important for EEE Job viva Question & Answer click Here

EEE Job viva Question & Answer-1

EEE Job viva Question & Answer-2

EEE Job viva Question & Answer-3