Aux 2 Unit 7 Q1

Electrical load sharing, in the context of generators, refers to the process of distributing the electrical load demand among multiple generators connected in parallel. This ensures that no single generator is overloaded and that the total power demand is met efficiently and reliably.

Key Aspects of Load Sharing:

• Parallel Operation: Load sharing is typically implemented in systems where multiple generators are connected in parallel to a common busbar. This allows for increased power capacity, redundancy, and flexibility in operation.
• Proportional Sharing: The ideal scenario is to have each generator share the load proportionally based on its capacity or rating. This ensures that all generators operate within their safe limits and contribute equitably to the overall power supply.
• Active and Reactive Power Sharing: Load sharing involves distributing both the active power (kW) and reactive power (kVAr) demand among the generators.
• Control Systems: Load sharing is achieved through sophisticated control systems that monitor the load, generator outputs, and other parameters to ensure balanced and stable operation.

Mechanisms for Load Sharing:

• kW (Real Power) Sharing: Primarily controlled by the governors on each generator’s prime mover. These governors adjust the fuel input to maintain a desired speed or frequency, and a slight speed droop characteristic ensures that the load is shared proportionally based on the generator’s capacity.
• kVAr (Reactive Power) Sharing: Controlled by the Automatic Voltage Regulators (AVRs) on each generator. The AVRs adjust the generator’s field excitation to control its terminal voltage and reactive power output. A slight voltage droop or cross-current compensation mechanisms can be used to ensure balanced reactive power sharing.

Benefits of Load Sharing:

• Increased Reliability: By distributing the load among multiple generators, the risk of any single generator failing is reduced, ensuring a continuous power supply.
• Increased Capacity: Parallel operation with load sharing allows for increased power capacity to meet higher demand.
• Improved Efficiency: Load sharing helps optimize the operation of each generator by ensuring they operate at their most efficient load points, reducing fuel consumption and emissions.
• Flexibility: Load sharing allows for adding or removing generators from the system as needed to match the changing load demand, providing operational flexibility.
• Maintenance: Individual generators can be taken offline for maintenance without disrupting the overall power supply, enhancing system availability.

Challenges and Considerations:

• Synchronization: Before connecting generators in parallel, they must be properly synchronized to ensure that their voltage, frequency, and phase sequence match.
• Control System Complexity: Load sharing requires sophisticated control systems and algorithms to ensure stable and balanced operation.
• Communication and Coordination: Effective communication and coordination between the generators and the control system are essential for successful load sharing.

Applications:

Load sharing is commonly used in various applications where multiple generators are employed, such as:

• Power Plants: To meet varying power demands and provide redundancy.
• Marine Vessels: To power onboard systems and provide backup in case of main engine failure.
• Industrial Facilities: To ensure continuous power supply for critical processes.
• Data Centers: To provide reliable power for servers and other IT equipment.

In conclusion, electrical load sharing is a vital aspect of operating multiple generators in parallel. It ensures efficient, reliable, and safe power generation by distributing the load proportionally among the generators and maintaining system stability.

Let’s break down what happens when one generator’s excitation is reduced in a parallel setup with equal load sharing:

Initial State (Before Excitation Reduction)

• Two identical generators are running in parallel.
• The switchboard ensures they each provide 50% of the total load, both in terms of real power (kW) and reactive power (kVAr).
• The system is stable, with both generators at the desired voltage and frequency.

Excitation Reduction on One Generator:

• Reduced Reactive Power Output: Lowering excitation directly weakens the magnetic field in that generator. This means it produces LESS reactive power (kVAr).
• Switchboard Compensation: Since the total load demand remains constant, the switchboard will try to maintain the balance. It senses the drop in kVAr from one generator and commands the other generator to increase its kVAr output to compensate.
• Voltage Dip: However, the under-excited generator now has a slightly lower terminal voltage. This can cause a slight dip in the overall system voltage.
• Increased Current in Under-Excited Generator: To try and maintain its share of the REAL power (kW) load, the under-excited generator will start drawing more current. This is because with a weaker magnetic field, it needs more current to produce the same kW.
• Potential Overheating: This increased current can lead to the under-excited generator running hotter than normal. If the situation persists, it could lead to overheating and potential damage.

Consequences and Concerns:

• Unequal Load Sharing: While the switchboard tries to maintain equal kW sharing, the reactive power balance is disrupted. One generator is now doing more of the “heavy lifting” in terms of kVAr.
• System Voltage Dip: The slight drop in system voltage might affect sensitive equipment connected to the switchboard.
• Overheating Risk: The under-excited generator is at risk of overheating due to the increased current it’s drawing.
• Inefficiency: The system is no longer operating at its optimal point, as one generator is working harder than the other.

Corrective Action:

• Ideally: The excitation of the under-excited generator should be brought back to its correct level. This will restore the balance of reactive power sharing and reduce the risk of overheating.
• Monitoring: If the situation cannot be immediately corrected, the under-excited generator’s temperature and current should be closely monitored to prevent damage.

In summary, reducing the excitation of one generator in a parallel system disrupts the reactive power balance, potentially leading to voltage dips, overheating, and inefficiency. It’s essential to maintain proper excitation levels on all generators for optimal and safe operation.

Closing the main circuit breaker of a generator when it’s out of phase with the system, even if voltage and frequency match, can have severe consequences. This is because phase alignment is crucial for safe and stable parallel operation of generators.

Possible Effects:

1. High Circulating Currents:

• The most immediate and significant effect is the flow of large circulating currents between the out-of-phase generator and the rest of the system.
• These currents can be several times the generator’s rated current, causing:
  • Overheating: Severe overheating of the generator windings and other electrical components, potentially leading to insulation breakdown and permanent damage.
  • Mechanical Stress: The high currents can induce strong electromagnetic forces within the generator, causing mechanical stress and potential damage to the rotor and stator.
  • Tripped Breakers: The excessive currents may trip the generator’s breaker or other protective devices in the system.

2. System Instability:

• The out-of-phase connection can cause significant power fluctuations and voltage dips in the entire electrical system.
• This instability can disrupt the operation of other connected loads and potentially lead to a cascading effect, causing further equipment trips and even a complete system shutdown.

3. Equipment Damage:

• The high currents and voltage fluctuations can also damage sensitive equipment connected to the switchboard, such as electronic controls, motors, and transformers.

4. Safety Hazards:

• The intense electrical arcing and potential fire associated with high circulating currents pose a serious safety risk to personnel in the vicinity.

Severity of the Effects:

• The severity of these effects depends on the phase angle difference at the moment of breaker closure. A larger phase angle difference (closer to 180 degrees) results in more severe consequences.
• The system impedance and the generator’s internal impedance also play a role in determining the magnitude of the circulating currents.

Prevention:

• Synchronization Checks: Before paralleling generators, it’s essential to perform proper synchronization checks to ensure that their voltage, frequency, and phase sequence match.
• Protective Relays: The electrical system should have protective relays that prevent breaker closure if the generators are out of phase beyond a certain tolerance.
• Manual Intervention: In case of automatic synchronization failure, manual intervention may be necessary to correct the phase alignment before closing the breaker.

Conclusion:

Closing the main circuit breaker when generators are out of phase is a serious operational error that can lead to severe damage, instability, and safety hazards. Strict adherence to synchronization procedures and the use of protective relays are crucial to prevent such incidents and ensure the safe and reliable operation of paralleled generator systems.