Hello everybody,

I'm still on the synchronous gen/motor tip and have come across another snag.

When you parallel generators, you're supposed to have one generator at a slightly higher frequency than the other, and then as the power contributed from one generator increases, the frequency decreases until it is at the desired frequency and desired power contribution (I.e. like a house diagram). I'm unclear on how this happens.

How, for instance, does the generator stay in phase with the other generator if it is at a different frequency? How does lowering the frequency increase power contribution? I can't find any resources that explain this in much detail.

As always, your help is greatly appreciated!

 

At steady-state all motors or generators will be running @ synchronous speed. During transient changes (e.g. load change) the machine decelerates or accelerates to adjust to the required power (displacement angle δ).

 

My understanding is that you need to synchronize the two generators using a device such as a synchroscope. If the generators are not in phase when they are connected together you can damage them as they are suddenly forced into sync. This happens because one generator will deliver a large current into the other which causes it to act as a motor until the two are in phase.

 

In simpler terms, you sneak up on it. You adjust one generator until it is almost exactly the same speed as the other then watch your synchroscope until the phases match and then connect them. If you do it perfectly, they will lock together without so much as a click.

 

By the way not all multiple generator installations used a synchroscope and I have seen several older setups with twin generators in ships.

They were invariably three phase and there were three bulbs connected between the R S T phases of generator A and the R S T phases of generator B. ie R-R, S-S, T-T,
The operator would build up the speed of the off load generator until the bulbs started to get bright and dim, indicating the voltage difference of any pair. It doesen't matter whether they are in sync, just that the voltage on each sine wave was the same at any particular instant. Eventually as the speeds came close the bulbs would start to pulse off and on , the voltage difference interval got longer as the speeds got closer. At this stage the operator would simply throw a switch during an interval when the bulbs were off (low voltage difference) which avoided any damage to the switchgear by arcing etc. After that the two generators would automatically share the load between them and stay in phase.

 

What I'm unsure about is how, if you throw a switch when one generator is at a slightly higher frequency than the other, the two voltages stay in phase. Wouldn't the one with the higher frequency slowly get out of phase with the other? What forces are able to get the two gens in synch and ultimately lower the frequency of the oncoming gen to match the other one?

I also read about a "governor set point" but i don't understand how this works. I read that if you. Increase the set point, the starting frequency of the oncoming gen is increased and then when it reaches steady state frequency it is delivering more power to the network. How does this happen?

 

I suspect one must firstly have a clear understanding of the Torque vs Displacement angle relationship for a machine - motor or generator. This usually provides the basis / rationale for the notion of how synchronism is maintained under transient conditions. This relates to the angle (δ) between the machine pole centers and the terminal voltage rotating flux field.

This is also relevant to the more accessible concept either of the synchronization of a single machine onto an infinite bus or the response of a synchronized machine to a transient load demand change.

In the case of paralleling primary generators to a [possibly non-infinite] common bus the problem is more complex. Because the initial connection of a new machine to the bus is a transient condition, consideration must not only be given to the Torque vs displacement angle characteristics but also to a variety of other issues including prime mover governor & excitation control response, rotational mechanical parameters and machine transient current behavior.

 

What I'm unsure about is how, if you throw a switch when one generator is at a slightly higher frequency than the other, the two voltages stay in phase. Wouldn't the one with the higher frequency slowly get out of phase with the other? What forces are able to get the two gens in synch and ultimately lower the frequency of the oncoming gen to match the other one?

I also read about a "governor set point" but i don't understand how this works. I read that if you. Increase the set point, the starting frequency of the oncoming gen is increased and then when it reaches steady state frequency it is delivering more power to the network. How does this happen?

If you use a synchroscope or bulbs as cork_ie mentioned to match the phase between the two, then the frequency of the two generators is the same when the switch is thrown. After that the generators stay in phase because any change if phase between the two will increase or decrease the load on the generator, which tends to keep them in phase. Basically the generator output current is determined by the slight phase angle between the generator rotor field and the output voltage. Thus, for example, if the generator drive applied more torque to one generator, which would tend to speed up the generator, the increased generator load due to the slight change in phase angle will increase the generator load current, absorbing the extra torque and keeping the generators in sync. The two generators will thus always stay in lock step under normal operating conditions.

I don't exactly know what the "governor set point" is but it is apparently related to how much load current the generator will supply. But in any case the RPM and output frequency of the generator do not change once it is connected to the grid.