• isquaredr
isquaredr
TL;DR Summary
Intuitive explanation for generator load sharing
I came across this statement in a generator manual I was reading(manufacturer Rolls Royce)
If the fuel supply of one of the engines in the sys-
tem is increased, it will carry a greater part of the
total load, but the speed is not increased.

The documentation barely touches any theoretical explanation as to how this works.
I would like to ask specifically how this phenomena is achieved. If a generator's fuel output is increased, it produces more mechanical force to turn the rotor shaft: given this, how does this make the generator assume more load to itself from the busbar? Take note that the context is parallel generators supplying a busbar for power distribution.
I understand that a higher load(more current) produces a counter-torque to the generated magnetic field, hence the fuel supply must increase to make up for this change in torque(to maintain constant busbar voltage)
However, how does the reverse happen: that increasing the fuel supply will result in it taking more load from the bus? I am convinced in the order that more load -> must increase fuel, but to reverse it and say more fuel -> generator takes more load, seems counter-intuitive.
I fail to picture the link between these two statements.

AC bus? Then it's closer to the operation of grid level load distribution, and is indeed not really intuitive for regular EEs. Small changes in frequency, phase ending with drastic changes in loads...

isquaredr said:
TL;DR Summary: Intuitive explanation for generator load sharing

However, how does the reverse happen: that increasing the fuel supply will result in it taking more load from the bus?
I assume we are talking about diesel engines with fuel injection pumps. Diesels normally run with excess air, so increasing the fuel volume per power stroke, will result in a greater piston pressure, and so a greater crankshaft torque.

The speed of a diesel is regulated by the governor, which is part of the injection pump. The governor regulates the proportion of the maximum fuel setting that will be injected.

The available power is the product of engine torque and the grid synchronous RPM of the generator.

Now there are two situations.

1. Where the two generators are driving an "infinite" regional bus.
The RPM of the generators will be regulated by the bus frequency. The power transferred by the generator to the grid is determined by the few degrees of phase advance of the generator, relative to the grid. The power available is automatically transferred to the grid by balancing torque against phase advance.

2. Where the two generators are supporting and regulating a local grid.
The two diesel engines, driving separate grid tied generators, will have their centrifugal governors coupled. That will regulate the RPM and so the frequency of the local grid. The field excitation of the generators will be adjusted to regulate the grid voltage. The phase of the two generators will self adjust to balance the available power from each generator. The multiplication feature of the diesel governors will maintain the power ratio of the two engines and generators.

DaveE, isquaredr and Tom.G
Baluncore said:
The speed of a diesel is regulated by the governor, which is part of the injection pump. The governor regulates the proportion of the maximum fuel setting that will be injected.
I assume that each diesel generator has a self-regulating mechanism to keep it at rated speed that is a part of the fuel injection system? Is it correct to say that the diesel's governor with a rated speed setting(the arrangement of the linkages) will regulate itself to run at a specific baseline rpm? And then the electronic actuated movement of the governor(for more fuel supply) and AVR control only comes into play when we speak of applying it into a network of generator to supply a bus?

My confusion also stems from my assumption that the idea of increasing a diesel generator's fuel supply is a function of the load demand; the speed regulator sends a fuel increase signal to the governor to meet the torque demand..... now, where I can not wrap my mind is how increasing the fuel inverses this function: that instead of "x then y", it is also possible for "y then x". X being higher load demand and Y being fuel increase.
Baluncore said:
The power transferred by the generator to the grid is determined by the few degrees of phase advance of the generator, relative to the grid. The power available is automatically transferred to the grid by balancing torque against phase advance.
Do you mean to say that the phase advance of a diesel generator(DG) dictates it's capability to assume a higher load relative to the other parallel DGs? Can you expound more on this one if my understanding of your statement is correct?

isquaredr said:
Can you expound more on this one if my understanding of your statement is correct?
First, you need to define the system and not leave me guessing what you are considering.
Second, can you simplify the system by considering a single diesel generator driving a three-phase infinite bus.

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