The parallel operation of three-phase generators requires that in the circuit formed by connecting the generators to the grid, no inrush current should be generated. The following conditions must be met: (1) The voltages of the generators should be the same in magnitude and phase; (2) The frequencies should be the same; (3) The phase sequence should be exactly the same.
To verify whether these conditions are met, a voltmeter can be used to check the voltages. The method of connecting directly or in a cross configuration for parallel operation is called the “approximate step method”. At this time, there is also a “self-step method” for parallel connection, which is described as follows. 1. Connect the test generators in parallel using the step meter.
(1) Direct connection method,
(2) Cross connection method,
(3) Connect the test generators to the grid using the step meter. The parallel connection device in the actual generator is generally set up when the phase sequence is correct beforehand, and then the parallel method is considered. Some adopt an operationally simple but complex automatic control device, which only requires operation buttons; some use the step meter (also called the synchronous indicator) on the “synchronous panel” to find the closing timing for parallel operation. The “synchronous panel” also has two voltmeters and two frequency meters to monitor the voltage and frequency of the synchronous generator and the grid. The principle of connecting the synchronous generator to the parallel operation using the step meter is essentially the same as the light method, that is, using the step meter to indicate the frequency difference and phase difference between the synchronous generator and the grid, but the phase sequence must be correct. In production practice, the issue of phase sequence is generally considered during installation, and in the laboratory, the phase sequence can be measured using a phase sequence meter to make it consistent with the grid’s phase sequence. The commonly used step meters are divided into electric type and electromagnetic type according to their principles. Their wiring and usage methods are slightly different.
2. Connect the synchronous generator in parallel using the self-step meter. The advantage of using the above method for parallel connection is that the newly connected generator and another generator will not or only slightly be affected by the impact. However, manual operation is time-consuming. Of course, when using an automatic control device, the new generator can be connected to the grid and operate, which is somewhat better. But the device is relatively complex. When an accident occurs in a certain unit of the grid and it suddenly stops running, requiring another unit to be started and connected to the grid in parallel, due to the continuous changes in the voltage and frequency of this grid, it is difficult to meet the conditions for precise step. The situation is also very urgent. Therefore, the “self-step method” is adopted to achieve the parallel operation of the generator. The method is as follows: “First, short-circuit the excitation group of the generator through a resistor approximately 10 times the resistance of the excitation group. When the generator’s speed rises to the synchronous speed (the frequency difference between the generator and the grid is 5%), close the parallel switch first, then add the excitation. The motor’s “self-step effect” can be utilized to quickly bring it into synchronization.
3. Power regulation after generator grid connection
After the synchronous generator is connected to the grid, it often needs to change its active power and reactive power. If you want to increase the output active power of the generator, according to the principle of power balance, you only need to increase the input power of the original motor. According to the theory of DC motors, if the speed remains unchanged, to increase the output power of the DC motor, the excitation current of the DC motor should be reduced or the armature voltage of the DC motor should be increased.
When the active power is constant, to change the reactive power of the grid-connected synchronous generator, according to the formula
P2 = mUIaCOSΦ = constant and P2 = m(UE0)/Xs*sinδ = constant Both formulas are necessarily satisfied
IaCOSΦ = constant
E0sinδ = constant
From the phasor diagram analysis, when the active power remains unchanged, the method to adjust the reactive power of the generator is to change the excitation current of the generator.
For more questions about the generator set, please call the Beidou Power team. More than ten years of professional production and sales of power generation equipment experience, more professional engineer team to serve you, choose Beidou power is to choose rest assured, welcome on-site factory inspection.
Post time: Feb-02-2026