Aux 2 Unit 7 Q2

In the context of load sharing of AC generators, speed droop refers to the intentional and controlled decrease in a generator’s rotational speed in response to an increase in its load. It is expressed as a percentage and represents the relationship between the change in speed and the change in load.

Mathematically:

Speed Droop (%) = [(No-Load Speed - Full-Load Speed) / No-Load Speed] * 100

Where:

• No-Load Speed: The speed of the generator when it’s not supplying any load.
• Full-Load Speed: The speed of the generator when it’s supplying its rated full load.

Key Points:

• Purpose: Speed droop is intentionally built into the governor system of a generator to achieve stable load sharing when multiple generators operate in parallel.
• Mechanism: When a generator’s load increases, the governor senses the slight decrease in speed and responds by increasing the fuel or steam supply to the prime mover (engine or turbine), trying to bring the speed back to its set point.
• Load Sharing: In a parallel system, each generator has its governor set with a specific droop characteristic. When the total load increases, the generators with lighter loads will experience a larger speed drop, causing their governors to respond more aggressively and take on a greater share of the increased load. This self-adjusting mechanism ensures that the load is shared proportionally among the generators based on their capacity.
• Stable Operation: Speed droop helps prevent instability and hunting (oscillations in speed and power output) in parallel generator systems. By allowing a slight decrease in speed with increased load, it creates a natural damping effect that stabilizes the system.

Typical Droop Values:

• 4-5%: A typical speed droop setting for generators operating in parallel.
• Adjustable: The droop setting can be adjusted depending on the specific application and system requirements.

In summary, speed droop is an intentional characteristic of generator governors that allows for stable and proportional load sharing in parallel operation. By responding to load changes with a slight decrease in speed, it creates a self-regulating mechanism that ensures efficient and reliable power generation.

When two or more generators operate in parallel with different speed droop settings, even if they initially share the load equally, the system’s behavior can become unbalanced and inefficient over time.

Understanding Speed Droop:

• Speed droop is the intentional decrease in a generator’s rotational speed in response to an increase in load. It’s expressed as a percentage and represents the relationship between the change in speed and the change in load.
• A higher droop percentage means the generator’s speed will decrease more significantly when the load increases.

Effects of Different Droop Settings:

1. Unequal Load Sharing:

• The generator with the higher droop setting will experience a larger speed drop for the same increase in load compared to the generator with the lower droop setting.
• This leads to the generator with the lower droop setting taking on a disproportionately larger share of any load increase, potentially leading to overload.
• Conversely, the generator with the higher droop setting will shed more load when the overall demand decreases.

2. Frequency Instability:

• The varying speed responses of the generators can lead to fluctuations in the system frequency.
• The generator with the higher droop setting will contribute more to frequency deviations, potentially causing instability and affecting the operation of sensitive equipment connected to the system.

3. Reduced Efficiency:

• Unequal load sharing can result in one or more generators operating at suboptimal load points, leading to reduced overall system efficiency and increased fuel consumption.

4. Potential for Overload and Damage:

• If the load increases significantly, the generator with the lower droop setting may become overloaded, potentially leading to overheating, damage, or even a trip.

Mitigation Strategies:

• Matching Droop Settings: Ideally, all generators operating in parallel should have the same or very similar speed droop settings to ensure proportional load sharing and stable operation.
• Active Load Control: If it’s not possible to match droop settings, active load control systems can be employed to monitor and adjust the load distribution among the generators based on their capacity and droop characteristics.
• Regular Monitoring and Adjustments: Continuous monitoring of the system’s performance and making adjustments to the droop settings as needed can help maintain balanced load sharing and prevent overload situations.

In summary, operating generators in parallel with different speed droop settings can lead to unequal load sharing, frequency instability, reduced efficiency, and potential overload. It’s essential to ensure proper matching of droop settings or implement active load control measures to achieve stable and efficient operation of the paralleled generator system.

When generators run in parallel with differing speed droop settings and a sudden load increase occurs, an imbalance in load sharing arises.

Understanding the Scenario

• Parallel Generators: Multiple generators are connected to a common busbar, sharing the electrical load.
• Different Speed Droop Settings: Each generator’s governor has a unique speed droop setting. Recall that droop is the intentional decrease in speed as load increases.
• Sudden Load Increase: The electrical load demand on the system rises abruptly.

The Effect

1. Initial Frequency Dip: The sudden load increase causes a momentary dip in system frequency.
2. Unequal Response:
   • Lower Droop Generator: This generator, designed to resist speed changes more, will slow down less in response to the frequency dip. Its governor will react less aggressively, increasing fuel input only slightly.
   • Higher Droop Generator: This one, with a greater sensitivity to frequency changes, will slow down more significantly. Its governor will react strongly, increasing fuel input considerably to try and catch up to the set speed.
3. Unequal Load Pickup:

• As a result, the higher droop generator takes on a disproportionately larger share of the new load. It’s essentially “picking up the slack” more readily than the lower droop unit.

Consequences

• Overloading: If the load increase is substantial, the higher droop generator might end up overloaded, potentially leading to:
  • Overheating and Damage: Excessive current flow can cause overheating of the generator’s windings and other components.
  • Tripping: Protective devices might trip, taking the overloaded generator offline. This could further destabilize the system if the remaining generators can’t handle the full load.
• Inefficiency: The system operates inefficiently as the load isn’t shared proportionally based on each generator’s capacity.

Mitigation

• Ideally, match droop settings: All paralleled generators should have the same (or very close) speed droop settings for optimal load sharing.
• Active Load Control: If mismatched droops are unavoidable, sophisticated control systems can actively manage load distribution, overriding the natural droop response to prevent overload.
• Operator Awareness: Operators must be vigilant, monitoring load sharing and potentially making manual adjustments if automatic controls aren’t sufficient.

In essence, mismatched droop settings lead to an unequal and potentially dangerous response to load changes. Proper configuration or active control is vital for ensuring a balanced and stable parallel generator system.