Is there reverse current flow in a generator during starting? If so, why?

When the generator is started, the so-called "reverse current" phenomenon may be observed, but this usually refers to the reverse electromotive force (Back EMF) that occurs at the moment of starting, rather than the true reverse current. The following explains this and why:

Back EMF (electromotive force)

When the generator first starts up, its rotor starts to rotate. According to Faraday's law of electromagnetic induction, when the rotor cuts the magnetic field in the stator windings, an induced electromotive force is generated in the windings. The initial direction of this induced electromotive force depends on the initial rotation direction of the rotor and the direction of the magnetic field. If the direction of rotation of the rotor is opposite to the preset generator output direction, a reverse electromotive force may be observed at the moment of startup.

Cause analysis

• Initial rotation direction: At the moment of start-up, if the rotation direction of the rotor is opposite to the direction of the magnetic field generated by the current in the stator winding, then the induced electromotive force generated will also be opposite.

• Magnetic field build up: At startup, the magnetic field inside the generator is not fully established, so the direction of the electromotive force initially generated may be different from the expected direction.

• Excitation system: For synchronous generators, the starting order of the excitation system may also affect the initial electromotive force direction. If the excitation system does not respond in time, it may cause a temporary reverse electromotive force phenomenon.

Reverse current

True reverse current refers to the flow of current in the opposite direction to the normal operation of the generator. This usually does not happen during startup unless there is a fault in the system or a flaw in the design. Here are some situations that can cause reverse currents:

• Failure to start: If the generator does not successfully start and enter normal operation, then there may not be enough electromotive force to drive the current, but there may be a reverse flow of current from the load or other power sources into the generator.

• Control system failure: If the control system is set up incorrectly or malfunctions, the current direction may be wrong.

• External effects: In some cases, such as a sudden change in grid voltage, current may temporarily flow in reverse.

How to deal with

• Check the starting procedure: Make sure the generator starting procedure is correct, especially for synchronous generators, you need to set up the excitation system correctly.

• Check the control system: check whether the control system is working properly and confirm that there are no setting errors or faults.

• Protective measures: Install appropriate protective devices, such as reverse current protectors, to prevent damage to the equipment caused by possible reverse current during startup.

• Monitoring and commissioning: Monitoring and commissioning before and after starting to ensure the normal operation of the generator.

Sum up

When the generator is started, it is usually the reverse electromotive force that is observed rather than the true reverse current. This phenomenon is usually due to the effect of incomplete establishment of the magnetic field at the moment of start-up or the initial rotation direction of the rotor. True reverse currents are less common, but when they do occur, they may be due to a control system failure or other external factors. Proper startup procedures, control system Settings, and appropriate protection measures can effectively avoid these problems.