同步发电机带电容性负载时，会产生助磁性质的电枢反应，所谓助磁，就是电枢电流产生的磁场会加强主极磁场，使得负载时的气隙磁场大于空载气隙磁场，因此其输出电压会升高。

至于停机，题主没有描述清楚，只是跳闸了还是电机失步停转了？无论哪种情况，可能有以下几个原因：

1 发电机突然接入较大的容性无功负载会引起电压突然升高，电压达到设定的跳闸阈值时会引起过压保护跳闸，这种情况一般是在很短时间内即跳闸。

同理，如果发电机突然接入一个较大的感性负载柴油发电机端电压会急剧降低，也可能会导致欠压跳闸。

2 同步发电机带较大容性负载时，因电枢反应的助磁效应使得端电压升高，励磁系统为了保持恒定电压必须减小励磁电流，减小励磁电流意味着发电机的反电势减小，电机的带载能力（带有功的能力）急剧下降，此时如果发电机还带有足够大的有功功率，会使功角急剧增大，当功角大于90°时，发电机会失步，从而引起停机。这种情况一般持续时间可能较长，需要特别指出的是，同步发电机的失步是一种非常严重的故障，甚至可以说是事故，很容易造成联轴器损坏甚至断轴的严重后果。

When a synchronous generator is loaded with a capacitive load, it generates armature reactions with magnetic assistance properties. The so-called magnetic assistance refers to the magnetic field generated by the armature current that strengthens the main pole magnetic field, causing the air gap magnetic field under load to be greater than that under no-load, resulting in an increase in its output voltage.

As for the shutdown, the questioner did not describe it clearly, was it just a trip or did the motor stop running due to out of step? In any case, there may be several reasons:

The sudden connection of a generator to a large capacitive reactive load can cause a sudden increase in voltage. When the voltage reaches the set tripping threshold, it will trigger overvoltage protection tripping, which usually occurs within a short period of time.

Similarly, if the generator is suddenly connected to a larger inductive load, the voltage at the generator end will sharply decrease, which may also cause undervoltage tripping.

When a synchronous generator carries a large capacitive load, the auxiliary magnetic effect of armature reaction causes the terminal voltage to rise. In order to maintain a constant voltage, the excitation system must reduce the excitation current. Reducing the excitation current means that the back electromotive force of the generator decreases, and the carrying capacity (carrying power) of the motor sharply decreases. At this time, if the generator still has sufficient active power, the power angle will increase sharply. When the power angle is greater than 90 °, the generator will lose step, causing shutdown. This situation may generally last for a long time, and it should be noted that the loss of synchronization of a synchronous generator is a very serious fault, even an accident, which can easily cause serious consequences such as coupling damage or even shaft breakage.