Hydraulic Governors in Parallel Alternators:
(a) Adjustable Integral Action:
Hydraulic governors in parallel alternators have adjustable integral action for two main reasons:
- Eliminating Steady-State Speed Errors: In a system with multiple generators sharing a load, slight differences in governor characteristics or mechanical imperfections can lead to steady-state speed errors between generators. Integral action in the governor accumulates the error signal over time, gradually adjusting the fuel supply until the speed error is eliminated. This ensures all generators maintain the same speed (synchronism) regardless of minor variations, promoting stable parallel operation and load sharing.
- Achieving Desired Droop Characteristics: In parallel operation, governors are configured with a specific “droop” characteristic. This refers to the decrease in generator speed with increasing load. Adjustable integral action allows fine-tuning of the droop, which determines how much each generator contributes to the total load change. By adjusting the integral gain, engineers can achieve the desired load-sharing ratio between generators, such as a 50/50 split as specified in your question.
(b) Load/Frequency Diagram and 50/50 Load Sharing:
A load/frequency diagram helps visualize the behavior of generators in parallel operation. Here’s how two generators achieve a 50/50 load share:
- Initial Conditions: Let’s assume both generators share the total load equally (50% each) and operate at the nominal frequency (f_0). This point is represented by the intersection of the individual generator droop curves (G1 and G2) with the total load line (P_tot).
- Load Change: When the total load increases (P_tot increases), both generators experience a slight decrease in speed due to their inherent droop characteristics. The speed drop is larger for the generator with a steeper droop curve (greater integral gain).
- Integral Action and Load Sharing: The integral action in each governor integrates the speed error over time. In response, the governor with the larger speed drop adjusts its fuel supply more significantly, causing its frequency to recover faster and take on a larger share of the increased load. The other generator’s frequency recovers slower, resulting in a smaller load increase.
- Equilibrium: This process continues until a new equilibrium is reached where the speed difference between generators is minimal, and they share the increased load according to their droop characteristics. With properly adjusted integral gains, both generators can achieve a 50/50 load share at the new operating point.
Diagram Illustration:
Imagine two downward-sloping curves (G1 and G2) representing the droop characteristics of generators. A horizontal line (P_tot) represents the total load. The intersection of these lines represents the operating points at different load levels. When the load increases, both curves shift down, but the one with higher integral gain shifts more, leading to a new equilibrium point where the load is shared equally.