Currently, there are 3 types of alternators operating in the power systems of countries around the world:
① Synchronous generator;
② asynchronous generator;
③ Asynchronous synchronous generator.
Regardless of the type of generator, the interaction of the stator magnetic field and the rotor magnetic field is required to achieve power transfer. In order to make the power transfer stable, the stator magnetic field and the rotor magnetic field must move synchronously. The difference between the three motors is only in the way of acquiring the rotor magnetic field, which makes their working principles different.
There is an excitation winding on the rotor of the synchronous generator, and the excitation current is provided by an independent excitation device. The centerline of the field winding is called the d-axis (also called the straight axis), and the axis orthogonal to it is called the q-axis (also called the quadrature axis). The excitation current provided by the excitation device generates a magnetic field in the air gap between the generator stator and the rotor, and the electromotive force generated by the stator winding cutting the magnetic field is called synchronous electromotive force. Therefore, the synchronous generator is not only an active power source, but also a reactive power source or a reactive load, and its reactive power state is mainly determined by the relationship with the terminal voltage. At present, the capacity of synchronous generators in the power system has an absolute advantage.
There is no independent excitation device on the rotor of the asynchronous generator to provide excitation current. It can only generate excitation current by cutting the stator magnetic field by the rotor windings due to the asynchronous rotation of the rotor and the stator magnetic field, so it must absorb the reactive power of the power system. This kind of asynchronous generator is called stator excitation asynchronous generator, which is an active power source and a reactive load. Because of its simple structure, low cost, reliable operation and easy to realize unmanned duty, it is widely used in wind power generation.
The load of modern power system changes drastically, so it is required to improve the mobility of the power supply, that is, to improve the ability of active power-frequency regulation and reactive power-voltage regulation. Reactive power requires a two-way adjustment method, that is, not only a positive (sending) reactive power source, but also a negative (suction) reactive power source is required to ensure the quality of the supply voltage and optimize the reactive power flow of the system. Therefore, the asynchronous generator is used as a reactive power regulator. The method has good application value.
Synchronous generators can only run at synchronous speed, and the anti-interference ability (ie stability) is poor. Especially for large-capacity synchronous generators connected to high-level power grids, the electromagnetic time constant increases, and the per-unit value of rotational inertia decreases. At the same time, it often requires It absorbs reactive power under the light load state of the system, so the problem of running stability is more serious. In order to solve this problem, Russian scholars first proposed the idea of asynchronizing the synchronous generator, and thus developed the “asynchronous synchronous generator”. This kind of generator can run at both synchronous speed and asynchronous speed, and also has high running stability under the state of deep absorption of reactive power. At the same time, since the asynchronous synchronous motor can adjust the speed in a small range, it can also be used in pumped-storage power stations, so that the speed can be adjusted under the two operating states of power generation and water extraction to achieve the best operating efficiency. In the mid-1980s, the former Soviet Union invested two 200MW asynchronous synchronous steam turbine generators in Ukraine’s Bullstown Thermal Power Plant. The actual operation and test proved the good performance of this type of generator set. An asynchronous synchronous generator is essentially a synchronous generator, but it can operate asynchronously. In order to distinguish from the aforementioned two generators, the aforementioned synchronous generator is referred to as a conventional synchronous generator, and the aforementioned asynchronous generator is referred to as a stator-excited (or conventional) asynchronous generator.